Nuevos fotoánodos y nuevos materiales transportadores de huecos para la fabricación de celdas solares fotovoltaicas basadas en perovskita

New Photoanodes and Hole Transporting Materials for the Fabrication of Perovskite-Based Solar Cells University of Seville Director: Shahzada Ahmad Tutor: Agustín Rodríguez González-Elipe Fco. Javier Ramos Mellado PhD Program: “Ciencia y Tecnología de Nuevos Materiales” 2016 ABENGOA RESEARCH III Acknowledgments/Agradecimientos First of all, I would like to acknowledge Abengoa Research for giving me the opportunity to start my doctoral work. It was a great pleasure being part of this company, particularly, taking part of this pioneer project called Abengoa Research: doing cutting edge research in Spain, in sustainable energy field, looking for future business sounded as too many fantasies together but we combated and overcame them. Long live AR! Thanks also to University of Seville and specially to responsibles of Science and Technology of new Materials Program, since organizing a PhD Program under continuous changing conditions is never easy and you were able to consolidate it successfully. Very important and personal thanks to my supervisor at Abengoa Research and the director of this thesis: Shahzada Ahmad, for trusting me and letting me work my way in a very exciting and motivating environment offering me always, the best was in your hand. I have learnt a lot from you, but the best is, I can really say I have a good friend for future. My sincere thanks to Agustin Rodriguez González-Elipe, tutor of this thesis for giving me interesting and fruitful guidelines. Your immense knowledge was always instructive and motivational. Next paragraph is booked for all my colleagues at Abengoa Research, in particular for all those worked next to me at lab: Samrana, Manuel, Laura, Elena, Gonzalo and all of them who helped me to complete this research work. I cannot forget Manuel Doblaré, as main responsible of Abengoa Research project. You always had your door opened for any problem or question being able to solve everything was in your hand bringing a smile. Very special thanks to Michael Grätzel and M.K. Nazeeruddin. It was an unforgettable opportunity working at EPFL with you. It has been probably the most inspirational experience in my life. I have learnt not only more science I could ever imagine there, but also I discovered myself living in other country. Thanks also to all my colleagues at EPFL, they always helped and taught me in the right direction. La prochaine reconnaissance est pour la famille Burrus. Xavier et Diane, vous m’avez fait sentir plus qu’un simple locataire à St. Sulpice, vous savez très bien que vous êtes pratiquement mes parents suisses parce que je me sens comme un serpeliou-espagnol. Merci beaucoup pour votre énorme gratitude et attention. Merci beaucoup de vos leçons linguistiques car le français que je suis capable de parler est grâce à vous. J’ai bien joui des débats et discussions, de mon étage à Zermatt et de la magnifique opportunité de découvrir la gastronomie suisse : raclette, fondue, papet vaudois et des fromages. Merci beaucoup et à bientôt! IV A continuación, me tomaré la licencia de escribir unas pocas líneas en español. Mamen y Saioa. Una parte bien importante de esta tesis es gracias a vosotras. Os voy a echar mucho de menos. Os deseo lo mejor, que es lo que me habéis dado, me habéis animado y me habéis hecho pasarlo bien olvidando el tedio del día a día. Gracias por no poder evitar sonreír aunque, a veces, vengan mal dadas. Vuestro positivismo y alegría es altamente contagioso. Seguid cultivándolo. Me gustaría agradecer a todos los compañeros doctorandos en Abengoa, los que quedan y aquellos que pasaron. Me gustaría no perder nunca el contacto a través de los grupos de whatsapp Martes de cañas y DMHe. Ha sido una época apasionante de aprendizaje multidisciplinar y de continuo cultivo de la curiosidad, germen de toda ciencia y conocimiento. (Espero encontrar candidatos que custodien la Constitución de DMHe). Para todos mis amigos del pueblo del que: “Cual nevada paloma / que del azul del cielo / vino a posar el vuelo / en lo alto de una loma / castellana, / irradia su blancura / hacia la gran llanura / el Campo de Criptana. / Así la vio Cervantes / y la juzgó tan bella / que formó cien gigantes, tan blancos como ella, / para que con sus brazos extendidos / le dieran el más firme de los nidos,” porque pese a la distancia que me ha acarreado esta etapa en mi vida, siempre me siento como en casa al recordar cuando alguna vez hemos entonado esos versos. Espero que perdure por muchos años el espíritu del Palomino’s Club aunque, a veces, no lo podamos rememorar cuánto quisiéramos. Sin moverme de geografía, a toda mi familia. Principalmente a mis padres y mi hermano. Papá y mamá, espero que seáis conscientes de que vuestro continuo apoyo y ánimo han sido los principales causantes de seguir siempre hacia delante con paso seguro y firme, sin desistir, sin dejar de esforzarse. Muchas gracias por vuestro sacrificio generoso y por vuestros sabios consejos. A veces, estúpidamente, estas cosas cuesta decirlas; aquí las tenéis para cuando queráis leerlas. Carlos, siento ser un manco y cansarte a menudo con mis torpezas informáticas; me gustaría devolverte la ayuda que me ofreces y poder vernos más porque siempre que estoy contigo aprendo mucho y me lo paso en grande. Por cierto, ahora ya me vas a poder cantar el “ Me hago doctor ” con razón. Además, un cariñoso recuerdo a mi abuela Josefina, a la que me cuesta recordar sin lágrimas al pensar en la alegría que hubiera sentido de haber visto completado este proceso. Por último, a Miriam, mi novia y espero que, dentro de poco, mucho más. Sabes que has sido la principal sufridora de mis largas estancias lejos, mis malos humores en momentos estresantes y divagaciones varias que suponen esta tesis. Muchas gracias por tu apoyo, tu comprensión y tu cariño. Ten presente que, al final de una etapa se aprecia con añoranza y nostalgia el camino recorrido y con pasión y entusiasmo lo que está por llegar, “o sea / resumiendo / estoy jodido / y radiante / quizá más lo primero / que lo segundo / y también / viceversa”. “Ipsa scientia potestas est” (knowledge itself is power) - Sir Francis Bacon In Meditationes Sacrae (1597) “Any problem can be solved using the materials in the room” -Edwin Herbert Land. In Peter C. Wensberg, Land's Polaroid: A Company and the Man Who Invented It (1987). VII Abstract Perovskite Solar Cells (PSC) is one the most recent photovoltaic approaches to transform directly sun light into electricity. Though perovskite type materials are known for more than a century, the employment of novel hybrid organic-inorganic perovskite materials as absorbers in photovoltaic was recently developed, firstly in 2009 using perovskite with a liquid electrolyte based devices and afterwards in 2012 with the start of the so-called perovskite fever employing a solid-state configuration . During this short period of existence, the evolution of PSC field has been prompt and substantial, and power conversion efficiencies over 20% at lab scale are being obtained. In a typical PSC configuration, perovskite absorber layer is placed between electron-transporting material (ETM) and hole-transporting material (HTM). Normally, ETM layer consisted of mesoporous films made of inorganic metal oxides particles, principally TiO2, other metal oxide as well as n -type organic or inorganic based materials. At lab scale, ETM can be easily dispensed through solutions process spin coating technique to prepare the photoanode of the PSC. However it is unscalable and high material consuming process. For the HTM, some molecules are extensively reported in the literature due to its performance such as 2,2',7,7'- Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) and Poly(triaryl amine) (PTAA). However, they have some disadvantages such as their high price, difficult synthesis and the requirement of adding certain compounds for high performance which reduce the final stability of devices. In order to overcome the cited drawbacks of existing ETM and HTM for PSC fabrication, some novel alternatives have been exposed in the present work to solve the state of the art issues. Firstly, ZnO porous columnar thin films grown by plasma enhanced chemical vapor deposition (PECVD) were used as effective photoanodes in PSC. With this method it is also possible to control the deposition conditions over large area. Furthermore, columnar TiO2 films grown by physical vapor deposition at oblique angle incidence (PVD-OAD) were also applied as photoanodes in PSC; the resulting thickness and porosity of thin films can be modified by tuning the incidence angle. In addition, with the employment of PVD-OAD technique, rational design of 1-dimensional photonic crystals (1-DPC) layers were implemented as photoanodes in PSC exhibiting enhanced optical and absorbing properties. The techniques employed here for the photoanode development were solvent free and present good control of the film properties and homogeneity over large area. On the other hand, novel HTM molecules were explored in PSC fabrication. Moreover, all the studied molecules presented highest occupied molecular orbital (HOMO) levels compatible with perovskite valence band, making them candidates to be used in PSC. The soluble pentacene derivatives such as TIPS-pentacene, was reported for first time as efficient and stable HTM in PSC without the use of any additive. A non-aggregated phthalocyanine molecule (coded as TT80), soluble in organic apolar solvents has been described as HTM in PSC as well. Two novel molecules, which were easy to synthesize and were based on triazatruxene VIII compounds (named as HMDI and HPDI), were design as such to ensure high solubility in apolar solvents, improved stability and performance in PSC. In conclusion, by introducing novel ETM architectures and HTM compounds, the spectrum of possibilities in the field of PSC has been expanded. This offers new pathways for future PSC development and commercialization. IX Resumen Las celdas solares de perovskita (PSC, del inglés perovskite solar cells ) se consideran uno de los ejemplos de tecnología fotovoltaica más recientes para transformar de manera directa la luz solar en electricidad. Aunque las perovskitas son materiales que se conocen por más de un siglo, la utilización de perovskitas híbridas orgánicas-inorgánicas como materiales absorbedores en dispositivos fotovoltaicos es relativamente reciente, siendo usadas por primera vez en 2009 en dispositivos fotovoltaicos con electrolito líquido y, poco después, al comienzo de la fiebre de la perovskita en 2012, en configuraciones en estado sólido. En este breve lapso de tiempo, la evolución en este campo fotovoltaico ha sido rápida y sustancial, habiéndose logrado eficiencias en la conversión superiores al 20% a escala de laboratorio. En una configuración de PSC típica, la capa de perovskita que funciona como absorbedor de luz se encuentra ubicada entre un material transportador de electrones (ETM, del inglés electron-transporting material ) y un material transportador de huecos (HTM, del inglés hole-transporting material ). Normalmente, la capa de ETM consiste en una lámina mesoporosa hecha de partículas de óxidos metálicos (TiO2, principalmente), pero también se pueden encontrar otros materiales semiconductores tipo-n, tanto orgánicos como inorgánicos. A escala de laboratorio, el ETM se puede dispensar fácilmente a través de técnicas de spin-coating para preparar los fotoánodos de las PSC. Sin embargo, este proceso no es escalable y requiere un gran consumo de productos. En el caso de los HTM, algunas moléculas como 2,2',7,7'-Tetrakis[N,N-di(4- metoxifenil)amino]-9,9'-spirobifluoreno (Spiro-OMeTAD) y Poli(triarilamina) (PTAA) han sido extensamente documentadas en bibliografía debido a su contrastado comportamiento. No obstante, dichos compuestos presentan algunos inconvenientes como su elevado precio, su difícil síntesis, así como la necesidad de añadir ciertos aditivos en su composición que disminuyen la estabilidad de los dispositivos fotovoltaicos. Para solventar los problemas encontrados en el estado del arte que tienen relación con el uso de ETM y HTM en la fabricación de PSC, se han propuesto algunas nuevas alternativas en el presente trabajo. De este modo, en primer lugar, se han implementado láminas delgadas porosas basadas en columnas de ZnO crecidas mediante deposición química en fase vapor mejorada por plasma (PECVD, del inglés plasma-enhanced chemical vapor deposition ) como fotoánodos de PSC, siendo eficientes y habiéndose empleado técnicas fácilmente escalables y homogéneas sobre grandes superficies para su preparación. En segundo lugar, mediante la técnica de deposición física en fase vapor bajo ángulo de incidencia oblicuo (PVD-OAD del inglés physical vapor deposition at oblique angle deposited) , se han preparado films nanocolumnares de óxido de titanio que se han introducido como ETM poroso de PSC, donde ciertas propiedades del fotoánodo tales como la porosidad pueden ser controladas de manera precisa modificando el ángulo de incidencia. Además, con el uso de la misma técnica se han diseñado cristales fotónicos porosos de 1-dimensión (1-DPC, del inglés 1 -dimensional photonic crystal ) que sirven de fotoánodos de PSC con propiedades ópticas y de absorción mejoradas en los dispositivos fotovoltaicos. XVI 3. Global summary of results - 51 - 3.1 PSC based on nanocolumnar plasma deposited ZnO thin films - 53 - 3.2 Nanocolumnar 1-dimensional TiO2 photoanodes deposited by PVD-OAD for PSC fabrication - 56 - 3.3 Porous 1-dimensional nanocolumnar structures as effective photonic crystal for PSC - 61 - 3.4 A dopant free linear acene derivative as a HTM for PSC - 65 - 3.5 Non-aggregated Zn(II)octa(2,6-diphenylphenoxy) phthalocyanine as a HTM for efficient PSC - 68 - 3.6 Rational design of triazatruxene based hole conductors for PSC - 70 - 4. Global discussion of results - 75 - 4.1 PSC based on nanocolumnar plasma deposited ZnO thin films - 77 - 4.2 Nanocolumnar 1-dimensional TiO2 photoanodes deposited by PVD-OAD for PSC fabrication - 78 - 4.3 Porous 1-dimensional nanocolumnar structures as effective photonic crystal for PSC - 80 - 4.4 A dopant free linear acene derivative as a HTM for PSC - 82 - 4.5 Non-aggregated Zn(II)octa(2,6-diphenylphenoxy) phthalocyanine as a HTM for efficient PSC - 84 - 4.6 Rational design of triazatruxene based hole conductors for PSC - 85 - 5. Conclusions - 87 - Appendix - 91 - Presentation of doctoral thesis (official document) - 93 - Appropriateness report - 95 - Contribution report - 97 - Report of scientific relevance of the publications - 99 - List of publications - 103 - Copy of publications contained in this thesis - 105 - International mention (official document) - 185 - International experts reports - 187 - Certificate of stays - 191 - List of stays - 193 - List of patents - 193 - List of conferences - 194 - 1. Introduction 1. Introduction - 3 - 1.1 Brief history of PV Solar cells transform sunlight directly into electricity with the employment of semiconductor materials through photovoltaic (PV) effect. Although PV installation has increased considerably in recent years, and the total power installed capacity in the world was ~233GWp by the end of 2015 1 and will be between 396-540 GWp by the end of 2019 [1] (Figure 1.1). The growth of PV technologies has been long and irregular. In the following sections, a short historical overview of PV technologies will be presented. Figure 1.1. World solar PV cumulative market scenarios until 2019. Source: Solar Power Europe (SPE) [1]. 