Efficient and Stable Inverted Planar Perovskite Solar Cells Employing ...

DOI: 10.1002/ente.201700422

Efficient and Stable Inverted Planar Perovskite Solar Cells Employing CuI as Hole-Transporting Layer Prepared by Solid–Gas Transformation Haoxin Wang,[a] Ze Yu,*[a] Xiao Jiang,[b] Jiajia Li,[a] Bin Cai,[a] Xichuan Yang,*[a] and Licheng Sun[a, c] The inorganic p-type semiconductor CuI possesses several unique characteristics such as high transparency, low-production cost, high hole mobility, and good chemical stability and is a promising hole-transporting material candidate that can be explored in solar-cell devices. Herein, we adopt a simple solid–gas reaction method to fabricate a uniform CuI film by exposing a thermally evaporated copper film to iodine vapor and apply it as a hole-transporting layer (HTL) in inverted planar perovskite solar cells (PSCs). The optimized devices display a promising power conversion (PCE) efficiency of

14.7 %, with an open-circuit voltage of 1.04 V, a short-circuit current density of 20.9 mW cm@2, and a fill factor of 0.68. This is one of the highest PCE values reported so far for CuI-based HTL in PSCs. Moreover, the devices studied also exhibit good long-term stability at ambient atmosphere, arising from the hydrophobicity of CuI HTL. The results highlight that CuI fabricated using the simple and low-temperature processing method presented here holds great promise as low-cost alternative HTL material for the development of efficient and stable inverted planar PSCs in the future.

Introduction Organic–inorganic metal halide perovskites recently attracted considerable research interest as light absorbers in solidstate thin film solar cells owing to some of their excellent optoelectronic properties, including direct band gap, low exciton binding energy, a broad light absorption spectrum range, high absorption coefficient, and a long electron–hole diffusion length.[1] A substantially high power conversion efficiency (PCE) of 22.1 % was achieved in just few years.[2] Moreover, the perovskites are fabricated from inexpensive starting materials and can be produced by solution-processable techniques. The high overall performance and facile fabrication process render PSCs a very promising low-cost next-generation solar cell technology. The state-of-the-art cell configuration (n-i-p, transparent conducting oxide/n-type semiconductor/active layer/p-type semiconductor/metal) of PSCs was originally derived from solid-state dye-sensitized solar cells, where the perovskite absorber was infiltrated into a mesoporous TiO2 scaffold (electron-transporting layer, ETL). A hole-transporting layer (HTL), such as 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD), is further coated on top of the perovskite layer.[3] Alternatively, due to the ambipolar semiconducting characteristic of the perovskite, PSCs can also be fabricated in an inverted p-i-n fashion, so-called inverted planar PSCs.[4] The perovskite layer is sandwiched between a HTL, such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT: PSS), and a ETL, typically a fullerene derivative such as [6,6]-phenyl-C61butyric acid methyl ester (PCBM).[5] Inverted planar PSCs showed excellent PCEs of 19–20 %.[6] Remarkably, they can

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be fabricated at low-temperature conditions, which makes it possible to be potentially applied in flexible devices.[7] PEDOT:PSS is the most widely used HTL in inverted planar PSCs and showed remarkable PCEs as high as 20.1 %.[8] However, the acidic and hygroscopic characteristics of PEDOT:PSS are harmful for the long-term stability of cell devices, which severely limits its practical application in the future. Therefore, it is highly desirable to develop alternative HTLs to replace PEDOT:PSS for inverted planar PSCs. [a] H. Wang, Prof. Dr. Z. Yu, J. Li, B. Cai, Prof. Dr. X. Yang, Prof. Dr. L. Sun State Key Laboratory of Fine Chemicals Institute of Artificial Photosynthesis DUT-KTH Joint Education and Research Center on Molecular Devices Dalian University of Technology (DUT) 116024 Dalian (PR China) E-mail: [email protected] [email protected] [b] Dr. X. Jiang Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education) School of Environmental Science and Technology Dalian University of Technology 116024 Dalian (PR China) [c] Prof. Dr. L. Sun Department of Chemistry KTH Royal Institute of Technology SE-100 44 Stockholm (Sweden) Supporting Information for this article can be found under: https://doi.org/10.1002/ente.201700422. This publication is part of dual Special Issues on “Perovskite Optoelectronics”, published in Energy Technology and ChemSusChem. Please visit the Energy Technology issue at http://dx.doi.org/10.1002/ente.v5.10, and the companion issue of ChemSusChem at http://doi.org/10.1002/cssc.v10.19.

