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Time-Resolved Infrared Spectroscopy Directly Probes Free and Trapped Carriers in Organo-Halide Perovskites Kyle T. Munson, Christopher Grieco, Eric Kennehan, Robert J. Stewart, and John B. Asbury ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00033 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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Time-Resolved Infrared Spectroscopy Directly Probes Free and Trapped Carriers in OrganoHalide Perovskites Kyle T. Munson, Christopher Grieco, Eric R. Kennehan, Robert J. Stewart, and John B. Asbury*

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania USA, 16802 *Corresponding Author: [email protected]

Abstract Free carrier dynamics in organo-halide perovskites can directly reveal information about their carrier lifetimes, and indirectly reveal information about trap state distributions, both of which are critical to improving their performance and stability. Time-resolved photoluminescence (TRPL) spectroscopy is commonly used to probe carrier dynamics in these materials, but the technique is only sensitive to radiative decay pathways and may not reveal the true carrier dynamics. We used time-resolved infrared (TRIR) spectroscopy in comparison to TRPL to show that photo-generated charges relax into free carrier states with lower radiative recombination probabilities, which complicates TRPL measurements. Furthermore, we showed that trapped carriers exhibit distinct mid-infrared absorptions that can be uniquely probed using TRIR spectroscopy. We used the technique to demonstrate the first simultaneous measurements of trapped and free carriers in organo-halide perovskites, which opens new opportunities to clarify how charge trapping and surface passivation influence the optoelectronic properties of these materials. Table of Contents Graphic excitation

FCA Free Carrier Abs.

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(N-H) TRIR Spectrum

FCA vs PL Decay

Time (s) CH3NH3PbI3

1 ACS Paragon Plus Environment

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Photovoltaics based on organo-halide perovskite absorbers such as CH3NH3PbI3 have attracted much attention due to their ease of fabrication1-3, high absorption coefficient 4-6, long charge carrier diffusion lengths7-9 , tunable band gaps5, 10-12, and high power conversion efficiencies13. Their remarkable

qualities

are

further

highlighted

by

higher

open-circuit

voltages

and

photoluminescence quantum yields in comparison to other materials.14-15 Yet, there is growing evidence from electrical measurements of perovskite devices that their charge recombination processes continue to be dominated by non-radiative recombination centers.16-19 Unfortunately, TRPL

does

not

directly

measure

non-radiative

recombination

processes

because

photoluminescence results from radiative decay that is indirectly influenced by such processes.17 Without an independent measure of charge carrier dynamics in perovskite materials, it is challenging to determine to what extent TRPL studies reveal the true charge carrier dynamics in materials involving both radiative and non-radiative pathways.19-23 Electrical measurements of these processes are complicated by the need to collect charges at electrodes, which convolute charge transport, charge injection and charge recombination processes into a single measurement. ‘Contactless’ transport measurements such as time-resolved microwave conductivity or terahertz spectroscopy19-33 can access intrinsic properties of charge carriers. However, these measurements do not necessarily provide direct probes of the charge traps that may be involved in non-radiative recombination processes. Previous time-resolved mid-IR (TRIR) spectroscopy measurements of PbS colloidal quantum dot photovoltaic materials revealed that absorptions of trapped carriers could be measured in quantum dots solids.34-35 These trapped carrier absorptions, termed trap-to-band transitions were similar to prior reports of mid-IR transitions of trapped carriers observed in silicon, GaSb and InSb electronic materials.24, 36-38 Here, we used TRIR spectroscopy to examine charge carrier dynamics


