High-Working-Pressure Sputtering of ZnO for Stable and Efficient

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High-working-pressure Sputtering of ZnO for Stable and Efficient Perovskite Solar Cells Abhishek Thote, Il Jeon, Hao-Sheng Lin, Sergei Manzhos, Takafumi Nakagawa, Donguk Suh, Junho Hwang, Makoto Kashiwagi, Junichiro Shiomi, Shigeo Maruyama, Hirofumi Daiguji, and Yutaka Matsuo ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00105 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Applied Electronic Materials

High-working-pressure Sputtering of ZnO for Stable and Efficient Perovskite Solar Cells Abhishek Thote,† Il Jeon,† * Hao-Sheng Lin,† Sergei Manzhos,‡ Takafumi Nakagawa,† Donguk Suh,† Junho Hwang,† Makoto Kashiwagi,† Junichiro Shiomi,† Shigeo Maruyama,†,§* Hirofumi Daiguji†* and Yutaka Matsuo†,⊥* † Department of Mechanical Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Mechanical Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576, Singapore § Research Institute for Energy Conservation, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan ⊥ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China KEYWORDS perovskite solar cells, ZnO, sputtering, perovskite degradation, high working pressure

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ABSTRACT

Owing to its high mobility and low sintering temperature, ZnO is a promising electron-transporting layer for flexible and tandem applications of perovskite solar cells. However, ZnO inevitably triggers the degradation of perovskite materials. Such degradation can be inhibited when ZnO films with improved stoichiometry and minimized defects are used. In this work, a high efficiency with substantial stability of ZnO-based perovskite solar cells is achieved using a high-workingpressure-sputtering technique. The high-working-pressure-sputtering process can produce higher quality ZnO films with fewer surface defects compared with conventional sputtering or sol-gel ZnO solution processes. A power conversion efficiency of 17.3% is recorded. In addition, the stability of these devices is significantly higher than that of the conventional ZnO-based perovskite solar cells. This work showcases that ZnO can be a good candidate for the electron-transporting layer in perovskite solar cells, particularly for flexible and tandem applications when right sputtering conditions are used.

1. INTRODUCTION The photovoltaic community has witnessed a rapid development of perovskite solar cells (PSCs) in recent years, because of their high power conversion efficiency (PCE). As much as the perovskite light-absorbing layer in PSCs is important, the charge-transporting layers play an equally vital role.1 ZnO electron transporting layer (ETL) in PSCs has been investigated by many researchers as an alternative to the widely used TiO2.2–5 This is because ZnO requires a low sintering temperature, which is a prerequisite for printability on flexible substrates.6-8,56 Moreover, ZnO has electron mobilities between 200 and 300 cm2 V s-1, which are approximately one order


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of magnitude higher than those of TiO2.9–11 However, ZnO in PSCs has not been preferred over TiO2, because it leads to a faster degradation of the perovskite layer.3,12,13 It was observed that residual chemicals,14,15 adsorbed water molecules,16,17 ZnO interstitials,18,19 and the basicity of ZnO20–22 contribute to the degradation. To avoid this problem, several solutions, such as adding a layer on top of ZnO,14,23,24 doping with Al,25,26 and a two-step deposition method, have been proposed thus far.8,27 Nevertheless, these methods require numerous sequential fabrication steps, which is not ideal for achieving a facile PSC process.28 Amongst the proposed solutions, sputtering process of ZnO is deemed the most promising, and has already been demonstrated by number of researchers.29–31 Contrary to the general perception, ZnO sputtering does not increase the fabrication cost, because it is an extension of the indium tin oxide (ITO) deposition, which is also sputtered.32 Furthermore, sputtering does not require stabilizing ligands, which exacerbate the degradation of perovskite layer.14,15 However, sputtered ZnO still incorporate many defects on its surface, which leads to the formation of hydroxyl groups, triggering the degradation of the perovskite layer.33–35 Previously, we reported a novel sputtering method for generating ZnO films with high conductivity and crystallinity by controlling the working pressure inside the sputter chamber.32 High working pressure (HWP) during the sputtering reduced the energy of the high-energy particles, such as Ar, by controlling the bombardment energy, resulting in reduced recombination centers and fewer defects (Figure S1). Organic solar cells fabricated on this HWP-sputtered ZnO showed a significant increase in the device performance. In this work, we used HWP-sputtered ZnO films in PSCs and demonstrated that they improved both the efficiency and stability significantly. This work reveals that the HWP-sputtering improves the surface properties of ZnO


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which have a direct impact on the stability of the perovskite materials on top. Ultimately, a PCE of 17.3%, was obtained from the HWP-sputtered ZnO-based PSCs with high stability.

