Room-Temperature Processed Nb2O5 as the Electron-Transporting

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Room-Temperature Processed Nb2O5 as the Electron Transporting Layer for Efficient Planar Perovskite Solar Cells Xufeng Ling, Jianyu Yuan, Dongyang Liu, Yongjie Wang, Yannan Zhang, Si Chen, Haihua Wu, Feng Jin, Fu-peng Wu, Guozheng Shi, Xun Tang, Jiawei Zheng, Shengzhong (Frank) Liu, Zhike Liu, and Wanli Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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ACS Applied Materials & Interfaces

Room-Temperature Processed Nb2O5 as the Electron Transporting Layer for Efficient Planar Perovskite Solar Cells Xufeng Ling,1 Jianyu Yuan,1 Dongyang Liu,1 Yongjie Wang,1 Yannan Zhang,1 Si Chen,1 Haihua Wu,1 Feng Jin,3 Fupeng Wu,1 Guozheng Shi,1 Xun Tang,1 Jiawei Zheng,1 Shengzhong (Frank) Liu,2 Zhike Liu*,2 and Wanli Ma*,1 1

Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China 2 Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China 3 Shanghai Ultra-precision Optical Manufacturing Engineering Research Center, and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China ABSTRACT: In this work , we demonstrate high-efficiency planar perovskite solar cells (PSCs) using room-temperature sputtered niobium oxide (Nb2O5) as the electron transporting layer (ETL). Widely spread ETL like TiO2 often requires high-temperature (> 450 ºC) sintering, which is not desired for the fabrication of flexible devices. The as sputtered amorphous Nb2O5 ETL (labeled as a-Nb2O5) without any heat treatment can lead to a best power conversion efficiency (PCE) of 17.1% for planar PSCs. Interestingly, the crystalline Nb2O5 (labeled as c-Nb2O5) with high-temperature (500 ºC) annealing results in very similar PCE of 17.2%, indicating the great advantage of aNb2O5 in energy saving. We thus carried out systematical investigation on the properties of the amorphous a-Nb2O5 film. The Hall Effect measurements indicate both high mobility and conductivity of the a-Nb2O5 film. Kelvin probe force microscopy (KPFM) measurements define the Fermi levels of a-Nb2O5 and c-Nb2O5 as -4.31 eV and -4.02 eV, respectively, which allow efficient electron extraction at the Nb2O5/perovskite interface regardless of the additional heating treatment on Nb2O5 film. Benefitting from the low-temperature process, we further demonstrated flexible PSCs based on a-Nb2O5 with a considerable PCE of 12.1%. The room temperature processing and relatively high device performance of a-Nb2O5 suggest great potential for its application in optoelectrical devices.

KEYWORDS: Room-temperature, Nb2O5, Electron transporting layer, Perovskite solar cells, Flexible. 1. Introduction Organic-inorganic hybrid perovskites (typically CH3NH3PbX3 with X = I, Br, or Cl) are extremely promising materials for next generation photovoltaic application owing to their high light absorption coefficient, low exciton binding energy and long charge carrier diffusion length. 1-5 During the past 8 years, the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have rapidly increased from 3.6% to 22.1%situating at the forefront of solution processing photovoltaic devices. 6-16 With respect to the “inverted” planar heterojunction PSCs, 17-20 so far the record efficiency PSCs commonly consist of a mesoporous TiO2 scaffold, a perovskite active layer and an organic hole transporting layer. However, the TiO2 mesoporous scaffold normally requires high-temperature (> 450 ºC) sintering, 2, 12-15 which is not desired for large-scale manufacturing and incompatible with flexible substrates. Therefore, it is critical to exploit novel electron transporting layer (ETL) materials with desirable properties and relatively low processing temperature, which in consequence, will simplify the devices fabrication process and be compatible with common flexible polymeric substrates. Fortunately, few alternative transparent metal oxides, such as ZnO 21-24 and SnO2, 25-28 have been explored and applied as the ETLs in conventional planar PSCs since they exhibit similar or even better electrical properties and can be processed at low temperature compared to TiO2. In 2013, Kelly et al. first successfully demonstrated pla-

