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Ultrafast Imaging of Carrier Cooling in Metal Halide Perovskite Thin Films Sanghee Nah, Boris Spokoyny, Chan M.M. Soe, Costas Stoumpos, Mercouri G. Kanatzidis, and Elad Harel Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04520 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018
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Nano Letters
Ultrafast Imaging of Carrier Cooling in Metal Halide Perovskite Thin Films Sanghee Nah,† Boris M. Spokoyny,† Chan M.M. Soe,†,‡ Costas C. Stoumpos,†,‡ Mercouri G. Kanatzidis,†,‡ and Elad Harel†,* †
Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
‡
Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, IL 60208, USA *Corresponding author E-mail:
[email protected] Understanding carrier relaxation in lead halide perovskites at the nanoscale is critical for advancing their device physics. Here, we directly image carrier cooling in polycrystalline CH3NH3PbI3 films with nanometer spatial resolution. We observe that upon photon absorption, highly energetic carriers rapidly thermalize with the lattice at different rates across the film. The initial carrier temperatures vary by many multiples of the lattice temperature across hundreds of nanometers, a factor that cannot be accounted for by excess photon energy above the bandgap alone, nor in variations of the initial carrier density. Electron microscopy suggests that morphology plays a critical role in determining the initial carrier temperature and that carriers in small crystal domains decay slower than those in large crystal domains. Our results demonstrate that local disorder dominates the observed carrier behavior, highlighting the importance of making local rather than averaged measurements in these materials. Keywords: Perovskite, microscopy, ultrafast, carriers, cooling One Sentence Summary: Hot carrier cooling at the nanoscale in polycrystalline perovskite particles is directly visualized, revealing a significant impact of local morphology on the carrier relaxation rates. Hybrid lead halide perovskites have emerged as potential materials for optoelectronics, such as photovoltaics, low-threshold lasing devices and light emitting diodes.1-3 Carrier lifetime is one of the critical factors accounting for the energy-transfer efficiency in such perovskite-based optoelectronic devices. Ultrafast transient absorption (TA) spectroscopy has been employed previously to directly investigate photogenerated carrier lifetime, carrier diffusion lengths, and carrier mobilities.4-6 For example, recent TA studies have shown that the polycrystalline hybrid perovskites exhibit longer carrier lifetime than highly crystalline inorganic semiconductors such as GaAs.4, 7 The TA technique has been utilized to study the overall picture of carrier relaxation dynamics including carrier-carrier scattering, carrier-phonon scattering, and carrier-impurity scattering.6, 8-10 However, such ensemble TA measurements only extract average relaxation dynamics of hot carriers in methylammonium lead iodide (CH3NH3PbI3) as they do not take into account local morphology in perovskite films. However, the average properties may mask much of the underlying physics. For instance, recent work from our lab showed that both free-carrier and exciton population co-exist in close proximity but that the average spectra show no signs of 1
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bound exciton states. In this work, we explicitly interrogate spatially dependent hot carrier decay dynamics in single perovskite particles. We reveal both the average and distribution of carrier relaxation rates at a spatial resolution of 200 nm in polycrystalline CH3NH3PbI3 perovskite thin films by using spectrally resolved ultrafast broadband TA microscopy.11 We show that highly energetic carrier populations created by above bandgap excitation undergo rapid thermalization through carrier-phonon scattering with different cooling rates which depend on the pump power and perovskite crystal sizes.12, 13 As the pump power increases, a phonon bottleneck effect becomes more pronounced. Photogenerated carriers near the center area in the particle exhibit relatively slow decay rates, while carriers on the outer region of the particle show faster relaxation rates by as much as 300% greater than the average. While the exact mechanism of these large variances remains unknown, high-resolution scanning electron microscopy suggest that these differences may be due to crystal size and shape. What is clear, however, is that local morphology plays an important role in the efficiency of carrier-phonon scattering. Our “singleparticle” measurements reveal that while the average decay rate is relatively insensitive to the average carrier density under these conditions, the distribution of rates is not. The ultrafast imaging method described here directly measures both the mean and distribution of carrier properties, attributes key to defining and improving performance in devices utilizing heterogeneous materials. Hybrid CH3NH3PbI3 perovskite thin films were fabricated as previously reported (details available in Figure S1 of SI). While both continuous and low-coverage films were measured, here we focus here on the latter in order to evaluate the influence of grain boundaries and edges on carrier relaxation (see SI for discussion). The synthesis procedure used consistently formed large polycrystalline particles several microns in diameter, with predominately larger crystallites on the periphery and small crystallites in the center. Each perovskite film was positioned