Defect Passivation of CsPbIBr2 Perovskites for High-Performance

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Letter 2

Defect passivation of CsPbIBr perovskites for highperformance solar cells with large open-circuit voltage of 1.28 V Jiajun Lu, Shan-Ci Chen, and Qingdong Zheng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01430 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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ACS Applied Energy Materials

Defect Passivation of CsPbIBr2 Perovskites for High-Performance Solar Cells with Large OpenCircuit Voltage of 1.28 V Jiajun Lu,†,‡,§,‖ Shan-Ci Chen,† and Qingdong Zheng*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, Fujian 350002, China. ‡

University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China.

§

School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road,

Shanghai 201210, China. ‖

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai

200050, China.

ABSTRACT: As one of all-inorganic perovskites, cesium lead halide-mixed perovskite (CsPbIBr2) has a bright photovoltaic application prospect owing to its ambient stability,

ACS Paragon Plus Environment

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appropriate bandgap and distinctive color. However, the defect states in grain boundaries and surface of CsPbIBr2 polycrystalline film lead to nonradiative carrier recombination which subsequently reduces the open-circuit voltage (VOC) and final power conversion efficiency (PCE) of the corresponding perovskite solar cells (PSCs). In this work, polyethylene glycol (PEG) is used to passivate the defect states of pure CsPbIBr2 film by improving the film morphology and coverage. The best-performance PSC based on PEG-passivated CsPbIBr2 exhibits a VOC of 1.28 V, a short-circuit current (JSC) of 8.80 mA cm-2, a fill factor (FF) of 0.649, and a PCE of 7.31%. However, the reference best-performance device based on pure CsPbIBr2 shows an inferior PCE of 6.36% with a lower VOC of 1.10 V, a comparable JSC of 8.81 mA cm-2 and a similar FF of 0.656. The VOC of 1.28 V is the highest among all CsPbIBr2 PSCs. Furthermore, the PEGpassivated PSC shows improved shelf stability in comparison with the reference device without PEG-passivation. This work provides a facile strategy to fabricate CsPbIBr 2 PSCs with enhanced PCE, enlarged VOC and improved stability by defect passivation.

KEYWORDS: perovskite solar cell, polyethylene glycol, open-circuit voltage, defect passivation, stability

Emerging attention has been paid on the photovoltaic application of organic-inorganic hybrid perovskite materials owing to their excellent optoelectronic properties such as long carrier diffusion length, large absorption coefficients and suitable optical bandgaps.1-5 Nowadays, the highest power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (PSCs) has approached 23.3%.6 However, organic cations in the organic-inorganic hybrid


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perovskites such as methylammonium (MA+) and formamidinium (FA+), are easily degraded in ambient environment which results in poor stability of the corresponding devices. Compared with the organic-inorganic hybrid perovskites, all-inorganic perovskites (CsPbX3, X=halide) show improved thermal stability and photostability due to removal of the unstable cations.7-9 Therefore, increasing efforts have been focused on all-inorganic perovskite solar cells in recent several years.10 It is noteworthy that all-inorganic perovskites with different counter-ions (CsPbX3) have varied advantages and disadvantages. For example, CsPbBr3 perovskite shows outstanding moisture and thermal stability. But its large bandgap (Eg = 2.30 eV) limits the maximum photons which can be harvested by the corresponding device.11 And cubic CsPbI3 perovskite has a more appropriate bandgap of 1.73 eV, but it would degrade to orthorhombic CsPbI3 (Eg = 2.82 eV) rapidly when exposed to ambient environment.12 It should be noted that the phase transition temperature of orthorhombic CsPbI3 to cubic CsPbI3 is over 300 oC.13 Later on, researchers started to focus on bromide-containing mixed-halogen perovskites (CsPbI3-xBrx) which exhibit reduced phase transition temperature and adjustable bandgaps. CsPbI2Br has better stability than CsPbI3, but ambient humidity could cause the CsPbI2Br perovskite to decompose and form a non-perovskite phase.14 It was reported that air stability was improved with the increasing content of bromine in the cesium lead mixed-halogen perovskites,15 and the phase transition temperature of bromide-rich CsPbIBr2 perovskite could be reduced to around 100 oC.16 Therefore, the CsPbIBr2 perovskite is a promising active material for photovoltaic devices with its low phase transition temperature. There are several strategies to improve the efficiencies of CsPbIBr2 PSCs. For example, Liu et al. demonstrated a PCE enhancement of CsPbIBr2 PSCs from 1.30% to 5.52% by optimizing the thickness of MoOX buffer layer.17 Lau et al. reported a


