Anisotropic Optoelectronic Properties of Melt-Grown Bulk CsPbBr3

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Anisotropic Optoelectronic Properties of Melt-Grown Bulk CsPbBr3 Single Crystal Peng Zhang, Guodong Zhang, Lin Liu, Dianxing Ju, Longzhen Zhang, Kui Cheng, and Xutang Tao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01945 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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Anisotropic optoelectronic properties of melt-grown bulk CsPbBr3 single crystal Peng Zhang, Guodong Zhang*, Lin Liu, Dianxing Ju, Longzhen Zhang, Kui Cheng, Xutang Tao*

State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, P. R. China

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Phone: 86-531-88364963 * E-mail: [email protected]; Phone: 86-531-88369099

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ABSTRACT: All-inorganic perovskite CsPbBr3 has been considered as one of the star semiconductors due to its inspiring optoelectronic properties and higher stability than the organic-inorganic hybrid counterparts. The preparation of large size single crystals with low trap density and the performance optimization on the devices still challenge the commercial application of this material. Here, the large transparent CsPbBr3 single crystal (ɸ 24 mm × 90 mm) was grown by a modified Bridgman method. With the determination of crystallographic directions, the anisotropic optoelectronic properties were investigated for the first time. The result shows a high electron mobility (11.61 cm2/Vs) along b axis, one order of magnitude higher than that along c axis. Moreover, the photoresponse measurement yields a high responsivity (5.83 A/W) and EQE (1360%) on the (001) plane irradiated by the 532 nm laser diode with 1 mW/cm2 under 10 V bias, which is a 305% enhancement compared with the (010) plane. Our study on anisotropic optoelectronic properties of CsPbBr3 will provide a significant approach to enhance the performance of single-crystalline devices. TOC GRAPHICS

KEYWORDS halide perovskite, CsPbBr3, single crystal growth, crystal orientation,

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photodetector, anisotropic optoelectronic properties Recently, halide perovskites with the general formula ABX3 (where A = CH3NH3, CH(NH2)2, Cs; B = Pb, Sn, Ge; and X = Cl, Br, I) have attracted intense attention due to the inspiring optoelectronic properties, such as large light absorption coefficient,1 high carrier mobility2 and long carrier diffusion length3. Combining with the merits of high defect tolerance,4 facile synthesis and low cost,5-6 this family of compounds show great potential in the optoelectronic devices including solar cells,7-10 light-emitting diode ,11-12 photodetectors,13-14 X-ray and γ-ray detectors15-17 and laser18-20. Compared with the organic-inorganic hybrid perovskites, all-inorganic CsPbBr3 possesses improved thermal and moisture stability due to the replacement of the volatized organic cation by inorganic Cs+, without deteriorating the optoelectronic properties.21-22 As is well-known, the anisotropy of the crystal often affects the properties of the material. In the past few years, a lot of efforts have been made to reveal the influence of the structural anisotropy on the characteristics of halide perovskite materials23-29. Motta et al. calculated the carrier transport properties of MAPbI3 in theory and found a significant asymmetry of the mobility which depends on the charge transport direction with respect to the molecular axis.23 Cho et al. studied the anisotropic charge transport properties in orientationally pure crystalline MAPbI3 film, showing the widely application of crystal orientation control in optoelectronic devices.24 Jurow et al. reported the controllable anisotropic photon emission resulting from the vertically aligned transition dipole moments in the film of CsPbBr3 nanocubes.25 Except for

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these crystalline perovskite films, on the other hand, some researchers have also investigated the anisotropic optoelectronic properties of MAPbX3 (X = Cl, Br, I) single crystals (SCs) with the development of halide perovskite SCs growth technique.26-29 For example, Zuo et al. reported a 153.33% enhancement of responsivity for (110) plane of MAPbBr3 SC compared with the (100) plane.26 Ding et al. observed a 135% increased responsivity and 128% heightened external quantum efficiency (EQE) on (112) plane of MAPbI3 SC than those on (100) plane.28 The above studies show the crystal orientation strongly affects the properties of halide perovskite materials. CsPbBr3 crystallizes in the orthorhombic system (Pnma space group) at room temperature, implying the possible intense anisotropic optoelectronic properties. Some groups have reported the bulk CsPbBr3 SCs growth from the melt1, 16, 30-31 or the solutions14,

