Low-Temperature-Processed WO x as Electron Transfer Layer for Planar Perovskite Solar Cells Exceeding 20%
Ef fi ciency
Cong Chen, Yue Jiang,* Yehui Wu, Jiali Guo, Xiangyu Kong, Xiayan Wu, Yuzhu Li, Dongfeng Zheng, Sujuan Wu, Xingsen Gao, Zhipeng Hou, Guofu Zhou, Yiwang Chen, Jun-Ming Liu, Krzysztof Kempa, and Jinwei Gao*
1. Introduction
Over the last decade, organic–inorganic lead halide perovskite solar cells (PSCs) have experienced a rapid development, with the power conversion efficiency (PCE) increasing from 3.8% to 25.2%.[1] In part, the progress has been due to the efforts in perfecting perovskite materials, including achieving a suitable bandgap, high absorption coefficient, proper morphology, and stability.[2] In parallel efforts, the optimization of device
structure and processing technology, together with perfecting the interfacial materials, strongly impacted the prog- ress.[3] In particular, improvements in the electron transfer layer (ETL), which enables efficient electron transfer and hole blocking, have played a vital role in device improvements.[4]
At present, TiO2is widely adopted as an ETL, mainly due to its favorable band struc- ture and mature fabrication process.
However, its high-temperature sintering process (>450C) hinders any low-cost and large-scale applications, and is unsuit- able for the fabrication of flexible PSCs based on plastic substrates.[5] Therefore, low-temperature-processed metal oxide materials, such as ZnO, SnO2, and ZrO2, have been studied as a replacement. Liu and Kelly[6]were the first to use low-temperature-processed ZnO to fabricate a planar PSC, which achieved an efficiency of 15.7%. However, ZnO was found to corrode perovskite above 100C. Shin et al.[7]
fabricated PSCs with an ETL made by using the tedious-to- process ternary Zn2SnO4, but with a low processing tempera- ture of 100C, achieving a PCE of 15.3%. Even though SnO2 is considered the best low-temperature-processed ETL material at the moment, the annealing temperature (150C) is still too C. Chen, Dr. Y. Jiang, Y. Wu, J. Guo, X. Kong, X. Wu, Y. Li, D. Zheng,
Prof. S. Wu, Prof. X. Gao, Dr. Z. Hou, Prof. J.-M. Liu, Prof. K. Kempa, Prof. J. Gao
Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology
Academy of Advanced Optoelectronics South China Normal University Guangzhou 510006, China
E-mail: [email protected]; [email protected] Prof. G. Zhou
Guangdong Provincial Key Laboratory of Optical Information Materials and Technology and Institute of Electronic Paper Displays
South China Academy of Advanced Optoelectronics South China Normal University
Guangzhou 510006, China
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/solr.201900499.
DOI: 10.1002/solr.201900499
Prof. Y. Chen
Institute of Polymers and Energy Chemistry College of Chemistry
Nanchang University Nanchang 330031, China Prof. J.-M. Liu
Laboratory of Solid State Microstructures Nanjing University
Nanjing 210093, China Prof. K. Kempa Department of Physics Boston College
Chestnut Hill, MA 02467, USA
Low-temperature, solution-processed metal oxides are of great interest as alternative materials for electron transport layers in perovskite solar cells. WOx
is a promising candidate that could truly enable low-temperature (<100C) processing. However, its amorphous-state form typically obtained with the solution process suffers from high defect density. This causes large charge recombination, and consequently significant deterioration of the solar cell efficiency. Herein, an ultra-low-temperature processed (50C) nanocrystalline WOxas the electron transport layer, free of this problem, is demonstrated. This material is obtained by the reaction of tungsten chloride with hexanol, which induces transformation of the precursor solution into stable colloidal particles.
The best solar cell, with the WOxelectron transport layer, achieved an efficiency of 20.77%, which is a record performance for this class of perovskite solar cells.
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high for the commonly used plastic substrates.[8]Thus, discov- ering a truly low-temperature (<100C) processed ETL, with high performance and simple fabrication, is of fundamental importance.