2. EXPERIMENTAL 2.1. ZnO preparation. Patterned indium-doped tin oxide substrates of dimensions 15 × 15 mm (Techno Print Co. Ltd., ~ 9 Ω sq.-1) were used for the experiments. They were sequentially cleaned with water, acetone, and 2-isopropanol for 15 min each, and subsequently subjected to UV/O3 treatment for 15 min. For sol-gel ZnO, a 0.1 M solution of zinc acetate dihydrate (Zn(CH3COO)2•2H2O) (Wako, 99.0%) in ethanol (Wako, 99.5%) was prepared and subsequently subjected to rigorous stirring for 2–3 h at 80 °C. Subsequently, an ethanolamine stabiliser (28% in weight) was added and the solution was stirred for further 12–15 h at 60 °C. Radio frequency (RF) magnetron sputtering was employed to obtain both conventionally sputtered ZnO and HWPsputtered ZnO. ZnOx ceramic target obtained from Kojundo Chemical Laboratory Co., Ltd was used for sputtering. The RF power used for the deposition process was 100 W. The sputter deposition time was varied proportionately for the sputtered and HWP-sputtered films to obtain a similar thickness. The deposition was performed in an environment of Ar and O2, where O2 partial pressure was maintained at 1% of the pressure of the gas mixture. The normal sputter deposition was performed at a pressure of 2.6 × 10-1 Pa, whereas the HWP-sputtering was performed at a pressure of 2.6 × 100 Pa. 2.2. Device Fabrication. The photoactive layer of MAPbI3 is prepared using a 1:1:1 molar mixture of MAI (TCI), PbI2 (TCI), and dimethyl sulfoxide (DMSO) (Sigma Aldrich) at 50 wt% in a nitrogen environment. The mixture was stirred without heating and left for complete dissolution. Subsequently, 50 µL of this solution was spin-coated on pre-annealed ZnO thin films at 4000 rpm


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for 30 s. Further, 0.1 mL of diethyl ether was deposited 7.5 s after the spin-coating process was initiated. The CH3NH3I.PbI2.DMSO adduct film was annealing on a hot plate at 100 °C for 10 min to obtain a brown-coloured perovskite thin film. To prepare the hole transport material solution, 80 mg spiro-MeOTAD, 15 µL stock solution of lithium bis(trifluoromethylsulphonyl)imide in acetonitrile (520 mg mL-1), and 22.5 µL 4-tert-butylpyridine were dissolved in 1 mL chlorobenzene. Subsequently, 15 µL of this solution was spin-coated on the perovskite layer to form the hole transport layer. Finally, ~50-nm-thick gold electrodes were deposited on the substrates via thermal evaporation in vacuum. 2.3. Characterizations. The J–V characteristics were measured using a software-controlled source meter (Keithley 2400 SourceMeter) under dark conditions and the simulated sunlight irradiation of 1 sun (AM 1.5G; 100 mW cm-2) using a solar simulator (EMS-35AAA, Ushio Spax Inc.) with a Ushio Xe short arc lamp 500. The source meter was calibrated using a silicon diode (BS-520BK, Bunkokeiki). SEM analysis of the perovskite films was performed using an S-4800 (Hitachi). The SEM and EDX measurements of ZnO films were performed using JSM-7001F (JEOL) and JED-2300F (JEOL), respectively. Shimadzu UV-3150 was used for the UV–vis–NIR measurement. The PL measurements were performed using JASCO Spectrofluorometer (FP8300). The valence band and Fermi levels measurements of ZnO films were performed using Riken Keiki PYS-A AC-2 and Kelvin probe spectroscopy in air (ESA), respectively. The photoemission measurements were performed using XPS (PHI5000, Versa Probe) with monochromatic Al Kα radiation. The water contact angle measurements were performed using a contact angle meter (DMo-501, Kyowa Interface Science Co., Ltd.). The substrates were annealed on a hot plate at 110 °C for 10 min before performing water contact angle measurements to


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evaporate the deposited vapor on the films. Transmission FTIR measurements were performed using Shimadzu IRAffinity-1S.

