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High Photoluminescence Efficiency and Optically-Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors Felix Deschler, Michael Price, Sandeep Pathak, Lina Klintberg, David Dominik Jarausch, Ruben Higler, Sven Huettner, Tomas Leijtens, Samuel David Stranks, Henry J. Snaith, Mete Atature, Richard T. Phillips, and Richard H. Friend J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz5005285 • Publication Date (Web): 24 Mar 2014 Downloaded from http://pubs.acs.org on April 1, 2014
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High Photoluminescence Efficiency and OpticallyPumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors †
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Felix Deschler , Michael Price , Sandeep Pathak †
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, Lina Klintberg , David-Dominik Jarausch ,
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Ruben Higler , Sven Hüttner , Tomas Leijtens , Samuel D. Stranks , Henry J. Snaith , Mete †
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Atature , Richard T Phillips ,and Richard H. Friend
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Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
AUTHOR INFORMATION Corresponding Author * Prof. Richard H Friend, E-mail:
[email protected], Prof. Henry J. Snaith, E-mail:
[email protected] ACS Paragon Plus Environment
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The study of the photo-physical properties of organic-metallic lead-halide perovskites, which demonstrate excellent photovoltaic performance in devices with electron- and hole-accepting layers, helps to understand their charge photo-generation and recombination mechanism and unravels their potential for other optoelectronic applications. We report surprisingly high photoluminescence (PL) quantum efficiencies, up to 70%, in these solution-processed crystalline films. We find that photoexcitation in the pristine CH3NH3PbI3-xClx perovskite results in free charge carrier formation within 1ps, and that these free charge carriers undergo bimolecular recombination on timescales of 10s to 100s of nsecs, To exemplify the high luminescence yield of the CH3NH3PbI3-xClx perovskite, we construct and demonstrate the operation of an opticallypumped vertical cavity laser comprising a layer of perovskite between a dielectric mirror and evaporated gold top mirrors. These long carrier lifetimes together with exceptionally high luminescence yield are unprecedented in such simply prepared inorganic semiconductors and we note that these properties are ideally suited for photovoltaic diode operation.
TOC GRAPHICS
KEYWORDS Mixed-halide lead perovskites, spectroscopy, lasing, photovoltaics, charge generation
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Organo-lead mixed halide perovskite-based solar cells have shown a breakthrough in power conversion efficiency1. Solution-processed devices with power conversion efficiencies of 1012% 2-6 were reported in 2012, and have more recently exceeded 15% in devices processed by evaporation 7 and sequential deposition 8. There is evidence from optical studies 9-10 that the diffusion range for the photoexcitations can be substantial, up to 1 µm, and this allows efficient light harvesting across the thickness of the perovskite layer (typically 400 nm). Nevertheless, it has not been clear whether the mobile species is a neutral exciton or free charge, nor the origin of the high open-circuit voltages. As was shown by Lee et al. 6 the mixed halide perovskite is capable of both electron and hole charge conduction, consistent with the excellent performance of evaporated planar heterojunction structures reported by Liu et al. 7. It was also demonstrated by Burschka et al. 8 that the triiodide perovskite is an efficient sensitizer, and can even operate as a hole-conductor sustaining reasonable efficiencies without a hole acceptor layer 11. Twodimensional layer structures with PbIx layers separated by organic ligands have been extensively investigated by Mitzi et al and show luminescent properties with excitonic character.12-16. However, the mixed halide perovskites used in the presented study are three-dimensional semiconductors. A fundamental question is whether the photoexcitation in the perovskite remains as a bound exciton and is later ionised at the electron and/or hole accepting heterojunctions (either distributed or planar) or whether charge generation occurs in the pristine perovskite material. In order to address this and also to study the long-time recombination/luminescent processes we carried out transient photoluminescence and transient absorption (TA) measurements on spin-coated perovskite films spin-cast onto glass substrates and also of working devices comprising mixed halide perovskite on a compact titanium dioxide electron accepting electrode with a hole acceptor layer formed by spin-coating the hole-
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transporter 2,20,7,70-tetrakis(N,N-di-p-methoxyphenyl-amine)9,90-spirobifluorene (spiroMeOTAD). For sample preparation methylamine (CH3NH2) solution 33 wt% in absolute ethanol was reacted with hydroiodic acid (HI) 57 wt % in water with excess methylamine under nitrogen atmosphere in ethanol at room temperature. Typical quantities were 24 mL methylamine, 10 mL hydroiodic acid and 100 mL ethanol. Crystallization of methylammonium iodide (CH3NH3I) was achieved using a rotary evaporator; a white coloured powder was formed indicating successful crystallization. Methylammonium iodide and lead (II) chloride (PbCl2) was dissolved in anhydrous N,N-Dimethylformamide at a 3:1 molar ratio of CH3NH3I to PbCl2, to produce a mixed halide perovskite precursor solution. Thin film samples were prepared by spin-coating followed by annealing on a hotplate at 100ºC for 60 min (full details in SI).
Photoluminescence from these materials is emitted in a relatively narrow band centred at 1.6 eV just below the absorption edge, as is shown in figure S1. Figure 1a shows the photoluminescence quantum efficiency (PLQE) as a function of excitation fluence for a thin perovskite film on glass.
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(a)
60 PLQE (%)
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500 1000 1500 Excitation power (mW/cm²)
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Figure 1. a: Photoluminescence quantum efficiency of CH3NH3PbI3-xClx films on spectrosil, annealed in air, and capped with a thin PMMA layer. Excitation was performed with a cw laser at 2.33eV. Integrated PL intensity was taken from 1.77-1.46 eV, measured in a Labsphere integrating sphere. The increase in PLQE with excitation fluence indicates a competition between bimolecular recombination and monomolecular, trap-assisted charge recombination. The high values shown here demonstrate that at higher fluences the dominant mechanism of recombination is radiative.
The PLQE is relatively low at low fluences (