33-36

as well as the unique properties in optoelectronic detection and

radiation detection. However, to our best knowledge, there is no systemically report about the optoelectronic anisotropy of CsPbBr3 SC. It may be caused by the difficulty in large-sized CsPbBr3 SCs growth and the crystal orientation and processing technique. Therefore, it is impending to process the work of CsPbBr3 SCs growth and orientation, and investigate the anisotropic optoelectronic properties of CsPbBr3 SC. In this work, we report a large CsPbBr3 SC with the size of ɸ 24 mm × 90 mm grown by a modified vertical Bridgman method. Moreover, the crystallographic orientations of the CsPbBr3 SC were determined by the X-ray orientation technique. Based on these achievements, the optoelectronic anisotropy of CsPbBr3 SC was

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investigated in detail, and the mechanism of the anisotropy were discussed from the crystallographic view. Figure 1a shows the schematic diagram of the crystal growth furnace. By carefully controlling the temperature gradient and cooling procedure, a crack-free CsPbBr3 SC with the size of ɸ 24 mm × 90 mm was successfully grown, which is the largest volume (>40 cm3) CsPbBr3 SC up to now (Figure 1b).1,

14, 16, 30-38

With the

optimization of the cutting and polishing technique, a CsPbBr3 crystal wafer (ɸ 24 mm×2 mm ) with good transparency was obtained, as shown in lower left of Figure 1b. CsPbBr3 crystallizes in the orthorhombic system (Pnma space group) at room temperature and the crystal structure was illustrated in Figure 1c.31 Based on that, the crystallographic orientations of CsPbBr3 SC were determined by the X-ray Laue back diffraction technique (Figure S2) and further confirmed by the XRD patterns (Figure 1d). Finally, an oriented cuboid CsPbBr3 SC with polishing finely were obtained for the optoelectronic performance measurement (Figure 1b). The detailed description of the crystal growth and processing technique can be found in the Supporting Information.

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Figure 1. (a) Schematic diagram of the vertical Bridgman furnace for CsPbBr3 single crystal growth. (b) Photographs of the as-grown CsPbBr3 single crystal (upper), polished wafer (lower left), and oriented cuboid CsPbBr3 SC (lower right). (c) Crystal structure of CsPbBr3. (d) XRD patterns of the (100), (010) and (001) crystal wafers. Figure 2a shows the ultraviolet-visible (UV-vis) absorption spectrum and photoluminescence (PL) spectrum measured by the CsPbBr3 powder, and the inset is the Tauc plot curve converted by the Kubelka-Munk equation for bandgap calculation.39 CsPbBr3 exhibits a sharp absorption edge at 560 nm in the UV-vis spectrum, corresponding to the bandgap of 2.25 eV which is slightly larger than that of solution-grown crystals (2.16 eV).14 The steady state photoluminescence (PL) spectrum peak locates at 547 nm without any defect level related emission band, showing an abnormal blue-shift (17 meV) compared to the absorption-derived bandgap which can also be found in the previous reports.31, 36 The shorter wavelength

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and sharper PL peak of melt-grown CsPbBr3 than that of solution-grown crystals also indicated the less defects and impurities in the melt-grown crystal.36-37 The time resolved PL spectrum (Figure 2b) includes three components, a short-lifetime process (τ1 = 1.29 ns, 45.77%), an intermediate-lifetime process (τ2 = 7.41 ns, 36.37%) and a long-lived component (τ3 = 43.16 ns, 17.86%). The calculated average lifetime of 10.9 ns is close to the previous study on CsPbBr3 SC (8.5 ns) and nanocrystal (8.9 ns) under 442 nm excitation.1,

40

The Ultraviolet photoemission spectroscopy (UPS)

measurement (Figure 2c) shows that the VBM lies at 1.8 eV below Fermi level and the secondary electrons cutoff is 17 eV, leading to the work function about 4.22 eV for CsPbBr3.41 Based on the band gap and UPS measurement, the energy band diagram of CsPbBr3 was drawn in Figure 2d, showing a n-type semiconductor for the melt-grown CsPbBr3.

