Solution-Processed MoOx Hole-Transport Layer with F4-TCNQ

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Solution-processed MoOx hole-transport layer with F4-TCNQ modification for efficient and stable inverted perovskite solar cells Lijun Chen, Qiaomu Xie, Li Wan, Wenxiao Zhang, Sheng Fu, Haitao Zhang, Xufeng Ling, Jianyu Yuan, Lijing Miao, Cai Shen, Xiaodong Li, Wenjun Zhang, Bo Zhu, and Hai-Qiao Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01004 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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ACS Applied Energy Materials

Solution-processed MoOx hole-transport layer with F4-TCNQ modification for efficient and stable inverted perovskite solar cells Lijun Chen,a,b Qiaomu Xie,b Li Wan,b Wenxiao Zhang,b Sheng Fu,b Haitao Zhang,b Xufeng Ling,d Jianyu Yuan,d Lijing Miao,b Cai Shen,b Xiaodong Li,b Wenjun Zhang,b Bo Zhua and Hai-Qiao Wang*b, c a

Department of Polymer Materials, School of Materials Science and Engineering, Shanghai

University, Nanchen Road 333, Shanghai, 200444, China. b

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

Ningbo, 315201, China. Corresponding E-mail: [email protected] c

University of Chinese Academy of Sciences, Beijing, 100049, China.

d

Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren-Ai

Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China. KEYWORDS: perovskite solar cell, MoOx hole-transport layer, F4-TCNQ modification, interface property, efficiency, stability

ABSTRACT:

Besides high quality perovskite, precisely designed interface is always necessary to achieve top performance perovskite solar cells. However, this inevitably introduces complexity for fabrication,

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increases cost, and consequently hinders the commercialization. Thus developing efficient, easy processable and inexpensive interface is critically important for commercialization of perovskite photovoltaics. In this work, via solution processed MoOx HTL and F4-TCNQ modification, we demonstrated efficient and stable inverted MAPbI3 perovskite solar cells, due to improved optoelectronic properties at the interface and perovskite. Champion PCE of 16.26% was achieved for the optimized device with negligible hysteresis. Equal importantly, huge improvement is also demonstrated for device stability, by retaining over 95% of its initial PCE after 150 h in ambient condition (RH: ~45%), and 95% after 40 h in operational situation under continuous AM 1.5G illumination, respectively. Our work highlights that efficient and stable perovskite solar cells can be accomplished with easy processable and inexpensive inorganic interlayer, and provides referential strategy and methodology for this target. Which would be beneficial for the commercialization of PSC technology.

1. INTRODUCTION Perovskite solar cell (PSC) is considered a promising renewable energy technology of the next generation owing to its huge progresses achieved in the past ten years. Especially for organicinorganic PSCs, the power conversion efficiency (PCE) was improved from below 4% to over 23%.1 However, the commercialization of PSCs is still limited mainly due to the device stability and cost issues.2 Those top performance PSCs (PCE ~20%) have initially and commonly been fabricated in conventional configuration with mesoporous TiO2 as the electron-transport layer (ETL) and usually combined with an efficient holt-transport layer (HTL). 3-5 The TiO2 ETL can cause photocatalytic degradation of device.6-7 And for which a high temperature sintering (450550 °C) is inevitable.8 The HTL is typically composed of organic compound like 2,2’,7,7’-tetrakis (N,N-dip-methoxyphenylamino)-9,9’-spirobifluorene

(spiro-oMeTAD),9

poly(triarylamine)


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(PTAA),10 FDT,11 and 9-(2-ethylhexyl)-N,N,N,N-tetrakis(4-methoxyphenyl)-9H-carbazole-2,7diamine (EH44).12 Most of them are complicated and time consuming to synthesis.13, 14 And almost for all of them, additive dopants such as LiTFSI, t-BP and FK209 are indispensable to enhance device performance.

