Degradation Mechanism and Stability Improvement of Dopant-Free ZnO/LiF<i><sub>x</sub></i>/Al Electron Nanocontacts in Silicon Heterojunction Solar Cells
Wenjie Lin, Mathieu Boccard, Sihua Zhong, Vincent Paratte, Quentin Jeangros, Luca Antognini, Julie Dréon, Jean Cattin, Jonathan Thomet, Zongtao Liu, Zhiming Chen, Zongcun Liang, Pingqi Gao, Hui Shen, Christophe Ballif
Abstract
Dopant-free passivating contacts for photovoltaics have the potential to be deposited at low costs while providing excellent surface passivation and low contact resistance. However, one pressing issue of dopant-free carrier selective contacts is their lower environmental stability compared to conventional silicon-based contacts. In this contribution, we study the degradation in the ZnO/LiFx/Al electron selective nanocontact with experiments and simulations and suggest design modifications for higher performance and stability. Using a thicker metallization and optimal ZnO deposition temperature (130 °C), we improved open-circuit voltage and fill factor, together with improved stability with retention of over 93 and 88% of the initial open-circuit voltage and fill factor after storage in air for 380 h. The champion device has reached an efficiency of 21.3% with VOC of 727 mV, JSC of 37.6 mA/cm2, and FF of 78.0%. Furthermore, the enhanced stability in vacuum, scanning transmission electron microscopy (STEM) images, and the current-exchange simulation suggests that the degradation of the a-Si:H(i)/ZnO/LiFx/Al contact is caused by a drop of the LiFx/Al work function, due to interaction with air. This work has developed a deep understanding of the degradation mechanism and the methodology of stability analysis for dopant-free silicon solar cells.