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in organo-halide perovskite films as a way to independently measure the dynamics of free carriers and their interactions with trap states and charge recombination centers. We investigated the extent to which TRIR spectroscopy provides an alternative to measure charge carrier dynamics in organo-halide perovskites that is more sensitive to the presence of charge traps and recombination centers than conventional TRPL spectroscopy. We first benchmarked the properties of the CH3NH3PbI3 films examined here using TRPL spectroscopy because this permits direct comparison to prior work reported in the literature.9, 15, 39-44 Figure 1a depicts absorption and photoluminescence spectra of CH3NH3PbI3 films that were prepared on CaF2/mesoporous alumina substrates using a two-step deposition method as previously described (see the Supporting Information).44-45 The absorption and emission spectra of the films measured before (solid curves) and after (dashed curves) surface passivation by triphenylphosphineoxide (TPPO) are characteristic of similar films reported in the literature.15, 44 An SEM image of the film appears in the Supporting Information (Figure S1) and exhibits the cube-like structure that is typical of perovskite films deposited using the two-step approach.45-49 Represented in Figure 1b are time-resolved photoluminescence (TRPL) decay traces of the CH3NH3PbI3 films before and after surface passivation with TPPO. The excitation conditions used in the experiment (10 ns, 532 nm pulses at 50 nJ/cm2 energy density) and corresponding PL decay kinetics are similar to those reported by others.9, 39-43 The data demonstrate that passivation of surfaces of CH3NH3PbI3 crystals with TPPO enhances the charge carrier lifetime nearly six-fold. Overlaid on these TRPL decay traces are best fit curves using a biexponential function. The fitting procedures are described in the Supporting Information with the best fit parameters tabulated in Table S1. Average PL decay lifetimes of 70 ± 10 ns and 410 ± 40 ns were obtained from analysis of the TRPL kinetics for the unpassivated and TPPO passivated CH3NH3PbI3 films, respectively.


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We chose TPPO to passivate the CH3NH3PbI3 film because a similar ligand, trioctylphosphine oxide, was recently reported to passivate surfaces of organo-halide perovskite films by Ginger and co-workers.15 In our hands, we found that TPPO provided more complete removal of surface trap states indicated by a longer TRPL lifetime than the alkane-based phosphine oxide. We note that the mechanism of passivation remains unclear, which will be the subject of further investigation. The excitation energy density dependence of the TRPL decay traces of samples prepared using the same procedures have been reported previously.44 These measurements demonstrated that the perovskite films examined here exhibited the inverse relationship between recombination rate and

a

b

TPPO

d

c Unpassivated Film

TPPO Passivated Film Free carrier absorption

Free carrier absorption Trap state absorption

Figure 1. a. Visible absorption spectra of a CH3NH3PbI3 film with corresponding time-integrated PL spectra before and after surface passivation using TPPO. b. Photoluminescence decay curves before and after surface passivation with TPPO. Time-resolved mid-IR transient absorption spectra of CH3NH3PbI3 films before c. and after d. TPPO surface passivation. Fourier transform infrared spectra of the CH3NH3PbI3 films are depicted below the main panels highlighting the N-H stretch band of CH3NH3+ ions. The dotted vertical line in d. is a guide to the eye highlight the shift in frequency of the vibrational feature relative to the FTIR spectrum.


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pump fluence that is expected on the basis of reports by others.10, 50 At higher excitation densities, higher order annihilation processes cause the carrier dynamics to exhibit non-single exponential behavior as observed in Figure S4. Figure 1c depicts nanosecond TRIR spectra measured at several time delays following pulsed excitation of a CH3NH3PbI3 film at 532 nm prior to surface passivation with TPPO (see the Supporting Information for experimental details). We note that a higher excitation density of 2.0 J/cm2 was used to measure the TRIR spectra to reduce the data collection time to a few hours needed to achieve a reasonable signal to noise quality over the entire mid-IR region. The spectra are characterized by an absorption feature increasing monotonically with lower frequency in the 2250 – 1500 cm-1 (0.28 – 0.18 eV) range that is super-imposed on a broad absorption peak spanning the higher frequency portion of the mid-IR spectral range. Figure 1d depicts TRIR spectra of the same CH3NH3PbI3 film passivated with TPPO and measured under identical conditions. Similar to the unpassivated film, the TRIR spectrum at 30 ns exhibits a transient absorption feature that increases monotonically with decreasing frequency. We note that such monotonically increasing transient absorption features are characteristic of free carrier absorptions in semiconductor materials such as silicon and GaAs in the mid-IR spectral range.24, 36-38, 51 Represented below the main panels of Figure 1c and 1d are Fourier transform infrared (FTIR) spectra of the corresponding CH3NH3PbI3 films before and after surface passivation with TPPO. The gray shaded boxes emphasize the N-H stretch region of CH3NH3+ ions in the films that appear around 3300 cm-1. The TRIR spectra of the TPPO treated film in Figure 1d exhibit a narrow feature at slightly higher frequency than the N-H vibrational mode in the FTIR spectrum and that overlaps the broad electronic transition. Similar narrow features overlapping electronic transitions were


