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Low-Temperature Electron Beam Deposition of Zn-SnOx for Stable and Flexible Perovskite Solar Cells Zonglong Song, Wenbo Bi, Xinmeng Zhuang, Yanjie Wu, Boxue Zhang, Xinfu Chen, Cong Chen,* Qilin Dai, and Hongwei Song* role in suppressing recombination and rectifying the photocurrent generated in perovskite photoactive films.[7] So far, a number of inorganic n-type semiconductors have been applied to PSCs as electron transporting materials, including TiO2, SnO2, ZnO, WOx, and Nb2O5.[8,9] To fabricate flexible and wearable PSCs on flexible substrates which could be used for commercial purposes, the ETL must be prepared at a low-temperature condition.[10,11] According to previous reports, the PSCs based on SnO2 under low temperature can reach a PCE of more than 20%.[12,13] Compared with other kinds of ETLs, SnO2 has some advantages, including excellent energy-level alignment and high mobility. It has been widely considered as a typical candidate to replace TiO2 as an ETL in PSCs.[13–16] SnO2 as an ETL in PSCs can reduce the degradation effect of the perovskite photoactive layer induced by TiO2 and result in better stability under continuous illumination.[17] There are various methods to deposit SnO2 on the FTO substrate, including the chemical bath deposition method, etc.[18] The fabrication of a compact SnO2 film is a challenge by the spin-coating technique. First, the agglomeration issue of the nanoparticles in the ink will make the SnO2 thin film form pinholes and nonuniformity over a large area. In addition, the SnO2 film has a poor wetting property with the perovskite precursor solution and it must be plasma pretreated to modify this interface.[15] Moreover, it is difficult to prepare the precise thickness of SnO2 by the spin-coating technique. Solutionprocessed SnO2 ETLs were limited in laboratory research and they were not easy to develop for large-scale and low-cost applications.[19] For commercialization, it is quite urgent to find a controllable-thickness and low-cost method for preparing SnO2 films. Physical vapor deposition can manufacture a quite dense and thickness-controllable SnO2 film under low temperatures. SnO2 deposited by E-beam evaporation technology with a lowtemperature process is beneficial for manufacture at a large scale, low cost, and uniformity. The use of E-beam can be simultaneously equipped with hundreds of SnO2 substrates, which are conductive to continuous and automated production. The fabricated SnO2 film with high electron mobility, good antireflection, and orientated crystallinity played important roles in highperformance PSCs. More importantly, low-temperature preparation of ETLs and the excellent performance of the device can accelerate the application of flexible and wearable PSCs.

Perovskite solar cells (PSCs) attract tremendous interest due to their feasibility, high power conversion efficiency (PCE), light weight, and flexible architecture. However, some challenges are still present for cheap mass fabrication in commercial applications. Herein, efficient Zn-SnOx electron transport layers (ETLs) are used by the low-temperature (100
C) electron beam (E-beam) method. Doping Zn2þ in SnO2 improves conductivity, suppresses charge recombination, and optimizes the energy level structure of SnO2, leading to an improved PCE from 18.95% to 20.16%. More importantly, the PCE of the modified device is more than 80% of its initial values for 800 h in ambient air with a relative humidity of 40%. The flexible device exhibits a PCE of 15.25% and remains at an initial PCE of 83% after 100 bending cycles. The efficient and flexible PSCs are potentially used as wearable energy power sources. The low-temperature preparation of ETL and the excellent performance of devices present great commercial potential for future applications.

1. Introduction Perovskite solar cells(PSCs) have received great attention in recent years due to their excellent photovoltaic performance.[1] The certified power conversion efficiency (PCE) of 24.2% has been achieved.[2] The marvelous performance benefits from the excellent properties of perovskite materials, such as a high optical absorption coefficient and an adjustable optical bandgap.[3–5] However, some challenges prevent PSCs from becoming a competitive business technology, including limited spectral response, poor air stability, and flexibility.[6] It is obvious that the research and exploration of any of the above challenges will accelerate the progress of PSCs in practical applications. In the device structure of PSCs, the electron transport layer (ETL) plays an irreplaceable Z. Song, W. Bi, X. Zhuang, Y. Wu, B. Zhang, X. Chen, C. Chen, Prof. H. Song State Key Laboratory on Integrated Optoelectronics College of Electronic Science and Engineering Jilin University 2699 Qianjin Street, Changchun 130012, P. R. China E-mail: [email protected]; [email protected] Prof. Q. Dai Department of Chemistry, Physics, and Atmospheric Sciences Jackson State University Jackson, MS 39217, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/solr.201900266.

