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The Journal of Physical Chemistry Letters
Exciton Binding Energy and the Nature of Emissive States in Organometal Halide Perovskites Kaibo Zheng, † Qiushi Zhu, §Mohamed Abdellah, †,# Maria Messing,‡ Wei Zhang, † Alexander Generalov,ǁYuran Niu,ǁ Lynn Ribaud,□Sophie E. Canton, ∆,◊,* Tõnu Pullerits†,* †
Department of Chemical Physics, Lund University, Box 124, 22100, Lund, Sweden
#
Department of Chemistry, Qena Faculty of Science, South Valley University, Qena 83523, Egypt
§
Department of Synchrotron Radiation Instrumentation, Lund University, Box 118, 22100, Lund, Sweden
‡
Deptartment of Solid State Physics, Lund University, Box 118, 22100, Lund, Sweden
ǁ
MAX IV Laboratory, Lund University, Box 118, 22100, Lund, Sweden
□
X-ray Science Division, Advanced Photon Source and Materials Science Division, Argonne National
Laboratory, Argonne, Illinois, 60439, United States ∆
IFG Structural Dynamics of (Bio)chemical Systems, Max Planck Institute for Biophysical Chemistry,
Am Fassberg 11, D-37077 Goettingen, Germany ◊
FS-SCS, Structural Dynamics with Ultra-short Pulsed X-rays Deutsches Elektronensynchrotron DESY,
Notkestrasse 85, D-22607 Hamburg, Germany
AUTHOR INFORMATION Corresponding Authors * Tonu Pullerits:
[email protected]; Sophie E. Canton:
[email protected] .
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The Journal of Physical Chemistry Letters
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ABSTRACT: Characteristics of nano-scale materials are often different from the corresponding bulk properties providing new, sometimes unexpected, opportunities for applications. Here we investigate the properties of 8 nm colloidal nanoparticles of MAPbBr3 perovskites and contrast them to the ones of large micro-crystallites representing a bulk. X-ray spectroscopies provide an exciton binding energy of 0.32±0.1 eV in the nanoparticles. This is 5 times higher than the value of bulk crystals (0.084±0.010 eV), and readily explains the high fluorescence quantum yield in nanoparticles. In the bulk, at high excitation concentrations, the fluorescence intensity has quadratic behavior following the Saha-Langmuir model due to the non-geminate recombination of charges forming the emissive exciton states. In the nanoparticles, a linear dependence is observed since the excitation concentration per particle is significantly less than one. Even the bulk shows linear emission intensity dependence at lower excitation concentrations. In this case the average excitation spacing becomes larger than the carrier diffusion length suppressing the non-geminate recombination. From these considerations we obtain the charge carrier diffusion length in MAPbBr3 of 100 nm.
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KEYWORDS. exciton binding energy; fluorescence; nanoparticles; perovskites; photoelectron spectroscopy
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The Journal of Physical Chemistry Letters
Organometal halide perovskites have attracted considerable attention as materials for low-cost opto-electronic devices. The advantages of such materials lay in facile solution-based chemical synthesis and high absorption coefficient in a broad visible spectral range.1-8 Furthermore, rapid dissociation of excitons to charges and slow recombination lead to long lifetime of the separated charges, which together with high charge mobility insure simultaneously large diffusion length and efficient carrier transportation.9-12 Besides photovoltaics applications, these materials have shown promise for light emitting diodes (LED) and lasing devices.13-17 In films of the material the exciton binding energy (Eb) is small which explains the efficient exciton dissociation at room temperature.18 Electrons and holes can meet again forming excitons and emit photons. The yield of emission critically depends on the electron-hole overlap whereas the non-radiative trap/defectmediated recombination reduces the emission efficiency.9-10,19-20 Solution-based synthesis typically leads to high concentration of such traps/defects.21-22 Semiconductor nanocrystals often show fascinating physical properties and opportunities for applications.23 Recently self-assembled methylammonium (MA) lead bromide perovskite (CH3NH3PbBr3) nanocrystals in a solid organic matrix were demonstrated,24 and Schmidt et al. reported synthesis of colloidal nanoparticles (NP) of MAPbBr3 with mean size of less than 10 nm.25 The NPs show quantum confinement and high photoluminescence (PL) quantum yield (QY) ϕf = 17% – significantly more than in the corresponding bulk material (
