Optimization of PbI2/MAPbI3 Perovskite Composites by Scanning Electrochemical Microscopy

A variety of PbI2/MAPbI3 perovskites were prepared and investigated by a rapid screening technique utilizing a modified scanning electrochemical microscope (SECM) in order to determine how excess PbI2 affects its photoelectrochemical (PEC) properties. An optimum ratio of 2.5% PbI2/MAPbI3 was found to enhance photocurrent over pristine MAPbI3 on a spot array electrode under irradiation. With bulk films of various PbI2/MAPbI3 composites prepared by a spin-coating technique of mixed precursors and a one-step annealing process, the 2.5% PbI2/MAPbI3 produced an increased photocurrent density compared to pristine MAPbI3 for 2 mM benzoquinone (BQ ) reduction at −0.4 V vs Fc/Fc. As a result of the relatively high quantum yield of MAPbI3, a time-resolved photoluminescence quenching experiment could be applied to determine electron−hole diffusion coefficients and diffusion lengths of PbI2/MAPbI3 composites, respectively. The diffusion coefficients combined with the exciton lifetime of the pristine 2.5% PbI2/MAPbI3 (τPL = 103.3 ns) give the electron and hole exciton diffusion lengths, ∼300 nm. Thus, the 2.5% PbI2/MAPbI3 led to an approximately 3.0-fold increase in the diffusion length compared to a previous report of ∼100 nm for the pristine MAPbI3 perovskite. We then demonstrated that the efficiency of liquid-junction solar cells for 2.5% excess PbI2 of p-MAPbI3 was improved from 6.0% to 7.3%. ■ INTRODUCTION Organic−inorganic hybrid lead-based perovskites exhibit remarkable properties, including high absorption coefficients, long exciton lifetimes and diffusion lengths, high charge-carrier mobilities, and low exciton binding energies, and have yielded power conversion efficiencies (PCE) of ∼20% in photovoltaic cells. These photovoltaic cells usually are titanium dioxide (TiO2) dye-sensitized solar cells (DSSC) 9,10 and solid-state solar cells. In a previous paper, we described liquid junction PEC solar cells involving MAPbI3 perovskites. 18 The MAPbI3 perovskites are extremely sensitive to moisture and unstable in polar solvents. However, dichloromethane (CH2Cl2) can be used for liquid junction PEC solar cells with reasonable stability because of its relatively higher dielectric constant and is also useful for fundamental studies. More importantly, the use of liquid electrolytes allows easy combinatorial synthesis and screening of new perovskite materials in arrays and testing the effects of various dopants on them. In this article, we describe such studies using robotic synthesis and rapid screening based on scanning electrochemical microscopy (SECM). MAPbI3 perovskites were synthesized with equimolar mixture of MAI and PbI2. Dittrich et al. showed excess PbI2 can be used to passivate perovskite grain boundaries and decrease carrier recombination lifetime for improving the performance of perovskite-based solar cells. Burda et al. then confirmed this passivation effect of excess PbI2 by femtosecond time-resolved transient absorption spectroscopy (fs-TA) of MAPbI3 perovskite films. The peak intensities of perovskite TA were used to estimate relative amounts of excess PbI2 in the samples, while powder X-ray diffraction (XRD) can independently confirm the existence of excess PbI2. Time-resolved transient absorption demonstrated that perovskite films with less excess PbI2 displayed faster relaxation rates. These fast dynamics are assigned to charge carrier trapping at perovskite grain boundaries, and the slower dynamics in samples containing PbI2 are attributed to a passivation effect. 22 However, these studies did not show the amount of PbI2 required for optimal passivation. Here, our contribution is to screen different quantitative combinations of PbI2 and MAPbI3 perovskites efficiently by utilizing SECM imaging. ■ EXPERIMENTAL SECTION Materials. Methylamine (CH3NH2, 2 M in methanol, Alfa Aesar), hydroiodic acid (HI, 57 wt % in water, Alfa Aesar), lead iodide (PbI2, 99.9985% metals basis, Alfa Aesar), N,Ndimethylformamide (DMF, ≥99.9%, Sigma-Aldrich), methylene chloride (CH2Cl2, anhydrous, ≥99.9%, Sigma-Aldrich), tetrahydrofuran (THF, anhydrous, ≥99.9%, Sigma-Aldrich), ethyl acetate (EA, anhydrous, ≥99.8%, Sigma-Aldrich), p-benzoquinone (BQ , ≥99.5%, Sigma-Aldrich), tetrabutylammonium Received: August 3, 2016 Revised: August 4, 2016 Published: August 9, 2016 Article