perovskite interface with oxygen plasma treatment

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Performance Enhancement of Perovskite Solar Cells via Modification of the TiO2 /Perovskite Interface with Oxygen Plasma Treatment Yumeng Wang , Dongdong Wang , Hao Qu , Jiushan Cheng , Yi Fang , Chunmei Zhang , Qiang Chen PII: DOI: Reference:

S0040-6090(20)30003-1 https://doi.org/10.1016/j.tsf.2020.137786 TSF 137786

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Thin Solid Films

Received date: Revised date: Accepted date:

7 May 2019 22 December 2019 3 January 2020

Please cite this article as: Yumeng Wang , Dongdong Wang , Hao Qu , Jiushan Cheng , Yi Fang , Chunmei Zhang , Qiang Chen , Performance Enhancement of Perovskite Solar Cells via Modification of the TiO2 /Perovskite Interface with Oxygen Plasma Treatment, Thin Solid Films (2020), doi: https://doi.org/10.1016/j.tsf.2020.137786

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Highlights 

The TiO2 layer was fabricated via atomic layer method at low temperature.

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The wettability of TiO2 surface was suitably tailored by oxygen plasma.

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Charge recombination was suppressed at the interface of perovskite/TiO2.

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Oxygen plasma treatment improved the performance of the devices.

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Performance Enhancement of Perovskite Solar Cells via Modification of the TiO2/Perovskite Interface with Oxygen Plasma Treatment Yumeng Wang a, b, Dongdong Wang b,*, Hao Qu a, b, Jiushan Cheng a, b, Yi Fang c, Chunmei Zhang a, b,*, Qiang Chen a, b a

Lab of Plasma Physics and Materials, Beijing Institute of Graphic Communication, Beijing 102600,

China b

Beijing Key Laboratory of Printing & Packaging Materials and Technology, School of Printing and Packaging Engineering, Beijing 102600, China

c

Beijing Engineering Research Center of Printed Electronics, Beijing Institute of Graphic

Communication, Beijing 102600, China Abstract High-temperature (~500 oC) sintering is a routine post-treatment to fabricate compact Titanium dioxide (TiO2) electron selective layer (ESL) in high-performance perovskite solar cells (PSCs). Here, we propose an effective low-temperature (<150 oC) approach: pinhole-free and compact ultrathin TiO2 layers are fabricated via thermal atomic layer deposition (ALD) following a low-pressure oxygen plasma treatment. The hydrophilicity of TiO2 surfaces can be suitably tailored, which favors precursor solution being well spread and the growth of perovskite film. In addition, effective removal of oxygen vacancies serving as unproductive quenching sites in TiO2 surface is observed, which is attributed to atomic oxygen radical, the most reactive species generated in oxygen plasma. The PSC device with oxygen-plasma-treated ALD-TiO2 ESL achieves an efficiency of 14.9%, which is better than 13.3% of the device with 500 oC- sintered ALD-TiO2 ESL and comparable to 14.3% of the device with conventional 500 oC-sintered Sol-gel-TiO2 ESL under our conditions. Keywords

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Atomic layer deposition, oxygen plasma, atomic oxygen radical, perovskite solar cell, low temperature processing Introduction In the last few years, organic-inorganic hybrid perovskite solar cells (PSCs) have received tremendous interests as a promising next-generation solar photovoltaic technology because of their high power conversion efficiencies (PCEs) and low-cost. The PCE has substantially increased to 23.7% after several years of active research [1-3]. Solution processability and low-temperature (<150 o

C) fabrication are key advantages, which enable PSC technology cost-competitive and compatible

