Passivation of interstitial and vacancy mediated trap-states for efficient and stable triple-cation perovskite solar cells

Journal of Power Sources 383 (2018) 59–71

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Passivation of interstitial and vacancy mediated trap-states for efficient and stable triple-cation perovskite solar cells

T

Md Arafat Mahmuda,∗, Naveen Kumar Elumalaia,∗∗, Mushfika Baishakhi Upamaa, Dian Wanga, Vinicius R. Gonçalesb, Matthew Wrighta, Cheng Xua, Faiazul Haquea, Ashraf Uddina a b

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW, 2052, Australia School of Chemistry, University of New South Wales, Sydney, NSW, 2052, Australia

H I G H L I G H T S ozone (UVO) treatment reduces interstitial and vacancy mediated traps of ZnO film. • Ultraviolet treated ZnO ETL based perovskite solar cell (PSC) demonstrates 16.73% efficiency. • UVO film on UVO treated ZnO exhibits higher inter-particle connectivity and large grain. • Perovskite treatment reduces microstrain and dislocation density values of PSCs. • UVO • UVO treated ZnO based PSC retains 88% of its initial efficiency even after a month.

A R T I C L E I N F O

A B S T R A C T

Keywords: Low temperature UV-Ozone Oxygen vacancy ZnO ETL Perovskite solar cell Electrode polarization

The current work reports the concurrent passivation of interstitial and oxygen vacancy mediated defect states in low temperature processed ZnO electron transport layer (ETL) via Ultraviolet-Ozone (UVO) treatment for fabricating highly efficient (maximum efficiency: 16.70%), triple cation based MA0.57FA0.38Rb0.05PbI3 (MA: methyl ammonium, FA: formamidinium, Rb: rubidium) perovskite solar cell (PSC). Under UV exposure, ozone decomposes to free atomic oxygen and intercalates into the interstitial and oxygen vacancy induced defect sites in the ZnO lattice matrix, which contributes to suppressed trap-assisted recombination phenomena in perovskite device. UVO treatment also reduces the content of functional hydroxyl group on ZnO surface, that increases the inter-particle connectivity and grain size of perovskite film on UVO treated ZnO ETL. Owing to this, the perovskite film atop UVO treated ZnO film exhibits reduced micro-strain and dislocation density values, which contribute to the enhanced photovoltaic performance of PSC with modified ZnO ETL. The modified PSCs exhibit higher recombination resistance (RRec) ∼40% compared to pristine ZnO ETL based control devices. Adding to the merit, the UVO treated ZnO PSC also demonstrates superior device stability, retaining about 88% of its initial PCE in the course of a month-long, systematic degradation study.

1. Introduction Organic-inorganic metal halide perovskite solar cells (PSCs) are considered to be one of the most promising third-generation photovoltaic technologies with a rapid surge in their power conversion efficiency (PCE) up to 22.10% [1] from an initial PCE of 3.8% [2] in less than a decade. Remarkably, PSCs, fabricated in low-temperature process [3–11] demonstrate unprecedented potentials for commercially lucrative, mass scale production of photovoltaic devices on flexible substrates via roll-to-roll process. To that regard, ZnO electron transport

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layer (ETL) based PSCs bid fair to be an ideal candidate for such technology, as they can be fabricated in a simple, solution-processed route [4,9–15]. Nonetheless, low-temperature processed, pristine ZnO film contains numerous defect states within its energy bandgap [16,17], which can adversely affect the photovoltaic performance of PSC by acting as recombination centres for the photo-generated charge carriers from perovskite film [15,18]. To address this shortcoming of low-temperature processed ZnO ETL, metal (Al [19,20], Mg [21], Cs [18], Li [15]) doping or a secondary organic ETL [polyethylenimine (PEI) [22], [6,6]-phenyl C61 butyric

Corresponding author. Corresponding author. E-mail addresses: [email protected] (M.A. Mahmud), [email protected] (N.K. Elumalai).

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https://doi.org/10.1016/j.jpowsour.2018.02.030 Received 6 November 2017; Received in revised form 31 January 2018; Accepted 12 February 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.

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2. Experimental section

acid methyl ester (PCBM) [23]] layer has been applied to passivate the trap-states of ZnO film. However, organic layer like PCBM is thermally unstable at any temperature around 100 °C, which (PCBM) has been reported to aggregate or diffuse during the annealing-induced perovskite crystallization stage [22] in normal structured (n-i-p) PSC. Additionally, the electron mobility of the organic ETL films is several orders of magnitude lower compared to ZnO [24]. On the other hand, recent studies [15,18] show that the metal doping only passivates the interstitial trap-states in pristine ZnO film, but the oxygen vacancy mediated trap-states of ZnO ETL remain unaffected by such dopant incorporation. In general, oxygen vacancies act as deep-level (∼2.05 eV below the conduction band [25]) donor defects within the ZnO bandgap, which initially capture the photo-generated holes and thus eventually work as recombination centres for photo-induced electrons [16]. Besides, the oxygen vacancies reduce the material crystallinity and n-type conductivity of ZnO ETL [16,26], which can deteriorate the photovoltaic performance of a PSC. Thus, it is of paramount importance to explore a ZnO ETL modification method that can simultaneously passivate the interstitial as well as oxygen vacancy induced trap-states in ZnO film for enhanced performance of PSC. In relation to this, the ultra-violet ozone (UVO) treatment of ZnO thin film ETL can be an insightful approach, since such treatment has been reported to reduce the oxygen vacancy of ZnO nanowire [27] or nanofilm [16] with organic solar cells (OSCs). However, the optimum UVO exposure time for the ZnO nanostructures reported in these studies [16,27] vary over a long range (20 s [16] to 600 s [27]) and may not be suitable with sol-gel processed ZnO thin film ETL for PSCs, which (sol-gel ZnO) offers more facile fabrication pathway compared to nanostructured ETLs [19,28]. Moreover, there remain some contradictory reports with ZnO nanorod [29] or thin film [30] in OSCs, where UVO treatment was reported to deteriorate the photovoltaic performance of the solar cell. Thus, it is still a topic of open debate whether the UVO exposure on metal oxide ETL is benign to photovoltaic devices and to date, it remains uninvestigated in case of PSCs. Therefore, it is an intriguing prospect to explore the effect of UVO treatment on the passivation of oxygen vacancy and interstitial trap-states prevalent in low temperature processed sol-gel ZnO ETL and its (UVO's) concomitant influence on the photovoltaic performance [31], photo-current hysteresis [32] and device stability [33] of PSCs, incorporating such modified ZnO ETLs. Our current work is focused along this research direction. In this work, we have reported UVO-treated sol-gel ZnO ETL for highly efficient [maximum PCE: 16.73%], organic-inorganic hybrid triple cation based MA0.57FA0.38Rb0.05PbI3 (MA: methyl ammonium, FA: formamidinium, Rb: Rubidium) PSC. An optimum duration of UVO exposure on ZnO ETL has been found out, which ensures enhanced device performance of MA0.57FA0.38Rb0.05PbI3 PSC, compared to PSCs incorporating pristine ZnO ETL. The optimized UVO-treated ZnO film demonstrates significantly suppressed interstitial as well as oxygen vacancy mediated trap-states which were quantitatively analyzed with the deconvolution of high resolution X-Ray photoelectron spectroscopy (XPS) spectra. The charge transport characteristics and recombination phenomena of the modified device have also been compared with the control device using electrochemical impedance spectroscopy (EIS) and capacitance vs voltage (C-V) characterization. The effects of UVO treatment on the surface morphology of the overlying perovskite film have also been explored and their impact on photovoltaic performance has been investigated. The electrode polarization phenomena of the fabricated devices have been compared from the respective frequencydependent capacitive spectra and their impact on photo-current hysteresis is examined. A month-long, degradation study is conducted to comprehend the device stability of the PSCs. The superior device stability with UVO treated ZnO ETL has been investigated with C-V characterization of aged device and the quantitative analysis of the functional groups present on the concerned ETL surfaces.

2.1. Fabrication method At first, patterned ITO/glass substrates were sequentially cleaned with alkaline (Hellmanex III) soap water, deionized (DI) water, anhydrous acetone and 2-propanol solvents. After the glass cleaning, a 45 nm thick, low temperature (140 °C) processed sol-gel ZnO film was deposited on the substrate in the same manner as reported in our earlier works [28,34]. For UVO treated ZnO film, a UV-ozone pro-cleaner™ (Bioforce Nanoscience Inc.) was used to expose UVO on the deposited ZnO films for different durations. For the photo-active perovskite (MA0.57FA0.38Rb0.05PbI3) layer, a precursor solution of anhydrous DMF (N, N–dimethylformamide) solvent was prepared by mixing required quantity of MAI, FAI, RbI and PbI2 powder in the solvent. The perovskite solution was spin-coated on the ETL coated films using nucleation-assisted gas-quenching method [35]. After the spin-coating, the coated substrates were annealed at 100 °C temperature for 10 min in restricted volume solvent annealing (RVSA) method [36]. For hole transporting layer (HTL), a precursor solution of 2,2′,7,7′-tetrakis[N,Ndi(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD) (spiro-OMeTAD) in anhydrous chlorobenzene (72.3 mg/ml) along with 28.8 μL 4-tert-butylpyridine (4-tbp) and 17.5 μL bis(trifluoromethane) sulfonimide lithium (Li-TFSI) (520 mg/ml in acetonitrile) was prepared and spin-casted with a spin-rate of 3000 rpm for 30s. Lastly, a 100 nm silver (Ag) layer was thermally evaporated on the substrate inside a high vacuum (∼1 × 10−6 mbar) thermal evaporator chamber. The thermal evaporation rate was 2.3 Å/s and a shadow, metal mask was used to confine the active device area to be 4.5 mm2 while doing the Ag evaporation.