Tungsten oxides (WOx) are promising potential candidates, with wide bandgaps (2–3 eV), high electron mobility (10–20 cm2V1s1, monocrystalline[9]), good chemical stability, and processing temperature below 100C. Wang et al.[9] reported a WOxfilm by spin coating an isopropyl alcohol solution of tungsten chloride, followed by annealing at a very high temperature of 150C. They also developed a WOx–TiOx composite ETL, processed at an ultralow annealing temperature of 70C, but the cell efficiency was only 13.45%.[9] These devices, made of theamorphousform of WOxsuffer from low open circuit voltage (usually less than 0.95 V), and lowfill factor (FF)[10]. In addition, the relatively low Fermi level of WOx(4.65 eV) also contributes toVOCreduction.[11]
In this work, we have developed a route to fabricatenanocrys- tallineWOxfilms at a record low temperature, as low as 50C.
We also discovered that using hexanol (H-WOx) leads to strongly reduced charge recombination, and increase of the Fermi level, facilitating strong improvements inVOCand FF.
The corresponding n–i–p planar PSCs achieved record high efficiencies. Specifically, with the MAPbI3 absorber the efficiency reached 19.16%, and it was further increased to the record high of 20.77%, with the (FAPbI3)1x(MAPbBr3)x absorber.
2. Results and Discussion
2.1. Synthesis and Morphology
WOxwas synthesized in a two-step process.[9,12]In step one, the reaction was
WCl6þxCnH2nþ1OH!WCl6xðOCnH2nþ1ÞxðblueÞ þxHCl"
(1) It shows that WCl6reacts with alcohol through the process of the electron-deficient W6þinteracting with the electron-rich alkoxyl. In the second step, the reaction was
WCl6xðOCnH2nþ1Þxþ ð6xÞCnH2nþ1OH
!WðOCnH2nþ1Þ6ðtransparentÞ þ ð6xÞHCl" (2) The reaction was completed with the formation of transparent W(OCnH2nþ1)6. Therefore, the lighter the color of thefinal pre- cursor solution, the higher is the yield of the fully substituted product.
In this work, we eliminated organic binders, which have been used before to stabilize the obtained precursor solution of small-size nanoparticles, and might have been retained on the surface of the metal oxide, forming trap centers.[13]
We used butanol and hexanol as reactants instead, which lead to larger nanoparticle sizes, while simultaneously increas- ing the electrophilic substitution rate with their stronger electron donating property (see Supporting information for
more details). The resulting precursor solutions (as well as the corresponding films) are referred to as E-WOx, B-WOx, and H-WOx.
Figure 1a shows images of the precursor solutions, illuminated with a laser beam. The H-WOx solution shows a strong Tyndall effect, revealing larger colloidal particle size, in agreement with the result shown in Figure S1, Supporting Information. When the precursor solution was spin coated on fluorine-doped tin oxide (FTO) glass, followed by thermal annealing at 50C and 20 min with UV ozone, the E-WOx, B-WOx, and H-WOx ETLs became colorless (see Figure S2, Supporting Information).
The morphologies of WOx films were characterized with high-resolution transmittance electron microscopy (HRTEM).
It is clear that E-WOx is amorphous (Figure 1b), whereas H-WOx is well crystallized (Figure 1d), with the interplane spacingd¼0.37 nm, corresponding to (200) lattice planes of monoclinic WO3. B-WOx (shown in Figure 1c) has a quasi- crystalline character.[14] The scanning electron microscopy (SEM) images of Figure S3, Supporting Information, reveal the superior conductivity of H-WOx as compared to E-WOx and B-WOx.
X-ray photoelectron spectroscopy (XPS) was applied to analyze the surface chemical states of WOx. The full XPS spectra survey provided in Figure 2a shows the presence of O and W, but no residual Cl (Figure 2b). In Figure 2c, the two characteristic peaks at the binding energy of36 and 38 eV correspond to states W 4f7/2 and W 4f5/2 of the W6þ state, respectively. In Figure 2d–f, the peak due to O (1s) was decomposed into a peak characteristic of the lattice oxygen atoms in a fully coordinated environment (W—O—W, at lower binding energy), and a peak characteristic of hydroxide species (W—OH, at higher binding energy).[15]Since the W—O—W backbone acts as the electron- conducting pathway, the higher peak intensity ratio in H-WOx (85.3%) implies higher electron mobility and thus transfer effi- ciency, as compared to E-WOx(66.99%) and B-WOx (77.13%).
In contrast, W—OH, due to incomplete oxidation of the oxide lattice, serves as a shallow trap site.[15] Therefore, we expect lowest density of traps in H-WOx.
