High-efficiency, hysteresis-less, UV-stable perovskite solar cells with

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High-Efficiency, Hysteresis-Less, UV-Stable Perovskite Solar Cells with Cascade ZnO−ZnS Electron Transport Layer Ruihao Chen, Jing Cao, Yuan Duan, Yong Hui, Tracy T Chuong, Daohui Ou, Faming Han, Fangwen Cheng, Xiaofeng Huang, Binghui Wu,* and Nanfeng Zheng*

J. Am. Chem. Soc. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/22/18. For personal use only.

State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National & Local Joint Engineering Research Center of Preparation Technology of Nanomaterials, College of Chemistry and Chemical Engineering, Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: Perovskite solar cells (PSCs) have reached certified efficiencies of up to 23.7% but suffered from frailness and instability when exposed to ambient atmosphere. Zinc oxide (ZnO), when used as electron transport layer (ETL) on PSCs, gives rise to excellent electronic, optic, and photonic properties, yet the Lewis basic nature of ZnO surface leads to deprotonation of the perovskite layer, resulting in serious degradation of PSCs using ZnO as ETL. Here, we report a simple but effective strategy to convert ZnO surface into ZnS at the ZnO/perovskite interface by sulfidation. The sulfide on ZnO−ZnS surface binds strongly with Pb2+ and creates a novel pathway of electron transport to accelerate electron transfer and reduce interfacial charge recombination, yielding a champion efficiency of 20.7% with improved stability and no appreciable hysteresis. The model devices modified with sulfide maintained 88% of their initial performance for 1000 h under storage condition and 87% for 500 h under UV radiation. ZnS is demonstrated to act as both a cascade ETL and a passivating layer for enhancing the performance of PSCs.

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fer.18,19 However, the photodegradation effect of TiO2 has caused the decrease of PSC performance and thus limited the operational stability and commercialization of PSCs.20 Choosing substitute materials to replace TiO2 is a vital path toward creating high-performance PSCs. Noticeably, the electron mobility (0.1−10 cm2 V−1 s−1) of TiO2 is several orders of magnitude lower than that of Zn2SnO4 (100−200 cm2 V−1 s−1), SnO2 (100−200 cm2 V−1 s−1), and ZnO (200− 300 cm2 V−1 s−1),21−23 thereby limiting electron transfer and increasing the probability of charge recombination. ZnO is of interest for its direct wide bandgap, high electron mobility, and transparent conductivity.24 However, in previous studies, the defects on ZnO surface such as hydroxyl groups and oxygen vacancies inevitably brought about serious degradation of PSCs when using ZnO nanostructures as ETLs.23,25−27 Recently, strategies to address the above-mentioned issues include modification of ZnO surface by introducing organic and inorganic materials to obstruct the direct contact between perovskite and ZnO,28,29 or doping Li+, Cs+, or Al3+ ions to improve the device performance.30,31 Among the mentioned research, the efficiencies of all ZnO-based PSCs barely exceed 18%, and long-time stability has been seldom achieved.32 Therefore, interfacial engineering on the ZnO/perovskite interface has remained an appealing challenge.

INTRODUCTION Perovskite solar cells (PSCs) have achieved power conversion efficiencies (PCE) of up to certified 23.3% in the past few years.1−3 Stability and hysteresis are still challenging issues that limit the commercialization of PSCs.4,5 A typical device structure for PSCs contains an n-type electron transport layer (ETL), light absorbing layer, and p-type hole transport layer (HTL). For light absorbing layers, MA-, FA-, or Cs-based perovskites (methylammonium (MA), CH3NH3+; formamidinium (FA), CH(NH2)2+; cesium (Cs)) have possessed high extinction coefficients and high charge mobilities (∼20 cm2 V−1 s−1),6−10 while Cs-spiked triple-cation PSCs display better reproducibility, higher long-term stability, and efficiency.11,12 For HTLs, Li- or Co-doped 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9 spirobifluorene (Spiro-OMeTAD) and Li-spiked poly triarylamine (PTAA) are the most effective and common candidates due to their easy solution-processing and moderate hole mobilities (1.6 × 10−3 and 2.4 × 10−3 cm2 V−1 s−1, respectively).13,14 In terms of ETLs, titania (TiO2) is most widely adopted for its proper band gap and high transmittance. A variety of methods have been developed to improve the efficiency of TiO2-based PSCs and reduce their hysteresis.15,16 Recently, Tan et al. adopted chlorine-capped TiO2 colloidal nanoparticles to effectively form chemical binding with Pb2+ ions to achieve high performance.17 It is worth mentioning that metal sulfides have been used to modify TiO2 surface to improve the electron extraction and trans© XXXX American Chemical Society

