Enhanced perovskite electronic properties via a modified lead(II) chloride Lewis acid-base adduct and their effect in high-efficiency perovskite solar cells

Methylammonium lead triiodide (MAPbI(3)) perovskite solar cells have gained significant attention with an impressive certified power conversion efficiency of 22.1%. Suppression of recombination at the interface and grain boundaries is critical to achieve high performance perovskite solar cells (PSCs). Here, we report a simple method to improve the performance of PSCs by incorporating a lead chloride (PbCl2) material into the MAPbI(3) perovskite precursor through a Lewis acid-base adduct. The optimal concentration of PbCl2 that helps increase the grain size of MAPbI(3) with introduction of the ideal amount secondary phases (lead iodide and methylammonium lead tri-chloride) is 2.5% (molar ratio, relative to lead iodide). Examination by steady-state photoluminescence and time-resolved photoluminescence has shown that devices based on MAPbI(3)-2.5% of PbCl2 facilitated longer charge carrier lifetime and electron-hole collection efficiency which is ascribed to reduced defects and concurrent improved material crystallinity. Electrochemical impedance spectra (EIS) of the corresponding PSCs have revealed that, compared to the pristine MAPbI(3) perovskite film, the 2.5% PbCl2-additive increased the recombination resistance of the PSCs by 2.4-fold. Meanwhile, measurement of the surface potential of the perovskite films has indicated that the PbCl2-additive modifies the electronic properties of the film, shifting the fermi-level of the MAPbI(3) film by 90 meV, leading to a more favourable energetic band matching for charge transfer. As a result, the performance of PSCs is enhanced from an average efficiency of 16.5% to an average efficiency of 18.1% with maximum efficiency reaching 19% due to the significantly improved fill-factor (from 0.69 to 0.76), while the hysteresis effect is also suppressed with the PbCl2-additive.