Photoexcited Dynamics in Metal Halide Perovskites: From Relaxation

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Photoexcited Dynamics in Metal Halide Perovskites: From Relaxation Mechanisms to Applications Zhongguo Li, Yubin Chen, and Clemens Burda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11347 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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The Journal of Physical Chemistry

Photoexcited Dynamics in Metal Halide Perovskites: From Relaxation Mechanisms to Applications a,c b a,b Zhongguo Li, Yubin Chen, and Clemens Burda * a

Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA

b

International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China c

College of Physics and Electronic Engineering, Changshu Institute of Technology, Changshu 215500, China

AUTHOR INFORMATION Corresponding Author: *E-Mail: [email protected],

ABSTRACT The past decade has witnessed a growing interest in metal halide perovskite (MHPs) materials, driven by their promising applications in photovoltaics and optoelectronics. The further pursuit of improved performance and stability will rely on a clear understanding of the fundamental properties of these materials. In this Feature Article, we outline a representative set of studies detailing how ultrafast laser spectroscopy can be used to access the optoelectronic properties and photophysical mechanisms of halide perovskites. Beginning with a concise synopsis of metal halide perovskite history and structural properties, the dynamic processes in MHPs related to the photovoltaic performance are discussed. Then, a brief overview of the representative time-resolved optical spectroscopic techniques in MHP research is provided. Afterward, the recent advances in

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exploring the carrier, lattice and spatially resolved dynamic processes in halide perovskite are summarized. The last section is devoted to a range of applications using halide perovskites. Finally, a conclusion and outlook for the field with some predictions for future opportunities will round this article off.


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1. INTRODUCTION Within the last decade, metal halide perovskites (MHPs) have attracted a significant amount of research effort due to their fascinating properties including long carrier lifetimes, strong optical absorption, low-cost fabrication and band-gap tunability. These remarkable features have fueled the development of photovoltaic and optoelectronic devices, such as solar cell, LED, laser, photodetectors, all of which tend to yield high performance and are at the same time easy-tomanufacture and flexible.1-5 In just six years, the power conversion efficiency (PCE) of perovskite solar cells has soared dramatically to 23%,6 and the external quantum efficiency (EQE) of perovskite light emission devices (LED) has reached 20%.7 These results indicate their potential to impact our daily life. Nevertheless, stability and toxicity are two critical concerns that must be addressed before future commercialization.8 In recognition of both the challenges and opportunities, a clear understanding of the structure-property relationship for metal halide perovskite represents an exciting and challenging new area of research for materials, physics and chemistry. Consequently, a wide range of complementary experimental techniques is required to characterize the fundamental properties of MHPs. Optical spectroscopy has played a crucial role in investigating the underlying dynamic processes in various chemical and optoelectronic systems such as solar fuels and artificial photosynthesis, respectively.9,10 It is interesting to ask why time-domain optical spectroscopy has become so central in the study of MHPs? The answer is that the conversion of light to electricity or vice versa in materials relies on a series of ultrafast processes of excited charge carriers, for which the competitive kinetics determine the functionality and efficiency of the resulting devices. Timedomain optical spectroscopy enables to directly probe these photogenerated carrier dynamics at timescales of electronic and lattice motion. A better understanding of charge relaxation, trapping,



diffusion, interfacial transfer and lattice distortion dynamics can help to optimize and improve the performance of relevant devices. The scope of optical spectroscopy is very broad and already has been subject to extensive reviews.9-12 Therefore, this article mainly focuses on the field of timeresolved optical spectroscopy with laser pulses in the UV-visible to near-IR regime. The sequence of time-resolved optical (pump-probe) spectroscopy is simple: first, one excites the sample material with a short (usually hundreds of fs) pump pulse, and then, the subsequent photo-physical processes can be monitored with a time-delayed probe pulse. In most cases, the pump pulses are generated from a femtosecond laser source such as Ti:Sapphire laser or optical parametric amplifiers. The probe pulses can span the electromagnetic spectrum, from terahertz to X-ray, and exploit various light-matter interactions such as absorption, luminescence, scattering, reflection and diffraction. By changing the experimental parameters including pump wavelength, pulse duration, probe spectral bandwidth and pump-probe time delay, a quantitative characterization of the photoinduced kinetics can be evaluated. The details of which are expanded upon in the chapters below. At present, the time-resolved optical absorption and luminescence spectroscopy have become robust and mature standard techniques to investigate various dynamic processes in MHPs. However, new experimental approaches (such as time-resolved X-ray absorption spectroscopy,13 femtosecond electron scattering,14 time-resolved photoemission spectroscopy,15 etc.) have emerged in recent years. These novel techniques will continue to play a central role in MHP research. This perspective is arranged as follows: Section 2 presents a brief overview of the spectroscopically relevant properties of MHPs and a description of dynamic processes in photoexcited MHPs related to solar cell and optoelectronic applications. Section 3 highlights interesting optical spectroscopy techniques in MHP research, providing background information


