Letter pubs.acs.org/JPCL
Direct Experimental Evidence for Photoinduced Strong-Coupling Polarons in Organolead Halide Perovskite Nanoparticles Kaibo Zheng,†,§,‡ Mohamed Abdellah,†,∥,‡ Qiushi Zhu,†,§,‡ Qingyu Kong,⊥ Guy Jennings,⊥ Charles A. Kurtz,⊥ Maria E. Messing,# Yuran Niu,∇ David J. Gosztola,⊥ Mohammed J. Al-Marri,§ Xiaoyi Zhang,⊥ Tönu Pullerits,*,† and Sophie E. Canton*,○ †
Department of Chemical Physics and Nanolund, Lund University, Box 124, 22100 Lund, Sweden Gas Processing Center, College of Engineering, Qatar University, PO Box 2713, Doha, Qatar ∥ Department of Chemistry, Qena Faculty of Science, South Valley University, Qena 83523, Egypt ⊥ X-ray Science Division, Advanced Photon Source and Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States # Department of Solid State Physics and Nanolund, Lund University, Box 118, 22100 Lund, Sweden ∇ MAX IV Laboratory, Lund University, Box 118, 22100 Lund, Sweden ○ Center for Ultrafast Imaging, University of Hamburg, 22761 Hamburg, Germany §
S Supporting Information *
ABSTRACT: Echoing the roaring success of their bulk counterparts, nano-objects built from organolead halide perovskites (OLHP) present bright prospects for surpassing the performances of their conventional organic and inorganic analogues in photodriven technologies. Unraveling the photoinduced charge dynamics is essential for optimizing the optoelectronic functionalities. However, mapping the carrier−lattice interactions remains challenging, owing to their manifestations on multiple length scales and time scales. By correlating ultrafast time-resolved optical and X-ray absorption measurements, this work reveals the photoinduced formation of strong-coupling polarons in CH3NH3PbBr3 nanoparticles. Such polarons originate from the self-trapping of electrons in the Coulombic field caused by the displaced inorganic nuclei and the oriented organic cations. The transient structural change detected at the Pb L3 X-ray absorption edge is well-captured by a distortion with average bond elongation in the [PbBr6]2− motif. General implications for designing novel OLHP nanomaterials targeting the active utilization of these quasi-particles are outlined.
O
associated with their effective masses denoted by me and mh. The free propagation can be altered by scattering events with other carriers or with the inorganic lattice and its defects, before being halted by various recombination processes including firstorder (geminate recombination), second-order (bimolecular e− h recombination), and third-order (Auger recombination).12 The carriers can also be trapped in sub-bandgap states, which are often related to defects.13 The carrier mobilities observed experimentally are higher than for conventional organic semiconductors but still surprisingly low considering the effective masses predicted by first-principles electronic structure calculations (me = 0.15m0 and mh = 0.1m0).14 This finding can be rationalized by the recent realization that the carrier− phonon interaction in OLHP materials is likely strong,15 so that the motion of a free carrier can polarize the surrounding atoms and shift their equilibrium positions within the lattice.16 This in
ptimizing the performances and the stability of organolead halide perovskites (OLHP) as superior photoconverting materials, e.g., for photodetection, photovoltaics, and light-generation applications, requires unravelling the complex photoinduced dynamics across a wide range of time scales, length scales, and incident fluences.1−5 In practice, this involves identifying the various relaxation pathways and quantifying the associated (often-coupled) transfer processes. Because bulk OLHP (microcrystals, single-crystals, or films) are direct band gap materials, photoabsorption near the band edge creates Wannier excitons (e.g., bound electron−hole pairs).6,7 Owing to their very low binding energy (BE), these excitons dissociate quasi-instantaneously and generate charge carriers as electrons (e) and holes (h).6,8,9 In a reverse process, the electrons and holes can recombine, forming excitons. The overall thermodynamic balance depends upon the temperature and the concentration of excitations as determined by the incident fluence through the Saha−Langmuir model.6,10,11 Thermalization completes on the subpicosecond to picosecond time scale, while e/h transport takes place with a mobility © 2016 American Chemical Society
Received: September 7, 2016 Accepted: October 28, 2016 Published: October 28, 2016 4535
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Figure 1. (a) HR-TEM image of the MAPbBr3 NPs under high magnification with the inset showing the interference fringes of the atomic lattice. (b) Ground-state absorption (GSA) spectrum of the NPs. The inset shows the X-ray photoelectron spectrum in the valence region for an incident photon energy of 200 eV (black) and 70 eV (red), corresponding to an escape depth of 8 and 5 Å, respectively. For 70 eV, the band edge tail increases compared to that for 200 eV. Therefore, this component originates from the surface of the NPs. (c) TR-OAS spectra of the NPs for an excitation wavelength λexc of 400 nm and a fluence of 1015 photons/cm2/pulse. (d) The long-lived spectral component from the SVD global fitting of the TR-OAS spectra shown in panel c. The inset contains the corresponding kinetics.
