Temperature-dependent electronic properties of inorganic-organic hybrid halide perovskite (CH3NH3PbBr3) single crystal Xiaolei Cui, Sijian Yuan, Huotian Zhang, Xin Zhang, Pengfei Wang, Li Tu, Zhengyi Sun, Jiao Wang, Yiqiang Zhan, and Lirong Zheng
Citation: Appl. Phys. Lett. 111, 233302 (2017); View online: https://doi.org/10.1063/1.5005005 View Table of Contents: http://aip.scitation.org/toc/apl/111/23 Published by the American Institute of Physics
APPLIED PHYSICS LETTERS 111, 233302 (2017)
Temperature-dependent electronic properties of inorganic-organic hybrid halide perovskite (CH3NH3PbBr3) single crystal Xiaolei Cui,1 Sijian Yuan,1 Huotian Zhang,1 Xin Zhang,2 Pengfei Wang,1 Li Tu,1 Zhengyi Sun,2 Jiao Wang,1 Yiqiang Zhan,1,a) and Lirong Zheng1,3 1
State Key Laboratory of ASIC and System, SIST, Fudan University, Shanghai 200433, China Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Centre for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211800, China 3 Royal Inst Technol KTH, iPack VINN Excellence Ctr, S-16440 Stockholm, Sweden 2
(Received 15 September 2017; accepted 15 November 2017; published online 5 December 2017) In this paper, the temperature-dependent electronic properties of inorganic-organic hybrid halide perovskite (CH3NH3PbBr3) single crystals are investigated. The dynamic current-time measurement results at different temperatures directly demonstrate that the electrical properties of the perovskite single crystal are dependent on the work temperature. We find that the Poole-Frankel conduction mechanism fits the current-voltage curves at small bias voltage (0–1 V) under darkness, which is mainly attributed to the surface defect states. The capability of carriers de-trapping from defects varies with different work temperatures, resulting in an increased current as the temperature increases under both darkness and illumination. In addition, the different transient photocurrent responses of incident light at two wavelengths (470 nm, 550 nm) further confirm the existence of defect states on the single crystal surface. Published by AIP Publishing. https://doi.org/10.1063/1.5005005 Inorganic-organic hybrid halide perovskites, CH3NH3PbX3 (e.g., MAPbBr3), have attracted considerable attention due to their superb optoelectronic properties, such as low trapping density,1 broadband light absorption,2 and long diffusion length.3 Thereupon, the optical and electrical devices based on perovskites have been investigated and demonstrated with remarkable performances, such as solar cells,4,5 light-emitting diodes (LEDs),6,7 photodetectors,8,9 field effect transistors (FETs),10,11 and so on. The power conversion efficiency of perovskite solar cells has increased from 4%4 to over 20%5 in the past few years. To date, the applications of CH3NH3PbX3 as stated above are mainly based on polycrystalline thin films, while the dramatical influences of the grain boundaries and defect densities on the optoelectronic performances are considerable. For example, the trap states can lead to the phenomenon of hysteresis which reduces FET performance.10 Compared to their polycrystalline film counterpart with small grain sizes, CH3NH3PbX3 single crystals are now considered to be the more suitable alternatives for future optoelectronic applications because of their superior intrinsic properties, for instance, lower trapstate densities and longer carrier diffusion length.12,13 Either for the polycrystalline film or for the single crystal, it is very important to understand the influence of temperature, which not only is of fundamental interest to figure out the mechanism but also aims to identify the practical applications of the devices based on these perovskites.14 Both the annealing temperature during the formation of the crystal and the ambient temperature varying after the formation of devices have a great influence on the optoelectronic properties of finial perovskite devices.15–19 The former is closely related to the solution-process crystallite size, and a)
Author to whom correspondence should be addressed:
[email protected]
0003-6951/2017/111(23)/233302/5/$30.00
great efforts have been made to enlarge the grains to reduce the trap densities, the non-radiative recombination centers, and, consequently, the high charge-carrier mobility and improved performance.15,16 The latest work demonstrates that the optimal annealing temperature is 60 6 5 C for MAPbBr3 because the single crystal possesses a highly symmetrical relaxed cubic crystal structure at this temperature.17 With regard to the ambient temperature, it could give rise to the band-gap modulation, structural phase transition, charge-carrier diffusion, length change, etc., which exerts an influence on the optoelectronic properties of devices.14,18,19 A crystalline material is viewed as the ideal platform for analyzing the intrinsic properties like electronic features.20 Up to now, the fundamental studies for the single crystal mainly focus on the crystal-growth mechanism and methods linked to its quality as well as the surface and bulk optical properties.1,12,13,17,20–23 Although previous studies have reported the dark current-voltage characteristic associated with the trap density21 and the difference between the photocurrent and dark current in air and vacuum,24 a fundamental understanding of ambient temperature-dependent electronic features in connection with traps is rarely involved. Our previous work has revealed the existence of defect states on the MAPbBr3 single crystal surface through experiments and theoretical calculations.25 In this article, we investigate the electronic properties of the MAPbBr3 single crystal within the specific temperature range without phase transition in air. The dynamic current-time (I-t) measurement results directly show that both the dark current and the photocurrent under the incident light with wavelengths of 470 and 550 nm are positively correlated with the air temperature variation. The trap-limited conduction (Poole-Frankel, P-F) mechanism is applied to explain these phenomena due to its good fitting of the current-voltage (I-V) curves at small bias voltage (0–1 V)
111, 233302-1
Published by AIP Publishing.