2.2. Device characterization The device performance of the fabricated devices was analyzed with a Keithley 2400 source meter with a 1000 W/m2 (AM 1.5G) simulated sunlight. Before measuring the current density-voltage (J-V) characteristics of the PSCs, the light intensity was calibrated using a reference silicon solar cell, certified from national renewable energy laboratory (NREL). To prevent the over-calculation of the short-circuit current density (JSC) of the PSCs, a non-reflective, black aperture mask was put on top of the solar cells during the J-V measurement. An ESCALAB250Xi machine (Thermo Scientific Inc.) was used to perform the XPS measurement using mono-chromated AlKα (hυ = 1486.68 eV) anode (120 W, 13.8 kV, 8.7 mA). The background pressure of the chamber was maintained at 2 × 10−9 mbar. For optical characterization (transmittance, reflectance and absorbance) of the films, an ultraviolet visible near infra-red (UV–vis–NIR) spectrometer (Perkin Elmer–Lambda 950) was used. The x-ray diffraction (XRD) characterization was performed with PANalytical Empyrean thin-film XRD machine. The XRD characterization was performed with CuKα radiation, maintaining a step-scanning interval of 0.02°. Surface morphology images were captured using Carl Zeiss AURIGA cross beam scanning electron microscopy (SEM) machine. Surface topography was analyzed with Bruker Dimension ICON SPM atomic force microscopy (AFM) machine and the surface roughness was calculated using Nanoscope Analysis 1.7 version software. Water contact angle measurement on ETL films was conducted using a Ramé-hart contact angle goniometer (Model 200) and the relevant images were analyzed in DROPimage advanced software for calculating the contact angles. An Autolab PGSTAT-30 impedance analyzer was used to investigate the charge transport characterization and recombination phenomena of the fabricated devices by electrochemical impedance spectroscopy. During the impedance analysis, a 20 mV alternating current (AC) was retained to ensure a liner response from the impedance analyzer. The frequency sweeping was conducted from 100 mHz to 1 MHz. 60

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Fig. 1. (A) Schematic representation of fabricated PSCs (ZnO or UVO treated ZnO device), J-V curves (both FB-SC and SC-FB direction at 0.05 V/s) of the best performing (B) ZnO device and (C) O-ZnO device, Stabilized current density and stabilized PCE as a function of time for the best performing (D) ZnO and (E) O-ZnO device.

3. Results and discussion

(short-circuit to forward bias) and FB-SC (forward bias to short-circuit) scan directions. From Table 1, O-ZnO PSCs demonstrate about 16% higher average PCE (ZnO PSC: 13.65%, O-ZnO PSC: 15.81%) compared to ZnO devices. Fig. 1(B) and (C) depict the J-V curves of the best performing ZnO and O-ZnO devices, respectively. The champion O-ZnO PSC exhibits a PCE of 16.73% in FB-SC scan direction, while the corresponding value for the ZnO device is 14.37% (Table 1). The stabilized PCE and current density of the fabricated PSCs at maximum power point (MPP) were also measured (Fig. 1(D)-1(E)), since these two are critical yardsticks for the apprehension of photovoltaic performance of a PSC, exhibiting photo-current hysteresis phenomena [32]. The numeric values of steady-state PCE and current density of the two PSCs are listed in Table S2 for facilitating quantitative comparison. From Fig. 1(D)–1(E), the stabilized PCE and current density for O-ZnO device are 15.76% and 20.62 mA/cm2, respectively, while the related values for ZnO PSC are 13.65% and 19.97 mA/cm2, respectively, which reconfirms the enhanced device performance with O-ZnO PSC. We have

3.1. Photovoltaic performance Pristine ZnO and UVO-treated ZnO films were used as ETLs along with normal structured MA0.57FA0.38Rb0.05PbI3 PSC. The device architecture of the PSC is: ITO/ZnO or UVO treated ZnO ETL/ MA0.57FA0.38Rb0.05PbI3 perovskite/Spiro-OMeTAD HTL/Ag (Fig. 1(A)). We have conducted an optimization experiment to find out the optimum UVO exposure duration on ZnO ETL that ensures the highest average PCE with our fabricated PSCs (Table S1). From Table S1, the optimum UVO exposure duration for ZnO ETL is 10 min, which renders an average PCE of 15.81%. For further referencing, we will term the pristine ZnO and optimized UVO treated ZnO ETL as ZnO and O-ZnO ETL and corresponding PSCs as ZnO and O-ZnO device, respectively. Table 1 compares the average and the best photovoltaic performance of ZnO and O-ZnO PSCs at a scan rate of 50 mV/s in both SC-FB 61

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Table 1 Average and best photovoltaic performance with ZnO and O-ZnO PSCs at both FB-SC (forward bias to short circuit) and SC-FB (short circuit to forward bias) directions at a scan rate 0.05 V/s. The average values have been presented with corresponding standard deviation values of ten samples from a random batch. Device

Average/Best

Open Circuit Voltage, VOC (mV)

Short Circuit Current Density, JSC (mA/cm2)

Fill Factor, FF (%)

Efficiency (%)

Series Resistance, RS (Ω cm2)

Shunt Resistance RSh (Ω cm2)

ZnO

Average Best (FB-SC) Best (SC-FB) Average Best (FB-SC) Best (SC-FB)

956.09 ± 17.36 971.85 910.65 1015.61 ± 6.92 1013.22 959.92

23.28 ± 0.42 23.69 23.25 23.26 ± 0.24 23.64 23.45

61.34 ± 2.19 62.42 58.38 66.93 ± 1.42 69.85 66.55

13.65 ± 0.40 14.37 12.36 15.81 ± 0.43 16.73 14.98

9.29 ± 2.10 8.19 10.98 5.91 ± 0.90 4.46 4.82

1292 ± 165 1179 1426 1788 ± 981 2786 2524

O-ZnO

photo-generated holes and electrons. During the UVO exposure, ozone (O3) decomposes to atomic oxygen (O) by absorbing the ultra-violet ray according to the following reaction [16]:

also calculated the JSC values from the external quantum efficiency (EQE) characterization (Fig. S1) and compared the JSC values from EQE curve along with the results from J-V characterization conducted under 1 sun illumination. From the EQE curves presented in Fig. S1, the integrated short-circuit current density (JSC) of ZnO and O-ZnO PSCs are 22.80 mA/cm2 and 22.64 mA/cm2, respectively, which match closely with the JSC values attained from J-V measurement. The small difference between the Jsc values obtained from EQE and J-V curve could originate from multiple sources, such as inconsistency in solar simulator and tungsten lamp and characterization time span [37,38]. Overall, both the control and reference devices demonstrate good reproducibility, which is illustrated with a statistical chart-box in Fig. S2, showing the variation in the basic J-V parameters (PCE, VOC, JSC and FF). Corresponding numeric values have been presented in Table S3 and Table S4, respectively for ZnO and O-ZnO PSCs. Here, it is worth mentioning that the PCE (16.73%) demonstrated by O-ZnO device in this work is the highest among the PSCs reported with sol-gel ZnO ETL [15,18,21,39–44]. Although, higher PCEs with ZnO ETL based PSCs have already been reported, all those studies have incorporated nanostructured ZnO network (nano-particle [45], nano-pillar [46] etc.) as ETLs, which necessitate time-consuming and sophisticated synthesis process, as compared to the facile deposition technique of sol-gel ZnO ETL. From Table 1, the enhanced device performance with O-ZnO PSCs mostly comes along with the significant enhancement in VOC and FF values in them relative to ZnO devices, as the average JSC values for both the devices are almost identical. In the following sections, we have demonstrated the results obtained from a number of characterization techniques, aimed at discerning the superior device performance with O-ZnO device compared to ZnO PSC.