with the flexible and wearable electronic devices. A typical PSC is composed of a transparent conductive oxide (TCO) (indium tin oxide or fluorine-doped tin oxide (FTO)) electrode, a thin electron selective layer (ESL), a perovskite photoactive layer, a hole transporting layer and a metal contact. The perovskite absorbs incident light and efficiently converts photons into charge carriers. The photogenerated electrons and holes coexist in perovskite, and then electrons can be injected into ESL (with subsequent transport to TCO electrode) and holes into hole-transport layer (HTL) (with subsequent transport to metal electrode). Collecting and transporting electrons and blocking holes effectively through ESL, which can facilitate electron extraction from the perovskite and suppress electron-hole recombination near TCO electrode, are crucial to the performance of the PSCs [4-12]. Titanium dioxide (TiO2) is the most common choice as ESL material due to its transparency across the visible spectrum, favorable electron mobility, appropriate band structure and photochemical stability [13-16]. Various techniques have been used to prepare TiO2 ESL in PSCs, such as sol-gel method, radio frequency (RF) magnetron sputter, pulsed laser deposition, spray pyrolysis and chemical vapor deposition. Most of these approaches rely on a routine high temperature treatment to improve crystallization and conductivity of TiO2 film. Besides high temperature, relatively thick film (>30 nm) is always required to prevent pinholes in crystalline TiO2 film, which is to avoid leakage current generated by direct contact between perovskite active layer and TCO electrode but result in resistance to collect the electrons. Recently, atomic layer deposition (ALD) technique has been introduced to manufacture TiO2 ESL in PSC [17-19]. The technique can deposit a pinhole-free,

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uniform and conformal ultrathin layer with precisely-controlled thickness at low temperature. However, the wettability of TiO2 surface is affected by ALD process parameters such as the deposition temperature as well as the deposition thickness [20]. The surfaces of TiO2 films deposited at lower temperature are hydrophobic, which may hinder perovskite/TiO2 interfacial contact and result in the relatively low device PCE. The growth of perovskite film is sensitive to the substrate surface properties, and perovskite/TiO2 interfacial engineering has been identified as an efficient approach to improve device performance. A lot of treatment techniques [18, 21-32], such as titanium tetrachloride, graphene quantum dots, amino acids, thiols and carboxyl groups, silane monolayers, self-assembled fullerene monolayers, and UV-O3 treatments have been employed to modify TiO2 surface. Oxygen plasmas at low pressure are of great interest in plasma processing such as surface modification and activation. In this work, we employed a facile room-temperature low-pressure oxygen plasma treatment to modify the thermal-ALD TiO2 surface, and used the treated ultrathin TiO2 film as ESL in the planar halide PSCs. The hydrophilicity of TiO2 surface was found can be properly controlled, which favored the spread of precursor solution and the growth of perovskite film. In addition, atomic oxygen radical generated in oxygen plasma effectively removed oxygen vacancies serving as the charge carrier quenching sites in TiO2 surface. The PSC device with oxygen-plasma-treated ALD-TiO2 ESL achieved an efficiency of 14.9%, which was better than 13.3% of the device with 500 oC-sintered ALD-TiO2 ESL and comparable to 14.3% of the device with conventional 500 oC -sintered Sol-gel-TiO2 ESL under our conditions. Our studies confirmed that appropriate oxygen plasma treatment onto ALD-TiO2 surface can improve the TiO2/perovskite interfacial contact, facilitate charge extraction and suppress charge recombination. The results suggested a simple and effective route to modify the ALD-TiO2 ESL surface for better device performance, and more importantly, all the process can be conducted under low temperature. Experimental section Preparation of TiO2 electron selective layer: Patterned FTO electrodes were cleaned ultrasonically in acetone, isopropanol, ethanol and deionized water sequentially for 15 min, respectively, then dried by N2 stream and exposed to ultraviolet light for 15 min. For ALD deposition, titanium isopropoxide