nar PSCs based on ZnO nanoparticles through roomtemperature solution processing techniques, with the highest PCE reaching 15.76%. [21] Quite recently, You et al. reported a PCE of ~20% planar PSCs by adopting SnO2 as the ETL at low temperature (~150 ºC). 28 The high transparency and electron mobility (up to 240 cm2 V−1 s−1), together with favorable Fermi level, can facilitate electron extraction at ETL/perovskite interface and therefore enhance the solar cells performance. 28 Besides, a few other high-mobility metal oxides have been developed, such as In2O3 (∼20 cm2 V−1 s−1) 29 and WOx (10-20 cm2 V−1 s−1), 30, 31 whereas the PCEs of the PSCs based on these novel ETLs are below 15%. Thus, it is crucial to exploit other potential metal oxides as the ETLs in planar PSCs and further improve the devices performance. Interestingly, niobium oxide (Nb2O5) is a promising ntype semiconductor exhibiting similar optical band gap and electronic properties, more significantly, improved chemical stability relative to that of conventional TiO2. 32 The conduction band (CB) edge of Nb2O5 is generally considered to be close to that of TiO2. 32-34 Previous reports of dye-sensitized solar cells (DSSCs) have demonstrated Nb2O5 as an effective blocking layer of surface recombination. 34-38 Recently, Miyasaka et al. 39 and Graeff et al. 40 have reported the application of Nb2O5 thin film as hole blocking layer in PSCs independently. The Nb2O5 films were prepared through a sol-gel solution process and reactive magnetron sputtering, respectively. However, the devices in both cases consisted of a thick

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mesoporous layer and required high-temperature (∼500 ºC) heat treatment. Moreover, the PCEs of these PSCs were far from satisfaction (< 13%). 39, 40 Herein, we demonstrate a dense amorphous Nb2O5 (aNb2O5) film deposited at room temperature using a facile radio frequency (RF) magnetron sputtering technique. 41 The amorphous a-Nb2O5 film is highly transparent across the visible spectrum. The crystalline Nb2O5 (c-Nb2O5) film can be achieved through high-temperature (500 ºC) annealing. Interestingly, we found that the low-temperature processed aNb2O5 exhibits similar excellent optical and electrical properties compared to that of crystalline c-Nb2O5. In addition, the Fermi level of the a-Nb2O5 film (-4.31 eV) matches better with the CB of the CH3NH3PbI3 perovskite (-3.9 eV). By incorporating the a-Nb2O5 film as the ETL, we have successfully fabricated the conventional planar PSCs with a PCE as high as 17.1%, which is comparable to the PCE of devices based on high-temperature heated c-Nb2O5. Moreover, flexible PSCs were fabricated using a-Nb2O5 as the ETL on poly(ethylene terephthalate) (PET)/ indium tin oxide (ITO) substrates, exhibiting a maximum efficiency of 12.1%, indicating great potential to flexible devices application. 42-45 Hence, our findings in this work not only demonstrate the efficacy of Nb2O5 ETL for high-performance planar PSCs but also provide a facile and low-temperature technique for metal oxides ETL preparation. 2. Experimental Section 2.1 Materials and Synthesis CH3NH3I (MAI) was synthesized according to the reported procedures.2 24 mL methylamine (33 wt.% in absolute ethanol, Sigma-Aldrich) and 10 mL hydroiodic acid (57 wt.% in water, Alfa Aesar) were mixed in a 250 mL round-bottom flask at 0 ºC for 2 h under continuous stirring. The precipitate was obtained through removing the solvents at 50 ºC using a rotary evaporator. The product was recovered after redissolved in ethanol and recrystallized with diethyl ether. Finally, the powder was dried at 60 ºC under vacuum for 48 h. 2.2 Nb2O5 Films Deposition The Nb2O5 films were at room temperature by RF magnetron sputtering (PVD-75, Kurt J. Lesker, U.S.A) in an argon (99.999%) atmosphere using a pure Nb2O5 target (99.99%, ZhongNuo Material). The distance between the target and substrate was 15 cm and the sputtering power was set as 100 W. Initially, at a base pressure of 2 × 10−6 Torr, the target was exposed to the pure Ar gas with a flow rate of 20 sccm. leading to a chamber pressure of 2.2 × 10-3 Torr. Later on, the target was pre-sputtered for 10 min to remove the contaminants on the target surface. The thicknesses of the Nb2O5 films were varied through different deposition time and determined by a surface profiler (AMBiOS XP-200). For comparison, some Nb2O5 films were heated at 500 ºC for 45 min in air before the deposition of perovskite films. 2.3 Devices Fabrication The fluorine-doped tin oxide (FTO)/glass or PET/ITO substrates were ultrasonic cleaned and treated with UV-Ozone for 20 min. The Nb2O5 films were deposited on the substrates through the methods above and treated with UV-Ozone for 10 min before the deposition of perovskite films. The MAPbI3 perovskite films were deposited through a one-step spincoating process with an anti-solvent dripping. 10, 14 The MAP-