T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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With this regard, several inorganic p-type semiconductors, such as pristine and doped nickel oxide (NiOx),[2, 9] copper oxide (CuOx),[10] and copper thiocyanate(CuSCN)[11] were applied as HTLs in inverted planar PSCs, showing satisfactory PCEs together with improved air stability. The inorganic p-type semiconductor CuI, which shows high transparency, large band gap (3.1 eV), high hole mobility, high chemical stability, and low production costs, also attracted attention as HTL in PSCs.[12] CuI was initially studied as HTL in conventional n-i-p PSCs. However, the PCEs were rather low, with a maximum value of merely 7.5 %.[12c] Recently, CuI was also applied as HTL in inverted planar PSCs. Chen et al. fabricated planar inverted PSCs based on solution-processed CuI film as a HTL, obtaining a PCE of 13.6 %.[12d] The CuI films were fabricated on fluorine-doped tin oxide (FTO) conducting glass by spin-coating CuI solution in a low-boiling-point solvent acetonitrile. Bian and coworkers further boosted the performance by using a fast deposition–crystallization method to produce high-quality perovskite layer on CuI film.[12e] However, it was found that some small islands can be detected for the as-prepared CuI film, indicating that the FTO substrate was not fully covered by CuI fabricated by this method.[12d] This may result in leakage paths between FTO substrate and perovskite layer. Therefore, it is of great importance to identify an alternative method to fabricate highly uniform CuI films for inverted planar PSCs. The solid–gas reaction method has been proven to be a facile approach to prepare CuI films.[13] A sputtered copper

film was transformed to CuI by simply exposing it to iodine vapor. The thickness of the CuI film can be controlled by adjusting the thickness of the Cu layer. Recently, Moshaii and co-workers successfully fabricated CuI by using this method in conventional n-i-p PSCs.[12c] A uniform and well-controlled CuI film was fabricated on a perovskite layer by exposing a Cu film to iodine vapor. However, a maximum PCE of only 7.5 % was achieved for the conventional PSCs. In this work, we adopted the solid–gas reaction method to fabricate a uniform CuI film on FTO substrate and applied it as a HTL in inverted planar PSCs with a cell structure of FTO/CuI/CH3NH3PbI3 (MAPbI3)/PCBM/polyethylenimine (PEI)/Ag. Under optimized conditions, the best devices displayed an impressive PCE of 14.7 % measured at 1 sun illumination (100 mW cm@2, AM 1.5G). Moreover, the devices studied also exhibited good long-term stability at ambient atmosphere, mainly because of the hydrophobicity of the CuI HTL. The results suggest that CuI fabricated from simple and low-temperature processing method holds a great promise as an alternative HTL material for the development of efficient and stable inverted planar PSCs in the future.

Results and Discussion Figure 1 a presents the schematic illustration of the fabrication process of CuI by exposing a thermally evaporated Cu film to iodine vapor, according to a previously reported method.[13] The Cu film was thermally evaporated on FTO substrate, which was fixed by double-sided tape onto a Petri

Figure 1. a) Schematic illustration of the formation of CuI using the vapor iodization method. b) Photographs of Cu film before and after the iodization. c) XRD patterns of FTO, FTO/CuI, FTO/CuI/perovskite. d) XPS spectra of (left) Cu 2p and (right) I 3d core level peaks.