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and H2O were used as precursor and oxidant, respectively. The deposition temperature was set at 140 ºC, and N2 gas with 99.999 % purity was used as the purge gas. The thickness of the TiO2 film was controlled by the number of reaction cycles, and the growth rate was 0.034 nm/cycle. For comparison, the Sol-gel TiO2 was prepared by adding the titanium isopropoxide into the ethanol. The TiO2 layer was coated by spin-coating method with a speed of 2000 rpm for 50 s, and then the film was thermally annealed at 500 oC for 30 min. Oxygen plasma treatment: Capacitively coupled plasma was used to treat the ALD-TiO2. The vacuum chamber was pumped down to a base pressure of 3 Pa after the TiO2/FTO substrates were transferred into the chamber. Then oxygen gas was introduced into the chamber via a mass flow controller. After the pressure stabilized at 20 Pa, the oxygen plasma was generated by the 13.56 MHz RF power source, and the applied RF power was varied from 10 to 25 W. The time of plasma treatment was 30 s. Perovskite solar cell fabrication: The (FAPbI3)0.85(MAPbBr3)0.15 precursor solution was prepared in a glovebox, which consisted of FAI (1 M), PbI2 (1.1 M), MABr (0.2 M) and PbBr2 (0.2 M) in anhydrous dimethyformamide:dimethylsulfoxide 4:1 (V:V). We employed vacuum-flash assisted solution processing method to prepare the perovskite layer. The spin coating procedure was done by a two-step spin-coating process at 1000 rpm for 10 s and 4000 rpm for 10 s. After that, the sample was put into a chamber connected to a vacuum-pumping instrument. The perovskite film was exposed to the low pressure at 20 Pa for 10 s, followed by introduced ambient air into the chamber. Subsequently the substrate was annealed at 150 °C for 10 min. The HTL was deposited on top of the perovskite film by spin-coating at 4000 rpm for 30 s using a chlorobenzene solution, which contained 65 mg/mL of Spiro-OMeTAD, 20 µL/mL of tert-butylpyridine, and 70 µL/mL of bis(trifluoromethane) sulfonimide lithium salt (170 mg/mL in acetonitrile). Then Au electrode was deposited on Spiro-OMeTAD by thermal evaporation. The area of each cell is 4.5 mm2. Device Characterization: The cross-sectional view image of the complete PSC device and the top-view images of the perovskite films were taken using a field emission scanning electron

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microscope (SEM) (SU8020, Hitachi). The atomic force microscopy (AFM) was employed in tapping mode to characterize the TiO2 and perovskite surface morphology (DI-INNOVA, Veeco). The hydrophilicity of TiO2 surfaces was investigated by water contact angle measurement (DSA 100, KRÜSS). Optical emission spectra (OES) of the gas discharge plasma were monitored with a fiber spectrometer (FLAME-S, Ocean Optics). The surface chemical composition of pristine and plasma treated TiO2 were characterized by X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250 Analysis System (Thermo Scientific) with an Al Kα (1486.6 eV) monochromatic X-ray source (150 W, 20 eV pass energy, 500 μm2 spot size). The samples were analyzed after exposure to ambient air but without any pre-sputter cleaning. The base pressure in the analysis chamber was better than 5 × 10−8 Pa. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon contamination. XPS spectra were analyzed using XPSPEAK 4.1 software. The film crystallinity was analyzed by X-ray diffraction (XRD) (D/max-2200 PC, Rigaku) using Cu Kα radiation over a 2Ɵ range from 10° to 60°, with a scan rate of 5°/ min and a step size of 0.05°. The current density-voltage (J-V) characteristics were gained with a Keithley 4200 source meter under the simulated light intensity of 100 mW/cm2 (ABET Technologies AM1.5). Steady-state photoluminescence (PL) spectra were measured with a spectrofluorophotometer (RF-5301PC, SHIMADZU) using a 470 nm excitation line. The external quantum efficiency (EQE) measurement was conducted by using a Zolix test system (Scan100). Results and discussion We fabricated a series of PSC devices in the FTO/TiO2/Perovskite (600nm)/Spiro-OMeTAD (200 nm)/Au (80 nm) configuration by varying TiO2 ESL layer: thermal ALD-deposited without and with oxygen plasma treatment (20 nm), thermal ALD-deposited with 500 oC post-annealing (20 nm), and conventional Sol-gel-deposited with 500 oC post-annealing (~40 nm). The cross-sectional SEM image of a PSC device with the ALD-TiO2 ESL was shown in Figure 1. Uniform and compact ALD-TiO2 layer clearly appeared between FTO and perovskite layers. Due to the excellent film conformity and uniformity [16, 17], the TiO2 thin layer presented a homogenous and full-range coating, which replicated the surface morphology of the underlying FTO layer.

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The OES spectrum recorded during plasma treatment was shown in Figure 2(a). In the spectral range from 200 to 900 nm, the spectrum was dominated by the two atomic oxygen lines at 777 and 844 nm, corresponding to the oxygen atom transition O(3p 5P → 3s5S) and O(3p3P → 3s3S), respectively [33, 34]. Two main excitation mechanisms had been reported for the atomic oxygen emission: [35, 36] (1) electron impact excitation of ground state oxygen atoms, and (2) dissociative excitation of oxygen molecules,