2.2. Electronic and Optical Properties of WOxLayers and Structures
The UV–vis absorption spectra and transmittance are almost iden- tical for the threefilms, as shown inFigure 3a (and in Figure S4, Supporting Information), with the bandgaps estimated to be 3.72, 3.77, and 3.79 eV, corresponding to E-WOx, B-WOx, and H-WOx, respectively. The band structure was further characterized by UV photoelectron spectroscopy (UPS) (see Figure 3b). The Fermi levels (EF), calculated from the intercept of the higher binding energy with the incident photon energy, are4.62,4.28, and 4.30 eV, for E-WOx, B-WOx, and H-WOx, respectively.[16]The values of the valance band maximum (VBM) were estimated to be8.24,8.02, and7.99 eV, respectively, leading to the follow- ing conduction band minima (CBM):4.52,4.25, and4.20 eV.
We note that B-WOxand H-WOxare more energetically compati- ble with the perovskite and FTO than E-WOx(see also Figure S5, Supporting Information).
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Figure 3c shows the current voltage (J–V) scans, where H-WOx exhibits a steeper slope compared with E-WOx and B-WOx- based devices, resulting from its superior conductivity.
This is confirmed by the conductive atomic force microscopy (CFM) shown in Figure S6, Supporting Information, where the average current of H-WOx(16.10 nA) is higher than those Figure 2. XPS spectra of WOxfilms. a) Full XPS spectra survey. b) Cl 2p. c) W 4f. d–f ) O 1s of E-WOx, B-WOx,and H-WOx.
Figure 1.Optical and HRTEM images of WOx. a) Optical images of WOxprecursor solutions. HRTEM images of b) E-WOx, c) B-WOx, and d) H-WOx
films. All scale bars are 5 nm.
of E-WOx (1.30 nA) and B-WOx (14.8 nA)[17]. The electron mobility was measured using the space charge-limited current (SCLC) method,[18] with the electron- selective device structure FTO/WOx/phenyl-C61-butyric acid methyl ester (PCBM)/Ag (Figure 3d). The electron mobility for the H-WOxfilm was found to be6.61104cm2V1s1, which is one order of magni- tude higher than that for E-WOx (5.62105cm2V1s1), and twice as high as B-WOx (3.04104cm2V1s1). As dis- cussed earlier (XPS results in Figure 2), the low trap density in H-WOxis the origin of its better electronic properties.[15]
The charge transfer efficiency between the perovskite absorber and the WOx ETLs was investigated by the steady- state photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) for the structure: perovskite/
WOx/FTO. The emission peak at 773 nm (Figure 3e) originated from MAPbI3 was mitigated by the insertion of E-WOx and B-WOx and significantly quenched when the perovskite film was deposited on H-WOx. This suggests the fast electron extrac- tion and transport efficiency of the structure, resulting from its overall higher electron mobility and conductivity.[19]The nor- malized TRPL (Figure 3f ) further confirms the highest electron transfer efficiency of H-WOxby its low PL decay time (τ1andτ2) (see also Table S1, Supporting Information).τ1is related to trap- assisted recombination, andτ2represents free carrier recombi- nation.[20]τ1reduced gradually from 3.18 (ETL-free sample) to 2.96 (E-WOx), 2.38 (B-WOx), and 2.29 ns (H-WOx), while τ2
decreased from 128.86 to 110.98, 97.38, and 82.83 ns, corre- spondingly. Clearly, the lower trap density and enhanced charge transfer efficiency at the H-WOx/perovskite interface led to the enhanced device electronic performance.
2.3. Perovskite Solar Cells Based on MAPbI3
The photovoltaic performance of planar PSCs based on our WOx ETLs was examined in the device configuration FTO/
WOx/MAPbI3/2,20,7,70-tetrakis(N,N-di-p-methoxyphenylamine)- 9,9-spirobifluorene (spiro-OMeTAD)/Ag.[21]Devices were fabri- cated in the same batch, with identical procedures and parameters.
Figure 4a shows the cross-sectional SEM image of the devices. The thickness of WOx, perovskite, spiro-OMeTAD, and Ag electrodes was controlled to be about 28, 410, 200, and 70 nm, respectively.