DOI: 10.1002/solr.201900266

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In recent years, it has become a research hotspot to improve the conductivity of SnO2 and match the perovskite energy level by doping other metal ions. Park et al. employed Liþ in SnO2 ETLs to increase the PCE from 15.29% to 18.20%.[20] Anaraki et al. formed a Nb-SnO2 device to achieve a PCE of 20.5% compared with an undoped device.[21] According to previous reports, Zn2þ doping in SnO2 has been used in dye-sensitized solar cells.[22,23] A small concentration of Zn doping could improve the electrical conductivity of SnO2 and optimize energy levels with the perovskite material, leading to the improvement of PSCs.[24] Herein, we fabricated efficient and flexible PSCs based on the Zn-SnOx ETL by a typical E-beam method. The E-beam method has the advantages of low temperature, controllable thickness, and low cost, which benefit mass production and commercial applications.[25]

2. Result and Discussion Figure 1a shows the procedure of the SnO2 or Zn-SnO2 deposition process by the E-beam method, which is beneficial to achieving large-scale commercial production. Figure 1b shows the schematic diagram of the completed devices with the structure of FTO/Zn-SnOx/perovskite/spiro-OMeTAD/Au from the bottom to the top layer. Figure 1c shows a cross-sectional scanning electron microscope (SEM) image of the prepared PSCs with an ordered and hierarchical structure. Each layer of FTO/Zn-SnOx/ perovskite/spiro-OMeTAD is quite smooth, which can be conducive to transportation. The photo of the SnOx film with and without annealing is depicted in Figure S1, Supporting Information. It can be clearly found that the color becomes lighter after annealing, which indicates that the valence of Sn may change during

the process of deposition. X-ray photoelectron spectroscopy (XPS) was used to test the SnOx film. The banding energy of the Sn core for the unannealed sample shift toward the left was compared with the annealed SnOx film, which could be ascribed to Sn0 state. Therefore, to ensure the valence state of the Sn element in the deposited film, annealing during deposition is an essential step. Figure 1d,e shows 2D atomic force microscope (AFM) images of SnOx films. The annealed SnOx film shows a smoother surface with a roughness measurement of the surface (RMS) of 15.53 nm than that of the as-evaporated film (25.81 nm). Figure 1f shows the X-ray diffraction (XRD) patterns of pure SnO2 and Zn-doped SnO2 samples with different molar ratios (1%, 3%, and 5%). All the diffraction peaks can be indexed to rutile SnO2 (JCPDS 77-0451). Previous studies proved that the radius of Zn2þ is 0.73 Å and the radius of Sn4þ is 0.71 Å.[26] According to Bragg’s equation (2dsinθ ¼ nλ), the introduction of Zn2þ will make the XRD peak position of SnO2 shift toward the smaller diffraction angle. It is obvious that the (110) diffraction peaks shift toward the smaller diffraction angle (right of Figure 1f ). It can be attributed to the substitution of Zn2þ for Sn4þ, and Zn2þ possesses a larger size than Sn4þ.[27] The XRD results illustrated that a part of Sn4þ has been replaced by Zn2þ, indicating that Zn2þ was successfully doped in the lattice of SnO2. Furthermore, to confirm the existence of Zn2þ doping, the film of Zn-SnOx was tested by the energydispersive X-Ray spectroscopy (EDX) method. According to the Figure S3, Supporting Information, we can find that Zn2þ was distributed uniformly in the film. The pure SnOx film and Zn-doped SnOx were tested by XPS to determine the change in binding energy of Sn and O after doping. The Sn 3d5/2 and 3d3/2 transition peaks of the films before and after doping are shown in Figure S4, Supporting Information. Compared with

Figure 1. a) Schematic illustration of Zn-SnOx film deposition process by E-beam equipment. b) The device structure of prepared PSCs. c) The crosssectional SEM image for the prepared PSCs. d) AFM image of unannealed SnOx film. e) AFM image of annealed SnOx film. f ) X-ray diffraction patterns of undoped SnO2 films and Zn (1%, 3%, 5%) doped SnO2 films.