3. RESULTS AND DISCUSSION The faster perovskite degradation on ZnO is due to the intrinsic surface properties of ZnO. There are two aspects of discussion: the first aspect is the degradation through moisture, which is accelerated in the presence of ZnO. It is well known that CH3NH3PbI3 (MAPbI3) dissociates into CH3NH3I and PbI2 by water (1).16 CH3NH3I can further decompose into CH3NH2 and HI (2).22 Unlike TiO2, ZnO reacts with HI to produce ZnI2 and H2O, and consequently, the degradation is accelerated (3).36

𝐶𝐻3𝑁𝐻3𝑃𝑏𝐼3⇌𝐶𝐻3𝑁𝐻3𝐼 + 𝑃𝑏𝐼2

(1)

𝐶𝐻3𝑁𝐻3𝐼⇌𝐶𝐻3𝑁𝐻2 + 𝐻I

(2)

𝑍𝑛𝑂 + 2𝐻𝐼⇌𝑍𝑛𝐼2 + 𝐻2O

(3)

(4) 𝑍𝑛(𝑂𝐻)2 +4𝐶𝐻3𝑁𝐻2 ∙ 𝐻𝐼→[𝑍𝑛(𝐶𝐻3𝑁𝐻2 ∙ 𝐻𝐼)4]2 + +2𝑂𝐻 ― The second aspect is the defect sites on the surface of ZnO. The hexagonal wurtzite structure of ZnO account for empty octahederal sites that could accommodate Zn interstitials. According to the Kröger–Vink equation, defects formation mechanism on ZnO surfaces are given in equation •• 𝑋 𝑋 •• (5), (6), (7), (8) and (9). Here, 𝑍𝑛𝑋𝑍𝑛, 𝑍𝑛𝑋𝑖, 𝑉𝑋𝑍𝑛, 𝑍𝑛•• 𝑖 , 𝑂𝑂, 𝑉𝑂, 𝑉𝑂 , 𝑉′′ 𝑍𝑛, 𝑒′ and ℎ are neutral zinc

atom, neutral zinc interstitials, neutral zinc vacancy, positively charged zinc interstitials, neutral oxygen atom, neutral oxygen vacancy, negatively charged zinc vacancy, an electron and a hole, respectively. During the sputtering, high energy oxygen particles would enhance the ionization of these defects.59,60


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𝑍𝑛𝑋𝑍𝑛⇌𝑍𝑛𝑋𝑖 + 𝑉𝑋𝑍𝑛

(5)

𝑍𝑛𝑋𝑖⇌𝑍𝑛•• 𝑖 + 2𝑒′

(6)

𝑂𝑋𝑂⇌𝑉𝑋𝑍𝑛 + 𝑉𝑋𝑂

(7)

𝑉𝑋𝑂⇌𝑉•• 𝑂 + 2𝑒′

(8)

𝑉𝑋𝑍𝑛⇌𝑉′′𝑍𝑛 + 2ℎ•

(9)

CH3NH3I terminations on the surface of the perovskite films interact with ZnO chiefly by forming I–Zn bonds if there are uncoordinated Zn, Zn interstitials and anion vacancy sites. The deprotonation can result in methylammonium (CH3NH3+) next to Zn producing methylamine (CH3NH2) with the released protons being absorbed by vicinal oxygen or cation vacancies.13 This is not observed in the case of perovskite on TiO2.37 These uncoordinated Zn, Zn interstitials and oxygen vacancies lead to the formation of hydroxyl groups and Zn(OH)2 species, which can react with CH3NH3I to etch the ZnO film (4).38 Therefore, it is imperative that ZnO be free from both water and surface defects. As it is important to prevent the initiation of the perovskite degradation in ZnO-based PSCs, all the processes were performed inside a glove box. Further, all ZnO films were pre-annealed before the perovskite deposition to dry off even a trace amount of water molecules.17 Such pre-annealing of ZnO was crucial in obtaining high PCEs and improving reproducibility (Table S1). 10 min of pre-annealing was enough to obtain perovskite grains of sufficient size as ZnO induces a faster growth of perovskite crystals via the formation of a tetragonal phase.39–41 We prepared four different types of ZnO ETL films: two splits of sol-gel ZnO films annealed at 300 °C and 500 °C, sputtered ZnO films, and HWP-sputtered ZnO films (Figure 1a). The HWP-sputter process is briefly illustrated in Figure 1b. We also assessed the accelerated degradation by depositing