,

. Direct atomic excitation

was regarded as dominant for O* emission at 844 nm whereas dissociative excitation was more important for O* emission at 777 nm. It’s interesting that the intensity of two O* peaks showed different tendency with varying the applied power (Figure 2(b)). Increasing the power from 10 W to 25 W, the intensity of O* emission at 777 nm increased monotonically. However, the intensity of O* emission at 844 nm increased at first, reached its maximum at 15 W, and then declined. The atomic oxygen emitting intensity depends on the population of the relevant excited states and not directly on the density of atomic oxygen at the ground state. The actinometry results showed that with increasing the applied power, electron dissociative excitation with the oxygen molecules (for 777 nm emission) kept growing, while electron direct excitation with the ground state oxygen atoms (for 844 nm emission) increased at first, reached peak at 15 W and then declined. The increased dissociative excitation can be well understood since that electron density and electron temperature would both rise with increasing power, and oxygen molecules kept serving as the source of 777 nm emission. On the other hand, the declined direct atomic excitation after 15 W may arise from the exothermic attachment of one-electron and neutral atomic oxygen, O + e→ O-, because of the big negative electron affinity of neutral oxygen atom (~1.46 eV) [37, 38]. Higher electron density with increasing power may consume more neutral oxygen atoms, which caused a reduction in atomic direct excitation between electrons and oxygen atoms. Low-temperature oxygen plasma treatment at low-pressure is widely used in surface modification and activation, e.g. cleaning the surface of TCO substrates in organic light-emitting diode and organic photovoltaic cell devices [39-41]. As shown in Figure 3, the static contact angle of deionized water, which was 69.9° on the as-grown ALD-TiO2 surface, decreased to 20.7° on the 15 W plasma-treated and 17.6° on the 25 W plasma-treated ALD-TiO2 surfaces. Oxygen plasma treatment

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effectively tailored the hydrophilic properties of ALD-TiO2 surface. Meanwhile, carbon contamination on TiO2/FTO surface was not significantly altered from the XPS analysis. Same tendency of the TiO2 surface wettability change was reported by using UV/O3 treatments and explained by TiO2-mediated photo-oxidation [11, 41-43]. Electron-hole pairs were generated upon the incidence of photons with higher energy than the bandgap of TiO2 (TiO2 + hν → e- + h+), diffused to the surface and reacted with adsorbed molecules such as O2, H2O, generating OH* radicals (h+ + H2O → OH* + H+) which altered the surface hydrophilicity. During oxygen plasma treatment, neutral atomic oxygen had been recognized as the most reactive specie with strong oxidability. The high-energy atmosphere oxidized the TiO2 surface directly, brought about hydrophilic functional groups such as –OH and –O, and resulted in the enhanced wettability [34, 44-48]. The surface morphologies of TiO2/FTO substrates and perovskite films deposited on TiO2/FTO substrates were studied by AFM. As shown in Figure 4, although the surface topographical properties of TiO2/FTO substrates weren’t obviously changed by oxygen plasma treatment, the altered hydrophilicity of TiO2/FTO surface affected the growth of perovskite film significantly. The maximum height of the roughness profile R z (the sum of the maximum value of profile peak height on the profile curve and the maximum value of profile valley depth in a sampling length) was 78.9 nm for the pristine TiO2/FTO surface, 58.6 nm after 15 W-oxygen-plasma treatment and 68.4 nm after 25 W-oxygen-plasma treatment. The root mean square roughness (Rq) of TiO2/FTO surface changed from 49 to 28.8 and 30.4 nm accordingly. However, the Rz of above-covered perovskite film decreased from 345.2 nm (on pristine TiO2/FTO) to 173.7 nm (on 15 W-oxygen-plasma-treated TiO2/FTO) and 184.8 nm (on 25 W-oxygen-plasma-treated TiO2/FTO). In addition, the Rq of perovskite surface changed from 49 nm to 21 nm and 30 nm. Oxygen plasma is chemically reactive, in which atomic oxygen radical has been recognized as the most reactive specie. With 15 W-oxygen-plasma treatment, chemical etching by monatomic oxygen, the so-called plasma ashing, may remove the residual organic species (remnants of ligands incompletely removed during the low-temperature thermal ALD process), and thus reduce the roughness of ALD TiO2 surface. When the power increased to 25 W, as the actinometry results shown in Figure 2(b), more neutral oxygen