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dish. A certain amount of iodine was placed into another Petri dish. The latter Petri dish was put onto a hotplate and then covered by the former Petri dish with Cu/FTO substrates. To accelerate the reaction, the hotplate was set to a fixed temperature of 100 8C. Iodine was melted during the heating process, and the iodine vapor reacted with the Cu films. Figure 1 b shows a photograph of a sample before and after iodization. A significant difference can be clearly observed. Due to the large band gap of CuI, the sample after iodization became transparent to the naked eyes. To confirm this hypothesis, X-ray diffraction (XRD) patterns of the asprepared CuI film on FTO conducting glass substrate were recorded (Figure 1 c). The CuI film shows an intense diffraction peak at 25.468, corresponding to the (111) reflection of polycrystalline g-phase CuI.[12d] To further confirm the conversion of Cu to CuI, we carried out X-ray photoelectron spectroscopy (XPS) measurements of the as-prepared CuI film. The Cu 2p and I 3d photoelectron peaks are shown in Figure 1 d, and the survey spectrum is presented in Figure S1 in the Supporting Information. The binding energies of

Cu 2p3/2 and I 3d5/2 appear at 932.0 and 619.2 eV, respectively, which are in good agreement with the values as reported previously.[14] Moreover, the compositional ratio I/Cu is estimated to be 1.09. We can infer that the conversion of Cu to CuI is complete. Figure 2 a shows the top view of a scanning electron microscopy (SEM) image of the as-prepared CuI film ( & 40 nm thick) on a FTO substrate. It clearly shows that the FTO substrate is completely covered by CuI nanoparticles. This implies that the prepared CuI film is effectively prevented from coming into direct contact with the perovskite layer and FTO substrate, which reduces unwanted charge recombination at the FTO/perovskite interface, improving the overall performance. The quality of a perovskite film is affected by several factors, such as the type of additional solvent, the concentration of the perovskite precursor solution, and the dropping time of an antisolvent.[15] Dimethyl sulfoxide (DMSO) as a Lewis-base adduct of PbI2 was used in the precursor solution for the perovskite as reported previously by Ahn et al. to form a homogeneous and dense perovskite

Figure 2. Top view of SEM images of a) vapor iodization-processed CuI film on FTO and b) perovskite film on CuI/FTO substrate. c) Energy-level alignment of different components in PSCs. d) device structure of inverted planar PSCs studied (left) and corresponding cross-sectional SEM image (right).

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film.[16] The perovskite film was prepared by using an antisolvent engineering technique according to a method reported previously.[15b] The XRD patterns of the perovskite absorber MAPbI3 deposited on the CuI film is depicted in Figure 1 c. It shows intense diffraction peaks at 2q = 14.018, 28.408, and 31.568, corresponding to the (110), (220), and (310) planes of the tetragonal MAPbI3 phase, consistent with previous results.[17] In the top-view SEM image in Figure 2 b, a smooth and uniform perovskite film on top of the CuI film can be identified. Although some cracks between the perovskite particles can be observed that arise from the supersaturation of the perovskite precursor solution, the formed perovskite crystals are quite large with an average size of more than 300 nm. This is anticipated to be helpful for the transport and collection of photo-generated holes due to a decreased number of grain boundaries. The as-prepared CuI film was further adopted as a HTL in inverted planar PSC devices with a structure of FTO/CuI/ MAPbI3/PCBM/PEI/Ag. The energy-level diagram of different components and cell configuration of PSCs are presented in Figure 2 c and d The valance band position of CuI is @5.2 eV,[3] which matches the valence-band position of MAPbI3 (@5.4 eV).[12d] This implies that CuI can effectively extract photo-generated holes from the perovskite absorber. Moreover, the conduction-band position of CuI is estimated to be @2.1 eV, which is more negative than that of MAPbI3

(@3.9 eV). This is expected to effectively retard the electron flow from the perovskite layer to the FTO conducting substrate and to effectively reduce charge recombination at the interface. The cross-sectional SEM image (Figure 2 d) displays a uniform and dense MAPbI3 film with a thickness of about 350 nm on CuI. PCBM ( & 60 nm) was further spin coated on top of the perovskite layer as an ETL, and a thin layer of PEI was introduced between PCBM and the Ag back contact electrode ( & 80 nm) to serve as a cathode buffer layer. The photocurrent density–voltage (J–V) characteristics of inverted planar PSCs employing different thicknesses of CuI as HTLs measured under 100 mW cm@2 (AM 1.5G) are displayed in Figure 3 a, and the corresponding photovoltaic parameters are summarized in Table 1. A control device without any HTL was also prepared for comparison, the best device of which exhibits a rather low PCE of only 3.3 %, with an open-circuit voltage (Voc) of 0.86 V, a short-circuit current density (Jsc) of 10.1 mW cm@2, and a fill factor (FF) of 0.38. It can be easily noted that the introduction of the CuI HTL dramatically improved all photovoltaic parameters. It emphasizes once again that the incorporation of the CuI HTL effectively prevented the direct contact between FTO and perovskite layer, thus circumventing undesirable charge recombination at the FTO/perovskite interface and improving the overall performance. For an optimized thickness

