Perovskite CH3NH3PbI3–xBrx Single Crystals with Charge-Carrier

Apr 11, 2017 - The long carrier lifetimes in perovskite single crystals have drawn significant attention recently on account of their irreplaceable co...
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Perovskite CH3NH3PbI3−xBrx Single Crystals with Charge-Carrier Lifetimes Exceeding 260 μs Fengying Zhang,†,‡ Bin Yang,‡ Xin Mao,‡ Ruixia Yang,‡ Lei Jiang,‡ Yajuan Li,‡ Jian Xiong,‡ Yang Yang,‡ Rongxing He,*,† Weiqiao Deng,*,‡ and Keli Han*,‡ †

Key Laboratory of Luminescence and Real-Time Analytical chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China ‡ State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China S Supporting Information *

ABSTRACT: The long carrier lifetimes in perovskite single crystals have drawn significant attention recently on account of their irreplaceable contribution to highperformance photovoltaic (PV) devices. Herein, the optical and optoelectronic properties of CH3NH3PbI3 and CH3NH3PbI3−xBrx (with five different contents of Br doped) single crystals were investigated. Notably, a superior carrier lifetime of up to 262 μs was observed in the CH3NH3PbI3−xBrx (I/Br = 10:1 in the precursor) singlecrystal PV device under 1 sun illumination, which is two times longer than that in the CH3NH3PbI3 single crystal. Further study confirmed that the ultralong carrier lifetime was ascribed to the integrated superiority derived from both the low trap-state density and high charge-injection efficiency of the device interface. On this basis, appropriate incorporation of Br is useful in the design of better PV devices. KEYWORDS: carrier lifetime, photovoltaic device, single crystal, trap-state density, charge-injection efficiency

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INTRODUCTION Organolead halide perovskites have undergone rapid development and drawn widespread attention since they were first reported in 2009,1 involving various applications in photovoltaic (PV) cells,2,3 light-emitting diodes,4,5 lasers,6 and photodetectors.7 It is worth mentioning that efficiencies exceeding 22% have been realized in solar cells employing these perovskite materials.8−11 This remarkable efficiency is believed to be closely related to the long carrier lifetime and low trap-state density of perovskite absorbers.12−17 Aiming at further improving the crucial carrier diffusion process, a mixture of multiple halogen perovskites has been developed.18−20 Br-doped perovskite solar cells have been reported to show a better performance than that of CH3NH3PbI3.21−23 However, the mechanism of Br doping is still unclear because all of these studies are based on perovskite thin films, for which it is hard to separate the device performance from morphology. Fortunately, the emergence of single-crystal perovskite helps overcome the limitations of perovskite films, such as plenty of surface defects, voids and boundaries, therefore attracting significant attention.24−27 Furthermore, the intrinsic properties of mixed halogen perovskites can be studied because of the free grain boundaries. Here, we conducted a comprehensive investigation on Brdoped CH3NH3PbI3−xBrx (I/Br = 60:1, 30:1, 10:1, 8:1, and 6:1 in the precursor) single crystals to explore their optical and optoelectronic properties. Br incorporation has a significant © 2017 American Chemical Society

influence on the crystal structure as well as the steady-state absorption and photoluminescence (PL) spectra. The trap-state density, charge-carrier diffusion, and transfer processes were further studied using the space-charge-limited current technique, impedance spectroscopy (IS) system, and Kelvin-probe force microscopy, respectively, and we found that appropriate incorporation of Br (I/Br = 10:1 in the precursor) contributed to lowering the trap-state density and improving the chargecarrier lifetime.

