Pierre Besson

Effect of the encapsulant temperature on the angular and spectral response of multi-junction solar cells

Multi-junction solar cells (MJSC) operating at working conditions under concentration are subjected to temperatures (T) for which the optical coupling provided by their anti-reflective coatings (ARC) has not been optimized. High temperatures and wide ray angles produced by the concentrator on the optical interface of the cell can significantly modify the ARC performance. This effect is especially pronounced for ARCs adapted to silicone encapsulant because the silicone refractive index (n) is significantly sensitive to temperature, modifying the optimal design thickness and material composition. This effect is magnified for tilted rays whose optical path length through the ARC layer is most modified. In this work, an absolute external quantum efficiency (EQE) characterization system is adapted to perform angular and temperature spectral response analysis, allowing to quantify the impact of the optical mismatch caused by the increase of encapsulant temperature for each of their junctions. The intricacies of this upgraded characterization technique are explored, providing insight on important unexpected measurement variables such as finger orientation with respect to the incident ray bundle. A significant spectral mismatch between junctions due to the change in silicone temperature has been observed, leading to short-circuit current (ISC) losses as high as 6% with respect to the design conditions (T=25°C) for rays impinging the cell with tilt angles of 70° and more realistic operation temperatures of 65°C. The losses arise from current mismatch between subcells produced by variations in the optical coupling. Lessons from this analysis may be taken into account by future CPV system designers.

On the sensitivity of 4 different CPV module technologies to relevant ambient and operation conditions

The sensitivity of four different CPV module technologies to most operating conditions relevant to CPV systems has been studied, namely DNI, spectrum, cell and lens temperature and clearness of the sky. In order to isolate the influence of a single operation parameter, the analysis of long-term outdoor monitoring data is required. The effect of lens temperature on cell current has been found to vary greatly between modules due to the different optical architectures studied. Maximum sensitivity is found for silicone-on-glass primary lenses. The VOC thermal coefficient was found to vary between module technologies, probably due to differences in maximum local effective concentration.

Advances on multijunction solar cell characterization aimed at the optimization of real concentrator performance

Multijunction solar cells (MJSC) are usually developed to maximize efficiency under test conditions and not under real operation. This is the case of anti-reflective coatings (ARC), which are meant to minimize Fresnel reflection losses for a family of incident rays at room temperature. In order to understand and quantify the discrepancies between test and operation conditions, we have experimentally analyzed the spectral response of MJSC for a variety of incidence angles that are in practice received by a concentrator cell in high-concentration photovoltaic (HCPV) receiver designs. Moreover, we characterize this angular dependence as a function of temperature in order to reproduce real operation conditions. As the refractive index of the silicone is dependent on temperature, an optical mismatch is expected. Regarding other characterization techniques, a method called Relative EL Homogeneity Analysis (RELHA) is applied to processed wafers prior to dicing, allowing to diagnose the wafer crystalline homogene...

Effects of lens temperature on irradiance profile and chromatic aberration for CPV optics

Lens-based optical concentrators are currently the most common in concentrator photovoltaic systems. This paper discusses experimental results to quantify the effects of temperature on the primary optical elements of three commercial Fresnel-based designs. The designs are: Silicon on Glass Primary with no secondary, PMMA Primary with a Truncated Inverted Pyramid secondary, and a PMMA 4 quadrant Fresnel – Kohler system. We quantify the effects of temperature on the irradiance profile with the variation in peak to average ratio of from 25 – 50 ° C. Furthermore, the effect of temperature on chromatic aberration is represented with the ratio of the top subcell current to middle subcell for a standard triple junction germanium-based cell. For the system with the lowest peak-to-average ratio, PMMA-based 4 Fresnel-Kohler design, we observed a change of 2% in the subcell ratio when the lens was heated to 50 °C.

CPV-specific test procedures for evaluating on-grid inverters

A new protocol for testing on-grid inverters accounting for the operating conditions specific to CPV arrays is introduced. First, a CPV-specific weighted average for providing the static conversion efficiency according to typical frequency distributions of CPV yield throughout the year. Then, a set of representative scenarios for analyzing the dynamic MPPT efficiency of the inverter: slow ramp (clear sky), fast cloud passage, self-shading between arrays (typical of sunrise and sunset) and a wavering current response typical of imperfect tracking. The procedures have been applied to two different string-size inverters at the CEA inverter test bench, which have revealed strong differences in their dynamic MPPT efficiency. The CPV-specific weighted average proposed was found to provide lower performance figures than the classical European and CEC efficiencies for the two studied inverters. This protocol provides important information for CPV plant dimensioning and equipment selection, as the differences in performance revealed can result in significant deviation on the final energy yield.