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atoms may be consumed by exothermic attachment with higher electron density. On the other hand, higher plasma density and electron temperature brought about increased bombardment of the ALD TiO2 surface with ions, such as O -, and the stronger physical etching increased the surface roughness. Figure 5 showed the top-view SEM images of the perovskite films. It was found that polydisperse micrometer-sized perovskite islands with poor surface coverage were loosely distributed on the untreated TiO2/FTO substrate. After 15 W-oxygen-plasma treatment, dense and pinhole-free perovskite film homogenously covered on the TiO2 ESL. However, with 25 W-oxygen-plasma treatment, several nanometer-sized pinholes appeared. The residual organic species on as-grown ALD-TiO2 surface, mainly remnants of ligands incompletely removed during the low-temperature ALD process, can result in somewhat hydrophobic [19, 20]. The hydrophobicity was detrimental to the formation of above-covered film and may result in rough perovskite film with poor coverage. After exposed to 15 W oxygen plasma, the hydrophilicity and roughness of TiO2 surface were improved effectively, which favored precursor solution being well spread and the growth of perovskite film significantly [13, 18]. However, 25 W-oxygen-plasma treatment may increase the surface roughness, which brought about pin-holes in perovskite film. To further study the impact of oxygen plasma treatment on the growth of perovskite film, XRD patterns were compared in Figure 6. Based on the (001) peak broadening of the reflection at 14.1°, the average crystallite size can be estimated using the Scherrer equation. While 15 W-oxygen-plasma treatment on TiO2/FTO substrates increased the crystallites from ~229 nm to ~257 nm, 25 W-oxygen-plasma treatment deteriorated the crystallite to ~232 nm. The XRD results were consistent with the surface morphology characterization, which showed that properly tailored surface hydrophilicity via oxygen plasma treatment onto TiO2/FTO substrate was crucial to the growth of above-covered perovskite film. High-resolution XPS spectra of O1s were obtained to analyze the chemical composition of ALD-TiO2 surface (Figure 7). O1s core level spectra were fitted with peaks at 529.9 eV (O I), 530.3 eV (OII) and 531.6 eV (OIII) [41, 49, 50]. The OIII peak at 531.6 eV was related to O-H, O-C or O-O bonds adsorbed on the TiO2 surface, the OII peak at 530.3 eV was corresponded to the

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oxygen-deficient in the TiO2 matrix, and the OI peak at 529.9 eV was relevant to the oxide lattice without oxygen vacancies. As shown in Figure 7, the intensity of peak OII distinctly decreased relative to the intensity of peak OI after oxygen plasma treatment, which indicated the decrease of oxygen-deficient concentration. The ratios of OII/OI decreased from 1.07 to 0.75 and 0.79 after the samples were treated with 15 and 25 W plasma, respectively. This suggested that oxygen vacancies are effectively reduced by oxygen plasma treatment. Oxygen plasma produced the atomic oxygen radical, and Ti3+ was active site because of its unsaturated coordination. As a result, atomic oxygen radical was expected to diffuse and bond with Ti3+，ameliorating the oxygen vacancies and lowering the oxygen vacancy concentration at the surface of TiO2. The decrease of electronic trap sites was expected to facilitate the transfer of electrons and benefit the perovskite solar cell performance [41]. It was worthy to note that with increasing the plasma power from 15 to 25 W, no significant difference was observed in lowering the concentration of oxygen vacancy. Similar to the tendency of direct atomic excitation (844 nm emission) change, this may be due to the increased consumption of radical species via attachment of one-electron and neutral atomic oxygen as well. The kinetics of charge transfer at perovskite/TiO2 interfaces, and external quantum efficiency (EQE) data, J-V curves and statistical PCE values of the corresponding PSC devices with the structure of FTO/ESL/perovskite/HTL/Au were shown in Figure 8. Consistent with other research [51], the steady-state photoluminescence (PL) spectra of perovskite films exhibited peaks at 770 nm. The significant PL quenching was observed for the perovskite film deposited on 15 W-oxygen-plasma-treated substrate. For the perovskite film deposited on 25 W-oxygen-plasma-treated TiO2/FTO substrate, PL intensity increased inversely (Figure 8(a)). PL quenching results meant that the charge transferred at perovskite/TiO2 interface can be more efficient by suitable modification of TiO2 surface with oxygen plasma treatment, indicating the effective suppression of charge recombination at the surface of TiO2. In agreement with the above study of charge transfer kinetics at perovskite/TiO2 interfaces, the integrated current densities from the EQEs were 19.0 (as-grown ALD-TiO2 ESL), 20.0 (15 W-oxygen-plasma-treated ALD-TiO2 ESL), and 19.4(25 W-oxygen-plasma-treated ALD-TiO2 ESL) mA/cm2 (Figure 8(b)). The derived values of photovoltaic performance parameters from J-V curves (Figure 8(c)) were summarized in Table 1.

