Diffuse radiation influence on the performance of luminescent solar concentrators

Diffuse radiation influence on the performance of Luminescent Solar Concentrators Miguel Barragán Sánchez-Lanuza [1][2], Isidoro Lillo-Bravo [2], Amador Menéndez-Velázquez [3], Jose-Maria Delgado-Sanchez [1] [1] Dpt. Applied Physics, University of Seville, Ctra. Utrera km 1, 41013 Seville (Spain) [2] Dpt. Energy Engineering, University of Seville, Avda. de los Descubrimientos s/n, 41092 Seville (Spain) [3] Photoactive Materials Research Unit, IDONIAL Technology Center, Calle Ablanal 8, 33428 Asturias (Spain) ABSTRACT Luminescent Solar Concentrators (LSCs) are often purported to perform pretty well under both direct and diffuse solar irradiance conditions, yet the underlying rationale supporting this assumption remains scant. Furthermore, a significant portion of previous research on these photovoltaic devices relies on indoor equipment for efficiency characterization, thereby limiting insights into their performance under real operating conditions. In this study, we provide a theoretical framework elucidating the principles governing the superior performance of LSCs under diffuse irradiance. Specifically, we demonstrate that the isotropic distribution of dipoles within the LSC optical window aligns favorably with the Lambertian distribution of diffuse radiation, thereby facilitating enhanced photon absorption. This theoretical foundation is bolstered by experimental investigations involving prototypes utilizing Lumogen Red dye, conducted under adverse irradiance conditions, including sunny, hazy and partially cloudy skies. Our experimental findings reveal that light scattering, whether arising naturally from diffuse radiation or via scattering from neighboring surfaces, significantly amplifies the short circuit current of LSC devices compared to that generated under clear sky conditions. Keywords: Luminescent Solar Concentrator; Diffuse radiation; dye absorption; photovoltaics; dipole isotropic distribution; diffuse fraction coefficient. 1. Introduction A Luminescent Solar Concentrator (LSC) is an optoelectronic device composed of luminescent species, such as luminophores or fluorophores, embedded in a host material and typically encapsulated in glass-glass with solar cells attached to it, usually at the edges, to effectively convert light into electricity. This device captures solar irradiance through its front face and utilizes a total internal reflection mechanism to guide the captured light towards the solar cells (Figure 1). Luminescent emitters within the LSC absorb a specific spectral range of solar irradiance, subsequently re-emitting it at different wavelengths through the photoluminescence process. This dual functionality imparts two significant design advantages to the solar cell. Firstly, the irradiance spectrum undergoes modification to achieve a better match with the solar cell bandgap. Secondly, the geometric concentration factor, determined by the ratio between the front face of the LSC and the solar cell area, enhances the photocurrent generated. The basic concept of LSC was first proposed in the early 1970s [1-3]. Initially, the concept aimed to achieve a reduction in the size of the solar cells by over 90%. Due to the cost-effectiveness of the materials used in manufacturing LSC, this was intended to result in a considerable reduction in module process. Furthermore, the LSC waveguide is constructed using polymeric materials, such as poly-methylmethacrylate (PMMA), which are lighter than conventional silicon photovoltaic (PV) modules. This characteristic poses a challenge for new PV applications in buildings [4]. Additionally, the front face of the LSC can be transparent or cast in any color, making them visually appealing to architects. Subsequently, various authors have explored alternative approaches to enhance LSC performance. Organic materials are extensively used in LSCs. The most commonly employed organic dyes in LSC include molecules of the Coumarin family, such as Coumarin 6 (C6) [5], perylenes, such as PM597 [6], and perylenebisimides, like Lumogen Red (LR305) [7]. These organic dyes offer the advantage of exhibiting high quantum yield and good solubility in polymer matrices. However, they limited absorption ranges and small Stokes-shift, leading re-adsorption. Alternative designs involve combining different types of dyes to improve the absorption [8] or emission [9] spectra range. Other explored strategies to enhance their performance include considering back reflectors, such as Lambertian back reflectors [10] or Bragg back reflectors [11]. The suitability of the LSC for operating