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on their working principles and features. Section 4 summarizes several representative studies, divided into three general categories: carrier dynamics, lattice dynamics and spatially resolved dynamics in MHPs. Section 5 presents examples of applications of MHP materials and devices. Section 6 concludes with a brief synopsis and outlook for the future of MHP research. Lastly, we would like to point that, while we did our best to highlight current spectroscopic developments in the vast field of metal halide perovskites, this feature cannot be a comprehensive review and we apologize to anybody whose work is not mentioned here.

2. OVERVIEW OF HALIDE PEROVSKITE 2.1 Research history of MHPs Perovskites, named after the Russian mineralogist Lev A. Perovski, form a class of compounds which have the crystal structure of ABX 3 . The first perovskite mineral, CaTiO 3 , was discovered by the German mineralogist Gustav Rose in 1839.2,3 Traditionally, perovskite oxides (ABO 3 ) hosting transition metals, have dominated the research field of perovskite due to their intriguing properties derived from the interplay of charge, spin, orbital and lattice correlations.16 In contrast, perovskites with halide ions (ABX 3 , X=Cl, Br or I) have gained less attention for a long time. In 1978, the first organic-inorganic halide perovskite (MAPbX 3 ) was reported by Weber.17 In 1994, David Mitzi found that layered organic-inorganic halide perovskites have semiconductor-to-metal transitions, which could be used in thin film transistors (TFT) and LED applications.18 Metal halide perovskites have only recently, within the last decade, started to gain traction within the materials research community. Particularly, the potential applications in solar cells stoke interest in these materials. In 2009, Miyasaka et al. reported the first lead halide perovskite solar cell using MAPbI3 as sensitizer in a conventional dye-sensitized solar cell (DSSC) configuration. The PCE of their cell was 3.8%.19 Two years later, Park and co-workers further improved the energy conversion ACS Paragon5Plus Environment


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efficiency to 6.5% by carefully optimizing the perovskite coating procedure.20 In 2012, three breakthrough publications on MHP solar cells with PCEs larger than 10% triggered the research eruption of this material.21-23 This trend can be visualized by the number of research papers published regarding "halide perovskite" on the Web of Science during the last decade.

2.2 Crystal and Electronic Structures of MHPs To better understand its fascinating properties, one must first consider the microscopic structure of MHPs. The unit cell of metal halide perovskites with formula ABX 3 is shown in Fig. 1a, in which A and B are the cations, and X is the anion that coordinates to B. Currently, the most-studied MHPs usually have an organic molecule (e.g., methylammonium (MA) or formamidinium (FA)) or an inorganic atom (e.g., caesium) as A-site cation. The B-site cation and X-site anion are metal (Pb or Sn) and halide (Cl, Br, I, or some mixture thereof), respectively. The B-site cation and Xsite anion form a BX6 octahedron. These octahedra are corner-shared to form a 3D framework, in which the A-site cations are located inside the cavities. The ratio of ionic sizes that perovskite architectures can tolerate is indicated by the Goldschmidt tolerance factor (t). The Goldschmidt tolerance factor can be calculated using the following expression:3

t=

rA + rX 2(rB + rX )

