Defect-Tolerant Sodium-Based Dopant in Charge Transport Layers for Highly Efficient and Stable Perovskite Solar Cells

To extract charges more efficiently through charge-transporting layers (CTLs), various dopants are necessary. Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) is the most widely used dopant in electron- and hole-transporting layers. However, Li+ ions easily migrate into the perovskite and deteriorate the device performance. To address this issue, several efforts such as introducing a buffer layer have been tried, but the issue is still not fully resolved. Thus it is required to find a simple way without additional treatments. In this work, we propose a simple strategy to use defect-tolerant dopant in CTLs, sodium bis(trifluoromethanesulfonyl)imide (Na-TFSI), to improve both the efficiency and the stability of perovskite solar cells (PSCs). The PSCs with Na-TFSI for both the electron-transport layer and the hole-transport layer show the highest power conversion efficiency up to 22.4%. In addition, the device with Na-TFSI exhibited better long-term operating stability at 45 °C, maintaining >80% of the initial performance even after 500 h of continuous 1 sun illumination.