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high PCE of 6.30% for the CsPbIBr2 PSCs fabricated by spray-assisted deposition.18 More recently, Liang et al. incorporated manganese cations into the CsPbIBr2 films to improve the device performance.19 The performance of perovskite solar cell is directly affected by the surface and electronic properties of corresponding perovskite film. Nonradiative recombination of charge carriers occurred in defect states at the grain boundaries and surface of perovskite thin film would lead to decreased photoluminescence (PL) intensity and shorter carrier lifetime thereby affecting the open-circuit voltage (VOC) and PCE of PSCs.20 In addition, defects may allow water and oxygen to infiltrate into the perovskite film and thus accelerate its decomposition, ultimately affecting the stability of PSCs.21 In this regard, it’s essential to develop perovskite films with low density of defect states and excellent film coverage. To reduce the defect states of perovskite films, a variety of defect-passivating agents have been developed so far, such as P3HT,22 alkylphosphonic acid ω-ammonium cations,23 glycol ether,24 fullerene25 and quaternary ammonium halides.26 As a frequently-used agent in biomedical and biological fields, polyethylene glycol (PEG) has been used as polymer additives to enhance the performance of organic-inorganic perovskite solar cells.27,28 In addition, Song et al. introduced PEG into allinorganic perovskite CsPbBr3 to passivate the crystal grains of CsPbBr3 films thus improving the luminance, current efficiency and external quantum efficiency of the corresponding lightemitting diodes.29 However, PEG has never been used as a passivating agent to enhance the performance of all-inorganic perovskite CsPbIBr2 solar cells, to the best of our knowledge. Herein, we propose a facile strategy to enhance the PCE of CsPbIBr2 PSCs by mixing a small amount of PEG (10 mg/mL) in the CsPbIBr2 precursor solution. With its three-dimensional network skeleton, PEG can act as a supporting matrix to improve coverage of the perovskite ﬁlm


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on TiO2 layer. At the same time, the wettability of the precursor solution can also be improved after the introduction of PEG. Furthermore, the self-assembled PEG network can slow down crystal growth and restrain aggregation of perovskite crystals during the process of perovskite phase formation. Then, the defect states at grain boundaries and surface of CsPbIBr2 bulk film was passivated effectively thereby leading to a more uniform perovskite film with less voids. Consequently, PSCs based on CsPbIBr2 with PEG-passivation exhibit enhanced photovoltaic performance with an enlarged VOC up to 1.28 V and an enhanced PCE of 7.31% in comparison with the pristine CsPbIBr2 PSCs with a lower VOC of 1.10 V and an inferior PCE of 6.36%. The pure CsPbIBr2 precursor solution was prepared by dissolving PbBr2 and CsI (with a 1:1 molar ratio) in dimethyl sulphoxide (DMSO) with a concentration of 0.8 mol/L. Then PEG was added into the precursor solution with a weight concentration of 10 mg/mL to form a PEG-mixed precursor solution. By spin-coating the PEG-mixed precursor solution onto the corresponding substrate followed by subsequent annealing, PEG-passivated CsPbIBr2 film was formed. The CsPbIBr2 film without PEG was prepared by using pure CsPbIBr2 precursor solution by the same procedure as that used for the PEG-passivated CsPbIBr2 film.


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a)

b)

d)

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c)

f)

Figure 1. Top-view SEM (a, d), AFM height (b, e) and three-dimensional view (c, f) images of the CsPbIBr2 films without (top row) and with PEG (bottom row), respectively. The width of AFM images is 3.0 µm.

The quality of perovskite films plays a significant role in determining the photovoltaic performance of PSCs. A compact and well-crystallized perovskite film is a premise for high efficiency solar cells. Fig. 1 shows top-view scanning electron microscope (SEM) images (Fig. 1a, d) and atomic force microscopy (AFM) images (Fig. 1b, c, e, f) of the CsPbIBr2 films without and with PEG-passivation, which were prepared on compact TiO2 (c-TiO2)/FTO/glass substrates. It can be found that there are many gaps and voids in the grain boundaries of neat CsPbIBr2 film. Nevertheless, after adding PEG to the pure CsPbIBr2 film, defects at the grain boundaries and surface of CsPbIBr2 film are passivated effectively, resulting in a film with better continuity and fewer voids, improved compactness, and decreased roughness. The improved morphology would suppress charge recombination and improve extraction of carriers in the interface, and eventually