O3 ⇄ O2 + O

(1)

The free atomic oxygen (O) demonstrates a high oxidizing capability that modifies the chemical state of oxygen to be more stoichiometric in O-ZnO film by filling the oxygen vacancies [27]. Besides, as observed from Fig. 2(A)–2(B), the peak positions of O1s A, O1s B and O1s C spectra shift to higher binding energy in O-ZnO film, with respect to ZnO film. We can observe similar high energy shifts for Zn2p(1/2) and Zn2p(3/2) XPS spectra in O-ZnO film from the high resolution Zn2p spectra of the ETL films (Fig. 2(C)). The simultaneous high energy shifts of O1s and Zn2p XPS spectra for O-ZnO ETL indicate that more interstitial ZnO sites [27] are filled with oxygen atoms in O-ZnO film, which (interstitial sites) can work as trapping centres for photo-induced carriers [17]. Thus, both the interstitial and oxygen vacancy mediated trap-states are more suppressed in O-ZnO ETL film, compared to ZnO ETL. This is also consistent with the enhanced average VOC values, demonstrated with O-ZnO PSCs (Table 1) due to reduced trap-assisted recombination [47]. Referring back to Fig. 2(A)–2(B), the integrated area ratio S (S′ = S + S3 + S ) [48] for O-ZnO ETL is slightly higher (ZnO: 29.72%, O1 2 3 ZnO: 30.21%) compared to ZnO ETL film. The slightly higher area ratio, S′ for O-ZnO ETL denotes that the adsorbed species on O-ZnO ETL is relatively higher [48] in contrast with ZnO ETL. The higher oxygen related adsorbed species in O-ZnO film is consistent with the passivation of oxygen vacancy mediated trap-sites in it (O-ZnO), which (the trap sites) tend to absorb atomic (O) or molecular (O2) oxygen [27] during the UVO exposure process. However, to quantitatively investigate the content of various chemisorbed species in the two ETL films, we have analyzed the high resolution C1s XPS spectra of ZnO (Fig. 2(D)) and O-ZnO (Fig. 2(E)) films. The C1s XPS spectra of ZnO ETL film can be de-convoluted in two Gaussian sub-peaks, whereas, there is an additional sub-peak for O-ZnO ETL at a binding energy of 289.29 eV. The two common sub-peaks for ZnO and O-ZnO ETL films can be termed as C1s A and C1s C spectra, while the additional sub-peak for ZnO as C1s B spectra. Generally, C1s A, C1s B and C1s C spectra are related to non-oxygenated sp3 hybridized carbon bond (C-C), the bond between carbon atom and hydroxyl group (C-OH) and the bond associated with ether/epoxy group (C-O-C) [50,51], respectively. From Fig. 2D), the presence of C1s B peak for ZnO film denotes that the amount of hydroxyl group is higher on ZnO ETL, compared to O-ZnO ETL film. In line with this observation, the elemental analysis of the XPS survey curves of the two ETL films (Fig. S3) also shows that ZnO film contains 1.90% hydroxyl functional group on its surface, whereas the hydroxyl group is undetected in O-ZnO ETL film (Tables S5–S6). The presence of functional group like hydroxyl on ZnO surface is likely to reduce the electron mobility [47] of ZnO ETL. Thus, combined with the oxygen vacancy, higher content of hydroxyl groups in ZnO film are expected to decrease its (ZnO ETL's) n-type conductivity [50], compared to O-ZnO ETL. The reduced n-type conductivity of ZnO ETL is also evident from the lower dark injection current [52] in the forward

3.2. Material and optical characterization of ETL films To investigate the modification of pristine ZnO surface with UVO treatment, we have conducted XPS elemental analysis of ZnO and OZnO films on ITO/glass substrate. The envelop curve and the de-convoluted Gaussian sub-peaks of high resolution O1s XPS spectra of ZnO and O-ZnO films are presented in Fig. 2(A) and Fig. 2(B), respectively. Table S5 and Table S6 list the related peak binding energies, peak full width at half maximum (FWHM) and atomic elemental percentage of ZnO and O-ZnO ETL films, respectively. As observed from Fig. 2(A)–2(B), both the ETL films demonstrate three Gaussian subpeaks, namely O1s A, O1s B and O1s C, respectively in ascending order of their peak binding energies (Tables S5–S6). Generally, the sub-peaks O1s A, O1s B and O1s C of ZnO film are associated with the chemical bond between neighboring oxygen and zinc atoms (Zn-O bond) [47], the oxygen deficient or oxygen vacancy regions [48], and the bonds related to the chemisorbed species [hydroxyl (-OH), adsorbed O2 etc.] on ZnO surface [49], respectively. As marked in Fig. 2(A)–2(B), the S integrated area ratio (S = S +2S ) is lower for O-ZnO film, compared to 1 2 ZnO film (ZnO: 46.58%, O-ZnO: 45.32%). The lower area ratio, S for OZnO film denotes that the oxygen vacancies are suppressed with UVO treatment [47], which can otherwise act as recombination centres for 62

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Fig. 2. Deconvoluted Gaussian subpeaks and envelop curve of high resolution O1s XPS spectra for (A) ZnO and (B) O-ZnO ETL film, (C) High resolution XPS spectra of Zn2p for ZnO and OZnO ETL films on ITO/glass substrate, Deconvoluted Gaussian subpeaks and envelop curve of high resolution C1s XPS spectra for (D) ZnO and (E) O-ZnO ETL film, (F) Dark I-V of ZnO and O-ZnO PSCs (the inset shows amplified view of the forward injection region).

conjunction with perovskite film [22,54]. With regard to these characterization results, the trap-state mitigation and the superior n-type conductivity of O-ZnO ETL can be illustrated with a schematic presentation (Scheme 1). Scheme 1(A) shows the wurtzite ZnO lattice structure, depicting the interstitial as well as oxygen vacancy mediated trapping or recombination centres. The ZnO surface also contains large amount of functional hydroxyl groups (Scheme 1(A)), that reduces the n-type conductivity of ZnO ETL. With UVO treatment, the decomposed ozone gas releases oxygen atoms, which passivates the interstitial and oxygen vacancy mediated defect sites (Scheme 1(B)). Thus, O-ZnO ETL ensures lower trap-assisted recombination and enhanced charge transfer property at perovskite/ETL interface for O-ZnO PSC, which contributes to higher VOC, FF and henceforth enhanced PCE values attained with them (Table 1). A detailed investigation into the charge transfer property and the trapmediated carrier recombination phenomena of the fabricated devices,

bias region (applied bias > 0.7 V) [53] of ZnO device from the dark I-V characteristics of the two PSCs (Fig. 2(F)). This also conforms to the higher average series resistance, RS (ZnO PSC: 9.29 Ω cm2, O-ZnO PSC: 7.83 Ω cm2) values in ZnO PSC, compared to O-ZnO device from light JV measurement (Table 1). The increased series resistance of ZnO ETL implicates towards less n-type conductivity of ZnO ETL and less efficient charge extraction property at perovskite/ETL interface for ZnO PSC, which is consistent with the lower average FF value (Table 1) in them, relative to O-ZnO device. Here, it is worth mentioning that it would be an intriguing prospect for future study to conduct Hall-effect measurement for observing the conductivity, carrier concentration, and mobility of ZnO layer before and after UVO treatment to evaluate how UVO treatment affects the mobility of bulk ZnO. Referring back to Tables S5–S6, although O-ZnO demonstrates slightly higher ether groups (ZnO: 4.24% O-ZnO: 4.74%) on its surface, compared to ZnO film, the ether groups are not as detrimental as the hydroxyl groups, in 63

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Scheme 1. Fig. (A) depicts ZnO wurtzite lattice structure for pristine ZnO ETL, demonstrating the interstitial and oxygen vacancy mediated defect states, which can work as charge trapping sites and cause the photo-generated carrier recombination during the operation of perovskite solar cell. Pristine ZnO also contains a large number of functional hydroxyl groups on its surface, that reduces the n-type conductivity of ZnO ETL. Fig. (B) shows the ZnO lattice matrix for O-ZnO ETL. During UV exposure, ozone decomposes to atomic oxygen, which passivates the interstitial and oxygen vacancy mediated trap sites. Moreover, the content of hydroxyl group reduces on O-ZnO ETL by the UV-ozone treatment. Thus, O-ZnO ETL exhibits higher n-type conductivity and OZnO PSC renders enhanced photovoltaic performance, compared to ZnO PSC.

We have also characterized the optical absorption of identically fabricated perovskite films on top of two different ETL films (Fig. S4). As comprehended from Fig. S4, the absorption profiles of the perovskite films on top of ZnO and O-ZnO ETL films are not significantly different, which is consistent with the close-ranged JSC values of ZnO and O-ZnO PSCs from J-V characterization (Table 1). The optical bandgaps of ZnO and O-ZnO films are also nearly identical (ZnO: 3.32 eV, C-ZnO: 3.34 eV), as calculated from the corresponding Tauc plots, shown in Fig. S5.

by means of electrochemical impedance spectroscopy, has been presented in the following section of this work. We have also examined the optical transmittance of both the ETL films using UV–vis–NIR spectrometer (Fig. 3(A)), since, an ETL film is required to be highly transparent in a normal structured (layer sequence: n-i-p) PSC so that adequate sunlight can be transmitted to the overlying photo-active perovskite film. The calculated average visible (370–740 nm) transmittance (AVT) [55,56] values of ZnO and O-ZnO films are 86.83% and 88.37%, respectively, which denote both the ETL films exhibit befitting optical transparency for a normal structured PSC.