8-11

However, due to intrinsic hygroscopic property of the dopants, intense

degradation processes within device can be caused.15 In addition, these organic hole-transport materials are often costly. For instance, the price of spiro-OMeTAD and PTAA are more than ten16 and fifty17 times higher than that of gold, respectively. Although new efficient conventional PSCs were design and developed to avoid the deficiencies in TiO2 ETL based PSCs.15 The process complexity, device stability and fabrication cost issues are still challenging, which limits the further progress and commercialization of PSCs.2 However, inverted PSC can be prepared with simpler device architecture, more facile fabrication, lower cost and suppressed hysteresis,18, 19 which is more compatible with largescale, flexible and printing manufacture. Especially combined with stable and inexpensive inorganic HTL, it possesses advantages to overcome the above mentioned issues/drawbacks. However, till now only few copper20, 21 and nickel based22-23 HTL systems have achieved relative high efficiency. And it is still remaining challenge for most inorganic HTLs24, 25 to obtain efficient and stable performance, especially for those solution processed inorganic HTLs.4, 18, 26 As a p-type contact, molybdenum oxide (MoOx) has been widely applied in electronic devices owing to its advantages of low cost, high hole-mobility, good stability and suitable work function.27,28 Furthermore, its low toxicity29 and good compatibility in largescale/flexible fabrication27,30 fulfil the requirement of future commercialization of PSCs. Actually its application in conventional PSCs has been demonstrated, deposited by vacuum thermal evaporation.31,32,33 However, the research in inverted PSCs is quite limited. Owing to imperfect interfacial properties


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like rough surface, trap states, low work function relative to perovskite HOMO level and chemical reaction with perovskite, low efficiency and weak device stability was obtained for MoOx HTL based inverted PSCs.25, 34, 35 Thermal-evaporated single MoOx HTL delivered efficiency of ~6% for inverted PSCs due to the low FF.34 Even thermal-evaporated MoOx/NPB35 and solutionprocessed MoOx/PEDOT:PSS36 bilayer-structures produced relatively low efficiency (PCE of 13.7% and 14.8% respectively) as well. Therefore modification of MoOx HTL is essential to improve the device performance. As a p-type dopant, 2,3,5,6-Tetrauoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is a strong electron acceptor with a low lowest unoccupied molecular orbital (LUMO, -5.2 eV) energy level, which could down-shift the fermi-level/increase the work function of MoOx,31 thus reduce energy level offset at the contact and benefit device performance. Meanwhile, this molecular doping can also contribute to film morphology and defects passivation.37-38 It has been successfully utilized in PSCs to modify the interface39-43 and absorbing layer44-45 to improve device performance. Herein we report the application of solution processed inorganic MoOx HTL in inverted MAPbI3 PSCs and demonstrate huge improvement of device efficiency and stability via F4-TCNQ modification on MoOx HTL. With single MoOx HTL, decent efficiency (PCE 12.06%) with modest device stability was recorded. While after modification, greatly enhanced PCE of 16.26% was achieved with negligible hysteresis. More importantly, dramatically improved device stability was also demonstrated by the device degradation test in ambient condition and operational situation. As far as we know, this is the best performance of inverted perovskite solar cells reported in literatures based on MoOx HTL. 2. EXPERIMENTAL SECTION


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2.1. Materials. Molybdenum (V) isopropoxide (5%w/v in isopropoxide, 99.6%), isopropanol (IPA), chlorobenzene (CB), methanol (MeOH), N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Alfa Aesar. Lead iodide (PbI2), methylammonium iodide (MAI), (6, 6)-phenyl-C61-butyric acid methyl ester (PCBM), bathocuproine (BCP) were ordered from Xi’an Polymer Light Technology Corp. The F4-TCNQ (99.5%) was purchased from AGLAIA TECH and it was dissolved in methanol and heated at 60 °C for 2 h. The perovskite precursor solution of 1.45 M was prepared by mixing PbI2 and MAI in DMF/DMSO co-solvent. 2.2. Device Fabrication. Patterned ITO substrates (ITO/glass: Rs ≤ 15 Ω per square) were cleaned by detergent, deionized water, acetone and isopropanol for 15 min in sequence. Then the ITO substrates were transferred to O2-Plasma cleaner treating for 5 min. After that, the molybdenum isopropoxide was diluted 20 times by isopropoxide to prepare the molybdenum solution. And it was spin coated on ITO at 4000 r.p.m. for 30 s, then thermal annealed at 150 °C for 20 min in ambient atmosphere. F4-TCNQ solution with different concentration (0.5, 1, 1.5, 2 mg/ml), was deposited on top of MoOx layer by spin coating at 4000 r.p.m. for 20 s in nitrogen filled glovebox. Immediately, the substrates were thermal annealed on hotplate at 80 °C for 20 min. Then the perovskite precursor solution was spin coated on the MoOx or MoOx/F4-TCNQ HTL at 4800 r. p. m. for 20 s, with anti-solvent CB treatment at last 12 s of the procedure, followed by two steps thermal annealing on a hotplate at 60 °C for 30 s and at 80 °C for 2 min. The crystalline perovskite films were covered by the PCBM solution (20 mg/ml) at 2000 r.p.m. for 30 s. Finally, 8 nm BCP and 100 nm Ag were thermally evaporated under high vacuum. The active area of the cell is 9 mm2. 2.3. Characterization and Measurements. The current density-open circuit voltage (J-V) characteristics are measured by using calibrated solar simulator of Keithley 2400 source