Figure 3. a) J–V curves of PSC devices with different CuI thicknesses. b) IPCE spectrum of the best device with CuI HTL. c) PCE histogram of 24 PSCs based on CuI as HTL. d) J–V curves of the best PSC devices based on CuI HTL measured under different scan directions.

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Table 1. Photovoltaic parameters of PSCs with various thicknesses of CuI HTL measured under 100 mWcm@2 illumination (AM 1.5G).[a]

Thicknesses of CuI [nm]

Voc [V]

Jsc [mA cm-2]

FF

PCE [%]

Rs [Wcm2]

Rsh [Wcm2]

w/o[b] 20 40 60

0.84 : 0.04 (0.86) 0.93 : 0.05 (0.97) 1.04 : 0.01 (1.04) 1.03 : 0.02 (1.01)

8.2 : 3.3 (10.1) 16.0 : 1.7 (17.5) 18.8 : 1.5 (20.9) 15.9 : 0.5 (16.3)

0.30 : 0.07 (0.38) 0.45 : 0.05 (0.41) 0.66 : 0.01 (0.68) 0.54 : 0.01 (0.54)

2.0 : 0.9 (3.3) 6.5 : 0.4 (7.0) 13.0 : 1.0 (14.7) 8.8 : 0.2 (9.0)

52.7 8.0 4.0 14.9

82.7 293.7 597.3 244.9

[a] Average parameters were calculated along with the standard deviation from 10 devices for each condition. The data for the best-performing cells are shown in parentheses. [b] w/o represents devices without any HTLs.

( & 40 nm CuI), the PSC device shows a considerably higher PCE of 14.7 %, with a Voc of 1.04 V, a Jsc of 20.9 mW cm@2, and a FF of 0.68. Moreover, the series resistance (Rs) and the shunt resistance (Rsh), which were extracted from the J–V measurements, also demonstrate the properties of CuI HTM with different thicknesses. A PSC based on a 40 nm-thick CuI layer exhibits the lowest Rs and the highest Rsh, which is consistent with the device performance. A 40 nm-thick CuI layer was chosen for all following studies. The incident photon-to-current efficiency (IPCE) spectrum of PSCs based on CuI as a HTL is shown in Figure 3 b, which exhibits plateau IPCE values of over 75 % between 400 and 700 nm. The integrated current density is calculated to be 20.0 mW cm@2, which is basically in good agreement with the result from the J–V measurement (Figure 3 a). The PCE histogram plot in Figure 3 cof 24 independent PSCs was. The distribution of the PCEs is mainly located in a small range from 12 % to 14 %, and half of the devices exhibit PCEs above 13 %. We further examined the hysteresis in the J–V curves by measuring the device under different scan directions as shown in Figure 3 d, and the corresponding photovoltaic parameters are summarized in Table 2. Notably, the device based on CuI shows a negligible hysteresis. This result suggests that a CuI film can effectively facilitate hole extraction and reduce charge accumulation at the interface, which would give rise to a negligible hysteresis.[18] To better understand the hole-transfer property of the asprepared CuI film, we carried out steady-state photoluminescence (PL) and time-resolved PL decay measurements of CuI/perovskite films. As shown in Figure 4 a, the pristine per-

Table 2. Photovoltaic parameters obtained for the best PSC devices base on CuI as HTL for different scan directions. Mode

Voc [V]

Jsc [mA cm@2]