233302-2
Cui et al.
FIG. 1. (a) Device structure; the inset shows an image of an MAPbBr3 single crystal; (b) scanning electron microscopy (SEM) top surface image of a single crystal; steady-state photoluminescence (PL) of crystals (c) without polymethyl methacrylate (PMMA) and (d) with PMMA. The peak positions are fitted by the Gaussian method.
in the dark, and the temperature changes the capability of carriers de-trapping from defect states. The different transient photocurrent responses of photons with two wavelengths further verify the hypothesis that trap states mainly exist on the surface of the single crystal. As shown in the inset of Fig. 1(a), the MAPbBr3 single crystals can be prepared from solution by the modified inverse temperature crystallization (ITC) method.21 The structural phase of the crystal is the cubic Pm3m space group at room temperature and does not transform when the temperature is greater than 236.9 K.26 Figure 1(a) displays the device with symmetrical Au electrodes, between which the channel is coated with the polymethyl methacrylate (PMMA) to avoid the degradation of the single crystal surface during the measurement because of the environmental sensitivity properties
Appl. Phys. Lett. 111, 233302 (2017)
of perovskites.24,27 Figure 1(b) presents the SEM top surface image of a single crystal. The tetragonal shapes in parallel indicate that the seed crystal has good uniformity and the defects exist on the surface. Figures 1(c) and 1(d) show the evolution of steady-state photoluminescence (PL) spectra of samples without and with PMMA in the temperature range of 30 C to 50 C, respectively. Gaussian fitting peak results indicate that the PL spectra are centered at about 544 nm for the former and 543 nm for the latter, which turns out that PMMA coating has no evident influence on MAPbBr3 crystals’ bandgap in this condition. It is worth noting that there is a blue-shift of the center emission wavelength with increasing temperature in a prevenient study of the MAPbBr3 polycrystalline film,18 but our result is not consistent with this result. These phenomena can be attributed to the difference in laser fluence and wavelength28 as well as the atmosphere environment for characterization.24,29 Of course, the grain orientation and grain boundaries in polycrystalline film may also cause these phenomena. To shed light on the influences of temperature on the single crystal electronic properties, the dynamic current-time (I-t) properties of MAPbBr3 single crystal devices in the continuous heating and cooling process are detected. As shown in Fig. 2(b), the dark current (blue curve) reaches a steady value after a period of time at 25 C. During the heating (25 C ! 50 C) and cooling (50 C ! 25 C) processes, the current [red curve in Fig. 2(b)] obviously shows positive correlation with the temperature variation. The photocurrents show the same variation behaviour, as diagrammed in Fig. 2(c) (470 nm) and Fig. 2(d) (550 nm). The measurement of the fixed temperature is to provide a reference for the current characteristics of temperature changes. Two variables should be defined before quantifying the current variation value relevant to temperature: temperature variation circle (TVC) and current increment rate (CIR). TVC is the process from high temperature (50 C) to low temperature (25 C or 30 C), and CIR is expressed as follows:
FIG. 2. (a) The testing schematic diagram of the photocurrent; the dynamic (b) dark current-time and photocurrenttime characteristics under (c) 470 nm wavelength and (d) 550 nm wavelength LEDs with the same light intensity in the specific temperature range at 1 V bias.