Fig. 3. (A) Transmittance curves of ZnO and O-ZnO films on ITO/glass substrate, (B) XRD patterns of MA0.57FA0.38Rb0.05PbI3 perovskite films fabricated on top of ZnO and O-ZnO ETL films (on ITO/glass substrate) showing the major diffraction peaks and the corresponding crystal orientation, (C) Relative normalized peak intensity and crystallite dimension of the perovskite films fabricated on top of two ETL films from the XRD spectral peak analysis, (D) Microstrain and dislocation density of perovskite films on top of ZnO and O-ZnO ETL films from the XRD spectral peak analysis.

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contributes to enhanced FF values in O-ZnO PSC, leading to its enhanced device performance with reference to ZnO PSCs. We have also analyzed the surface topology of the perovskite films on top of both the ETL films using atomic force microscopy (AFM) (Fig. 4(E)-4(H)). Table S10 lists the average as well as the root mean square (RMS) surface roughness of the perovskite films on ZnO and OZnO ETL films from AFM characterization. As observed from Fig. 4(E)–4(H) and Table S10, with the increase in perovskite grain size on O-ZnO ETL, the surface roughness of perovskite film slightly increases, which is consistent with previous literature [15]. We have also calculated the micro-strain and dislocation density [15,36] values of perovskite films on both the ETLs from the XRD spectral peak fitting (Fig. 3(D)). In principle, micro-strain (ε ) quantifies the amount of point defects in a nano-crystalline material like perovskite and is mathematically expressed as [63]:

3.3. Material, surface morphology and topology characterization of perovskite film To investigate whether the bulk perovskite property varies on top of two different ETLs, we have analyzed the XRD patterns of identically fabricated MA0.57FA0.38Rb0.05PbI3 perovskite films over ZnO and OZnO ETL films (Fig. 3(B)). During the XRD characterization, identical film thickness, substrate size and X-Ray intensity were maintained for both the samples, so as to eliminate any variation in volume fraction considered for each measurement. For both the perovskite films, (110), (220), (310), (224) and (314) XRD peaks ascertains the formation of tetragonal perovskite structure [57,58]. From the XRD peak spectral fitting, the normalized peak intensity and the crystallite dimension [59] of the perovskite films have been presented in Fig. 3(C). Table S7 lists the related values for facilitating numeric comparison. From Fig. 3(C), the normalized peak intensity or the crystallinities of the two perovskite films are almost identical, related to all the major diffraction peaks. However, the individual crystallite dimension is somewhat higher in perovskite atop O-ZnO film, specially pertaining to (220), (310) and (224) characteristic XRD peaks. To investigate more into it, we have compared the perovskite grain size on ZnO and O-ZnO ETL films from the top view surface morphology image of the perovskite films with scanning electron microscopy (SEM) (Fig. 4). From Fig. 4, we can observe that most perovskite grains grown on O-ZnO ETL demonstrates relatively higher grain size compared to the same (perovskite) on top of ZnO ETL. Here, it is worth mentioning that every individual perovskite grain on O-ZnO ETL is not larger than the same (each perovskite grain) on ZnO ETL. This is consistent with the fact that the crystallite dimension of perovskite film on O-ZnO ETL (as calculated from XRD spectral analysis in our work) is larger than the same (perovskite on ZnO ETL) with respect to most of the perovskite characteristic peaks, but not for all the XRD peaks. For example, with regard to (110) peak, perovskite crystallite dimension is larger on ZnO ETL than O-ZnO ETL [Fig. 3(C)]. To discern the origin of overall higher perovskite grain size on OZnO ETL, we have conducted wettability characterization of the two ETL films with water droplets (Fig. S6), since less hydrophilic ETL surface can contribute to large perovskite growth on top of it (ETL) [60]. Table S8 lists the values of water contact angles on various positions of ZnO and O-ZnO ETL surfaces. From Table S8, the average values of water contact angles on ZnO and O-ZnO films are (33.4 ± 1.5)0 and (28.3 ± 1.3)0, respectively, denoting that UVO treatment makes the ZnO surface somewhat more hydrophilic. So, our results suggest that the perovskite grain growth, in our case, was not heavily affected by the surface wettability of the two underlying ETL films. Thus, the larger perovskite grain growth on O-ZnO ETL film can be attributed to the lower content of functional hydroxyl groups on its surface, which (functional groups) can otherwise reduce the inter-particle connectivity and thus hamper the perovskite grain-growth [61] on ETL film. In this regard, we have conducted atomic force microscopy (AFM) characterization of the two ETL films (Fig. S7). Corresponding numeric values of average and root mean square (RMS) surface roughness of the ETL films have been listed in Table S9. As observed from Fig. S7 and Table S9, UVO treatment reduces the surface roughness of ZnO film, which facilitates more enhanced inter-particle connectivity of perovskite grain and thus contributes to larger perovskite grain growth on O-ZnO ETL film, compared to ZnO ETL. The larger perovskite grain in O-ZnO device contributes to its enhanced device performance, compared to ZnO PSC. In principle, the ionized donor-acceptor pair (DAP) recombination phenomena are significantly suppressed in a perovskite film having large grain [62], which explains the higher average VOC value in O-ZnO PSC, relative to ZnO device. Additionally, large perovskite grain in O-ZnO PSC decreases the quantity of grain-boundaries in it (perovskite), which reduces the inter-particle contact resistance and thus lowers the series resistance [61] in O-ZnO device (Table 1). Reduced series resistance

ε=

() β 4

(2)

cotθ

where, θ and β refer to Bragg diffraction angle and full width at half maximum (FWHM) of a characteristic diffraction peak of perovskite, respectively. On the other hand, dislocation density (δ) gives a quantitative idea regarding the imperfection in the crystalline lattice structure and is expressed by the following Williamson and Smallman's equation [64]:

δ=

n d2

(3)

where, n and d denote a constant (n ≈ 1 considering lowest possible dislocation [12]) and crystallite dimension from Debye Scherrer equation [59], respectively. From Fig. 3(D), the micro-strain and dislocation density values of ZnO film is relatively higher compared to O-ZnO film, specially pertaining to (220), (310) and (224) characteristic perovskite peaks. This denotes that the point defects and crystalline imperfection are relatively higher in perovskite atop ZnO ETL, compared to O-ZnO ETL. This result from the XRD spectral fitting conforms to the higher leakage current in ZnO PSC at the negative bias region (applied bias < −0.5 V) from the dark I-V curve (Fig. 2(F)). Due to the higher leakage current, the reverse dark saturation current density (J0) in ZnO PSC is expected to be larger than that of O-ZnO device. Usually, the VOC of a PSC is mathematically expressed as: [65]

VOC =

kT ⎛ JSC ⎞ ln q ⎝ J0 ⎠ ⎜

⎟

(4) −5

where, k, T and q stand for Boltzmann's constant (8.62 × 10 eV/K), absolute temperature in Kelvin scale and electron charge, respectively. As comprehended from Eq. (4), higher J0 results in reduced VOC for ZnO PSCs. The higher leakage current in ZnO PSC is also congruent with the reduced shunt resistance in them, as compared to O-ZnO device from Table 1. Therefore, O-ZnO PSCs demonstrate superior photovoltaic performance, compared to ZnO PSCs. 3.4. Charge transport characteristics To gain further insight into the charge transfer properties and the trap-assisted recombination phenomena of the fabricated devices, we have conducted EIS characterization [66,67]. Fig. 5(A) depicts the Nyquist plots of ZnO and O-ZnO PSCs in dark condition under a forward bias of 0.95 V. The inset of Fig. 5(A) shows the equivalent electronic circuit model [13,28,68,69], which was used to fit the experimental data from the Nyquist plots. The equivalent circuit consists of two parallel (resistance-capacitance) R-C components, along with a series resistance, where, RC, CC, RRec, Cμ and RSE refer to interfacial contact resistance pertaining to perovskite/ETL or perovskite/HTL interface, capacitance related to the bulk (perovskite) property, recombination resistance, chemical capacitance and the resistance owing to the wire 65

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Fig. 4. Top view Scanning Electron Microscopy (SEM) images of MA0.57FA0.38Rb0.05PbI3 perovskite on top of (A)–(B) ZnO and (C)–(D) O-ZnO ETL films. Two and three dimensional atomic force microscopy (AFM) images of MA0.57FA0.38Rb0.05PbI3 perovskite on top of (E)–(F) ZnO and (G)–(H) O-ZnO ETL films.