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measurement under simulated AM 1.5G irradiation with intensity of 100 mW cm-2, and the J-V curves are measured in glovebox under forward scan from 1.2 V to -0.2 V (reverse scan from -0.2 V to 1.2 V) with dwell time of 50 ms. The external quantum efficiency measurement is conducted in ambient atmosphere from 300 nm to 800 nm. The Newport quantum efficiency measurement system (ORIEL IQE 200TM) combined with a 150 W Xe lamp is calibrated by a standard Si solar cell. The surface roughness and surface potential of the hole-transport layer are tested by atomic force microscopy (AFM) (Dimension 3100, Vecco, America). The surface morphology of perovskite and HTLs are measured by scanning electron microscope (SEM) (S4800, Hitachi, Japan), as well as cross-section of device, with an accelerating voltage of 8 kV. UV-vis spectra were recorded on a Perkin Elmer Lambda 950 spectrophotometer. Photoluminescence (PL) and time-resolved photoluminescence (TRPL, emission at 760 nm) spectra were recorded on a fluorescence spectrophotometer (F-4600, Hitachi Ltd., Tokyo, Japan) with excitation wavelength of 466 nm and 532 nm. The X-ray diffraction (XRD) patterns were conducted on X-ray diffractometer (Bruker AXS D8 Advance, Germany). X-ray photoelectron spectroscopy (XPS) measurements are carried out using a Kratos AXIS ULTRA DALD XPS system, adjusted according to the C 1s peak at (284.7 ± 0.1) eV. 3. RESULTS and DISCUSSION Inverted device ITO/MoOx/F4-TCNQ/MAPbI3/PCBM/BCP/Ag and its reference device without F4-TCNQ layer were fabricated. The schematic device structure, cross-section SEM image of the whole device, chemical structure of molecular F4-TCNQ and energy level of the corresponding materials discussed are shown Figure 1, parts of the energy levels are referenced from literatures.30, 40

Due to strong withdraw ability of F4-TCNQ, electrons transfer from the valence band of MoOx

to the LUMO of F4-TCNQ. Hole concentration in valence band of MoOx increases. Which would


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increase the work function of MoOx and optimize the band alignment between perovskite and anode, leading to reduced charge carrier recombination and large hole mobility in MoOx, thus promoting the hole extraction.41, 46 The J-V curves of PSCs with MoOx and MoOx/F4-TCNQ HTL were presented in Figure 2a and Table S1. With single MoOx HTL, modest performance with average PCE of 11.85% and the highest PCE of 12.06% was recorded. While after modification with thin F4-TCNQ layer, the device provided remarkably improved performance due to the all enhanced photovoltaic parameters. The champion PCE was promoted to 16.26% (average value 15.83%), with hugely enhanced FF (from 69.32% to 76.01%), JSC (from 17.06 to 20.17 mA cm-2) and slightly improved VOC (from 1.02 to 1.06 V). The enhancement of FF could be attributed to the reduced charge recombination after F4-TCNQ passivation, which would reduce the energy lose at interface and improve the Voc. And the improved JSC could be explained for by the improved hole extraction and transport after F4-TCNQ modification. Negligible hysteresis behavior was recorded after the modification compared to the large hysteresis of the reference device (Figure 2a). Which further demonstrates the promoted carrier transport in device with the MoOx/F4-TCNQ HTL. The efficiency is dependent on the concentration of F4-TCNQ solution used to deposit (Figure 2b). The best PCE was obtained with 1 mg/ml F4-TCNQ solution. To validate the JSC values, the corresponding external quantum efficiency (EQE) of device was recorded (Figure 2c). Compared with the reference device, the MoOx/F4-TCNQ HTL based device presented higher photon to electron conversion efficiency throughout the entire visible range from 300 to 800 nm. From which, integrated photocurrent densities of 16.93 mA/cm2 and 19.65 mA/cm2 were determined for the reference and sample devices respectively. Both of them agree well with the JSC values obtained in J-V measurements.