under diffuse irradiance has been frequently underscored in literature as one of its potential advantages [12-15]. Authors commonly attribute this capability to the capacity to concentrate light obviating the need for a tracking system, a distinctive feature when compared to conventional concentrated photovoltaic (CPV) systems. So, it is assumed that LSCs can capture incident sunlight from different angles, but to the best of our knowledge, this assumption has not been yet proved. In contrast to CPV systems, which exclusively harness the direct component of solar irradiance, it is asserted, albeit not demonstrated, that LSC possesses the capacity to concentrate both direct and diffuse radiation (DHI). Notably, the majority of the experiments to evaluate the LSC performance, as reported by other researchers, are carried out under indoor conditions utilizing a solar simulator that is characterized by an incident collimated beam of light replicating the direct normal incident (DNI) component of solar irradiance [16-19]. Despite the prevalent acknowledgement of the LSC’s advantage to operate under diffuse irradiance, a comprehensive exploration into the underlying reasons sustaining this phenomenon remains conspicuously absent from existing research. Noteworthy exceptions include the work of Debije et al [20,21], who monitored in outdoor conditions a set of LSC prototypes observing that their performance increases during sunrise and sunset. Their findings attribute this enhancement to the spectral change in solar irradiance spectrum, and to the fact that short wavelength light is scattered more effectively in the atmosphere, enhancing the blue component under diffuse radiation conditions, aligning well the absorption spectrum of the specific used dye. Similar explanation is concluded by Sethi et al [22] who also characterized the LSC performance in outdoor conditions LSC: in diffuse solar irradiance conditions the blue light ratio is higher in the solar spectrum, and in that range the reported LSC has higher absorption. However, despite this insightful contribution, a holistic examination of the myriad intricacies influencing LSC performance under diffuse irradiance conditions is yet to be undertaken within the scientific community. Therefore, the aim of this research study is to comprehensively explore the multifaceted factors (irradiance component, incident angle of the light, and spectrum matching) contributing to the LSC performance under diffuse irradiance conditions. This inquiry seeks to fill the existing gap in understanding and potentially pave the way for further advancements in solar energy technologies. Figure 1. LSC architecture representing isotropic emission by the fluorophore, guide light produced by total internal reflection and escape angle losses. 2. Materials and methods The LSC prototypes (Figure 2a) developed in this study are based on Lumogen Red (LR305) from BASF. LR305 was dissolved in a solution comprising polymethyl methacrylate (PMMA) and chloroform, both sourced from Sigma Aldrich. Polymerization was conducted at room temperature under continuous stirring using a magnetic stirrer. A mixture of 300 mg of PMMA and 4 mL of chloroform was stirred for 48h to ensure a homogeneous solution with a dye concentration of 1.5% w/w. Subsequently, the dye solution was deposited on glass by spin-coating (Laurell, WS-650Mz-8NPP). The chloroform solvent was evaporated at 50 ºC on a hot-plate within a covered Petri dish to increase the vapor pressure and prevent dust particles deposition on the coated film. The LSC prototypes were designed to accommodate CIGS thin-films solar cells (100 x 10 mm2) along the edges in a front-face configuration. The optical window, measuring 100 x 100 mm2, encompasses the four solar cells positioned at the edges. For encapsulation, each prototype was laminated with commercial EVA and glass sheets using a laminator from P energy (model L036LAB). To ensure statistical reliability, each prototype was replicated 10 times. As a reference, a set of prototypes with similar geometry was manufactured without incorporating any luminescent dye [23]. Indoor characterization during the manufacturing process involved transmittance and reflectance measurements conducted with a white halogen lamp (Newport) coupled to a monochromator (Oriel Cornstone 260). The samples were placed inside an integrating sphere (Labspehere), and light modulation was achieved through a mechanical chopper (Stanford Research Systems Inc). The modulated light was then monitored using a silicon detector (Newport model 7610) connected to an external Lock-In Amplifier (Stanford Research Systems SR830). For