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CH3NH3PbI3 on these films and annealed them at 105 °C in air (Figure 1c). It was apparent even with the naked eye that the sol-gel ZnO at 300 °C showed the lowest stability, and the HWPsputtered ZnO showed the highest stability (Figure 1d). The instability of sol-gel ZnO was due to the high concentration of hydroxyl and acetate groups produced via the hydrolysis reaction.12–15 The sol-gel ZnO annealed at 500 °C was relatively more stable than the sol-gel ZnO annealed at 300 °C owing to evaporation of aminoethan-1-ol ligands as demonstrated using energy-dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) (Figure S2 and S3). ZnO annealed at 500 °C shows reduced amount of carbon and nitrogen according to EDX, which indicates the reduced amount of aminoethan-1-ol (Figure S2). Also, XPS shows that the carbon intensity decreased for the ZnO samples annealed at 500 °C, corroborating our hypothesis (Figure S3a). The stability difference between sputtered ZnO and HWP-sputtered ZnO was visibly unclear, hence UV-vis spectroscopy was used to differentiate them (Figure 1e and f). Both sputtered and HWP-sputtered ZnO substrates were subjected to high temperatures (100oC) and humidity (R.H ~ 40%) during the test. It is evident from the data that the absorption of CH3NH3PbI3 on sputtered ZnO was reduced after aging for 1h (Figure 1e), whereas the absorption of CH3NH3PbI3 on HWP-sputtered ZnO did not change for 1 h (Figure 1f). Understanding the stability difference between the sputtered ZnO and the HWP-sputtered ZnO is much more challenging. Therefore, detailed investigation of the interface of ZnO is necessary. As mentioned above, more surface defects can lead to a faster degradation of perovskite. Although both Sputtered and HWP-sputtered ZnO films had similar Zn/O ratio (Zn/OHWP ≈ 0.77, Zn/OSputter ≈ 0.79), XPS oxygen peaks and Zn peaks (Figure S3c) confirm that HWP-sputtered ZnO possesses fewer defects than the sputtered ZnO, which is consistent with our previous finding.32 The intensity of oxygen-deficient peak, Odef, at binding energy at ~531.1 eV,


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HWP representing oxygen vacancy sites, is reduced for the HWP-sputtered ZnO (OHWP def /OZn ≈ 0.68,

/OSputter ≈ 0.75 ) (Figure S3b). 57-58 Furthermore, the chemisorbed -OH peak at ~532.2 OSputter def Zn HWP Sputter eV, OOH, was lower for HWP-sputtered ZnO films (OHWP /OSputter ≈ 0.68 OH /OZn ≈ 0.59, OOH Zn

). The low intensity of OOH component indicate lack of non-stoichiometric OH for HWP-sputtered ZnO films. In addition, Zn 2p peaks for the HWP-sputtered ZnO films are shifted to a lower binding energy by 1.2 eV, which indicates that more Zn is bonded to oxygen, indicating fewer interstitials at the interface. To support this, we performed photoelectron yield spectroscopy (PYS) measurement for the HWP-sputtered ZnO and sputtered ZnO films. The data denotes that the HWP-sputtered ZnO films possess a higher valence level by approximately 0.3 eV compared with the sputtered ZnO films (Figure S4). We observed that the two films have different surface properties. This indicates that HWP-sputtered ZnO possesses fewer interstitials than conventionally sputtered ZnO as the oxygen non-stoichiometry is reported to downshift the valence edge.42-44


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Figure 1. a) Structural schematics of a PSC using ZnO as ETL and four types of ZnO films with different surface properties. b) Graphical depiction of the inside of the sputtering chamber. c) Illustration of the aging process. d) Degradation images of perovskite films on different ZnO films during the aging process. UV–vis absorption spectra of the perovskite films on e) sputtered ZnO and f) HWP-sputtered ZnO during the aging process.