(1)

where r A , r B and r X are the ionic radii of the A cation, B cation, and the X anion, respectively. For most existing MHPs, the Goldschmidt tolerance factor t is found in the range of 0.8 to 1.0.1-3 Fig. 1b demonstrates the density of states (DOS) of the representative halide perovskite MAPbI3 calculated via density functional theory.24 Such calculations show that the valence band maximum (VBM) of MAPbI 3 is determined by hybridization of Pb-s and I-p orbitals, whereas the conduction


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band minimum (CBM) has mostly Pb-p orbital character.24 The electronic bandgap of MAPbI3 is governed by the B-X bond of the inorganic octahedra. The high-Z nature of the lead and iodine ions leads to large spin-orbit coupling, which lowers the band gap and splits the CBM.25 In general, the A-site cation does not have significant contribution to the band edge state. However, the A-site cation plays an essential part in determining the crystal structure of MHPs, as it can distort the inorganic octahedra through hydrogen bonding. The disorder of A-site cations could couple to the crystal structure of MHPs.26 It is also known that the crystal structure of MHPs is temperaturedependent. For example, the room-temperature phase of MAPbI 3 is tetragonal, but it will change to orthorhombic when the temperature decreases below 160 K.3

Figure 1. (a) Unit cell of the MAPbI 3 in the cubic phase: the dark spheres inside the octahedra represent Pb atoms. The red spheres at the octahedral corners are I atoms and the molecule inside the cuboctahedral cavity is CH 3 NH 3 . (b) Electronic band structure and density of states of MAPbI 3 . Reproduced from ref 24. Copyright 2014 Springer Nature.

2.3 Photo-induced Dynamic Processes in MHPs The performance of photovoltaic and optoelectronic devices is ultimately determined by the relaxation dynamics of the photogenerated charge carriers in the MHP materials. Furthermore, the ACS Paragon7Plus Environment


actual device, based on MHPs, consists of several functional layers such as perovskite light harvesting layer, the electron and hole transport layers (ETL and HTL), and the collection electrodes. Therefore, to achieve higher performance, it is also necessary to have a clear understanding on the interfacial charge transfer processes. The charge-carrier dynamics in MHPs have been the subject of numerous reviews.10-12,27,28 Below, we give a brief summary of the current understanding on charge-carrier relaxation in MHP materials and devices. In MHP materials, photo-induced carriers are generated instantaneously and the following recombination and diffusion processes mainly occur on the microsecond time scale. However, unlike in other conventional semiconductors such as silicon or GaAs, in MHPs the ion mobility (on the time scale of seconds or even longer) also has a significant impact on the performance and stability of MHP devices.28 The diagram in Figure 2a summarizes the timescales of various processes in MHPs after photo-excitation. Immediately after photoexcitation, the charge-carrier distribution inside MHPs is nonthermal. The nonequilibrium charge carriers will then rapidly thermalize within one picosecond via carrier-carrier scattering.10,27 These thermalized carriers have a carrier temperature significantly higher than the lattice temperature, thus are termed hot carriers. After carrier thermalization, the subsequent hot carrier cooling is established through carrierphonon scattering. In traditional semiconductors such as Si, this carrier-phonon scattering process is accomplished within a few picoseconds. However, pervious papers reported that the hot carrier relaxation time in MHP is longer than tens of picosecond.29 After carrier-phonon scattering, the carriers relax to the edge of CBM and VBM. Then the carrier recombination can occur radiatively or non-radiatively on the nanosecond to microsecond time scale. Alternatively, the carriers could also be trapped by defects or grain boundaries on timescales ranging from picoseconds to nanoseconds. Transport of carriers and ions in MHPs occurs on a timescale of ns to μs and ms to


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second, respectively.11,27,28 There are also processes related to ion transport that happen on longer timescales, but they are not under the purview of this review.

Figure 2. (a) Photoinduced processes that occur on various times scales in MHPs. (b) Schematic diagram of various processes in photoexcited MHP devices.