the assessment of luminescent dye emission, a fluorimeter (Edinburg Instrument FLSP920) was employed. The IV curve of both solar cells and LSC prototypes was characterized using a solar simulator (Abet 3000 AAA). Outdoor characterization (Figure 2b) involved monitoring the short-circuit current (Isc) under various irradiance conditions using a datalogger (Agilent 34970A). A nearby meteorological station recorded solar irradiance components, ambient temperature, and wind speed. The total diffuse (DHI), direct (DNI) and global (GHI) irradiance were measured using first-class calibrated Kipp and Zonen radiometers. Instantaneous spectral irradiance was captured with a factorycalibrated EKO MS-711N radiometer connected to an RSB-01 rotating shadow band unit. Spectral global and diffuse horizontal data were recorded every minute with a resolution of 0.5 nm, covering a bandwidth from 300 to 1100 nm. The outdoor experiments were carried out Seville (Spain: 37.41 N, 6.01 W). 3. Results and discussion The majority of LSC prototypes, akin to other solar energy devices documented in the literature, are conventionally characterized under indoor conditions using solar simulators with a standard AM 1.5 spectrum. However, it is imperative to acknowledge that the solar spectrum outdoors deviates from the AM 1.5 standard due to specific location factors, ambient humidity, the presence of intermittent clouds, suspended particulate matter, among other variables. In order to furnish results that closely align with real outdoor operating conditions, we characterized the solar irradiance spectrum under three contrasting scenarios: sunny sky, hazy sky, and partially cloudy sky days. The spectral components of GHI, DNI, and DHI for three distinct adverse days selected for this research study are shown in Figure 3, 4 and 5. Additionally, Figure 6 illustrates the absolute components of solar irradiance for the selected days. On sunny days, the solar spectrum typically exhibits a predominant DNI component, accompanied by a reduced DHI component. Conversely, hazy days, and particularly pronounced in partially cloudy conditions, tend to show a higher influence of the DHI component. Additionally, the spectral profiles of DNI and DHI diverge, with DHI displaying heightened intensity in the blue range, while the peak intensity of DNI is situated within the visible spectrum [24]. Consequently, a comparative analysis of sunny, hazy, and partially cloudy days reveals alterations not only in intensity but also in spectral distribution. The spectral shifting observed in the irradiance spectrum is also evident during sunrise in sunny sky days, especially in the winter seasons or when the day is characterized by the presence of humidity, aerosols or dust in the atmosphere. During such conditions, the irradiance energy is Figure 2. a) Illustration of the luminescent solar concentrator prototype developed specifically for this research study using LR305; b) Outdoor experimental facilities designed to monitor the photocurrent generation in relation to the varying components of solar irradiance. a) b ) influenced by absorption and scattering by water vapor molecules. Through mechanisms associated with Rayleigh and Mie scattering, water vapor or a high density of particles in the atmosphere enrich the DHI component at ground level in short wavelengths, particularly in the blue region range. Another distinction between DNI and DHI lies in the fact that the former can be collimated with a single point of origin, while the latter originates from every direction. Figure 3. Spectral irradiance experimentally measured during a sunny sky day: a) GHI, b) DNI, and c) DHI. Figure 4. Spectral irradiance experimentally measured during a hazy sky day: a) GHI, b) DNI, and c) DHI. To understand the LSC performance operating under real environmental conditions, two identical 10x10 cm2 prototypes were manufactured using Lumogen Red as optical window, as it is one of the most common references in this research field [25]. They were installed outdoor to monitor the short-circuit current relative to a reference sample evolution under different environmental conditions (solar irradiance and ambient temperature). The reference sample consists in one identical prototype to LSC ones but without any dye embedded in glass-glass. It just considers four solar cells at the edges of the glass substrate, keeping the same geometry of the LSC prototypes. Figure 7 illustrates the normalized short-circuit current of the LSC compared to the reference sample (glass-glass lamination with the same specifications as the LSC prototype but without any embedded dye) under