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bI3 precursor solution was prepared by an equimolar mixture of PbI2 (99.999%, beads, Alfa Aesar) with MAI in anhydrous dimethyl sulphoxide (DMSO): γ-butyrolactone (GBL) (3:7, v/v) at room temperature and the total concentration is 1.2 M. The solution was coated onto the Nb2O5 substrate and spun at 1000 and 4000 rpm for 10 and 40 s, respectively. During the high-speed spin-coating step, 120 µL chlorobenzene was swiftly dripped onto the substrate 20 s preliminary to the end of the program. The substrate was then dried at 100 ºC for 10 min. A solution of 2,2’,7,7’-tetrakis(N,N-dipmethoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD, 90 mg, 99.5%, Xi’an Polymer Light Technology Corp.) in anhydrous chlorobenzene (1 mL) was mixed with 22 µL of lithium bis- (trifluoromethanesulfonyl) imide solution (520 mg mL-1 in anhydrous acetonitrile) and 36 µL 4-tert-butylpyridine and spin-coated on MAPbI3 film at 5000 rpm for 30 s. Finally, 100 nm Au was deposited by thermal evaporation under a vacuum of 2 × 10-6 mbar. The active area of the cell was defined as 7.25 mm-2 through a shadow mask. 2.4 Measurement and Characterization The current density-voltage (J-V) curves of the devices were obtained through a Keithley 2400 digital source meter under simulated AM 1.5G spectrum at 100 mW cm-2 with a solar simulator (Newport, Class AAA, 94023A-U). The light intensity was calibrated by an NREL-certified Oriel Reference Cell (91150V). The incident photon-to-electron conversion efficiency (IPCE) spectra were recorded through a certified IPCE equipment (Zolix Instruments, Inc. Solar Cell Scan 100). The steady-state and time-resolved photoluminescence (PL) spectra were obtained from a FluoroMax-4 Spectrofluorometer (HORIBA Scientific). The field emission scanning electron microscope (Carl Zeiss Supra 55) was used to capture the scanning electron microscopy (SEM). The ultraviolet-visible (UV-vis) absorption and transmittance spectra were recorded on a Perkin Elmer model Lambda 750 instrument. The X-ray diffraction (XRD) measurements were carried out on a PAN alytical 80 equipment (Empyrean, Cu Kα radiation). The Xray photoelectron spectra (XPS) were obtained though a Kratos AXIS UltraDLD ultrahigh vacuum photoemission spectroscopy system with an Al Kα radiation source. The atomic force microscopy (AFM) images (tapping mode) were captured through a Veeco Multimode V instrument. Surface potentials of Kelvin probe force microscopy (KPFM) were measured through an Asylum Research Cypher S AFM microscope. The electrochemical impedance spectroscopy (EIS) measurements were carried out on an impedance analyzer (Metrohm AUTOLAB PGSTAT302N) under open-circuit condition with frequency from 100 Hz to 1 M Hz. Hall Effect measurements were performed through an Ecopia HMS-3000 with the sample structure of glass/a-Nb2O5 (or c-Nb2O5)/Ag, where the thickness of a-Nb2O5 (or c-Nb2O5) and silver contact is 115 and 100 nm, respectively. 3. Results and Discussion The conventional n-i-p planar PSCs were fabricated with the device structure as shown in Figure 1a. We introduced transparent electrode (FTO or ITO) as the bottom cathode, and spiro-OMeTAD and gold as the hole transporting layer (HTL) and top anode, respectively. The MAPbI3 perovskite absorber was prepared through an anti-solvent dripping, 10, 14 and we incorporated Nb2O5 as the ETL in this study. Figure 1b depicts the cross-sectional SEM image of the optimized