The charge transport processes in MHP devices are schematically depicted in Figures 2b. After photogeneration, the carriers can recombine or travel to the interface of the perovskite with the ETL or HTL. At the interface, the carriers can undergo charge injection into neighboring layers. The time required for this injection process depends on the interfacial properties. During these processes, a portion of the carriers could be lost through interfacial recombination.10,28 Finally, the remaining carriers make their way through the ETL and HTL to be collected by the electrodes. ACS Paragon9Plus Environment


3. TIME-RESOLVED OPTICAL SPECTROSCOPIC TECHNIQUES Various optical spectroscopy techniques, such as transient absorption/reflection,30 time-resolved photoluminescence,31 time-resolved THz spectroscopy,32 time-resolved microwave conductivity (TRMC),33 transient photovoltage/photocurrent,34 etc, have been used to investigating the photophysical properties and mechanism of perovskite materials. In this section, we give a brief discussion on several representative optical spectroscopic techniques used in metal halide perovskite (MHP) research. Transient absorption/reflection: In transient absorption/reflection (TA/TR) measurements, a strong laser pulse (pump) is used to excite the sample material. And the subsequent photoinduced carrier relaxation and recombination processes are monitored by a variably delayed probe pulse. The probe pulse can be monochromatic or a broadband white light continuum and the excited (pumped) carrier distribution can cause either an increase or decrease to the probe light intensity relative to unpumped samples, which is called "bleaching" and "photoinduced absorption", respectively. The information obtained from absorption and reflection measurement is basically the same, and the pump and probe wavelengths can be varied. By using different pump-to-probe pulse delays, TA/TR measurements can cover time scales from sub-picosecond to microseconds to reveal the carrier relaxation and recombination dynamics.10,11 Femtosecond laser pulses used in TA/TR measurements can be very short (tens of fs), meaning they have a very broad spectrum. These short pulses could excite the phonon vibrational mode in MHPs. Hence the measured TA/TR decay curves are overlapped by temporal oscillations. These oscillation signals are then Fourier transformed to obtain the vibrational spectra. Correspondingly,

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the phonon vibration in MHPs can be analyzed from the vibrational spectra. In this case, the TA/TR measurements can be called impulsive vibrational spectroscopy (IVS).35,36 Time-resolved photoluminescence: The principle of time-resolved photoluminescence spectroscopy (TRPL) is similar to that of TA/TR technique. In TRPL, the measurements are performed by monitoring the luminescence from the sample excited by a pump laser pulse. And the carrier dynamics is probed as a function of the delay time after pump excitation. The configuration of TRPL can be either up-conversion or single-photon counting, and the detector can be a streak camera or photomultiplier tube. Photoluminescence signals rely on the radiative recombination from band-to-band transitions or trap states. Therefore, the TRPL spectroscopy gives access to information about the carrier distribution at and within the bandgap of MHPs.11 The detection time window of TRPL ranges from sub-picoseconds to seconds. As a result, the TRPL and TA method have become the benchmark techniques for carrier dynamics studies of MHP materials. Two-dimensional infrared (2D-IR) spectroscopy: Traditional TA/TR measurement only contain two laser pulses (pump and probe). Hence TA/TR measurement can only resolve carrier populations without information about phase. 2D-IR is analogous to multidimensional nuclear magnetic resonance (NMR) spectroscopy, which can be seen as an extension of conventional TA spectroscopy. In 2D-IR, two identical collinear pump pulses are used, with the time delay between them (coherence time, t 1 ) being scanned for a fixed delay between the second pump and the probe (population time, t 2 ). The probe pulse is dispersed in a spectrometer, providing resolution in the detection frequency. By using Fourier transform (FT), the 2D-IR spectra can be obtained from the probe spectrum. There are two kinds of signals in 2D-IR spectra: diagonal and off-diagonal peaks, which are related to the population and coherence of different energy levels, respectively. Indeed,



