Quantum Efficiency Measurements to Deduce Non-Ideal Solar-Cell Features
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1012-Y12-03
Quantum Efficiency Measurements to Deduce Non-Ideal Solar-Cell Features Timothy J. Nagle, Alan R. Davies, and James R. Sites Department of Physics, Colorado State University, Fort Collins, CO, 80523
ABSTRACT Appropriate interpretation of quantum-efficiency (QE) measurements made on non-ideal solar cells often reveal subtle features of the photodiodes. QE measurements on CuIn1-xGaxSe2 and CdTe thin-film solar cells with CdS buffer layers demonstrate some of these features, and in each case we identify the electrical processes responsible. One well-known complication in cells with CdS buffer layers is ìphotodopingî, where blue light exposure increases the n-type CdS carrier density. The resulting modification of the p-n junction alters the carrier collection for all wavelengths and can lead to misinterpretation of results. The use of a white, DC-bias light during QE measurements generally solves this problem, but it has not been clear what bias light level is sufficient for accurate results. Measurements with varying intensities of DC-bias light show that complications due to CdS photoconductivity are generally mitigated near 0.05 sun intensity. Other factors which can cause misinterpretation of QE measurements include the presence of secondary barriers, and photoconductivity in the absorber layer. Numerical simulations of band profiles under various light-bias conditions are consistent with the experimental data. The analysis is used to suggest a standard set of measurement conditions for reliable QE analysis. INTRODUCTION This article will focus on quantum-efficiency (QE) measurements of thin-film CdTe and CuIn1-xGaxSe2 (CIGS) solar-cells which use CdS window layers. Under standard conditions, QE measurements quantify the spectral sensitivity of a solar cell. The idealized QE measurement would hold the cell at zero voltage-bias and use one-sun light-bias while a chopped monochromatic probe beam is used as an AC perturbation [1,2]. These ideal conditions are virtually never used in practice, primarily because of the difficulty in measuring a small AC perturbation superimposed on a large DC bias current. In addition to this technical limitation, we often choose to measure QE at non-zero voltages where light-bias effects are larger. When nonstandard measurements (e.g. in the dark, under voltage bias) yield results that are clearly different from the idealized measurement, we use the term apparent quantum efficiency (AQE). These AQE results can differ significantly from the standard measurement, but they can also provide us with valuable information about the band structure of the device, such as the amount of photoconductivity in the window layer (CdS) and the change in space-charge width under illumination. We present experimental curves taken under a variety of light-bias conditions,
compare our results to numerical simulations to suggest which conditions are sufficient for valid results, and offer physical explanations for the non-ideal behavior. In this paper we continue the convention introduced in
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