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planar PSC, which clearly shows each of the corresponding layers. The thickness of each constituent layer of the PSC is measured to be 85, 300 and 180 nm for Nb2O5, MAPbI3, and spiro-OMeTAD, respectively. Figure 1c depicts the energy level diagram of the device with different Nb2O5 ETLs, 17, 46, 47 which indicates good electron transfer between Nb2O5 and MAPbI3. In this work, we deposited the Nb2O5 films using RF magnetron sputtering at room temperature through a pure Nb2O5 target. The top-view SEM image illustrated in Figure 1d (inserted) exhibits the surface morphology of the Nb2O5 film deposited on FTO/glass substrate. This uniform and dense film shows no apparent pinholes, indicating the high film quality by sputtering deposition. The surface root-mean-square (RMS) roughness of the Nb2O5 film on FTO/glass is 27.2 nm compared to 27.9 nm for the bare FTO substrate (Figure S1, Supporting Information (SI)). Upon thermal annealing at 500 ºC for 45 min, the Nb2O5 film (c-Nb2O5) surface became slightly rougher with a RMS roughness of 29.0 nm, likely due to the film crystallization. A homogeneous MAPbI3 perovskite film with excellent surface coverage on both a-Nb2O5 and cNb2O5 can be subsequently formed (Figure S2, SI) with an average grain size of ca. 220 nm, which is beneficial to light absorption and charge carriers generation. 14 The electronic states of the as-deposited Nb2O5 film were characterized by Xray photoelectron spectroscopy (XPS). As shown in Figure 1d, only the characteristic peaks of Nb and O can be observed in the XPS spectra, indicating the compositional purity of the deposited films. 41 As shown in the Nb 3d core level spectra (Figure S3, SI), the doublet peak at binding energies of 210.3 eV and 207.5 eV is observed, which is corresponding to Nb 3d3/2 and Nb 3d5/2, respectively, indicating the presence of five-valent niobium (Nb5+) in the deposited films. 41 The core level spectra of O 1s exhibit a certain degree of asymmetry in its shape (Figure S3, SI). The peak at lower binding energy of 530.6 eV corresponds to O2- in the Nb2O5, and peak at higher binding energy of 532.2 eV can be attributed to the surface oxygen-group absorbance. 28, 46, 48 The crystal structure and Fermi levels of the Nb2O5 films with or without thermal treatment were also characterized by X-ray diffraction (XRD) and Kelvin probe force microscopy (KPFM), respectively. Figure 1e displays the XRD patterns of the as-sputtered Nb2O5 film and the film after heated at 500 ºC for 45 min on glass. It is distinct that no peaks exist in the asdeposited Nb2O5 film, indicating the amorphous nature of the as sputtered a-Nb2O5 film. However, after heating at 500 ºC for 45 min, the peaks emerge at 22.61º, 28.58º, 36.7º, 46.23º, 50.67º, 55.18º, 56.14º and 58.97º, which are corresponding to the (001), (100), (101), (002), (110), (102), (111) and (200) diffraction peaks in hexagonal phase Nb2O5 (c-Nb2O5), respectively (Reference code：JCPDS 28-0317). 49 The Fermi levels of Nb2O5 films were measured by KPFM with the surface potential images shown in Figure S4 (SI). It is found that the Fermi level of a-Nb2O5 is -4.31 eV, deeper than that of -4.02 eV for c-Nb2O5. The difference in Fermi level could be ascribed to the oxygen vacancies filling induced by hightemperature heating in air. 42, 50 It was reported that oxygen vacancies formed during the deposition of a-Nb2O5 films by RF magnetron sputtering, 41 generating impurity density of states above the valence band. 50 It leads to the downshift of the Fermi level of the amorphous film, and after thermal annealing in the ambient; the vacancies are filled by oxygen, 42, 51 yielding an upshifted Fermi level of -4.02 eV for c-Nb2O5 film. Similar trend can be found in the XPS results (Figure S3, SI),