the signal from traditional TA can be seen either as a cut or an integral of the 2D-IR spectra. Therefore, a large amount of information, especially about electron-phonon coupling and cation orientation in MHPs, can be extracted from the 2D-IR spectra at different t 2 . A detailed review on 2D-IR spectroscopy can be found in reference 37.37 Time-resolved optical Kerr effect spectroscopy: Time-resolved optical Kerr effect (OKE) spectroscopy can be viewed as a kind of pump-probe measurement technique with the difference that the OKE technique monitors the transient birefringence of the photoexcited samples materials. The principle of OKE spectroscopy is as follows. The strong electromagnetic field of a pump laser pulse can create induced dipoles in the sample material. Then, the induced dipoles interact with the laser light field, forcing the sample to align parallel to the laser polarization. This alignment will create a transient birefringence in the sample that can be probed with a second laser probe pulse. The time evolution of the birefringence reveals information about microscopic structure dynamics in the samples. Over the past several decades, the OKE method has been widely used in studying the solvation dynamics in various liquids.38 Recently, the OKE technique has been used in research of MHPs, which reveals intriguing information about the cation orientation dynamics in MHPs.29 The details of these results will be discussed below. Light-induced transient grating technique: The transient grating (TG) method is a type of pump-probe technique that employs two pump beams. The interference between the two pump beams creates a periodic light field in the sample, forming a transient grating with period Λ determined by the input angle θ and wavelength of the pump λ. As a result, the photoinduced carrier density also has a grating-like spatial distribution. The decay kinetics of carrier density is monitored by the time-delayed probe beam, whose intensity is then correlated to both carrier lifetime τ and ambipolar diffusion coefficient D. The rate of the carrier diffusion depends on the


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grating period, while that of carrier lifetime does not, which allows for distinguishing between these two processes. Due to this unique feature, TG technique has been widely used for studying the carrier dynamics in many inorganic semiconductors such as GaN, ZnO, etc.39 TA/TRPL microscopy: In conventional TA or TRPL measurements, the laser spot size focused on the sample material is usually tens of micrometers in diameter. As a result, the information of spatial heterogeneity in the sample is lost because the measured signal provides spatially averaged information from different areas within the pump laser spot. For MHPs thin films, the impact of microstructure on carrier dynamics is a vital process in defining the macroscopic performance. Therefore, optical spectroscopy with high spatial resolution can made a major contribution to understanding the influence of heterogeneity on carrier dynamics of MHPs. The most common way of performing TA/TRPL microscopy is to combine conventional TA/TRPL measurements with far-field optical microscopy. In this case, the pump beam is focused at the sample with a high numerical aperture objective lens. And the second time-delayed probe or photoluminescence beam can be detected by another lens or the same objective lens in transmission or reflection configuration, respectively. Due to the high numerical aperture objective lens, the spatial resolution of the pump beam can be enhanced to the sub-micrometer level, enabling to image change in dynamics induced by defects and grain boundaries in MHPs. Moreover, TA/TRPL microscopy can also be measured by raster scanning the sample through the laser focus or vice versa by using a piezo stage or galvo-scanning mirrors. Time resolved images of these types of measurements can reveal the impact of spatial heterogeneity on carrier transport in MHPs.40-42



4. DYNAMIC PROCESSES IN METAL HALIDE PEROVSKITES This section presents a comprehensive overview of the dynamic processes in metal halide perovskites (MHPs), including carrier dynamics, lattice dynamics, and recent developments in spatially-resolved dynamics studies on MHP materials and devices. 4.1 Carrier dynamics in MHPs

Carrier relaxation and recombination processes are central in determining the performance of MHPs. Accordingly, one finds an ever-increasing number of studies investigating the carrier dynamics in MHPs materials and devices. Hot carrier cooling in MHPs: Price et al. reported the first carrier thermalization and hot carrier cooling dynamics in MHP via TA spectroscopy.43 Their results showed that the carrier thermalization after photoexcitation with an ultrashort laser pulse is accomplished within 100 fs, which was later confirmed by Richter et al.44 Moreover, they also found that there is a phonon bottleneck at high carrier densities, which slows down the cooling of hot carriers. Similar results were also reported by Yang et al.45 and Beard et al.46 On the other hand, a recent study showed that this slow hot carrier cooling behavior was also observed in MAPbBr 3 single-crystal microplatelets even at low carrier densities, well below the hot-phonon bottleneck regime (Figure 3a). This result was attributed to the screening effect resulting from photoinduced large polarons.29 Carrier recombination in MHP materials: Since the exciton binding energy for MAPbI 3 and MAPbBr 3 is sufficiently low (

Photoexcited Dynamics in Metal Halide Perovskites: From Relaxation

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