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observed in prior measurements of PbS quantum dot films.34-35, 52 It is noteworthy that the narrow feature in the N-H stretch region is not clearly observed in the TRIR spectra of the unpassivated CH3NH3PbI3 film. Prior work examining defects in PbS colloidal quantum dot films as well as silicon and GaSb electronic materials reveal that defects occupied by trapped carriers can exhibit broad electronic transitions in the mid-IR spectral range.24, 34-38 This spectral range in particular corresponds to photon energies of 0.05 – 0.6 eV that are characteristic of a distribution of shallow to deep trap states. The dotted curve in Figure 1c overlaid on the 30 ns TRIR spectrum is a guide to the eye emphasizing the presence of such broad electronic absorptions of trapped carriers in the unpassivated CH3NH3PbI3 film. To isolate the transient absorption spectra of trapped carriers in the unpassivated CH3NH3PbI3 film (Figure 1c), we measured TRIR spectra at longer time delays at which the free carriers had already decayed. The TRIR spectra appearing in Figure S5 were measured at time delays between 1.0 and 3.0 s. Unlike the spectra measured on the 30 – 100 ns time scale in Figure 1c, the shape of the spectra measured on this longer time scale do not change with time within experimental precision, confirming that the free carriers had already decayed in the unpassivated film by the microsecond time scale. We therefore assign the mid-IR spectra in Figure S5 to the absorptions of trapped carriers in the unpassivated CH3NH3PbI3 film. From this assignment, we predicted that a TRPL decay trace measured under these conditions would exhibit negligible radiative emission from the sample after 1.0 s time delay. The TRPL kinetic trace of the unpassivated film represented in Figure S4 was measured after excitation at 532 nm with 500 nJ/cm2. The data confirm the decay of PL intensity on the few hundred nanosecond time scale with negligible emission occurring at 1.0 s or longer at this elevated excitation density.


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Figure 2a displays the 2.0 s TRIR spectrum reproduced from Figure S5 that represents the trapped carrier absorption (TCA) in the unpassivated film. The spectrum is overlaid with the sum of two peaks; a broad peak around 0.4 eV describing the higher energy portion of the TCA, while a narrower Gaussian centered at 0.17 eV captures the lower energy portion. We refer to the sum of these functions as the fit of the TCA in the film. The lower energy portion of the TCA spectrum corresponds closely to the ~0.16 eV trap energy recently reported in CH3NH3PbI3 films using admittance spectroscopy.53 It is currently unclear whether the higher energy portion of the TCA around 0.4 eV is associated with deeper traps or whether it is a transition from a trap to higher energy sub-bands within the valence or conduction bands.54-55

Unpassivated CH3NH3PbI3 Film

a

TCA

FCA

Localization of carriers into nonemissive trap states

e–

h+

CB

excitation

energy

b

PL

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VB

Figure 2. a. Time-resolved infrared spectrum at 30 ns time delay of an unpassivated CH3NH3PbI3 film overlaid with a best fit spectrum. The fit consists of a sum of the trapped carrier absorption spectrum (red) and free carrier absorption spectrum (blue). b. Schematic diagram illustrating the presence of trap states that absorb in the mid-IR region. Color-coded arrows indicate the optical transitions corresponding to the free carrier absorption and trapped carrier absorption spectra.