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enhance the VOC and PCE of PSCs.30 According to the surface topography images in AFM, the root-mean-square (RMS) roughness of the neat CsPbIBr2 film is 102.2 nm, while the RMS roughness of PEG-passivated CsPbIBr2 film is reduced to 26.3 nm. Also, it reveals that the coverage of CsPbIBr2 film is improved with the PEG-passivation. The greatly reduced surface roughness for the PEG-passivated CsPbIBr2 film is in agreement with its reduced voids and smoother surface. It is known that large surface roughness and inhomogeneous distribution of grains could lead to bad contact between the perovskite layer and the charge extraction layers, eventually resulting in an increased possibility of charge recombination.31 X-ray diffraction (XRD) analysis was performed in order to know phase properties of the CsPbIBr2 films with and without PEG-passivation. Fig. 2a presents XRD patterns of CsPbIBr2 with and without PEG-passivation. As shown in Fig. 2a, no impurity peak shows up with the incorporation of PEG. The XRD peaks located at 14.9°, 21.06° and 30.1° correspond to the planes of (100), (110) and (200) of the CsPbIBr2 film. The interplanar spacings are calculated through Bragg equation and the lattice parameters are deduced, which are consistent with a cubic phase CsPbIBr2 reported previously by others.32 It should be noted that the peak intensities of CsPbIBr2 with PEG-passivation slightly decrease, which may be attributed to the reduced crystallinity of the PEG-passivated CsPbIBr2.


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a)

b)

FTO

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c)

e)

d)

Figure 2. (a) XRD patterns, (b) XPS spectra, and UPS spectra for (c) valence band edge, (d) full spectra and (e) secondary electron cutoff edge of the CsPbIBr2 films without and with PEGpassivation.

The elemental compositions of CsPbIBr2 films with and without PEG-passivation were also investigated by the X-ray photoelectron spectroscopy (XPS) technique. Fig. 2b presents the XPS spectra of films with all related elements. Clearly, for the photoemissions of Cs 3d, Pb 3d, Pb 4f, Br 3d and I 3d, the binding energy peaks for both films are the same (Fig. S1a-d). It should be pointed out that before the XPS test, the samples were prepared in the air. The C 1s and O 1s spectra can be used to identify the types of surface C and O species presented. Based on the C 1s spectra (Fig. S1e), the binding energy peaks of 284.8 eV and 286.2 eV can be assigned to C-


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H/C-C groups and C-O group/adsorbed CO2, respectively.33,34 The carbon groups originate from the PEG and DMSO (the annealing temperature did not reach the boiling point of DMSO). According to the O 1s spectra (Fig. S1f), the binding energy peaks of 531.0 eV and 532.8 eV can be ascribed to the surface adsorbed O/OH groups and C-O group/adsorbed water, respectively.35,36 The XPS results indicate that this facile mixing method does not cause any remarkable influence on the chemical states of the CsPbIBr2 perovskite. To observe influence of PEG on the energy levels of CsPbIBr2 film, ultraviolet photoelectron spectroscopy (UPS) was carried out. Fig. 2d shows the UPS spectra as well as valence band edge (Fig. 2c) and secondary electron cutoff edge (Fig. 2e) of CsPbIBr 2 films without and with PEG-passivation. The values of Eonset and Ecutoff can be obtained from the intersection points of the linear parts. Then, the value of valence band maximum (VBM) is calculated by using the formula of VBM = 21.22 eV - (Ecutoff - Eonset). The CsPbIBr2 film exhibits Eonset = 1.92 eV and Ecutoff = 17.03 eV, thus the corresponding VBM value is calculated to be 6.11 eV. Similarly, the CsPbIBr2 film with PEG-passivation exhibits Eonset = 1.91 eV and Ecutoff = 17.07 eV, and the corresponding VBM value is determined to be 6.06 eV. In general, there is a negligible change in energy position of the valence band for CsPbIBr2 film after the PEGpassivation. The optical properties of CsPbIBr2 films with and without PEG-passivation were also investigated by Ultraviolet–visible spectroscopy. As shown in Fig. 3a, the linear absorption bands for both films are basically the same, especially for their optical bandgaps which can be calculated according to their absorption edges. The calculated Eg of 2.07 eV for CsPbIBr2 film with or without PEG-passivation is in accordance with the previous literature.18 The linear absorption spectra suggest that the perovskite structure is not disrupted by the PEG-passivation,


