Impact of Na Dynamics at the Cu₂ZnSn(S,Se)₄/CdS Interface During Post Low Temperature Treatment of Absorbers

Cu2SnZn(S,Se)4 (CZTSSe) solar cells based on earth abundant and nontoxic elements currently achieve efficiencies exceeding 12%. It has been reported that, to obtain high efficiency devices, a post thermal treatment of absorbers or devices at temperatures ranging between 150 and 400 °C (post low temperature treatment, PLTT) is advisable. Recent findings point toward a beneficial passivation of grain boundaries with SnOx or Cu-depleted surface and grain boundaries during the PLTT process, but no investigation regarding alkali doping is available, even though alkali dynamics, especially Na, are systematically reported to be crucial within the field. In this work, CZTSSe absorbers were subjected to the PLTT process under different temperatures, and solar cells were completed. We found surprisingly behavior in which efficiency decreased to nearly 0% at 200 °C during the PLTT process, being recovered or even improved at temperatures above 300 °C. This unusual behavior correlates well with the Na dynamics in the devices, especially with the in-depth distribution of Na in the active CZTSSe/CdS interface region, indicating the key importance of Na spatial distribution on device properties. We present an innovative model for Na dynamics supported by theoretical calculations and additional specially designed experiments to explain this behavior. After optimization of the PLTT process, a Se-rich CZTSSe solar cell with 8.3% efficiency was achieved.