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RESULTS AND DISCUSSION On the basis of the inverse temperature crystallization (ITC) method,27 CH3NH3PbI3 and CH3NH3PbI3−xBrx (I/Br = 60:1, 30:1, 10:1, 8:1, 6:1) single crystals were synthesized. More details on the synthesis methods are provided in the Experimental Section. Photographs of Br-doped crystals are displayed in Figure S1, and they all present a similar morphology to that of CH3NH3PbI3 in Figure 1a. According to the X-ray photoelectron spectroscopy (XPS) results (Figure 1b), peaks at around 68 eV appeared more clearly at an increased Br concentration, indicating that Br was successfully incorporated into the perovskite. The XPS analysis for all elements is illustrated in Figure S2, and the detected intensity Received: February 4, 2017 Accepted: April 11, 2017 Published: April 11, 2017 14827

DOI: 10.1021/acsami.7b01696 ACS Appl. Mater. Interfaces 2017, 9, 14827−14832

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Figure 1. (a) Photographs of the CH3NH3PbI3 crystal synthesized by the ITC method. (b) The signal intensity of bromine in CH3NH3PbI3−xBrx single crystals detected using XPS measurement. (c) Powder X-ray diffraction (XRD) profiles of six as-grown single crystals. (d) Steady-state absorption and PL spectra of ground state CH3NH3PbI3 and CH3NH3PbI3−xBrx (60:1, 30:1, 10:1, 8:1, 6:1) crystals.

Figure 2. (a) Schematic diagram of the PV device. (b) The carrier lifetimes obtained from IS.

The phase purity and crystal structures were characterized by XRD conducted on powder obtained from large crystals (Figure 1c). All patterns showed several characteristic diffraction peaks at 14.1, 28.4, 31.8, 40.5, and 43.1°, corresponding to the (110), (220), (114), (224), and (314) planes, respectively, suggesting similar crystalline structures.27 From the enlarged XRD pattern in the range of 27.7−29.0°, it was observed that the diffraction peaks shifted consistently to higher angles with an increase in the content of Br in CH3NH3PbI3−xBrx crystals, denoting decreased lattice constants. However, the (004) diffraction peak disappeared when the I/Br ratio in the precursor reached as high as 10:1, which was ascribed to the phase transition caused by the large amount of doping.30 Steady-state absorption spectra showed a characteristic absorption peak at about 837 nm, consistent with the literature,24,31 which blue-shifted gradually (Figure 1d),18,32 generating the corresponding band gaps (Eg) of 1.53, 1.54, 1.55, 1.60, 1.61, 1.64 eV with an increase in the Br doping amount. As shown in Figure S5, the significant linear correlation between the Eg value and Br content revealed that Br was added proportionally and is beneficial in enhancing the band gap. Also, a blueshift from 768 to 751 nm appeared in the PL spectra in Figure 1d when the I/Br ratio increased to 6:1. The characteristic peak in pristine CH3NH3PbI3 was also in

of Br showed a good linear relationship with the doping ratio (I/Br) in the precursor. From the quantitative XPS analysis, the surface composition ratios of I and Br in all doped single crystals (I/Br = 60:1, 30:1, 10:1, 8:1, 6:1 in the precursor) were 13.65, 7.85, 3.10, 2.77, and 1.79, respectively. The computed ratios and precursor ratios for I and Br show a good linear relationship (Figure S2). More precisely, we employed the ion chromatography method to determine the molar ratio of I and Br in the final perovskite crystals and identify the exact formula of CH3NH3PbI3−xBrx, which is believed to be a more direct and accurate method compared with XPS analysis.28,29 The concentrations of iodide and bromine anions in the perovskite single crystals were calculated from the calibration curves in Figure S3. On the basis of the results, the final contents of Br in the doped crystals were found to be in proportion to the initial contents in the precursor, which is consistent with the XPS results. The average molar ratios of I and Br after repeat measurements were 20.73, 11.99, 4.86, 3.14, and 2.11 for CH3NH3PbI3−xBrx (I/Br = 60:1, 30:1, 10:1, 8:1, and 6:1 in the precursor) crystals, respectively. Besides, energy-dispersive spectroscopy analysis and energy-dispersive X-ray (EDX) elemental mapping were carried out using a scanning electron microscope to investigate the uniformity of doped Br in the crystals. The results revealed that Br was distributed uniformly throughout the single crystals (Figure S4). 14828

DOI: 10.1021/acsami.7b01696 ACS Appl. Mater. Interfaces 2017, 9, 14827−14832

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Figure 3. PL time decay for all investigated single crystals at λ = 770 nm, showing a fast component and a slower decay by biexponential fits.