the oxygen content in c-Nb2O5 film increases compared to that in a-Nb2O5, implying the oxygen vacancies filling may occur during the thermal annealing process. Hence, benefitting from the relative larger offset in energy levels between the CB of perovskite and the Fermi level of a-Nb2O5 ETL, the electron transfer from the absorber layer to the ETL may be more efficient compared to that in c-Nb2O5 system. 42 Finally, the optical transmittance of the a-Nb2O5 and c-Nb2O5 film was investigated. As shown in Figure 1f, both Nb2O5 films show high transmittance of 90% on average in the wavelength range from 400 to 800 nm, while a-Nb2O5 shows slightly higher transmittance in the visible region than c-Nb2O5. Consequently, there is no apparent difference between the absorbance of perovskite films deposited on the two Nb2O5 substrates (Figure S5, SI). The current density-voltage (J-V) curves of the champion planar PSCs under AM 1.5G, 100 mW cm-2 illumination with optimized Nb2O5 ETLs are shown in Figure 2a, measured under different scan directions. The relevant photovoltaic parameters are listed in Table 1. The thickness of Nb2O5 ETL was carefully optimized (Table S1 and Figure S6, SI). The optimal Nb2O5 film thickness was determined to be around 85 nm. Quite unexpectedly, the device integrating the assputtered a-Nb2O5 as the ETL exhibits an impressive PCE of 17.1%, with a relative high short-circuit current density (Jsc) of 22.9 mA cm-2, an open-circuit voltage (Voc) of 1.04 V and a high fill factor (FF) of 0.72 under reverse scan. In contrast, the optimal device based on high-temperature annealed c-Nb2O5 achieves a very close PCE of 17.2%, with a Jsc of 22.4 mA cm2 , a Voc of 1.05 V and a FF of 0.73. When tested under the forward scan direction, the FF and PCE of the devices are 0.66 and 15.1% for a-Nb2O5-based device and 0.67 and 15.1% for c-Nb2O5-based device, respectively. According to previous reports on the hysteresis in TiO2-based PSCs, the J-V hysteresis in Nb2O5-based devices is possibly due to the unbalanced electrons and holes flux from perovskite to ETLs and HTLs, respectively, 52-54 which could be alleviated or even eliminated through interfacial modifiers. 55-57 The J-V results indicate that the PSCs show quite similar device performance using both Nb2O5 film as the ETL, even though c-Nb2O5 is hightemperature processed and has much higher crystallinity. The incident photon-to-electron conversion efficiency (IPCE) spectra ware also measured, as shown in Figure 2b. The Jsc calculated by integrating the IPCE curve with an AM 1.5G reference spectrum is within 7 % error compared to the corresponding Jsc obtained from the J-V curves. To further confirm the reproducibility of the planar PSCs based on a-Nb2O5 and c-Nb2O5 ETLs, we fabricated and tested 26 individual devices. The PCE distribution histograms of the cells are shown in Figure 2c with the detailed photovoltaic parameters listed in Table 1, validating that the PCEs are highly reproducible with little variation. It is found that the average Jsc increases from 21.8 mA cm-2 for c-Nb2O5 to 22.0 mA cm-2 for a-Nb2O5, which may be due to its higher transmittance, suitable energy level and smoother surface. In comparison with the c-Nb2O5 based device, the average Voc decreases from 1.05 V to 1.03 V for a-Nb2O5 ones, which may be attributed to its lower Fermi level than that of c-Nb2O5. 58 Interestingly, the average FF of c-Nb2O5 based devices is 0.71, only slightly larger than the 0.70 for a-Nb2O5 based ones despite of their significant difference in crystallinity. Hence, as a result of comparable Jsc, Voc and FF, the planar PSCs using quite simply processed a-Nb2O5-based can exhibit an average PCE of 15.9%, which is very similar with the 16.3% PCE of c-