Received: October 24, 2018 Published: December 10, 2018 A

DOI: 10.1021/jacs.8b11001 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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study was FTO/ETL/m-TiO2/perovskite/Spiro-OMeTAD/ Au.38 Figure 1d shows the cross-sectional SEM image of one typical PSC based on ZnO−ZnS-450 ETL. The thickness of m-TiO2/perovskite layer was ∼550 nm, and the top-view SEM image indicates the uniform and high-quality perovskite film, which was further proved by XRD and SEM measurements (Figures S4−S6).39,40 Photovoltaic Property of PSCs with ZnS Interlayer. To explore the role of ZnS interlayer in the photovoltaic performance of PSCs, we used ZnO−ZnS-450 films as ETLs and measured the all-round photovoltaic parameters. As shown in Figure 2a, the ZnO−ZnS-450 device with an active area of

Here, we propose to convert ZnO surface into ZnS (denoted as ZnO−ZnS) as a cascade ETL. Similar to ZnO, ZnS is also an n-type semiconductor with a wide direct band gap, and has higher electron mobility (100−150 cm2 V−1 s−1)33 than TiO2, in favor of electron extraction and transfer.34 It has been reported that strong coordination between S and Pb2+ ions exists.35,36 Here, we found that S atoms on ZnO−ZnS surface coordinated with Pb2+ ions at the interface of ETL/perovskite (Figure 1a), thereby forming an exquisite pathway of electron

Figure 1. Structural characterization of ZnO−ZnS ETL and its PSC device. (a) Illustration of ZnO−ZnS-based PSC device. (b) XRD patterns of powder scraped from ZnO and ZnO−ZnS-450 films. (c) HAADF-STEM image and elemental maps of nanoparticles taken from ZnO−ZnS-450 film. (d) Device structure and cross-sectional SEM image of PSC. Inset is top-view SEM image of perovskite film.

Figure 2. Photovoltaic performance of ZnO−ZnS-based PSCs. (a) Best J−V curves and (b) PCE of 25 individual ZnO and ZnO−ZnS450 devices. (c) IPCE spectra and corresponding integrated short currents Jsc of the best-performance ZnO and ZnO−ZnS-450 devices. (d) PCE evolution of unencapsulated devices of ZnO and ZnO−ZnS450 under dark storage in a drybox (25 °C, RH 30%). Error bars indicate the standard deviation.

transfer to accelerate charge extraction and mitigate hysteresis. Furthermore, ZnS acted as a passivation interlayer to enhance the overall device stability. Consequently, the combination of ZnO−ZnS with a light absorbing layer of triple-cation CsFAMAPb(IBr)3 perovskite produced high-performance PSCs with >20% PCE, negligible hysteresis, and high longterm stability, showing great potential for commercialization.

0.1 cm2 exhibited Jsc of 24.10 mA cm−2, Voc of 1.121 V, and FF of 76.66%, giving an overall PCE of RS 20.71% (reverse scan), whereas the bare ZnO device showed an overall PCE of RS 17.14% with a Jsc of 22.22 mA cm−2, Voc of 1.072 V, and FF of 71.93%. In terms of hysteresis appearance, the ZnO−ZnS-450 PSC showed negligible hysteresis with efficiency of FS 20.33% (forward scan) with a Jsc of 23.93 mA cm−2, Voc of 1.113 V, and FF of 76.35%, while the bare ZnO PSC exhibited large hysteresis with relatively low efficiency of FS 15.21% with a Jsc of 22.15 mA cm−2, Voc of 1.081 V, and FF of 63.54%, similar to previous reports on ZnO.41,42 Figures 2b and S7 show PCE distribution of 25 individual devices for each kind of ZnO and ZnO−ZnS-450 substrates. The average efficiencies are RS 20.3 ± 0.4% and FS 19.9 ± 0.4% for ZnO−ZnS-450 PSCs, and RS 16.8 ± 0.3% and FS 14.8 ± 0.4% for ZnO PSCs. The average efficiencies of PSCs scattered narrowly, suggesting the excellent duplicability of CsFAMA-based PSCs. The incident photon-toelectron conversion efficiency (IPCE) spectrum (Figure 2c) reached greater than 90% for ZnO−ZnS-450 devices in the visible region. Photocurrent densities calculated from the above spectrum data based on ZnO−ZnS-450 and ZnO were 23.34 and 21.36 mA cm−2, respectively, in good consistency with their J−V tests results (mismatched by 3.2%). The analogous surface morphologies of perovskite films on ZnO