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the SnOx film, the binding energy of Sn in the Zn-SnOx film varies from 487.1 to 486.1 eV in the peak of Sn 3d5/2. Zn doping decreases the binding energy of Sn and this can be attributed to oxygen deficiency.[28] The transition peak of O 1s also shifts from 530.1 to 529.45 eV due to the doping of Zn2þ. A strong binding energy of Zn appears in the position 1053 eV, indicating that Zn2þ is successfully doped into the lattices. To further explore the effect of SnOx thickness on the performance, we deposited thin films with different thicknesses (30, 40, 50, and 60 nm) on the bare FTO. From Figure S5a, Supporting Information, it can be observed that the PCEs first increase with the increasing thickness of the SnOx film from 30 to 50 nm, then decline with further increase in the thickness to 60 nm. As is illustrated in Figure S5a, Supporting Information, the prepared PSCs with 50 nm SnOx ETLs can exhibit a Voc of 1.09 V, a Jsc of 22.44 mA cm2, an FF of 0.753, and a corresponding PCE of 18.43%. Statistical distributions of the photovoltaic parameters for PSC devices based on the SnOx ETLs with different thicknesses are summarized in Table S1, Supporting Information. Figure S5b, Supporting Information, shows the transmittance spectra of the SnOx films on the FTO with different thicknesses (30, 40, 50, and 60 nm).[28] It can be seen that the SnOx film shows a high transmittance in the range of wavelength from 440 to 800 nm, as the thickness is 30 and 40 nm. It is observed that the absorption edge is gradually reduced with the increase in the thickness of the SnOx film. This may be one of the reasons why PCE begins to decay over 50 nm. The test results are consistent with the previous report. However, the PCE of the PSCs based on SnOx evaporated by the E-beam method does not necessarily decay from the thickness of 40 nm. From Figure S5a, Supporting Information, we can make a prediction that the PSCs should have the maximum PCE between the thicknesses of 40 and 50 nm. Therefore, to further optimize the thickness of the SnOx film by using the E-beam method, the films with thickness of 45 nm were deposited on the bare FTO. From the testing results in Figure S6a, Supporting Information, a champion PCE of 18.95% was achieved with Jsc of 22.72 mA cm2 and Voc of 1.11 V from 45 nm SnOx ETL-based PSCs. Figure S6a, Supporting Information, shows square resistance of the SnOx films tested by the M-3 Mini type four-probe tester (SuZhou Jingge Electronic Co.). The square resistance is derived from the resistance of SnOx ETLs. The PSCs based on the ETL of 45 nm SnOx show the smallest square resistance in different thicknesses, indicating the superior interfacial performance. Fitting results from the Nyquist plots of the PSCs with a frequency range from 0.1 to 100 000 Hz in Figure S6b, Supporting Information. The values of Rreb are proportional to the performance of the device. This is direct evidence that the PSC device performs better based on the SnOx film at the thickness of 45 nm. The hysteresis of the device has been a nonnegligible problem in planar PSCs.[29] We have prepared the PSCs based on SnO2 ETLs by solution and E-beam (35 devices per group) methods (Figure S7, Supporting Information). After testing, the optimized PCE values achieved in the solution-processed SnO2based devices were 15.20% and 18.38%, respectively, from the forward and reverse J–V scans. The E-beam depositedSnOx-based PSCs show PCE values of 17.07% and 18.95%, respectively. It can be concluded that the solution¼processed

