Influence of Se Grading on the Free Carrier Profile of CdSeTe/CdTe Solar Cells

CdTe solar cells have shown great improvements in recent years due partly to selenium alloying, leading to strong interest in CdSeTe/CdTe devices. The effects of selenium inclusion on the electronic properties of CdTe is becoming better understood, but uncertainties remain. Capacitance-voltage (CV) measurements offer a quick and accessible method for profiling the free carriers in photovoltaic devices, however little has been done to study the effect of Se alloying in CdTe via this procedure. Here we use numerical simulation of CdSeTe/CdTe photovoltaic devices to analyze the effects that Se could have on the CV behavior. We show that the charge distribution in the device changes significantly depending on how Se influences the energetic and electronic properties of the absorber, and that a poor back contact is expected to obscure the ability of CV to detect these changes. We then discuss these predictions in the context of CV profiling of devices with various Se distributions fabricated via thermal evaporation. We conclude that Se affects the carrier profile in a more complicated manner than assumed in our model and that limited spatial resolution near the front interface, likely because of back contact barriers, precludes precise extraction of information on passivation or compensation via this technique.