http://pubs.acs.org/journal/aelccp
Transient Sub-Band-Gap States at Grain Boundaries of CH3NH3PbI3 Perovskite Act as Fast Temperature Relaxation Centers Xinyi Jiang,† Justin Hoffman,† Costas C. Stoumpos,†,‡ Mercouri G. Kanatzidis,†,‡ and Elad Harel*,† †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Argonne−Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois 60208, United States
Downloaded via NOTTINGHAM TRENT UNIV on July 30, 2019 at 15:20:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Extensive spectroscopic studies have been performed on grain boundaries (GBs) of thin-film metal halide perovskites, which inevitably form with current fabrication methods, but direct, on-site determination of the transition energy and dynamics of the associated defect states and their impact on local carrier behaviors have remained elusive. Here, scanning electron microscopy (SEM) correlated to transient absorption microscopy (TAM) on CH3NH3PbI3 perovskite particles is used to identify a defect state ∼60 meV into the band gap at GBs, which accelerates carrier cooling and act as additional energy acceptors. An in-depth statistical analysis performed on a large data set (806 distinct spatial locations) reveals that the shallow defect state, generally considered to be benign, plays a significant role in accelerating carrier cooling, which is detrimental to hot carrier solar cells.
M
supported by big-data statistics. In this Letter, we present the first realization of SEM correlated transient absorption microscopy (TAM) on polycrystalline perovskites, which directly maps the formation of a sub-band-gap state at GBs. The dynamics of these states are investigated together with their influence on the local carrier behaviors, suggesting the electrically charged nature and the potential role of GBs as centers for fast carrier cooling. Furthermore, the microscope setup and the specially fabricated films gave us the capability to study hundreds of data points in a single map with multiple film morphologies included. A thorough statistical analysis with both parametric and nonparametric tests performed on such a big data set enables us to draw a reliable connection among the formation of the sub-band-gap state, the doping effect at GBs, and the accelerated carrier cooling process. Our findings indicate that, contrary to the conventional wisdom that the shallow defect state in perovskite GBs does not significantly change carrier dynamics (“benign”),23,30,32,33 the cooling dynamics are strongly affected by additional energy acceptors formed by these states, which speeds up the process and leaves limited time for hot carrier extraction. The outcome of this effect, which is an accumulation of carriers at the band edge,
etal halide perovskites have received intense attention as active layers in solar cells for their high power conversion efficiency (over 22%1) and the relatively low technical barrier in fabrication.2 Contributing to their idiosyncratic carrier behavior, like low recombination rates, 3−9 low cooling rates, 6,8−10 and long diffusion lengths,11−16 are the unique band structures that may arise from different film morphologies. The impact of the local nanostructure, such as at grain boundaries (GBs), on carrier properties has been widely studied using various microscopic methods,17 mainly photoluminescence (PL) microscopy18−22 and atomic force microscopy (AFM),23 along with ensemble pump−probe spectroscopy by comparing samples with different bulk morphologies.24−26 However, the nature of the GBs and the associated defect states remain unclear. Many previous studies either performed ensemble measurements averaging over tens of microns in films, thus convolving signals from many individual grains, or studied properties like the local PL lifetime at the band gap,18−22 carrier diffusion,27 and other properties23,28 to indirectly probe the dynamics at the defect state, including theoretical works.29−31 Until now, there has not been a direct probe of the transition energy of the defect state and population dynamics in a locally precise way so that only regions of films (e.g., interior, near-edge, boundary, etc.) or single grains are probed while simultaneously measuring hot carrier dynamics. Further, comparison of carrier properties between different morphologies has not, until now, been © 2019 American Chemical Society
Received: April 24, 2019 Accepted: June 17, 2019 Published: June 17, 2019 1741
DOI: 10.1021/acsenergylett.9b00885 ACS Energy Lett. 2019, 4, 1741−1747
Letter
Cite This: ACS Energy Lett. 2019, 4, 1741−1747
Letter
ACS Energy Letters
Figure 1. Spectrally resolved TA spectra at the GI and GB of CH3NH3PbI3. (a) SEM image of the perovskite particle. The yellow solid curve traces the edge of the particle, and the black dashed curves trace the GBs between grains. The ones in the inner circle are not marked as they are too dense. Scale bar: 1 μm. (b) TA surface and (c) TA spectra with fits (black solid curves) at different pump−probe delays at one position picked in GIs. (d) TA surface and (e) TA spectra with fits (black solid curves) at different pump−probe delays at one position picked at GBs. (f) Sub-band-gap feature extracted from fit, with spectrally integrated (1.570 ± 0.015 eV) sub-band-gap intensity (red curve) and single-exponential fit (black solid curve). The population lifetime is labeled. (g) Cuts of TA spectra at GBs at 1.0 (green) and 1.8 ps (blue), with fits (black solid curve) and residues (dashed curve, scaled). An arcsin-scaled color map is applied on (b), (d), and (f).
air can be found in the Supporting Information (ST4, Figure S14). The TA spectra across the whole particle within the solid curve (Figure 1a) are measured with a single-wavelength pump at 2.41 eV and a broad-band white-light probe from 1.50 to 1.87 eV, which covers spectral ranges both below and above the band gap at 1.63 eV.35 The TA spectra of two representative points picked from the grain interior (GI) and GB areas are displayed in Figure 1b−e. The conspicuous difference between the two spectra is the subpeak below the band gap at the GB (Figure 1d), which shows up as a growing feature in the time-resolved spectra (Figure 1e). To extract this sub-band-gap peak, a band-filling model was constructed (Supporting Information, MM4), which took into account the three prominent features and the related photophysics observed in previous ensemble measurements3−5,9 and the GI TA spectra here: (1) the growing positive ground-state bleaching (GSB) representing the increasing amount of band edge carriers; (2) the high-energy tail of the GSB representing the population and cooling of hot carriers; and (3) the negative photoinduced absorption (PIA) below the band gap representing the renormalized band gap due to the presence of photoexcited carriers. All three features are considered in the band-filling model and later subtracted from the spectra to extract the sub-band-gap state. The sub-band-gap peak centered at ∼1.57 eV and the dynamics are then extracted (Figure 1f,g). Notably, the high-energy tail is much broader in the GI (Figure 1c) compared to that in the GB (Figure 1e) at 2 ps, indicating a higher quasi-temperature of the carriers in the GI. Different fit algorithms were used and all gave similar results. The center part of the particle made of small crystallites (