Figure 4b shows the J–V curves of the champion devices based on different WOxETLs, with detailed parameters, includ- ing the open-circuit voltage (VOC), short-circuit current density (JSC), FF, and PCE. The PCE of the E-WOx-based device was 14.32%, with VOC¼0.96 V, JSC¼22.18 mA cm2, and FF¼67.45%. VOC and FF were significantly improved by replacing E-WOxwith B-WOx: VOC¼1.06 V and FF¼74.19%, yielding a PCE¼17.85%, withJSC¼22.65 mA cm2. This was further improved with H-WOx: VOC¼1.08 V, FF¼77.97%, JSC¼22.71 mA cm2, and PCE¼19.16%. The corresponding external quantum efficiency (EQE) spectra are shown in Figure 4c.
The EQE value, in the range of wavelengths 300–800 nm, was between 70% and 85%. The EQE integrated JSC of 21.93, 22.27, and 22.53 mA cm2, respectively, for the E-WOx, B-WOx, and H-WOx-based devices, agree well with the directJ–Vmeas- urements (Figure 4b).
The steady-state output was recorded at the maximum power bias points (Vmp) of 0.73, 0.88, and 0.91 V, for the E-WOx, B-WOx, and H-WOx-based PSCs, respectively, as shown in Figure 4d. The stabilized PCEs are 12.31%, 17.24%, and Figure 3. Electronic, optical, and charge transfer properties. a) Absorption spectra of WOxdeposited on quartz. b) UPS of WOxdeposited on indium tin oxides/glass. c) Conductivity of WOx. d) SCLC of FTO/WOx/PCBM/Ag. e) Steady-state PL and f ) TRPL of FTO/WOx/MAPbI3.
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18.56% with current density of 16.91, 19.51, and 20.36 mA cm2 for E-WOx, B-WOx, and H-WOx, respectively, in accordance with the values obtained fromJ–Vcurves. In addition, the reproduc- ibility of the device performance was checked with one batch of 25 devices for each ETL. The corresponding PCE distribution histograms are shown in Figure 4e (and Figure S7, Supporting Information). The averages of PCEs obtained for these histo- grams are 11.66%, 15.84%, and 18.24%, for E-WOx, B-WOx, and H-WOx, respectively. Interestingly, H-WOx-based devices are not only the best, but also show the best repeatability of the PCE.
To study the device stability, PCEs of the unencapsulated devi- ces, stored at an ambient atmosphere with humidity around 30%, was monitored for over two weeks. The results are presented in Figure 4f. H-WOxexhibits the best stability, with the PCE retain- ing 82.9% of the initial value. PSCs based on B-WOxand E-WOx
retained only 73.4% and 47.3% of their initial values, respec- tively. Therefore, the nanocrystalline nature of the H-WOxnot only benefits the photovoltaic performance, but also favors long-term stable output.
2.4. Perovskite Solar Cells Based on (FAPbI3)1x(MAPbBr3)x
and H-WOxETL
To further demonstrate the great potential of the H-WOxETL in PSCs, we developed PSCs based on the (FAPbI3)1x(MAPbBr3)x perovskite, in the following device structure configuration: FTO/
H-WOx/(FAPbI3)1x(MAPbBr3)x/spiro-OMeTAD/Ag.Figure 5a shows the J–V curves of the best device. A record PCE of 20.77% (VOC¼1.06 V,JSC¼24.82 mA cm2, FF¼79.13%) was achieved, which is the highest efficiency reported so far for WOx-based PSCs. The device has a negligible hysteresis.
Figure 4. Structure and photovoltaic performance of PSC. a) Cross-sectional SEM of the MAPbI3-based solar cell. b)J–Vcurves, with parameters shown in the inset. c) EQE (left) and the corresponding integrated current density (right). d) Steady-state PCE and current density at a constant bias voltageVmp. e) PCE distribution histogram of PSCs. f ) Stability test.
Figure 5.Performance of the (FAPbI3)1x(MAPbBr3)x-based PSCs with the H-WOxETL. a)J–Vcurves with parameters in the inset. b) EQE (left) and integrated current density (right). c) Steady-state PCE and current density at a constant bias voltageVmp.
The corresponding EQE (see Figure 5b) is more than 85% in the spectral range 300–800 nm, and the integrated JSC of 24.18 mA cm2 is in excellent agreement with the J–V result (Figure 5a). The steady-state output in Figure 5c gave the stabi- lized PCE and current density of 19.97% and 22.07 mA cm2, respectively.