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The 30 ns TRIR spectrum of the unpassivated CH3NH3PbI3 film from Figure 1c is also reproduced in Figure 2a for comparison to the 2.0 s TRIR spectrum. The amplitudes of the 2.0 s TRIR spectrum and TCA fit have been scaled to match the amplitude of the 30 ns spectrum around 0.4 eV. The comparison demonstrates that the higher frequency portion of the 30 ns TRIR spectrum of the unpassivated film is dominated by the absorptions of trapped carriers. Comparing the 30 ns and 2.0 s spectra in Figure 2a, the difference on the lower frequency side indicates the absorption of free carriers that are present at 30 ns in the unpassivated film. We used the Drude model56 to describe the mid-IR absorption of free carriers because the shapes of free carrier absorptions in this model result from the excitation of mobile carriers among momentum states within the bands. Therefore, the Drude model can be used to describe the absorption spectrum of mobile carriers. A description of the Drude model and its adaptation to fit the TRIR spectra are presented in the Supporting Information. We fit the 30 ns TRIR spectrum of the unpassivated film with a sum of the free carrier absorption spectrum from the Drude model and the TCA spectrum. The best-fit was calculated by optimizing the free carrier density and mobility terms in the Drude model (Table S3) while also varying the relative weight of the trapped and free carrier absorption spectra to obtain the best linear regression (Table S4). The optimized free carrier absorption spectrum obtained from this procedure is reproduced in Figure 2a as the solid blue curve. Figure 2b depicts a schematic diagram of the optical transitions that appear in the TRIR spectra. The diagram arbitrarily focuses on electrons for clarity recognizing that similar free carrier absorptions and trapping processes may arise from photo-generated holes. Bandgap excitation of the unpassivated film produces electrons and holes that exhibit free carrier absorptions. These carriers can later become trapped, resulting


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in the absorption of trapped carriers described by the TCA spectrum. This trapping process also shortens the PL lifetime measured in comparison to the TPPO passivated film (Figure 1b). In contrast to the unpassivated film, surface treatment with TPPO extended the PL lifetime of the passivated CH3NH3PbI3 film nearly six-fold. Fitting the TRPL kinetic decays of the perovskite films in Figure 1b with the kinetic model developed by Snaith and co-workers18-19 suggests the density of non-radiative trap states decreased nearly 100-fold with TPPO passivation (Table S2).

TPPO Passivated CH3NH3PbI3 Film

a 30 ns spectrum Drude model fit to FCA

b

Figure 3. a. Time-resolved infrared spectrum of the TPPO passivated CH3NH3PbI3 film at 30 ns time delay overlaid with the free carrier absorption spectrum obtained from a fit using the Drude model to describe the free carrier absorption spectrum. b. Comparison of time-resolved infrared decay traces for a TPPO treated film measured at 1700 cm-1 and 3400 cm-1 showing that they have the same decay kinetics. The vertical lines indicate the time delays at which TRIR spectra appearing in the inset were measured. The inset depicts time-resolved infrared spectra measured at a range of time scales throughout the free carrier decay dynamics. The comparison demonstrates that charge carriers in the TPPO passivated film remain free throughout their lifetime.


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We fit the 30 ns TRIR spectrum of the TPPO passivated CH3NH3PbI3 film with the Drude model and present the comparison in Figure 3a. The data reveal that unlike the unpassivated film, the TRIR spectrum of the TPPO passivated CH3NH3PbI3 film was characterized by a single population of free carriers with an absorption spectrum well described by the Drude model across the full spectral range (0.18 – 0.63 eV). The best fit parameters for the Drude model describing this sample are also represented in Table S3 for comparison to the parameters for the unpassivated film. In Figure 3b we present a normalized comparison of kinetics traces measured at 0.2 eV (~1700 cm-1) and 0.4 eV (~3400 cm-1) in the TPPO passivated film. The vertical lines appearing in Figure 3a mark the frequencies where the kinetics were measured. The similarity of the decay kinetics further demonstrate that a single population of free carriers absorb at both probe energies. Finally, the inset of Figure 3b compares TRIR spectra of the TPPO passivated film at several time delays that have been scaled to facilitate comparison of their frequency dependent shapes. The vertical lines in the main panel of Figure 3b highlight the 240 ns and 1000 ns time delays at which the TRIR spectra appearing in the inset were measured. The comparison demonstrates that little change occurs in the shape of the spectra with time in contrast to the unpassivated film (Figure 2a), again confirming the assignment that the TRIR spectra of the TPPO passivated film are dominated by a single population of free carriers with negligible contribution coming from trapped carriers throughout their lifetime. The analysis of the TRIR spectra of the perovskite films before and after TPPO passivation demonstrates that TRIR spectroscopy provides an approach to examine the dynamics of free carriers directly through their mid-IR absorptions.24, 36-38, 51 We took advantage of this to compare in Figure 4a the dynamics of free carriers in the TPPO passivated film measured directly at 0.2 eV with the corresponding TRPL decay trace of the same film measured under the same excitation