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The dark J-V curves (Figure 2d) of the PSCs show that the dark current density of PSC based on MoOx/F4-TCNQ HTL is about one order of magnitude lower than that of the reference device. And higher rectification ratio can be determined, which indicates that charge carriers are dissociated effectively instead of charge recombination in device47 and could explain for the improved device parameters. In addition, better device reproducibility was obtained for the MoOx/F4-TCNQ HTL based devices. But broader distribution and lower values were recorded for the reference devices (Figure 3 and Table S1). Apart from efficiency, the device stability is studied by monitoring the device degradation in ambient atmosphere (relative humidity: ~45%) and operational situation (in N2 filled glovebox under continuous AM 1.5G illumination). As shown in Figure 4a, the reference device shows poor stability, losing over 50% of its initial efficiency after 150 h in ambient atmosphere, similar to previously reported result,25 while the MoOx/F4-TCNQ based device presents obviously enhanced stability. Over 95% and 70% of its initial PCE was retained after 150 h and 300 h, respectively. Again, much improved long-term device stability was demonstrated for the MoOx/F4-TCNQ based device (Figure 4b), by retaining 95% of its initial efficiency after 40 h, under continuous AM 1.5G full solar spectrum irradiation without any UV-filter in glovebox. However, the reference device degraded to 73% of its initial PCE after only one hour. In addition, PSCs with different HTLs were probed at MPP under AM 1.5G illumination to track the output. Steady power output of 15.04% and current density of 19.79 mA/cm2 were obtained for MoOx/F4-TCNQ based device (Figure 4c), but obviously unsteady output was recorded for reference device. These results agree well with the measured hysteresis characteristic. It should be noted that all the tests have been conducted on device without encapsulation. The results demonstrate that the stability of MoOx HTL based PSCs can be effectively improved by the modification of F4-TCNQ.


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It is known that the charge transport at interface is strongly influenced by the interfacial morphology and surface coverage.48, 49 To understand how the F4-TCNQ layer influences the HTL morphology and device performance, AFM measurements were conducted for the HTL before and after the deposition of F4-TCNQ, and as well as the perovskite layer on top of which. As shown in Figure 5a and b, the single MoOx layer forms rough surface with pin holes. After the deposition of F4-TCNQ, smoother film surface was recorded with a root-mean-square (RMS) roughness of 1.32 nm, compared to 2.2 nm of the MoOx film. Which is beneficial for the subsequent deposition and growth of high quality perovskite film. In addition, the smooth and compact MoOx/F4-TCNQ HTL can reduce shunt current and prevent the chemical reaction between MoOx and perovskite,31 which is beneficial for the device performance, as verified by the measured JSC and the stability results. To further reveal how the perovskite film was influenced by the HTL, SEM measurement was conducted to study the growth and accumulation of perovskite crystals. As shown in Figure 5c and d, more homogeneous and smoother surface was recorded for the perovskite film on MoOx/F4TCNQ, and with larger crystalline size. While smaller grain size and more pinholes can be observed in the reference perovskite film. Again, it is confirmed by the AFM topography images (Figure 5e and f). Smaller RMS roughness of 8.12 nm was obtained for the perovskite film on MoOx/F4-TCNQ, compared to 12.7 nm of that on MoOx. Less grain boundary/gaps implies less trap defect/recombination centers. Compact/dense interface helps to prevent direct contact of perovskite to the electrode. All these improvements could have contributed to the device efficiency and stability. The current-voltage traces and trap density of perovskite films were measured with hole-only devices by using the space charge limited current (SCLC) method. Figure 6a and b show the J-V


