High-performance large-area blade-coated perovskite solar cells with low ohmic loss for low lighting indoor applications

High-performance large-area blade-coated perovskite solar cells with low ohmic loss for low lighting indoor applications Zhuoneng Bi,1,2 Xueqing Xu,1,2* Xia Chen,1 Yanqing Zhu,1 Chang Liu, 3 Hua Yu, 3* Yupeng Zheng,1,2 Pavel A. Troshin,4,5 Antonio Guerrero, 6 Gang Xu1,2 1 Key Laboratory of Renewable Energy, Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China 2 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China 3 Institute of Photovoltaics, Southwest Petroleum University, Chengdu 610500, P. R. China 4 Silesian University of Technology, Akademicka 2A, 44-100 Gliwice, Poland 5 Institute for Problems of Chemical Physics of the Russian Academy of Sciences (IPCP RAS), Academician Semenov avenue 1, Chernogolovka, Moscow region, 142432 Russian Federation 6 Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castelló, Spain * Corresponding author email: [email protected], [email protected]m Abstract Emerging hybrid organic-inorganic perovskites with superior optoelectronic property demonstrate promising prospect for photovoltaic applications, in particular for low-lighting indoor applications e.g. within internet of things (IoT) networks or lowenergy wireless communication devices. In order to prepare devices with high power output under low-illumination conditions, scalable fabrication techniques are preferred for large-area perovskite solar cells. In addition, one of the key parameters to achieve high-efficiency large-area perovskite solar cells is to minimize the ohmic loss to further boost the solar cell efficiency. Herein, a one-step blade-coating method assisted by hexafluorobenzene (HFB) was developed to deposit large-area smooth and defect-free perovskite films with low ohmic loss. The as-fabricated devices demonstrated power conversion efficiency (PCE) of 20.7% (area of 0.2 cm2) and 16.5% (1 cm2), respectively, under standard 1000 mW/cm2 (AM1.5G) illumination conditions. Besides, the large-area (1 cm2) devices demonstrated a remarkable PCE of ~32% under 0.1 mW/cm2 (~285 lux) illumination provided by white light-emitting diode (LED) lamp for indoor lighting which strongly indicate that the fabricated large-area devices exhibited practical applications to be used in the vast majority of low-lighting indoor conditions (≥100 lux). The analysis using a single diode model suggests that the high performance of the large-area devices under low-lighting indoor conditions is highly associated with the largely reduced ohmic losses of the perovskite films by a facile and scalable blade-coating method. The ohmic loss for the 1 cm2 device was estimated as low as 1.66% at ~285 lux. The presented scalable approach paves the way to designing high-performance perovskite solar cells for a variety of emerging indoor PV applications. Keywords: perovskite solar cells, blade-coating, large area, low-lighting indoor applications, ohmic loss Introduction Hybrid organic-inorganic perovskite solar cells (PSCs) have been demonstrated to be one of the most promising candidates for the next generation photovoltaics due to their high power conversion efficiency (PCE) and low materials and fabrication cost 1, 2. PSCs have versatile promising applications in portable electronics, such as remote energy-independent sensors for the internet of things networks, or low-energy wireless communication devices 3-5. Most of these devices need to work continuously under low indoor illumination conditions (100 ~ 1000 lux) 5. Low-lighting indoor photovoltaics has a very high demand in the current and future power-supply market 4. Hence, the low-lighting indoor perovskite photovoltaics have recently emerged as one of the promising and practical technology for indoor power supply 6-11. Organic-inorganic perovskite absorber materials is one of the ideal candidates for indoor photovoltaics since their absorption spectra perfectly match the emission spectra of LED or fluorescent lamps (400 - 800 nm) 12. Recently, Liu et al. achieved an impressive PCE of 40.1% under indoor illumination conditions (824.5 lux) for the perovskite solar cell with a small active area of 0.08 cm2 12. There are also multiple other reports on small-area PSCs delivering PCEs of 30 – 40% under indoor conditions. However, the small area device is not practically for commercialization use and the development of large-area indoor PSCs is impeded by large ohmic loss. Feng et al. have reported one of the highest records to date for large-area devices (2.25 cm2) demonstrating excellent PCE of 30.6% under indoor condition (1000 lux)13. Generally, there are two popular approaches to improve PCE of PSCs under indoor illumination conditions. The first approach requires the devices achieving high shunt resistance (Rsh), which can effectively minimize ohmic losses under low-light conditions 14-16. Interface modification is a common strategy to obtain high shunt resistance devices 6, 14. The other approach is based on suppressing the charge carrier recombination losses, which is manifested in the diode ideality factors (n) close to 1 10. The ideality