3. Discussion
The superior properties of H-WOxdemonstrated in this work are largely due to the nanocrystalline structure, as shown in the HRTEM images in Figure 1. This minimizes the bulk defect den- sity, thus improving the carrier mobility, enhancing the conduc- tivity, and leading to the record highVOC. This is confirmed also inFigure 6a, which shows an excellent quality of the PSC junc- tion, by showing the nearly perfect linear dependence of the open-circuit voltage (Voc) on the log of the light intensity,[22]with the slope given bynkT/q, wherekis the Boltzmann constant,T the absolute temperature, andqthe electron charge.nis the ide- ality factor, which scales with the trap-assisted recombination rate;n¼1 corresponds to the absence of such recombination.[23]
The extractednvalues are 1.38 for H-WOx, 3.35 for E-WOx, and 1.92 for B-WOx-based cells. Clearly, H-WOx, having n closest to 1, has the lowest level of trap-assisted recombination.[24]
This is fully consistent with our experiments discussed previ- ously (see Figure 3c,d, etc.) and further confirmed by transient photovoltage (TPV) studies, as shown in Figure 6b: the PSC with the H-WOxETL has the longestVOCdecay time. The highest conductivity of H-WOx leads to the smallest series resistance, as shown in Figure 6c (14.44Ω of H-WOx vs 17.76Ω of
E-WOx and 15.21Ω of B-WOx), and thus leads to the largest FF (see Figure 4b).
Furthermore, H-WOxhas the best electron transfer efficiency as confirmed by electrochemical impedance spectroscopy (EIS) in Figure 6c, where the Nyquist plots of the cell using H-WOx present the smallest transfer resistance Rtr¼121.9Ω, as compared toRtr¼219.3Ωfor E-WOxandRtr¼140.9Ωfor B-WOx.[25]The PL and TRPL measurements shown in Figure 3 also agree with this result. Dark J–V curves are shown in Figure 6d. While both PSCs with B-WOxand H-WOxETLs show low leakage current density, the steeper current increase at high bias voltage (larger than 0.8 V) for the solar cell based on H-WOx reveals its most efficient electron injection capability.[26]Last but not least, H-WOxis most compatible with perovskites, as men- tioned previously.
4. Conclusions
In summary, we have thoroughly studied WOxfilms obtained by tungsten chloride reacting with different alcohols (ethanol, butanol, and hexanol), and their application as the ETL in PSCs.
We found that hexanol induces colloidal transformation of the precursor solution and leads to the formation of a nanocrystalline structure of thefilms. This dramatically reduces the trap density and consequently boosts the electron mobility, and thus also the conductivity. The planar PSCs based on a H-WOx ETL show excellent photovoltaic performance, with PCE above 20% and VOC approaching 1.1 V. Our work has solved the fundamental recombination problem in WOx, making application of this material very promising in PSCs.
Figure 6. Charge transfer properties of PSCs with WOxETLs. a)Vocversus light intensity. b) TPV results. c) EIS results. d) DarkJ–Vcurves.
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5. Experimental Section
Materials: Spiro-OMeTAD, methylammonium iodide (MAI) (Yingkou You Xuan Trade Co. Ltd), bis(trifluoromethanesulfonyl)imide (Li-TFSI), tert-butylpyridine (t-BP) (Sigma-Aldrich), lead iodide (PbI2 99.9%) (Xi’an Polymer Light Technology Cory.), and WCl6(Macklin) were used as received.
The solvent was dimethyl sulfoxide (DMSO, 99.9%), chlorobenzene (CB, 99.8%), acetonitrile, and dimethyl formamide (DMF, 99.8%). FTO glasses (7Ωsq1) were received from Yingkou You Xuan Trade Co. Ltd.
Preparation of Colloidal WOxPrecursor Solution: The WOxprecursor solu- tion was prepared by dissolving WCl6in ethanol, butanol, and hexanol, sep- arately, reacting 24 h in a glovebox. The obtained sky blue transparent clear WOxprecursor solution wasfiltered with 0.45μm polytetrafluoroethylene filter before use.
Perovskite Precursor Preparation: The perovskite precursor was prepared as reported.[21] Typically, 922 mg of PbI2 and 320 mg of MAI were dis- solved in 1.6 mL mixed solvent of DMF and DMSO (7:3 V/V), stirring at 80C for overnight before use.