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curves of the reference device ITO/MoOx/MAPbI3/spiro-OMeTAD/Ag and the sample device ITO/MoOx/F4-TCNQ/MAPbI3/spiro-OMeTAD/Ag. The trap density Ntrap is calculated according to the equation: VTFL=eNtrapL2/(2εε0),50 where VTFL is the trap-filled limit voltage, e is the electronic charge, L is the thickness of perovskite film, ε is the relative dielectric constant of MAPbI3, and ε0 is the vacuum permittivity. Decrease of trap state density from 1.47×1016 cm-3 to 9.81×1015 cm-3 was determined after F4-TCNQ modification, due to the passivation function of F4-TCNQ. The steady state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were measured to figure out the role of F4-TCNQ in the process of charge carrier extraction.51 As shown in Figure 7a, the effect of PL quenching is more pronounced for the MoOx/F4-TCNQ interface than the single MoOx layer, suggesting a more efficient hole-carriers extracting from perovskite.

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The decrease of charge recombination or PL in perovskite on

MoOx/F4-TCNQ HTL could also indicate a higher quality of perovskite. Which is consistent with the results of the trap density analysis (Figure 6). The measured TRPL decays curves (Figure 7b) were fitted with a bi-exponential decay function52 and the extracted lifetimes are shown in the insert table. The fast decay lifetime τ1 can be attributed to the interfacial quenching effect, i.e. free carriers transporting from perovskite layer to HTL. And the slow decay lifetime τ2 is assigned to the radiative recombination property in perovskite. The perovskite film on MoOx has a fast-decay lifetime of 2.66 ns with weight fraction of 19.76% and a slow decay lifetime of 65.82 ns with weight fraction of 80.24%. The modification with F4-TCNQ significantly decreased the fast decay lifetime to 1.67 ns with increased weight fraction of 35.09% and the slow decay lifetime to 44.67 ns with decreased weight fraction of 64.91%. These results suggest more efficient hole extraction


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from the perovskite to the anode and depressed recombination after the F4-TCNQ modification, which is consistent with the obtained performance parameters. To gain further insight and understand the mechanism behind the improvement, chemical and electronic structure of the interface was studied by X-ray photoelectron spectroscopy (XPS). Except for the F and N signals (Figure 8c and d), no other new chemical components are observed in the XPS spectra (Figure 8a), indicating the incorporation of F4-TCNQ at HTL after modification. Mo (VI) states (with Mo 3d5/2 core level peak at 232.92 and 232.57 eV respectively) were recorded for both HTLs after thermal annealing (Figure 8b), suggesting transformation of Mo (V) to Mo (VI), i.e. from molybdenum (V) isopropoxide to MoO3. The shift (0.35 eV) toward higher binding energy of the 3d doublet of Mo after modification, could be attributed to the strong electron affinity of F4-TCNQ.38 Which convinces us of electron transfer from MoOx to F4-TCNQ, and is consistent with previous report.19 This is confirmed again by the surface potential change (from 0.65 V to 0.79 V) (Figure S1) after the modification. In addition, the crystalline characteristics of perovskite films were investigated by X-ray diffraction (XRD), as shown in Figure S2. Similar XRD patterns are observed for the perovskite films on MoOx/F4-TCNQ and MoOx HTL, except for a slight enhancement of the representative diffraction of lattice plane (110) and (220). The enhanced diffraction peaks indicate a preferred growth of the crystals in the corresponding planes. It could suggest an improved perovskite crystalline quality. And this is consistent with the SEM observations and the J-V characteristics. 4. CONCLUSIONS In conclusion, we demonstrate the fabrication of inverted PSCs based on solution-processed inorganic MoOx HTL and huge improvement of device efficiency and stability via F4-TCNQ modification. A champion PCE of 16.26% (with VOC of 1.06 V, JSC of 20.17 mA/cm2 and FF of


