High Resolution Mapping of Two-Photon Excited Photocurrent in

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

High Resolution Mapping of Two-Photon Excited Photocurrent in Perovskite Micro-plate Photodetector Bin Yang, Junsheng Chen, Qi Shi, Zhengjun Wang, Marina Gerhard, Alexander Dobrovolsky, Ivan G. Scheblykin, Khadga Jung Karki, Keli Han, and Tõnu Pullerits J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02250 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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The Journal of Physical Chemistry Letters

High Resolution Mapping of Two-Photon Excited Photocurrent in Perovskite Micro-plate Photodetector Bin Yang†‡#, Junsheng Chen†§#, Qi Shi§, Zhengjun Wang§, Marina Gerhard§, Alexander Dobrovolsky§, Ivan G. Scheblykin§, Khadga Jung Karki§*, Keli Han†*, Tõnu Pullerits§* †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics

(DICP), Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, China. ‡

University of the Chinese Academy of sciences, Beijing 100039, P. R. China.

§

Department of Chemical Physics and NanoLund, Chemical Center, Lund University, P.O. Box

124, 22100 Lund, Sweden. AUTHOR INFORMATION Corresponding Author Khadga Jung Karki ([email protected]) Keli Han ([email protected]); Tõnu Pullerits ([email protected])

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ABSTRACT: We fabricate photodetectors based on solution-processed single CH3NH3PbBr3 micro-crystals (MCs) and map the two-photon absorption (TPA) excited photocurrent (PC) with spatial resolution of 1 µm. We find that the charge carrier transport length in the MCs depends on the applied electric field, which can be increased from 5.7 µm (with 0.02 V bias, dominated by carrier diffusion) to 23.2 µm (with 2 V bias dominated by carrier drift). Furthermore, PC shows strong spatial variations. Combining the PC mapping results with time-resolved photoluminescence microscopy, we demonstrate that the spatial distribution of PC mainly originates from the inhomogeneous distribution of trap-states across perovskite MCs. This suggests that there is still large margin for improvement of perovskite single crystal devices by better controlling of the traps.

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Organometal halide perovskites (OHPs) with outstanding optoelectronic properties and cheap synthesis methods have received broad attention.1-22 The remarkable performance of perovskite-based optoelectronic devices is closely related to the long charge carrier diffusion length.2-4 Numerous studies, based on perovskite polycrystalline thin films or centimeter-sized single crystals, have reported largely diverging values of the high diffusion length and suggested its various possible origins.5-9 Consequently, intrinsic charge carrier recombination and transport characteristics still remain elusive. Thin films suffer from large differences between the grains and the grain boundaries.10 Centimeter-sized single crystals on the other hand are far larger than the charge carrier diffusion length (µm) and the photocarriers cannot be extracted efficiently.4 In this respect, individual perovskite nano- or micro-structures with the size comparable to the carrier diffusion length and without contact with other crystals are preferable to study the fundamentals of the charge carrier recombination and transport processes.11-15 The charge carrier diffusion in perovskite single-crystal nanostructures has been earlier monitored by using either scanning photoluminescence (PL) microscopy16 or scanning PC microscopy (SPCM).17 Still, the carrier distribution and transport mechanism in devices with electric field applied are complex and not fully understood.23-27 All previous studies of CH3NH3(MA)PbX3 (X=Cl, Br, I) perovskite single crystal photodetector are based on one-photon absorption,18-21 in which the light beam can only penetrate the surface regions and the crystal is not excited homogeneously. Since trap-states have significant influence on the PL properties in perovskite28, an interesting question rises: is the trap-state distribution homogeneous within the perovskite single crystal and how do the trapstates affect the optoelectronic properties in the devices. These fundamental issues need to be clarified to further improve the performance of the perovskite-based optoelectronic devices.

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In this study, single CH3NH3(MA)PbBr3 micro-crystals (MCs) are used as an active material for sensitive photodetectors. MAPbBr3 is much more stable than the iodine (I) counterpart and it is easy to grow. We use SPCM based on two-photon absorption (TPA) to study the PC mapping in single MAPbBr3 MCs, in which the near-infrared light can penetrate the whole crystal.29,30 It is found that the hole transport length (LD) in the MCs depends on the applied electric field. Under negligible bias, LD is determined by charge carrier diffusion (LD = 5.7 µm at a bias of 0.02 V). At higher bias, LD increases and is mainly determined by the drift of the carriers (LD = 12.3 µm at 1 V, LD = 23.2 µm at 2 V). Importantly, we found that PC exhibits a spatial variation, which is mainly due to the inhomogeneous distribution of trap-states in the MCs. MAPbBr3 MCs were synthesized by a modified solution inverse temperature crystallization method, 31,32 see Supporting Information (SI) for the details. Scanning electron microscope (SEM) image shows the MCs with length scales of a few tens of micrometers (Figure S2) and with thickness of about 2-5µm. X-ray diffraction (XRD) patterns prove the phase purity and high crystallinity of the MCs (Figure S3a). The absorption spectrum of the MAPbBr3 MCs grinded to powder is shown in Figure S3b. Due to the possible distortion of the absorption edge by combination of high optical density of the individual grains together with incomplete sample coverage of the substrate,33 we used one-photon excited PL spectrum maximum to estimate the bandgap of about 2.254 eV Figure S3b). These results are in good agreement with the earlier reports for MAPbBr3 single crystals.31,32,34

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Figure 1. (a) Schematics of the MC based photodetector and the measurement arrangement. (b) High resolution SPCM scheme. Glass substrates with coated ITO contacts (with 5 µm channel width and 1000 µm length) were used to fabricate the perovskite MC based photodetectors. After the fast crystallization and the annealing process, the 5 µm ITO gap was covered by several MCs (Figure S4). Figure 1a illustrates the measurement arrangement of the single MC based photodetector, in which SEM image (up right) show a single MAPbBr3 MC, which lies on top of the 5 µm gap between two ITO electrodes. The MC displays well defined facets with smooth and clean surfaces. Figure 1b illustrates the high resolution photocurrent mapping system. A laser beam (Ti:Sapphire oscillator Synergy, Femtolasers with 780 nm center wavelength and 80 MHz repetition rate) was focused (spot size about 1 µm, see Figure S5) onto a single MC (details of the instrument are provided in SI).35 The ITO-MAPbBr3 interface is expected to produce a rectifying junction,36 which enables low dark current (< 10 nA under 3 V bias), as shown in Figure 2a. Under laser illumination, the current increases significantly due to the photoinduced charge carriers (Figure 2a).

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Figure 2. (a) I-V curves in the dark and with laser excitation light density from 0.2 to 1.2 mJ/cm2/pulse at 780 nm. (b) Excitation light density dependent PC on a log-log scale. (c) Excitation light density dependent responsivity under V = 2.5 V bias. (d) Frequency-dependent PC under V = 2.5 V bias. Since the excitation wavelength at 780 nm is well below the band gap of MAPbBr3 MCs, the photoinduced charge carriers should mostly originate from the TPA process.29 Ideally, the TPA generated PC should show a quadratic dependence on the excitation fluence I=kφ2, where

φ is the excitation fluence, I is the PC, and k is the two-photon current coefficient, which is related to the TPA coefficient β through the relation k =

βd , with the effective space charge 2hω

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layer width d and the photon energy hω .37 Since in our experiment TPA is not the only excitation channel, the quadratic dependence needs to be modified as

I = kφ n (1) where n is a fitting parameter. The PC dependence on excitation fluence is shown on a log-log scale in Figure 2b. The slope increases from n

High Resolution Mapping of Two-Photon Excited Photocurrent in

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