Chlorine-terminated MXene quantum dots for improving crystallinity and moisture stability in high-performance perovskite solar cells

In n-i-p planar perovskite solar cells, high-crystallinity perovskite film and long-term stability against high humidity are critical for high photovoltaic performance and future commercialization. In this work, the Cl-terminated Ti3C2 quantum dots (denoted as Ti3C2Clx QDs) are introduced as additives in perovskite precursor solution by one-step deposition method. The strong interaction between the Cl terminations of Ti3C2Clx QDs and Pb2+ ions can retard the crystallization rate and induce the preferred grain orientation during the film crystallization process, thus delivering a high-quality perovskite film with high crystallinity, fewer trap-states, and smaller residual tensile strain. Meanwhile, due to its top–bottom increasing gradient distribution, the addition of Ti3C2Clx QDs can also accelerate the charge extraction and benefit band alignment between the tin oxide (SnO2) electron transport layer (ETL) and the perovskite layer. As a result, the device with Ti3C2Clx QDs can achieve an enhanced efficiency of 21.31% with an ultrahigh open-circuit voltage of 1.19 V and negligible hysteresis. In addition, by substituting the OH terminations of Ti3C2Tx, Cl-terminated Ti3C2Clx QDs can avoid possible deprotonation of protonated organic amine in perovskite, improving the overall stabilities. As a result, the unencapsulated device can exhibit outstanding long-term humidity stability by maintaining over 84% of its initial PCE after being aged for 1000 h under 40% relative humidity in the dark at room temperature.