fact n can be estimated from the dependence of the open-circuit voltage (VOC) on the incident light intensity 13. Achieving good ideality factors requires suppressing trap-assisted recombination within the device. The passivation of traps and defects in the perovskite films is an effective way to reduce VOC losses 12. For potential practical applications, low-lighting perovskite solar cells should be produced using scalable techniques compatible with the roll-to-roll process. However, most of the perovskite films reported so far prepared by non-scalable methods such as spin-coating. To date, large-area deposition methods remain insufficiently investigated. The first reports on blade-coating were published in 2015, and presented PSCs with the PCEs of 11-15% produced using DMF as a solvent 17, 18. More recently, meniscus- assisted technique produced large-grain perovskite films, which boosted the device efficiency up to 20% (using DMSO as a solvent) 19. Solvent mixtures of DMSO and gamma-butyrolactone (GBL) were previously used to fabricate PSCs with the PCE of about 18% 20, 21. Further development of the blade-coating techniques for PSCs fabrication is commonly considered as one of the most promising steps towards largescale commercial applications of PSCs 2, 22-25. In this work, we present a one-step blade-coating assisted by HFB approach to deposit high-quality defect-free perovskite films for indoor perovskite photovoltaics. The proposed method produces highly crystalline perovskite films in a single step by using engineered inks based on a low inhalation toxic solvent mixture of DMSO, GBL and HFB. The preheating of the precursor ink enables complete dissolution of PbI2 and MAI in the mixed solvent close to the solubility limit. It is found that the HFB additive effectively smooths down the perovskite films and passivate the defects, thus, the asfabricated devices demonstrate high power conversion efficiency (PCE) of 20.7% on a small area of 0.2 cm2 and 16.5% on the area of 1 cm2 under standard 1 sun (AM1.5G) illumination conditions. Furthermore, the large-area (1 cm2) devices deliver a remarkable PCE of ~32% under 0.1 mW/cm2 (~285 lux) illumination provided by lightemitting diode (LED) lamp. Results and discussion Figure 1. a) Schematic diagram of the HFB-assisted one-step blade-coating method to produce high-quality defect-free MAPbI3 film. b) Photographs of MAPbI3 precursor solutions with and without HFB at room temperature and at 70 oC. Figure 1a shows the schematic diagram of the blade-coating setup to produce MAPbI3 perovskite films. In the experimental process the substrate is fixed on a hotplate with a blade set above the substrate forming the slit of around 100 m. The perovskite precursor ink is injected into the slit and the pre-heating blade immediately moves horizontally on the heating substrate. With the solvent evaporating, the black perovskite film forms in a couple of seconds. The one-step blade coating process is very simple and adapts to different process variations. In addition, for further improve the quality of perovskite films on this one-step blade coating approach the solvent may be easily modified. In particular we observe that the use of a co-solvent HFB in the perovskite precursor is beneficial for the film quality. Further details of the one-step blade-coating process are given in the experimental section. Figure 2. a, d) AFM images of the Control and HFB 15% MAPbI3 films. b, e) Topview and c, f) cross-section SEM images of the Control HFB 15% samples. Figure 1b shows a image of the reference precursor solution and the modified ink loaded with HFB additive. HFB is a low inhalation toxic organic solvent that is commonly used in medicine 26. HFB is immiscible with water, but it can be miscible with DMSO in a certain proportion 27. At room temperature, perovskite inks with HFB additive are non-transparent, which indicates incomplete material solubilization but to the presence of a bad solvent (or anti-solvent) such as HFB. However, preheating the precursor solution at 70 oC results in complete solubilization of PbI2 and MAI and formation of transparent solution with material concentrations close to the solubility limit leading to saturated solutions. Figure S1 shows the scanning electron microscopy (SEM) images of the MAPbI3 films prepared from the inks with different HFB loadings and marked as Control (corresponds to HFB 0%), HFB 5%, HFB 10%, and HFB 15%. All samples exhibit spherulitic growth and large gain size with no voids or pinholes. The Control and HFB 15% samples display obvious difference between as illustrated by the SEM images shown in Figures 2b and 2e. The HFB 15% sample exhibits a flat and uniform crystalline grain surface, whereas the Control sample has wavy surface morphology. The surface topography of the films was further studied by atomic force microscopy (AFM) as shown in Figure 2a and 2d. The root-mean-square roughness values for the Control and HFB 15% samples were determined as 28.1 and 20.5 nm, respectively, which is consistent with the SEM images. The cross-section SEM images (Figures 2c, 2f and S2) reveal the average thickness of perovskite