turn modifies the nuclear potential experienced by the carrier, which can then become self-trapped. Such a quasi-particle comprising the net lattice distortion and the self-trapped carrier is called a strong-coupling polaron.15 Its renormalized velocity is necessarily slower than that of the free carriers because its diffusion hinges upon the correlated motions of carriers and nuclei. Possibly explaining the modest carrier mobilites in OLHP despite their small effective mass,15,17,18 polaron formation has also been invoked for analyzing the rapid healing and slow degradation of devices based on these materials under working conditions,17,19 with support from density functional theory (DFT) calculations.16 However, the direct experimental observation of these quasi-particles has not been reported to date.19 As for conventional organic and inorganic semiconductors, unique physicochemical attributes emerge when the characteristic length scale of the OLHP building blocks falls into the nanometer range. For example, recently improved synthesis protocols have produced CH3NH3PbX3 (X= I, Br, or Cl) nanoparticles (NPs) with emission quantum yield now exceeding 70%,20 holding great promise for cutting-edge optoelectronic applications. More generally, controlling the response to photoillumination is crucial for tailoring advanced functionalities in novel OLHP NPs because the photoactivated carrier dynamics are expected to be significantly modified compared to bulk materials. Specifically, the increased surface− volume ratio induces the appearance of delocalized surface states and deep trap states localized around under-coordinated lattice atoms or dangling bonds. In addition, both the long-
range and short-range orders govern the band edge properties that clearly differ from the bulk phase, causing the exciton BE to exceed the values anticipated from the particle-in-a-box model, 21 with distinct repercussions on the radiative processes.22 However, the precise role of the carrier−lattice interactions in OLHP NPs is only partially understood because their spatial and temporal manifestations span several orders of magnitude. In particular, the possible participation of polarons would have a far-reaching impact on the balance between electronic, radiative, and thermal processes. By combining ultrafast time-resolved optical absorption spectroscopy (TROAS) and ultrafast time-resolved X-ray absorption spectroscopy (TR-XAS), this work reveals the formation of strongcoupling polarons in CH3NH3PbBr3 (MAPbBr3) NPs. Figure 1a shows a high-resolution transmission electron microscopy (HR-TEM) image of the MAPbBr3 NPs under high magnification. The inset evidences the interference fringes due to the regular atomic lattice. The mean size of the particles is 8 ± 2 nm. Figure 1b displays the ground state absorption (GSA) spectrum of the NPs in the ultraviolet (UV)−visible region. The absorption band edge at 530 nm is attributed to the Wannier−Mott excitons,23 the optically allowed promotion of an electron from the valence band (VB) maximum to the conduction band (CB) minimum being slightly higher in energy. The absorbance also presents a featureless tail extending well into the red region. Comparison with the Xray photoemission spectrum measured for two incident photon energies, namely 200 and 70 eV (red trace and black trace in the inset of Figure 1b), allows assigning this feature to the 4536
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The Journal of Physical Chemistry Letters population of sub-band gap states. Because the intensity of the photoelectron signal below 2.4 eV clearly increases with decreasing incident photon energy (i.e., with decreasing photoelectron mean free path), these occupied states are predominantly located toward the surface of the NPs. TR-OAS is then employed to investigate the photoinduced dynamics. Figure 1c shows the corresponding optical pump−probe spectrograms acquired at different time delays, for an excitation wavelength (λexc) of 400 nm and a fluence of 1015 photons/ cm2/pulse. Following a complex interplay between thermalization and recombination processes, a single spectral component appears and lives for the 3 ns window spanned by the delay line (see Figure 1d). A dedicated analysis of the ultrafast relaxation is beyond the scope of the present work and will be reported elsewhere. The profile shown in Figure 1d consists of a positive band centered around 520 nm and a negative band with its minimum at the absorption edge (535 nm). It constitutes the spectral fingerprint of photogenerated excited electrons. The amplitude of the bleach at wavelengths larger than 550 nm is quite small, and the contribution of the sub-band gap states (being here predominantly hole traps) can be discarded in the subsequent analysis of the long-lived species generated by photoexcitation. While the TR-OAS measurement pinpoints the participation of excited electrons, it is not possible to unambiguously