Band gap fluctuations in Cu(In,Ga)Se 2 thin films
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F16.4.1
Band gap fluctuations in Cu(In,Ga)Se2 thin films
Julian Mattheis, Thomas Schlenker, Martin Bogicevic, Uwe Rau, and Jürgen H. Werner Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart e-mail: [email protected]
ABSTRACT A simple statistical model describes measured absorption and photoluminescence data of Cu(In1-x, Gax)Se2 thin films. The broadening of the transition peak in the absorption spectra stems from band gap fluctuations. The extent of the spatial inhomogeneities as expressed in the standard deviation σ Eg reaches a maximum of σ Eg = 90 meV for films with equal amounts of indium and gallium, indicating alloy disorder as one possible source of the band gap fluctuations. The fluctuations observed lead to a decrease ∆VOC of the maximum possible open-circuit voltage VOC of almost 150 mV. However, the experimentally measured, low VOC of solar cells with high gallium content cannot be explained by band gap fluctuations alone. Consequently, our analysis suggests that the dominant recombination process in Cu(In1-x, Gax)Se2 thin film solar cells with high gallium content is not governed by the band gap energy, but is more likely due to deep levels within the forbidden gap. INTRODUCTION Lateral band gap inhomogeneities have a strong influence on the power conversion efficiency of solar cells [1]. This statement holds for the radiative efficiency limit as well as for real solar cells that are dominated by non-radiative recombination processes [2]. Due to the exponential dependence of the recombination current on the (effective) band gap, the saturation current for a sample with fluctuations increases drastically compared to a homogeneous device with a spatially uniform band gap. Therefore, the open-circuit voltage VOC and consequently the conversion efficiency deteriorate significantly. Polycrystalline thin-film solar cells exhibit band gap fluctuations because of the high degree of disorder in the absorber materials. In polycrystalline Cu(In1-x, Gax)Se2 the disorder may arise from structural defects, from spatial fluctuations of stoichiometry and / or the alloy composition [2]. This contribution investigates the absorption and photoluminescence (PL) of various Cu(In1-x, Gax)Se2 films by comparing the experiments with the predictions of a simple statistical model [1] that assumes a Gaussian distribution for the fundamental energy gap of the semiconductor. This model has only two free parameters, namely the mean band gap energy E g and the standard deviation σ Eg . The model fits absorption and PL data simultaneously and yields a quantitative measure for the band gap fluctuations in terms of the standard variation σ Eg ranging
F16.4.2
from σ Eg = 25 meV to σ Eg = 90 meV . The open-circuit voltage losses ∆VOC resulting directly from spatial disorder range from ∆VOC = 10 mV to ∆VOC = 150 mV . EXPERIMENTAL We fabricate CIGS thin films with a thickness around 1.7 µm by co-evaporating the elements onto soda lime glass substrates with a substrate t
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