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Figure 2. (a) Ultraviolet-visible absorption spectrum, Tauc plot curve (inset) and fluorescence emission spectrum for CsPbBr3. (b) The time resolved PL decay transient measurement of CsPbBr3. (c) Ultraviolet photoemission spectroscopy of CsPbBr3. (d) Energy band diagram of CsPbBr3. The anisotropic electron transport properties in CsPbBr3 SC were determined for the first time by the space charge limited current (SCLC) measurement with a simple sandwich structure of Ti/CsPbBr3 SC/Ti, as shown in the inset of Figure 3a-c. Ti electrodes (70 nm thickness, 6 mm length and 3 mm width) were applied to form electron-injection devices on account of the shallow-work-function.42 In the current-voltage (I ≈ Vn) curves, the Ohmic (n = 1), trap-filling (n > 3) and Child (n = 2) regions were all observed with the increase of bias voltage. The trapping state densities were calculated using the equation:

=

 

(1)



where VTFL is the trap-filled limit voltage, L is the thickness of the sample, ε is the relative dielectric constant for CsPbBr3 (measured to be ≈ 22.9, 23.9, 23.2 along a, b, c direction, respectively), ε0 is vacuum permittivity, and e is the elementary charge. Hence, the trap density of three crystal planes were calculated to be 1.65×1010 cm-3, 6.31×1010 cm-3 and 1.08×109 cm-3 for a, b, c direction, respectively, which is comparable with the reported halide perovskite single crystalline CsPbBr3,1 MAPbI3,3 FAPbI3,43 and MAPbBr344. The electron mobility was estimated from the Child region according to Mott-Gurney’s equation:

=

 

(2)

 

where JD is the current density, V is the applied voltage. According to the equation, the higher electron mobility of 11.61 cm2/Vs is derived along b direction, whereas it is

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only 3.54 cm2/Vs and 1.62 cm2/Vs for a and c direction, respectively, showing an obvious anisotropy of electron transport properties. The better electron transport ability along b direction can be explained by the difference of the carrier transport paths in the crystal structure, i. e. the [PbBr6]4- octahedra align in b direction, leading to the Br-Pb-Br bonds continuous (inset of Figure 3b). The interconnected [PbBr6]4octahedra form straight chains along b direction which benefit the carrier transport. In contrast, the [PbBr6]4- and Cs+ array alternately with respect to the a or c direction, in other words, the octahedra form zigzag chains along a or c direction (inset of Figure 3a and c), which reduce the overlap of the Br-p orbital, suppressing the carrier transport.23

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Figure 3. Dark current-voltage characteristics of CsPbBr3 SC measured by SCLC method along (a) a direction, (b) b direction and (c) c direction. To investigate the optoelectronic anisotropy of the crystal, the photoresponse measurements of the (100), (010), and (001) planes were performed. As schematically illustrated in Figure 4a-c, a pair of gold interdigital electrodes were deposited on each facet, with the figure width of 200 µm and the effective illuminated area of 8.25 mm2 (Figure S3). By applying this type of electrodes, carriers will transport along c, a, b direction in (100), (010) and (001) plane, respectively. A xenon lamp with exciting light from 365 nm to 555 nm was applied to determine the wavelength dependence of

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photoresponse performance, and we found the optimal excitation wavelength is near 532 nm for all three devices (Figure S4). Thus, a laser diode (LD) with the wavelength of 532 nm was adopted for all of the following photoresponse measurements. Figure 4d-f show the current–voltage (I–V) curves of the three devices measured under dark condition and 532 nm LD irradiation with the power density increasing from 1 mW/cm2 to 16.5 mW/cm2. The highest photocurrent of (100), (010) and (001) plane was obtained under the condition of 16.5 mW/cm2 and 10 V bias about 2.50 mA, 0.88mA and 2.73mA, respectively. Moreover, the dark currents under 10V bias were only 1.43 µA, 1.38 µA and 1.84 µA for (100), (010) and (001) plane, respectively. The current of all three devices increases with the enhancement of the light power density and applied voltage. Notably, the photocurrent steeply increases with enhancing the bias voltage in the low voltage region (about 0~5 V), and then the photocurrent tends to saturate in the high voltage region (about 5~10 V). Furthermore, the smaller irradiation power, the lower voltage needed for the saturation photocurrent. It is reasonable for the photocurrent to change with this trend. The number of the photo-generated carriers is proportional to the irradiation power, at the same time, a corresponding voltage is needed to promote the separation of carriers. When the voltage exceeds the critical value, the photocurrent will not increase because of the thermal recombination of carriers. The spectral responsivity (R), external quantum efficiency (EQE) and detectivity (D*) are three key parameters to evaluate the performance of a photoresponse device upon an incident light, were calculated by equations:

=

 

(3)



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EQE = $∗ =

∙!"