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in agreement with the observation found in the XRD experiment. The carrier dynamics of perovskite films can be evaluated by PL and time-resolved PL (TRPL) measurements (Fig. 3b, c). The PL intensity of CsPbIBr2 film with PEG-passivation is enhanced by a factor of 2 compared to that of neat CsPbIBr2 film. As shown in Fig. 3b, the TRPL for both perovskite films can be theoretically fitted by curves of double-exponential decay with two best fitting decay constants. The PL lifetime consists of a fast decay component (short lifetime τ1) and a slow decay component (long lifetime τ2) which correspond to the free electron-hole bimolecular recombination and the trap-assisted recombination, respectively.37,38 It should be noted that the CsPbIBr2 film with PEG-passivation exhibits τ1 = 2.56 ns (proportion = 0.23) and τ2 = 17.31 ns (proportion = 0.77). However, the neat CsPbIBr2 film exhibits τ1 = 2.08 ns (proportion = 0.27) and τ2 = 4.78 ns (proportion = 0.73). The TRPL results indicate that lifetimes of both fast and slow decays are extended obviously owing to the PEG-passivation. The longer decay lifetime means longer diffusion length which is beneficial for enhanced photovoltaic performance of the corresponding PSC. In theory, the decrease of defects can reduce the Shockley-Read-Hall type recombination with the larger Fermi level splitting, eventually yield more non-equilibrium carriers and finally lead to a larger VOC.31 The results suggest that the PEG-passivation can reduce the defect states of CsPbIBr2 film which is in favor of a larger VOC for the corresponding photovoltaic device.


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Figure 3. (a) Absorption spectra, (b) normalized steady-state PL spectra and (c) TRPL spectra of CsPbIBr2 films without and with PEG-passivation.


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8 6 V oc

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d) 80

EQE (%)

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0.649

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0

Wavelength (nm)

Figure 4. (a) Cross section SEM image of the perovskite solar cells; (b) Current-voltage characteristic and (c) PCE statistical distribution histograms of 25 individual PSCs; (d) EQE spectra and the corresponding integrated JSC curves of the champion PSCs.

Later on, we fabricated planar solar cells with a device structure of FTO glass/cTiO2/perovskite/spiro-OMeTAD/Au, where FTO and spiro-OMeTAD are fluorine-doped tin oxide and 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spiro-bifluorene, respectively. The cross section SEM image of the PSC are presented in Fig. 4a. To intuitively compare the performance of two different types of devices, current-voltage (J-V) curves and EQE spectra of


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the best-performance CsPbIBr2 PSCs without and with PEG-passivation are shown in Fig. 4b, d. The best-performance PSC based on CsPbIBr2 without PEG-passivation exhibits a PCE of 6.36% with a VOC of 1.10 V, a short-circuit density (JSC) of 8.81 mA cm-2 and a fill factor (FF) of 0.656. For the best-performance PSC based on PEG-passivated CsPbIBr2, the JSC and FF are 8.80 mA cm-2 and 0.649, respectively, very close to those of the control device (no PEGpassivation). To our surprise, the VOC increases from 1.10 V to 1.28 V, and the PCE increases from 6.36% to 7.31%. The VOC of 1.28 V is the highest among all CsPbIBr2 PSCs (Table S1). The small change in JSC value agrees well with the little difference between EQE spectra of the two best-performance devices. Both devices have optical responses in the range from 300 to 600 nm. The EQE spectra for both devices are in agreement with the corresponding absorption spectra of CsPbIBr2 films. The highest EQE values for the devices with and without PEGpassivation are 77.4% and 78.7%, respectively. It should be noted that the calculated JSC values by integrating the EQE data with the AM 1.5G spectrum are 8.58 mA cm -2 (with PEGpassivation) and 8.71 mA cm-2 (without PEG-passivation), respectively, which match well with the measured JSC values based on the J-V curves within 3% errors. Furthermore, under forward scans, the best-performance PSCs based on CsPbIBr2 with and without PEG-passivation exhibit PCEs of 5.11% and 4.34% with hysteresis indices of 30.1% and 31.8%, respectively (Fig. S2). The results suggest that the PEG passivation slightly helps to eliminate the hysteresis of PSCs. The detailed device parameters are shown in Table 1. Comparing the devices based on pure CsPbIBr2 with an average PCE of 5.50%, the devices based on PEG-passivated CsPbIBr2 show a higher average PCE of 6.53%. A great enhancement of average VOC from 1.10 to 1.24 V is observed in going from the pure CsPbIBr2 device to the PEG-passivated CsPbIBr2 device. The average FF also increases in going from the pure CsPbIBr2 device to the PEG-passivated


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CsPbIBr2 device which contributes to the improved average PCE for the later one. 25 individual solar cells were fabricated to evaluate the reproducibility of devices (detailed photovoltaic parameters are shown in Tables S2 and S3). The PCE statistical distributions are shown in Fig. 4c, demonstrating an improved PCE for PSCs based on the PEG-passivated CsPbIBr2.

