Optical Management with Nanoparticles for a Light Conversion

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Optical Management with Nanoparticles for Light Conversion Efficiency Enhancement in Inorganic #-CsPbI3 Solar Cells Lei Liang, Miao Liu, Zhiwen Jin, Qian Wang, Haoran Wang, Hui Bian, Feng Shi, and Shengzhong Liu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04842 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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Optical Management with Nanoparticles for Light Conversion Efficiency Enhancement in Inorganic γ-CsPbI3 Solar Cells Lei Liang1,#, Miao Liu1,#, Zhiwen Jin2*, Qian Wang2, Haoran Wang1, Hui Bian1, Feng Shi1,*, and Shengzhong Liu3,* #

These authors contributed equally to this work. Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology; School of Materials Science & Engineering, Shaanxi Normal University, Xi’an, 710119, P. R. China. 1Key

2School

of Physical Science and Technology & Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, P. R. China. 3Dalian

National Laboratory for Clean Energy; iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, P. R. China. E-mail: [email protected], [email protected], [email protected]

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Abstract Recently, γ-CsPbI3 perovskite solar cells (PSCs) have shown potential applications in optoelectronic devices, due to their high thermal stability. However, the incomplete utilization of the solar spectra especially in near-infrared (ca. 46%) range significantly limits the power conversion efficiency (PCE). Herein, core-shell-structured NaLuF4:Yb,Er@NaLuF4 upconversion nanoparticles (UCNPs) have been successfully synthesized and integrated into the hole transport layer for improving PCE in γ-CsPbI3 PSCs. Compared with the reference one, the short-circuit current density (JSC) and PCE of the optimized device reached up to 19.17 mA/cm2 (18.81 mA/cm2) and 15.86% (15.51%), respectively. Actually, due to the ultra-low photoluminescence quantum yield (PLQY, 1%.43,44 In addition, too many UCNPs (covered with insulator organic matters) used to enhanced PLQY could block carrier transmission through the device.42 However, it is still unclear whether such lower PLQY can effectively broaden the absorption spectrum of PSCs to


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the NIR region. Hence, it is the key to understanding the underlying mechanism of the UCNPs for further improving PCE of PSCs. In this work, we have successfully enhanced the conversion efficiency in γ-CsPbI3 PSCs via introducing UCNPs into the hole transport layer (HTL). The results demonstrated that both short-circuit current density (JSC) and PCE were significantly increased by as much as 1.91% and 2.26%, respectively. Further characterizations proved the generally recognized the UC effect of UCNPs in PSCs to improve their PCE was negligible. Moreover, the enhanced performance actually was attributed to light scattering from the UCNPs and reflection in PSCs, which resulted in increasing the optical path length and facilitating the perovskite layer re-absorption.

Results and discussion The device structure which is consisted of glass substrate/fluorine-doped tin oxide (FTO)/TiO2/γ-CsPbI3/UCNPs doped poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA)/Au), and cross-section scanning electron microscope (SEM) images are presented in Figure 1a. The planar ETL TiO2 layer was deposited on a neat FTO glass substrate by a hydrothermal deposition route to collect and transport electrons. Then, the γ-CsPbI3 film (~ 300 nm) was fabricated on the planar TiO2 layer by a single step spin-coating process. X-ray diffraction (XRD) pattern of the fabricated film was measured to confirm the γ-CsPbI3 crystal structure (Figure S1).30 SEM and atomic force microscope (AFM) images of the obtained γ-CsPbI3 film were also given in Figure S2. After annealing, the HTL PTAA film was coated on the γ-CsPbI3 layer, and


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a controlled thickness of 100 nm was fabricated to gather holes and facilitate carrier transport. Finally, gold cathode was evaporated to accomplish the device. From the spectrum in Figure 1b, the utilizable solar spectrum is approximately a wide wavelength ranging from 300 nm to 1100 nm, including UV range, visible range, and NIR range.42 It can be seen that the external quantum efficiency (EQE) curve of γCsPbI3 was only sensitive to UV and visible light for the active layer. And there was no photoelectric response in the NIR region. Thus, the UCNPs were added into the HTL layer aiming to broaden the NIR absorption, from 900 nm to 1100 nm.34,42,45 It is attainable that the UCNPs act as energy reservoir can absorb NIR light and convert it into visible light, which resulted in broadening absorption spectrum for a device, such as ranging from 500 nm to 700 nm. Combining with the UC mechanism, we aimed to utilize NIR spectrum via incorporating with a γ-CsPbI3 layer. To understand the underlying mechanism and achieve enhanced UC luminescence (UCL), core-shell structure and nonnuclear shell structure platforms were used. The two types of core-shell structures, NaYF4:Yb,Er@NaYF4 and NaLuF4:Yb,Er@NaLuF4 NPs, were prepared using a modified two-step co-precipitation process.46 Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) were used to investigate the evolution of the morphology and phase of the as-synthesized samples. TEM images (Figure 2a-d) of the core-only and core-shell NPs depict structures with uniform particle size and morphology. The average sizes of core-only (NaYF4:Yb,Er and NaLuF4:Yb,Er) NPs were about 35 nm and 25 nm, respectively. After coating a passivated-shell,