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76%) is accomplished with negligible hysteresis. Which is enhanced by ~35% compared to the PCE of reference device. More importantly, great improvement of device stability is also demonstrated by degradation test both in ambient atmosphere and operational situation. Over 95% of its initial PCE was retained for the optimized device after 150 h in ambient condition (RH: ~45%) while the reference device lost over 50% of its initial efficiency. Improved long-term stability was verified as well for the optimized device by retaining 95% of its initial PCE after 40 h, under continuous AM 1.5G spectrum illumination without any UV-filter in glovebox. But the PCE of reference device degraded to 73% of its initial value after only one hour. It is considered that the improvements of the device after F4-TCNQ modification could be mainly attributed to the upgraded conductivity, film morphology and energy level alignment, passivated defects and prevented chemical reactions between MoOx and perovskite. This work provides strategy for optimizing PSCs based on this class of easy-process and inexpensive inorganic materials, which could benefit the commercialization of PSC technology.

ASSOCIATED CONTENT Supporting Information Surface potential characteristics of the MoOx and MoOx/F4-TCNQ films on ITO substrate; X-ray diffraction spectra of perovskite films on MoOx and MoOx/F4-TCNQ HTLs; UV-vis absorption spectra of MoOx/perovskite/PCBM and MoOx/F4-TCNQ/perovskite/PCBM film; I–V curves of devices in structure of ITO/MoOx/F4-TCNQ/Al, with different F4-TCNQ films; Contact angle of water droplets on MoOx and MoOx/F4-TCNQ surfaces; the best performance parameters of PSCs with MoOx and MoOx/F4-TCNQ HTL.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.-Q. Wang). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the Major Project of "Science and Technology Innovation 2025" of Ningbo China (2018B10055), National Natural Science Foundation of China (21875272), Natural Science Foundation of Zhejiang Province of China (LQ19E030008).

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unbalanced charge distribution inside a perovskite-sensitized solar cell. Nat Commun 2014, 5, 5001.

Figure 1. (a) Schematic illustration of the inverted architecture PSCs employing MoOx/F4-TCNQ as the HTL, and the chemical structure of F4-TCNQ molecule. (b) Cross-sectional SEM image of the whole device. (c) Schematic illustration of energy level alignment of materials in device.


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Figure 2. (a) and (d) J-V curves of the optimal devices based on MoOx and MoOx/F4-TCNQ HTL, under AM 1.5G illumination and dark condition respectively; (b) Influence of F4-TCNQ concentration on the device performance; (c) EQE curves of the optimal device based on MoOx and MoOx/F4-TCNQ HTL layers.


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Figure 3. The J-V metrics of the MAPbI3 PSCs with MoOx and MoOx/F4-TCNQ HTL, based on 20 separated cells. (a) VOC, (b) JSC, (c) FF and (d) PCE.

Figure 4. (a) Air stability of PSCs based on MoOx and MoOx/F4-TCNQ HTLs, tested in ambient atmosphere (relative humidity ~45%); (b) Operational stability of PSCs based on MoOx and MoOx/F4-TCNQ HTLs, examined under continuous AM 1.5G illumination in glovebox. (c) Steady-state output at the maximum power point of the PSC devices with the MoOx (0.72 V) and MoOx/F4-TCNQ (0.76 V) HTLs, under continuous simulated AM 1.5G illumination.


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Figure 5. (a) and (b) AFM topography images of the MoOx and MoOx/F4-TCNQ HTL on ITO substrate; (c) and (d) Top-view SEM images of perovskite films on the MoOx and MoOx/F4-TCNQ HTLs; (e) and (f) AFM topography images of the perovskite films on MoOx and MoOx/F4-TCNQ HTLs.

Figure 6. (a) Current-voltage traces and trap density of perovskite films on MoOx. (b) Currentvoltage traces and trap density of perovskite films on MoOx/F4-TCNQ.


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Figure 7. (a) Steady state PL spectra, and (b) time-resolved PL decay curves of perovskite films deposited onto the MoOx and MoOx/F4-TCNQ HTLs.

Figure 8. (a) Full XPS spectra of MoOx and MoOx/F4-TCNQ films. High resolution XPS spectra of the MoOx and MoOx/F4-TCNQ films, b) Mo 3d core level; (c) F 1s; (d) N 1s.


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Abstract Graphic


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Solution-Processed MoOx Hole-Transport Layer with F4-TCNQ

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