A Study of Light-Sensitive Ideality Factor and Voltage-Dependent Carrier Collection of CdTe Solar Cells in Forward Bias

Two features that distinguish CdS/CdTe solar cells from traditional c-silicon cells have been widely confirmed by experiment: 1) The ideality factor of the diode current is sensitive to photon flux input, and 2) the photocurrent is voltage dependent. Here, numerical simulation of one-sided p-type CdTe junctions has been performed in order to interpret the experimental characteristics under forward bias. In these calculations, we assumed a distribution of trapping states within the CdTe bandgap. Detailed discussions have been divided into two parts: light-dependent forward current and voltage-dependent carrier collection. The former has demonstrated that recombination loss increases with increased optical generation. As a result, the diode current, in a certain forward bias region, depends on illumination. With this mechanism, the ideality factor of simulated J-V curves shows a variation consistent with experimental curves. Carrier collection loss under voltage bias is the combined result of carrier diffusion, drift, and recombination during transport. Simulations that are based on our assumption have demonstrated that collection loss increases at higher forward bias and depends on wavelength, carrier mobility, and carrier lifetime.