1.1.1 Early PV approaches The PV effect was discovered by A. E. Becquerel in 1839 where silver halides coated electrodes improved the generated current in the presence of blue and violet light [2]. Further, in 1877, W.G. Adams and R.E. Day noticed the generation of a current between two platinum wires connected at the edges of a vitreous selenium cylinder [3], i.e. the first evidence of PV effect in a solid state device ever. During the Universal exhibition 1878 in Paris, A. Mouchot displayed the first solar power generator. Following this, in 1883, C. Fritts was able to fabricate a thin film solar cell sandwiching 25-125 µm of Se between a metal and a gold leaf manually pressed to the Se surface [4]. Although these Se thin film PV prototypes were poorly efficient (<1% Power Conversion Efficiency, PCE), it is worth to mention the 1 To be updated by June 2016 in the 2016 SPE report. Ph.D. Thesis. Fco. Javier Ramos Mellado - 4 - overall configuration was quite similar to the current silicon solar cell technology. C. Fritts was the first to demonstrate the capability of this field, identifying the use of low cost materials and process, and the power storage as critical points for a future development of this technologies [5], being both topics are still essential nowadays. After several decades, during early 1930’s there was a renaissance in PV technologies due to the copper-cuprous oxide junctions (Cu-Cu2O). Almost 40 scientific works were reported in Grondahl’s review in 1933 for this junction couple. [6]. A typical configuration of the original Cu-Cu2O PV cells consisted on cuprous oxide grown over metallic copper; a Pb wire was pressed to cuprous oxide with a glass plate [5]. For the first time, a transparent side was described in a layered architecture to allow the light come into the cell. Taking advantage of the Cu-Cu2O impulse, selenium based solar cells were studied further and improved, converting in the dominant variety of PV those days [7]. During that decade, more precisely in 1832, the PV effect in Cadmium selenide (CdSe) was demonstrated by Audobert and Stora. In 1839, Nix and Treptwo reported thallous sulphide (Tl2S) as semiconductor for PV [8]. 1.1.2 1st Generation PV J. Czochralski invented the method to produce single crystal metal ingots from molten solutions with the help of a seeding agent by 1918 [9]. Nevertheless, until thirty years later, this process was not adapted to grow semiconductor singlecrystal ingots. Czochralski process variation to grow crystalline germanium and later silicon was conceived in 1948 by G. Teal and J. Little [10], being considered as the starting point for subsequent first generation PV. Another decisive development in the PV field took place in 1941, when doping was utilized for first time creating p- and ntype doped Si [11]. During 1950’s, Bell Labs were focused in the fabrication of solar cells for space purposes. In 1954, the first silicon solar cell with a PCE of ~6% based on a p-n junction was fabricated by the Bell Lab team composed of Daryl Chapin, Calvin Fuller and Gerald Pearson [12]. That was a major breakthrough in the field of PV technologies, since only 0.5% PCE were obtained earlier this advance. For first time, a functional solar cell showed a decent PCE that allowed its implementation in several applications. After further optimization, Bell Labs was able to produce silicon solar cell with 11% PCE. That was considered as the birth for 1st generation PV. 1st Generation PV comprises both monocrystalline and polycrystalline silicon solar cells. The main difference between mono- and polycrystalline silicon relies on the crystal size. While monocrystalline silicon has a grain size over 10 cm, polycrystalline has much smaller, and is normally between 1 µm-1 mm (when grain size is between 1 mm-10 cm it is often termed as multicrystalline) [13]. From an industrial prospects, monocrystalline silicon is being grown by Czochralski process; whereas polycrystalline silicon is produced by either Siemens or fluidized bed reactor (SBR) processes to achieve very high purity silicon (solar grade). 1. Introduction - 5 - Currently, first generation PV solar cells are able to reach 25.6% of PCE for monocrystalline and 21.25% for polycrystalline at lab scale (Figure 1.2). While in a solar modules 22.9% and 19.2% PCE is being offered respectively [14]. Presently, PV market is clearly dominated by 1st generation silicon crystalline solar panels with a ~90% of the global market [15]. 1.1.3 2nd Generation PV In order to find other alternatives inside PV technology, which consuming less material and potentially more efficient and cheaper than the ones based on silicon, thin film based technology was developed. They were termed as 2nd generation PV and appeared in 1970’s. These type of thin film PV were made of either III-V ( e.g. GaAs), II-VI (such as CdTe and CdS) or I-III-VI2 (for example CIGS) semiconductors or amorphous silicon. In these configurations, the absorber is presented as a thin film, in order to save material and hence reducing the cost. One important highlight during this period was the first GaAs heterostructure created by the Nobel prize winner, Z. Alferov and coworkers in 1970 [16]. GaAs is one of the most interesting alternatives to replace crystalline silicon with certified PCE of 28.8% in single junction solar cells and 24.1% for modules [14]. Copper Indium Gallium Selenide (CIGS) and Cadmium telluride (CdTe) thin films based absorbers were also classified in this category. PCE of 22.3% for CIGS and 22.1% for CdTe at lab scale and modules with 17.5% and 18.6% respectively have been reported recently [14]. Additionally, in 1976, D. Carlson and C. Wronski developed the first amorphous silicon (a-Si) solar cell with 1.1% of PCE [17]. Two years later, the first solar calculator was developed using this technology. Although at that moment the performance was limited, it paved the route to take advantage of a material which today dominates the world of mini PV applications, such as calculators and small fans. PCE of 10.2% for a-Si and 13.6% using a multijunction in combination with nanocrystalline silicon were certified leading this sector [14]. Today, approximately the 10% of the market is covered by thin film technologies [15]. However, certain issues as toxicity and the use of low abundant materials must be solved. 1.1.4 3rd Generation PV The most recent PV impulse has been the so-called 3rd Generation PV; a diverse variety of different PV approaches belongs to this group. 3rd Generation PV consist in diverse technologies, not fully commercially exploited yet, which could allow a reduction in the final watt peak price and they have been developed since 1990’s. They are potentially cheap and easy to fabricate. These PV technologies still need more research to push the efficiencies and to circumvent certain issues to be reliable for industrial scale. Dye-Sensitized Solar Cells (DSSC) are considered as the first approach inside this category. DSSC were presented by B. O’Regan and M. Grätzel in 1991 using a mesoporous TiO2 (mp-TiO2) photoanode, a Ru complex dye and iodide/triiodide Ph.D. Thesis. Fco. Javier Ramos Mellado - 6 - redox liquid electrolyte with PCE over 7% [18]. To avoid leakage and corrosion issues caused by liquid redox electrolytes, solid-state dye-sensitized solar cells (ss- DSSC) were developed placing an organic hole transporting material (HTM) instead of liquid redox shuttle. First report for ss-DSSC was published by U. Bach and coworkers in 1998 showing a 0.74% PCE [19]. Nowadays, DSSC technologies have shown interesting results, their working principles have been understood and a wide variety of approaches and alternatives have been created [20]. The Japanese company Sharp have certified efficiencies of 11.9% in the case of liquid DSSC (8.8% for a submodule) [14] while lower performances was reported for ss- DSSC, a respectable PCE of 7.2% was reported when small organic solid molecule 2,2',7,7'-Tetrakis[ N,N -di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) was placed as HTM [21], and 10.2% for inorganic HTM based on perovskite structure [22]. Organic Photovoltaics (OPV) belongs to the area of electronics where conductive organic polymers and/or small organic molecules are responsible for the absorption of light and generation of charges by PV effect. They have been developed since early 2000’s. Similarly to DSSC, OPV has offered plenty of alternatives with potentially cheap solutions [23]. Champion cells certified in this field have shown PCE of 11.0% for lab scale and 8.7% for a module, both made by Toshiba [14]. When a crystal of semiconductor material such as: PbS, PbSe, CdS, CdSe or CdTe is small enough (in the order of nanometers) quantum confinement in the three directions of space can be found forming quantum dots. These semiconductor nanoparticles can absorb light and transmit its energy to a second semiconductor which is sensitized by the quantum dots transferring the generated charges. Quantum Dot Solar Cells (QDSCs) are based on this principle. Although the sophisticated physics is underlying these devices, the fabrication approach is simple, being possible the preparation through solution process in a simple manner with a well control of the bandgap and an easy tuning of the absorption spectra by tuning the size of the dots [24]. PCE of 10.6% was certified using QDSC by Toronto University. Finally, the most recent technology discovered inside 3rd generation PV has been perovskite solar cells that will be explained more in detail in subsequently section. 1.1.5 Perovskite solar cells (PSC) Hybrid organic-inorganic perovskite solar cells (PSC) have been the most recent advance in 3rd Generation solar cells. Perovskites with PV interest are based in halides with central cations having oxidation state +2 such as lead (II) or Sn (II) and organic cations +1 like methylammonium (MA) or formamidinium (FA). Mitzi and coworkers from IBM Center developed several organic-inorganic perovskites with other organic cations for diverse electronic applications such as transistors and light emitting diodes [25]–[29]. In 2009, Miyasaka and coworkers published the first approach using MAPbI3 and MAPbBr3 perovskites as absorbers in DSSC with a limited PCE of 3.8% [30]. After that, Park’s group showed a QDSC using perovskite with a very promising 6.5% PCE [31], however the structure still uses corrosive liquid electrolytes. 1. Introduction - 7 - In 2012, methylammonium lead triiodide perovskite (MAPbI3) was introduced in a solid state devices forming a continuous film of perovskite, sandwiched between the electron transporting material (ETM) and hole transporting material (HTM) showing efficiencies over 9% [32]. Another significant characteristic has amplified the interest over PSC was the demonstration of ambipolar conductivity, i.e. both electrons and holes can be transported in hybrid organic-inorganic perovskites [22], [33], [34]. Grätzel and coworkers pushed this research field by reporting and certifying a highly efficient solar cell using sequential deposition route [35]. Nowadays efficiencies >20% have been reported in literature [36], [37] with a certified record of PCE=22.1% from KRICT/UNIST, Korea. In addition, efficiencies of 15.6% have been certified recently using active areas bigger than 1 cm2 [38]. 1.1.6 Summary and efficiency chart A summary of the best efficiency research cell efficiencies is shown in Figure 1.2. This work is compiled by National Renewable Energy Laboratory (NREL). Crystalline Si is represented in blue, 2nd generation PV cells are in green (except GaAs is in purple), and emerging 3rd generation PV are depicted in orange-red. Heterojunctions are represented in purple (except the abovementioned GaAs case). Heterojunctions were not explained before because they are out of the scope of these work but, essentially, they consist on devices with several semiconductors with different band gaps where each semiconductor absorbs certain range of wavelength. Figure 1.2. Best research-cell efficiencies chart for different PV technologies. Extracted from NREL website. On the basis of absorption of light and charge carrier take place together (in the same material) or separated (in two different ones), PV cells can be classified in semiconductor and sensitized solar cell respectively. Whereas in semiconductor solar cells 1st, 2nd generation PV are included; between sensitized solar cells DSSC (both liquid and solid) and QDSC are counted in. Ph.D. Thesis. Fco. Javier Ramos Mellado - 8 - The case of OPV is more complex, since, light absorption occurs in the donor molecule and the generated Frenkel exciton is diffused to the interface where the splitting of excitons takes place; then charges travel from that interface to the edges. They are not purely semiconductors because they need an interface to dissociate the high energy Frenkel exciton into electron and holes. PSC was seen as quantum dots sensitizing mp-TiO2 during the first approaches of this technology [30], [31]; however, currently it has been established as semiconductor where transport of charges happens in the bulk of perovskite as well. Based on previous classification and the generation they belong to, a conceptual scheme is presented in Figure 1.3 including all PV technologies. Figure 1.3. Scheme of the different generations of PV showing the different types included in each category. 