233302-3
Cui et al.
Appl. Phys. Lett. 111, 233302 (2017)
CIR ¼
Ihigh ; Ilow
(1)
where Ihigh and Ilow are the current at the high and low temperatures, respectively. The data in Table I are extracted and calculated from the I-t curves. It is clear that the average CIR for the dark current is greater than that for the photocurrent in spite of the small discrepancy in the temperature variation range, which indicates the different mechanisms of the temperature effect on the two types of currents. Due to the related fabrication process, the defect states have been found and studied in perovskite polycrystalline thin films, which mainly arise from the grain boundaries and the molar ratio of the precursor in solution preparation.10,30 Actually, the experimental and theoretical calculations have also revealed the existence of defect states on single crystal surfaces where they resemble polycrystalline thin films.1,21,22,31,32 The point defects in halide perovskites would contribute shallow states on account of their lower formation energies.33,34 This suggests that point defects should be more inclined to contribute trap centers rather than non-radiative ones, and so, the current may increase due to the release of carriers from traps. Compared to the device structures in previous reports,21,35 it is likely that holes play the dominant role in trapping and detrapping processes in our device. Poole-Frankel conduction (P-F) is what can explain the trap-limited current density and described by the following equation:36 pffiffiffi Et exp bPF E ; J ¼ r0 E exp (2) kB T where Et is the trap energy, kB is the Boltzmann constant, T is the absolute temperature, r0 is the zero field conductivity, E is the electric field, and bPF ¼ kB1T Sqrt
e3 pee0
, where e is the
dielectric constant of the medium and e0 is the vacuum dielectric constant. Because the intrinsic carrier density of the single crystal is small, it is expected that the main conduction mechanism of the dark current would be the trap limited P-F by the defects. The time-resolved PL measurement is a helpful method to explore the presence of defects in the inorganic-organic hybrid perovskite materials.1 As shown in Fig. 3, the charge recombination lifetimes at different temperatures are very similar. These short lifetimes (average value 35.3 ns, in close proximity to the previous report22) indicate that the defects (trap states) can easily capture the charges generated TABLE I. Current increment rate (CIR) extracted and calculated from the I-t curves.
CIR (%)/D25 C
FIG. 3. The time-resolved PL spectra and their corresponding fitting results at three temperatures. The inset is expanded to show a negligible difference among three spectra.
by pulsed laser illumination. The I–V curves with the depenpffiffiffi dence of ln (I/E) vs. E for various temperatures of the dark current are shown in Fig. 4(a), where E ¼ VL , in which V is the applied voltage and L is the channel length. It indicates an approximately linear variation below the electric field value of about 1/14 kV/cm (consistent with dynamic measurement voltage, 1 V), and here, the P-F mechanism may govern the conduction, which agrees with the previous report.37 It also shows that the dark current augments with the increase in temperature. Besides, Fig. 4(b) shows the negative linear correlation variation of ln (I/E) with 1/kBT. The activation energy (Et) for E ¼ 1/14 kV/cm is calculated to be 0.22 eV from the slope, which reconfirms that defect states are shallow because the depth of trap energy level states is close to the bandedge rather than over the bandgap.34 It is obvious that the extent of carrier de-trapping depends on factor exp kEB tT
in P-F conduction, which is
two times larger at 50 C than at 25 C, closely identical to the value obtained from the dark current average CIR calculation. Because the intrinsic carrier concentration of the single crystals is small and the implantation of photogenerated carriers reduces the influence of defects on trapping carriers as a result of filling, the photocurrent average CIR is much smaller than that of the dark current. In order to further verify the existence of defects and mainly those existing on the surface, transient photocurrent responses corresponding to two wavelengths of incident light (470 nm and 550 nm) with the same illumination intensity
CIR (%)/D20 C Photocurrent
TVC1 TVC2 TVC3 TVC4 Average
Dark current
550 nm
470 nm
246.2 246.8 247.6 … 246.9
28.0 27.0 26.1 … 27.0
30.1 28.0 27.6 26.0 27.9
FIG. 4. (a) The variation of ln (I/E) vs. E1/2 for different temperatures, and (b) shows the variation of ln (I/E) vs. 1/kBT for E with the value of 1/14 kV/cm.
233302-4
Cui et al.