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SnO2-based PSCs exhibit a more serious hysteresis phenomenon. According to previous reports, the uniformity of the carrier transporting layer is an important factor affecting the hysteresis.[30] E-beam-deposited SnO2 can be more conducive to the transmission of carrier. Simultaneously, we have prepared the PSCs based on SnO2 with the methods of solution and E-beam (35 devices per group). The distribution diagram of PCE is shown in Figure S8, Supporting Information. The PCE distribution of devices prepared by E-beam is relatively concentrated, while the PCEs of the solution-processed SnO2-based PSCs exhibit wider distribution. These results indicate that the E-beam method is more suitable for future commercial applications. The SEM images of the SnOx and Zn-SnOx films using the E-beam method are shown in Figure S9, Supporting Information. It can be seen that there is no significant change in the grain before and after doping. Simultaneously, the SEM photos of the perovskite on the top of the SnOx or Zn-SnOx are shown in Figure S10, Supporting Information. The grain size of perovskites has no obvious change after doping, suggesting that Zn doping in SnO2 has no influence on the morphology of perovskite films. From Table S2, Supporting Information, it is seen that the FF of PCEs first increases with increasing doping concentration of Zn from 1% to 3%, then decreases from 3% to 5%. As shown in Figure 2a,b, the optimized PSC with 3% Zn-SnOx exhibits a Jsc of 23.410 mA cm2, a Voc of 1.12 V, an FF of 77.1% and a corresponding PCE of 20.16% with increased PCE by 6.39% compared with the device based on bare SnOx. The enhanced Jsc can be partly attributed to the improved photoelectric response in the UV region and favorable energy-level matching, as is depicted from the incident photon-to-current efficiency (IPCE) spectra in Figure 3d. The enhanced FF is attributed to the improved charge collection and conductivity. To quantitatively characterize the hysteresis behavior of SnOx and Zn-SnOx in Figure 2a,b, HI was used to quantify hysteresis effect, which is defined as[31] OC Z

HI ¼

OC Z

ð J RS ðVÞ  J FS ðVÞÞ dðVÞ= SC

J RS ðVÞdðVÞ,

(1)

SC

where ∫ OC SC ðJ FS ðVÞdðVÞÞ represents the forward scan area of the J–V curve and ∫ OC SC ðJ RS ðVÞdðVÞÞ represents the reverse scan area of the J–V curve. The value of HI is inversely proportional to the hysteresis characteristic of the device. HIs of the PSCs based SnOx and Zn-SnOx were shown in Figure S11, Supporting Information. It is easy to deduce that the HIs of the PSCs are 0.087 and 0.068, respectively, for the SnO2 and Zn-SnOx-based devices. This result reveals that the device based on Zn-SnOx has the faster extraction speed of electrons and a more balanced carrier transport between the perovskite layer and ETLs.[32] The n-type conduction can be improved by doping with Zn2þ, this is due to the formation of zinc interstitials (Zni) and/or oxygen vacancies (Vo), which can reduce the distance between the Fermi level and the conducting band resulting in improved conductivity.[33] Simultaneously, the conductivity of the semiconductor can also be improved by reducing the defect density.[34] From Figure S12, Supporting Information, it can be seen that the RMS

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Figure 2. a,b) Forward and reverse J–V curves of PSCs under simulated AM 1.5 illumination. c) Resistance of the ETLs based on SnOx and Zn-SnOx. d) J–V curves for SnOx and Zn-SnOx sample.

Figure 3. a) Steady-state photoluminescence (PL) curves measured from FTO/SnOx/perovskite and FTO/Zn-SnOx/perovskite samples. b) TRPL spectroscopy for the S1 and S2 films c) Fitting results from the Nyquist plots of PSCs at 0.8 V bias. d) IPCE of the S1 and S2 samples based PSCs.

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of Zn-SnOx decreased from 15.22 to 9.17 nm and the size of crystal lattice was larger with Zn doping. A larger lattice size means a smaller defect density.[35] Therefore, doping Zn2þ in the SnO2 can reduce the resistance of the ETLs. The conductivity of the thin films can be expressed by σ, which is defined as[36] σ ¼ d= AR,

(2)

