Thickness-Dependent Performance of Undoped Polycrystalline CdSeTe-Only Solar Cells: n-type Behavior and Hole Diffusion Length From Modeling

Selenium has improved carrier lifetime and luminescence in CdSexTe1-x/cadmium telluride (CdTe) bi-layer absorbers. Without a CdTe layer, uniform bandgap, without any intentional doping (undoped), CdSeTe-only (or CST-only) absorbers exhibited higher external radiative efficiency and carrier lifetimes compared to bi-layer absorbers. However, this did not translate into device performance. This has been attributed to poor hole mobility in the undoped CdSeTe-only absorber due to the presence of hole traps. In this study, we demonstrate that the limited device performance stems from the n-type nature of undoped CdSeTe-only absorber within p-type device architecture. A significant improvement in power conversion efficiency from <5% to 13.4% was achieved by reducing the absorber thickness to less than one micron, one of the highest reported for this technology. Poor device performance in undoped thicker absorbers is attributed to limited hole (minority carrier) diffusion. Through solar cell capacitance simulator (SCAPS) modeling, hole diffusion length in these absorbers, with electron concentration of 3 x 1015 cm-3 (extracted from C-V measurement), was estimated to be in the range of 0.5-0.9 ..mu..m.