three solar adverse irradiance conditions: (a) clear and sunny sky; (b) hazy sky; and (c) partially cloudy sky. To correlate the normalized short-circuit current Figure 5. Spectral irradiance experimentally measured during a partially cloudy sky day: a) GHI, b) DNI, and c) DHI. Figure 6. Absolute irradiance experimentally measured during three adverse sky conditions: a) sunny, b) hazy, and c) partially cloudy. with the solar irradiance component, it is presented alongside the diffuse fraction coefficient (𝑘𝑑), which is calculated as [26]: 𝑘𝑑=𝐷𝐻𝐼 𝐺𝐻𝐼×100 [𝐸𝑞.4] The 𝑘𝑑 is interpreted such that on sunny days with a predominant DNI component, it tends towards zero, whereas under atmospheric conditions characterized by a dominant DHI component, it tends towards 100%. Thus, in Figures 7a to 7c, we can compare the relationship between changes in the normalized short-circuit current of the prototype with the incident radiation conditions. (a) (b) In Figure 7a, it is observed that under sunny sky conditions, the short-circuit current monitored in the LSC prototype is approximately 20% higher than the reference sample, with significant enhancements during the sunrise and sunset (around 35%). The explanation for these peaks is lies in two factors. First, during these periods, the irradiance spectra undergo a slight spectral change to a more pronounced response in the UV wavelengths, as Debije et al [10] pointed out, thereby favoring the matching with the dye absorption spectrum. However, if this were the sole explanation for this phenomenon, it would fail to account for the enhanced performance observed experimentally under hazy or partially cloudy skies. To provide a more comprehensive understanding, it is necessary to consider the influence of the Lambertian DHI distribution, and the isotropic distribution of the dipoles. Second, it is also noted that during these times, the 𝑘𝑑 increases significantly, indicating a more pronounced DHI component in the solar spectrum. With the dominance of the DHI component, the photoresponse of the LSC is heightened due to the increased probability of absorption by the dipoles. The aforementioned conclusion is further corroborated upon examining Figures 7b and 7c, depicting hazy and partially cloud conditions, respectively, with higher average values for 𝑘𝑑. In these scenarios, where solar irradiance exhibits a higher DHI component alongside an isotropic nature in the direction of incident light vectors, the ratio of short circuit current in the LSC prototypes normalized to the reference sample displays a notable increase compared to sunny days. This observation underscores the notion that under diffuse conditions, the probability of photon absorption by the dipoles constituting the dyes is significantly augmented. This (c) Figure 7. Normalized short circuit current of LSC prototype relative to a reference sample under three adverse solar irradiance conditions: a) sunny and clear sky; b) hazy sky; and c) partially cloudy sky. specific absorption spectrum of each dye, as illustrated in Figure 10. The nuanced differences in absorption ratios underscore the intricate interplay between the spectral characteristics of incident radiation and the spectral absorption properties of individual dyes within LSC devices. (2) the order parameter of the selected dye. Order parameter (𝑠) is usually defined as [31]: 𝑠= 𝐴∥−𝐴⊥ 𝐴∥+2𝐴⊥ [𝐸𝑞.7] where 𝐴∥ and 𝐴⊥ are the absorbance values of the sample, when incident light was polarized parallel and perpendicular to the alignment direction of the host LCs, respectively. A completely random molecule has an order parameter of 𝑠=0, whereas perfect alignment of the dye with its absorption axis fully parallel to the incident light yields 𝑠=1 (Figure 12). Previous studies [32- 34] have reported that Lumogen Red and Lumogen Yellow absorption is slightly deviated from the plane, with an order parameter close to 𝑠=0.2 (this means that the parallel and perpendicular ratio is 7:3). In contrast, the dyes DCM, and PM650 demonstrate preferential absorption aligned with their molecule axis (𝑠=0.7) (this means that the parallel and perpendicular ratio is 9:1). In outdoor LSC applications, solar irradiation reaches the dipole at oblique incident angles. Assuming that the dye molecules are isotropically distributed within the host material, the effective absorption coefficient (𝛼) is calculated, deriving from the Eq. 7, as [35]: 1 𝛼2=𝑐𝑜𝑠2𝜃 𝛼∥2+𝑠𝑒𝑛2𝜃 𝛼⊥ 2 [𝐸𝑞.8] where 𝜃 represents the relative angle of the dipole orientation to the sun elevation, while 𝛼∥ and 𝛼⊥ denoted the parallel and perpendicular absorption coefficients, respectively. Correlating this characteristic