onto a piezoelectric nanostage, which was placed on a home-built Peltier cooling stage holder, keeping the films at 6 °C to reduce photoinduced degradation. For the TA measurement, the output of a Yb:KGW amplifier system (200 KHz, 190 fs, 1030 nm) was split into two beams. One portion of the beam was focused on a beta barium borate (BBO) crystal generating a pump beam centered at 2.40 eV for above bandgap excitation (bandgap 1.63 eV). The other portion of the beam was focused on a yttrium aluminum garnet (YAG) crystal generating a white light continuum probe beam. Collinearly propagating pump and probe beams through a dichroic mirror were focused on a perovskite particle using a 74× reflective aluminum objective (NA 0.65) to reduce chromatic aberration. The spot size of the pump and probe were measured as about 0.45 and 0.5 microns, respectively. Transmitted signals were collected through a 100× objective (NA 0.7). Spatially filtered signals through a pinhole were then sent to a spectrometer, and spectrally dispersed signals were detected by a high-speed electron multiplying charge coupled camera (EMCCD). The pump beam was modulated at 2.5 KHz using a Pockels cell, and the EMCCD camera was triggered at 5 KHz. The probe power was set to 1.78 , and the pump power varied from 0.26 to 1.08 . Using pump power greater than 1.08 resulted in accumulated photo-damage of the perovskite particles during raster scanning. To keep the beam exposure time as short as possible at each spot in the particle, scans were only collected for delay times up to 1.92 ps. The stage was moved in steps of 200 nm, which was below the diffractionlimited spot size of the pump. The quoted spatial resolution of the measurement corresponds to the smallest step size in which a change in the pump-induced differential probe transmission may be reliably measured. In analogy with super-resolution methods, both the pump-probe delay and probe spectral changes offer opportunities to distinguish certain properties of two nearby points
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in space by means of their nonlinear optical response. Therefore, the spatial resolution may below the spot size of the pump and probe beams, in contrast to linear measurements such as transmission or reflection. For our particular measurement and given the high signal-to-noise ratio of these experiments, we estimate a lateral spatial resolution of about 200 nm with respect to carrier temperature maps, while the axial resolution is limited by the sample thickness (0.95 were kept. Bottom: Histograms of decay times assembled by performing statistics on all points within the field-of-view. Fitting parameters and R2 maps are available in SM.
In summary, we investigated hot carrier relaxation rate distributions in individual hybrid perovskite particles by analyzing spatially and spectrally resolved ultrafast broadband TA spectral dynamics. Our results reveal that the average carrier cooling rates increase with excitation pump power, and high power can prolong carrier cooling rates due to the phonon 6
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bottleneck effect. Critically, we find that the local structure affects the carrier cooling rates dramatically. While further study is needed to connect the underlying morphology and microstructure to carrier properties, scanning probe measurements suggest that large crystals contribute to fast decay rates, while smaller domains yield slow decay rates. In addition to morphology, it is possible that surface passivation with organic cations at grain boundaries and defect sites may play a role in the spatial distribution of the hot carrier cooling dynamics. Therefore, spatially correlated analyses in conjunction with local probes on individual perovskite particles are needed to fully understand the ultrafast photophysics in perovskite materials and to facilitate the development of next-generation optoelectronic devices. The addition of spatiallydependent ultrafast spectroscopy is an important tool to obtain both the average and distribution of carrier properties which determine the overall function and efficiency of devices made using highly heterogeneous materials. Acknowledgments: E.H. acknowledges support by the Air Force Office of Scientific Research (FA9550-14-1-0005), and the Packard Foundation (2013-39272) in part. M.K. acknowledges support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (DE-SC0012541, synthesis and physical characterization of samples). The electron microscopy work made use of the EPIC facility of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. The authors declare no competing financial interest. Supplementary Materials: Figure S1. Characterization of perovskite films Figure S2. Probe absorbance and integrated photoluminescence for the same particle analyzed in paper Figure S3. Spectrally integrated pump transmission Figure S4. Spatial distribution of R-squared values for fit of TA spectra to band-filling model Figure S5. Spatial distribution of R-squared values for fit in carrier decay rate maps to singleexponential model Figure S6. Ultrafast carrier cooling maps for a second perovskite particle Figure S7. Spatial distribution of R-squared values for fit of TA spectra of particle two to bandfilling model Figure S8. Carrier cooling decay maps and histograms for particle two Figure S9. Spectrally integrated TAM image at T = 120 fs. Figure S10. Integrated photoluminisence as a function of laser power. Movie S1-S3: Movies of carrier cooling (AVI) at three pump intensities
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