where A represents the active area of the PSCs (0.1 cm2 ¼ 0.2  0.5 cm), R is the resistance of ETL calculated by V ¼ IR, and d is the thickness of the SnOx and Zn-SnOx film (45 nm). From Figure 2c, the average resistance of the effective area for the SnOx and Zn-SnOx were calculated to be 5.92 and 4.37 Ω. By the equation, σ of the SnOx and Zn-SnOx was roughly calculated as 7.6  106 and 1.05  105 S cm1, respectively. The enhanced conductivity of Zn-SnO2 will reduce the hysteresis effect slightly. According to the previous study, Zn2þ doped in the lattice interstitials can improve the n-type conducting properties due to zinc interstitials (Zni), and native defects such as oxygen vacancies (Vo) could acts as donors, which result in the decrease in resistivity of the SnO2 thin film.[33] From the test results, it is believed that there can be a certain number of zinc ions in the lattice interstitials. The UV–Vis absorption spectra of the perovskite films based the SnOx and Zn(1%, 3%, and 5%)-doped SnOx films were shown in Figure S13, Supporting Information. With the increased doping concentration of Zn2þ, absorption increases obviously at the wavelength of 350–500 nm, suggesting that Zn2þ doping can enhance the absorption of perovskite in the UV region. High performance reproducibility of the device was demonstrated by testing the performance of 35 devices and the result is shown in Figure 2d. The average PCEs of the controlled device, the Zn-SnOx device, were 17.70% and 18.94%, respectively. Figure 3a depicts the PL spectra of the FTO/Zn-SnOx/ perovskite film and FTO/SnOx/perovskite film. It is obvious that the intensity of PL has a significant reduction when Zn2þ is introduced. Based on the fact that PL quenching originates from the carrier extraction across the interface between perovskite and ETLs,[37,38] therefore, it is obvious that Zn2þ doping improves charge transfer and suppresses the accumulation of electrons between the perovskite and ETL, resulting in improvement of the charge extraction rate of perovskite.[20] The time-resolved photoluminescence (TRPL) spectroscopy was used to further explore the carrier extraction behavior of ETLs with and without doping. Figure 3b shows the decay dynamics of PL intensity for perovskite on SnOx and Zn-SnOx. As shown, the film with Zn2þ doping shows a little faster PL decay than pure SnO2. The effect of material properties of ETLs on charge extraction efficiency is one of the factors determining the Jsc value in PSCs. Therefore, the carrier extraction rate can be slightly increased by doping Zn2þ in SnO2.[23,39] Figure 3c shows the Nyquist plots of the devices obtained by electrochemical impedance spectroscopy (EIS) analysis with the bias of 0.8 V. The equivalent circuit in the upper right corner of the picture is utilized to fit the data. The low frequency region of Rreb usually corresponds to the resistance of the interface between the ETL layer and the perovskite layer.[40,41] It can be concluded that the Zn2þ-SnOx-based device can exhibit higher carrier recombination resistance (Rreb) (320.76 Ω) than that of Sol. RRL 2019, 1900266

the control device (288.46 Ω). The value of Rreb is related to suppressed carrier recombination.[42] A higher Rreb means greater the reorganization resistance, lesser the charge recombination loss, and better the device performance. Above all, it can be concluded that Zn2þ doping in SnO2 can improve the performance of the PSCs. From Figure 3d, it was found that IPCE increases evidently in the region of 350–430 nm after Zn2þ doping in SnO2. Therefore, we can draw a conclusion that Zn2þ doping reduces the photoelectron recombination process, which improves the UV region in IPCE as well as increases photocurrent. To explore the effect of Zn2þ doping effect on the energy level, the relevant energy levels of the devices were tested using the ultraviolet photoemission spectroscopy (UPS) method. As shown in Figure 4a, the ECB of SnOx increases markedly from 4.05 to 3.85 eV after Zn doping. The increase in ECB promotes the electron transfer, resulting in improving performance of the PSCs. The work functions (φ) of the bare SnOx and Zn-SnOx samples were calculated to be 5.57 and 5.44 eV by the equation ϕ ¼ 21.22  (Ecut-off  Ei), respectively (Figure S14, Supporting Information).[43] There, Ecut-off is low kinetic energy threshold in photoelectron spectroscopy, and Ei is defined as 0 eV. For the SnOx and Zn-SnOx samples, the energy gaps between the valence band and the Fermi level (Ef ) are 2.85 and 2.99 eV, respectively, and the corresponding valence band positions are 8.56 and 8.29 eV, respectively. The optical band gaps (Eg) of the bare SnOx and the Zn-SnOx samples by UV–Vis absorption spectroscopy are 4.51 and 4.44 eV, respectively. The conduction band edges for control and Zn-SnOx are 0.25 eV (ΔE0) and 0.05 eV (ΔE1), indicating that Zn2þ doping could reduce the band gap difference between perovskite and ETL, resulting in a smaller voltage loss between Zn-SnOx and perovskite.[44,45] Figure 5a shows the PCE values as a function of time for bare SnOx and Zn-SnOx device under continuous light irradiation for 400 s at the maximum power point (0.9 V). All of the devices presented outstanding stability, which further embodies the advantages of the E-beam deposition method. After 400 s, the current density of the Zn-SnOx-based device still retains 93% of the initial values, whereas the bare SnOx-based device retains only 84%. Therefore, device stability can be enhanced by Zn2þ doping in SnOx ETLs. According to the previous report, the thermal stability of the perovskite films on different ETLs is mainly due to the difference in hydroxyl groups on ETL films.[46] This indicates that the devices based on these ETLs can present a similar trend during storage process, which can be attributed to the fact that hydroxyl-induced chemical degradation occurs slowly even at room temperature and the degradation pathway may be more dominant than other degradation pathways. As shown in Figure 5b, the device based on Zn-SnOx exhibits significantly enhanced stability in room temperature and ambient environment with a relative humidity of 40%. It is notable that the PCEs of devices based on bare SnOx rapidly decline in the first 360 h of storage time. Though the PCE of Zn-SnOx-based devices attenuated faster in the last storage time, the PCE of the modified devices can maintain over 85% of the initial values after 800 h. The stability of the devices based on SnOx and Zn-SnOx was tested under 365 nm continuous illumination for 100 h. It can be found that the stability of Zn-SnOx had a significant improvement from Figure S15, Supporting Information. After 100 h of