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(4)

# 

(5)

&  ⁄'

where Ipc and Idark are the photocurrent and dark current for the device, respectively, P0 is the irradiation power density, S is the effective area of the detector, h is Planck constant, c is speed of light, λ is wavelength of irradiation and e is the elementary charge. As shown in Figure 4g-l, these three parameters have a positive correlation to the applied voltage under a fixed light power density. Furthermore, the values of the three parameters decrease with the enhancement of the irradiation power density under a constant bias voltage, as shown in the Figure S5. The highest R (5.83 A/W), EQE (1360%) and D* (2.5 × 1012 Jones) were obtained for the (001) plane under the 532 nm light with 1 mW/cm2 and 10V bias, whereas they were 1.44 A/W, 336% and 6.2×1011 Jones for (010) plane and 4.45 A/W, 1037% and 1.9×1012 Jones for (100) plane, respectively. Our photoresponse devices exhibit a better performance compared with the reported results (Table 1), probably due to both the high quality of our melt-grown crystal and the optimization of the devices in orientation and irradiation wavelength. Table 1. Performance comparison of the CsPbBr3 Single crystal photodetectors Active layer

CsPbBr3 SC

R (A/W) @ Vbias

EQE

on/off ratio

(light source, intensity)

(light source, intensity)

@Vbias

5.83 @ 10 V

12

103 @ 2 V

Our work

1360% 2

2@5V

2.5×10 @10 V 2

(532 nm, 1 mW/cm ) CsPbBr3 SC

Ref.

D* (Jones) @Vbias

(532 nm, 1 mW/cm ) 460%

-

103 @ 2 V

1

7.00%

1.8×1011 @ 5 V

100 @ 5 V

12

105 @ 0 V

34

(535 nm, 0.31 mW/cm2) CsPbBr3 SC

0.028 @ 5 V (450 nm, 1 mW)

CsPbBr3 SC

0.028 @ 5 V (550 nm, 10 mW/cm2)

(450 nm, 1 mW) 6%

1.7×1011 @ 5 V (550 nm, 10 mW/cm2)

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As shown above, these three CsPbBr3 SC devices have an obviously facet-dependent anisotropic optoelectronic property. The R and EQE of (001) plane exhibit a 305% enhancement compared with (010) plane, and slightly larger than that of (100) plane. This facet-dependent optoelectronic property can be explained from the structural anisotropy of CsPbBr3 which determines the generation and transport of the charge carriers. According to the previous calculation,4 Br and Pb dominate the state of band gap, while Cs has no contribution. Thus, the density and the arrangement of [PbBr6]4- units intensely affect the generation, dissociation and transport of electron-hole pairs.27, 45 In the (001) plane, the higher distribution density of [PbBr6]4than (010) plane contributes to the stronger photoconductivity effect,27 and the excitons are easily dissociated into free charge carrier because of the low exciton bonding energy.45 Moreover, the channel in b direction further benefits the transport of the photo-generated carriers in (001) plane as discussed above. Noticeably, despite the similar distribution density of [PbBr6]4- octahedron in the (001) and (100) planes, the R and EQE on the (001) plane are slightly higher (5.83 A/W and 1360% for R and EQE, respectively) than that of the (100) plane (4.45 A/W and 1037% for R and EQE, respectively). It could attribute to the different carrier transport paths on the (001) and (100) plane. As described above, b direction in (001) plane is more favorable for the carrier transport than c direction in (100) plane, which response for the better photoresponse performance of (001) plane.

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Figure 4. (a)-(c) the schematic diagrams of the three devices on (100), (010) and (001) plane respectively. (d)-(f) logarithmic characteristics of I-V (left part) and I-V (right part) under dark and 532 nm illumination for (100), (010) and (001) plane respectively. (g)-(i) responsivity (R) and external quantum efficiency (EQE) for (100), (010) and (001) plane respectively. (j)-(l) Detectivity (D*) of these three devices. The photocurrent-time response measurements of (100), (010) and (001) planes were conducted to determine the switching behavior and photocurrent stability. Figure 5a-c show the time-dependent photocurrent of the three devices with power density of 1 mW/cm2 at different applied voltage. The photocurrents increased and recovered rapidly before and after the light illumination with good repetition, indicating an excellent photocurrent stability. The dark currents along a, b, c direction under 4 V bias are 470 nA, 701 nA and 246 nA, respectively. The higher dark current along b direction can be attributed to the stronger carrier transport ability. The high on/off

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ratio of 103 was obtained for all three devices under 2 V bias, indicating the excellent sensitivity for the photodetectors based on CsPbBr3 SCs.