Figure 4. Current−voltage traces of all investigated single-crystalline perovskites.

agreement with the that reported in the literature.33 Both the absorption and luminescence results further demonstrated that Br was successfully introduced into the mixed CH3NH3PbI3−xBrx single crystals. Carrier lifetimes of perovskite crystals were investigated using IS on the basis of PV devices in order to simulate their real working conditions under solar irradiation.33 The IS system was operated through a simple equivalent circuit, as described in Figure S6, and this method has been widely used for the study of PV cells, including dye-sensitized solar cells,34 organic solar cells,35 and perovskite solar cells.36 As illustrated in Figure 2a,

Au, with a thickness of 25 nm, was deposited on the surface of the crystals as anodes and gallium (Ga), on the other side as cathodes. On the basis of the IS spectra in Figure S6, the calculated average carrier lifetimes were 136 ± 21, 202 ± 25, 233 ± 26, 262 ± 39, 171 ± 27, and 94 ± 17 μs under 1 sun for CH3NH3PbI3 and CH3NH3PbI3−xBrx, with I/Br ratios of 60:1, 30:1, 10:1, 8:1, and 6:1 in the precursor, respectively, as shown in Figure 2b. Repetitive measurements were carried out on similar PV devices but with different crystals to eliminate errors. Obviously, the carrier lifetime was prolonged on increased Br incorporation initially, but it decreased distinctly with a 14829

DOI: 10.1021/acsami.7b01696 ACS Appl. Mater. Interfaces 2017, 9, 14827−14832

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ACS Applied Materials & Interfaces ηinj ≈ [1 + fg exp(β × ΔE)]−1

continuous increase in the Br amount. Here, CH3NH3PbI3−xBrx with an I/Br ratio of 10:1 in the precursor showed the longest carrier lifetime, as long as 262 μs, and was considered to be a good candidate for PV devices, better than CH3NH3PbI3. To provide a thorough explanation on how Br incorporation affects the carrier transmission process, nanosecond transient PL spectroscopy on the nanosecond scale was used, excluding the influence of interfaces on IS measurements in PV devices. Notably, two time components, including a fast decay (τ1) and a slower component (τ2), were observed, as shown in Figure 3, and they were considered to be closely related to the surface defects and carrier recombination in the bulk, respectively.27 In terms of the surface carrier constants (fast component), there was no significant difference for the six single crystals investigated. However, the bulk carrier constants (slow component) were considered to be sensitive to the bulk trapstate density, which increased with Br incorporation. When the I/Br ratio in the precursor reached up to 10:1, a maximum value of 914 ± 93 ns was obtained by fitting. Subsequently, there was some decrease in the values of τ2 as the Br content continued to increase. Furthermore, trap-state densities were estimated by dark current measurements in single-crystal hole-only devices, in which single crystals were sandwiched between two layers of 25 nm thick gold (Au) electrodes realized by thermal evaporation. In Figure 4, a kink was observed in the dark I−V curve and divided the curve into three parts: the region of an ohmic response, wherein current increases linearly with voltage; the trap-filled limit voltage (VTFL) zone, corresponding to the kink point; and the child area, wherein there is a quick nonlinear increase in current. Here, VTFL is determined from trap-state densities, implying the trap states are fully filled with injected carriers25,33 VTFL =

where fg and β are constants and ΔE = EV − EC is defined as the energy gap between the valence band of perovskite and the work function of the metal.38 The local surface potentials of all as-grown crystals were characterized thoroughly to clarify the effect of Br incorporation on the energy levels by using Kelvinprobe force microscopy (Figure S7). The results revealed that the surface potentials of CH3NH3PbI3 and CH3NH3PbI3−xBrx (I/Br = 60:1, 30:1, 10:1, 8:1, and 6:1 in the precursor) were −171.49, −146.72, 251.22, 207.85, 270.51, and 279.08 mV, respectively. In the range of allowable error, the impact of trace amounts of bromine doping on the surface potential was very small, but it became larger when the proportion of bromine doping increased. From the work-function results obtained from photoelectron spectroscopy in air (Figure S8), valence band maximum (VBM) values of −5.41, −5.41, −5.42, −5.48, −5.49, and −5.51 eV for I, 60:1, 30:1, 10:1, 8:1, and 6:1 were obtained, and conduction band minimum (CBM) values of −3.88, −3.87, −3.87, −3.88, −3.88, and −3.87 eV were obtained from integrating both evaluated valence bands and band gaps. These values show that the VBM decreased with increasing Br concentration, but changes in the CBM were tiny. According to eq 2, we simulated the trend of electronic injection rates based on assumptive constants, as provided in Figure S9. There is a significantly increased injection efficiency when the energy gap is low; however, it is almost unchanged on further increasing the energy gap. In terms of the decreased valence band value after doping with bromine, there would be a better electronic injection rate between Au and the valence band of the perovskite so as to decrease the free carrier recombination rate. However, it is inapplicable to further reduce the valence band. A schematic overview for expounding charge transfer in two interfaces (perovskite/Au and perovskite/Ga) of the PV device is provided in Figure 5. Both the decreased bulk trap-state