1. Introduction - 9 - 1.2 Solar Cell Characterization 1.2.1 Solar spectra Sun irradiance is not homogeneous over Earth surface. Hence, the selection of the appropriated solar spectra adequate to test the PV devices is a key step in the subsequent evaluation and comparison of the solar cells. To compare results and extract conclusions from parts of the world with different irradiance, an international standard is needed. The standard used is made by American Society for Testing and Materials (ASTM). Air Mass Zero spectra (AM0) is considered as the extraterrestrial irradiance i.e. the irradiance at the top of the atmosphere (black line in Figure 1.4). It is worthy to say AM0 correlates quite well with the black body irradiation at 5800 K. 1366.1 W m-2 is given by the total integrated irradiance of this spectrum. AM0 irradiance spectrum is reported in ASTM E-490. Once light has reached the atmosphere, a part is attenuated by the absorption and scattering before attaining terrestrial surface. UV and visible light is absorbed by ozone (O3) and oxygen (O2), near infrared by nitrous oxide (N2O) and methane (CH4) while carbon dioxide (CO2) and water (H2O) absorb both in near and far infrared. Another factor to take in consideration is the angle of incidence of incoming light which depends on the latitude and the moment of the day. To standardize it, the concept air mass (AM) is applied. AM is the optical path length through the atmosphere (𝐿) normalized by the path length vertically upwards i.e. the zenithal one (𝐿0). Accordingly, AM is calculated as: 𝐴𝑀 =𝐿 𝐿0≈1 cos⁡(𝜃) (1.1) Where 𝜃 is the angle formed between the zenith and the shortest path direction of incoming sun light. ASTM G-173-03 establishes an angle of 48.19º giving AM=1.5. Furthermore, for this standard, both direct and diffuse light is considered giving 1000.4 W m-2 for AM1.5G. If no more information is specified, AM1.5G will be the sun irradiance standard employed in this whole work. AM0 and AM1.5G are depicted in the following Figure 1.4 showing the reduction caused by absorption of atmospheric gases. Figure 1.4. Solar irradiance spectra AM0 and AM1.5G extracted from ASTM E-490 and ASTM G- 173-03 respectively. 0500 1000 1500 2000 2500 3000 3500 0.0 0.5 1.0 1.5 2.0 2.5 Solar Irradiance (W m-2 nm-2) Wavelength (nm) AM0 AM1.5G Ph.D. Thesis. Fco. Javier Ramos Mellado - 16 - structures with ABX3 formula regarding their ionic radii (equation 1.19). Therefore, when halide perovskites (ABX3) with tolerance factor comprised between 0.71< 𝑡 <1, also require an octahedral factor of 0.41<𝜇 < 0.732 for the stability of perovskite crystal structure. 𝜇 =𝑅𝐵𝑅𝑋 ⁄ (1.19) Considering organic-inorganic hybrid perovskites halides with PV interest, ionic radii of different ions are [53]:  𝑅𝐴=0.18 nm for methylammonium (MA), 0.23 nm for ethylammonium (EA) and 0.19-0.22 nm in the case of formamidinium (FA).  𝑅𝐵=0.119 nm for Pb2+ and 0. 110 nm for Sn2+.  𝑅𝑋=0.181 nm for Cl-, 0.196 nm for Br- and 0.220 nm in the case of the most extended I-. Consequently, the crystal structure of typical CH3NH3PbI3 (𝑡=0.834 and 𝜇=0.541) corresponds with a stable perovskite with octahedral rotations and the organic linear methylammonium cation in the center as it is depicted in Figure 1.8c. Other stable perovskites with possible PV interest such as are included in Figure 1.8d. Figure 1.8. a) Ideal cubic-symmetry perovskite structure with ABX3 formula. Taken from Eames, C. et al. [54], b) detail of the 12-fold cuboctahedral coordination for the A cation in a ABX3 perovskite. Extracted from Gao, P. et al. [55], c) structure of CH3NH3PbI3 showing octahedral rotations and the linear methylammonium cation in the center. Also from Eames, C. et al. [54] and d) tolerance (𝑡) and octahedral factors (𝜇) for different perovskites with photovoltaic interest. From Green et al. [53]. a) b) c) d) 1. Introduction - 17 - On the other hand, a point group that includes an inversion center as one of its symmetry elements is called centrosymmetric [56]. Certain perovskite materials, such as BaTiO3 and PbTiO3 are not centrosymmetric due to B cation displacement. For non-centrosymmetric perovskite materials, e.g. the above-mentioned BaTiO3 and PbTiO3, B-site off-centering drives ferroelectric behavior. Generally, ferroelectricity is only present in perovskites with 𝑡~1, since it is reduced by A-site antipolar movement in perovskites as octahedral rotations increases (when 𝑡 is reduced) [43]. Nevertheless, octahedral rotations are not responsible by itself of ferroelectric suppression, since some perovskites show ferroelectric behavior together with octahedral rotations ( e.g. GdScO3 or CaZrO3). Actually, it is the A- site displacements that accompany the octahedral rotations which produce the decisive role in suppressing ferroelectricity [43]. B-displacement has not been shown in organic-inorganic lead halide perovskite family (APbX3) due to their ionic radii of their components. So, theoretically, ferroelectricity driven by B-site offcentering is not possible as usual. Moreover, authors reported complete absence of ferroelectricity in hybrid lead halides showing only remarkable polarization using piezoelectric force microscopy (PFM) [57]. However, for perovskites for PV interest, it becomes much more complex to analyze this phenomenon since A cation is not represented as a point in the space but a linear structure. Hence, in some reports, A is represented as a dipole which causes ferroelectricity [58]. Regarding the effects of temperature on crystal structure, different perovskite phases are expected, being 𝛼-phase the high temperature one, and then other 𝛽 and 𝛾 phases are formed as temperature is decreased. Depending on the type of organic cations and halide anions employed in the perovskite synthesis variations in crystal phases are expected. First approach studying temperature dependence of crystal lattice for methylammonium lead halides was accomplished by Poglitsch and Weber [59]. In this study, three different phases were distinguished for CH3NH3PbI3 perovskite: a cubic 𝑃𝑚3𝑚 𝛼-CH3NH3PbI3 over 54ºC, a tetragonal 𝐼⁡4/𝑚𝑐𝑚 𝛽-CH3NH3PbI3 between 54ºC and -111ºC, and orthorhombic𝑃𝑛𝑎21 𝛾-CH3NH3PbI3 phase for temperatures lower than -111ºC. More recent works done by Stoumpos et al. [60] disclosed similar tendency, however 𝛼 phase is based on not perfectly cubic but an almost cubic 𝑃4𝑚𝑚 tetragonal space group >50ºC (57ºC according Baikie et al. [61]), while 𝛽-CH3NH3PbI3 is reported as tetragonal 𝐼⁡4𝑐𝑚 for -111ºC<T<50ºC and orthorhombic 𝛾-CH3NH3PbI3 lower than that temperature. One important consideration is at room temperature, PV active 𝛽-phase is present, however on increase in temperatures such as 60ºC, which is a normal working temperature for any PV technology, active 𝛽-phase is transformed into 𝛼- CH3NH3PbI3. Thus the, implementation in future industrial methods, could imply heat sink systems design. The introduction of compositional engineering utilizing other types of perovskites, can limit or even avoid this potential issue. For example, HC(NH2)2PbI3 exhibit a PV active trigonal 𝛼-phase (𝑃3𝑚1) for⁡𝑇>-73ºC, another trigonal 𝛽-HC(NH2)2PbI3 ( 𝑃3 ) for -143ºC> 𝑇>-73ºC and orthorhombic 𝛾- HC(NH2)2PbI3 below -143ºC. It is noticeably that for temperatures lower than 87ºC in the presence of solvents, a non-perovskite like yellow compound 𝛿-HC(NH2)2PbI3 with hexagonal structure (𝑃63𝑚𝑐) is formed inside the mother liquor [60], [62]. Ph.D. Thesis. Fco. Javier Ramos Mellado - 18 - A summary of different cubic lattice parameters and the distances between ions in the perovskite crystal structure is presented in Table 1.1 showing very similar parameters independently the technique employed for the measurements. Table 1.1. Crystal structure parameters for cubic (pseudocubic) 𝛼-CH3NH3PbI3 phase. Reference Technique employed Cubic lattice parameter (Å) I-I separation (Å) Pb-I bond length (Å) C-N bond length (Å) C-H bond length (Å) N-H bond length (Å) Poglitsch, A. and Weber, D. J. Phys. Chem. (1987) [59] Dispersive polarizing millimeterwave interferometer 6.3285 - - - - - Kawamura, et al. J. Phys. Soc. Jpn. (2002) [63] X-Ray diffraction 6.29 - - - - - Baikie, T. et al. J. Mater. Chem. A (2013) [61] X-Ray diffraction 6.28 - - - - - Stoumpos, C. C. et al. Inorg. Chem. (2013) [60] X-Ray diffraction 6.31 4.46 3.16 1.48 - - Weller, M. et al. Chem. Commun. (2015) [64] Neutron diffraction 6.32 4.47 3.16 1.35 0.99 0.99 Eames, C. et al. Nat. Commun. (2015) [54] DFT 6.28 4.45 3.16 1.48 1.1 1.04 1.3.3 Optical properties Hybrid organic-inorganic perovskites shows very strong absorption properties in the UV-visible part of spectra. From past PV approaches [30], [31] to more recent developments [36], [65] incident photon-to-current efficiencies (IPCE) of PSC showed outstanding light harvesting and collecting capabilities to take maximum profit from the incident photons along its whole absorption spectra. In addition to its excellent absorption, the value of the optical bandgap is another key parameter to explain the exceptional behavior in the PSC. Earlier studies were developed explaining the optimum band gap to maximize final PCE is ~1.5eV [66]– [68] which matches with hybrid organic lead halide perovskites. Moreover, perovskite bandgap is a direct band gap, [69] thus no crystal momentum (k-vector) is required for photon absorption. 1. Introduction - 19 - In more recent works, optical band gap configuration of CH3NH3PbI3 forming a continuous media ( i.e. 3-dimmensional perovskite) has been proposed, showing the valence band is formed by Pb6s-I5p sigma 𝜎-antibonding orbital while conduction band consists on Pb6p-I5s 𝜎-antibonding and Pb6p-I5p 𝜋-antibonding orbitals [69], [70], as it is represented in Figure 1.9. However, in recent contributions even a more complex distribution was suggested , since two valence bands were found [71]–[74] or even an excited charge transfer state over the conduction band [75]. Figure 1.9. Bonding diagram of 3-dimmensional CH3NH3PbI3 crystal, showing the valence band (VB) and conduction band (CB) orbitals. Adapted from Umebayashi, T. et al. [69]. Furthermore, optical bandgap tuning has been a topic with an increasing interest. Modifications in bandgap were achieved principally by two ways: varying MA-FA ratio and I-Br ratio. With the control over methylammonium-formamidinium ratio, an extension into the near infrared absorption spectra was achieved since the onset was displaced from ~780 nm to over 800 nm [44]. The variation of iodidebromide composition contributes not only with the optical band gap variation, but also with open circuit voltage and perovskite color tuning [47]. Therefore, perovskite compositional control has been demonstrated as an effective technique to improve optical properties, and then overall PV behavior in PSC [36], [65], [76]. Finally, other optical characterization findings showed absorption spectra is divided into Beer-Lambert (until ~525 nm) and cavity regions (from ~525 nm to cut-off wavelength) [77] and the justification of a capping layer also for mesoporous based configuration in order to achieve high efficiency solar cells [78]. Ph.D. Thesis. Fco. Javier Ramos Mellado - 20 - 1.3.4 Electronic properties and charge recombination On light illumination on the perovskite absorber, incident photons gets excited, an electron-hole pair called exciton is formed. The required energy to split that exciton into separated electron and hole charges is named as binding energy (𝐸𝑏). Regarding 𝐸𝑏, excitons are normally divided in two categories: Frenkel excitons and Mott-Wannier ones. Frenkel exciton are normally presented in low permittivity systems where screening of charges are not significant, so columbic interactions are important producing excitons with small radius difficult to break, so when 𝐸𝑏 is high (from 100-1000 meV); Frenkel excitons are usual in OPV systems. In contrast, Mott-Wannier excitons are distinctive from semiconductors, since they are presented in high permittivity media where screening effects reduce the columbic interactions creating much bigger and then, much easier to break excitons; therefore, 𝐸𝑏 in these PV systems is lower than Frenkel ones but higher than thermal energy: 𝑘𝐵𝑇 (typically around tens of meV but lower than 100 meV). Binding energies for several types of hybrid organic-inorganic lead halide perovskites are reported in Table 1.2. In layered perovskites, e.g. (C9H19NH3)2PbI4, (NH2C(I)=NH2)3PbI5 (CH3NH3)4PbI6·2H2O, binding energies are high, showing 𝐸𝑏 values typical for Frenkel excitons. In contrast, for methylammonium lead halides, 𝐸𝑏 are much lower but, discrepant. Most of the reported values for CH3NH3PbI3 are consistent with Mott-Wannier excitons (𝑘𝐵𝑇<𝐸𝑏<100 meV). However, in some reports it is advised the mechanism could be non-excitonic due to the very low value of 𝐸𝑏 and nature of perovskite [79], having a direct splitting into free charges (electrons and holes), just by thermal activation, i.e. when 𝐸𝑏<𝑘𝐵𝑇. Considering at 25ºC, 𝑘𝐵𝑇=25.7 meV, non-excitonic nature of perovskite is also supported in recent studies [77], [80]. 