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RESULTS AND DISCUSSION Surface Conversion of ZnO into ZnS Interlayer for PSCs. In this work, we designed a simple strategy to obtain ZnO−ZnS films as ETLs. ZnO blocking layers were first synthesized via thermal decomposition of zinc acetate on FTO (fluorine-doped tin oxide) according to a previously reported method,37 then immersed in an alcoholic solution of sulfurcontaining material (e.g., thiourea), and finally annealed in a muffle furnace in air at elevated temperatures (referred to as ZnO−ZnS-X; X is the annealing temperature in the unit of degree centigrade). X-ray diffraction (XRD) patterns (Figures 1b and S1) showed ZnS as-formed via postsurface conversion of ZnO was indexed to wurtzite structure, similar to the hexagonal structure of the ZnO underlayer. The surface sulfidation of ZnO maintained the morphology of ZnO layer (Figure S2). Elemental maps confirmed that S was uniformly distributed on ZnO−ZnS surface (Figure 1c). By tuning the annealing temperatures from 200 to 600 °C, the intensity of ZnS(0002) peaks in XRD gradually decreased (Figure S1), while the S content of ZnO−ZnS got precise control (Figure S3). The typical configuration of mesoporous PSCs in this B

DOI: 10.1021/jacs.8b11001 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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to the charge recombination, the higher frequency belonged to charge transfer at ETL/perovskite interfaces because the perovskite/HTL interfaces were identical in both cases. For the ZnO−ZnS-450 device, the recombination resistance (Rrec) was significantly larger than that of the bare ZnO device and the transfer resistance (Rt) of ZnO−ZnS-450 device was lower. From another observation, no considerable difference was found in electron mobilities of ZnO−ZnS-450 and ZnO (Figure S12). Furthermore, steady-state and time-resolved photoluminescence (TRPL) spectroscopies were used to explore the chargetransfer kinetics at the ETL/perovskite interface. As compared to the perovskite-only film with a decay time of τ = 52.3 ns, the quenched photoluminescence behavior (Figure 3c,d) of the perovskite film with ZnO−ZnS-450 confirmed that electron transfer across the perovskite/ZnO−ZnS-450 interface was faster and more effective (10.5 ns) than that across the perovskite/ZnO interface (21.4 ns). Because of these improvements, the novel devices with negative-hysteresis and long-time stability were successfully fabricated. Therefore, we concluded that sulfur conversion of ZnO/perovskite interface improved the device performance by enhancing interfacial electron extraction and transfer, as well as suppressing the charge recombination. Interfacial Pb−S Coordination between ZnS and Perovskite Layers. Previous studies have indicated that strong Pb−S coordination could be formed when S and Pb2+ ions met together.35,36 Here, we confirmed the strong Pb−S interaction at the interface of ZnO−ZnS/perovskite via both direct and indirect evidence. To get the direct evidence, Fourier transform infrared spectra (FTIR) of ZnO−ZnS/ perovskite films were investigated (Figure S13). A novel absorption peak at 610 cm−1 was assigned to the Pb−S bond, which confirmed the in situ formation of Pb−S at the interface between ZnO−ZnS-450 and perovskite.45 For the indirect evidence, ZnO−ZnS-450 films and powder from ZnO−ZnS450 films were studied by mixing with Pb2+ ions. The ZnO− ZnS-450 film treated by Pb2+ (ZnO−ZnS-450−Pb2+) was analyzed by low-energy ion scattering spectroscopy (LEISS).46,47 With the sputtering time of He+ ions increasing, the intensity of both S and Pb peaks at the kinetic energy of 1.71 and 2.77 keV, respectively, became weakened during He+ sputtering (Figure 4a−c). After sputtering treatment, ZnO− ZnS-450−Pb2+ surface almost changed to ZnO. The ZnO− ZnS-450−Pb2+ film was also evaluated by X-ray photoelectron spectroscopy (XPS). Zn, Pb, O, and S peaks were observed (Figure S14). The peak located at ∼162 eV was assigned to the binding energy of S 2p. After Pb2+ was introduced on ZnO− ZnS-450 surface, the Pb 4f7/2 and Pb 4f5/2 peaks were emerging at the binding energies of 138.4 and 142.8 eV, respectively.48,49 It is noteworthy that the peak of Pb 4f5/2 was merely ascribed to PbS, excluding similar interference of PbBr2 (143.9 eV).50 In addition, light yellow powder taken from ZnO−ZnS-450 film was also studied by immersing into alcoholic Pb(NO3)2 solution, which quickly bound Pb2+ ions and turned dark blue (Figure S15). For comparison, the offwhite ZnO powder was unchanged in color after Pb2+ treatment. The samples after Pb2+ treatment were further measured by X-ray fluorescence (XRF), and the outcome further verified the existence of PbS on ZnO−ZnS-450−Pb2+ surface (Table S3). All of the above results demonstrated that ZnS interlayer could strongly bind with Pb2+ ions of perovskite layer, and the as-formed excellent