Device Fabrication and Characterization: MAPbI3 perovskite precursor was prepared by dissolving 922 mg of PbI2 and 320 mg of MAI in 1.6 mL mixed solvent of DMF and DMSO (7:3 V/V), stirring at 80C over- night before use. MAPbI3-based devices were fabricated with the structure FTO/WOx/perovskite/spiro-OMeTAD/Ag, with the thickness of each layer about 28, 410, 200, and 70 nm. Typically, FTO-coated glass substrates were ultrasonically cleaned with detergent, deionized water, acetone, and isopropanol in a sequence for 20 min. All FTO substrates were cleaned by UV ozone for 15 min before spin coating. The WOxETLs were deposited by spin coating the WOxprecursor solution on FTO substrates at 3000 rpm for 30 s, followed by thermal annealing at 50C for 40 min and ultraviolet ozone treatment for 25–30 min to totally remove residual solvent. Then, the MAPbI3precursor solution was spin coated on FTO/WOxsubstrates at 500 rpm for 3 s and 4000 rpm for 30 s, with quickly dripping 400μL chlo- robenzene onto the rotating perovskitefilm at the beginning of 8–10 s of the second spin-coating step. For (FAPbI3)1x(MAPbBr3)x, 1.3Mof PbI2in DMF:DMSO¼9.5:0.5 was spin coated on WOxat 1500 rpm for 30 s and then annealed at 70C for 1 min; the mixture solution of FAI:MABr:
MACl¼60 mg:6 mg:6 mg in 1 mL IPA was spin coated onto the PbI2at 1500 rpm for 30 s, with further thermal annealing at 30–40% humidity condition for 130C and 25 min.[27] Spiro-OMeTAD and Ag were then sequentially deposited on FTO/ WOxsubstrates as reported.[28]
The morphology was investigated byfield emission scanning electron microscopy (ZEISS Ultra55) and TEM (JEM-2100). The crystal struc- ture and phase of the WOx ETLs were characterized using XRD (PANalyticalX’Pert PRO) with Cu Kαradiation under operation condi- tions of 40 kV and 40 mA. The absorption spectra of ETLs deposited on quartz and the transmittance of FTO/ETL were measured by a UV–vis spectrometer (Shanghai Jinhua Technology Instrument Co., Ltd. UV-759) in a wavelength range of 300–800 nm. UPS characteriza- tion was performed by the ThermoFisher ESCALAB 250Xi. Steady-state PL and TRPL spectra were measured by afluorescence spectrometer (HITACHI F-5000) excited at 470 nm and an FLS980 with the excitation wavelength of 773 nm. TheJ–Vcharacteristics and steady-state perfor- mance atVmp(0.73, 0.88, and 0.91 V for E-WOx, B-WOx, and H-WOx) of the devices were measured with a Keithley 2440 source under a simu- lated AM 1.5G spectrum. The light intensity of the solar simulator (Newport, 91160) was calibrated using a standard silicon solar cell device by the National Renewable Energy Laboratory. The EQE measure- ments of photovoltaic devices were conducted using a QE-C system (Taiwan, Enlitech). The EIS and transient photovoltage measurements were performed on the Zahner Zennium electrochemical workstation.
The electron mobilities of WOxfilms were measured using the SCLC model, using an electron-only device of FTO/ WOx/PCBM/Ag, where the SCLC is described by J¼(9/8) εr ε0μe(V 2/L3), where ε0 is the permittivity of free space,εris the dielectric constant of the metal oxide layers,μeis the electron mobility,Vis the voltage drop across the device, andLis the metal oxide thickness (110 nm). The dielectric constantεrof WOx was assumed to be 14. The surface potentials and conductivity of WOxfilms were obtained from the CFM (Asylum Research, Cypher).
XPS was performed on a Thermo Scientific ESCALAB 250xi and particle size analysis was performed on a Malvern Zetasize nano zs90.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
The authors are thankful for the financial support from the NSFC–
Guangdong Joint Fund (No. U1801256), National Key R&D Program of China (No. 2016YFA0201002), NSFC (Nos. 51803064, 51571094, 51431006, 51561135014), Guangdong Provincial Foundation (2016KQNCX035), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No. IRT_17R40), and Guangdong Innovative Research Team Program (No. 2013C102). The authors are also thankful for the support from the Guangdong Provincial Engineering Technology Research Center for Transparent Conductive Materials, National Center for International Research on Green Optoelectronics (IrGO), MOE International Laboratory for Optical Information Technologies and the 111 Project. K.K. acknowledges partial support by the Guangdong Innovative and Entrepreneurial Team Program (No. 2016ZT06C517).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
electron transfer layers, low-temperature WOx, perovskite solar cells Received: November 4, 2019
Revised: January 6, 2020 Published online: January 31, 2020
[1] a) Best Research-Cell Efficiency Chart, https://www.nrel.gov/pv/cell- efficiency.html (accessed: August 2019); b) A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050;
c) H. Kim, C. Lee, J. Im, K. Lee, T. Moehl, A. Marchioro, S. Moon, R. Humphry-baker, J. Yum, J. E. Moser, M. Gra,Sci. Rep.2012,2, 1;
d) M.M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science2012,338, 643; e) N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. Il Seok,Nature2014,517, 476; f ) Q. Jiang, Y. Zhao, X. Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin, J. You,Nat.