Table 1. Photovoltaic parameters of solar cells based on CsPbIBr2 with and without PEG-passivationa Active layers

VOC (V)

JSC (mA cm-2)

FF

PCE (%)

Pure CsPbIBr2

1.10 (1.10 ± 0.04)

8.81 (8.26 ± 0.36)

0.656 (0.606 ± 0.039)

6.36 (5.50 ± 0.44)

CsPbIBr2 with

1.28 (1.24 ± 0.04)

8.80 (8.23 ± 0.43)

0.649 (0.642 ± 0.026)

7.31 (6.53 ± 0.40)

PEG aThe

passivationb average values are based on 25 devices;

b

The PEG concentration in the precursor solution is 10

mg/mL.

In fact, the amount of PEG additive plays a significant role in determining the performance of PSCs. As shown in Table S4, when the PEG concentration in the precursor is 5 mg/mL, the average VOC, FF and PCE increase slightly compared to those of the pure CsPbIBr2 device. When the PEG concentration in the precursor increased to 10 mg/mL, the device parameter enhancements become more obviously. However, when the PEG concentration further increased to 15 mg/mL, the JSC and PCE decrease sharply which could be due to the fact that too much charge-insulating PEG is not beneficial for good charge transportation (Fig. S3). As shown Fig. S3, mobility of the electron-only devices decreases with the increasing PEG concentration in the precursor. Without any encapsulation, the devices based on CsPbIBr2 with and without PEGpassivation were stored in ambient environment at 20 oC and with ~25% relative humidity to


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evaluate their air stability. The device stability for both PSCs over time is shown in Fig. S4. As shown in the figure, the PEG-passivated PSC can retain more than 50% of its initial PCE after 100 hours storage. However, the PCE of pure CsPbIBr2 device is reduced to 40% of its initial PCE after only 75 hours storage. The improved stability for PEG-passivated PSC can be attributed to its more uniform and dense film (Fig. 1d) which helps to prevent the infiltration of water or oxygen efficiently. In order to examine the morphological evolution over time of the PEG-passivated CsPbIBr2 film and pure CsPbIBr2 film, we placed both films in ambient environment at 20 oC and with ~25% relative humidity. From the SEM images, we cannot find significant morphological changes over time for both films (Fig. S5). However, the appearance (color) of the pure CsPbIBr2 film changed greatly over time compared with the slight appearance change for the PEG-passivated CsPbIBr2 film (Insets in Fig. S5). This result is further confirmed by changes in the absorption spectra of the PEG-passivated CsPbIBr2 film and pure CsPbIBr2 film shown in Fig. S6. It should be noted that Li salt in the hole transport layer is highly hygroscopic and can result in the degradation of perovskite film.39 In summary, with an incorporation of PEG into the CsPbIBr2 perovskite film, we have demonstrated an enhanced PCE of 7.31% for the best-performance CsPbIBr2 PSC compared to the control PSC without the PEG-passivation (6.36%). The improved PCE is mainly attributed to the greatly increased VOC for the PEG-passivated PSC. The passivation of surface and grain boundaries by PEG leads to a reduced probability of nonradiative recombination and thus extends the lifetime of charge carriers, all of which contributes to the significantly increased VOC as well as PCE. In the stability experiment, the best-performance PEG-passivated PSC showed improved stability compared to the control PSC without PEG-passivation. Therefore, our present work offers a facile strategy to simultaneously enhance the efficiency and stability of CsPbIBr 2


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PSCs by using a novel passivating agent of PEG. With their short-wavelength light-harvesting feature, the CsPbIBr2 PSCs would be promising sub-cell candidates in tandem solar cell applications.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org. Materials, characterization and fabrication of solar cells, XPS spectra, SEM images, hysteresis of best-performance PSCs, current-voltage characteristics of electron-only devices, device stability, photovoltaic parameters of solar cells fabricated under different conditions, and other related data. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT We acknowledge the support from the National Natural Science Foundation of China (No. U1605241), the Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) (No. XDB20000000), and the Key Research Program of Frontier Sciences, CAS (No. QYZDBSSW-SLH032). We thank Mr. Zihao Zhang for a help with SEM measurements (Nanjing University of Science and Technology).

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TOC VOC=1.10 V

Au Spiro-OMeTAD Perovskite c-TiO2 FTO Glass

VOC=1.28 V


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Defect Passivation of CsPbIBr2 Perovskites for High-Performance

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