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To enhance the observation of the interface properties of ZnO, we analyzed the ZnO surface before and after the interaction of ZnO with MAPbI3 (Figure 2a). By depositing and degrading MAPbI3 on ZnO, and thereafter washing it off using DMF, we could induce a greater difference between HWP-sputtered ZnO and sputtered ZnO (convention) surfaces. The interaction between ZnO and MAPbI3 is summarised and graphically depicted in Figure 2c. Water contact angle tests, fourier-transform infrared spectroscopy (FTIR), and XPS were used to observe the difference before and after the interaction. Water contact angle data show that sputtered ZnO is more hydrophilic than HWP-sputtered ZnO before the deposition of MAPbI3 (Figure 2b). Previous studies reported that water molecules are easily absorbed on zinc and oxygen vacancy sites.61-63 The dissociation of a water molecule on zinc oxides surface would probably require strong bonding between the oxygen in the water molecule and a cation site on the sputtered surface, as well as a short distance between this cation site and an anion site on the substrate, thus enabling hydrogen bonding to the surface which weakens the O-H bond in the molecule. The difference in the water contact angle increases after the interaction as both films become more hydrophilic. This can be explained by surface condition of ZnO. As more charged species on the ZnO surfaces attract polar water molecules via ionic interaction, sputtered ZnO with more defect sites such as uncoordinated Zn2+ and non-stoichiometric OH will be more hydrophilic than HWP-sputtered ZnO. The increased hydrophilicity of the films after the deposition and degradation of MAPbI3 indicates that there are more charged species generated from the interaction between ZnO and MAPbI3. This can be explained by equations (3) and (4), according to which hygroscopic ZnI2, Zn(OH)x, and [Zn(CH3NH2•HI)4]2+ are generated on the ZnO surface when MAPbI3 degrades.13 The sputtered ZnO became even more hydrophilic than HWP-sputtered ZnO after washing off MAPbI3 because


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more charged species were generated, corroborating our hypothesis that HWP-sputtered ZnO possesses a better interface, leading to slower degradation.

Figure 2. (a) Illustration of analysis before and after the deposition and washing off MAPbI3 from ZnO. b) Water contact angles of sputtered ZnO (red) and HWP-sputtered ZnO (blue) before (left) and after (right) the interaction of ZnO with MAPbI3. c) Graphical illustration of the interaction of ZnO with MAPbI3 before the contact (left) and during the degradation (right). VestaTM was used for the illustration. FTIR data of sputtered ZnO (red) and HWP-sputtered ZnO (blue) before (left) and after (right) the interaction of ZnO with MAPbI3.


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Transmission FTIR analysis provided chemical evidence for this phenomenon. Both the sputtered ZnO and HWP-sputtered ZnO on Si substrates have Zn–O peaks at 450 cm-1 (Figure 3a and b).40,45,46 After depositing and washing off MAPbI3, the peak corresponding to the Si–O peak from the Si substrates at 1050 cm-1 and at 1400 cm-1 intensified in the case of the sputtered ZnO ( Sputter HWP Sputter FTIRHWP 1050/FTIR1050 ≈ 0.80, FTIR1400/FTIR1400 ≈ 0.90 ) (Figure 3b). The enhancement of

the peak at 1050 cm-1 can be attributed to C–N and that at 1400 cm-1 can be attributed to C–H and O–H. Considering that the peak of the Si substrate should have the same intensity, we suspected that these peaks indicate the generation of [Zn(CH3NH2•HI)4]2+ and Zn(OH)x species. XPS was used to investigate the ZnO interface further. Figure S5 shows XPS Zn 2p3/2 peaks of sputtered ZnO and HWP-sputtered ZnO at a high resolution after calibrating their binding energies for comparison. We can observe that the full width half maximum of Zn 2p3/2 peak for sputtered ZnO Sputter is wider than HWP-sputtered ones (ZnHWP FWHM ≈ 1.9 eV, ZnFWHM ≈ 2.2 eV ), indicating the

existence of Zn species bonded to hydroxides.47 Figure 3c and d show Zn 2p3/2 peaks before and after the interaction with MAPbI3. We can observe that, in both sputtered ZnO and HWP-sputtered ZnO, the Zn peaks decreased due to presence of small amount of unwashed MAPbI3 on ZnO surface. However, the peak positions were different. The Zn 2p3/2 peak of sputtered ZnO was shifted to a slightly lower binding energy than that of HWP-sputtered ZnO by 0.4 eV (Figure 3c and 3d). This can be attributed to the fact that more [Zn(CH3NH2•HI)4]2+ complexes are formed for the sputtered ZnO due to the greater number of intrinsic defect sites (4). However, HWPsputtered ZnO mostly constitutes ZnI2 due to interaction with HI (3). The presence of [Zn(CH3NH2•HI)4]2+ complexes for sputtered ZnO can be further confirmed from I 3d3/2 peak, shifted to higher binding energies by ~2 eV (Figure S7). XPS O 1s peaks also point to the same