MA0.57FA0.38Rb0.05PbI3 perovskite to neighboring O-ZnO ETL. This is also consistent with the higher n-type conductivity of O-ZnO ETL compared to ZnO ETL, due the reduced content of functional hydroxyl group in it (O-ZnO ETL). The lower RC value with O-ZnO PSC from EIS characterization also complies with its reduced series resistance [43] and superior FF value from J-V analysis. From Table 2, O-ZnO PSC also exhibits about 39% higher recombination resistance, RRec (ZnO PSC: 67.05 Ω cm2, O-ZnO PSC: 93.15 Ω cm2) than ZnO PSC. Higher RRec value with O-ZnO PSC from EIS analysis also complies with the higher shunt resistance [43], RSh for O-ZnO device from J-V characterization (Table 1). Since, during the EIS characterization under dark, the external voltage supply is the only source of injected charge carriers, the higher RRec with O-ZnO PSC

and metal connections, respectively. In our work, we have attained a reasonable fitting [chi-squared value (χ2): 0.00219] for our experimental data incorporating the resistive and capacitive element alone in the equivalent electronic circuit [70,71]. Therefore, we have not included any CPE element in our equivalent circuit. All the values of fitted electronic parameters have been listed in Table 2. From Table 2, ZnO PSC demonstrates about 91% higher value of contact resistance, RC (ZnO PSC: 26.78 Ω cm2, O-ZnO PSC: 14.04 Ω cm2), compared to O-ZnO PSC. Since, the HTL layer (spiro-OMeTAD) is common for both the PSCs, the variation in contact resistance between the two devices is expected to originate from perovskite/ETL interface [13]. Therefore, the lower RC value with O-ZnO PSC reconfirms the enhanced charge extraction phenomena [28,43] from the photo-active 66

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Fig. 5. (A) Nyquist plots of ZnO and O-ZnO devices at 950 mV bias under dark (The inset shows the equivalent electronic circuit used to fit the experimental data to extract various electronic parameters of the device, (B) Mott Schottky and C-V curves of ZnO and O-ZnO devices at 10 KHz frequency under dark, (C) Capacitive response of the fabricated PSCs as a function of frequency at zero external bias under dark (The inset shows the amplified view of the low frequency capacitive response).

Fermi level modulation of minority carriers [73] and thus can play an adverse role on the device performance. Therefore, O-ZnO PSC exhibits enhanced device performance, relative to ZnO PSC.

Table 2 Fitted values of different electronic parameters from Nyquist plots of ZnO and O-ZnO PSCs at a bias of 950 mV under dark. Device

RSE (Ω cm2)

RC (Ω cm2)

RRec (Ω cm2)

CC (nF/cm2)

Cμ (nF/cm2)

ZnO O-ZnO

2.45 2.91

26.78 14.04

67.05 93.15

26.00 24.89

226.67 131.11

3.5. Hysteresis As comprehended from the qualitative observation of the J-V curves (Fig. 1(B)-1(C)), both the PSCs demonstrate photo-current hysteresis, to some extent. To quantitatively compare the hysteretic behavior of the two devices, we can calculate the corresponding hysteresis index (HI) values from the following expression [10,32]:

denotes mitigated charge trapping phenomena at the defect or dislocation sites [72] of the bulk perovskite or at the interface between perovskite and O-ZnO ETL film. The reduced trap-assisted recombination phenomena are thus conforming to the suppressed interstitial and oxygen vacancy mediated trap-centres in O-ZnO ETL and lower microstrain and dislocation density values of perovskite atop O-ZnO film. To investigate further into the variation in the photovoltaic performance between the two PSCs, we have accomplished Mott-Schottky and C-V characterization of ZnO and O-ZnO devices (Fig. 5(B)). As comprehended from the Mott Schottky curves presented at Fig. 5(B), there is a direct transition between geometrical capacitance (Cg) region and accumulation capacitance (CS) region, with no distinct region for depletion layer capacitance (Cdl) [73]. This direct transition denotes that the fabricated perovskite layers in both ZnO and O-ZnO PSCs are highly crystalline and they do not contain large enough defect density (∼1017 cm−3), as to be detected from the Mott-Schottky analysis [73]. Thus, in our case, determining the flat-band potentials from MottSchottky analysis is not feasible. However, as observed from the corresponding capacitance vs voltage (C-V) curves, O-ZnO PSC demonstrates lower capacitance value in the high bias accumulation capacitance (CS) region, as compared to ZnO PSC. In general, accumulation capacitance is related with the ionic accumulation mechanism occurring at the perovskite/ETL interface [74,75], which can lead to the

JFB − SC HI =

( )−J ( ) J ( ) (5) ( ) and J ( ) refer to current densities at a bias Voc

SC − FB

2

FB − SC

where, JFB − SC

( ) Voc

Voc

2

Voc

Voc

2

2

SC − FB

Voc

2

of 2 for FB-SC and SC-FB scan directions, respectively. The values of the hysteresis indices for ZnO and O-ZnO PSCs have been listed in Table S11. From Table S11, the HI value (ZnO PSC: 0.021, O-ZnO PSC: 0.014) and hence the photo-current hysteresis [10,76] are relatively lower in O-ZnO PSC, compared to ZnO device. Here, it is notable that the hysteresis index (0.014) demonstrated by O-ZnO device is lower than many other reported high efficiency PSCs, based on ZnO ETL [15,18,21,41,45,77,78]. Although, some prior perovskite studies have reported less hysteretic behavior (as comprehended qualitatively from the respective J-V curves, since the value of HI is not explicitly reported) [42,46,79–83] compared to the current work, however, either it requires delicate nano-structured network [46] or additional fullerene derivative [42] to attain such low hysteretic behavior or the reported efficiencies are lower [79–83] than that of the current work. To probe into the hysteretic behavior of the fabricated devices, we 67

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Fig. 6. Normalized device performances of ZnO and O-ZnO devices as a function of sample storage time in a N2 filled glove box: (A) Normalized PCE, (B) Normalized VOC, (C) Normalized JSC and (D) Normalized FF. All the J-V parameters were obtained with FB-SC (forward bias to short-circuit) scan direction with a scan rate of 0.05 V/s, (E) Mott Schottky and C-V curves of aged (30 days) ZnO and O-ZnO devices at 10 KHz frequency under dark.

the electrode polarization process is more suppressed in them (O-ZnO PSC), contributing to lower photocurrent hysteresis in O-ZnO device, compared to ZnO PSC. The mitigated electrode polarization phenomena in O-ZnO PSC can be ascribed to the suppressed interstitial and oxygen vacancy mediated trap-states in O-ZnO ETL.

can refer to the frequency-dependent capacitive spectra of ZnO and OZnO PSCs at zero bias under dark (Fig. 5(C)). Generally, with regard to photo-current hysteresis, the region of interest in the capacitive spectra is the low frequency zone (frequency range: 0.1–1 Hz), which is related to the inherent electrode polarization phenomena of PSCs [74]. The electrode polarization process is linked with the slow ion migration kinetics [74,84] to the external electrodes, which can alter the local electric field distribution in a PSC. The imbalance in the local electric field causes a variation in the time required for reaching the equilibrium condition in each scan during the J-V characterization, which leads to the hysteretic photo-current behavior of PSCs [74,85]. The electrode polarization phenomena are exhibited by a high capacitive response in the capacitive spectra of PSC, specially at the low frequency zone [74]. As comprehended from the inset of Fig. 5(C), the low frequency capacitance for O-ZnO PSC is relatively lower, which denotes

3.6. Device stability A month long, degradation study was conducted to assess the device stability of the fabricated ZnO and O-ZnO PSCs. The un-encapsulated devices were kept inside a N2 filled glovebox under dark [61,86,87] and the J-V characterization was conducted on a regular basis in a humidity-controlled (relative humidity: 35%–40%), ambient atmosphere. Fig. 6(A), (B), (C) and (D) depict the normalized PCE, VOC, JSC and FF values, respectively of ZnO and O-ZnO PSCs as a function of storage 68

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resulting in mitigated photo-current hysteresis phenomena in it, compared to ZnO PSC. O-ZnO PSC also demonstrates superior device stability, relative to ZnO device, retaining about 88% of its (O-ZnO device) initial PCE even after a month. The enhanced device stability with OZnO PSC has been found to be associated with the lower content of functional hydroxyl groups and the suppressed interstitial and oxygen vacancy mediated trap-states in O-ZnO ETL. Thus, the reported UVO treated sol-gel ZnO ETL demonstrates promising prospects towards the roll-to-roll production of mass scale, flexible substrate based PSCs in low-temperature route.

time. Table S12 and Table S13 tabulate the day-wise J-V parameters for ZnO and O-ZnO devices, respectively. From Fig. 6(A), O-ZnO device holds about 88% of its commencing PCE even after a month, whereas the corresponding value for ZnO PSC is about 75% over the same period of time. This indicates O-ZnO PSC demonstrates superior device stability, as compared to ZnO device. As comprehended from Fig. 6(B)–6(D), the enhanced stability of O-ZnO PSC comes along with higher normalized VOC, JSC and FF values retained in it, as compared to ZnO PSC over the course of degradation study. From Fig. 6(B), O-ZnO PSC retains about 98% normalized VOC value after 30 days, while it drops down to about 90% for ZnO device. To probe into the higher normalized VOC value with aged O-ZnO PSC, we have conducted the Mott-Schottky and C-V characterization of the aged ZnO and O-ZnO devices (Fig. 6(E)). From the Mott-Schottky curves presented in Fig. 6(E), we observe similar direct transition between Cg and CS region, as seen in fresh PSCs. However, the aged ZnO PSC demonstrates an order of higher CS compared to O-ZnO PSC, whereas, for the fresh devices, the difference between the CS values of the two devices was not so large. This denotes that the ionic accumulation mechanism and concomitant Fermi level modulation are more rapid in aged ZnO PSC, compared to aged O-ZnO PSC. This explains the higher normalized VOC values retained in aged O-ZnO PSCs, with reference to aged ZnO devices. In addition to the reduced normalized VOC, aged ZnO PSC also demonstrates relatively lower normalized JSC and FF values, compared to aged O-ZnO PSC. The lower JSC and FF values in aged ZnO PSC can be attributed to two possible mechanisms. Firstly, ZnO ETL exhibits higher content of functional hydroxyl groups on its surface, compared to OZnO ETL. In earlier literature [20,22], hydroxyl groups have been reported to play an adverse role in terms of perovskite decomposition. The hydroxyl groups can decompose the perovskite structure by breaking the ionic interaction [22] between the organic/inorganic hybrid cation (MA0.57FA0.38Rb0.05NH2+) and the PbI3− anion. Therefore, the hydroxyl group induced perovskite decomposition process is expected to be more accelerated in ZnO PSC, which conforms to the lower normalized JSC and FF values in it than the O-ZnO PSC. Secondly, with aging, the trapped charges at the interstitial and the oxygen vacancy sites of ZnO ETL can electrostatically deform the perovskite crystal [88], which facilitates the water molecule penetration into perovskite structure. This can accelerate the water-induced perovskite degradation phenomena [89] in ZnO device. Therefore, overall, aged O-ZnO PSC demonstrates higher normalized JSC and FF and hence enhanced device stability, as compared to ZnO device.