Appl. Phys. Lett. 111, 233302 (2017)
are compared, as shown in Figs. 5(a) and 5(c). One-photon and two-photon excitations are the methods commonly used to study the surface and bulk optical properties of the single crystal, respectively.20,22 But in virtue of the filter effect of hybrid halide perovskites38 and long carrier diffusion length in the single crystal,1,21 when photons with different energies are comparable to the bandgap, they could generate charge carriers at different depths in the crystal. In addition, CH3NH3PbBr3 is the direct bandgap material,34 and the absorption coefficient (a) can be described by the following equation:1 a ¼ cðhv Eg Þc ;
(3)
where hv is the photon energy, Eg is the bandgap with the value of 2.21 eV, c is a constant of the material, and Ç is 1/2 for direct bandgap semiconductors. However, a is a constant when hv is greater than a certain value. Furthermore, the photon penetrate length h ¼ a1 , and so, the penetrate lengths of 470 nm and 550 nm photons are 100 nm and 20 lm according to the absorption coefficient of the MAPbBr3 single crystal in the latest work, respectively, which are obtained from the combination of transmission and reflection spectroscopy data.39 It is clear that the photons of these two wavelengths can produce charge carriers at different depths of the single crystal and the short-wavelength (470 nm) photons generate charge carriers mostly near the surface, which is confirmed by the different transient photocurrent responses in Fig. 5. Figure 5(a) presents the photoswitching of the devices at a frequency of 0.5 Hz, a wavelength of 470 nm, and a light intensity of 1.1 mW/cm2. The current shows a typical increase at the illumination, and it decreases in the dark state. The increase is related to the generation and trapping of photoinduced carriers on the single crystal surface, and the decrease should be mainly related to carrier recombination and de-trapping processes.40,41 With consideration of the density of defects being certain, the establishment of equilibrium is the result of the joint effect of the trapping and de-trapping. Figure 5(c) shows a different transient characteristic, where the photocurrent is attenuated from a high level to a steady state value instead of gradually increasing
to the steady state. This is because the photon generates carriers in a deeper position of the single crystal. A part of them could diffuse to the single crystal surface nearby due to the relatively small absorption coefficient and then may be trapped by traps. The other part may be directly collected by Au electrodes as a result of the applied field,12 and so, there is a decay tendency to a steady state. This similar decay experimental phenomenon has also appeared in a previous work, and when the bias voltage increased, the decay trend became smooth because of the enhanced ability of electrodes to collect carriers.42 As for the increasing behaviour of the dark current from a low level, it may be due to the additional carriers provided by the photon produced at a deeper position of the crystal for the trapping and de-trapping process at the moment the LED is switched off. The two kinds of transient photocurrent responses maintain the same variation tendency at the temperatures of 40 C and 50 C in Fig. S1 (supplementary material). In summary, we investigate the direct correlation of the atmosphere temperature with the trap states of the perovskite single crystal. Our experimental results intuitively demonstrate that both the dark current and the photocurrent are positively correlated with the temperature variation. The relatively large concentration of carriers at high temperature due to the detrapping of carriers, consequently, causes the relatively large current. This phenomenon could be well explained by the P-F conduction mechanism, and the I-V characteristic data are well fitted in the low voltage range. The activation energy (Et) is calculated to be 0.22 eV. Additionally, the presence of surface defect states is also confirmed by the different photocurrent responses of incident light at two typical wavelengths. These findings provide a way to understand the effect of the actualwork temperature on the single crystal device performance. Researchers also prompt to unceasingly search methods to further optimize the performance of single crystals and devices based on these rapidly developing photoelectric materials. See supplementary material for the transient photocurrent response data at the temperatures of 40 C and 50 C as well as experimental details. This work was financially supported by the National Natural Science Foundation of China (61774046), the Natural Science Foundation of Shanghai (17ZR1402100), and the National Key Research and Development Program of China (2016YFE0110700). 1
FIG. 5. Transient photocurrent responses under (a) 470 nm and (c) 550 nm wavelength LEDs at a pulse frequency of 0.5 Hz at the temperature of 30 C. Schematic of the device at (b) 470 nm and (d) 550 nm incident light.
