Letter pubs.acs.org/NanoLett
Cite This: Nano Lett. 2018, 18, 827−831
Transient Sub-bandgap States in Halide Perovskite Thin Films S. Nah,† B. Spokoyny,† X. Jiang, C. Stoumpos,† C. M. M. Soe,†,‡ M. G. Kanatzidis,†,‡ and E. Harel*,† †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Argonne-Northwestern Solar Energy Center, Northwestern University, Evanston, Illinois 60208, United States
‡
S Supporting Information *
ABSTRACT: Metal halide perovskites are promising solar energy materials, but their mechanism of action remains poorly understood. It has been conjectured that energetically stabilized states such as those corresponding to polarons, quasiparticles in which the carriers are dressed with phonons, are responsible for their remarkable photophysical properties. Yet, no direct evidence of polarons or other low-energy states have been reported despite extensive efforts. Such states should manifest as below bandgap features in transient absorption and photoluminescence measurements. Here, we use single-particle transient absorption microscopy on MAPbI3 (MA = methylammonium) to unambiguously identify spectrally narrow sub-bandgap states directly; we demonstrate that such signals are completely averaged away in ensemble measurements. Carrier temperature-dependent studies suggest that hot carriers are directed toward transient low-energy states which are immune from permanent defects and traps, thereby giving rise to low carrier recombination rates and ultimately high power conversion efficiency in devices. The utilization of short-lived sub-bandgap states may be a key design principle that propels widespread use of highly heterogeneous materials in optoelectronic applications. KEYWORDS: Metal halide perovskite, transient absorption microscopy, ultrafast spectroscopy, polaron states, spatially resolved measurements
H
Briefly, we use above-bandgap excitation tuned to 2.4 eV and a broadband continuum probe that spans 1.5−2.0 eV. The bandgap of MAPbI3 lies around 1.63 eV. Twenty-four individual perovskite locations were selected at random and spectrally resolved TA spectra were measured at delay times ranging from −0.1 to 2 ps. The photoluminescence (PL) spectrum was simultaneously recorded and subtracted from each TA measurement. SEM images (see SI) reveal that the crystalline domains in our thin film sample show two prominent crystal morphologies: small crystallites in the 100−300 nm range and larger, elongated crystallites in the 700−1000 nm range. As the spatial resolution of our optical microscope is about 500 nm, most regions selected are from single crystalline domains, whereas a few have contributions from a small (