Acknowledgement The authors gratefully acknowledge the financial support provided by Future Solar Technologies Pty. Ltd. for this research work. Author V.R.G. thanks ARC for the Australian Laureate Fellowship (FL150100060) attributed to Scientia Prof. J. Justin Gooding (School of Chemistry, UNSW-Sydney). The authors would also like to acknowledge the endless support from the staffs of Photovoltaic and Renewable Energy Engineering School, Electron Microscope Unit (EMU) and Solid State and Elemental Analysis Unit under Mark Wainwright Analytical Centre, UNSW. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2018.02.030. References [1] W.S. Yang, B.-W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells, Science 356 (2017) 1376–1379. [2] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [3] H. Tan, A. Jain, O. Voznyy, X. Lan, F.P. García de Arquer, J.Z. Fan, R. QuinteroBermudez, M. Yuan, B. Zhang, Y. Zhao, F. Fan, P. Li, L.N. Quan, Y. Zhao, Z.-H. Lu, Z. Yang, S. Hoogland, E.H. Sargent, Efficient and stable solution-processed planar perovskite solar cells via contact passivation, Science (2017) eaai9081. [4] Q. An, P. Fassl, Y.J. Hofstetter, D. Becker-Koch, A. Bausch, P.E. Hopkinson, Y. Vaynzof, High performance planar perovskite solar cells by ZnO electron transport layer engineering, Nano Energy 39 (2017) 400–408. [5] Y. Li, L. Meng, Y. Yang, G. Xu, Z. Hong, Q. Chen, J. You, G. Li, Y. Yang, Y. Li, Highefficiency robust perovskite solar cells on ultrathin flexible substrates, Nat. Commun. 7 (2016) 10214. [6] M. Park, J.-Y. Kim, H.J. Son, C.-H. Lee, S.S. Jang, M.J. Ko, Low-temperature solution-processed Li-doped SnO2 as an effective electron transporting layer for highperformance flexible and wearable perovskite solar cells, Nano Energy 26 (2016) 208–215. [7] X. Yin, P. Chen, M. Que, Y. Xing, W. Que, C. Niu, J. Shao, Highly efficient flexible perovskite solar cells using solution-derived NiOx hole contacts, ACS Nano 10 (2016) 3630–3636. [8] S.S. Shin, W.S. Yang, J.H. Noh, J.H. Suk, N.J. Jeon, J.H. Park, J.S. Kim, W.M. Seong, S.I. Seok, High-performance flexible perovskite solar cells exploiting Zn2SnO4 prepared in solution below 100 °C, Nat. Commun. 6 (2015) 7410. [9] K. Mahmood, B.S. Swain, A. Amassian, 16.1% efficient hysteresis-free mesostructured perovskite solar cells based on synergistically improved ZnO nanorod arrays, Adv. Energy Mater. (2015) 1500568. [10] R.T. Ginting, E.-S. Jung, M.-K. Jeon, W.-Y. Jin, M. Song, J.-W. Kang, Low-temperature operation of perovskite solar cells: with efficiency improvement and hysteresis-less, Nano Energy 27 (2016) 569–576. [11] D. Liu, T.L. Kelly, Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques, Nat. Photon. 8 (2014) 133–138. [12] M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, F. Haque, M. Wright, C. Xu, A. Uddin, Controlled nucleation assisted restricted volume solvent annealing for stable perovskite solar cells, Sol. Energy Mater. Sol. Cell. 167 (2017) 70–86. [13] D. Liu, J. Yang, T.L. Kelly, Compact layer free perovskite solar cells with 13.5% efficiency, J. Am. Chem. Soc. 136 (2014) 17116–17122. [14] M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, B. Puthen-Veettil, F. Haque, M. Wright, C. Xu, A. Pivrikas, A. Uddin, Controlled Ostwald ripening mediated grain growth for smooth perovskite morphology and enhanced device performance, Sol. Energy Mater. Sol. Cell. 167 (2017) 87–101. [15] M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, V.R. Goncales, M. Wright, C. Xu, F. Haque, A. Uddin, A high performance and low-cost hole transporting layer

4. Conclusion We have reported UVO treated sol-gel ZnO ETL in conjunction with highly efficient (maximum PCE: 16.73%) organic/inorganic triple cation based MA0.57FA0.38Rb0.05PbI3 PSCs in a low temperature fabrication route, suitable for flexible substrates. The optimum UVO exposure time on ZnO ETL has been found to be 10 min, which provides about 16% higher average PCE, compared to pristine ZnO ETL based PSCs. UVO treatment on ZnO ETL passivates the interstitial as well as the oxygen vacancy mediated trap-states on pristine ZnO surface, that contributes to suppressed trap-assisted recombination phenomena and hence enhanced photovoltaic performance with O-ZnO PSC. UVO treatment also reduces the content of functional hydroxyl groups on pristine ZnO ETL, that leads to better inter-particle connectivity and larger perovskite grain growth on top of O-ZnO ETL. Larger perovskite grain-size atop O-ZnO film contributes to reduced inter-particle resistance and lower DAP recombination, which leads to superior device performance with O-ZnO PSC. The EIS characterization conducted in our study also confirms that the interfacial contact resistance at perovskite/ETL interface decreases and the device recombination resistance increases with the UVO exposure on ZnO ETL. As a result, the electrode polarization process is more suppressed in O-ZnO PSC, 69

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[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]