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Figure 4. a,b) UPS spectra describing the cut-off energy (Ecut-off ) and Fermi edge (EF, edge) for SnOx and Zn-SnOx films, respectively. c) Energy diagram of band alignment for devices based on the SnOx and Zn-SnOx.

illumination, the PCEs of the modified devices can remain at 80% of their initial values, while the devices based on the bare SnOx can only stay at 70%. This can be attributed to the carrier extraction rate of doped devices that is faster under ultraviolet irradiation, which reduces the loss of carrier recombination.[47] Moreover, due to Zn2þ doping, the ultraviolet absorption of ETL can be enhanced, leading to reduction in the damage of UV light to the perovskite layer. Due to the low temperature and large-scale preparation advantages of E-beam deposition, we further explore its application in flexible PSCs. Figure 5c depicted the J–V characteristic for the flexible PSCs based on the PET/ITO substrate. The optimized device could exhibit a PCE of 15.25% with Jsc of 20.61 mA cm2, Voc of 1.10 V, and FF of 67.26%. The reason for the declined PCE in the flexible PSCs could be estimated to be the following points: 1) the PET/ITO exhibits a larger sheet resistance of 14.6 Ω sq1 compared with that of the glass/FTO substrates (7.4 Ω sq1). 2) The light transmittance of PET/ITO decreased by 14% compared with glass/FTO. To characterize the bending resistance of flexible devices, we conducted the bending test of the flexible devices. The PCE of flexible devices based on SnOx and Zn-SnOx exhibits a slight decrease under a radius of 26 mm (corresponding to the bending angles of 55.4
) after 40 bending times in Figure 5d. From 40 to 100 times, the PCE of SnOx-based devices drops sharply relative to the values obtained from Zn-SnOx-based devices. After 100 bending times, Sol. RRL 2019, 1900266

the Zn-SnOx device can still retain a PCE over 11%, indicating excellent flexural stability.

3. Conclusions In conclusion, an E-beam deposition method was employed to prepare SnOx ETLs in the planar PSCs. The large-scale preparation of ETLs by the E-beam method provides another way of thinking for the commercialization of PSCs. By optimizing the doping concentration of Zn2þ in SnO2 and film thickness, the PCE and stability of PSCs can be significantly improved. The Zn-SnOx-based PSCs could obtain a PCE up to 20.16%. The flexible PSCs with an optimized PCE of 15.25% can remain at 84% of the initial values after 100 bending times. This article provides a workable method to employ large-scale, highperformance, and flexible PSCs for future applications.

