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

• Published in: IEEE journal of photovoltaics Vol. 8; no. 4; pp. 1125 - 1131
• Main Authors: Watthage, Suneth C., Phillips, Adam B., Liyanage, Geethika K., Song, Zhaoning, Gibbs, Jacob M., Alfadhili, Fadhil K., Alkhayat, Rabee B., Ahangharnejhad, Ramez H., Almutawah, Zahrah S., Bhandari, Khagendra P., Ellingson, Randy J., Heben, Michael J.
• Format: Journal Article
• Language: English
• Published: IEEE 01.07.2018
• Subjects: Back contact; bifacial; Cadmium compounds; CdTe; II-VI semiconductor materials; methylammonium iodide; perovskite; Photovoltaic cells; Schottky barrier; Surface morphology; Surface treatment; Te layer; Temperature measurement; X-ray diffraction
• ISSN: 2156-3381; 2156-3403
• DOI: 10.1109/JPHOTOV.2018.2830302

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.