1. Introduction - 21 - Table 1.2. Binding energy (𝐸𝑔) measured by different methods for different perovskite materials. Adapted from Sum, T. and Mathews, N. [81]. Reference Perovskite Dimensionality Method 𝑬𝒃 (meV) Koutselas, I.B. et al. J. Phys.: Condens. Matter (1996) [82] CH3NH3PbI3 3D Optical absorption 30 Hirasawa, M. et al. Physica B. (1994) [83] Magneto-absorption 37 Ishihara, T. J. Lumin. (1994) [84] Temperature dependent PL 45 Tanaka, K. et al. Solid State Commun. (2003) [85] Magneto-absorption 50 Sun, S. et al. Energy Environ. Sci. (2014) [80] Temperature dependent PL 19±3 D'Innocenzo, V. et al. Nat. Commun. (2014) [79] Optical absorption 55±20 Lin, Q. et al. Nature Photon. (2015) [77] Optical absorption ~2 Tanaka, K. et al. Solid State Commun. (2003) [85] CH3NH3PbBr3 3D Magneto-absorption 76 Koutselas, I.B. et al. J. Phys.: Condens. Matter (1996) [82] Optical absorption 150 Zhang, W. et al. Nano Lett. (2013) [86] CH3NH3PbI3-xClx 3D Temperature dependent PL 98 Ishihara, T. J. Lumin. (1994) [84] (C9H19NH3)2PbI4 2D Temperature dependent PL ≥330 Koutselas, I.B . et al. J. Phys.: Condens. Matter (1996) [82] (NH2C(I)=NH2)3PbI5 1D Optical absorption ≥410 Koutselas, I.B. et al. J. Phys.: Condens. Matter (1996) [82] (CH3NH3)4PbI6·2H2O 0D Optical absorption 545 Another characteristics which make hybrid organic-inorganic lead halide perovskite interesting for PV purposes is their high mobility [87] and ambipolar conductivity i.e. perovskite can work for both transporting electrons and holes. Ambipolar behavior was demonstrated in initial studies, since Chung et al. employed a tin halide perovskite (CsSnI3) as HTM in ss-DSSC exhibiting remarkable efficiency (PCE>10%) [22] proving ptype conduction for that compound. In addition, ptype conduction for CH3NH3PbI3 based solar cells was validated by Etgar et al. using a PSC configuration without HTM [34]. Alternatively, Snaith and coworkers confirmed n -type conduction employing a non-injecting electron mesoscopic Al2O3 scaffold between anode and CH3NH3PbI3, where perovskite was not only the absorbing material but also the responsible to transport electrons to anode. After charges have been formed and separated, electrons and holes have to reach anode and cathode respectively. If during their path, electrons make contact with holes they recombine immediately resulting in losses since generated charges were Ph.D. Thesis. Fco. Javier Ramos Mellado - 22 - not collected at electrodes. Concentration of charges (both electrons and holes) with time is expressed by general continuity equation (equation 1.20) 𝑑𝜌𝑁 𝑑𝑡 =𝐺−𝑘1𝜌𝑁+𝑘2𝜌𝑁2+𝑘3𝜌𝑁3 (1.20) Where 𝜌𝑁represents charge carrier density in number of particles per volume (either electrons or holes), meanwhile 𝐺 is the charge generation term,𝑘1𝜌𝑁, 𝑘2𝜌𝑁2and 𝑘3𝜌𝑁3 are the terms related with monomolecular, bimolecular and trimolecular recombination mechanisms respectively. An energy level diagram explaining different recombination mechanisms process is shown in Figure 1.10. Figure 1.10. Energy level scheme for the different types of recombination mechanisms takes place inside a PSC. Regarding recombination mechanisms, some differences can be recognized:  Monomolecular recombination ( 𝑘1). Non-radiative trap-assisted recombination. It is principal mechanism in PSC [74], [88], [89]. Two different subtypes can be distinguished:  Shockley–Read–Hall (SRH) recombination: due to deep traps. It can be reduced controlling the morphology and grain boundaries inside the perovskite layer.  Surface recombination: traps at the interfaces. Very important in PSC  Bimolecular recombination (𝑘2). A radiative due to band to band recombination. It is the desired, as it allows making the process reversible. It is function of the photoluminescence. Bimolecular recombination Radiative. Band-to-band recombination CB VB Monomolecular recombination Non-radiative. Trap-assisted recombination Trimolecular recombination Non-radiative. Auger recombination 𝐸 𝐸𝑡 𝑘2 𝑘1 𝑘3 Surface recombination Shockley-Read-Hall (SRH): due to defects 1. Introduction - 23 -  Trimolecular recombination (𝑘3). It is also non-radiative. It is called Auger recombination and less important for PSC. In Table 1.3 recombination rate constants (𝑘1,⁡𝑘2 and 𝑘3) reported in literature for different perovskites calculated using optical pump THz probe (OPTP) spectroscopy are shown. Charge carrier mobility values were also included in this table. Type of perovskite, deposition method and architecture are detailed in below table for comparison. Table 1.3. Summary of recombination rate constants and charge carrier mobilities for different hybrid organic-inorganic perovskites. Adapted from Johnston, M. and Herz, L. [90]. Reference Type of perovskite Deposition method. Architecture Chargecarrier mobility (cm2 V-1 s-1) Rate Constants 10-6 (s-1) 1010 (cm3 s-1) 1028 (cm6 s-1) Wehrenfening et al. Adv. Mater. (2014) [88] MAPbI3-xClx Solution process. Perovskite over mp-Al2O3 11.6 4.9 0.87 0.99 Wehrenfening et al. Energy Environ. Sci. (2014) [89]/ Wehrenfening et al. J. Phys. Chem. Lett. (2014) [91] Vapor. Planar 33 12 1.1 0.2 Wehrenfening et al. Adv. Mater. (2014) [88] MAPbI3 Solution process. Perovskite over mp-Al2O3 8.2 14 9.2 1.3 Milot et al. Adv. Funct. Mater. (2015) [92] Solution process. Planar 35 15 0.6 1.6 Rehman, W. et al. Adv. Mater. (2015) [93] FAPbI3 Solution process. Planar 27 7 1.1 0.2 Piatkowski, P. et al. , J. Phys. Chem. Lett. (2016) [94]a Solution process. Planar 40 - 75 0.000152 2.97 7.31 Rehman, W. et al. Adv. Mater. (2015) [93] FAPbBr3 Solution process. Planar 14 21 11 1.5 Noel, et al. Energy Environ. Sci. [50] FASnI3 Solution process. Perovskite over mp-TiO2 1.6 8 14 N.R. aIn this case OPTP Spectroscopy was employed in combination with Flash photolysis. N.R. Not Reported. Previously, different processes which are taking place into the perovskite have been discussed, i.e. photoexcitation, generation of charges, charge carrier and recombination. A global summary of the different process inside PSC are detailed in Table 1.4. and Figure 1.11, considering TiO2 as ETM and any HTM. Desirable Ph.D. Thesis. Fco. Javier Ramos Mellado - 24 - processes (green) were differentiated from undesirable ones (red): charge recombination and exciton annihilation. Table 1.4. Summary of different desirable an undesirable processes into PSC containing TiO2 as ETM and any HTM. Adapted from Marchioro, A. et al. [95]. Desirable process Undesirable process 0. Photoexcitation of perovskite Perovskite+ h υ → (e-…h+)perovskite 3. Exciton annihilation via photoluminescence (e-…h+)perovskite → h υ 1. Electron injection into the TiO2 conduction band (e-…h+)perovskite → e-CB(TiO2) + h+ (perovskite) (1a) h+ (perovskite) → h+ (HTM) (1b) 4. Non-radiative exciton annihilation (e-…h+)perovskite → ∇ 2. Hole injection into HTM (e-…h+)perovskite → h+ (HTM) + e- (perovskite) (2a) e- (perovskite) → e-CB(TiO2) (2b) 5. Recombination between electron from TiO2 and hole from perovskite e-CB(TiO2) + h+ (perovskite) → ∇ 6. Recombination between hole from HTM and electron from perovskite h+ (HTM) + e- (perovskite) → ∇ 7. Interface TiO2/HTM recombination e-CB(TiO2) + h+ (HTM) → ∇ Figure 1.11. Summary of different desirable an undesirable processes into a PSC containing TiO2 as ETM and Spiro-OMeTAD as HTM. Adapted from Marchioro, A. et al. [95]. 1. Introduction - 25 - Another issue which was observed in PSC was the presence of unbalanced charge distribution where electrons are not collected in a balanced way with holes at electrodes, since undesirable accumulation of charges, principally holes sit inside the perovskite capping layer not being extracted sufficiently [96]. As a result of different recombination processes presented, carrier lifetime and diffusion lengths were demonstrated to be ~1 micron in PSC [71], [72]. However, the microstructure employed has been revealed as an essential parameter in carrier lifetime enhancement [97]. On the other hand, some perovskite oxides such as LaMnO3 or LaFeO3 were proven as effective ionic conductors. Consequently, ion migration has been suggested in several works as the reason underlying several effects [44], [98]. However, ionic conduction in PSC have not been checked until recently [54], [99]. Eames et al. were able to evaluate defect formation energies, concluding iodide vacancies were much easier to be moved than any other ones, suggesting an excess of PbI2 during perovskite synthesis in order to minimize issues derived from defect chemistry. Moreover, demonstration of CH3NH3PbI3 as an effective mixed ionic-electronic conductor has been reported by several methods [54], [100]. The mixed conductivity nature of both CH3NH3PbI3 and mixed (CH3NH3)1-x(NH2- CH=CH2)xPbI3 was also confirmed by Yang et al. proposing ionic transport is even more important than electronic one inside perovskite materials [99]. Another noticeable phenomenon detected in PSC is the presence of hysteresis in J- V characterization curves. Hysteresis in PSC is the performance dependency of PV devices regarding the voltage sweeping direction. The first observation of hysteresis in PSC was noticed by Snaith and coworkers [101] and it has always been explained as an adverse effect. It is remarkable to mention in many cases, bad preconditioning, wrong measurement practices or extreme sweeping speeds were wrongly attributed to hysteresis effect [102], [103]. Hysteresis origin is still not fully clear. Principal hysteresis explanations were ascribed to undesirable traps in perovskite [104], ionic perovskite conduction [98], [105], [106] or even a mixture of them [107]. 1.3.5 Device architecture Any PV device, independently the type of technology employed or the final architecture prepared, consist of an absorber material placed in between the HTM and the ETM one; adjacent to the transporting materials, two electrodes must be placed, the anodic one next to the ETM and the cathodic next to the HTM, with the limitation that one of two electrodes has to be transparent to let the light come into the PV device. However, this five functional materials i.e. anode-ETM- Absorber-HTM-Cathode, could be condensed in less than five material layers if one material is able to wrap several functions at the same time or, in an opposed way, presenting more than five layers when a function is divided in two different materials. An example for the former case is silicon solar cell where between p- and ntype silicon absorption and transporting tasks are covered while for the latter case the combination of compact and mesoporous as ETM in DSSC or PSC can be considered as example. Ph.D. Thesis. Fco. Javier Ramos Mellado - 32 - instance, to avoid water-driven perovskite degradation, the introduction of hydrophobic agents as polymer-carbon nanotube composites were tested as an effective approach to limit moisture-driven degradation even under moderate heating conditions [137]. Other possibility to control perovskite stability has consisted in removing organic compounds from solar cell architecture being replaced by inorganic ones [117], [118], since certain additives habitually employed to reach high performance using organic semiconductors, like Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) are highly hygroscopic and other ones directly reacts or dissolves perovskite as 4- tert -Butylpyridine (t-BP). Additional possibilities to enhance long term stability of PSC also reported in literature comprised interfacial modifications such as the introduction of chromium oxide-chromium contact to improve air stability [138], CsBr/titania interface modification to improve UV-light instability [139], and the introduction of alkylphosphonic acid ω-ammonium chlorides as crosslinkers, where not only the stability is improved due to an interfacial improvement between perovskite and mesoporous titania, but also the efficiency was enhanced as a result of crosslinking of neighboring perovskite grains [140]. The introduction of hybrid perovskites in multijunctions has been proposed as a potential way to pave the development of mixed PV tandem technologies with superior harvesting properties [141]. Up to now, some efforts have been made to create perovskite tandem multijunctions in combination with silicon [142], [143], Cu2ZnSn(S,Se)4 kesterite [144] and CIGS [143] in order to enhance PV characteristics, but the results were not completely satisfactory since the output power of the best PV individual device were practically not improved by the introduction of tandem architecture. Other perovskite usages with potential commercial interest reported recently consist on water-splitting devices combining perovskite with NiFe layered double hydroxide [145] or BiVO4 [146]. 1.4 Motivation and scope of this work. Considering the previous state of the art, the search of novel porous photoanodes and new easy to fabricate and cost effective HTM are considered as a necessary requisite to promote and enhance the knowledge of this emerging technology making possible for future commercialization. New HTMs are required in order to replace the expensive Spiro-OMeTAD or Poly(triarylamine) (PTAA). Those molecules were demonstrated as effective candidates for research and lab scale, however their cost could limit the progress of PSC cell technology. Thus, new small organic molecules easy to fabricate and cost effective were proposed showing properties and behavior remarkable considering the state of the art. The molecules proposed in this thesis were: TIPS- pentacene, where the novelty relies on the use of that molecule as HTM for PSC since it is an classical well studied, high mobility and cheap organic semiconductor; new Zn(II)–based non-aggregated phthalocyanine abbreviated as TT80 and triazatruxene-based compounds (called HMDI and HPDI) with extremely good solubility and HOMO level available to be implemented in PSC as HTM. Regarding ETM, although planar architecture approaches were proved in literature, best performances and characteristics have been reached with the use of porous inorganic ETM (TiO2 or ZnO) over a dense buffer or blocking layer using regular 1. Introduction - 33 - architectures. These materials combine excellent properties with scalable approach. Nevertheless, the techniques employed at lab scale, such as spin-coating or spray coating, are not possible to upscale correctly, are time and material consuming and poorly reproducible being difficult and inadequate for future commercial approaches. Hence, new deposition techniques both easy scalable and highly reproducible and low material consuming are essential for future applications [147]. In this work, solvent-free techniques, such as plasma deposition for ZnO nanowires (PECVD), and physical vapor deposition at oblique angle deposited (PVD-OAD) for TiO2 nanocolumns and mixed TiO2-SiO2, 1-dimensional nanocolumnar photonic crystals were employed and analyzed in order to fabricate fully controllable and easy to upscale porous photoanodes for PSC. Ph.D. Thesis. Fco. 