FF

PCE [%]

forward reverse

1.04 1.04

20.2 19.9

0.68 0.69

14.3 14.3

ovskite film shows a high PL emission peak centered at 760 nm. A significant quenching effect is clearly observed in the presence of the CuI HTL. To gain further insight on the quenching effect, we also implemented PL kinetic measurements by monitoring the emission maximum of 760 nm (Figure 4 b). In contrast to the pristine perovskite film with a decay time of 50.4 ns, the introduction of the CuI HTL under the perovskite layer substantially shortened the PL decay time to 6.3 ns. This result further confirms that the CuI film can effectively transfer the holes from the perovskite. Finally, we investigated the long-term stability of PSC devices based on CuI as HTLs, which were stored without encapsulation in ambient atmosphere in the dark. For comparison, the stability of cells incorporating the most widely used HTL PEDOT:PSS were also tested as a reference. The best device based on PEDOT:PSS shows a PCE of 12.8 % (Voc = 0.91 V, Jsc = 20.2 mW cm@2, FF = 0.69; Figure S2 in the Supporting Information). Figure 5 displays the changes in photovoltaic parameter of the representative devices. Notably, the CuI-based device maintained 80 % of its initial efficiency after 210 h in air. However, the devices employing PE-

Figure 4. a) Steady-state PL spectra of the FTO/perovskite films with and without CuI. b) Time-resolved PL decay kinetics of the FTO/perovskite films with and without CuI monitored at 760 nm.

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Figure 5. Changes of photovoltaic parameters for PSC devices employing CuI and PEDOT:PSS as HTLs measured under 100 mWcm@2 illumination (AM 1.5G). The devices were kept without encapsulation at ambient atmosphere (temperature of & 25 8C and 25 % < relative humidity < 40 %).

DOT:PSS exhibited a fast decay in PCE, with less than 30 % of its initial performance remaining after the same testing period in air. The better long-term stability of the CuI-based device should be mainly attributed to the hydrophobic nature of CuI as observed previously.[12d,e]

film as HTL for the development of low-cost, efficient, and stable inverted planar PSCs.

Experimental Section Materials

Conclusions We fabricated inverted planar perovskite solar cells (PSCs) by incorporating CuI as a hole-transporting layer (HTL) prepared by using a simple solid–gas reaction method. The introduction of the CuI HTL effectively prevented the direct contact between the fluorine-doped tin oxide (FTO) substrate and the perovskite layer, which was expected to reduce charge-carrier recombination losses at the interface and to improve the overall performance. The best devices based on CuI as HTLs exhibited a decent power conversion efficiency (PCE) of 14.7 % with negligible hysteresis measured at 1 sun illumination (100 mW cm@2, AM 1.5G). This is one of the highest PCE values reported so far for CuI-based HTLs in PSCs. The studied devices exhibited good long-term stability at ambient atmosphere, mainly because of the hydrophobicity of the CuI HTL. It is also worth noting that the low-temperature processing may potentially enable the fabrication of PSCs on flexible substrates. The present work demonstrated a simple and effective approach to fabricate a CuI Energy Technol. 2017, 5, 1836 – 1843

All chemicals and reagents were used as received from chemical companies. Methylammonium iodide (CH3NH3I, 99.5 %) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM, 99.5 %) were purchased from Xi’an Polymer light Technology (Shaanxi, China). PbI2 (99.99 %) was purchased from Sigma–Aldrich (USA). Iodine (99.8 %), polyethyleneimine (PEI, MW = 70 000, 50 wt % aqueous solution), isopropanol (99.8 %), chlorobenzene (99.8 %), and 1, 2-dichlorobenzene were purchased from Aladdin Reagents (Shanghai, China).