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With the as-grown ALD-TiO2 ESL, the device showed an open-circuit voltage (VOC), a short-circuit photocurrent density (JSC), and a fill factor (FF) of 1.04 V, 19.11 mA/cm2 and 0.64, respectively, leading to a PCE of 12.7%. For the device with 15 W-oxygen-plasma-treated ALD-TiO2 ESL, The PCE reached 14.9% with a VOC of 1.05 V, a JSC of 20.18 mA/cm2 and a FF of 0.70. Further increasing applied power deteriorated the device performance. Examining the correlation among actinometry OES measurement, wettability and chemical composition of ALD-TiO2 surface, surface morphology, roughness and crystallinity of above-covered perovskite film, PL spectra and PSC device performance, we can find that atomic oxygen generated in oxygen plasma was the essential radical species for the surface modification of ALD-TiO2 and performance enhancement of perovskite solar cells. The hydrophilicity of TiO2 surfaces can be suitably tailored, which favored precursor solution being well spread and the growth of perovskite film. Combined with the effective removal of oxygen vacancies serving as unproductive quenching sites, efficient charge extraction and suppressed charge recombination facilitated a better device performance. The deterioration of device performance with further increasing power may be caused by the following factors: oxygen plasma caused slow etching which may degrade the surface morphology of TiO2, and downward shift of the Fermi level of TiO2 may worse the energy level matching at perovskite/TiO2 interface [34, 52-53]. The evolution of photovoltaic performance of PSC devices with as-grown and 15 W oxygen-plasma-treated ALD-TiO2 ESLs was shown in Figure 9. The devices were stored in ambient (Relative humidity 30±5%, Room Temperature 25 oC) without encapsulation. It has been reported that the introduction of hydrophobic top-covered ESL can benefit PSCs ambient stability significantly by preventing water penetration [54]. In our case, since the ALD-TiO2 ESL was placed at the bottom of PSC device, the improved hydrophilicity of TiO2 surface did not caused additional water/oxygen absorption. After 8 hour exposure, PCE of the PSC device with plasma-treated ALD-TiO2 ESL decreased from 14.3% to 13.1%, and that of the device with as-grown ALD-TiO2 ESL decreased from 12.9% to 12.1% (Figure 9). No significant differences in PCE stability were observed.

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High temperature annealing of TiO2 film in air or oxygen was the common route to improve crystallization and remove oxygen vacancy. It was noteworthy that ALD-TiO2 ESL with room temperature oxygen plasma treatment can achieve better performance than ALD-TiO2 ESL with 500 o

C annealing, and comparable performance to 500 oC-sintered Sol-gel conventional TiO2 ESL

(Figure 8(c)). Since oxygen plasma treatment cannot induce crystallization as 500 oC-annealing did, the performance improvement mainly came from the interface effect other than bulk effect. Other oxidizing atmospheres, such as UV-O3 and O3, had been explored before [19]. As was pointed out by Mercouri et al., UV-O3 had slightly less effect on the performance improvement than 500oC-annealing, while O3 had no effect on the performance improvement. Atomic oxygen generated in room-temperature oxygen plasma exhibited much more reactive than UV-O3 and O3. The results suggested that oxygen plasma treatment was a simple, fast, effective route to modify the ALD-TiO2 ESL surface in a PSC for better device performance. Conclusion In conclusion, ALD is a powerful deposition tool for preparing pinhole-free, uniform and conformal ultrathin layer with precisely-controlled thickness at low temperature. However, the hydrophobic surface (organic remnants of ligands) and poor crystallinity (amorphous phase thin-film) under low-temperature thermal-ALD process often limits device performance. In this work, we demonstrated a facile room-temperature processing route: using oxygen plasma to tailor surface hydrophilicity and ameliorate oxygen vacancies. The PSC device with oxygen-plasma-treated ALD-TiO2 ESL achieved better performance than that with 500 oC-sintered ALD-TiO2 ESL and was comparable to the device with conventional thicker 500 oC-sintered Sol-gel-TiO2 ESL under our conditions.