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Taking into account aforementioned evidences, exploration of novel alternatives for photoanodes and new HTMs with matching energetic levels compatible with perovskite are essential requirements for PSC cell optimization. Furthermore, the search of cost-effective materials, easy to fabricate compounds, and homogeneous and large scale feasible deposition techniques are also central requirements in order to drive a potential and realistic upscaling and future industrial application. For that reason, the principal aim of this work was proposing and proving new photoanodes and innovative HTM for PSC fabrication. More precisely, regarding new photoanode materials, since best performing devices are being still achieved with porous photoanodes based on non-expensive inorganic semiconductors using regular configuration, it was decided to keep the nature of the compounds but modifying the deposition technique employed. At lab scale, spin coating techniques are useful for research purposes. However, they are poorly reproducible, not able to be scaled and extremely time, energy and material consuming. Hence, the employment of porous inorganic semiconductor films prepared by dry chemistry techniques, with easy controllable conditions, homogeneous in large area and easy to scale as effective photoanodes in PSC was considered as a principal goal for the current study. Consequently, in this thesis, diverse alternatives were tested, suggesting ZnO nanowires, TiO2 nanocolumns and 1-dimmensional nanocolumnar photonic crystals (1-DPC) as PSC photoanodes. For each case, particular objectives can be detailed:  ZnO nanowire-based photoanodes. Demonstration of ZnO nanowires films with porous structure grown by plasma-enhanced chemical vapor deposition (PECVD) as photoanodes in PSC was projected. Device optimization and characterization were also shown.  TiO2 nanocolumnar photoanodes. Proof of concept of tilted TiO2 nanocolumnar films as porous photoanodes in PSC was demonstrated. Device optimization, study of morphological characteristics on final performance ( i.e. porosity and thickness), infiltration of perovskite inside photoanode film and stability were analyzed.  1-DPC as photoanodes. Different mesoporous nanocolumnar multilayered films with alternating refractive indices were studied as photonic crystal like structures for light management in order to show improved absorption properties. Device integration, optimization and statistical analysis were accomplished as well. On the other hand, new HTMs are needed to replace the expensive and hard to synthetize Spiro-OMeTAD and PTAA. Thus, implementation and optimization of unusual or original HTMs presenting easy synthesis and low cost are considered central tasks of this work. Moreover, in order to improve stability, some of the molecules were tested without dopants or additives (LiTFSI or t-BP), since long Ph.D. Thesis. Fco. Javier Ramos Mellado - 50 - term stability is reduced by those compounds though PV properties were enhanced. For the selected HTMs: TIPS-pentacene, Zn(II)octa(2,6-diphenylphenoxy) phthalocyanine (coded as TT80) and triazatruxene based HTMs (named as HMDI and HPDI), some particular objectives can be distinguished:  TIPS-pentacene as HTM. For this case, testing an old existing molecule showing remarkable properties and reproducibility not only with but also without the use of additives was studied.  Non-aggregated Zn(II) phthalocyanine as HTM. In this example, the proof of concept a metal-organic semiconducting phtalocyanines can work as HTM in PSC was desired. Device optimization and influence of diphenylphenoxy substituent were planned.  Triazatruxene based compounds as HTM. HTMs based on triazatruxene core were tested in PSC. Their PV properties, stability, thermal properties and statistical analysis were also analyzed. 3. Global summary of results 3. Global summary of results - 53 - More significant results obtained during this thesis are summarized in following chapter. Global discussions will be elucidated in chapter 4. Each subsection corresponds to one article, which can be found in appendix section in journal format. 3.1 PSC based on nanocolumnar plasma deposited ZnO thin films Main figures and results regarding new porous nanocolumnar ZnO thin films as photoanodes for PSC are contained in this section. Information has been taken from the article text available as annex. First of all, the new device architecture expected by the introduction of ZnO nanowires in regular (n-i-p) porous configuration is represented in Figure 3.1. In contrast, a scheme for standard regular (n-i-p) porous configuration using spherical semiconductor particles was depicted in introduction section (Figure 1.12a).The technique here selected to grow the nanocolumnar ZnO films was plasma enhanced chemical vapor deposition (PECVD) using diethyl zinc as precursor. Some of the advantages of implementing this technique are the absence of solvents during fabrication, the easy scalability and adequate uniformity over large areas making a good candidate for future industrial scale processes. Figure 3.1. Schematic diagram of a device fabricated using ZnO nanocolumnar structures. Dimensions are not at scale. In order to confirm the conceived structure, scanning electron microscopy images were developed. Cross-sectional and top view SEMs were prepared onto the photoanodes based on porous nanocolumnar ZnO thin films with different thicknesses (Figure 3.2a-c). In addition, a cross sectional SEM was prepared for the best performing device using 300 nm thick ZnO film. Ph.D. Thesis. Fco. Javier Ramos Mellado - 54 - Figure 3.2. SEM images (cross-sectional and top view) of ZnO films of: a) 400, b) 500, and c) 600 nm thickness and d) cross-sectional images of fabricated solar-cell devices containing 300 nm-thick ZnO films. Although nanocolumnar ZnO porous films with diverse thicknesses were prepared, only the ones with 300 nm were employed for device integration based on preliminary prospective analyses which reveal the best results with this thickness. These films were characterized by a high transparency and refractive index of 1.8 which denotes also its high porosity. In this case, sequential deposition was utilized according introductory chapter. Different perovskite deposition conditions were explored, such as concentration of PbI2 precursor, concentration of MAI solution for dipping and annealing temperature of perovskite film, analyzing its impact on device PV performance. Results are summarized in Table 3.1. It was observed that Spiro-OMeTAD based devices (PCE=3.9%, 𝐽𝑆𝐶=12.5 mA cm-2, 𝑉𝑂𝐶=792 mV and FF=0.391) showed better performance than PTAA based ones (PCE=1.3%, 𝐽𝑆𝐶=8.3 mA cm-2, 𝑉𝑂𝐶=481 mV and FF=0.327) in similar conditions. After annealing temperature optimization, devices with Spiro-OMeTAD reached PCE=4.8%, (𝐽𝑆𝐶=16.0 mA cm-2, 𝑉𝑂𝐶=718 mV and FF=0.412). 3. Global summary of results - 55 - Table 3.1. Summary of the performances obtained employing different experimental conditions under 1 Sun (100 mW cm-2) for PSC containing ZnO nanocolumnar photoanodes deposited by plasma-enhanced chemical vapor deposition (PECVD). [PbI2] (M) CH3NH3I concentration (mg mL-1) Annealing temperature (ºC) HTM used (mA cm-2) (mV) FF PCE (%) 1 8 70 Spiro-OMeTAD 12.5 792 0.391 3.9 1.25 8 70 Spiro-OMeTAD 5.3 660 0.321 1.2 1.35 8 70 Spiro-OMeTAD 1.4 631 0.350 0.3 1 10 70 Spiro-OMeTAD 6.0 761 0.406 1.9 1.25 10 70 Spiro-OMeTAD 2.7 621 0.397 0.7 1 8 100 Spiro-OMeTAD 16.0 718 0.412 4.8 1 8 70 PTAA 8.3 481 0.327 1.3 Furthermore, J-V curves both in dark and under 1 sun intensity for the optimized perovskite synthesis conditions ( i.e. [PbI2] =1.25 M and 8 mg mL-1 MAI concentration in 2-propanol) comparing different annealing temperatures are shown in Figure 3.3a. IPCE characteristic for champion device is also included (Figure 3.3b). Figure 3.3. a) J–V curves of the fabricated devices. Blue line show a cell annealed at 70ºC whereas red line represent a cell annealed at 100ºC, being both deposited using [PbI2]=1.25M, 8 mg mL-1 MAI concentration in 2-propanol. The corresponding dark currents are represented by dotted lines and b) IPCE of the ZnO nanocolumnar structure with CH3NH3PbI3 solar cells. Finally, more detailed discussions can be found in discussion section and in the main article attached in appendix part. -4 0 4 8 12 16 20 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Current density (mA cm-2) Voltage (V) 0 20 40 60 80 400 450 500 550 600 650 700 750 800 IPCE (%) Wavelength (nm) a) b) Ph.D. Thesis. Fco. Javier Ramos Mellado - 56 - 3.2 Nanocolumnar 1-dimensional TiO2 photoanodes deposited by PVD-OAD for PSC fabrication The main aim of this work consisted on the implementation of a new type of nanocolumnar TiO2 structures (NC-TiO2) deposited by physical vapor deposition at oblique incidence angle (PVD-OAD) creating a tilted NC-TiO2 porous photoanodes for PSC preparation. The utilization of a solvent-free technique was introduced in order to minimize operational costs and permitting homogeneous and controlled growth conditions over big deposition areas. An explicative scheme of the technique employed is depicted in Figure 3.4a, where it is observed by controlling the position of the sample and the distance to the source, the incidence glancing angle ( 𝛼) is tuned. In addition a schematic representation of PSC using the tilted nanocolumns as photoanode is included as well (Figure 3.4b). Figure 3.4. a) Schematic configuration for the physical vapor deposition at glancing incidence (PVD-OAD) technique used for titania nanocolumnar films and b) schematic representation of the PSC using the tilted nanocolumns as photoanode. A characteristic of PVD-OAD technique is by varying the incidence glancing angle (𝛼), the tilting angle of the deposited column (𝛽) is modified and then, the porosity of the film can be tuned with precision according the desired requirements. In the present study, several thicknesses and porosities were tested by varying the deposition time and incidence angle (𝛼) respectively in tilted NC-TiO2 porous films. For an evaporation angle 𝛼=70º a real tilting angle 𝛽=~34º is expected, while when 𝛼=85º the columns are expected to be more tilted (𝛽=~47º). Afterwards, those structures were employed as photoanodes in PSC being analyzed their morphologies and the influence of photoanodes over photovoltaic properties. For morphological characterization, SEM images were recorded (Figure 3.5). In Figure 3.5a and 3.5b cross sectional SEM images for tilted NC-TiO2 porous films with different 𝛼 are included to compare the changes in tilting angle (𝛽) and appreciate the variation in the porosity when the deposition angle is tuned. Top view SEM images for NC-TiO2 before and after perovskite deposition are included in Figures 3.5c and 3.5d. Both cross sectional SEM for a NC-TiO2 film with perovskite infiltrated and for a full device are also studied to observe the complete 3. Global summary of results - 57 - infiltration of perovskite across the NC-TiO2 porous film. For visualization the thickness of each layer, cross sectional SEMs of a device after perovskite infiltration and for a final device are presented in Figure 3.5e and 3.5f respectively. Figure 3.5. Cross sectional scanning electron microscopy (SEM) images of a) TiO2 tilted nanocolumnar TiO2 photoanode deposited by PVD-OAD with 70º as glancing angle (𝛼), b) cross section of the NC-TiO2 film deposited with 85º as 𝛼, c) top view image of the NC-TiO2 film, d) top view of the perovskite formed over the NC-TiO2 film, e) cross sectional SEM image of NC-TiO2 film with perovskite infiltrated and f) cross sectional SEM image of full device. Note the Figure 3.5a is focused and zoomed. For the sample (Figure 3.5e) composed by bl-TiO2/NC-TiO2/perovskite, time-of- flight secondary ion mass spectrometry (ToF-SIMS) measurements were performed in order to confirm the elemental distribution of the different materials along the cross section of the sample. Thus, the infiltration of perovskite inside the porous matrix made of NC-TiO2 can be studied. In the following Figure 3.6, results of ToF- SIMS are included for the species with interest: Ti+, Sn+, Pb+, PbO+, SiO+, I+ NH3+ Ph.D. Thesis. Fco. Javier Ramos Mellado - 64 - Figure 3.12. a) J-V characterization of the PSC containing TiO2 tilted nanocolumns at 𝛼=70º (black) and PC1 (green), b) J-V characterization of the PSC containing PC2 (blue) and PC3 (red), c) IPCE spectra for the PSC with TiO2 tilted nanocolumns at 𝛼=70º (black) and PC1 (green), and d) IPCE spectra for the PSC with PC2 (blue) and PC3 (red). The statistical results for PV parameters are summarized as box-whiskers chart as well in following Figure 3.13. Figure 3.13. Box-whiskers plot which shows all the photovoltaic magnitudes for the different devices containing 1-DPC porous structures: PC1 (green), PC2 (blue) and PC3 (red). Grey lines represent the average values for each magnitude in the case of base NC-TiO2 photoanode. Dots at the right side of the box mean the exact value of each sample. a) b) d) c) 3. Global summary of results - 65 - 3.4 A dopant free linear acene derivative as a HTM for PSC