Photonics2019,13, 460.
[2] a) V. L. Pool, B. Dou, D. G. Van Campen, T. R. Klein-Stockert, F. S. Barnes, S. E. Shaheen, M. I. Ahmad, M. F. A. M. Van Hest, M. F. Toney, Nat. Commun. 2017, 8, 1; b) W. Qiu, T. Merckx, M. Jaysankar, C. M. De La Huerta, L. Rakocevic, W. Zhang, U. W. Paetzold, R. Gehlhaar, L. Froyen, J. Poortmans, D. Cheyns, H. J. Snaith, P. Heremans, Energy Environ. Sci. 2016, 9, 484;
c) H. Zhang, J. Cheng, D. Li, F. Lin, J. Mao, C. Liang, A. K. Jen, M. Grätzel, W. C. H. Choy, Adv. Mater. 2017,29, 1; d) H. Tan, A. Jain, O. Voznyy, X. Lan, M. Yuan, B. Zhang, Y. Zhao, F. Fan, P. Li, L. N. Quan, Y. Zhao, Z. Lu, Z. Yang, S. Hoogland, E. H. Sargent, Science 2017, 726, 722; e) M. V Kovalenko, L. Protesescu, M. I. Bodnarchuk,Science2017,750, 745.
[3] a) L. Calió, S. Kazim, M. Grätzel, S. Ahmad,Angew. Chem. Int. Ed.
2016,55, 14522; b) M. Saliba, T. Buonassisi, M. Grätzel, A. Abate, W. Tress, A. Hagfeldt,Science 2017,744, 739; c) A. Fakharuddin, L. Schmidt-Mende, G. Garcia-Belmonte, R. Jose, I. Mora-Sero,Adv.
Energy Mater. 2017, 7, 1700623; d) S. Il Seok, M. Grätzel, N.-G. Park,Small2018,14, 1704177.
[4] a) M. Najafi, F. Di Giacomo, D. Zhang, S. Shanmugam, Small 2018, 14, 1702775; b) A. Fakharuddin, L. Schmidt-Mende, G. Garcia-Belmonte, R. Jose, I. Mora-Sero,Adv. Energy Mater.2017, 7, 1700623; c) Z. H. Bakr, Q. Wali, A. Fakharuddin, L. Schmidt-mende, M. Brown, R. Jose,Nano Energy2017,34, 271; d) D. Y. Son, J. W. Lee, Y. J. Choi, I. H. Jang, S. Lee, P. J. Yoo, H. Shin, N. Ahn, M. Choi, D. Kim, N. G. Park,Nat. Energy2016,1, 1.
[5] M. V Kovalenko, L. Protesescu, M. I. Bodnarchuk,Science2017,358, 745.
[6] D. Liu, T. L. Kelly,Nat. Photonics2013,8, 133.
[7] 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. Il Seok,Nat. Commun.2015,6, 7410.
[8] Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You,Nat. Energy2016,2, 16177.
[9] a) K. Wang, Y. Shi, B. Li, L. Zhao, W. Wang, X. Wang, X. Bai,Adv.
Mater.2016,28, 1891; b) J. M. Berak, M. J. Sienko,J. Solid State Chem.1970,2, 109; c) K. Wang, Y. Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma,J. Phys. Chem. Lett.2015,6, 755.
[10] K. Wang, Y. Shi, L. Gao, R. Chi, K. Shi, B. Guo, L. Zhao,Nano Energy 2017,31, 424.
[11] a) R. Zhang, W. Liu, R. Hu, Y. Ma, Y. Sun, J. Zhang, Y. Pu,Sol. Energy Mater. Sol.2019,191, 183; b) Z. Zhang, H.-X. Wei, G.-F. Ma, Y.-Q. Li, S.-T. Lee, J.-X. Tang,Appl. Phys. Lett.2013,103133302.