D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent, and O. M. Bakr, Science 347, 519 (2015). 2 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, and M. Gr€atzel, Nature. 499, 316 (2013). 3 G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gr€atzel, S. Mhaisalkar, and T. C. Sum, Science 342(6156), 344 (2013). 4 A. Kojima, K. Teshima, Y. Shirai, and T. Miyasaka, J. Am. Chem. Soc. 131, 6050 (2009). 5 F. Zhang, W. Shi, J. Luo, N. Pellet, C. Yi, X. Li, X. Zhao, T. J. S. Dennis, X. Li, S. Wang, Y. Xiao, S. M. Zakeeruddin, D. Bi, and M. Gr€atze, Adv. Mater. 29, 1606806 (2017). 6 H.-K. Seo, H. Kim, J. Lee, M.-H. Park, S.-H. Jeong, Y.-H. Kim, S.-J. Kwon, T.-H. Han, S. Yoo, and T.-W. Lee, Adv. Mater. 29, 1605587 (2017).
233302-5 7
Cui et al.
M. Wang, Y. Shi, J. Bian, Q. Dong, H. Sun, H. Liu, Y. Luo, and Y. Zhang, Chem. Phys. Lett. 662, 176 (2016). 8 Y. Wang, R. Fullon, M. Acerce, C. E. Petoukhoff, J. Yang, C. Chen, S. Du, S. K. Lai, S. P. Lau, D. Voiry, D. O’Carroll, G. Gupta, A. D. Mohite, S. Zhang, H. Zhou, and M. Chhowalla, Adv. Mater. 29, 1603995 (2017). 9 L. Shen, Y. Fang, D. Wang, Y. Bai, Y. Deng, M. Wang, Y. Lu, and J. Huang, Adv. Mater. 28, 10794 (2016). 10 D. Li, H.-C. Cheng, Y. Wang, Z. Zhao, G. Wang, H. Wu, Q. He, Y. Huang, and X. Duan, Adv. Mater. 29, 1601959 (2017). 11 A. R. bin M. Yusoff, H. P. Kim, X. Li, J. Kim, J. Jang, and M. K. Nazeeruddin, Adv. Mater. 29, 1602940 (2017). 12 Y. Fang, Q. Dong, Y. Shao, Y. Yuan, and J. Huang, Nat. Photonics. 9, 679 (2015). 13 M. Cao, J. Tian, Z. Cai, L. Peng, L. Yang, and D. Wei, Appl. Phys. Lett. 109, 233303 (2016). 14 W. A. Saidi, S. Ponce, and B. Monserrat, J. Phys. Chem. Lett. 7, 5247 (2016). 15 Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan, and J. Huang, Adv. Mater. 26, 6503 (2014). 16 J.-C. Ke, Y.-H. Wang, K.-L. Chen, and C.-J. Huang, J. Alloys Compd. 695, 2453 (2017). 17 B. Murali, E. Yengel, W. Peng, Z. Chen, M. S. Alias, E. Alarousu, B. S. Ooi, V. Burlakov, A. Goriely, M. Eddaoudi, O. M. Bakr, and O. F. Mohammed, J. Phys. Chem. Lett. 8, 137 (2017). 18 R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, and L. M. Herz, Adv. Funct. Mater. 25, 6218 (2015). 19 M. I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, S. M. Zakeeruddin, U. Rothlisberger, and M. Gr€atzel, Sci. Adv. 2, e1601156 (2016). 20 Y. Yamada, T. Yamada, L. Q. Phuong, N. Maruyama, H. Nishimura, A. Wakamiya, Y. Murata, and Y. Kanemitsu, J. Am. Chem. Soc. 137, 10456 (2015). 21 Y. Liu, Z. Yang, D. Cui, X. Ren, J. Sun, X. Liu, J. Zhang, Q. Wei, H. Fan, F. Yu, X. Zhang, C. Zhao, and S. F. Liu, Adv. Mater. 27, 5176 (2015). 22 B. Wu, H. T. Nguyen, Z. Ku, G. Han, D. Giovanni, N. Mathews, H. J. Fan, and T. C. Sum, Adv. Energy Mater. 6, 1600551 (2016). 23 N. T. Shewmon, H. Yu, I. Constantinou, E. Klump, and F. So, ACS Appl. Mater. Interfaces 8, 33273 (2016). 24 H.-H. Fang, S. Adjokatse, H. Wei, J. Yang, G. R. Blake, J. Huang, J. Even, and M. A. Loi, Sci. Adv. 2, e1600534 (2016). 25 H. Zhang, Y. Liu, H. Lu, W. Deng, K. Yang, Z. Deng, X. Zhang, S. Yuan, J. Wang, J. Niu, X. Zhang, Q. Jin, H. Feng, Y. Zhan, and L. Zheng, Appl. Phys. Lett. 111, 103904 (2017).
