Thiol-Functionalized MoS2 Induced High Quality α-phase FAPbI3 Crystal for Efficient Perovskite Solar Cells

Formamidinium lead iodide (FAPbI₃) has become one of the most promising materials for high performance perovskite solar cells (PSC) due to its ideal band gap and broad light absorption spectrum. However, the readily transformation from black α-phase FAPbI₃ to photoinactive yellow δ-phase makes it difficult to prepare a high-quality phase-pure FAPbI₃ active layer. Meanwhile, the in-plane tensile strain and out-of-plane compressive strain during conventional solution-based film processing technique will cause the lattice mismatches and composition distribution in the polycrystalline multigranular grains, which would further limit the lifetime of PSCs.<br/><br/>Here, we introduced the thiol (3-Mercaptopropionic acid, 3-MPA) functionalized molybdenum disulfide (MoS₂) nanoflakes (NFs) into the FAPbI₃ perovskite layer. The MPA-MoS₂NFs are incorporated through mixing with FAPbI₃ precursors when preparing the ink. The NFs are believed to severe as the crystallization agent and passivates the defects in the as-prepared films. We tracked the crystallographic evolution during thermal annealing with synchrotron-based in situ grazing-incidence wide angle (GIWAXS). The result shows that the MPA-MoS₂ forms an intermediate structure with FAI at the initial stage then facilitate the orientated growth of FAPbI₃. Moreover, based on the depth-dependent GIWAXS, the FAPbI₃ films with MPA-MoS₂ shows a reduced compressive strain across the film thickness direction. The strong interaction between MPA-MoS2 and the uncoordinated Pb²⁺ makes the NFs attached closely to the grain boundaries, which forms efficient heat spreading channels thus modifying the thermal gradient during film preparation. As observed in SEM and XRD, respectively, the MPA-MoS₂ incorporated FAPbI₃ shows increases in average grain size with a 3-fold enhancement in crystalline peak intensity, suggesting the promotion of the perovskite crystal quality after the incorporation of the NFs. In addition, as witnessed in photoluminescence (PL) and lifetime decay measurements, the FAPbI3 with MPA-MoS2 NFs functionalized by monolayer of the MPA thiols exhibits quenching in PL intensity and lifetime, indicating more efficient the charge transport likely occurs in the NFs incorporated FAPbI3. PSCs with MPA-MoS₂ with a champion PCE of 22.3% have been achieved successfully, and the corresponding unencapsulated devices retain over 90% of their initial efficiencies in the ambient environment after 1600 h.