[12] K. Nishio, T. Sei, T. Tsuchiya,J. Ceram. Soc. Jpn.1999,107, 199.
[13] Y. Hou, C.O.R. Quiroz, S. Scheiner, W. Chen, T. Stubhan, A. Hirsch, M. Halik, C.J. Brabec,Adv. Energy Mater.2015,5, 1501056.
[14] K. Mahmood, B. S. Swain, A. R. Kirmani, A. Amassian,J. Mater. Chem.
A2015,3, 9051.
[15] H. Yu, H. I. Yeom, J. W. Lee, K. Lee, D. Hwang, J. Yun, J. Ryu, J. Lee, S. Bae, S. K. Kim, J. Jang,Adv. Mater.2018,30, 1.
[16] a) K.-H. Jung, J.-Y. Seo, S. Lee, H. Shin, N.-G. Park,J. Mater. Chem. A 2017, 5, 24790; b) Y. Kuang, V. Zardetto, R. Van Gils, S. Karwal, D. Koushik, M. A. Verheijen, L. E. Black, C. Weijtens, S. Veenstra, R. Andriessen, W. M. M. Kessels, M. Creatore, ACS Appl. Mater.
Interfaces2018,10, 30367.
[17] W. Chen, F. Z. Liu, X. Y. Feng, A. B. Djuriši´c, W. K. Chan, Z. B. He, Adv. Energy Mater.2017,7, 1.
[18] a) D. Yang, R. Yang, K. Wang, C. Wu, X. Zhu, J. Feng, X. Ren, G. Fang, S. Priya, S. (F.) Liu, Nat. Commun. 2018, 9, 3239;
b) C. Wang, C. Xiao, Y. Yu, D. Zhao, R. A. Awni, C. R. Grice, K. Ghimire, I. Constantinou, W. Liao, A. J. Cimaroli, P. Liu, J. Chen, N. J. Podraza, C. S. Jiang, M. M. Al-Jassim, X. Zhao, Y. Yan,Adv. Energy Mater.2017,7, 1.
[19] a) W. Chen, Y. Wu, J. Fan, A. B. Djuriši´c, F. Liu, H. W. Tam, A. Ng, C. Surya, W. K. Chan, D. Wang, Z. B. He,Adv. Energy Mater.2018,8, 1;
b) W. Chen, Y. Zhou, L. Wang, Y. Wu, B. Tu, B. Yu, F. Liu, H. W. Tam, G. Wang, A. B. Djuriši´c, L. Huang, Z. He,Adv. Mater.
2018,30, 1.
[20] L. Zuo, Z. Gu, T. Ye, W. Fu, G. Wu, H. Li, H. Chen,J. Am. Chem. Soc.
2015,137, 2674.
[21] R. Mai, X. Wu, Y. Jiang, Y. Meng, B. Liu, X. Hu, J. Roncali, G. Zhou, J. Liu, K. Kempa, J. Gao,J. Mater. Chem. A2019,7, 1539.
[22] A. You, M. A. Y. Be, I. In,Appl. Phys. Lett.2005,86, 123509.
[23] W. Tress, M. Yavari, K. Domanski, P. Yadav, B. Niesen, J. P. C. Baena, A. Hagfeldt, M. Graetzel,Energy Environ. Sci.2018,11, 151.
[24] P. S. Cells, J. Duan, Y. Zhao, B. He, Q. Tang,Angew. Chem. Int. Ed.
2018,57, 3787.
[25] T. Bu, J. Li, F. Zheng, W. Chen, X. Wen, Z. Ku, Y. Peng, J. Zhong, Y. Cheng, F. Huang,Nat. Commun.2018,9, 4609.
[26] H. Yi, D. Wang, A. Mahmud, F. Haque, C. Xu, L. Duan, A. Uddin, Appl. Energy Mater.2018,1, 6027.
[27] Q. Jiang, Z. Chu, P. Wang, X. Yang, H. Liu, Y. Wang, Z. Yin, J. Wu, X. Zhang, J. You,Adv. Mater.2017,29, 1.
[28] C. Chen, Y. Jiang, J. Guo, X. Wu, W. Zhang, S. Wu, X. Gao, X. Hu, Q. Wang, G. Zhou, Y. Chen, J. Liu, K. Kempa, J. Gao,Adv. Funct.
Mater.2019,29, 1900557.
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