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conditions (500 nJ/cm2 at 532 nm). We emphasize that the comparison in Figure 4a is of dynamics measured in the TPPO passivated film to avoid complications arising from charge trapping that occurs in the unpassivated perovskite film (Figure 2). Biexponential functions were used to fit the TRIR and TRPL traces to quantify the decay kinetics. The best fit parameters of the functions appear in Table S5 from which we obtain an average free carrier lifetime of 250 ± 50 ns, which is significantly longer than the average lifetime of 77 ± 3 ns measured at this higher excitation density using TRPL spectroscopy. We note that these lifetimes differ from those obtained at lower excitation density (see Table S1) and those that have been reported by recent terahertz or time resolved microwave conductivity studies20-22, 25-33 because of the fast annihilation processes that occur at elevated excitation densities.18 The data therefore reveal that the TRPL decay does not capture the longer time-scale portion of the true free carrier decay kinetics in the TPPO passivated perovskite film. We considered whether recent TRPL studies of large single crystals might provide insight about the shorter TRPL decay relative to the free carrier dynamics reported here.7, 57-60 These studies of large single crystals report shorter PL decays than the actual carrier dynamics due to the combined effects of carrier diffusion and self-absorption.59-60 Close inspection of TRPL studies of single crystals reveals that the shorter PL decay dynamics following one-photon excitation are peculiar to these samples. This is because the optical densities at the bandgap of large single crystals on the order of 1 mm thickness are more than 1000-fold larger than would be recommended for optical characterization studies that are free of artifacts arising from nonuniform absorption throughout the thickness of the crystals combined with photoluminescence efficiency losses due to self-absorption. These artifacts have been used to advantage for the


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examination of carrier diffusion dynamics over hundreds of micrometers in such large single crystals by comparing emission dynamics from one- versus two-photon excitation.60 However, studies of nanocrystalline thin films on the order of 250 – 300 nm thickness as reported here avoid such artifacts because the absorption profiles throughout the thickness of the films are much more uniform. For example, the optical densities of the films reported here were 1.5 at the 532 nm excitation wavelength (see Section S1). This indicated that the excitation pulses were absorbed throughout the front half of the nanocrystalline thin films, which provided a distribution of excitations that was already representative of the interfacial versus interior environments present in the films. Consequently, carrier diffusion from interfacial regions that so strongly influences TRPL studies of single crystals59-60 is not expected to have a significant TPPO Passivated CH3NH3PbI3 Film

a Free Carrier versus PL decay kinetics

FCA

FCA

Relaxation of free carriers into states with lower PL QY

e–

h+

CB

excitation

energy

b

PL

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VB

Figure 4. a. Comparison of free carrier dynamics measured in a TPPO passivated CH3NH3PbI3 film in the mid-IR versus the photoluminescence decay trace measured at 765 nm in the same film under identical conditions. The curves overlapping the data represent biexponential fits used to quantitatively compare the free carrier versus the photoluminescence decay kinetics. b. Schematic diagram illustrating a relaxation process of free carriers into states with lower radiative recombination probability. This relaxation process causes the photoluminescence kinetics trace to decay faster than the true free carrier dynamics.