and ZnO−ZnS-450 (Figure S5) indicated that the improvements in device performance were mainly attributed to the changes in the interfacial optoelectronic properties. Stabilized efficiency output of ZnO−ZnS-450 devices was higher than that of ZnO devices (Figure S8). Planar PSCs also exhibited the coincident experimental trend with mesoporous PSCs (Figure S9), and the detailed J−V data were displayed in Tables S1 and S2. Meanwhile, we examined the stability of ZnO and ZnO−ZnS-450 devices. Under dark storage, the statistical result from three devices and standard deviations have been carefully calculated. The ZnO−ZnS-450 devices showed substantially improved stability and maintained 18.23% efficiency (88% of the initial PCE) after storage in the dark for over 1000 h, while those based on bare ZnO rapidly dropped to 3.43% in efficiency within 200 h in a drybox (Figure 2d). The particular normalized PCEs were shown in Figure S10a. Meanwhile, the photostability of ZnO and ZnO− ZnS-450 devices under constant AM 1.5G illumination was compared. Under AM 1.5G, the ZnO−ZnS-450 devices showed significantly enhanced operating stability as compared to ZnO devices and retained 84% of their original performance after continuous operation at 25 °C for 50 h (Figure S10b). On the basis of all of the results mentioned above, we concluded that the ZnS interlayer effectively brought out the high performance of PSCs. Electron Transfer Across ZnS Interlayer. The interfacial electron transfer between ETL and perovskite layer was investigated to obtain insight into the performance improvement attributed to surface sulfidation of ZnO. Electrical impedance spectroscopy (EIS) was used as an effective tool to certify the charge transport and recombination processes.43,44 Figure 3a,b shows the Nyquist plots of ZnO and ZnO−ZnS-

Figure 3. Charge extraction and transfer in ZnO−ZnS-based PSCs. EIS of the PSCs with ZnO and ZnO−ZnS-450 measured at (a) 0 V and (b) 0.8 V bias under continuous one-sun illumination. (c) Steadystate and (d) time-resolved PL spectra of perovskite films on bare glass, ZnO, and ZnO−ZnS-450 films. The excitation fluence was ∼0.025 μJ cm−2.

450 cells measured at 0 and 0.8 V bias under one-sun illumination conditions, respectively. The Nyquist plots at different bias voltages were also obtained (Figure S11). In the high frequency region, the response of transport properties was observed. Two circles were further identified at considerable forward bias. While the circle with lower frequency was related C

DOI: 10.1021/jacs.8b11001 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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different relative humidities (Figure S23), the ZnO−ZnS-450 devices showed obviously improved UV stability. This is consistent with previous reports, of which metal sulfides were able to enhance the UV stability of PSCs.52 To avoid moisture degrading, the PSCs were sealed by graphene with excellent hydrophobicity.32 In this section, thin and uniform graphene was first synthesized and characterized. As shown in Figures S24 and S25, the graphene displayed an ultrathin morphology. After that, a graphene film was introduced on the configuration of FTO/ETL/m-TiO2/perovskite/Spiro-OMeTAD/Au (Figure 5a,b). As shown in Figure 5c, the graphene-sealed ZnO−

