Optimization of PbI₂/MAPbI₃ Perovskite Composites by Scanning Electrochemical Microscopy

A variety of PbI 2 /MAPbI 3 perovskites were prepared and investigated by a rapid screening technique utilizing a modified scanning electrochemical microscope (SECM) in order to determine how excess PbI 2 affects its photoelectrochemical (PEC) properties. An optimum ratio of 2.5% PbI 2 /MAPbI 3 was found to enhance photocurrent over pristine MAPbI 3 on a spot array electrode under irradiation. With bulk films of various PbI 2 /MAPbI 3 composites prepared by a spin-coating technique of mixed precursors and a one-step annealing process, the 2.5% PbI 2 /MAPbI 3 produced an increased photocurrent density compared to pristine MAPbI 3 for 2 mM benzoquinone (BQ) reduction at −0.4 V vs Fc/Fc + . As a result of the relatively high quantum yield of MAPbI 3, a time-resolved photoluminescence quenching experiment could be applied to determine electron–hole diffusion coefficients and diffusion lengths of PbI 2 /MAPbI 3 composites, respectively. The diffusion coefficients combined with the exciton lifetime of the pristine 2.5% PbI 2 /MAPbI 3 (τ PL = 103.3 ns) give the electron and hole exciton diffusion lengths, ∼300 nm. Thus, the 2.5% PbI 2 /MAPbI 3 led to an approximately 3.0-fold increase in the diffusion length compared to a previous report of ∼100 nm for the pristine MAPbI 3 perovskite. We then demonstrated that the efficiency of liquid-junction solar cells for 2.5% excess PbI 2 of p-MAPbI 3 was improved from 6.0% to 7.3%.