Superlattices and Microstructures
A modeling approach is presented for the first time to model the dependence of short-circuit current density (Jsc) and open-circuit voltage (Voc) on the grain size g in three thin film solar cells including the emerging perovskites. The variation of Jsc and Voc with the grain size (g) of three different solar cells with CdTe, CZTS and perovskite absorber layers are modelled and fitted with the experimental dataset collected from relevant literature. The experimental literature suggested that the grain size of absorber layers in solar cells is controlled during the deposition process by adjusting the growth rate, temperature and ambient. The model has been successfully applied to the experimental data obtained from CdTe, CZTS and MAPbI3 Perovskite solar cells by adjusting a few parameters which are affected by grain size such as mobility, diffusion length, and carrier lifetime. A bigger grain size leads to a higher short-circuit current density and open circuit voltage. However, the polycrystalline nature of thin film materials is still an impediment for the fabrication of large grain absorber layers compared to single crystalline Si materials. The modeling results show almost similar trends of dependency of device parameters on grain size: both Jsc and Voc follow the √{ g } trend (g1/2) and for larger grain sizes it tends to a constant level. CZTS and perovskite cells show an almost polynomial quadratic dependency on grain size. The grain size of 2 μm seems to be a turning point for the CdTe materials, 1.5 μm for CZTS and 250 nm for perovskites. However, in most cases, the increasing slope of the Jsc and Voc tends to a constant value because the effect of increasing the grain size on diffusion length and mobility (or carrier lifetime) becomes negligible.