Figure 4. Evidence of Pb−S coordination and effect of sulfidation degree on both surface work function and PCE. LEISS spectra of ZnO−ZnS-450−Pb2+ film: (a) Schematic of the sputtering process. (b) S peaks and (c) Pb peaks dependent on the sputtering time using 3 keV He+ ions. (d) Surface work function variations of ZnO, ZnO− ZnS-400, ZnO−ZnS-450, and ZnO−ZnS-500-based ETLs and corresponding PCEs.

Zn−S−Pb interface passivated the surface of ZnO, leading to enhanced efficiency and stability of PSCs. Effect of Sulfidation Degree on Energy Level of ZnS Interlayer. On the basis of the effect of surface sulfidation of ZnO on PSC performance, the sulfidation degree and energy levels of as-resulted ZnO−ZnS films are expected to be correlated and play a critical role in PSCs. We found that increasing the annealing temperatures from 200 to 600 °C in air would decrease the S content on ZnO−ZnS films. The ZnO−ZnS-450 film showed the highest performance when assembling into whole solar cells (Figures S16−S18), and the estimated thickness of corresponding ZnS converted from ZnO was ∼15 nm based on the S/Zn ratio (Figures S3 and S19). Scanning Kelvin probe microscopy (SKPM) was then applied to measure work functions of ZnO−ZnS-X (i.e., X = 400, 450, and 500 °C) and ZnO films with surface potential images (Figure S20). With the annealing temperatures increasing and S content decreasing, the work functions of ZnO−ZnS decreased from −3.67 eV for ZnO−ZnS-400 to −3.94 eV for ZnO−ZnS-450, and then to −4.15 eV for ZnO− ZnS-500 (Figure 4d). ZnO−ZnS-450 interlayer was the best candidate whose energy level was located at the middle of those of ZnO underlayer (−4.20 eV) and perovskite uplayer (−3.80 eV),51 and the corresponding PCE was highest (20.71%). According to the energy level alignment theory, ZnO−ZnS-450 with well-matched work function effectively promoted electron transfer, and lowered the possibility of charge recombination between ETL and perovskite.16 Enhancement of Ultraviolet Light (UV) Stability in ZnS-Based PSCs. After in-depth understanding of the function of ZnS interlayer and investigation of storage and photo stabilities (Figure 2d and S10), more realistic issues, such as ultraviolet light (UV) stability, should be considered. Interestingly, the slightly reduced transmission in the range of 310−380 nm for ZnO−ZnS-450 layer decreased the destruction by UV radiation (Figure S21). We assembled an apparatus of UV-stability test (Figure S22) and compared the UV stability of ZnO and ZnO−ZnS-450 devices. Under

Figure 5. UV light stability of graphene-encapsulated PSCs. (a) Schematic structure and (b) top SEM image of a ZnO−ZnS-450based mesoporous PSC device encapsulated with graphene. (c) Comparison of the stability of ZnO−ZnS-450-based and ZnO-based mesoporous PSC devices with graphene encapsulation under UV radiation (25 °C, RH ∼70%).

ZnS PSC displayed outstanding improvement of UV stability by retaining 87% of its initial PCE over 500 h under UV radiation at RH ∼70%. However, the graphene-covered ZnO PSCs rapidly dropped to 0.07% of their original efficiency within 150 h. The UV stabilities of unsealed devices were displayed in Figure S26 with similar trends. The outcomes demonstrated the advantage and potential of inserting ZnS at the ZnO/perovskite interface for UV stability.