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Nb2O5 based ones. Furthermore, the PSCs with both Nb2O5 ETLs can sustain over 70% of the initial PCE after 135 h of storage in dark ambient with controlled relative humidity and temperature of 40% and 25 ºC, respectively, which show better air stability than conventional TiO2 based devices (Figure S7, SI). The low-temperature process of a-Nb2O5 inspired us to fabricate flexible PSCs using it as the ETLs on PET/ITO substrates. The image and J-V curves of the best flexible device based on a-Nb2O5 are shown in Figure 2d, yielding an inspiring PCE of 12.1% with a Jsc of 21.2 mA cm-2, a Voc of 0.99 V and a FF of 0.59 under reverse scan. The PCE distribution histogram of the a-Nb2O5-based flexible PSCs is shown in Figure S8 (SI). The declined device performance of flexible PSCs in comparison with the device based on rigid substrates may be due to the fragile ITO on flexible PET substrate, further resulting in increased contact resistance and inferior film morphology of a-Nb2O5 or/and perovskite atop. 45, 59 These results indicate the promising future for the application of aNb2O5 as ETLs in high-efficiency flexible PSCs. 42-45 In order to acquire a better comprehending of interior reasons why the a-Nb2O5 ETL performs as good as the cNb2O5 for the PSCs, steady state photoluminescence (PL) spectroscopy was employed to reveal the charge transport occurring at perovskite/ETL interface. Figure 3a presents the steady state PL spectra of MAPbI3 perovskite films deposited on the bare glass, a-Nb2O5 and c-Nb2O5 substrate. The photoluminescence peak at 762 nm is attributed to the emission from MAPbI3. 45 Significant emission quenching is clearly observed for both samples with Nb2O5 ETL, indicating the efficient charge transfer at the interface for the two ETLs, which is consistent with their lower Femi levels compared to the CB of MAPbI3 films. To further tell their subtle difference in charge transfer, the time-resolved PL spectroscopy was also measured. The average PL decay lifetime of aNb2O5/perovskite and c-Nb2O5/perovskite is 4.73 and 5.54 ns, respectively, in comparison with that of 42.22 ns for perovskite film on bare glass. The slightly shorter PL lifetime for the a-Nb2O5/perovskite sample suggests more efficient electrons transport from MAPbI3 into a-Nb2O5, which may be ascribed to the superior interfacial contact (smoother surface) and larger energy level offset at the a-Nb2O5/perovskite interface. Furthermore, electrochemical impedance spectroscopy (EIS) was employed to investigate the interfacial charge transport and recombination dynamics in PSCs. 55, 60 Figure 3c shows the Nyquist plots of PSCs with a-Nb2O5 and c-Nb2O5 measured at open-circuit voltage under illumination. The equivalent circuit model for the PSCs is presented in Figure S9 (SI) and composed of the series resistance (Rs), two components for transfer resistance (Rtr) at the ETL/perovskite and perovskite/HTL interfaces, and the recombination resistance (Rrec). 42, 55 The Rtr and Rrec values correspond to the highfrequency element (left region) and the low-frequency one in EIS, respectively. Since the perovskite/HTL interface is same in both cases, the Rtr is associated with the Nb2O5/perovskite interfaces. 42 The values of the equivalent circuit are shown in Table S2 (SI). Apparently, the Rtr of the device with a-Nb2O5 is 186.1 Ω, significantly lower than that of 854.2 Ω obtained from the device with c-Nb2O5 ETL. These results evidently imply more efficient electrons extraction at the aNb2O5/perovskite interface and are consistent with the slightly greater PL quenching observed for the a-Nb2O5/perovskite

