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Numerical modeling is increasingly used to obtain insight in to the details of the physical operation of thin-film solar cells. Over the years several modeling tools specific to thin-film PV devices have been developed. A number of these tools have reached a mature status and are available to the PV community. Some of the most commonly used programs are presented and the possibilities as well as the shortcomings are discussed. Also, for the different thin-film PV devices (CdTe, CIGS, and, to a lesser extent, amorphous silicon and nano-structured solar cells) an overview is given of modeling efforts and achievements.Three-dimensional, numerical simulation is now almost indispensable for the design of silicon devices, from discrete transistors and optoelectronic devices to large-scale integration. Highly developed programs include, e.g., tunneling, optics, heat flow and other features. Numerical simulation is routinely performed in developing

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Recent developments in the following areas are briefly reviewed: a) the electrical structure of grain boundaries in CdTe absorbers, b) impurities and non-stoichiometry in CdTe solar cells and c) use of Sb 2 Te 3 in contacts to CdTe. Nominally identical solar cells fabricated using 99.999% pure CdTe feedstock from two different suppliers were compared. Differences in the photovoltaic response and absorber grain size were correlated with the purity of the feedstock, the purer material giving the higher V oc , FF and efficiency, and larger grain size. Quantum efficiency and C-V measurements indicated that the performance differences are most likely to result from reduced doping at the back contact surface in the less pure sample. A quantitative SIMS study of Sb-Te contacts to CdTe reveals that annealing in air at 400 C causes an influx of Sb and O into the absorber layer. Free energy calculations indicate that this is driven by the preferential reaction of O with Sb compared to CdTe oxidation.

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