the

core-shell

NPs

(NaYF4:Yb,Er@NaYF4


and

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NaLuF4:Yb,Er@NaLuF4) were well-dispersed and relatively uniform, with average sizes of 37 nm and 35 nm, respectively. To verify the crystal structure of as-obtained sample, the HRTEM images of the core-shell structure shown in Figure S3 which gives fine-resolved lattice fringes of the NPs. The dotted line represents the interface between the core and shell, and the lattice fringes are clearly. In Figure S3a, the two measured distance are 0.345 nm and 0.513 nm, which corresponding to the (001) and (100) planes of β-NaYF4 (JCPDS No. 28-1192), respectively. The measured distance in Figure S3b between the two adjacent fringes outside the line is 0.308 nm, which corresponding to the (110) lattice planes of β-NaLuF4 (JCPDS No. 27-0726) and another lattice spacing of 0.506 nm corresponding to (100) planes of β-NaLuF4. Due to the higher effective atom number of Lu3+ ions, the contrast of core is higher than the shell of the NaLuF4:Yb,Er@NaLuF4 core-shell structure and the interface is distinctly visible. Subsequently, the XRD patterns of samples are shown in Figure 2e-f. All of the XRD peaks matched well with the standard patterns of the -NaYF4 (JCPDS. No. 281192) and -NaLuF4 (JCPDS. No. 27-0726). To validate the effectiveness of the coreshell nano-structured design, the UC emission properties of these corresponding core and core-shell NPs were investigated (Figure 2g-h). Under 980 nm laser excitation, all of the samples exhibited characteristic emissions of Er3+ ions, and the intensity of UCL dramatically enhanced through fabricating core-shell structure. In particular, a negligible blue emission band centered at ~408 nm was observed, which was attributed to the 2H9/2  4I15/2 transition of Er3+.


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The typical UCL mechanism can be described by the energy level diagram in the Yb3+-Er3+ co-doped system, as shown in Figure 3a. Additionally, the UC mechanism for green (2H11/2  4I15/2 and 4S3/2  4I15/2) and red (4F9/2  4I15/2) emissions in NaYF4:Yb,Er/NaLuF4:Yb,Er NPs have been previously described, and it is always explained by the energy transfer process from Yb3+ to Er3+.43 To further support the enhancement of UCL via fabricating core-shell structure, we also carried out timedecay measurements of the Er3+ emission from the 4S3/2  4I15/2 transition (~ 540 nm) in different samples, as shown in Figure 3b. The lifetime decays of UCNPs for 4S3/2 of the Er3+ in core-shell NPs were far longer than those in core-only ones, which clearly demonstrated the role of surface passivation. For the active ions in UCNPs, a long lifetime usually means highly-efficient UCL. Obviously, the trend of decay lifetime variation is consistent with that of UCL intensity variation. These results demonstrate that core-shell structures play a significant role in enhancing UCL. In addition, the surface functional groups attached on the NPs were identified via fourier transform infrared spectroscopy (FTIR) studies, as shown in Figure 3c. The absorbance bands at 1565 cm-1 and 1467 cm-1 were attributed to the asymmetric and symmetric stretching vibration of the carboxylic group of oleic acid (OA), respectively.47 The 2930 cm-1 and 2860 cm-1 absorption bands were assigned to the asymmetric and symmetric stretching vibrations of methylene, respectively, and the absorption band around 3450 cm-1 was attributed to the stretching vibration of the hydroxyl group of OA.48 The UCNPS@PTAA as a HTL was used to fabricate the γ-CsPbI3 PSCs for investigating photovoltaic performance evaluation. The J-V curves and PCE