with the results presented in Figure 11, it is evident that Lumogen Red and Yellow are less sensitive to the cosine effect (Lambert’s Law) in the Direct Irradiation component of the solar irradiance spectrum compared to DCM or PM650. This result contrasts with the observations made in LSCs with aligned dyes, such as those based on liquid crystals or polymer stretching techniques. It suggests that dyes with dipole orientations slightly differing from the plane (such as Lumogen Red) might have a competitive advantage over purely linear dyes (such as DCM) for outdoor applications. This would add to the already reported advantages of Lumogen Red, such as broad absorption range, high quantum yield, photostability, and others reported by various authors [36]. It is widely acknowledged that one of the notable advantages of LSC devices lies in their applicability for photovoltaic energy generation in urban settings and integrated within building architecture [37-41]. In this research study, it has been elucidated the optical behavior of these devices under varying components of solar irradiance, with the overarching goal of deriving meaningful conclusions. Our findings, indicate that the performance of LSC devices is notably augmented when diffuse radiation predominates. This phenomenon is particularly evident in urban environments, where incident light may undergo scattering from surrounding surfaces, thereby disrupting the inherent directional nature of solar irradiance, even in locations where the DNI component prevails. In essence, LSC devices demonstrate a remarkable adaptability to diverse ambient conditions. Furthermore, our suggestions that their performance is particularly enhanced during sunrise period, and on hazy or partially cloudy days. This enhancement can be attributed to multiple factors, including the modification of the spectrum of solar irradiance observed under diffuse conditions, which aligns more favorably with the absorption spectra of the dye, but specially, the isotropic distribution of the light, which increases the likelihood of photon absorption by the dipoles withing the optical window of the LSC device. Figure 12. Schematic displaying dyes selected for this study with adverse order parameter and predominant absorption directions: a) Lumogen Red with 𝑠=0.2, and b) DCM with 𝑠=0.7. 4. Conclusions In conclusion, this study has provided a theoretical justification for the operational efficacy of LSC devices under both direct and diffuse solar irradiance conditions. Specifically, we have demonstrated that, for a LSC with isotropic orientation of luminescent dyes, diffuse radiation conditions are more favorable due to the Lambertian distribution of this radiation component, which increases the probability of photon absorption by the dipoles within the optical window when they exhibit an isotropic distribution. This conclusion holds true irrespective of the selected dyes. Furthermore, outdoor monitoring of LSC prototypes under diverse environmental conditions, including sunny, hazy, and partially cloudy days, has yielded valuable insights. We observed a notable increase in short circuit current (35%) when the diffuse fraction coefficient increases, indicating a prevalence of diffuse radiation in solar irradiance, compared to clear and sunny sky (25%). This empirical finding underscores the performance enhancing effect of diffuse radiation on LSC devices. Moreover, spectral measurements of DNI, DHI, and GHI have revealed an intensity boost in the UV spectrum region when the DHI component predominates, matching more closely with the absorption spectrum of the dye. This spectral alignment, observed during sunset, and during hazy or partially cloudy days, contributes to the observed increase in LSC performance. As a future research direction, our goal is to conduct similar outdoor measurement campaigns under varied climatic conditions and employing a wider range of dyes. Expanding the scope of experimental investigations will not only enhance our understanding of LSC behaviour but also contribute to the optimization of these devices for practical applications in diverse environmental settings. CRediT authorship contribution statement M. Barragán Sánchez-Lanuza: Writing, Data curation, Formal analysis. I. Lillo-Bravo: Writing, Conceptualization, Methodology, Supervision. A. Menéndez-Velázquez: Writing, Resources, Investigation. J.M. Delgado-Sanchez: Writing, Conceptualization, Methodology, Resources, Investigation, Formal analysis, Supervision. Declaration of Competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company. 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