Non-equivalent Tl doping for high performance perovskite solar cells: Crystal quality improvement with enhanced p-type character

Organic-inorganic hybrid perovskite solar cells, whose highest power conversion efficiency (PCE) already exceeds 25%, have been attracting considerable attention. However, there are still various challenges, such as their low repeatability, large number of intrinsic defects, and low environmental stability. In this study, we dope monovalent thallium (Tl+) into the precursor to form a pinhole-free perovskite film with improved photo-voltaic performance and environmental stability. The incorporation of Tl+ into CH3NH3PbI3 can improve the crystal quality of perovskite film by adjusting the rate of crystalline growth. Due to the non-equivalent doping of Tl+, the Fermi level is lowered by 0.31 eV toward the highest occupied molecular orbital, indicating that Tl behaves as a p-type dopant. An improved carrier dynamics and balanced charge transportation can be observed in MAPb(0.99)Tl(0.01)I(3). After the doping of 1 mol% Tl into the perovskite film, the average PCE of the cells is increased from 15.1 to 17.8%. Furthermore, the thallium-doped perovskite solar cell exhibits outstanding environmental stability because of the improved moisture resistance. After 200 h of storage under ambient conditions, the Tldoped perovskite solar cell retains 82.7% of its initial power conversion efficiency, which is higher than the retention of the intrinsic perovskite device (55.9%).