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distribution histograms are presented in Figure 4a-d. Remarkably, different contents of NaLuF4:Yb,Er@NaLuF4 UCPNs were added in PTAA HTL which shown various optical-electrical characteristics in the pristine and optimized solar cell devices. As shown in Figure 4a and Table 1, the PCE of γ-CsPbI3 PSCs was improved from 15.51% to the 15.86% when NaLuF4:Yb,Er@NaLuF4 UCNPs had been added in PTAA with a concentration of 2 mg/mL, due to the increased short-circuit current density (JSC) of the solar cells from 18.81 mAcm-2 to 19.17 mAcm-2. However, when the UCNPs concentration increased unceasingly, the PCE decreased inversely. The dark J-V curves with different concentrations of NaLuF4:Yb,Er@NaLuF4 UCNPs were compared in Figure S4. Clearly, the larger amount of the insulated UCNPs added into the PTAA film will increase the electronic resistance of the PTAA film and hinder the film carrier transmission. It could be concluded from film conductivity measurements.49 Hence, the quality of PTAA film could be affected by the excess insulating UCNPs. Apart from optimizing the concentrations (2 mg/mL) of UCNPs added in PTAA, we also investigated the PCEs of optoelectronic devices which were modified with the different types of UCNPs (Figure 4c and Table 2). When the concentration of UCNPs was 2 mg/mL, the PCEs of solar cells embellished with core-shell structure NPs were only slightly higher than the PCE of devices with noncore-shell structure NPs. This phenomenon could be induced by the decreased quenching of fluorescence and the improved light emitting yield which influenced the JSC boosting.34,43,46 To further verify the reproducibility of the devices, the PCEs of almost 30 devices were measured and their PCEs distribution histograms were shown in Figures 4b and 4d. It can be seen


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that the PCE values were centered on a narrow distribution which implied optimized devices exhibited favorable repeatability. The digital photos of 980 nm laser illuminating the PTAA solutions, glass/PTAA films and glass/γ-CsPbI3/PTAA films with and without NaLuF4:Yb,Er@NaLuF4 UCNPs are presented in Figures 5a and 5b. Upon irradiation with 980 nm laser, a beam of green light occurred in the PTAA solutions with UCNPs due to the UCNPs absorbed the NIR photons and converted them into visible light. Similarly, the analogical phenomenon took place in the films, including glass/PTAA, glass/UCNPS@PTAA, glass/γ-CsPbI3/PTAA, and glass/γ-CsPbI3/UCNPS@PTAA, as shown in Figure 5c-f. The absorption spectra and PL spectra of PTAA films with and without NaLuF4:Yb,Er@NaLuF4 UCNPS are shown in Figure S5. Clearly, after adding NaLuF4:Yb,Er@NaLuF4 UCNPs into the PTAA film, the absorption spectrum of PTAA films was slightly enhanced around 900-1000 nm. Upon 365 nm excited, negligible change was observed. While irradiation with 980 nm laser, the PTAA film with UCNPs exhibited characteristic emissions of Er3+ ions which was corresponding to the UC emission properties of UCNPs (as told above). These results indicated the asobtained HTL could absorb infrared light and then produce visible ones. Unfortunately, although the EQE spectrum shows an ideal integrated J value of approximately 4 mA/cm-2 in the NIR range from 900 nm to 1100 nm, the ultra-low PLQY (~1%)44 of the UCNPs could only contribute 0.04 mA/cm-2 to the γ-CsPbI3 PSCs, theoretically. It can be seen from Figure S6 that 2 mg/mL of NaLuF4 NPs without UC effect were also applied in PSCs, whereas the PCE of optoelectronic device were also improved. Such


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unusual phenomenon confirms there are other reasons which contributed to the improved PCE rather than the generally recognized the UC effect of UCNPs. Some researchers have indicated that a far from sunlight absorption can be ascribed to an insufficient optical light path.50-53 For example, as reported by Zhang et al. when the back of semiconductor/metal interface was roughness, where the contrast in the imaginary part of the refractive index was very large, the induced backscattering could largely reduce light escape from the solar cell structure.50 Hence, we believe the PTAA/Au surface is more important. Figure 5h-l exhibits the AFM images of the morphology information of the fabricated PTAA films in detail. Importantly, the surface roughness of the films slightly increased (from 4.81 nm to 8.10 nm) with increasing UCNPs concentration from 0 to 5 mg/mL. Meanwhile, the reflection of light at an interface between two different media depends on the refractive index of the materials.54,55 Hence, the synergistic effect of the rougher surface and the changed refractive index of UCNPs/PTAA in the heterogeneous UCNPs/PTAA coating could largely enhance light backscattering and re-absorption.41,56-58 As shown in the reflectance spectra (Figure 5m) and absorption spectra (Figure 5n), enhanced light harvesting was clearly found for the γ-CsPbI3 film with uniform and conformal UCNPs/PTAA coating. The averaged reflectance of PTAA coating was found to be ~30%, which was further reduced to ~20% in presence of the UCNPs (2 mg/mL). After incorporated UCNPs into the PTAA film, it was verified that the reflection could be further suppressed in the entire wavelength ranges. The absorption spectra also proved the superior light-trapping property, which could be relevant to the