Characterization of the influence of temperature on achromatic mirrors by means of METHOD

Our characterization tool, called METHOD which was previously built for refractive optics [1], has been adapted and optimized to evaluate a mirror from the APOLLON Project [2], characterized in the framework of the European CPVMatch project. Influence of the glass box of the thermal chamber, homogeneity of the temperature across the sample, and influence of the chromatic light on the energetic spot’s center are studied. We evaluate the mirror’s abilities at different working temperatures and cell-to-primary optical element (POE) distances.

METHOD: a tool for mechanical, electrical, thermal, and optical characterization of single lens module design

The optical characterization and electrical performance evaluation are essential in the design and optimization of a concentrator photovoltaic system. The geometry, materials, and size of concentrator optics are diverse and different environmental conditions impact their performance. CEA has developed a new concentrator photovoltaic system characterization bench, METHOD, which enables multi-physics optimization studies. The lens and cell temperatures are controlled independently with the METHOD to study their isolated effects on the electrical and optical performance of the system. These influences can be studied in terms of their effect on optical efficiency, focal distance, spectral sensitivity, electrical efficiency, or cell current matching. Furthermore, the irradiance map of a concentrator optic can be mapped to study its variations versus the focal length or the lens temperature. The present work shows this application to analyze the performance of a Fresnel lens linking temperature to optical and electrical performance.

Contributions to reproducible CPV outdoor power ratings

Methodologies that aim to obtain a reproducible power rating are still under discussion at the WG7 of the IEC and there is a need for feedback from real field application in order to validate or improve these methods. These procedures are evaluated through the outdoor rating of seven modules from four different CPV technologies, which have been measured at the CEA outdoor monitoring bench at the Institut National de l’Energie Solaire (INES) site. The benefit of introducing other procedural considerations is analyzed, namely the inclusion of spectrally-corrected irradiance, the utilization of lens temperature as a new parameter for regressions and the optimization of dataset filtering.

Improving Optical Performance of Concentrator Cells by Means of a Deposited Nanopattern Layer

Multijunction solar cells (MJSC) use anti-reflective coatings (ARC) to minimize Fresnel reflection losses for a family of light incidence angles. These coatings adapt the refractive index of the cell to that of the surrounding medium. Patterns with sizes in the range of the light wavelength can be used to further reduce reflections through diffraction. Transparent nanopatterns with a gradual profile, called moth-eye nanostructures, can adapt the refractive index of the optical interfaces (often with n∼1.5) used to encapsulate concentrator solar cells to that of the air (n air∼1). Here we show the effect of a nanometric moth-eye ARC with a round motif deposited on commercial MJSC that achieves short-circuit current (I SC) gains greater than 2% at normal incidence and even higher in the case of tilted illumination. In this work, MJSC with different moth-eye ARC are characterized under quantum efficiency (QE) as well as under concentrated illumination I-V in order to assess their potential. Simulations based on coupled wave analysis (RCWA) are used to fit the experimental results with successful results.

Reduction of front-metallization grid shading in concentrator cells through laser micro-grooved cover glass

Concentrator solar cell front-grid metallizations are designed so that the trade-off between series resistance and shading factor (SF) is optimized for a particular irradiance. High concentrator photovoltaics (CPV) typically requires a metallic electrode pattern that covers up to 10% of the cell surface. The shading effect produced by this front electrode results in a significant reduction in short-circuit current (ISC) and hence, in a significant efficiency loss. In this work we present a cover glass (originally meant to protect the cell surface) that is laser-grooved with a micrometric pattern that redirects the incident solar light towards interfinger regions and away from the metallic electrodes, where they would be wasted in terms of photovoltaic generation. Quantum efficiency (QE) and current (I)-voltage (V) characterization under concentration validate the proof-of-concept, showing great potential for CPV applications.