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Charge Carrier Lifetimes Exceeding 15 Microseconds in Methylammonium Lead Iodide Single Crystals Yu Bi, Eline M. Hutter, Yanjun Fang, Qingfeng Dong, Jinsong Huang, and Tom J. Savenije J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00269 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016
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The Journal of Physical Chemistry Letters
Charge Carrier Lifetimes Exceeding 15 Microseconds in Methylammonium Lead Iodide Single Crystals Yu Bi1, Eline M. Hutter1, Yanjun Fang2, Qingfeng Dong2, Jinsong Huang2*, Tom J. Savenije1* 1. Opto-electronic Materials Section, Department of Chemical Engineering, Delft University of Technology, 2628 BL Delft, The Netherlands. 2. Department of Mechanical and Materials Engineering and Nebraska center for Materials and Nanoscience, University of Nebraska-Lincoln, Nebraska 68588-0656, USA. *
Corresponding authors: Jinsong Huang:
[email protected], Tom J. Savenije:
[email protected] ACS Paragon Plus Environment
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ABSTRACT The charge carrier lifetime in organic-inorganic perovskites is one of the most important parameters for modeling and design of solar cells and other types of devices. In this work, we use CH3NH3PbI3 single crystal as a model system to study optical absorption, charge carrier generation and recombination lifetimes. We show that commonly applied photoluminescence (PL) lifetime measurements may dramatically underestimate the intrinsic carrier lifetime in CH3NH3PbI3, which could be due to severe charge recombination at the crystal surface and/or fast electron-hole recombination close to the surface. By using the time-resolved microwave conductivity (TRMC) technique, we investigated the lifetime of free mobile charges inside the crystals. Most importantly, we find that for homogeneous excitation throughout the crystal, the charge carrier lifetime exceeds 15 μs. This means that the diffusion length in CH3NH3PbI3 can be as large as 50 µm if it is no longer limited by the dimensions of the crystallites.
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The Journal of Physical Chemistry Letters
Organic-inorganic metal halide perovskites (OMHPs) are intensively investigated since these materials can be used as photoactive layer in solar cells reaching efficiencies up to 20%.1–11 Although there are many studies focusing on the material preparation, assembly and characterization of OMHP solar cells, there is limited information on the generation of charges by photo-excitation and collection of charges by the electrodes. The relationship between the structure of the material, presence of dopants or grain boundaries and the charge carrier dynamics is unclear. In particular, it is unknown to what extent the grain boundaries affect the charge carrier lifetime. This carrier lifetime determines the charge carrier diffusion length (LD), which delineates the optimal thickness of the perovskite photoactive layer.10,12 In previous studies on the diffusion length in perovskites, reported LD values were limited by the dimensions of the thin films.13–15 It has been shown that the material surface and grain boundaries impose a limitation to LD in the case that the charge recombination inside the grains is negligible.12,16 Direct visualization of the carrier density by transient absorption microscopy revealed a 2-8 folds longer lifetime in CH3NH3PbI3 (MAPbI3) polycrystalline films than in thin films with smaller crystallites, and underscored the importance of material morphology in controlling the charge transport in (polycrystalline) thin films.17,18 Recently, synthetic procedures have been developed to prepare large OMHP single crystals with sizes in excess of several millimeters, and in some case several centimeters.19–23 These crystals form an ideal platform for investigating intrinsic material properties such as the charge carrier generation yield and lifetime. Initial studies on the charge carrier lifetimes in MAPbI3 and MAPbBr3 single crystals show a large variation, which might be attributed to the different measuring techniques. However, reported values for single crystals were all longer than those measured in thin films.23–25 Additionally, electronic measurements, including transient photovoltaic and impedance spectroscopy, gave substantially longer carrier lifetimes (> 100 μs) than optical measurements such as time resolved photoluminescence (TRPL) (