4. Experimental Section Materials: Lead diiodide (PbI2, 99.9985%), lead dibromide (PbBr2, 99.999%), formamidinium iodide (FAI), Cesium iodide (CsI, 99.9%), and SnO2 colloidal solution were purchased from Alfa Aesar. Dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and SnO2 powders were purchased from MACKLIN. The SnO2 and Zn-SnO2 Target Preparation: The SnO2 film by the solution method: The SnO2 precursor solution was prepared by mixing SnO2

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Figure 5. a) PCE values as a function of time for the bare SnOx and Zn-SnOx device under continuous light irradiation. b) Durability of the SnOx and ZnSnOx devices exposed in the ambient air with a relative humidity of 40%. The test was on the devices without encapsulation. c) J–V curves of the flexible devices with Zn-SnOx as ETLs. Inset is a photograph of the flexible PSCs. d) Overall performance changes with the increasing times of bending cycles. All the PCE values were normalized to describe the device performance trends. There are over ten samples for every kind of device in this study. colloidal solution and deionized water (v:v ¼ 1:3). The SnO2 ETL was formed by spin coating SnO2 at 4000 rpm for 30 s and annealing at 175
C for 1 h in ambient air. The SnOx film prepared by E-beam: For the SnO2 target, SnO2 powder was dried in a drying oven at 200
C for 2 h, the dried powder was then transferred to a tablet press at a pressure of 22 MPa for 30 min. Finally, the target was sintered for 10 h at a high temperature in a muffle furnace. For the Zn-SnO2 target, the SnO2 and ZnCl2 powders were weighted with the molar ratios of 99:1, 97:3, and 95:5, then sintered in an oven at 200
C for 2 h; the dried powders were placed in a ball mill at 500 rpm s1 for 3 h, then the SnO2 target production steps were repeated. Solar Cell Fabrication: Laser-etched FTO was washed sequentially by absolute ethanol, water, isopropanol, and acetone. After drying them in nitrogen atmosphere, the SnO2/Zn-SnO2 films were deposited on FTO/ glass by the E-beam evaporated technique. The optimal deposition rate we explored was 0.5 nm s1. The film thickness was controlled by setting the deposition time and rate. During deposition, the substrates were annealed at 100
C. After being treated by UV–ozone for 20 min, the substrates were transferred to a glove box filled with dry nitrogen. The perovskite precursor solution was placed in the glove box in advance. FAI (1.0 M), PbI2 (1.1 M), CsI (0.05 M), PbBr2 (0.2 M), and MABr (0.2 M) were dissolved in anhydrous DMF: DMSO, 4:1 (v:v). The perovskite precursor solution was spin coated on the substrates at 1000 and 4000 rpm for 10 and 40 s, respectively; 400 mL anhydrous ether was fast dripped on the substrate within 20 s. The substrates were then annealed at 60
C for 10 min and 100
C for 1 h. The spiro-OMeTAD precursor was prepared by adding 72.3 mg of spiroOMeTAD, 28.8 μL of 4-tert-butylpyridine, and 17.5 μL Li-TFSI (predissolved in acetonitrile at 520 mg mL1) to 2 mL of chlorobenzene and then stirred for 8 h. The spiro-OMeTAD precursor was spin coated on the substrates at 3000 rpm for 30 s before depositing the Au electrode.

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The ETLs (45 nm) were deposited by using the E-beam method. The substrates were transferred to the muffle furnace after deposition. Then, the perovskite films were deposited on the surface of the E-beamevaporated SnOx/Zn-SnOx through a one-step spin-coating method. Finally, the spiro-OMeTAD precursor was spin coated on the substrates at 3000 rpm for 30 s before depositing the Au electrode. Characterizations: UPS and XPS measurements were characterized by a scanning microprobe (PHI 5000 VersaProbe, Ulvac-PHI) using HeI (21.2 eV) and monochromator Al Kα (1486.6 eV). J–V performance was tested using a Class A solar simulator (ABET Sun 2000) and a Keithley 2400 under AM 1.5G illumination. The intensity of light was extremely approximate AM 1.5G. IPCE curves were measured using Solar Cell Scan100 (Zolix, Beijing) with a wavelength from 350 to 800 nm. XRD measurements for the prepared target were obtained using Rigaku D/max 2550 X-Ray with a scan rate of 15
min1. The surface morphology of SnO2/Zn-SnO2 films deposited by the E-beam method was obtained by a SIRION field-emission SEM.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the Key Program of NSFC-Guangdong Joint Funds of China (U1801253), the National Key Research and Development Program (2016YFC0207101), the National Natural Science Foundation of

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China (Grant Nos. 11874181, 11674126, 61822506, 61874049, 61775080, 11674127, 61674067).

Conflict of Interest The authors declare no conflict of interest.

Keywords electron beams, flexibility, low temperatures, perovskite solar cells, Zn-SnOx Received: June 9, 2019 Revised: July 28, 2019 Published online:

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