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conclusion (Figure S6, Figure 3e and f). Before the deposition of MAPbI3, sputtered ZnO shows a more prominent oxygen-deficient and chemisorbed -OH component of the oxygen peak (Figure HWP HWP Sputter 3e) than that of HWP-sputtered ZnO (OHWP /OSputter / def /OOH /OZn ≈ 0.62:0.52:1, Odef OH

≈ 0.75:0.59:1) (Figure 3f) (Figure S6). After depositing MAPbI3 and washing it off with OSputter Zn DMF, the chemisorbed -OH and oxygen-deficient components were enhanced in both the ZnO samples. However, the chemisorbed -OH component was much more intensified for the sputtered ZnO sample (Figure 3e) than that of the HWP-sputtered ZnO sample, indicating the byproducts formed in equation (4) due to OH- (Figure 3f). The actual effect of the difference between the two types of sputtered ZnO films can be observed using photoluminescence (PL). PL data in Figure S8 show that the perovskite on HWP-sputtered ZnO became quenched more than that on sputtered ZnO. In addition, HWP-sputtered ZnO PL spectrum is slightly blue shifted (~7 nm) with a Sputter narrower full width at half maximum (PLHWP FWHM ≈ 40.8 nm, PLFWHM ≈ 50 nm ). The blue shift

denotes passivation of the trap states on the perovskite surface as they lead to spontaneous radiative recombination at the surface.48 Moreover, the narrower FWHM indicates a reduction in shallow trap density at the interface between HWP-sputtered ZnO and the perovskite, revealing that there are fewer defects at the HWP-sputtered ZnO interface.49 ZnO films and the perovskite films on ZnO were also analysed using scanning electron microscopy (SEM). The sol-gel ZnO films showed wrinkles consisting of dense agglomerated regions (Figure S9a and b). However, sol-gel ZnO films annealed at 500 °C showed improved morphology with a flatter surface. Both the sputtered and HWP-sputtered films, on the other hand, showed much better morphologies (Figure S10a and b). It is worth noting that the sputtered films displayed some regions of delamination whereas the HWP-sputtered films possesseed a better film quality. We conjecture the difference came from high residual stresses generated during sputtering processes as it has been reported that


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the film quality is directly related to the sputtering pressure.64 Perovskite films deposited on various ZnO films were also analysed using SEM (Figure S9c and d, Figure S10c and d). According to the SEM images in Figure S9c and d, the films of MAPbI3 on sol-gel ZnO were of poor-quality films with many voids, among which the samples annealed at 500 °C appeared slightly better. The MAPbI3 films on sputtered ZnO exhibited high film quality and were free of voids; the grain size of the perovskite on HWP-sputtered ZnO was marginally larger than that of sputtered ZnO (Figure S10c and d). This can be ascribed to the hydrophobicity of HWP-sputtered ZnO (Figure 2b) as a more hydrophobic substrate results in larger grain size.50


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Figure 3. a) FTIR data of sputtered ZnO (red) and HWP-sputtered ZnO (blue) b) FTIR data of sputtered ZnO (red) and HWP-sputtered ZnO (blue) after the interaction of ZnO with MAPbI3. XPS Zn 2p3/2 peaks of c) conventionally sputtered ZnO and d) HWP-sputtered ZnO before (solid line) and after (dotted line) the interaction of ZnO with MAPbI3. XPS O 1s peaks of e) conventionally sputtered ZnO and f) HWP-sputtered ZnO before (solid line) and after (dotted line)


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the interaction of ZnO with MAPbI3. e) Graphical illustration of the interaction of ZnO with MAPbI3 before the contact (left) and during the degradation (right). VestaTM was used for the illustration.

