Tracking Iodide and Bromide Ion Segregation in Mixed Halide Lead Perovskites during Photoirradiation Seog Joon Yoon, †,‡ Sergiu Draguta, ‡ Joseph S. Manser, †,§ Onise Sharia, § William F. Schneider, ‡,§ Masaru Kuno, ‡ and Prashant V. Kamat* ,†,‡,§ † Radiation Laboratory, ‡ Department of Chemistry and Biochemistry, and § Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States * S Supporting Information ABSTRACT: Mixed halide lead perovskites (e.g., CH 3 NH 3 PbBr x I 3-x ) undergo phase segregation creating iodide-rich and bromide-rich domains when subjected to visible irradiation. This intriguing aspect of halide ion movement in mixed halide films is now being tracked through excited-state behavior using emission and transient absorption spectroscopy tools. These transient experiments have allowed us to establish the time scale with which such separation occurs under laser irradiation (405 nm, 25 mW/cm 2 to 1.7 W/cm 2 ) as well as dark recovery. While the phase separation occurs with a rate constant of 0.1-0.3 s -1 , the recovery occurs over a time period of several minutes to an hour. The relative photoluminescence quantum yield observed for Br-rich regions (em. max 530 nm) is nearly 2 orders of magnitude lower than that of I-rich regions (em. max 760 nm) and arises from the fact that I-rich regions serve as sinks for photogenerated charge carriers. Understanding such cascading charge transfer to localized sites could further enable the design of gradient halide structures in mixed halide systems. A s we continue to explore the organic lead halide perovskites for photovoltaic and photonics applica- tions, 1-7 mixed halide perovskites have emerged as attractive candidates for continuous tuning of the bandgap. 8-10 For example, the bandgap of methylammonium lead iodide/ bromide (CH 3 NH 3 PbBr x I 3-x ) can be tuned between 1.55 and 2.43 eV by varying the composition of Br and I (x =0-3). 11,12 The use of mixed halide perovskites in solar cell and lasing applications motivates further investigation of the underlying optical and electronic properties of such systems. 4,13,14 The halide ions of perovskite films are chemically “softer” materials as compared to other semiconductors employed in photovoltaics because large ionic displacements have been noted at room temperature. 15,16 For example, halide ions are easily exchangeable by subjecting the iodide form of the film to other halides. Ion exchange from solution-based as well as gas- phase exposure to halogens 12,17-19 can also yield different forms of lead halide perovskites. A common method employed in designing mixed halide lead perovskites is to spin-coat a N,N- dimethylformamide (DMF) solution of stoichiometric Pb 2+ and halide (I - +Br - ) on a suitable substrate followed by annealing. 12,20,21 However, as shown in our recent study, the lead halide complexation chemistry dictates the nature of binding between Pb 2+ and Br - and Pb 2+ and I - , especially when excess halide ions are present. 22 Walsh and co-workers 23 explored mixing energies in the CH 3 NH 3 PbBr x I 3-x system with density functional theory. They found that mixing energies tend to be small and positive and that alloying is entropy-driven; thus, they predict the system to exhibit a miscibility gap. Similar calculations on the cesium lead mixed halide system come to the same conclusion. 24 These results are consistent with a facile halide exchange and with a tendency for phase segregation to occur under appropriate stimulation. Anomalous alloy proper- ties of mixed halide perovskites have also been studied using first-principle calculations together with cluster-expansion methods. 24 Another interesting aspect is the migration of halide ions and associated defect in perovskite films with relative activation energies as low as ∼0.1 eV. 25-28 This effect, predominantly seen in mixed halide perovskite (e.g., CH 3 NH 3 PbBr x I 3-x ) films as they undergo phase segregation when subjected to visible light irradiation, 19,29 is consistent with a small mixing energy. Hoke and co-workers observed trap-induced emission in the lower-energy region as the input of photons resulted in iodide- rich minority and bromide-enriched majority domains. 19 These segregation effects are reversible as the original spectral features were recovered after stopping the illumination. Even without Received: May 19, 2016 Accepted: June 19, 2016 Published: June 20, 2016 Letter http://pubs.acs.org/journal/aelccp © 2016 American Chemical Society 290 DOI: 10.1021/acsenergylett.6b00158 ACS Energy Lett. 2016, 1, 290-296