for efficient and stable perovskite solar cells, Phys. Chem. Chem. Phys. 19 (2017) 21033–21045. D. Li, W. Qin, S. Zhang, D. Liu, Z. Yu, J. Mao, L. Wu, L. Yang, S. Yin, Effect of UVozone process on the ZnO interlayer in[space]the inverted organic solar cells, RSC Adv. 7 (2017) 6040–6045. M. Smirnov, Electronic transport properties in pollycrystalline ZnO thin films, J. Adv. Res. Phys. 1 (2010). M. Arafat Mahmud, N. Kumar Elumalai, M. Baishakhi Upama, D. Wang, V.R. Gonçales, M. Wright, J. Justin Gooding, F. Haque, C. Xu, A. Uddin, Cesium compounds as interface modifiers for stable and efficient perovskite solar cells, Sol. Energy Mater. Sol. Cell. 174 (2018) 172–186. M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, M. Wright, T. Sun, C. Xu, F. Haque, A. Uddin, Simultaneous enhancement in stability and efficiency of lowtemperature processed perovskite solar cells, RSC Adv. 6 (2016) 86108–86125. X. Zhao, H. Shen, Y. Zhang, X. Li, X. Zhao, M. Tai, J. Li, J. Li, X. Li, H. Lin, Aluminum-Doped zinc oxide as highly stable electron collection layer for perovskite solar cells, ACS Appl. Mater. Interfaces 8 (2016) 7826–7833. J. Song, E. Zheng, L. Liu, X.-F. Wang, G. Chen, W. Tian, T. Miyasaka, Magnesiumdoped zinc oxide as electron selective contact layers for efficient perovskite solar cells, ChemSusChem 9 (2016) 2640–2647. Y. Cheng, Q.-D. Yang, J. Xiao, Q. Xue, H.-W. Li, Z. Guan, H.-L. Yip, S.-W. Tsang, Decomposition of organometal halide perovskite films on zinc oxide nanoparticles, ACS Appl. Mater. Interfaces 7 (2015) 19986–19993. J. Kim, G. Kim, T.K. Kim, S. Kwon, H. Back, J. Lee, S.H. Lee, H. Kang, K. Lee, Efficient planar-heterojunction perovskite solar cells achieved via interfacial modification of a sol-gel ZnO electron collection layer, J. Mater. Chem. A 2 (2014) 17291–17296. A.K. Chandiran, M. Abdi-Jalebi, M.K. Nazeeruddin, M. Grätzel, Analysis of electron transfer properties of ZnO and TiO2 photoanodes for dye-sensitized solar cells, ACS Nano 8 (2014) 2261–2268. L. Schmidt-Mende, J.L. MacManus-Driscoll, ZnO – nanostructures, defects, and devices, Mater. Today 10 (2007) 40–48. P. Adhikary, S. Venkatesan, N. Adhikari, P.P. Maharjan, O. Adebanjo, J. Chen, Q. Qiao, Enhanced charge transport and photovoltaic performance of PBDTTT-C-T/ PC70BM solar cells via UV-ozone treatment, Nanoscale 5 (2013) 10007–10013. C.X. Zhao, K. Huang, S.Z. Deng, N.S. Xu, J. Chen, Investigation of the effects of atomic oxygen exposure on the electrical and field emission properties of ZnO nanowires, Appl. Surf. Sci. 270 (2013) 82–89. M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, K.H. Chan, M. Wright, C. Xu, F. Haque, A. Uddin, Low temperature processed ZnO thin film as electron transport layer for efficient perovskite solar cells, Sol. Energy Mater. Sol. Cell. 159 (2017) 251–264. D.C. Olson, Y.-J. Lee, M.S. White, N. Kopidakis, S.E. Shaheen, D.S. Ginley, J.A. Voigt, J.W.P. Hsu, Effect of ZnO processing on the photovoltage of ZnO/poly(3hexylthiophene) solar cells, J. Phys. Chem. C 112 (2008) 9544–9547. S. Bai, Z. Wu, X. Xu, Y. Jin, B. Sun, X. Guo, S. He, X. Wang, Z. Ye, H. Wei, X. Han, W. Ma, Inverted organic solar cells based on aqueous processed ZnO interlayers at low temperature, Appl. Phys. Lett. 100 (2012) 203906. N. Elumalai, M. Mahmud, D. Wang, A. Uddin, Perovskite solar cells: progress and advancements, Energies 9 (2016) 861. N.K. Elumalai, A. Uddin, Hysteresis in organic-inorganic hybrid perovskite solar cells, Sol. Energy Mater. Sol. Cell. 157 (2016) 476–509. D. Wang, M. Wright, N.K. Elumalai, A. Uddin, Stability of perovskite solar cells, Sol. Energy Mater. Sol. Cell. 147 (2016) 255–275. M. Arafat Mahmud, N. Kumar Elumalai, M. Baishakhi Upama, D. Wang, F. Haque, M. Wright, K. Howe Chan, C. Xu, A. Uddin, Enhanced stability of low temperature processed perovskite solar cells via augmented polaronic intensity of hole transporting layer, Phys. Status Solidi (RRL) Rapid Res. Lett. 10 (2016) 882–889. F. Huang, Y. Dkhissi, W. Huang, M. Xiao, I. Benesperi, S. Rubanov, Y. Zhu, X. Lin, L. Jiang, Y. Zhou, A. Gray-Weale, J. Etheridge, C.R. McNeill, R.A. Caruso, U. Bach, L. Spiccia, Y.-B. Cheng, Gas-assisted preparation of lead iodide perovskite films consisting of a monolayer of single crystalline grains for high efficiency planar solar cells, Nano Energy 10 (2014) 10–18. M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, V.R. Goncales, M. Wright, C. Xu, F. Haque, A. Uddin, A high performance and low-cost hole transporting layer for efficient and stable perovskite solar cells, Phys. Chem. Chem. Phys. 19 (2017) 21033–21045. M. Ahmadi, K. Mirabbaszadeh, S. Salari, H. Fatehy, Inverted polymer solar cells with sol-gel derived cesium-doped zinc oxide thin film as a buffer layer, Electron. Mater. Lett. 10 (2014) 951–956. M.B. Upama, N.K. Elumalai, M.A. Mahmud, D. Wang, F. Haque, V.R. Gonçales, J.J. Gooding, M. Wright, C. Xu, A. Uddin, Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells, Org. Electron. 50 (2017) 279–289. J.-I. Park, J.H. Heo, S.-H. Park, K.I. Hong, H.G. Jeong, S.H. Im, H.-K. Kim, Highly flexible InSnO electrodes on thin colourless polyimide substrate for high-performance flexible CH3NH3PbI3 perovskite solar cells, J. Power Sources 341 (2017) 340–347. L. Zuo, Z. Gu, T. Ye, W. Fu, G. Wu, H. Li, H. Chen, Enhanced photovoltaic performance of CH3NH3PbI3 perovskite solar cells through interfacial engineering using self-assembling monolayer, J. Am. Chem. Soc. 137 (2015) 2674–2679. M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, A.M. Soufiani, M. Wright, C. Xu, F. Haque, A. Uddin, Solution-processed lithium-doped ZnO electron transport layer for efficient triple cation (Rb, MA, FA) perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 33841–33854. M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, L. Zarei, V.R. Goncales,

[43]

[44]

[45]

[46]

[47]

[48] [49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61] [62] [63]

[64]

[65] [66]

[67]

[68]

[69]

70

M. Wright, C. Xu, F. Haque, A. Uddin, Adsorbed carbon nanomaterials for surface and interface-engineered stable rubidium multi-cation perovskite solar cells, Nanoscale 10 (2018) 773–790. M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, M. Wright, K.H. Chan, C. Xu, F. Haque, A. Uddin, Single vs mixed organic cation for low temperature processed perovskite solar cells, Electrochim. Acta 222 (2016) 1510–1521. S.K. Chang, H.C. Lee, S.P. Lin, C.F. Lin, Low temperature two-step solution process for perovskite solar cells with planar structure, 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC), 2015, pp. 1–3. J. Song, L. Liu, X.-F. Wang, G. Chen, W. Tian, T. Miyasaka, Highly efficient and stable low-temperature processed ZnO solar cells with triple cation perovskite absorber, J. Mater. Chem. A 5 (2017) 13439–13447. Y.-Z. Zheng, E.-F. Zhao, F.-L. Meng, X.-S. Lai, X.-M. Dong, J.-J. Wu, X. Tao, Iodinedoped ZnO nanopillar arrays for perovskite solar cells with high efficiency up to 18.24%, J. Mater. Chem. A 5 (2017) 12416–12425. Y. Jung, W. Yang, C.Y. Koo, K. Song, J. Moon, High performance and high stability low temperature aqueous solution-derived Li-Zr co-doped ZnO thin film transistors, J. Mater. Chem. 22 (2012) 5390–5397. S.U. Awan, S.K. Hasanain, M.F. Bertino, G.H. Jaffari, Ferromagnetism in Li doped ZnO nanoparticles: the role of interstitial Li, J. Appl. Phys. 112 (2012) 103924. R.K. Singhal, A. Samariya, S. Kumar, Y.T. Xing, U.P. Deshpande, T. Shripathi, S.N. Dolia, E.B. Saitovitch, Switch ‘on’ and ‘off’ ferromagnetic ordering through the induction and removal of oxygen vacancies and carriers in doped ZnO: a magnetization and electronic structure study, Phys. Status Solidi 207 (2010) 2373–2386. G. Kakavelakis, T. Maksudov, D. Konios, I. Paradisanos, G. Kioseoglou, E. Stratakis, E. Kymakis, Efficient and highly air stable planar inverted perovskite solar cells with reduced graphene oxide doped PCBM electron transporting layer, Adv. Energy Mater. 7 (2017) 1602120. A. Morais, J.P.C. Alves, F.A.S. Lima, M. Lira-Cantu, A.F. Nogueira, Enhanced photovoltaic performance of inverted hybrid bulk-heterojunction solar cells using TiO2/reduced graphene oxide films as electron transport layers, PHOTOE 5 (2015) 057408. L. Jian-Feng, Z. Chuang, Z. Heng, T. Jun-Feng, Z. Peng, Y. Chun-Yan, X. Yang-Jun, F. Duo-Wang, Improving the performance of perovskite solar cells with glyceroldoped PEDOT: PSS buffer layer, Chin. Phys. B 25 (2016) 028402. J. Carrillo, A. Guerrero, S. Rahimnejad, O. Almora, I. Zarazua, E. Mas-Marza, J. Bisquert, G. Garcia-Belmonte, Ionic reactivity at contacts and aging of methylammonium lead triiodide perovskite solar cells, Adv. Energy Mater. 6 (2016) 1502246. J. Yang, B.D. Siempelkamp, E. Mosconi, F. De Angelis, T.L. Kelly, Origin of the thermal instability in CH3NH3PbI3 thin films deposited on ZnO, Chem. Mater. 27 (2015) 4229–4236. M.B. Upama, M. Wright, N.K. Elumalai, M.A. Mahmud, D. Wang, C. Xu, A. Uddin, High-efficiency semitransparent organic solar cells with non-fullerene acceptor for window application, ACS Photonics 4 (2017) 2327–2334. M.B. Upama, M. Wright, N.K. Elumalai, M.A. Mahmud, D. Wang, K.H. Chan, C. Xu, F. Haque, A. Uddin, High performance semitransparent organic solar cells with 5% PCE using non-patterned MoO3/Ag/MoO3 anode, Curr. Appl. Phys. 17 (2017) 298–305. M. Lv, X. Dong, X. Fang, B. Lin, S. Zhang, J. Ding, N. Yuan, A promising alternative solvent of perovskite to induce rapid crystallization for high-efficiency photovoltaic devices, RSC Adv. 5 (2015) 20521–20529. X. Guo, C. McCleese, C. Kolodziej, A.C.S. Samia, Y. Zhao, C. Burda, Identification and characterization of the intermediate phase in hybrid organic-inorganic MAPbI3 perovskite, Dalton Trans. 45 (2016) 3806–3813. H. Oh, J. Krantz, I. Litzov, T. Stubhan, L. Pinna, C.J. Brabec, Comparison of various sol–gel derived metal oxide layers for inverted organic solar cells, Sol. Energy Mater. Sol. Cell. 95 (2011) 2194–2199. C. Bi, Q. Wang, Y. Shao, Y. Yuan, Z. Xiao, J. Huang, Non-wetting surface-driven high-aspect-ratio crystalline grain growth for efficient hybrid perovskite solar cells, Nat. Commun. 6 (2015) 7747. C.-H. Chiang, C.-G. Wu, Film grain-size related long-term stability of inverted perovskite solar cells, ChemSusChem 9 (2016) 2666–2672. J. Chen, T. Shi, X. Li, B. Zhou, H. Cao, Y. Wang, Origin of the high performance of perovskite solar cells with large grains, Appl. Phys. Lett. 108 (2016) 053302. F. Haque, K.S. Rahman, M.A. Islam, M.J. Rashid, M. Akhtaruzzaman, M.M. Alam, Z.A. Alothman, K. Sopian, N. Amin, Growth optimization of ZnS zns thin films by RF magnetron sputtering as prospective buffer layer in thin film solar cells, Chalcogenide Lett. 11 (2014) 189–197. D. Pathinettam Padiyan, A. Marikani, K.R. Murali, Influence of thickness and substrate temperature on electrical and photoelectrical properties of vacuum-deposited CdSe thin films, Mater. Chem. Phys. 78 (2003) 51–58. M.A. Green, Solar Cells: Operating Principles, Technology, and System Applications, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1982. A. Guerrero, J. You, C. Aranda, Y.S. Kang, G. Garcia-Belmonte, H. Zhou, J. Bisquert, Y. Yang, Interfacial degradation of planar lead halide perovskite solar cells, ACS Nano 10 (2016) 218–224. A. Pockett, G.E. Eperon, T. Peltola, H.J. Snaith, A. Walker, L.M. Peter, P.J. Cameron, Characterization of planar lead halide perovskite solar cells by impedance spectroscopy, open-circuit photovoltage decay, and intensity-modulated photovoltage/ photocurrent spectroscopy, J. Phys. Chem. C 119 (2015) 3456–3465. Z. Zhu, C.-C. Chueh, F. Lin, A.K.Y. Jen, Enhanced ambient stability of efficient perovskite solar cells by employing a modified fullerene cathode interlayer, Adv. Sci. 3 (2016) 1600027-n/a. M.B. Upama, N.K. Elumalai, M.A. Mahmud, M. Wright, D. Wang, C. Xu, A. Uddin, Effect of annealing dependent blend morphology and dielectric properties on the

