Photoluminescence Lifetimes Exceeding 8 μs and Quantum Yields Exceeding 30% in Hybrid Perovskite Thin Films by Ligand Passivation

We study the effects of a series of postdeposition ligand treatments on the photoluminescence (PL) of polycrystalline methylammonium lead triiodide perovskite thin films. We show that a variety of Lewis bases can improve the bulk PL quantum efficiency (PLQE) and extend the average PL lifetime, ⟨τ⟩, with large enhancements concentrated at grain boundaries. Notably, we demonstrate thin-film PLQE as high as 35 ± 1% and ⟨τ⟩ as long as 8.82 ± 0.03 μs at solar equivalent carrier densities using tri-n-octylphosphine oxide-treated films. Using glow discharge optical emission spectroscopy and nuclear magnetic resonance spectroscopy, we show that the ligands are incorporated primarily at the film surface and are acting as electron donors. These results indicate it is possible to obtain thinfilm PL lifetime and PLQE values that are comparable to those from single crystals by control over surface chemistry. Organic−inorganic trihalide perovskites are a family of earth-abundant semiconductors with compositiontunable bandgaps that exhibit promising performance in a range of electronic and optoelectronic devices including solar cells, light-emitting diodes, lasers, and ambipolar transistors. Recently, high-quality perovskite single crystals have been grown using a variety of techniques and demonstrate exceptional optoelectronic properties that surpass their polycrystalline counterparts. In particular, it has been shown that single crystals possess lower defect concentrations, higher mobilities, longer carrier diffusion lengths, and longer carrier lifetimes, suggesting that the surface defects that are more prevalent in polycrystalline films limit performance. Despite the quality of single crystals, their growth can be timeconsuming and their structural properties make them more difficult to incorporate into many devices, particularly those requiring large areas such as solar cells. Therefore, there is still significant interest in processing perovskites rapidly from solution into polycrystalline thin films for device applications. Many studies have highlighted the “defect tolerance” of perovskites, underlining their apparent resistance to defects that would normally plague semiconductors processed at low temperature. While the performance of hybrid perovskite devices may be surprisingly good for a low-temperature, solution-processed thin-film technology, polycrystalline perovskite films still operate well below their theoretical performance limits, indicating that losses remain. Indeed, we recently used photoluminescence (PL) microscopy to reveal a subset of grain boundaries and “dark” regions in the films that serve as nonradiative centers, highlighting the role of heterogeneity and surface effects. Consistent with these studies, it has been shown that photoluminescence quantum efficiency (PLQE) and lifetime are limited by traps at carrier densities obtained under typical solar illumination, and only at high carrier densities (when the majority of traps are filled) are the highest PLQEs observed. These observations help explain why the carrier lifetimes and PLQEs of thin films of perovskites remain below those reported for single crystals and suggest that surface defect chemistry remains an important topic of study. Historically, controlling the surface chemistry in semiconductors such as Si and GaAs has been important in improving material properties for the development of solar cell technologies. In II−VI and IV−VI quantum dot systems, a substantial amount of work has focused on ligand passivation to modify surface states to improve PL emission properties as well as solar cell efficiencies. Received: June 28, 2016 Accepted: July 26, 2016 Published: July 26, 2016 Leter http://pubs.acs.org/journal/aelccp © 2016 American Chemical Society 438 DOI: 10.1021/acsenergylett.6b00236 ACS Energy Lett. 2016, 1, 438−444 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-0-417743 Erschienen in: ACS Energy Letters ; 1 (2016), 2. S. 438-444 https://dx.doi.org/10.1021/acsenergylett.6b00236