Appl. Phys. Lett. 111, 233302 (2017) 26
Q. Chen, N. De Marco, Y. Yang, T. Bin Song, C. C. Chen, H. Zhao, Z. Hong, H. Zhou, and Y. Yang, Nano Today 10, 355 (2015). C. Barugkin, J. Cong, T. Duong, S. Rahman, H. T. Nguyen, D. Macdonald, T. P. White, and K. R. Catchpole, J. Phys. Chem. Lett. 6, 767 (2015). 28 Y.-H. Qiu, F. Nan, Q. Wang, X.-D. Liu, S.-J. Ding, Z.-H. Hao, L. Zhou, and Q.-Q. Wang, J. Phys. Chem. C 121, 6916 (2017). 29 B. Murali, S. Dey, A. L. Abdelhady, W. Peng, E. Alarousu, A. R. Kirmani, N. Cho, S. P. Sarmah, M. R. Parida, M. I. Saidaminov, A. A. Zhumekenov, J. Sun, M. S. Alias, E. Yengel, B. S. Ooi, A. Amassian, O. M. Bakr, and O. F. Mohammed, ACS Energy Lett. 1, 1119 (2016). 30 Q. Wang, Y. Shao, H. Xie, L. Lyu, X. Liu, Y. Gao, and J. Huang, Appl. Phys. Lett. 105, 163508 (2014). 31 S. P. Sarmah, V. M. Burlakov, E. Yengel, B. Murali, E. Alarousu, A. M. El-Zohry, C. Yang, M. S. Alias, A. A. Zhumekenov, M. I. Saidaminov, N. Cho, N. Wehbe, S. Mitra, I. Ajia, S. Dey, A. E. Mansour, M. Abdelsamie, A. Amassian, I. S. Roqan, B. S. Ooi, A. Goriely, O. M. Bakr, and O. F. Mohammed, Nano Lett. 17, 2021 (2017). 32 T. Komesu, X. Huang, T. R. Paudel, Y. B. Losovyj, X. Zhang, E. F. Schwier, Y. Kojima, M. Zheng, H. Iwasawa, K. Shimada, M. I. Saidaminov, D. Shi, A. L. Abdelhady, O. M. Bakr, S. Dong, E. Y. Tsymbal, and P. A. Dowben, J. Phys. Chem. C 120, 21710 (2016). 33 W.-J. Yin, T. Shi, and Y. Yan, Appl. Phys. Lett. 104, 063903 (2014). 34 A. Buin, R. Comin, J. Xu, A. H. Ip, and E. H. Sargent, Chem. Mater. 27, 4405 (2015). 35 H. S. Rao, W. G. Li, B. X. Chen, D. Bin Kuang, and C. Y. Su, Adv. Mater. 29, 1602639 (2017). 36 A. Slonopas, B. J. Foley, J. J. Choi, and M. C. Gupta, J. Appl. Phys. 119, 74101 (2016). 37 E. J. Yoo, M. Lyu, J. Yun, C. J. Kang, Y. J. Choi, and L. Wang, Adv. Mater. 27, 6170 (2015). 38 M. I. Saidaminov, M. A. Haque, M. Savoie, A. L. Abdelhady, N. Cho, I. Dursun, U. Buttner, E. Alarousu, T. Wu, and O. M. Bakr, Adv. Mater. 28, 8144 (2016). 39 B. Wenger, P. K. Nayak, X. Wen, S. V. Kesava, N. K. Noel, and H. J. Snaith, Nat. Commun. 8, 590 (2017). 40 R. Dong, Y. Fang, J. Chae, J. Dai, Z. Xiao, Q. Dong, Y. Yuan, A. Centrone, X. C. Zeng, and J. Huang, Adv. Mater. 27, 1912 (2015). 41 Q. Han, S. H. Bae, P. Sun, Y. T. Hsieh, Y. Yang, Y. S. Rim, H. Zhao, Q. Chen, W. Shi, G. Li, and Y. Yeng, Adv. Mater. 28, 2253 (2016). 42 Y. Liu, J. Sun, Z. Yang, D. Yang, X. Ren, H. Xu, Z. Yang, and S. F. Liu, Adv. Opt. Mater. 4, 1829 (2016). 27