assign them as free or self-trapped carriers owing to rather similar spectral lineshapes in the visible range. Further insight is then obtained with TR-XAS, which is an element-sensitive probe of the bonding environment around a particular lattice site. Considering that the VB originates primarily from the antibonding combination of the Pb 6s and Br 4p orbitals, while the CB contains mainly Pb 6p orbitals, interrogating the Pb L3 edge following photoexcitation unveils the coupled changes in electronic and geometric structures.21 Figure 2a shows the normalized X-ray absorption coefficient μ around the Pb L3 edge attributed to the photoionization of an inner-shell 2p3/2 electron. The negative derivative signal [−dμ/ dE] is also displayed, with a minimum at 13.037 keV taken as the absorption edge position. The feature S is assigned to a shape resonance, which results from the temporary trapping of the outgoing photoelectron in the molecular field caused by the six surrounding Br−.24 Its energy position should follow Natoli’s rule.25 Figure 2b shows the transient difference spectrum [laser_on] − [laser_off] or an optical pump−X-ray probe time delay of 150 ps (blue curve) and a pump fluence of 3 × 1016 ph/cm2/pulse. The transient signal distinctively departs from a positive/negative derivative of the ground-state spectrum, which would approximate an XAS edge shift to lower/higher energy and a decrease/increase in Pb oxidation state.26 It exhibits a pronounced dip located at the first edge maximum (i.e., 13.045 keV). This feature is partly due to a decrease of the unoccupied DOS of nd character in the photoexcited state. The filling of the CB in the hundreds of picoseconds time window was also suggested by the TR-OAS measurement presented above. The postedge transient signal oscillates around zero, indicating some structural changes around the absorbing Pb2+ cation. As discussed in section 2 of the Supporting Information, a purely photothermal effect is highly unlikely. The shift of S to lower X-ray photon energy implies an overall elongation of Pb−Br bonds.25,27 More specifically, it is possible to compare the transient spectrum to simulations based on FEFF9.0 multiple scattering calculations, which have been systematically benchmarked on reference complexes containing Pb.28 When taking the tetragonal (quasi-cubic) structure established for the
Figure 2. (a) Normalized X-ray absorption coefficient, μ, at the L3 edge (black) and the negative derivative −dμ/dE (gray). (b) Transient difference signal (Diff) [laser_on] − [laser_off] (blue) acquired 150 ps after excitation at 400 nm, compared to FEFF9.0 simulations (model-NP) for the nanoparticles (red).
NPs (P4mm, with a = b 5.898 ± 2 Å ∼ c)21 as the ground-state structure, the red curve in Figure 2b results from an orthorhombic symmetry lowering, with an average Pb−Br bond elongation of ∼0.03 Å in the [PbBr6]2− octahedron motif29 with charge transfer. The simulations show qualitative agreement with the experimental trace. Extracting accurate bond elongations calls for an experimental strategy to estimate reliably the excited-state fraction. This task is complicated by the nonlinearity of the response,17 as shown by a preliminary study of the power dependency (for details see section 3 of the Supporting Information). The competition with Auger recombination, which sets in at high fluence, depletes the carrier population that could participate to the polaron formation, possibly explaining the effective saturation for the X-ray signal. The detailed modeling of the spectral shape in the near-edge region will also require quantifying the contribution from the delocalized excited electron population of nd character at 13.045 keV. Nevertheless, a photoinduced structural change is in line with the increased Raman modes in the 135−210 cm−1 region for illuminated free-standing MAPbI3 films.30 A photoinduced lattice dilatation also concurs with the recent report of a giant photostriction in single-crystal MAPbI3.17 The amplitude of this effect scaled linearly with the incident white light intensity up to 100 mW cm−2 (corresponding to about 1014 ph/cm2/pulse in the monochromatic condition used in this work), thereby establishing a direct correlation between the generation of free carriers and the macroscopic lattice expansion of 0.1% in this excitation regime.17 However, it 4537
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A quantitative correlation between the various rates calls for femtosecond studies with X-ray structural tools, in order to track on the atomic scale the carrier history, from the early events of carrier−phonon (LO and acoustic) scattering events to carrier localization (via polaron formation). Mirroring the wealth of studies dedicated to electron band edge properties engineering by tailored surface morpholophy and composition, this work should contribute to paving the way for photoactive carrier management within OLHP nano-objetcs.
should be noted that a simple isotropic bond elongation could not explain the present TR-XAS signal. Collectively, the simultaneous observation of long-lived excited electrons with TR-OAS and structural changes around Pb2+ with TR-XAS provide evidence for a photoinduced electron−lattice interaction that promotes the formation of strong-coupling polarons. As for the bulk materials,17,19 the appearance of polarons in photoexcited OHLP NPs should have a profound impact on the relaxation dynamics. In general, carrier−lattice interactions originate from two contributions, namely, the long-range Coulomb potential due to the displaced nuclei in the distorted lattice and the short-range deformation potential due to the alterations in local coordination and bonding. Depending on which of the two terms dominates, either “large” or “small” strong-coupling polarons are created. These quasi-particles can be distinguished through their transport properties. Large polarons exhibit bandlike coherent behavior, with modest mobilities (m > 1 cm2 V−1 s−1) that decrease with temperature. In contrast, the motion of small polarons is incoherent with low mobilities (m < 1 cm2 V−1 s−1) that increase with temperature because it occurs through thermally activated hopping from site to site. Considering that coherent transport necessitates a mean free path longer than about 10 times the lattice constant as estimated from linear scaling ab initio simulations,31 it can be proposed that small polarons are preferentially formed in the present NPs of 8 nm average diameter. However, the carriers in OLHP materials are dressed by the polarization field induced, not only by the rearrangement of the ions within the inorganic lattice but also by the organic cations, which can rapidly respond to perturbation by reorientation.32 Therefore, it is likely that both short-range and long-range potentials play a significant mechanistic role. A collective response of the NP could also be energetically favorable because their diameter (∼8 nm) matches the dimensions of supercells identified through simulations.31 The magnitude of the achievable photostriction also depends upon the ability of the lattice to distort. Because the capping agent exerts some additional pressure in selfstanding (colloidal) OLHP NPs,21 their photoinduced response is expected to be different from that of the bulk materials. Size reduction and dielectric confinement due to the capping agent are also likely to influence the polaron characteristics. Calculations with hybrid DFT predict that in charged isolated clusters of MAPbI3, the electrons are more readily trapped as polarons than the holes.16 As such, spatial separation between electrons and holes can occur. This should affect all the carrier lifetimes: geminate recombination would become less probable, while the second- and third-order recombination processes directly respond to variations in carrier concentrations. Because of charge accumulation, hot spots detrimental to the photostability may settle in the NPs. On the other hand, polaron formation could also serve as a channel capable of protecting the charge carriers until the concentrations drop below the level where ultrafast scattering with phonons or higher recombination switch off.33 This possibility should be examined in connection with the recent general frame of photorecycling.34 In conclusion, combined TR-OAS and TR-XAS measurements evidence the accumulation of long-lived excited electrons around the optical absorption edge concurrently to structural distortions around the Pb2+ cation. These observations are interpreted as the photoinduced formation of strong-coupling polarons that should affect the carrier transport, recombination, and trapping and hence the performances in light applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02046. Detailed experimental section including the sample preparation, material characterization, and time-resolved optical and X-ray absorption spectroscopies (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions ‡
K.Z., M.A., and Q.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The study was financially supported by the Knut and Alice Wallenberg Foundation, the Swedish Research Council, and by NPRP Grant NPRP7-227-1-034 from the Qatar National Research Fund (a member of Qatar Foundation). Collaboration within NanoLund is acknowledged. The use of the Advanced Photon Source and the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357.
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REFERENCES
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