The replacement of Spiro-OMeTAD as HTM in PSC for other alternatives which present frontier orbitals compatible with perovskite ( i.e. HOMO level higher than perovskite valence band), potentially less expensive, easier to synthesize and can work as dopant free in order to maximize the stability of the solar cells is a necessary requirement inside this field. In the present work, we have implemented for first time a high mobility organic semiconductor as HTM into PSC: 6,13- bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) with and without additives. In Figure 3.14a, TIPS-pentacene molecular structure is represented. To analyze the optical band gap and HOMO/LUMO levels of TIPS-pentacene, optical absorption and cyclic voltammetry measurements were made. In Figure 3.14b, cyclic voltammetry measurements are shown for pristine TIPS-pentacene, for TIPS- pentacene with additives (LiTFSI + t-BP) and for Spiro-OMeTAD. Results obtained for band gap, and frontier orbitals are summarized in Table 3.4 while the energy level diagram of the PSC containing TIPS-pentacene is shown in Figure 3.14c. In addition conductivity of TIPS-pentacene in film has been studied, showing 3.5×10-7 S cm-1 for pristine TIPS-pentacene while after the addition of the additives it was 1.0×10-5 S cm-1. Glass transition (𝑇𝑔) temperature of TIPS-pentacene was evaluated by DSC and found to be 𝑇𝑔=122ºC. Figure 3.14. a) Chemical structure of TIPS-pentacene, b) cyclic voltammograms of Spiro-OMeTAD and TIPS-pentacene in pristine form and with LiTFSI dopant and t-BP additive in dichloromethane solvent and c) corresponding energy level diagram for PSC containing TIPS-pentacene as HTM. Table 3.4. Summary of electrochemical and optical parameters of TIPS-pentacene and Spiro- OMeTAD. HTM 𝑬 ⁡ a (eV) 𝑬 ⁡ (V) HOMO (eV) LUMO b (eV) Pristine TIPS-pentacene 1.87 0.488 -5.4 -3.53 Spiro-OMeTAD 2.98 0.156 -5.07 -2.09 aOptical band gap (𝐸𝑔⁡𝑜𝑝𝑡) was calculated by onset of optical absorption. b𝐿𝑈𝑀𝑂 = 𝐻𝑂𝑀𝑂+𝐸𝑔⁡𝑜𝑝𝑡. Once the energy levels of TIPS-pentacene have been studied demonstrating its usefulness, it was as HTM in PSC, and devices were fabricated. To study the morphology and thickness of the fabricated PSC, cross sectional SEM pictures of PSC containing TIPS-pentacene as HTM are presented in Figure 3.15. a) b) c) Ph.D. Thesis. Fco. Javier Ramos Mellado - 66 - Figure 3.15. Cross-sectional SEM of the CH3NH3PbI3 perovskite based device with pristine TIPS- pentacene HTM, a) at 1 µm scale with low magnification and b) at 500 nm scale with higher magnification. Both images shows a clear interface between TIPS-pentacene and perovskite layer. Colors were applied to distinguish easily the different layers. Subsequently, photovoltaic characteristics of this PSC were analyzed. In Table 3.5 PV properties (𝐽𝑆𝐶,𝑉𝑂𝐶, FF, PCE) expressed as mean ± SD (standard deviation) for the solar cells containing different HTMs: TIPS-pentacene without additives, with additives and Spiro-OMeTAD as reference. Both J-V characterization and IPCE curves for a cell with average properties are shown in Figure 3.16 for each type of HTM here employed. Table 3.5. Photovoltaics parameters derived from J-V measurements of mesoporous CH3NH3PbI3 perovskite based devices with TIPS-pentacene and Spiro-OMeTAD as HTMa. Device (mA cm-2) (mV) FF PCE (%) Pristine TIPS-pentacene 20.84±0.01 913±5 0.60±0.01 11.51±0.31 TIPS-pentacene + LiTFSI + t-BP 16.51±0.49 920±4 0.54±0.01 8.20±0.23 Spiro-OMeTAD + LiTFSI + t-BP 18.61±0.78 843±5 0.62±0.01 9.77± 0.35 aAverage data with standard deviation were based in a single batch. Devices were masked with a black aperture to define the active area of 0.283 cm2. Figure 3.16. a) J–V characteristics of a CH3NH3PbI3 perovskite based device with average performance for pristine TIPS-pentacene (red), TIPS-pentacene with additives (blue) and Spiro- OMeTAD (black) as HTM in dark (hollow symbols) and under illumination at AM 1.5G (filled symbols), 100 mW cm-2 in a reverse bias scan; and b) corresponding IPCE spectra of the devices. a b 3. Global summary of results - 67 - Additionally, it is interesting to note the champion device was obtained from additive free TIPS-pentacene and it reached a PCE=11.82% presenting negligible hysteresis (see appendix section) and with remarking PV properties: 𝐽𝑆𝐶=20.86 mA cm-2, 𝑉𝑂𝐶=918 mV and a fill factor of FF=0.62. Finally, for further information, such as thermal characterization of TIPS-pentacene compound and hysteresis analysis, stability and statistical information of TIPS- pentacene containing PSC can be consulted in Appendix section. Ph.D. Thesis. Fco. Javier Ramos Mellado - 68 - 3.5 Non-aggregated Zn(II)octa(2,6-diphenylphenoxy) phthalocyanine as a HTM for efficient PSC In order to find novel compounds with interesting properties, capable to be used as HTM in PSC, a new phthalocyanine based compound: Zn(II)octa(2,6- diphenylphenoxy) phthalocyanine (coded as TT80) was tested. Molecular structure of TT80 compound is depicted in Figure 3.17a while absorption spectrum of the molecule in chloroform is shown in Figure 3.17b. Figure 3.17. a) Molecular structure of TT80, b) UV-Vis absorption spectrum of TT80 in CHCl3 solution (~1⨉10-5 M). After device integration, photovoltaic characteristics of PSC containing TT80 as HTM where analyzed through J-V and IPCE characterization. Three different deposition configurations were tested: pristine TT80 in Chlorobenzene, TT80 with additives (LiTFSI and t-BP) in chlorobenzene and additive add TT80 (with LiTFSI and t-BP) in toluene. In Table 3.6 photovoltaic parameters for champion cells in each configuration are detailed. Table 3.6. Photovoltaic parametersa obtained for the PSC employing phthalocyanine as holetransporting materials (HTMs). 𝑃𝑖𝑛, incident intensity of simulated AM1.5G solar lightb Additives Uptake Solvent ⁡(mW cm-2) (mA cm-2) (mV) FF PCE (%) - Chlorobenzene 96.8 11.90 578 0.362 2.6 LiTFSI + t-BP Chlorobenzene 97.4 16.35 797 0.503 6.7 LiTFSI + t-BP Toluene 97.4 16.91 796 0.450 6.2 aThe values presented were obtained for the best device in each configuration. For each configuration, several devices of equal quality were tested, and the uncertainties of PCE values were within 1–6% (see Table S1 in the Appendix section). b𝑃𝑖𝑛⁡ = 100 mW cm−2 for 1 sun irradiation. a) b) 3. Global summary of results - 69 - J-V curves for each champion cell previously reported are plotted in Figure 3.18a. IPCE curve for the optimized configuration, i.e. for TT80 + LiTFSI + t-BP using toluene as uptake solvent is found in Figure 3.18b. Figure 3.18. a) J–V curves under simulated full sun illumination (AM1.5G) for the three cells made with TT80 as HTM using different solvents and additives (PhCl = chlorobenzene; PhCH3 = toluene) and b) IPCE for the best cell made with TT80 under optimized configuration (TT80 + LiTFSI + t-BP in PhCl). Detailed photovoltaic parameters with statistical information for all the devices fabricated, current dynamic spectrum of TT80/PSC, copy of 1H NMR, MS and HRMS spectra of TT80 can be consulted in Appendix section where calculations to evaluate the molecular weight of TT80 molecule revealed a value of 2528.8g mol-1. 400 450 500 550 600 650 700 750 800 0 10 20 30 40 50 60 IPCE (%) Wavelength (nm) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 5 10 15 20 TT80 in PhCl TT80 + LiTFSI + t-BP in PhCl TT80 + LiTFSI + t-BP in PhCH3 Current Density (mA·cm-2) Voltage (V) a) b) Ph.D. Thesis. Fco. Javier Ramos Mellado - 70 - 3.6 Rational design of triazatruxene based hole conductors for PSC Persevering with the aim of finding new potentially low-cost materials with HOMO levels compatible with perovskite to be implemented in stable and efficient PSC, two novel compounds based on triazatruxene molecule, with substituents to obtain the necessary solubility to be deposited through solution process were proposed: 5,10,15-Trihexyl-3,8,13-trimethoxy-10,15-dihydro-5H-diindolo [3,2- a:30,20-c]carbazole (HMDI) and 5,10,15-Tris(4-(hexyloxy)phenyl)-10,15-dihydro- 5H-diindolo [3,2-a:30,20-c]carbazole (HPDI), observing a very high solubility for both (>100 mg mL-1). Figure 3.19 shows the synthetic route scheme for both compounds. Figure 3.19. Synthetic route for the HMDI and HPDI HTMs. In order to evaluate the band gap and frontier orbitals: HOMO/LUMO of these molecules, UV-Vis absorption spectra and cyclic voltammetry experiments were developed. In Table 3.7, band gap and molecular orbitals for HMDI and HPDI obtained by those experiments are detailed; glass transition temperature (𝑇𝑔) attained by differential scanning calorimetry (Appendix section) is also reported. In Figure 3.20, absorbance spectra and cyclic voltammetry plots for triazatruxene based compounds are included together with an energy level diagram for the here fabricated PSC. HPDI HMDI Triazatruxene 3. Global summary of results - 71 - Table 3.7. Summary of optical, electrochemical and thermal properties for the studied HTMs. Compound a (nm) 𝑬 ⁡ b (eV) 𝑬 ⁡ ⁡ ⁡ / c (eV) HOMOd (eV) LUMOe (eV) f (ºC) HMDI 395 3.14 0.108 -5.17 -2.03 1 HPDI 380 3.26 0.344 -5.41 -2.15 36 aAbsorption spectra measured in dichloromethane solution. bOptical band gap (𝐸𝑔⁡𝑜𝑝𝑡) was calculated by onset of optical absorption. c𝐸1 2⁡𝑜𝑥⁡𝑣𝑠⁡𝐹𝑐 /𝐹𝑐 was calculated from Cyclic Voltammetry as 𝐸1 2⁡𝑜𝑥⁡𝑣𝑠⁡𝐹𝑐 /𝐹𝑐 =1 2(𝐸𝑝𝑎 +𝐸𝑝𝑐). dHOMO was calculated as: 𝐻𝑂𝑀𝑂 =−(𝐸1 2⁡𝑜𝑥⁡𝑣𝑠⁡𝐹𝑐 /𝐹𝑐 + 064)in eV. eLUMO=𝐻𝑂𝑀𝑂+𝐸𝑔⁡𝑜𝑝𝑡. f𝑇𝑔 glass transition temperature calculated from DSC. Figure 3.20. a) UV-Vis absorbance spectra of HTMs measured in a dilute solution of chlorobenzene, b) cyclic voltammograms of HTMs measured in 0.1 M solution of tetra-n- butylammonium hexafluorophosphate in dichloromethane using a glassy carbon working electrode, Pt reference electrode, and Pt counter electrode with Fc/ Fc+ as the internal standard. Note: to match the scale the current on Y-axis was offset to 5.5 mA in case of HPDI curve and c) energy level diagram for the fabricated photovoltaic devices using HMDI and HPDI as HTMs. After checking the availability to introduce these novel compounds as HTMs in PSC since the HOMO level is compatible with the use of perovskite absorber and the solubility is adequate to be deposited by solution process methods, device integration of PSC containing HMDI and HPDI compounds were prepared. In Figure 3.21, cross sectional SEM pictures of a device containing HPDI as HTM (before gold cathode evaporation) is included to estimate the thickness of HTM layer and the intimate contact with perovskite layer is under it. a) b) c) Ph.D. Thesis. Fco. Javier Ramos Mellado - 72 - Figure 3.21. Cross sectional scanning electron microscopy image (SEM) image of the photovoltaic devices using HPDI as HTM. Finally, for PV characterization, both J-V and IPCE characterization measurements were developed as usual for PSC containing triazatruxene based HTMs. In Table 3.8 the principal PV parameters (𝐽𝑆𝐶,𝑉𝑂𝐶, FF, PCE) expressed as mean ± SD are indicated for cells prepared in the same batch employing three different HTM configurations: pristine HMDI, pristine HPDI and HPDI containing LiTFSI salt. In Figure 3.22, J-V and IPCE curves for champion devices prepared with each configuration are shown. Table 3.8. Photovoltaics parameters derived from J-V measurements for CH3NH3PbI3 perovskite based solar cells using the synthesized triazatruxene molecules.a HTM JSC (mA cm-2) VOC (mV) FF PCE (%) HMDI 14.43 938 0.637 8.62 Mean±SD 14.31±1.73 907±26 0.551±0.070 7.13±1.14 HPDI 16.51 897 0.663 9.83 Mean±SD 16.80±0.23 893±4 0.642±0.020 9.62±0.17 HPDI + LiTFSIb 19.16 976 0.576 10.82 Mean±SD 18.69±0.39 961±21 0.562±0.010 10.15±0.53 aAverage data and standard deviation calculated for 3 cells in the same batch. Mask size: 0.159 cm2. bMeasured at 99.5 mW cm-2 under AM 1.5G spectra. 3. Global summary of results - 73 - Figure 3.22. a) Current-Voltage characteristics for the PSC employing HPDI with LiTFSI salt as additive (red), pristine HPDI (blue) and pristine HMDI (black) and b) IPCE spectra for the devices shown in a). a) b) Ph.D. Thesis. Fco. Javier Ramos Mellado - 80 - 4.3 Porous 1-dimensional nanocolumnar structures as effective photonic crystal for PSC Three architectures of porous 1-dimensional photonic crystals (1-DPC) consisting on alternated nanocolumnar films with different refractive index deposited by physical vapor deposition at oblique angle (PVD-OAD) were implemented as effective photoanodes of CH3NH3PbI3 solar cells improving the PV characteristics by optical management of the incident light. The technique employed allows an easy large scale implementation. In Figure 3.9 the architectures of the different photonic crystal like structures were presented. For 1-DPC Type 1 (PC1), five TiO2 layers (~75 nm) with alternating refractive index (𝑛 =1.80/𝑛 =1.63/𝑛 =1.80/ 𝑛 =1.63/𝑛 =1.80) were prepared over a bl-TiO2. In this case the alternation in refractive index using the same material for all the prepared layers (TiO2) was induced by a careful control over the incidence glancing angle (𝛼), and then tuning the porosity and 𝑛 of the monolayer. Since the refractive index variation from layer to layer is modest (high 𝑛 =1.80 vs low 𝑛 =1.63), the number of layers in this case has to be increased up to 5 layers to have a suitable photonic band gap (see Figure 3.9g). In PC2, the first high refractive index layer (𝑛 =2.10) was made of compact TiO2 deposited by PVD using 𝛼=0º, while two nanocolumnar layers made of porous SiO2 (𝑛 =1.31) and porous TiO2 (𝑛 =1.77) were respectively deposited on the top utilizing 𝛼=70º. For PC3, TiO2 and SiO2 nanocolumnar films were grown over a thin bl-TiO2 under 𝛼=70º to create a photonic crystal like structure with alternating indices as following: 𝑛 =1.77/𝑛 =1.31/𝑛 =1.77. For all the cases analyzed here, the photonic band gap was shaped in the interval 400-500 nm (Figures 3.9g-i). In Figure 3.10, a precise and controlled growth of the nanocolumnar structures has been proved by cross sectional SEM pictures showing very low concentration of defects in the multilayered structure. For backscattered electron SEM images (left side of Figure 3.10), lighter areas