Selective Cd Removal From CdTe for High-Efficiency Te Back-Contact Formation

The presence of a Te-rich surface or an elemental Te layer is beneficial for the formation of a low-barrier back contact for high-efficiency CdTe solar cells. Etching processes are widely used to form Te-rich CdTe surfaces, while deposition processes such as evaporation are used to form elemental Te layers. Here, we show that a reaction between methylammonium iodide (CH 3 NH 3 I, MAI) and CdTe can be used to simply and controllably produce elemental Te over a wide processing window. Both X-ray diffraction and Raman spectroscopy confirmed the formation of a Te layer. The MAI-produced Te layer reduces the Schottky barrier height, improves the open-circuit voltage ( V OC ) and fill factor, and outperforms contacts formed with the evaporated Te. We examined the effect of MAI reaction temperature and the amount of Cu needed to optimize the device. CdS/CdTe stacks that were treated with a 125 mM MAI solution and heated to 125 °C for 10 min showed the best power conversion efficiency (PCE) of 14.1%, while the best efficiency of a standard device without treatment was 13.0%, and the best PCE of an evaporated Te layer was 13.8%. Notably, the improved efficiency for the MAI-treated devices was achieved with less Cu than was required for the standard device. With an indium tin oxide back electrode, the PCE was also improved from 11.0% to 12.2% with MAI treatment, providing a potential route for fabricating high-efficiency transparent or bifacial CdTe solar cells.