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different refractive index between the materials. According to the reflectance spectra and absorption spectra, the decreased in the reflection and increased in the absorption ratio of the γ-CsPbI3 film with increasing UCNPs concentration (0 mg/mL to 5 mg/mL) will increase sunlight absorption by the γ-CsPbI3 film. For the different types of UCNPs in PTAA films (2 mg/mL, Figure S7), the reflection values are nearly the same which indicates the reflection and absorption related to the amount of the UCNPs. Combining with extra regeneration visible light and remaining scattered and reflected sunlight affected by UCNPs, UCNPs is to trigger UC and extend the sunlight optical path, which is beneficial to accelerate photoelectric effects resulting in producing more photoelectric currents.41,59 Figure 6a shows that the typical J-V curves of the pristine and optimized γ-CsPbI3 PSCs are measured between reverse and forward scan directions, and the related parameters are summarized in Table 3. It should be noted that both of the devices show very little hysteresis because the effect of light-enhanced ion migration and photoinduced halide segregation are suppressed in such all-inorganic Cs systems, leading to essentially eliminated hysteresis in the present device.60 As shown in Figure 6b, we also measured the EQE spectra of the pristine and optimized PSCs, and the integrated products of the EQE curves with AM1.5G photon flux. Obviously, the EQE range from 500 nm to 750 nm was all enhanced slightly compared with the optimized PSCs, and this was proved attributing to the UCNPs induced back scattering which reduced the light escaped from the device structure.50 In addition, the UCNPs can rarely convert NIR photons to visible light to improve sunlight absorption of γ-CsPbI3 PSCs. This


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phenomenon can be explained by the ultra-low PLQY of UCNPs as told above. To ensure the reliability of the J-V measurements, the PCEs of the PSCs were recorded as a function of time, during which the cells were biased at their respective VMP values (0.90 V), as presented in Figure 6c. When the two devices were measured over 10 hours, the PCE of the best-performing reference device was maintained at 15.59%. Such PCE value was very close to those values which were obtained by the direct J-V measurements.

Conclusion In summary, in this work, we successfully fabricated γ-CsPbI3 PSCs integrated with UCNPs possessing high optoelectronic performance. The superior device performance was achieved with an increase in JSC by 1.91% and an increase in PCE by 2.26%, respectively. Consequently, a high PCE of 15.86% was attained. By investigating the NIR response of different PLQY-based UCNPs, the absorption spectrum of PSCs was broadened. In addition, we confirmed that the UC effect broaden the absorption spectrum of γ-CsPbI3 film to the NIR region but contributed negligibly to the device performance. From analysis of the absorption spectra, AFM images, and EQE curves, the addition of UCNPs mainly enhanced the light scattering and reflection to prolong optical path length, which led to visible light re-absorption and produced more photoelectric current. Hence, we believe that the presented results provide a credible way to mitigate solar energy loss and extend efficiently spectrum utilization to some degree. In other words, it is predictable that our investigation will supply a method


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to develop the utilization of solar spectrum and promote the photovoltaic devices performance.


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ASSOCIATED CONTENT Supporting Information Additional data, including Experimental section, XRD pattern, SEM and AFM images of the fabricated γ-CsPbI3 film, HRTEM images of the UCNPs, dark J-V curves, absorption spectra, PL spectra, and reflectance spectra of different types of UCNPs in PTAA films and the properties of the fabricated PSCs.

AUTHOR INFORMATION Corresponding Authors *(Z.J.) E-mail: [email protected] *(F.S) E-mail: [email protected] *(S.L) E-mail: [email protected]

Author Contributions L.L. and M.L. contributed equally to this work. L.L., M.L., and Z.J. performed the experiments and data analysis. The project was conceived, planned, and supervised by Z.J and F.S. The manuscript was written by Z.J., L.L. and F.S. All authors reviewed the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the National Key R & D Program of China (2017YFC0107400, 2017YFC0107401, and 2017YFA0204800), the National Natural Science Foundation of China (61704099, 11804214, 21641001, and 91733301), the Natural Science Foundation of Shaanxi Province (2018JQ6072 and 2017JQ2020), the Fundamental Research Funds for the Central Universities (GK201902003, GK201804002, and 2018CSLZ009), 111 Project (B14041), the basic scientific research business expenses of the central university, and Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University (LZUMMM2019001/LZUMMM2019005), and the Open Fund of the State Key Laboratory of Integrated Optoelectronics, Jilin University (IOSKL2018KF06).