entrapsL2 2ε0ε

(2)

(1)

where e is the electronic charge, L is the thickness of the single crystal, and ε0 and ε represent the vacuum permittivity and dielectric constant of the crystal, respectively. From eq 1, trapstate densities of (3.7 ± 0.4) × 1010, (3.1 ± 0.3) × 1010, (2.4 ± 0.3) × 1010, (1.9 ± 0.3) × 1010, (5.3 ± 0.3) × 1010, and (6.8 ± 0.4) × 1010 cm−3 were obtained for CH3NH3PbI3 without Br and with different doped amounts Br (I/Br = 60:1, 30:1, 10:1, 8:1, 6:1 in the precursor). Obviously, the trap-state density decreased with an increase in Br concentration initially, implying that the introduction of a trace amount of Br is beneficial for improving the crystalline structure. However, it started to increase when the I/Br ratio in the precursor exceeded 10:1, which may be attributed to breakage of the crystalline structure caused by excess Br. The lower trap-state densities facilitate repressing the carries recombination during the carrier diffusion process, therefore improving the carrier lifetime. Importantly, the trend of trap-state density greatly explains the law of PL decay. Different from a single crystal, the PV device has the more complex carrier-transfer process due to the effect of two metal interfaces (Au and Ga), giving rise to the extra interface charge transfer. Here, the interface charge transfer of the PV devices was evaluated indirectly from the electronic injection rate using a simplified equation, as follows (the detailed derivation process is presented in the Supporting Information)37

Figure 5. Simulated diagram of charge transfer in two interfaces of the PV device: perovskite/Au and perovskite/Ga.

density and improved charge-injection efficiency promoted carrier transport and extended the carrier lifetime at a low Br concentration. With a continuous increase in the ratio of Br incorporated, the charge-injection efficiency increased slowly, and as a result, the increased trap-state density dominated the carrier lifetime and had an adverse effect on carrier transport. 14830

DOI: 10.1021/acsami.7b01696 ACS Appl. Mater. Interfaces 2017, 9, 14827−14832

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two electrodes on opposite sides of the sample, which was kept in the dark. Surface potential measurements were characterized using a Veeco MultiMode 3D AFM, and the pinpoint is SCM-PIT Pt/Ircoated tips (2.8 N/m, 75 kHz, Pt/Ir reflective coating). It is a multifunction measuring instrument produced by Bruker (formerly Veeco company).

CONCLUSIONS In summary, th e int rinsic propert ies of hybrid CH3NH3PbI3−xBrx single crystals with trace amounts of bromine were explored, eliminating the effect of the morphology of the film. Importantly, their carrier lifetimes were investigated in detail and a high carrier lifetime of 262 μs under 1 sun illumination w as achieved in the CH3NH3PbI3−xBrx (I/Br = 10:1 in precursor) PV device, which was mainly ascribed to its reduced trap-state density and charge recombination as well as efficient electronic injection.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01696. Details of the theoretical derivation, synthetic methods, measurements, and characterization for all as-grown single crystals and supporting graphs and results referred to in the article (PDF)