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CONCLUSION We have demonstrated an effective strategy to create a ZnS interlayer as both a cascade ETL and a surface passivation layer for high-performance perovskite solar cells. ZnS at the ZnO/ perovskite interface bound strongly with Pb2+ ions, generated a novel pathway of electron transport to efficiently improve electron extraction and reduce interfacial recombination to obtain PSCs with the high efficiency, superb stability, and negligible hysteresis. Meanwhile, the ZnS interlayer acted as a cascade ETL, inhibiting charge recombination between ZnO and perovskite layers. In addition, ZnS passivated the basic surface of ZnO and avoided possible deprotonation of protonated organic amine in perovskite, improving the overall stabilities. Building such a cascade ETL and passivating layer offers a promising path to achieving high-performance devices ripe for commercialization. D

DOI: 10.1021/jacs.8b11001 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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ZnO and ZnO−ZnS-450 devices. The solvent was evaporated at 80 °C for 5 min to form the graphene film. Device Characterizations. Current−voltage characteristics were recorded on a solar simulator equipped with a Keithley 2400 source meter and a 300-W collimated xenon lamp (Newport) calibrated with the light intensity to 100 mW cm−2 at AM 1.5 G solar light condition by a certified silicon solar cell. Incident photon-to-electron conversion efficiencies (IPCE) were measured on a computer-controlled IPCE system (Newport) containing a xenon lamp, a monochromator, and a Keithley multimeter. The system was calibrated with the certified silicon solar cell, and IPCE data were collected at a DC mode. The IPCE values were calculated using the following equation:

EXPERIMENTAL SECTION

Materials. All chemicals and reagents were used as received from chemical companies without any further purification. CH3NH3Br (methylamine bromide, MABr) was synthesized and purified according to a literature method.53 To a stirring solution of methylamine in methanol (40 wt %, 24 mL) was slowly added aqueous hydrobromic acid (48 wt % in water) at 0 °C. After 2 h, the precipitate was collected by evaporation at 50 °C for 1 h. The asobtained product was washed with diethyl ether three times and then finally dried at 60 °C in a vacuum oven for 24 h to collect the desired pure MABr as white crystals. NH2CHNH2I (formamidine iodide, FAI) was synthesized by reacting 30 mL of hydroiodic acid (57% in water), 15 g of formamidine acetate in a 250 mL round-bottomed flask at 0 °C for 2 h with stirring. The precipitate was recovered by evaporating the solution at 60 °C for 1 h. The product was dissolved in ethanol, recrystallized using diethyl ether, and finally dried at 60 °C in a vacuum oven for 24 h.54 To prepare the graphene solution, reduced graphene oxide was synthesized via a two-step method of modified Hummers’ oxidation and further chemical reduction.55 In brief, graphite was mixed with H2SO4, and then KMnO4 was slowly added to the mixture in an ice bath. After the reaction, the brown graphene oxide (GO) was washed and centrifuged. Further, graphene was prepared by reducing GO with hydrazine at 80 °C. Finally, the obtained graphene was dispersed in isopropanol. The lead compounds (PbBr2, Pb(NO3)2, PbI2) and 2-methoxyethanol were purchased from Alfa Aesar; CsI was from TCI (Tokyo Chemical Industry Co., Ltd.). Zinc acetate, thiourea, monoethanolamine, ethanol, chlorobenzene, acetonitrile, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and isopropanol (IPA) were purchased from Sinopharm Chemical Regent Co., Ltd. 4-tertButylpyridine (TBP) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) were purchased from J&K Scientific Ltd. 2,2′,7,7′Tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD) was purchased from Luminescence Technology Corp. Solar Cell Fabrications. Fluorine-doped tin oxide (FTO) glass substrates with dimension of 2.0 × 2.0 cm2 were patterned by etching with zinc powder and 2 M hydrochloric acid. The substrates were then sequentially washed in ultrasonic baths of acetone, distilled water, and ethanol. A compact ZnO blocking layer was spin-coated onto the cleaned FTO substrate using 0.37 M zinc acetate and 0.37 M monoethanolamine in 2-methoxyethanol solution at 3000 rpm for 20 s. The substrate was heated at 120 °C for 15 min, and then annealed at 600 °C for 30 min. After being cooled to room temperature, the film was immersed into ethanol solution of 1 M thiourea at 70 °C for 30 min, dried at 80 °C for 10 min, and then annealed in air at various temperatures (e.g., 200, 300, 400, 450, 500, 600 °C) for 30 min to obtain ZnO−ZnS films (denoted as ZnO−ZnS-X; X is annealing temperature). For mesoporous PSCs, a mesoporous TiO2 film of ∼150 nm in thickness was deposited on the pretreated FTO/ZnO− ZnS substrate by spin-coating of TiO2 paste (Dyesol DSL 18NR-T) with ethanol (1:4, mass ratio) at 5000 rpm for 30 s, followed by heating at the same temperature of ZnO−ZnS-X for 30 min. For the perovskite layer, a mixture solution of 0.2 mmol of PbBr2, 1.05 mmol of PbI2, 0.1 mmol of CsI, and 87.5 μL of DMSO in 600 μL of DMF was spun on the ZnO−ZnS film at 3000 rpm for 20 s, and then a mixture solution of 70 mg of FAI and 12.35 mg of MABr in 1 mL of IPA was dropped on as-prepared substrate and dwelled for 13 s, then spun at 4000 rpm for 15 s. Finally, the substrate with perovskite precursor was heated at 110 °C for 25 min. After the preparation of CsFAMA perovskite layer, the hole transport layer solution was coated via solution process at 4000 rpm for 30 s, where SpiroOMeTAD/chlorobenzene (72 mg/mL) solution was employed with the additives containing 17.5 μL of Li-TFSI/acetonitrile (520 mg/ mL) and 28.8 μL of TBP. Finally, an 80 nm thick Au counter electrode was deposited via thermal evaporation under reduced pressure of 4 × 10−7 Torr. The active area was 0.10 cm2. For graphene encapsulation, the graphene solution was blade-coated on the top of