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Nano Letters

Figure 1. (a) Schematic structure and cross-sectional SEM image of the device; (b) solar spectrum, absorption spectrum, and emission spectrum of UCNPs, and EQE curve of γ-CsPbI3 PSC.


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Comparison of different types of UCNPs: (a-d) TEM images; (e-f) XRD patterns; (g-h) UCL spectra.


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Nano Letters

Figure 3. (a) Schematic energy diagram for Yb3+ and Er3+. (b) Luminescence decay curves of different types of UCNPs. (c) FTIR spectrum of NaLuF4:Yb,Er@NaLuF4.


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Comparison of the γ-CsPbI3 PSCs performance with different concentrations of NaLuF4:Yb,Er@NaLuF4 UCNPS in PTAA: (a) J-V curves, and (b) statistical PCE distribution histograms of 30 devices. Comparison of the PSCs performance with different types of UCNPs in PTAA (2mg/mL): (c) J-V curves, and (d) statistical PCE distribution histograms of 30 devices.


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Figure 5. Photos showing the phenomena of 980 nm laser illuminating the PTAA solutions, glass/PTAA films and glass/γ-CsPbI3/PTAA films: (a), (c) and (e) without UCNPs; (b), (d), and (f) with NaLuF4:Yb,Er@NaLuF4 UCNPS. (g) Integrated J in the NIR spectrum, under AM 1.5G theoretically. (h)-(l) AFM images of the PTAA films with different concentrations of NaLuF4:Yb,Er@NaLuF4 UCNPs on γ-CsPbI3 film. (m) Reflectance spectra. (n) Absorption spectra.


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Figure 6. (a) J-V characteristics under both the reverse and forward scan directions. (b) EQE curves and the integrated products of the EQE curves with AM1.5G photon flux. (c) Pristine and optimized PCEs of γ-CsPbI3 PSCs measured as a function of time.


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Table 1. The γ-CsPbI3 PSCs performance based on different concentrations of NaLuF4:Yb,Er@NaLuF4 UCNPS in PTAA (extracted from Figure 4a). Concentration

JSC

(NaLuF4:Yb,Er@NaLuF4) (mAcm-2)

VOC

FF

PCE

ΔJSC

ΔPCE

(V)

(%)

(%)

(%)

(%)

0

18.81

1.110

74.28

15.51

0

0

0.5 mg/mL

18.92

1.110

74.30

15.60

0.58

0.58

1 mg/mL

19.12

1.113

74.34

15.82

1.65

1.99

2 mg/mL

19.17

1.113

74.33

15.86

1.91

2.26

5 mg/mL

18.40

1.109

71.65

14.62

-2.18

-5.74

Table 2. The γ-CsPbI3 PSCs performance based on different types of UCNPs in PTAA (2mg/mL) (extracted from Figure 4c). Modified layer

JSC

VOC

FF

PCE

ΔJSC

ΔPCE

(2 mg/mL)

(mAcm-2)

(V)

(%)

(%)

(%)

(%)

without UCNPs

18.81

1.110

74.28

15.51

0

0

with NaYF4:Yb,Er

19.14

1.111

74.30

15.80

1.75

1.87

With aYF4:Yb,Er@NaYF4

19.18

1.112

74.31

15.85

1.97

2.19

with NaLuF4:Yb,Er

19.13

1.113

74.21

15.80

1.70

1.87

with NaLuF4:Yb,Er@NaLuF4

19.17

1.113

74.33

15.86

1.91

2.26

Table 3. Comparison of pristine and optimized γ-CsPbI3 PSCs measured under both the reverse and forward scan directions (extracted from Figure 6a and Figure 6b). Device

pristine

optimized

Scan

JSC

VOC

FF

PCE

JEQE

mode

(mA/cm2)

(V)

(%)

(%)

(mA/cm2)

reverse

18.81

1.110

74.28

15.51

forward

18.80

1.107

72.60

15.11

reverse

19.17

1.113

74.33

15.86

forward

19.16

1.108

72.40

15.37


18.33

18.60

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