Author contributions Yumeng Wang: Investigation, Data Curation

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Dongdong Wang: Conceptualization, Methodology, Supervision, Writing - Original Draft, Review & Editing

Hao Qu: Investigation

Jiushan Cheng: Validation, Formal analysis

Yi Fang: Investigation, Validation, Formal analysis, Visualization

Chunmei Zhang: Conceptualization, Methodology, Supervision, Writing - Review & Editing

Qiang Chen: Supervision

Declaration Of Interest Statement: The authors declare no conflict of interest.

Acknowledgements Project supported by the National Natural Science Foundation of China (Grant Nos. 11605012, 21604005, 51978372, 61474144, 61705003), the Education Ministry for Returned Chinese Scholars (Grant No. 10000200300), Beijing Municipal Education Commission (Grant No. KM201610015007, KM201710015011, KM201810015001, CIT&TCD201904050, KM201910015010), Initial Funding of BIGC (Grant Nos. 09000114/129, 27170115006), the collaborative innovation center of green printing & publishing technology (No. 20160113). Dongdong Wang and Chunmei Zhang would like to thank Prof. Lijuan Liang, Jiazi Shi, and Jianghao Liu for the sincere help and fruitful discussions. References

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Figure captions

Figure 1. Cross-sectional SEM image showing a general PSC device architecture based on FTO/ALD-TiO2/Perovskite/Spiro-OMeTAD/Au.

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Figure 2. (a) OES spectrum of the oxygen plasma at a pressure of 20 Pa and an applied power of 15 W. The inset optical image shows an ALD-TiO2/FTO substrate being exposed to the oxygen plasma. (b) Relation between applied RF power and intensity of atomic oxygen lines at 777nm and 844nm, respectively.

Figure 3. Static contact angles of deionized water on (a) as-grown, (b) 15 W, and (c) 25 W oxygen-plasma treated ALD-TiO2 surfaces.

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Figure 4. AFM images of (a) as-grown, (b) 15 W, and (c) 25 W oxygen-plasma-treated ALD-TiO2/FTO surfaces; AFM images of perovskite films deposited on (d) as-grown, (e) 15 W, and (f) 25 W oxygen-plasma treated ALD-TiO2/FTO substrates.

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Figure 5. Top-view SEM images of perovskite films deposited on (a) as-grown, (b) 15 W, and (c) 25 W oxygen-plasma-treated TiO2/FTO substrates. The scale bars are different in the corresponding upper and lower panels.

Figure 6. XRD patterns of perovskite films deposited on (a) as-grown, (b) 15 W, and (c) 25 W oxygen-plasma-treated ALD-TiO2/FTO substrates.

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Figure 7. XPS O 1s spectra of (a) as-grown, (b) 15W, and (c) 25 W oxygen-plasma-treated TiO2/FTO surfaces.

Figure 8. (a) Steady-state PL spectra of perovskite films deposited on as-grown, 15 W and 25 W oxygen-plasma-treated ALD-TiO2/FTO substrates; (b) EQE characteristics of PSC devices with as-grown, 15 W and 25 W oxygen-plasma-treated ALD-TiO2 ESLs; (c) Current-voltage characteristics of PSC devices with as-grown, 15 W and 25 W oxygen-plasma-treated ALD, and 500

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o

C-sintered ALD and Sol-gel TiO2 ESLs; (d) PCE values of PSC devices with as-grown, 15 W and

25 W oxygen-plasma-treated ALD-TiO2 ESLs. The symbols indicate the average values with the corresponding standard deviations from 10 samples.

Figure 9.

Degradation of Current-voltage characteristics of PSC devices with as-grown and 15 W

oxygen-plasma-treated ALD-TiO2 ESLs. The devices were stored in ambient without encapsulation.

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RF Power Jsc (mA/cm2) Voc (V) FF (%) PCE (%) 0W

19.11

1.04

64.06

12.7

15 W

20.18

1.05

70.13

14.9

25 W

19.69

1.02

66.18

13.3

Table 1. Photovoltaic parameters of the PSC devices with ALD-TiO2 ESLs as a function of applied RF power.

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