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sample, as revealed by the time-resolved PL. We further confirmed this result by the Hall Effect measurements (Table S3, SI). The Hall mobility of c-Nb2O5 is 26.77 cm2 V-1 s-1 and higher than that of 20.73 cm2 V-1 s-1 for a-Nb2O5, arising from the enhanced crystallinity. However, the latter ETL has a much higher carrier densities, likely due to the oxygen vacancies formed during the RF magnetron sputtering. Consequently, the a-Nb2O5 film exhibits higher conductivity of 2.053 × 10-5 S cm-1 than that of 1.748 × 10-5 S cm-1 for c-Nb2O5, leading to less resistance. Nonetheless, this reduction in Rtr is offset by a concomitant decrease in the Rrec, with the resistance of 3493 Ω for the a-Nb2O5-based device compared to that of 7692 Ω for the c-Nb2O5-based device. It is probably due to more serious carrier recombination caused by oxygen-induced defects or trap states existed in amorphous metal oxide films, 41, 42, 51 further leading to observed losses in Voc and FF of the devices with a-Nb2O5 ETLs. We compared our results to other room-temperature processed ETLs with high-efficiency (>15%) in conventional PSCs devices. Figure 4 provides a plot of PCE vs processing temperatures for a series of metal oxide ETLs used in reported high-efficiency PSCs. We note that only a very few metal oxides ETLs can achieve PCE higher than 17%. And the high device performance is usually achieved with the ETL processed at temperatures higher than 150 ºC, which is not ideal for the fabrication of flexible devices. In comparison, our aNb2O5 ETL can be processed under room temperature and achieve one of the best PCEs compared to other metal oxide ETLs with similar processing temperature. The fine balance between device performance and processing temperature for our ETL can simplify the solar cell fabrication procedures, reduce the manufacturing cost, and meanwhile maintain high efficiency. 4. Conclusions In conclusion, we have demonstrated a facile roomtemperature processed Nb2O5 by RF magnetron sputtering for PSCs application. The planar PSCs based on the a-Nb2O5 ETL achieve a PCE as high as 17.1%, comparable to devices with high-temperature processed Nb2O5 (17.2%). The high device performance is attributed to the excellent film quality, electron transport ability and favorable Fermi level of Nb2O5. The lowtemperature process enabled the application of a-Nb2O5 as the ETL on flexible PSCs based on ITO/PET, and consequently an optimal PCE of 12.1% was achieved. The room temperature processing and relatively high device performance suggest great potential for a-Nb2O5 in the application in flexible largearea solar cells and other optoelectrical devices. ASSOCIATED CONTENT Supporting Information. The material is available free of charge on the Internet at http://pubs.acs.org AFM images of FTO/Nb2O5 substrates; SEM images and UV-vis absorption spectra of perovskite films, XPS, Hall effects and KPFM measurements of Nb2O5 films; devices data including: Nb2O5 thickness optimization, PCE distribution histogram of flexible PSCs, stability and EIS parameters of devices. AUTHOR INFORMATION Corresponding Author


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[email protected] (Z. Liu)

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[email protected] (W. Ma) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The author thanks the Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University. This work was supported by the “111” projects, National Key Research Projects (Grant No. 2016YFA0202402), the National Natural Science Foundation of China (Grant No. 61222401, No. 61674111). And we also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES [1] Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C., Long-Range Balanced Electron-and Hole-Transport Lengths in OrganicInorganic CH3NH3PbI3. Science 2013, 342, 344-347. [2] Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid Solar Cells Based on MesoSuperstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. [3] Green, M. A.; Ho-Baillie, A.; Snaith, H. J., The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506-514. [4] Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. [5] Lin, Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P., Electro-Optics of Perovskite Solar Cells. Nat. Photonics 2015, 9, 106-112. [6] Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 60506051. [7] Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.H.; Moser, J. E.; Gratzel, M.; Park, N.-G., Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. [8] Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M., Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. [9] Liu, M.; Johnston, M. B.; Snaith, H. J., Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. [10] Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I., Solvent Engineering for HighPerformance Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897-903.