Figure 5. Time-dependent photocurrents of (a) (100) plane device. (b) (010) plane device and (c) (001) plane device. In conclusion, the CsPbBr3 single crystal with the dimensions of ɸ 24 mm × 90 mm was first grown by a modified Bridgman method, and the crystallographic directions were confirmed. The optoelectronic anisotropy which reflects in the anisotropic electron mobility and photoresponse properties was systematically investigated. For this behavior, a possible mechanism was discussed based on the anisotropic structure of CsPbBr3 crystal. These remarkable optoelectronic properties of CsPbBr3 SC will motivate researchers to utilize them for high-performance optoelectronic applications.

ASSOCIATED CONTENT Supporting Information

The supporting information is available free of charge on the ACS publications website at DOI: ……. Experimental details about the crystal growth and measurements. AUTHOR INFORMATION Corresponding Author

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* E-mail: [email protected]; Phone: 86-531-88364963 * E-mail: [email protected]; Phone: 86-531-88369099 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 51602178, 51321091), Program of Introducing Talents of Disciplines to Universities in China (111 project 2.0 No. BP2018013), and the Fundamental Research Funds of Shandong University. We greatly thank associate professor Qian Xin from the College of microelectronics, Shandong University for her help in the fabrication of electrons. We thank Dr. Yangyang Dang for his helpful discussion about the results of photoresponse properties. REFERENCES (1) Song, J.; Cui, Q.; Li, J.; Xu, J.; Wang, Y.; Xu, L.; Xue, J.; Dong, Y.; Tian, T.; Sun, H.; Zeng, H. Ultralarge All-Inorganic Perovskite Bulk Single Crystal for High-Performance Visible-Infrared Dual-Modal Photodetectors. Adv. Optical Mater. 2017, 5, 1700157. (2) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. M. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347, 519-522. (3) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths 175 µm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967-970.

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(4) Kang, J.; Wang, L. W. High Defect Tolerance in Lead Halide Perovskite CsPbBr3. J. Phys. Chem. Lett. 2017, 8, 489-493. (5) Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623-3630. (6) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692-3696. (7) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 1228604. (8) Gu, P. Y.; Wang, N.; Wu, A.; Wang, Z.; Tian, M.; Fu, Z.; Sun, X. W.; Zhang, Q., An Azaacene Derivative as Promising Electron-Transport Layer for Inverted Perovskite Solar Cells. Chem.-Asian J. 2016, 11, 2135-2138. (9) Wang, N.; Zhao, K.; Ding, T.; Liu, W.; Ahmed, A. S.; Wang, Z.; Tian, M.; Sun, X. W.; Zhang, Q., Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer. Adv. Energy Mater. 2017, 7, 1700522. (10) Gu, P.-Y.; Wang, N.; Wang, C.; Zhou, Y.; Long, G.; Tian, M.; Chen, W.; Sun, X. W.; Kanatzidis, M. G.; Zhang, Q., Pushing up the efficiency of planar perovskite solar cells to 18.2% with organic small molecules as the electron transport layer. J. Mater. Chem. A 2017, 5, 7339-7344. (11) Li, J.; Xu, L.; Wang, T.; Song, J.; Chen, J.; Xue, J.; Dong, Y.; Cai, B.; Shan, Q.; Han, B.; Zeng, H. 50-Fold EQE Improvement up to 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control. Adv. Mater. 2017, 29, 1603885. (12) Wang, N.; Liu, W.; Zhang, Q., Perovskite-Based Nanocrystals: Synthesis and Applications beyond Solar Cells. Small Methods 2018, 2, 1700380. (13) Fang, Y.; Dong, Q.; Shao, Y.; Yuan, Y.; Huang, J. Highly narrowband perovskite single-crystal photodetectors enabled by surface-charge recombination. Nat. ACS Paragon Plus Environment

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