Investigation of High Open Circuit Voltage in CdTe-based Solar Cells Using Oxide Back Buffer Layers

Polycrystalline CdTe-based photovoltaic (PV) devices currently account for ~5-10% of the PV market. This is despite the fact that the open circuit voltage (Voc) of these devices is only ~71 % of its detailed-balanced limit. Increasing the Voc will lead to further efficiency gains and potentially reduce the already low cost per Watt production. To improve the Voc will require reducing recombination, likely at the back interface for the current generation of Cu-doped devices. To do this, we employ sputtered oxide buffer layers, specifically, ${\mathrm{A}1_{2}\mathrm{O}_{3}}$ and ${\text{CuAlO}_{2}}$. The high bandgap and valence band positions deeper than that of CdTe suggest that these materials have the potential to reduce the recombination at the back interface. When thick oxide layers are used, we observe and s-kink in the current density-voltage curve, but Voc values above 900 mV are observed for both materials. As the thickness of the oxide buffers decreases the Voc decreases with an increase in the fill factor. These results point to a pathway to achieve high Voc while maintaining a high fill factor.