represents TiO2 films while SiO2 is placed in darker ones being observed a very homogeneous distribution of compounds and very drastic changes in material composition (and then in 𝑛) as it is required for a high optical quality in the formed photonic crystal architecture. A schematic image of the spatial distribution of the square amplitude of the electrical field for a photonic crystal like structure similar with PC3 was drawn in Figure 3.11. Although it was not at scale, the explanation of the optical effect able to push the absorption properties is expressed: the reduction in the group velocity of the light near the wavelength of a stop band in a periodic dielectric. As the photons are slowed down inside PC matrix, the chances to absorb them becomes higher, not only inside the optical band gap as it is expected from augmentation in reflections inside the device but also for other wavelengths outside the optical bandgap due to the concentration of the square of the electrical field inside the PC architecture. This phenomenon was confirmed in our experiment since the increase of IPCE was produced for the whole wavelength (Figures 3.12c-d). In Table 3.3, J-V curves (Figures 3.12a and b), and box whiskers charts (Figure 3.13), principal photovoltaic performance for the PSC prepared are shown for the three PC architectures used as photoanodes in PSC and compared with a similar 4. Global discussion of results - 81 - photoanodes based on tilted NC-TiO2 (analogous with the ones fabricated in the previous studies). The reference photoanode based on NC-TiO2 (300 nm thick) presented a performance of PCE=9.56±0.37% ( 𝐽𝑆𝐶 =17.87±0.53 mA cm-2, 𝑉𝑂𝐶=934.4±10.7 mV and FF=0.568±0.018). With the introduction of PC1, an enhancement in absorption, and subsequently in 𝐽𝑆𝐶 was observed exhibiting 𝐽𝑆𝐶=18.56±0.55 mA cm-2; however, the voltage was reduced in comparison with the reference (𝑉𝑂𝐶=867.7±11.9 mV) since the increase in photoanode thickness promotes higher recombination issues to obtain a final gain in the PCE (PCE=10.05±0.69% with FF=0.615±0.036). The performance of PC2 was poor (PCE=4.28±1.32%, 𝐽𝑆𝐶 =15.05±1.15 mA cm-2, 𝑉𝑂𝐶 =883.1±60.3 mV and FF=0.312±0.060), being mainly attributed to the use of a non-injecting oxide (NC- SiO2) just over the compact layer. In the case of PC3, an improvement of 𝐽𝑆𝐶 by the well design of PC structure, together with the shrink of photoanode which endorses an enhancement in terms of 𝑉𝑂𝐶 regarding reference photoanode, lead to reach a final overall PCE=11.52±0.30%, with 𝐽𝑆𝐶 =18.44±0.32 mA cm-2, 𝑉𝑂𝐶=1026.1±5.5 mV and FF=0.603±0.012, and showing for the champion cell a PCE=12.03 (𝐽𝑆𝐶=18.677 mA cm-2, 𝑉𝑂𝐶=1028 mV and FF=0.616) under 0.983 Suns (AM 1.5G). Finally, more detailed optical characterization and both top view and cross sectional SEMs before and after perovskite penetration are addressed in Appendix section. Ph.D. Thesis. Fco. Javier Ramos Mellado - 82 - 4.4 A dopant free linear acene derivative as a HTM for PSC A linear acene molecular organic semiconductor: 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), was successfully employed as HTM in PSC. This molecule is a potentially cheap material, showing HOMO level compatible with the valence band of perovskite (HOMO=-5.4 eV, LUMO=-3.53 eV), with interesting conductivity (3.5×10-7 S cm-1 in pristine form) and soluble in many organic solvents and widely reported in literature. It has been utilized without any additive as HTM in PSC showing improved stability and lower hysteresis than additive based one. In Figure 3.14a molecular structure of TIPS-pentacene is presented. Cyclic voltammograms using Fc/Fc+ as internal standard (Figure 3.14b) revealed a reversible redox couple, since anodic/cathodic peak ratio was close to unity, in both for pristine TIPS-pentacene and Spiro-OMeTAD; nevertheless, TIPS-pentacene displays lower HOMO level (more negative vs vacuum). After adding LiTFSI and t- BP to TIPS-pentacene, two trends were contrasted: the left shifting of cathodic peak suggesting higher HOMO level for additive add TIPS-pentacene in comparison with its pure form and the unusual absence of reversibility. The irreversibility in CV peaks for doped TIPS-pentacene was ascribed, in this case, with the introduction of trap sites or disorder in chain packing. Considering those results, frontier orbitals obtained for TIPS-pentacene and Spiro-OMeTAD are summarized in Figure 3.14c and Table 3.4, exhibiting HOMO= -5.4 eV and LUMO= -3.53 eV for TIPS-pentacene meanwhile for Spiro-OMeTAD HOMO=-5.07eV and LUMO=-2.09 eV. Regarding conductivity measurements in film, TIPS-pentacene shows remarkable behavior even before adding LiTFSI and t-BP (3.5×10-7 S cm-1), being increased when additives were incorporated (1.0×10-5 S cm-1). Cross sectional SEM images were made to analyze the morphology of the prepared solar cells (Figure 3.15). Successful complete penetration of perovskite into mesoscopic TiO2 photoanode (~250 nm) was achieved; being well differentiated part of the perovskite infiltrated into perovskite and a CH3NH3PbI3 capping layer over that. On the top of the capping layer, ~150 nm TIPS-pentacene were formed, presenting also comparable thickness when LiTFSI and t-BP were added to TIPS-pentacene. After trying several processing parameters such as solvent employed to dispense HTM and concentration of additives, the use of toluene as solvent to dispense TIPS-pentacene as such was found as preferable solvent. In Table 3.5 and Figure 3.16, photovoltaic behavior of PSC containing TIPS- pentacene as HTM is summarized. Pristine TIPS-pentacene as HTM showed an average performance of 𝐽𝑆𝐶 =20.84±0.01 mA cm-2, 𝑉𝑂𝐶 =913±5 mV and FF=0.60±0.01 for an average PCE=11.51±0.31%, with a champion cell reaching 𝐽𝑆𝐶=20.86 mA cm-2, 𝑉𝑂𝐶=918 mV and FF=0.62 to obtain PCE=11.82% under 1 Sun (AM 1.5G). On the other hand, the introduction of additives didn’t help to push the PV properties obtaining only 𝐽𝑆𝐶=16.51±0.49 mA cm-2, 𝑉𝑂𝐶=920±4 mV and FF=0.54±0.01 with an efficiency of PCE=8.20±0.23%. It is worthy to mention the low grade of hysteresis when additive free TIPS- pentacene was employed (PCE=11.82% in reverse scan while PCE=10.90% (it can be contrasted in Appendix section). 4. Global discussion of results - 83 - Moreover, since LiTFSI salt is well known as highly hygroscopic compound, it was expected to contribute in reducing the stability of the PV systems which contained it. Thus, stability of additive free TIPS-pentacene was demonstrated higher than the additives free one. After 3 months, PCE of pure TIPS-pentacene based devices was similar with the original one whereas for TIPS-pentacene + LiTFSI + t-BP, the PCE was reduced by half just after two weeks (see Appendix section). Ph.D. Thesis. Fco. Javier Ramos Mellado - 84 - 4.5 Non-aggregated Zn(II)octa(2,6-diphenylphenoxy) phthalocyanine as a HTM for efficient PSC TT80, a novel non-aggregated Zn(II)octa(2,6-diphenylphenoxy) phthalocyanine has been implemented as HTM for PSC fabrication by solvent deposition. This has been the first proof of concept to use phthalocyanine compound as effective HMT for PSC. The introduction of umbrella-like 2,6-diphenylphenoxy groups allows avoiding aggregation and helps with the dissolution of the metal-organic molecule in organic non-polar or poorly polar solvents. In Figure 3.17a, the molecular structure of TT80 is depicted presenting the umbrella-like 2,6-diphenylphenoxy substituents which are reported to avoid aggregation and assisting with its solubility. In Figure 3.17b, the UV-Vis absorption spectrum of TT80 in chloroform revealing a Q-band centered in 696 nm to show a deep green color. Three different configurations were tested to analyze the photovoltaic response of TT80 containing PSC dispensed by solution process: pristine TT80 in chlorobenzene, TT80 + LiTFSI + t-BP in chlorobenzene as well and TT80 + LiTFSI + t-BP in toluene. Photovoltaic response was summarized in Table 3.6 and Figure 3.18 disclosing a poor behavior when additives were not added to phthalocyanine compound in Chlorobenzene, presenting just 2.6% of PCE for best performing device using those conditions ( 𝐽𝑆𝐶 =11.90 mA cm-2, 𝑉𝑂𝐶 =578 mV and FF=0.362), but the performance was improved significantly with the introduction of LiTFSI and t-BP additives by 𝐽𝑆𝐶 and 𝑉𝑂𝐶 enrichment. In particular, for TT80 + LiTFSI + t-BP in chlorobenzene was achieved PCE=6.7% (𝐽𝑆𝐶=16.35 mA cm-2, 𝑉𝑂𝐶=797 mV and FF=0.503), whereas TT80 + LiTFSI + t-BP in toluene reached PCE=6.2% (𝐽𝑆𝐶=16.91 mA cm-2 𝑉𝑂𝐶=796 mV and FF=0.450) was attained for the champion device with each configuration respectively. The statistical information of the here treated cells can be found in Appendix section, where PSC containing TT80 were found reasonably reproducible. 4. Global discussion of results - 85 - 4.6 Rational design of triazatruxene based hole conductors for PSC New HTM based on triazatruxene core: (5,10,15-trihexyl-3,8,13-trimethoxy- 10,15- dihydro-5H-diindolo[3,2-a:30,20-c]carbazole) (named as HMDI) and (5,10,15- tris(4-(hexyloxy)phenyl)-10,15- dihydro-5H-diindolo[3,2-a:30,20-c]carbazole) (named as HPDI), were synthetized and applied as HTM in PSC. Alkoxy substituents were rationally assembled to triazatruxene core in order to increase the solubility in organic solvents and tuning the HOMO level of the molecules to make them compatible with the valence band of perovskite, Remarkable thermal properties and easy fabrication that potentially promote a low cost fabrication were suggested. In Figure 3.19, the synthetic route of triazatruxene based compounds is shown. An ease of fabrication of both compounds was found, requiring very few steps for the synthesis of HMDI (3 steps from triazatruxene) and HPDI (just in one step) from triazatruxene core. Due to the side alkoxy chains, both compounds presented a very high solubility (>100 mg mL-1) in organic solvents as chlorobenzene or toluene. Necessary characterization to evaluate the frontier orbitals (HOMO/LUMO) was exposed in Figure 3.20 and summarized in Table 3.7. 𝐸𝑔⁡𝑜𝑝𝑡 was determined through UV-Vis spectroscopy and HOMO level from cyclic voltammograms obtaining HOMO/LUMO levels of -5.17 eV/-2.03 eV for HMDI and -5.41 eV/-2.15 eV for HPDI. Since HOMO levels are higher than valence band of perovskite, HMDI and HPDI can be tested as HTM in PSC. On account of thermal characterization of HMDI and HPDI compounds, thermogravimetry analysis and differential scanning calorimetry were done (see Appendix section), showing very high thermal stability until 400ºC and low 𝑇𝑔, making possible to be more amorphous, so attaching well to irregular surfaces of perovskite capping layer crystals. For morphological study of devices, cross sectional SEM pictures were recorded (Figure 3.21). Over CH3NH3PbI3 capping layer, a uniform and well attached layer of HPDI of ~150 nm was observed. Photovoltaic parameters obtained from J-V and IPCE characterization curves are shown in Figure 3.22 and detailed in Table 3.8. HMDI and HPDI were firstly tried as HTM in PSC in their pristine forms, i.e. without any kind of additive. HMDI based PSC showed PCE=7.13±1.14%, with 𝐽𝑆𝐶=14.31±1.73 mA cm-2, 𝑉𝑂𝐶=907±26 mV and FF=0.551±0.070 while in the case of HPDI based ones, PCE=9.62±0.17%, with 𝐽𝑆𝐶 =16.80±0.23 mA cm-2, 𝑉𝑂𝐶 =893±4 mV and FF=0.642±0.020 were recorded with best cells reaching 8.62% and 9.83% respectively. In order to improve PV properties, in special photogenerated current, LiTFSI was included in the HPDI recipe showing PCE=10.15±0.53%, with 𝐽𝑆𝐶=18.69±0.39 mA cm-2, 𝑉𝑂𝐶 =961±21 mV and FF=0.562±0.010 with a champion device which reached PCE=10.82% (𝐽𝑆𝐶=19.16 mA cm-2, 𝑉𝑂𝐶=976 mV and FF=0.576) under Ph.D. Thesis. Fco. Javier Ramos Mellado - 86 - 99.5 mW cm-2 (AM 1.5G spectra). To complete PV characterization, the addition of t-BP didn’t result in an augmentation of PV properties (see Appendix section). Furthermore, full synthetic details and J-V hysteresis characterization can be found in the Appendix section. 5. Conclusions 5. Conclusions - 89 - In this thesis, two main general objectives were proposed: the investigation of new photoanode architectures and the implementation of innovative HTM in PSC. More in detail, regarding novel photoanodes, several porous 1-dimmensional structured photoanodes made of cost-effective inorganic metal oxides and using scalable and homogeneous over large area deposition techniques were addressed. Different architectures, such as plasma enhanced chemical vapor deposition (PECVD) and physical vapor deposition at oblique angle (PVD-OAD) were integrated in PSC and the performance of the PSC devices which contained those photoanodes was analyzed. Some specific conclusions can be also detailed:  Nanocolumnar ZnO thin films deposited by PECVD were employed as PSC photoanodes. Diverse properties, e.g. as microstructure, crystallinity, porosity, defect concentration of ZnO can be controlled by varying different conditions during deposition. Thickness of the film, annealing temperature of perovskite material and the use of an adequate HTM were found the most critical parameters in order to fabricate PV devices. After adjusting the optimal conditions during fabrication, PCE=4.8% PSC was attained. It has been as the first approach ever reported for a dry route synthetized photoanode implemented in PSC.  Porous tilted nanocolumnar titania (NC-TiO2) films deposited by PVD-OAD were satisfactory applied as photoanodes in PSC. The influence of morphological properties such as thickness and porosity of the film driven by glancing angle modification (𝛼) resulted critical in order to optimize the system. The impact of morphology on final PV properties was also studied. The effect of infiltration of perovskite inside the mesoporous photoanode scaffold was also proven and analyzed by ToF-SIMS. After device optimization, samples over 10% efficiency were prepared (PCE=10.53%). NC-TiO2 based structures resulted in more stable than the state of the art ones.  