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influence on the dynamics reported in our nanocrystalline thin films. Furthermore, the optical density at the bandgap of our films was 0.2 (see Figure 1), indicating that little self-absorption of the emitted photoluminescence occurred in our films, unlike studies of large single crystals. Therefore, the TRPL trace of the TPPO passivated perovskite film in Figure 4a represents an accurate measure of the emission dynamics of the perovskite films following 500 nJ/cm2 excitation without interference from non-uniform absorption profiles or self-absorption artifacts. Figure 4b depicts a schematic diagram of the electronic states and associated optical transitions that are suggested by the comparison of free carrier absorption versus TRPL decay traces. To simplify the figure, arrows indicating optical transitions associated the free carrier absorptions appear only for electrons in the diagram. Analysis of the TRIR spectra and TRPL decay traces demonstrate that TPPO passivation greatly reduced the density of charge traps. Consequently, the TRIR spectra and kinetics of the TPPO passivated film described photo-generated charges that were sufficiently mobile to exhibit free carrier absorptions throughout their lifetime. However, the free carrier absorption decay kinetics trace is significantly longer-lived in comparison to the TRPL data, indicating that these carriers relaxed into electronic states with lower radiative recombination probabilities despite being mobile throughout their lifetime. The appearance of transient vibrational features in the TRIR spectra of the TPPO treated film (Figure 1d) provides some insight about the nature of these relaxed electronic states in which carriers are mobile but have lower radiative recombination probabilities. The analysis of the TRIR spectra in Figures 2 and 3 demonstrate that the density of free carriers at 30 ns was markedly higher in the TPPO treated film. A more distinct transient vibrational feature was also observed in this film that had a higher frequency in comparison to the N-H stretch mode of CH3NH3+ ions in the perovskite crystals in their ground electronic state. Combining these observations, we suggest that


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the presence of free carriers may have caused a distortion of the lattice of the perovskite crystals leading to the observed perturbation of the N-H stretch in the TPPO treated film. The polarization of the lattice sufficient to perturb the N-H stretch of CH3NH3+ ions is indicative of electron-phonon coupling, which has been previously observed to quench radiative relaxation in other systems.61-64 The precise nature of this polarization and how it results in the transient absorption of the NH stretch band will be the subject further investigation. For example, it will be interesting to use computational modeling of free carriers in the perovskite crystals to investigate the mechanisms of electron-phonon coupling that may lead to the observed shift of N-H stretch mode with corresponding perturbation of the perovskite lattice. It is possible that the perturbation of the lattice causing the perturbation of the N-H stretch mode may be related to recent observations of polarization and symmetry breaking effects in perovskite crystals.65-69 We note that polarization of the organo-halide perovskite lattice resulting in electron-phonon coupling was recently reported from two-photon photoemission70 and temperature dependent photoluminescence studies.71 Although these measurements reported dynamics specifically at perovskite surfaces and in atomically thin 2D perovskites, it is possible that such lattice polarization may occur in the bulk as well, which would reduce the radiative recombination probability of free carriers and prevent TRPL spectroscopy from accurately capturing the true dynamics of free charges. These findings indicate that directly probing charge carriers themselves in the mid-IR may provide a more complete picture of how charge trapping and surface passivation influence carrier dynamics in perovskite materials to inform on-going work to enhance their performance and stability. Further investigations of the dynamics of mobile carriers through their mid-IR absorptions may provide a more accurate measure of charge carrier dynamics in organo-halide perovskites and reveal the underlying origins of such lattice polarization processes. Furthermore,


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on-going investigations of trapped carrier dynamics on ultrafast time scales in organo-halide perovskite materials may provide information about the nature of trap states, the mechanisms of ligand passivation and how these may modulate the surface chemistry and stability of the materials when used in photovoltaic and photodetector applications.

Acknowledgements KTM, CG, ERK and JBA are grateful for support of this work from the U.S. National Science Foundation under Grant Number CHE-1464735. RJS and JBA were also supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-SC0008120.

Supporting Information Available: (SEM Micrographs, PL kinetic modeling procedures and TRIR modeling procedures).


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