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Trap States and Their Dynamics in Organometal Halide Perovskite Nanoparticles and Bulk Crystals Kaibo Zheng, Karel Zidek, Mohamed Abdellah, Maria E Messing, Mohammed J. Al-Marri, and Tönu Pullerits J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00612 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016
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The Journal of Physical Chemistry
Trap States and Their Dynamics in Organometal Halide Perovskite Nanoparticles and Bulk Crystals
Kaibo Zheng,†,‡, * Karel Žídek,† Mohamed Abdellah,†,﹟ Maria E Messing,§ Mohammed J. Al-Marri,‡ Tõnu Pullerits†,*
†
Department of Chemical Physics, Lund University, Box 124, 22100, Lund, Sweden
‡ Gas Processing Center, College of Engineering, Qatar University, PO Box 2713, Doha, Qatar §
Department of Solid State Physics, Lund University, Box 118, 22100, Lund, Sweden
﹟
Department of Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt
Abstract Organometal halide perovskites have attracted tremendous attention for opto-electronic applications. Charge carrier trapping is one of the dominant processes often deteriorating performance of devices. Here, we investigate the details of trap behavior in colloidal nanoparticles (NP) of CH3NH3PbBr3 perovskites with mean size of 8 nm and the corresponding bulk crystals (BC). We use excitation intensity dependence of photoluminescence (PL) dynamics together with comprehensive simulation of charge carrier trapping and the trap-state dynamics. In bulk at very low excitation intensities the PL is quenched by trapping. A considerable fraction of the traps becomes filled if excitation fluence is increased. We identified two different traps, one exhibiting ultra-long lifetime (~70 µs) which leads to efficient accumulation of trap filling even at relatively low excitation intensities. In colloidal NPs, the average number of the surface traps is estimated to be 0.7 per NP. It means about 30% excitation would undergo trap-free radiative recombination. The trapping time-constant of 7 ns is orders of magnitude longer than the usual trapping times in typical colloidal quantum dots indicating semi-passivation of the trap states by a large barrier which slows down the
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process in the perovskite NPs. We also note that due to the localized character of photogenerated electron-hole pairs in NPs the trapping efficiency is reduced compared to the freely moving charges in BCs. Our results offer insight into the details of photophysics of colloidal perovskite nanoparticles which show promise for light emitting diode and laser applications.
1. Introduction: Organometal halide perovskites have recently been pointed out as promising materials for low-cost high efficiency opto-electronic devices since their strong absorption, rapid charge generation, slow recombination rate and high carrier mobility are advantageous for solar cells. 1–13 However, their application in emitting devices such as light emitting diodes (LED) and lasers is restricted by the low photon emission efficiency.14–16 Recently, synthesis of colloidal organometal halide perovskite nanoparticles (NPs) (CH3NH3(MA)PbIxBr3-X, CsPbIxBr3-X etc) has been reported with quantum confinement and superior photoluminescence quantum yield (QY) (~80%) compared to the bulk materials. 17
Our previous research reveals that one reason for such enhancement is the larger exciton
binding energy (Eb) in NPs compared with bulk crystals (BCs).18 The high Eb leads to the domination of Wannier-Mott excitons. Apart from Eb, the traps are essential for the emission dynamics. Existence of both surface and bulk traps in perovksite materials is widely reported.19–23 The surface traps are induced by surface dangling bonds or unsaturated atoms while the traps in the bulk volume can be generated by the vacancies or interstitials (e.g. I- ions).19,20 The trap states in perovskites can also be tuned by the morphology and geometry of materials.24 The trap passivation can be expected in perovskite NPs since the same organic capping agent (oleic acid) as conventional semiconductor NPs (e.g CdSe, PbS QDs) is used.25–29 This can strongly influence the emission properties of the NPs. 2
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The main goal of the current paper is to understand the details of how trapping influences the PL of perovskite materials and confirm that the enhanced PL in perovskite NPs can be related to the surface passivation. We compare the PL kinetics of MAPbBr3 perovksite NPs and BCs under different excitation intensities and model the balance between emitting excitons, charge carriers and the traps in the two samples. We show that in BCs two types of trap-mediated non-radiative recombinations dominate under low excitation intensity. One major type of trap states exhibits an ultra-long lifetime which gives rise to the accumulation of filled traps while the other is related to surface defects with much shorter lifetime. In NPs the density of trap states is less than one per NP, and ultra-long lived (up to microsecond) with slow trapping. Consequently the overall contribution of trap-mediated non-radiative recombination is negligible in the PL kinetics. Moreover, the localization of photogenerated electron-hole pairs in individual NPs reduces the possibility of trapping compared with free charges in BCs. These differences in emission dynamics explain the enhanced PL in perovskite NPs and point out the potential of such materials in emitting devices.