Three original porous 1-dimensional photonic crystals (1-DPC) deposited by PVD-OAD technique were used as photoanodes in PSC exhibiting enhanced absorption properties as proof of concept. Optical characterization, device integration and optimization were also completed showing >12% efficient devices. For the application of novel HTM into PSC, some materials were proposed gathering some important trends in order to be adopted as HTM into PSC, such as HOMO level compatible with hole injection from perovskite valence band, easy production which can potentially replace expensive Spiro-OMeTAD and adequate solubility in organic solvents. Some specific conclusions can be highlighted in each case:  Although TIPS-pentacene is a familiar organic semiconductor, its use as HTM in PSC has been addressed for first time in this thesis. 6,13- bis(triisopropylsilylethynyl) pentacene showed a compatible HOMO level to be used in PSC systems (HOMO = -5.4 eV/LUMO= -3.53 eV), with acceptable conductivity, soluble in many solvents and with the opportunity of avoiding the addition of highly hygroscopic Li salt additives, hence provoking more stable and hysteresis free photovoltaic devices. Thus, devices with power conversion efficiencies, of 11.8% were fabricated. Contribution report According to Article 9 of Agreement 9.1 / CG 04/12/19, as all the works contained in this thesis by publications have more than four coauthors, the contribution of each author is detailed as following.  F. J. Ramos, M. C. López-Santos, E. Guillén, M. K. Nazeeruddin, M. Grätzel, A. R. González-Elipe, and S. Ahmad, “Perovskite Solar Cells Based on Nanocolumnar Plasma- Deposited ZnO Thin Films,” ChemPhysChem, vol. 15, no. 6, pp. 1148–1153, 2014. FJRM performed the experiments and fabricated the devices. MCLS deposited ZnO films and ARGE supervised the work. EG performed EIS measurements. FJRM, ARGE and SA analyzed the data and prepared the manuscript while MKN and MG supervised the device fabrication. SA directed the project. All authors reviewed the paper.  F. J. Ramos, M. Oliva-Ramirez, M. K. Nazeeruddin, M. Grätzel, A. R. Gonzalez-Elipe, and S. Ahmad, “Nanocolumnar 1-dimensional TiO2 photoanodes deposited by PVD-OAD for perovskite solar cell fabrication,” J. Mater. Chem. A, vol. 3, pp. 13291–13298, 2015. FJRM performed the experiments, fabricated and optimized the solar cells. MOR deposited these structures under the supervision of ARGE and also made the ToF- SIMS measurement, while MKN and MG supervised the device fabrication part. SA directed the work. All authors discussed the results and commented on the manuscript  F. J. Ramos, M. Oliva-Ramirez, M. K. Nazeeruddin, M. Grätzel, A. R. Gonzalez-Elipe, and S. Ahmad, “Light management: porous 1-dimensional nanocolumnar structures as effective photonic crystals for perovskite solar cells,” J. Mater. Chem. A, vol. 4, pp. 4962 – 4970, 2016. FJRM performed the experiments, fabricated and optimized the solar cells. MOR deposited photonic crystal like structures under the supervision of ARGE and also made the optical characterization, while MKN and MG supervised the device fabrication part. SA directed the work. All authors discussed the results and commented on the manuscript  S. Kazim‡, F. J. Ramos‡, P. Gao, M. K. Nazeeruddin, M. Grätzel, and S. Ahmad, “A dopant free linear acene derivative as a hole transport material for perovskite pigmented solar cells,” Energy Environ. Sci., vol. 8, pp. 1816–23, 2015. Appendix - 99 - Report of scientific relevance of the publications Firstly, full reference of the works here compiled as a thesis by publications is provided in this section including: title of the publication, year of publication, name of the scientific journal with impact factor and quartile and number of citations of the work according both Google Scholar and CrossRef (Table A.1). Table A.1. Summary of the scientific relevance for the publications contained in this thesis. Last updated: 12.04.2016. Publication title Year Journal name Impact factor Quartile Citations (Google Scholar) Citations (CrossRef) Publications contained in this thesis Perovskite Solar Cells Based on Nanocolumnar Plasma- Deposited ZnO Thin Films 2014 ChemPhysChem 3.419 Q1 29 20 Nanocolumnar 1-dimensional TiO2 photoanodes deposited by PVD-OAD for perovskite solar cell fabrication 2015 J. Mater. Chem. A 7.443 Q1 4 5 Light management: porous 1-dimensional nanocolumnar structures as effective photonic crystals for perovskite solar cells 2016 J. Mater. Chem. A 7.443 Q1 0 0 A dopant free linear acene derivative as a hole transport material for perovskite pigmented solar cells 2015 Energy Environ. Sci. 20.523 Q1 16 16 Non-aggregated Zn(II)octa(2,6- diphenylphenoxy) phthalocyanine as a hole transporting material for efficient perovskite solar cells 2015 Dalton Trans. 4.197 Q1 15 10 Rational design of triazatruxene-based hole conductors for perovskite solar cells 2015 RSC Adv. 3.840 Q1 3 2 After that, a summary report has been also completed not only including first authorship publications but also for all the collaborations made by the candidate during the PhD in order to evaluate total number of citations, i10 index and h - index (Table A.2). Ph.D. Thesis. Fco. Javier Ramos Mellado - 100 - Table A.2. Summary of the scientific relevance for all the publications completed by the candidate during the PhD (both first author and collaborations). Last updated: 12.04.2016. Publication title Year Journal name Impact factor Quartile Citations (Google Scholar) Citations (CrossRef) Publications contained in this thesis Perovskite Solar Cells Based on Nanocolumnar Plasma- Deposited ZnO Thin Films 2014 ChemPhysChem 3.419 Q1 29 20 Nanocolumnar 1-dimensional TiO2 photoanodes deposited by PVD-OAD for perovskite solar cell fabrication 2015 J. Mater. Chem. A 7.443 Q1 4 5 Light management: porous 1-dimensional nanocolumnar structures as effective photonic crystals for perovskite solar cells 2016 J. Mater. Chem. A 7.443 Q1 0 0 A dopant free linear acene derivative as a hole transport material for perovskite pigmented solar cells 2015 Energy Environ. Sci. 20.523 Q1 16 16 Non-aggregated Zn(II)octa(2,6- diphenylphenoxy) phthalocyanine as a hole transporting material for efficient perovskite solar cells 2015 Dalton Trans. 4.197 Q1 15 10 Rational design of triazatruxene-based hole conductors for perovskite solar cells 2015 RSC Adv. 3.840 Q1 3 2 Other collaborations Highly efficient flexible cathode for Dye sensitized solar cells to complement Pt@TCO coatings 2014 J. Mater. Chem. A 7.443 Q1 6 7 Photoanode Based on (001)-Oriented Anatase Nanoplatelets for Organic − Inorganic Lead Iodide Perovskite Solar Cell 2014 Chem Mater. 8.354 Q1 20 15 Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell 2014 Nature Commun. 11.470 Q1 47 48 Elucidating Transport- 2014 J. Phys. Chem. C 4.772 Q1 31 27 Appendix - 101 - Recombination Mechanisms in Perovskite Solar Cells by Small- Perturbation Techniques Direct monitoring of ultrafast electron and hole dynamics in perovskite solar cells 2015 Phys. Chem. Chem. Phys. 4.493 Q1 16 13 Unraveling the Role of Monovalent Halides in Mixed Halide Organic- Inorganic Perovskites 2016 ChemPhysChem 3.419 Q1 0 0 Conference papers Fabrication and encapsulation of perovskites sensitized solid state solar cells 2014 40th PVSC, IEEEE (Conference paper) - - 2 1 Total citations 189 164 i10-index 7 7 h -index 7 7 Appendix - 103 - List of publications Complete reference of publications finished by the PhD candidate during his PhD is listed below by order of acceptation for publication:  J. Idígoras, E. Guillén, F. J. Ramos, J. A. Anta, M. K. Nazeeruddin, and S. Ahmad, “Highly efficient flexible cathode for Dye sensitized solar cells to complement Pt@TCO coatings,” J. Mater. Chem. A , vol. 2, pp. 3175–3181, 2014.  F. J. Ramos, M. C. López-Santos, E. Guillén, M. K. Nazeeruddin, M. Grätzel, A. R. González-Elipe, and S. Ahmad, “Perovskite Solar Cells Based on Nanocolumnar Plasma- Deposited ZnO Thin Films,” ChemPhysChem , vol. 15, no. 6, pp. 1148–1153, 2014.  F. J. Ramos, D. Cortes, A. Aguirre, F. J. Castano, and S. Ahmad, “Fabrication and encapsulation of perovskites sensitized solid state solar cells,” IEEE 40th PVSC , no. JUNE 2014, pp. 2584–2587, 2014.  M. I. Dar, F. J. Ramos, Z. Xue, B. Liu, S. Ahmad, S. A. Shivashankar, M. K. Nazeeruddin, and M. Grätzel, “Photoanode Based on (001)-Oriented Anatase Nanoplatelets for Organic − Inorganic Lead Iodide Perovskite Solar Cell,” Chem. Mater ., vol. 26, pp. 4675–4678, 2014.  V. W. Bergmann, S. A. L. Weber, F. J. Ramos, M. K. Nazeeruddin, M. Grätzel, D. Li, A. L. Domanski, I. Lieberwirth, S. Ahmad, and R. Berger, “Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell,” Nat. Commun ., vol. 5, p. 5001, 2014.  E. Guillén, F. J. Ramos, J. A. Anta, and S. Ahmad, “Elucidating Transport- Recombination Mechanisms in Perovskite Solar Cells by Small-Perturbation Techniques,” J. Phys. Chem. C , vol. 118, no. 40, pp. 22913–22922, 2014.  F. J. Ramos, M. Ince, M. Urbani, A. Abate, M. Grätzel, S. Ahmad, T. Torres, and M. K. Nazeeruddin, “Non-aggregated Zn(II)octa(2,6-diphenylphenoxy) phthalocyanine as a hole transporting material for efficient perovskite solar cells,” Dalton Trans ., vol. 44, no. 23, pp. 10847–51, 2015.  P. Piatkowski, B. Cohen, F. J. Ramos, M. Di Nunzio, M. K. Nazeeruddin, M. Grätzel, S. Ahmad, and A. Douhal, “Direct monitoring of ultrafast electron and hole dynamics in perovskite solar cells,” Phys. Chem. Chem. Phys ., vol. 17, no. 22, pp. 14674–14684, 2015.  F. J. Ramos, M. Oliva-Ramirez, M. K. Nazeeruddin, M. Grätzel, A. R. Gonzalez-Elipe, and S. Ahmad, “Nanocolumnar 1-dimensional TiO2 photoanodes deposited by PVD-OAD for perovskite solar cell fabrication,” J. Mater. Chem. A , vol. 3, pp. 13291–13298, 2015.  S. Kazim‡, F. J. Ramos‡, P. Gao, M. K. Nazeeruddin, M. Grätzel, and S. Ahmad, “A dopant free linear acene derivative as a hole transport material for perovskite pigmented solar cells,” Energy Environ. Sci ., vol. 8, pp. 1816–23, 2015. ‡ These authors contributed equally to this work. Ph.D. Thesis. Fco. Javier Ramos Mellado - 104 -  F. J. Ramos, K. Rakstys, S. Kazim, M. Grätzel, M. K. Nazeeruddin, and S. Ahmad, “Rational design of triazatruxene-based hole conductors for perovskite solar cells,” RSC Adv ., vol. 5, pp. 53426–32, 2015.  M. Deepa, F. J. Ramos, S. M. Shivaprasad, and S. Ahmad, “Unraveling the Role of Monovalent Halides in Mixed Halide Organic- Inorganic Perovskites,” ChemPhysChem , vol. 17, no. 6, pp. 913–920, 2016.  F. J. Ramos, M. Oliva-Ramirez, M. K. Nazeeruddin, M. Grätzel, A. R. Gonzalez-Elipe, and S. Ahmad, “Light management: porous 1-dimensional nanocolumnar structures as effective photonic crystals for perovskite solar cells,” J. Mater. Chem. A , vol. 4, pp. 4962 – 4970, 2016. Appendix - 193 - List of stays The following stays were completed at Laboratory of photonic and Interfaces (LPI), École Polytechnique Fédéral de Lausanne (EPFL), under the supervision of Prof. Michael Grätzel:  October 14th, 2012 – April, 30th, 2013 (6 and a half months)  May 31st, 2013 – August, 30th, 2013 (3 months)  September 30th, 2013 – March 30th, 2014 (6 months)  May 4th, 2014 – August 1st, 2014 (3 months)  December 15th – 22nd, 2014 (1 week)  March 1st – 7th, 2015 (1 week) List of patents  Title: “Compuesto aromático policíclico sustituido como material de transporte de huecos en células solares de estado sólido basadas en perovskita”. (“Substituted Polycyclic Aromatic Compound as a Hole Transport Material in Perovskite-Based Solid-State Solar Cells”). Inventors: S. Ahmad, F.J. Ramos, S. Kazim, M. Doblaré Castellano, M.K. Nazeeruddin, and M. Graetzel. Applicant: Abengoa Research, S.L. (100.0%) (ES). Publication number: ES2563361 (14.03.2016) Application number: P201431324 (12.09.2014)  Title: “Novel Compound and their Use as Hole Transport Material” Applicant: Abengoa Research, S.L. (100.0%) (ES). Publication number: Pending Application number: P201431776 (28.11.2014) Ph.D. Thesis. Fco. Javier Ramos Mellado - 194 - List of conferences A list of the conference contributions made during the PhD of the candidate by chronological order is detailed below, where presenting author is underlined and PhD candidate is highlighted in bold.  HOPV 14. Lausanne (Switzerland), May 11-14th, 2014. Appendix - 195 -  A. Douhal, P. Piatkowski, B. Cohen, F.J. Ramos, M.R. Di Nunzio, M.K. Nazeeruddin and S. Ahmad. “Interrogating the Dynamics of Electrons and Holes Within Perovskite-based Solar Cells”.  S. Ahmad and F.J. Ramos. “Device Characterization and Structure Property Relationship in Perovskites Based Solar Cell”.  E. Guillén, F.J. Ramos, J.A. Anta and S. Ahmad. “Small-perturbation Characterization of Perovskite Solar Cells and Identification of Transportrecombination Mechanisms” (Poster).  S. Kazim, F.J. Ramos, E. Guillén, P. Gao, M.K. Nazeeruddin, M. Graetzel and S. Ahmad. “An Efficient Organic Hole Transport Material for Mesoscopic Perovskite based Solar Cells” (Poster).  F.J. Ramos, M.K. Nazeeruddin, M. Oliva-Ramirez, M. Graetzel, A.R. Gonzalez-Elipe and S. Ahmad. “Nanocolumnar 1-D photoanodes deposited by PVD-GLAD for efficient perovskite solar cells fabrication” (Poster).  40th IEEE Photovoltaic Specialist Conference. Denver, Colorado (USA). June 8-13th, 2014. Poster and full text in book of Proceedings contributions. Poster nominated as finalist.  F.J. Ramos, D. Cortés, A. Aguirre, F.J. Castaño and S. Ahmad. “Fabrication and encapsulation of solid state perovskite solar cells” (Poster). Poster has been nominated for the best poster award of session 6 (Finalist)  F.J. Ramos, D. Cortés, A. Aguirre, F.J. Castaño and S. Ahmad. “Fabrication and encapsulation of perovskites sensitized solid state solar cells” (Conference paper). Ph.D. Thesis. Fco. Javier Ramos Mellado - 196 -  PSCO15. Lausanne (Switzerland), September 27-30th, 2015.  V. Bergmann, S.A.L. Weber, F.J. Ramos, M.K. Nazeeruddin, M. Grätzel, D. Li, I. Lieberwirth, S. Ahmad, R. Berger. “Unbalanced Charge Distribution inside a Perovskite-Sensitized Solar Cell in Real Space”.  F.J. Ramos, K. Rakstys, S. Kazim, M. Grätzel, M.K. Nazeeruddin and S. Ahmad. “Triazatruxene-based molecules as hole transporting material in Perovskite Solar Cells” (Poster).