Journal of Power Sources 383 (2018) 59–71

M.A. Mahmud et al.

[70]

[71]

[72]

[73]

[74]

[75] [76]

[77]

[78]

[79]

performance and stability of non-fullerene organic solar cells, Sol. Energy Mater. Sol. Cell. 176 (2018) 109–118. N.D. Pham, V.T. Tiong, P. Chen, L. Wang, G.J. Wilson, J. Bell, H. Wang, Enhanced perovskite electronic properties via a modified lead(ii) chloride Lewis acid-base adduct and their effect in high-efficiency perovskite solar cells, J. Mater. Chem. A 5 (2017) 5195–5203. Y. Jiang, E.J. Juarez-Perez, Q. Ge, S. Wang, M.R. Leyden, L.K. Ono, S.R. Raga, J. Hu, Y. Qi, Post-annealing of MAPbI3 perovskite films with methylamine for efficient perovskite solar cells, Mater. Horiz. 3 (2016) 548–555. A. Aprilia, P. Wulandari, V. Suendo, Herman, R. Hidayat, A. Fujii, M. Ozaki, Influences of dopant concentration in sol–gel derived AZO layer on the performance of P3HT: PCBM based inverted solar cell, Sol. Energy Mater. Sol. Cell. 111 (2013) 181–188. O. Almora, C. Aranda, E. Mas-Marzá, G. Garcia-Belmonte, On Mott-Schottky analysis interpretation of capacitance measurements in organometal perovskite solar cells, Appl. Phys. Lett. 109 (2016) 173903. O. Almora, I. Zarazua, E. Mas-Marza, I. Mora-Sero, J. Bisquert, G. Garcia-Belmonte, Capacitive dark currents, hysteresis, and electrode polarization in lead halide perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 1645–1652. O. Almora, A. Guerrero, G. Garcia-Belmonte, Ionic charging by local imbalance at interfaces in hybrid lead halide perovskites, Appl. Phys. Lett. 108 (2016) 043903. R.S. Sanchez, V. Gonzalez-Pedro, J.-W. Lee, N.-G. Park, Y.S. Kang, I. Mora-Sero, J. Bisquert, Slow dynamic processes in lead halide perovskite solar cells. Characteristic times and hysteresis, J. Phys. Chem. Lett. 5 (2014) 2357–2363. K. Mahmood, B.S. Swain, A. Amassian, 16.1% efficient hysteresis-free mesostructured perovskite solar cells based on synergistically improved ZnO nanorod arrays, Adv. Energy Mater. 5 (2015) 1500568. Q. Hu, Y. Liu, Y. Li, L. Ying, T. Liu, F. Huang, S. Wang, W. Huang, R. Zhu, Q. Gong, Efficient and low-temperature processed perovskite solar cells based on a crosslinkable hybrid interlayer, J. Mater. Chem. A 3 (2015) 18483–18491. J. Zhang, E.J. Juarez-Perez, I. Mora-Sero, B. Viana, T. Pauporte, Fast and low

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88] [89]

71

temperature growth of electron transport layers for efficient perovskite solar cells, J. Mater. Chem. A 3 (2015) 4909–4915. C.-Y. Chang, K.-T. Lee, W.-K. Huang, H.-Y. Siao, Y.-C. Chang, High-performance, air-stable, low-temperature processed semitransparent perovskite solar cells enabled by atomic layer deposition, Chem. Mater. 27 (2015) 5122–5130. F. Fu, T. Feurer, T. Jäger, E. Avancini, B. Bissig, S. Yoon, S. Buecheler, A.N. Tiwari, Low-temperature-processed efficient semi-transparent planar perovskite solar cells for bifacial and tandem applications, Nat. Commun. 6 (2015) 8932. A. Dymshits, L. Iagher, L. Etgar, Parameters influencing the growth of ZnO nanowires as efficient low temperature flexible perovskite-based solar cells, Materials 9 (2016) 60. J. Song, W. Hu, X.-F. Wang, G. Chen, W. Tian, T. Miyasaka, HC(NH2)2PbI3 as a thermally stable absorber for efficient ZnO-based perovskite solar cells, J. Mater. Chem. A 4 (2016) 8435–8443. H.J. Snaith, A. Abate, J.M. Ball, G.E. Eperon, T. Leijtens, N.K. Noel, S.D. Stranks, J.T.-W. Wang, K. Wojciechowski, W. Zhang, Anomalous hysteresis in perovskite solar cells, J. Phys. Chem. Lett. 5 (2014) 1511–1515. H.-S. Kim, N.-G. Park, Parameters affecting I–V hysteresis of CH3NH3PbI3 perovskite solar cells: effects of perovskite crystal size and mesoporous TiO2 layer, J. Phys. Chem. Lett. 5 (2014) 2927–2934. C.-G. Wu, C.-H. Chiang, Z.-L. Tseng, M.K. Nazeeruddin, A. Hagfeldt, M. Gratzel, High efficiency stable inverted perovskite solar cells without current hysteresis, Energy Environ. Sci. 8 (2015) 2725–2733. Y. Kato, L.K. Ono, M.V. Lee, S. Wang, S.R. Raga, Y. Qi, Silver iodide formation in methyl ammonium lead iodide perovskite solar cells with silver top electrodes, Adv. Mater. Interfaces 2 (2015) 1500195. N. Ahn, K. Kwak, M.S. Jang, H. Yoon, B.Y. Lee, J.-K. Lee, P.V. Pikhitsa, J. Byun, M. Choi, Trapped charge-driven degradation of perovskite solar cells, 7 (2016) 13422. J. Zhao, B. Cai, Z. Luo, Y. Dong, Y. Zhang, H. Xu, B. Hong, Y. Yang, L. Li, W. Zhang, C. Gao, Investigation of the hydrolysis of perovskite organometallic halide CH3NH3PbI3 in humidity environment, Sci. Rep. 6 (2016) 21976.