2. Experimental Section: Sample synthesis: CH3NH3PbBr3 (MAPbBr3) colloidal nanoparticles were synthesized by the hot injection methods analogous to the previous report.28 In brief, a solution of 1.5 mmol oleic acid in 10 ml of octadecene was first heated up to 80
o
C followed by addition of 0.3 mmol of
octadecylammonium bromide (OTABr). 0.2 mmol methylammonium bromide (MABr) dissolved in 0.5 ml DMF and 0.5 mmol lead(II) bromide dissolved in 0.5 ml DMF was subsequently added into the solution yielding a yellow dispersion of NPs, which were then precipitated by acetone. The NPs were finally dispersed in toluene. Here the ratio between OTABr and MABr is 6:4, which is the optimized value to obtain pure MAPbBr3 NPs. 28 In order to synthesize MAPbBr3 bulk crystals film, MAPbBr3 3
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precursor solution in DMF was first spin coated on glass substrate (2000 rpm, 30 sec), which was then annealed in atmosphere at 85 oC for half an hour. The crystal size was tuned by controlling the concentration of MAPbBr3 precursor solution. Typical concentrations of 50 mM, 10 mM, 5 mM and 1 mM produce crystals with mean sizes of 6 µm, 4 µm, 2 µm and 0.4 µm, respectively. For the details of synthesis of MABr, OTABr, and MAPbBr3 precursors see SI1. Steady-state spectroscopy: Ground-state absorption spectra were measured in a UV–Vis absorption spectrophotometer (PerkinElmer, Lambda 1050) equipped with integrated sphere to exclude signal due to light scattering. Steady-state photoluminescence was measured using a standard spectrometer (Horiba, Spex 1681) with excitation at 410 nm. Time-resolved photoluminescence: In the time-correlated single-photon counting (TCSPC) device (PicoQuant) a pulsed diode laser with 100k - 2.5 MHz repetition rate was used to excite the sample at 438 nm. Correspondingly, the time between pulses varied from 10 µs to 400 ns. Long-pass filter from 520 nm was used to pick up only the band-edge emission. The emitted photons were focused onto a fast avalanche photodiode (Micro Photon Device, SPAD). The response time of the photodiode is > 100 nm (for details of sample preparation and characterization see S1&S2). The steady-state absorption spectra of both samples are typical for MAPbBr3 with absorption edge close to 530 nm as shown in Fig. 1. Slightly blue-shifted absorption edge and emission spectrum of NPs compared with BCs indicate a limited quantum confinement effect since the size of NPs is still larger than the Bohr radius of MAPbBr3 (2 nm).29,30 The extra absorption features at lower wavelength (460 nm ~ 475 nm) of NP sample are attributed to minor contribution from 2D perovskite nanoplatelets.
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Our previous work shows that the influence of such
nanoplatelets on the emission dynamics is negligible.18 Here we will only focus on the main NPs phase. We also note that in our NPs no strongly red-shifted defect emission band is observed. Such band is common in conventional semiconductor quantum dots.34–36
Fig. 1 Steady-state UV-Vis absorption spectra and photoluminescence spectra of the NPs and BCs.
3.2 Time-resolved photoluminescence kinetics. The surface to volume ratio in nano- and micrometer objects strongly depends on the size. In this way, by changing the size of the crystals and following excitation dynamics, one may be able to distinguish between the surface and bulk traps. 37 Fig 2. shows the PL kinetics of MAPbBr3 BCs with 4 different sizes from 400 nm to ~ 6.2 µm in diameter as well as NPs at various excitation intensities 5
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measured by TCSPC. The NPs have significantly longer (> 100 ns) PL lifetime than BCs (< 100 ns). In colloidal semiconductor QDs (e.g. CdSe QDs) such longer excited states lifetimes can be attributed to the forbidden low energy transition prolonging the radiative lifetime. 26 In perovskite NPs no such dark state has been identified. Also the excitation intensity dependence of PL kinetics is significantly different for BCs and NPs. In BCs the PL lifetimes increase at higher excitation fluence, and this trend becomes more pronounced for smaller crystals. In NPs the lifetime increase is not so pronounced. In general, the charge recombination dynamics in MAPbX3 materials exhibits a combination of trap-mediated monomolecular recombination, bimolecular recombination and Auger recombination. According to the previous studies and our own results, the excitation density used in our TCSPC experiments (≤ 1015 cm-3, for details of excitation density calculation see S3) corresponds to the monomolecular recombination regime.
18,38
We can fit the PL kinetics by tri-exponential decays with one fast
component (0.5-1 ns), a middle component (4-7 ns), and a slow component (>10 ns) (details of fitting parameters see S4). We also proved that the tri-exponential model provide the best fitting compared with other kinetics models such stretched exponential decays, which is commonly used for disordered system with distribution of the rate constants (details see SI4). 39 This also indicates that PL kinetics in our samples is not likely to be affected by the inhomogeneity of crystals (i.e. size or morphologies) which is reported in MAPbI3 films.40 In equations we will denote the components by indexes 1 (fast), 2 (middle), and 3 (slow). The slow component can be attributed to the intrinsic trap free charge recombination,39 which is almost negligible here (