After investigating the surface properties of different ZnO films, we fabricated PSCs using these ZnO films and measured their photovoltaic performances (Table S2). As expected, the sol-gel ZnO-based PSCs exhibited much lower PCEs than the sputtered ZnO-based PSCs, although the sol-gel ZnO annealed at 500 °C exhibited higher PCEs than that annealed at 300 °C. The HWPsputtered ZnO ETL-based PSCs exhibited a much higher PCE of 17.3% than the sputtered ZnO ETL-based PSCs (15.7%) (Figure 4a, b, c, d, and e). Overall, the increases in all of the three photovoltaic parameters enhanced the PCE. We suspect that the higher short-circuit current density (JSC) and fill factor (FF) was due to larger grain size and higher conductivity owing to better oxygen stoichiometry (Figure 4a, c).51–53 The slight increase in open-circuit voltage (VOC) was probably due to the difference in ZnO interstitials18 and surface trap sites54 (Figure 4b). These surface trap sites induces recombination at the interface, resulting into lower device performance. This is also linked to the lower hysteresis of the HWP-sputtered ZnO ETL-based PSCs (Table S2).55 Moreover, the stability of the unencapsulated HWP-sputtered ZnO ETL-based PSCs in air was much higher than that of the sputtered ZnO ETL-based PSCs in the same condition (Figure 4f). The outcomes are reasonable considering the difference between surface properties of the two types of sputtered ZnO samples. The HWP-sputtered ZnO does not trigger the perovskite degradation as fast as the sputtered ZnO. The PCE of 17.3% obtained for the HWP-sputtered ZnO ETL-based PSC, which is one of the highest among the reported PCEs for single compact ZnO ETL-based PSCs.8


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Figure 4. Statistical analysis of the sputtered ZnO ETL-based PSCs (red diamonds) and the HWPsputtered ZnO ETL-based PSCs (blue squares) illustrating (a) JSC, (b) VOC, (c) FF, and (d) PCE distribution. (e) J–V curves under forward bias of PSCs using a sputtered ZnO (red circles) and HWP-sputtered ZnO (blue circles). (f) Stability test result of unencapsulated PSCs using a sputtered ZnO (red diamonds) and HWP-sputtered ZnO (blue squares) under constant illumination in air (20 °C, 30%).


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4. CONCLUSIONS In conclusion, ZnO films produced via a HWP-sputtering process possessed minimal defects. Accordingly, high-performance and stable ZnO-based PSCs were realised. The surface property of the ZnO films was analysed from various perspectives. In addition, the relationship between the surface property and the device performance was investigated by examining the interaction between ZnO and MAPbI3 during the degradation. As the sputtering process is used for the deposition of indium tin oxide electrode, the sputtering of ZnO is as effective as the solution process in terms of the fabrication cost and large-size application. In fact, as evidenced by our work, HWP-sputtered ZnO can be used to produce high-performance and highly stable ZnO ETLbased PSCs. As ZnO plays a vital role in the tandem application of PSCs, our next step in this research is to apply HWP-sputtering to silicon-perovskite tandem solar cells.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. HWP sputtering illustration; PCE of pre-annealed ZnO films; EDX analysis of sol-gel ZnO films; XPS analysis of sputtered films before and after the deposition and washing off MAPbI3; PYS analysis; PL quenching of MAPbI3 on sputtered and HWP-sputtered ZnO film; SEM analysis of ZnO and perovskite films; phototovoltaic performances of PSCs (PDF) AUTHOR INFORMATION Corresponding Author


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*I.J. [email protected] *S.M. [email protected] *H.D. [email protected], *Y.M. matsuo@ photon.t.u-tokyo.ac.jp Author Contributions A.T. and I.J. contributed equally. Notes The authors declare no competing financial interests. There are no conflicts to declare. ACKNOWLEDGMENT We gratefully acknowledge the Research and Education Consortium for Innovation of Advanced Integrated Science by Japan Science and Technology (JST) and Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Numbers JP15H05760, JP16H02285, 17K04970, and 17H06609. REFERENCES (1) Yu, Z.; Sun, L. Recent progress on hole‐transporting materials for emerging organometal halide perovskite solar cells. Adv. Energy Mater. 2015, 5, 1500213. (2) Son, D. -Y.; Im, J. -H.; Kim, H. -S.; Park, N. -G. 11% Efficient perovskite solar cell based on ZnO nanorods: an effective charge collection system. J. Phys. Chem. C 2014, 118, 16567– 16573.


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