Enhancing grain growth and boosting Voc in CZTSe absorber layers — Is Ge doping the answer?

In this study we present beneficial effects on the device performance with a Ge-assisted crystallization of nanocrystalline CZTSe precursors. For low Ge content layers, an increase in doping density is observed, which results in 8.6% efficiency devices and Voc values of above 470 mV, corresponding to Voc deficits of 583 mV, comparable to current record devices. High Ge content layers exhibit enhanced grain growth, however, they are also associated with deterioration in cell performance. Admittance spectroscopy measurements identified the appearance of a deep defect for high Ge doping. These results indicate that an accurate control of group IV (Ge, Sn) elemental composition seems mandatory for high device performance.