LONG AND PREZHDO VOL. 9 ’ NO. 11 ’ 11143–11155 ’ 2015 www.acsnano.org 11143 October 11, 2015 C 2015 American Chemical Society Dopants Control ElectronHole Recombination at PerovskiteTiO 2 Interfaces: Ab Initio Time-Domain Study Run Long * ,†,‡ and Oleg V. Prezhdo * ,§ † College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education, Beijing Normal University, Beijing, 100875, People's Republic of China, ‡ School of Physics, Complex & Adaptive Systems Lab, University College Dublin, Dublin, Ireland, and § Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States F ollowing the first report of a perovskite solar cell with the solar energy conversion efficiency of 3.8%, 1 organicinorganic halide perovskites, such as CH 3 NH 3 PbI 3 (MAPbI 3 ), have attracted intense attention. Perovskites have unique geometric and electronic properties, are good light absor- bers, and are cost-effective. 214 Currently, the highest reported conversion efficiency of perovskite-sensitized TiO 2 solar cells is 19.3%. 9 The MAPbI 3 band gap allows absorp- tion over a wide range of the solar spectrum, from visible to near-infrared. MAPbI 3 exhibits extremely long diffusion lengths for both electrons and holes, 4 around 100 nm, which is larger than the typical charge di ffusion lengths in other materials, on the order of 10 nm. The electron and hole diffusion lengths can increase with doping. For in- stance, mixed MAPbI 3x Cl x perovskites trans- port charge over distances exceeding 1 μm, an order of magnitude greater than in the pristine material. 15 Cl doping is possible only at relatively low concentrations. Some experiments demonstrate that Cl doping concentration can reach only 0.1% to 1%, with Cl atoms localized preferentially on the surface. 16,17 Such doping levels have little effect on the band gap. 18 Other papers report higher Cl doping concentrations. 19 In contrast, the Br and Sn dopants are com- mensurate with the MAPbI 3 lattice. 2022 MAPbI 3x Cl x can act as both light harvester and electron conductor. Meso-superstructured * Address correspondence to runlong.anh@gmail.com, prezhdo@usc.edu. Received for review July 26, 2015 and accepted October 11, 2015. Published online 10.1021/acsnano.5b05843 ABSTRACT TiO 2 sensitized with organohalide perovskites gives rise to solar-to- electricity conversion efficiencies reaching close to 20%. Nonradiative electronhole recombination across the perovskite/TiO 2 interface constitutes a major pathway of energy losses, limiting quantum yield of the photoinduced charge. In order to establish the fundamental mechanisms of the energy losses and to propose practical means for controlling the interfacial electronhole recombination, we applied ab initio non- adiabatic (NA) molecular dynamics to pristine and doped CH 3 NH 3 PbI 3 (100)/TiO 2 anatase(001) interfaces. We show that doping by substitution of iodide with chlorine or bromine reduces charge recombination, while replacing lead with tin enhances the recombination. Generally, lighter and faster atoms increase the NA coupling. Since the dopants are lighter than the atoms they replace, one expects a priori that all three dopants should accelerate the recombination. We rationalize the unexpected behavior of chlorine and bromine by three effects. First, the PbCl and PbBr bonds are shorter than the PbI bond. As a result, Cl and Br atoms are farther away from the TiO 2 surface, decreasing the donoracceptor coupling. In contrast, some iodines form chemical bonds with Ti atoms, increasing the coupling. Second, chlorine and bromine reduce the NA electronvibrational coupling, because they contribute little to the electron and hole wave functions. Tin increases the coupling, since it is lighter than lead and contributes to the hole wave function. Third, higher frequency modes introduced by chlorine and bromine shorten quantum coherence, thereby decreasing the transition rate. The recombination occurs due to coupling of the electronic subsystem to low-frequency perovskite and TiO 2 modes. The simulation shows excellent agreement with the available experimental data and advances our understanding of electronic and vibrational dynamics in perovskite solar cells. The study provides design principles for optimizing solar cell performance and increasing photon-to-electron conversion efficiency through creative choice of dopants. KEYWORDS: organohalide perovskites . TiO 2 . dopants . electronhole recombination . nonadiabatic molecular dynamics . time-domain density functional theory ARTICLE Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 8, 2019 at 00:09:45 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.