Fabrication of solar cells FTO-conducting glasses (NSG-Pilkington, 15 W per square) were patterned by etching with 2 m HCl and Zn powder. Then, the etched substrates were cleaned in detergent solution, deionized water, and isopropanol sequentially. FTO substrates were further cleaned by UV–ozone treatment for 30 min. Different thicknesses of Cu films were thermally evaporated onto the FTO substrates. The CuI films were prepared by exposing Cu films into iodine vapor as described previously.[13] Cu/FTO substrates were fixed by double-sided tape onto a Petri dish. About 100 mg


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iodine were put into another Petri dish with a smaller diameter. The latter Petri dish was covered by the former one. To accelerate the reaction, the hotplate was set to a fixed temperature of 100 8C. The iodization process was performed within 30 min. The thicknesses of CuI films could be simply controlled by adjusting the thicknesses of the Cu films. A CuI/Cu volume ratio of & 5 was used according to the previous report.[13] The resulting transparentCuI samples were immediately transferred into a nitrogenatmosphere glovebox (H2O and O2 < 0.1 ppm) for depositing the perovskite films. To prepare the perovskite precursor solution, PbI2 and CH3NH3I (PbI2/CH3NH3I = 1.05:1 molar ratio) were dissolved in a solvent mixture of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) (9:1, v/v) and then stirred at 60 8C for 12 h. The method for fabrication of the perovskite film was modified slightly from a previous report.[15b] The perovskite precursor solution was first dropped onto a CuI/FTO substrate. Then, the substrate was spin coated at 4000 rpm for 30 s and anhydrous chlorobenzene was quickly dropped onto the center of the substrate 7–9 s after the spin-coating process started. The perovskite film was then heated at 100 8C on a hotplate for 10 min. After cooling to room temperature, a PCBM solution in 1, 2-dichlorobenzene (30 mg mL@1) was spin coated on top of the perovskite film at 2000 rpm for 50 s. Subsequently, the PEI layer was spin coated on top of PCBM at 5000 rpm for 1 min from isopropanol solution (0.1 wt %). The sample was then transferred into a vacuum evaporation system and 80 nm of Ag was evaporated under high vacuum of 1 X 10@6 torr (1 torr = 133.322 Pa) by thermal evaporation.

Characterization X-ray powder diffraction measurements were performed in grazing-incidence X-ray diffraction (GIXRD) mode on the D8 Advance Diffractometer (Bruker AXS, Karlsruhe, Germany). X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi, USA) was carried out to characterize the binding energies of the as-prepared CuI film. The morphologies of CuI and perovskite films and the cross-sectional structure of PSC device were recorded by high-resolution field-emission scanning electron microscopy (HR-SEM) performed with FEI (Field Emission Instruments: Nova Nano SEM 450, USA). The photocurrent-voltage (J–V) curves of the PSC devices were recorded by using a Keithley 2400 Source-measure unit under illumination of simulated sunlight (AM 1.5G, 100 mW cm@2) provided by an Oriel Sol3A solar simulator (Newport USA, Model: 94023A) with an AM 1.5 filter in ambient air. Light intensity was calibrated by using a Newport calibrated standard Si reference cell (SER. No: 506/0358). The device area was 0.05 cm2. The J–V measurements were performed from forward bias to short-circuit potential at a scan rate of 50 mV s@1. The IPCE spectra were measured by using a Hypermono-light (SM-25, Jasco Co. Ltd., Japan). A standard silicon solar cell was used as a reference before the measurements. The temperatures and humidity of the stability measurements were controlled by a temperature humidity chamber (RP-80A, Beijing Hongzhan Instrument Co., Ltd.). Steady-state photoluminescence spectra were recorded using a Renishaw 2000 laser Raman microscope equipped with a 514.5 nm 20 mW argon ion laser of 2 mm spot size for excitation. Time-resolved PL (TRPL) decay measurements were carried out with a LP920 laser flash photolysis spectrometer (Edinburgh Instruments) by adoption of the literature method with minor modification as proposed by Stranks et al.[1c] Energy Technol. 2017, 5, 1836 – 1843

Acknowledgements This work was supported by the National Natural Science Foundation of China (21606039, 51661135021, 21507008, 91233201), the Fundamental Research Funds for the Central Universities, Swedish Foundation for Strategic Research (SSF), the Swedish Energy Agency, as well as the Knut and Alice Wallenberg Foundation.

Conflict of interest The authors declare no conflict of interest.

Keywords: copper iodide · hole-transporting perovskites · solar cells · solid-gas reaction

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Manuscript received: June 28, 2017 Revised manuscript received: July 25, 2017 Accepted manuscript online: July 26, 2017 Version of record online: August 23, 2017


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