EXPERIMENTAL SECTION

Material Synthesis. Methylamine (33 wt % in absolute ethanol), lead iodide (PbI2, 99%), and lead bromide (PbBr2) were purchased from Sigma Aldrich. Hydroiodic acid (HI, 57 wt % in water) was purchased from Alfa Aesar. All compounds were used without any further purification. CH3NH3I was synthesized by stirring 20 mL of hydroiodic acid and 25 mL of methylamine in a flask at 0 °C for 2 h under an argon atmosphere in an icewater bath. The original product, CH3NH3I, was obtained once the solvent (ethanol and water) was removed using a rotary evaporator at 55 °C. The crude white powder obtained was washed with isopropanol and finally recrystallized from the mixed solvent of ethanol and diethyl ether. The filtered product was dried at 60 °C in a vacuum oven overnight for later use. All of the single crystals (CH3NH3PbI3 and CH3NH3PbI3−xBrx) were grown using the ITC method proposed by Saidaminov et al.27 Lead iodide (4.61 g) and CH3NH3I (1.59 g) were dissolved in 10 mL of γ-butyrolactone (GBL) at 60 °C. The solution was heated up to 110 °C to obtain abundant seed crystals after filtration through a nylon filter with a 0.22 μm pore size. The large-sized CH3NH3PbI3 crystal was obtained from seed regrowth in a fresh 1 M precursor solution. Similarly, five Br-doped CH3NH3PbIxBr3−x single crystals were synthesized through the same procedure except for the different ratios of PbI2 and PbBr2 used. For instance, a single crystal of CH3NH3PbIxBr3−x (10:1) was prepared by dissolving 4.61 g of PbI2, 1.59 g of CH3NH3I, and 0.55 g of PbBr2 in 10 mL of GBL. Measurement and Characterization. XPS measurements were performed using Thermo ESCALAB 250Xi, and the excitation source was monochromatized AlK. For ion chromatography measurements, the perovskite single crystal was dissolved in an acidic aqueous medium and detected using a Dionex ISC 100 ion analyzer. Both iodine ion and bromide ion standard solutions were used to obtain the calibration curves. Scanning electron microscopy and EDX spectroscopy elemental mapping were performed using a field-emission scanning electron microscope (FESEM, 3 kV; JSM-7800F; JEOL). XRD was carried out on an X’pert PRO diffractometer equipped with Cu Kα X-ray (λ = 1.54186 Å) tubes using fine powders of crystals. UV−vis diffuse reflectance spectra were recorded at room temperature on a JASCO V-550 UV−vis absorption spectrometer with an integrating sphere attachment operating in the 190−900 nm region. PL spectra were recorded by laser scanning confocal microscopy (FV1000MPE; Olympus). IS spectra were recorded on VersaSTAT 3 (Princeton Applied Research) under 1 sun illumination, and the simulative sunlight was produced by Abet Technologies Sun 2000. The curve was fitted according to the equivalent circuit presented in the Supporting Information for our PV devices at different voltage and light biases. Rinternal and Cgeometry are related to the bulk properties, and Rrec and Cμ are associated with the internal-charge-transfer dynamics. The recombination lifetime is the reciprocal of the product of Rrec and Cμ. Rseries represents series contact resistances from the connection between the crystal and electrode, the PV device and LCR meter. PL decay measurements were performed using a homemade transient PL setup, and the sample (maintained in a dark environment) was excited using an ultraviolet laser source (λ = 355 nm) with a short pulse of ∼10−9 s. The time-dependent PL signal was recorded using timeresolved spectroscopy. Current, as a function of the applied voltage, was measured using Keithley 2400, with a rather simple geometry and

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.H.). *E-mail: [email protected] (W.D.). *E-mail: [email protected] (K.H.). ORCID

Rongxing He: 0000-0003-3100-2722 Weiqiao Deng: 0000-0002-3671-5951 Keli Han: 0000-0001-9239-1827 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge Xujun Liu and Jinwen Hu for help with recording the PL spectra. Funding was received from all funding sources, including grants and funding agencies. We are grateful to the National Basic Research Program of China (2013CB834604) and the National Natural Science Foundation of China (Grant nos: 21533010, 21321091, 21525315, 91333116, and 21173169) for their financial support.

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b01696 ACS Appl. Mater. Interfaces 2017, 9, 14827−14832