films as ~415 nm and ~438 nm for Control and HFB 15% samples, respectively. It is worth noting that the HFB 15% sample shows a more uniform structure and thickness of the perovskite film. Figure 3. Characterization of the MAPbI3 films with different HFB loadings on the glass/FTO/SnO2-PbO/SnO2 substrates. a) XRD diffraction patterns. b) UV-vis absorption spectra. c) Steady-state PL spectra. d) Time-resolved PL (TRPL) profiles. The effect of the HFB additive on the structural and optical properties of perovskite films is illustrated in Figure 3a and Figure 3b, respectively. All samples show similar peaks on the x-ray diffraction (XRD) patterns, indicating a high phase purity of the perovskites. The diffraction peaks at 14.12˚ 20.02˚ 28.46˚ 31.90˚ and 40.70˚ can be assigned to (110), (112), (220), (310), and (224) crystal planes of the tetragonal MAPbI3 perovskite lattice, respectively 18. The sharp and intense (112) diffraction peaks indicate a highly oriented crystal structure of the films. Interestingly, the addition of HFB increases the intensity of (110) peaks, suggesting improved film crystallization. The absorption spectra become significantly stronger with increasing the HFB loading from 0% to 15%. We believe that the uniform thickness of HFB-processed films helps to avoid absorption losses, while in the case of Control samples some light passes through the voids or deep valleys in the films. Thus, the obtained results reveal that HFB can not only make the perovskite films more uniform but also alter the crystal growth and enhance film absorbance. Figures 3c and 3d show steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra. The PL peaks of the HFB processed sample are red-shifted compared to that of the Control sample and the magnitude of the red-shift is proportional to the HFB loading with a ~7 nm peak shift for the HFB 15% sample. Furthermore, a red-shift of the low-energy absorption onset was observed, which in combination with PL data implies that the HFB-processed samples have a slightly lower bandgap as compared to the Control, which could be a consequence of the enhanced material crystallinity. The charge carrier lifetimes in the HFB-processed samples were considerably longer than that of the Control (  1= 36.0 ns and  2= 73.7 ns) sample and the lifetime increase was proportional with the HFB loading. In particular, the HFB 15% sample showed almost twice longer lifetimes (  1 = 52.7 ns and  2= 122.8 ns) as compared to the Control sample, which evidences that HFB additive effectively suppresses trap-assisted carrier recombination due to the formation of high-quality perovskite grains with a low density of defects. Fabrication and characterization of photovoltaic devices Figure 4. a) J-V curves measured under standard 100 mW cm-2 AM1.5G simulated illumination conditions. b) EQE spectra of the devices. c) Statistics for 20 Control and 20 HFB 15% devices. d) Stabilized output measurements in the maximum power point tracking regime under the standard illumination conditions. Photovoltaic devices were fabricated with device architecture of Glass/FTO/SnO2PbO/SnO2/MAPbI3/Spiro-OMeTAD/Au. The MAPbI3 films were prepared with different HFB loadings in the precursor ink, which are marked as Control, HFB 5%, HFB 10%, and HFB 15% samples. Further details on the device fabrication can be found in the Experimental section. It is worth mentioning that the deposition of MAPbI3 by the developed one-step blade coating method does not require any additional steps such as solvent annealing or anti-solvent bath, etc. The representative J-V curves measured under the simulated 1 sun illumination conditions are presented in Figure 4a, whereas the corresponding device parameters are summarized in Table S1. The Control devices show PCE exceeding 17%, which is in good agreement with the previously reported characteristics of the MAPbI3 devices fabricated in dry box conditions with a relative humidity between 10% to 20%. However, increasing of HFB amount in the ink improves the photovoltaic performance of the devices processed under the same conditions. Using the optimal amount of HFB (15%), the champion cells with a PCE of 20.7%, open-circuit voltage VOC of 1.13 V, short-circuit current density JSC of 23.4 mA/cm2, and fill factor FF of 78.4% was achieved. The stabilized output measurements of the PCE and photocurrent density of the champion cell delivered the values of 19.8% and 21.9 mA/cm2, respectively (Figure 4d). The obtained photocurrent density is in good agreement with the integrated JSC extracted from the EQE spectrum by its integration over the reference solar AM1.5G spectrum (Figure 4b). The statistics for 20 devices of each type (Figure 4c) show a remarkable improvement in average PCE from 16.6% to 19.1% when HFB is incorporated into the precursor inks. Furthermore, it was surprising that HFB additive also improves the environmental stability of the devices as shown in Figure S4. Thus, HFB could be considered as a highly promising co-solvent for perovskite inks and processing additive improving simultaneously the efficiency and stability of PSCs. 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