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Figure 1. (a) Device structure of the planar PSCs, (b) the cross-sectional SEM image of the optimized device and the scale bar is 200 nm, (c) energy level diagram of the PSCs, (d) top-view SEM image of the Nb2O5 film deposited on glass/FTO (inserted, the scale bar is 1 µm) and XPS survey scan of as-deposited Nb2O5 film, (e) XRD patterns of the Nb2O5 films and reference hexagonal crystal, and (f) transmittance spectra of the glass/FTO/Nb2O5 films.

Figure 2. (a) J-V curves of the champion devices based on a-Nb2O5 and c-Nb2O5 under different scanning directions, (b) the IPCE spectra of the champion devices, (c) the PCE distribution histogram of the PSCs using a-Nb2O5 and c-Nb2O5 as the ETLs, and (d) photograph and J-V curve of the champion flexible PSC on PET/ITO substrate under reverse scan.


8

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(a) perovskite a-Nb2O5/perovskite

Intensity (a.u.)

c-Nb2O5/perovskite

630

720

810

900

Wavelength (nm)

(b) 10

0

perovskite

Intensity (a.u.)

a-Nb2O5/perovskite c-Nb2O5/perovskite 10

-1

10

-2

0

(c)

100

200

300

400

Time (ns) 5k 4k

Z'' (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a-Nb2O5 c-Nb2O5

3k 2k 1k 0 0.0

2.0k

4.0k

6.0k

8.0k

10.0k

Z' (Ohm) Figure 3. (a) Steady state PL spectra of the perovskite, a-Nb2O5/perovskite and c-Nb2O5/perovskite films on FTO/glass, (b) time-resolved PL decay of the perovskite films on different substrates, and (c) Nyquist plots for the PSCs with aNb2O5 and c-Nb2O5.


9


22 TiO2

20

SnO2 ZnO2

18

PCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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AZO1

16

Zn2SnO4 WOx

14

In2O3

12

Nb2O5 Our work

10 8

Low

High

Medium

6 0

50

100

150

200

250

300

500

600

700

800

Processing Temperature (ºC) Figure 4. PCE distributions of conventional n-i-p PSCs based on different metal oxides ETLs processed at different temperatures. (The references are listed in Supporting Information, for TiO2: ref. 2, 3 (25 ºC), 4, 5 (70 ºC), 6-8 (150 ºC), and 9-19 (500 ºC). SnO2: ref. 20 (95 ºC), 21 (100 ºC), 22 (118 ºC), 23 (120 ºC), 24 (140 ºC), 25 (150 ºC), 26 (180 ºC), 27 (180 ºC) and 28 (185 ºC). ZnO: ref. 29, 30 (25 ºC), 31 (120 ºC), 32 (150 ºC), 33 (160 ºC), 34 (200 ºC), 35 (250 ºC) and 36 (290 ºC). AZO (Al-doped ZnO): ref. 37 (25 ºC). Zn2SnO4: ref. 38 (25 ºC), 39, 40 (100 ºC) and 41 (500 ºC). WOx: ref. 42, 43 (25 ºC), 42 (70 ºC), 43 (120 ºC), 44 (140 ºC), 42 (150 ºC) and 45 (500 ºC). In2O3: ref. 46 (200 ºC). Nb2O5: ref. 47 (500 ºC) and 48 (550 ºC)).

Table 1. Photovoltaic parameters of devices measured under one sun condition (100 mW cm-2, AM 1.5G). Scanning

Voc

ETL a-Nb2O5

c-Nb2O5

Jsc

PCE FF

direction

(V)

(mA cm-2)

Reverse

1.04

22.9

0.72

17.1

Forward

1.02

22.4

0.66

15.1

Average[a]

Reverse

1.03 ± 0.02

22.0 ± 0.5

0.70 ± 0.02

15.9 ± 0.8

Champion

Reverse

1.05

22.4

0.73

17.2

Forward

1.02

22.1

0.67

15.1

Reverse

1.05 ± 0.02

21.8 ± 0.5

0.71 ± 0.02

16.3 ± 0.7

Champion

Average[a]

(%)

[a]

Results are shown for 52 cells collected from 8 different batches. All devices were measured at an 80 mV s–1 scan rate.


10

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Toc:


11

Room-Temperature Processed Nb2O5 as the Electron-Transporting

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