IPCE(λ) =

1240 × J(λ) × 100% λ × P(λ)

where λ is the wavelength (nm), J(λ) is the photocurrent density (mA cm−2), and P(λ) is the incident power density of the monochromated light (mW cm−2). The Jsc (derived from IPCE) was integrated from the IPCE spectra according to the following equation:

JSC =

∫λ

λ2

1

qλ IPCE(λ)S(λ) dλ = hc

∫λ

λ2

1

λ IPCE(λ)S(λ) dλ 1240

where λ is the wavelength (nm), λ1 is 300 nm, λ2 is 800 nm, h is Planck constant, c is light speed, q is the elementary charge, and S(λ) is the intensity of Solar Spectrum at AM 1.5G Condition. XRD patterns were analyzed on an X-ray diffractometer (Rigaku, RINT2500) with a Cu Kα radiation source. Surface morphologies were recorded on a SEM-4800 field-emission scanning electron microscope (SEM). UV−vis spectra were measured on a cary-5000 UV−vis spectrophotometer. IR spectra were tested on a Thermo-Nicolet IR200 instrument. The time-resolved photoluminescence (PL) spectra were measured on an Edinburgh Instruments FLS920 spectrometer. Electrochemical impedance spectroscopy (EIS) was measured on a CHI 760D potentiostat in the frequency range from 106 to 0.1 Hz under illumination intensity (100 mW cm−2), in which the potential bias was applied at 0, 0.4, 0.6, and 0.8 V. UV radiation tests were on a UV lamp (Spectroline SB-100P/FA), and stable wavelength is 365 nm. For electron mobility of ZnO and ZnO−ZnS, electron-only devices (FTO/ETL/PCBM/Ag) were fabricated to calculate the electron mobility of the samples, including ZnO and ZnO−ZnS, by the space-charge-limited current (SCLC) model.16 Scanning Kelvin probe microscopy (SKPM) of ZnO and ZnO−ZnS work function measurements were carried out in air on an atomic force microscope (AFM, Agilent SPM 5500) equipped with a MAC III controller (providing three lock-in amplifiers) using a Pt/Ir-coated Si tip (FMG01/Pt, NT-MDT). For atomic force microscopy (AFM) of ZnO and ZnO−ZnS, AFM height profiles were obtained with a Cypher AFM (Asylum Research) atomic force microscope at a tapping mode under ambient conditions. The tip model was AC240TS-R3 (2 N/m, 70 kHz).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b11001.

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XRD, SEM, TEM, EDS, and XPS data of ZnO−ZnS layers or perovskite layers or reduced GO; and photovoltaic parameters and UV stabilities of the devices (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] E

DOI: 10.1021/jacs.8b11001 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Binghui Wu: 0000-0003-4015-9991 Nanfeng Zheng: 0000-0001-9879-4790 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Key R&D Program of China (2017YFA0207302), the National Natural Science Foundation of China (21731005, 21420102001, 21721001, and 21805232), and the fundamental research funds for central universities (20720180061).

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DOI: 10.1021/jacs.8b11001 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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DOI: 10.1021/jacs.8b11001 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX