Energy Level Gradients from Surface to Bulk in Hybrid Metal-Halide Perovskite Thin Films

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Energy Level Gradients from Surface to Bulk in Hybrid Metal-Halide Perovskite Thin Films

Sean A. Bourelle, Xie Zhang, Sascha Feldmann, Baiyu Zhang, Angus Mathieson, Lissa Eyre, Haralds Abolins, Thomas Winkler, Chris G. Van de Walle, and Felix Deschler

PRX Energy 3, 033001 – Published 26 July 2024

Abstract

Variations in local strain, defect densities, and composition of hybrid metal-halide perovskites have been reported to create heterogeneous energy landscapes in thin films, which impact charge-carrier diffusion and recombination dynamics. Here, we employ one- and two-photon transient absorption spectroscopy to selectively probe the dynamics of charge carriers from surface and bulk regions of methylammonium lead bromide thin films. Differences in the transient absorption spectra indicate that an energy gradient of approximately 100 meV is formed between the higher band-gap surface and lower band-gap bulk regions. Thus, during their lifetime, photoexcited carriers move away from the surface to recombine in the bulk, where our experiments detect long-lived charge populations despite the significant band splitting that has conventionally been assumed to inhibit efficient radiative recombination. Supported by first-principles calculations, we demonstrate that bright emission can still arise from the bulk with states that occupy a wide range of momenta in the vicinity of the band extrema, which show strong dipole transitions. Our results report that photoexcitations in the hybrid perovskites avoid defect-rich surface regions, and that particularly strong emission is generated from accumulated excitation populations in the bulk.

Received 14 December 2023
Revised 14 May 2024
Accepted 2 July 2024

DOI:https://doi.org/10.1103/PRXEnergy.3.033001

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Chemical Physics & Physical Chemistry

Energy applications Energy materials Functional materials

Film deposition Laser techniques Photoexcitation Sample preparation

Energy Science & TechnologyCondensed Matter, Materials & Applied PhysicsInterdisciplinary Physics

Authors & Affiliations

Sean A. Bourelle¹, Xie Zhang ^2,3, Sascha Feldmann ¹, Baiyu Zhang², Angus Mathieson¹, Lissa Eyre¹, Haralds Abolins¹, Thomas Winkler^1,4, Chris G. Van de Walle ^2,*, and Felix Deschler ^5,†

¹Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom
²Materials Department, University of California, Santa Barbara, California 93106-5050, USA
³School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
⁴Department of Physics and Astronomy, Aarhus University, Aarhus C, 8000, Denmark
⁵Institute for Physical Chemistry, University of Heidelberg, 69120 Heidelberg, Germany

^*Contact author: vandewalle@mrl.ucsb.edu
^†Contact author: deschler@uni-heidelberg.de

Popular Summary

Hybrid halide perovskites show impressive performance for optoelectronic applications, which raises key questions on the direct/indirect nature of the bandgap, their unexpected defect tolerance, and physical mechanism of bright luminescence. Combining theory and experiment, the authors show that the nature of the bandgap of hybrid halide perovskites spatially varies from a more direct surface region to a slightly indirect bulk. They find that this spatial energy level gradient diffuses carriers away from the defect-rich surface and into the bulk region, where they can avoid traps. Efficient bright emission is realized in the bulk of thin films, where Rashba-split bands provide strong radiative recombination due to a wide momentum range carrier occupation. These insights lay the foundation for the design of optimized interfaces and trap passivation in solar cells and light emitting diodes toward maximizing efficiencies.

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Vol. 3, Iss. 3 — July - September 2024

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Images

Figure 1
(a) Experimental concept of one-photon and two-photon optical spectroscopy. Schematic of the pump-probe scheme with one-photon excitation (1PE) at 3.1 eV and two-photon excitation (2PE) at 1.55 eV. Probe photon energies are centered at 2.2 eV (visible range) for one-photon transient absorption (1PTA), and at 1.15 eV (near-infrared region) for two-photon transient absorption (2PTA) to study photoexcitation energies and dynamics in a thin film (∼500 nm) of MA ${Pb Br}_{3}$ . Below: sketch of initial photoexcited carrier density as a function of film depth for 1PE surface excitation (blue) and 2PE bulk excitation (red). (b) Sketch of the conduction bands (CBs) and valence bands (VBs) in hybrid perovskites near a symmetry point, and the transitions that are probed with 1TPA and 2PTA. The shaded arrows indicate the transitions between the outer branches (VB₁ to CB₁) while full colors indicate the higher energetic transition starting at the symmetry point (involving VB₂ or CB₂). (c),(d) Maps of the energy-resolved 1PTA response following 1PE surface excitation and 2PE bulk excitation at fluences of 6 and 45 µJ/cm², respectively. (e) 1PTA spectra at 1-ps time delay following 1PE surface excitation (blue) and 2PE bulk excitation (red). (f) Kinetics of the ground-state bleach signal at a probe energy of 2.34 eV following 1PE (blue) and 2PE (red).
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Figure 2
(a),(b) Maps of the energy-resolved 2PTA response following 1PE surface excitation at 3.1 eV and 2PE bulk excitation at 1.55 eV. (c),(d) TA spectra for one-photon (1PTA) and two-photon (2PTA) probing following 1PE and 2PE, taken at time delays as indicated. (e),(f) Integrated 2PTA kinetics in the energy range around 1.1 eV (dotted line), which we attribute to signals from the bulk, and 1.2 eV (solid line), which we attribute to signals from the surface region, following 1PE and 2PE. (g) Sketch of the energy alignment of the observed optical transitions and carrier dynamics at surface and bulk regions of the studied hybrid perovskite films. The transition energies are extracted from our measured TA spectra shown in (c),(d) at a time delay of 1 ps.
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Figure 3
Calculated band structure and one-photon absorption of MA ${Pb Br}_{3}$ . (a) Transition energy between the lowest two CBs and the highest VB [as defined in (b)] for a slice (k_z = 0.5 in units of 2π/a) of k points in the Brillouin zone with MA oriented along [110]. The coordinates of the k points are defined relative to the R point (0.5, 0.5, 0.5). (b) Absorption coefficient of MA ${Pb Br}_{3}$ with MA oriented along [110]. E_g refers to the lowest energy band gap and E_g(R) denotes the band-gap energy at k = R. (c) Transition energy between the lowest two CBs and the highest VB for a slice (k_y = 0.5 in units of 2π/a) of k points in the Brillouin zone with MA oriented along [001]. (d) Absorption coefficient of MA ${Pb Br}_{3}$ with MA oriented along [001]. The insets in (b) and (d) show the band structures of MA ${Pb Br}_{3}$ along the k-point path where the Rashba splitting is most pronounced.
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Figure 4
(a),(b) Calculated lowest energy band gap (E_g) (a) and the band gap at k = R [E_g(R)] (b) of MA ${Pb Br}_{3}$ with MA oriented along [001] and [110] as a function of the biaxial strain. The blue and orange arrows next to the y axis indicate the experimentally measured band gaps. Under −1% strain, the surface has greater band gaps than the bulk, which explains the experimentally observed larger band gap at the MA ${Pb Br}_{3}$ surface than in the bulk. (c) Schematic band structure of MA ${Pb Br}_{3}$ , illustrating the Rashba splitting near the band extrema. ΔE_c and ΔE_v represent the Rashba splitting at the conduction-band minimum and valence-band maximum, respectively. (d),(e) Variations of ΔE_c and ΔE_v of MA ${Pb Br}_{3}$ with MA oriented along [001] and [110] as a function of the biaxial strain. The Rashba splitting at the surface ([100] orientation) is always weaker than that in the bulk ([110] orientation), irrespective of strain.
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