Solar Cell Performance Under Different Illumination Conditions
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Solar Cell Performance Under Different Illumination Conditions Christian Gemmer and Markus B. Schubert Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, D-70569 Stuttgart, Germany email: [email protected] ABSTRACT We numerically simulate performance data of hydrogenated amorphous silicon (a-Si:H) and copper indium gallium diselenide (CIGS) based solar cells for various illumination conditions. For ease of comparison, we model typical single junctions with the very same software. The study allows us to evaluate the cell feasibility in different hybrid electronic systems like smart cards, wrist watches, transponder systems, and mobile sensors. At an illumination intensity of 1 sun, the optical bandgap of the absorber material and the series resistances determine the spectral sensitivity of the solar cell to particular illumination spectra. For intensities of 10-2 suns and so-called D65 spectrum, which represents daylight under cloudy skies, the efficiency of a-Si:H solar cells nearly equates the CIGS cell performance although the AM 1.5 efficiency of the CIGS diode exceeds the one of our a-Si:H cell by more than a factor of two. Infraredweighted black body radiation leads to superior performance of the CIGS type, whereas for ultraviolet-weighted illumination the a-Si:H cell shows better performance. For intensities below 10-4 suns the external shunt resistance dominates the current-voltage characteristics of both cell types, resulting in poor performance independent of the incident spectrum. We complete our study by simulating the solar-powered charging process of a gold capacitor, which serves us as a model for the energy storage within a hybrid electronic system. The charging behavior under various realistic illumination conditions shows particular cell characteristics: high open circuit voltages qualify a-Si:H solar cells for electronic systems that require increased voltages and CIGS cells are suited for applications with higher current need. INTRODUCTION The optimization of solar cells usually proceeds for the standardized illumination spectrum AM 1.5 [1]. In many realistic cases, however, the illumination conditions remarkably deviate from this standard, resulting in substantially different cell performance. Typical examples are i) indoor applications, where the intensity Ptotal of predominantly red halogen light equals Ptotal = 1 Wm-2 (10-3 suns), and ii) outdoor applications, where cloudy skies exhibit a blue-shifted spectrum and intensities of 10-2 ... 10-1 suns [2]. Thus, studying the performance of solar cells under low light conditions and different illumination spectra is of high relevance for optimizing hybrid electronic systems like smart cards, wrist watches, transponder systems, and mobile sensors. In literature, only few publications treat low light level performance of solar cells. S. S. Hegedus used low intensity illumination on a-(Si,Ge) based cells to experimentally determine the carrier collection length [3]. Okamoto et al., and Hishikawa et al
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