halide perovskite interface: The impact of surface state passivation on energy alignment and photovoltaic performance of perovskite solar cells

halide perovskite interface: The impact of surface state passivation on energy alignment and photovoltaic performance of perovskite solar cells

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Journal Pre-proofs Full Length Article TiO2/halide perovskite interface: the impact of surface state passivation on energy alignment and photovoltaic performance of perovskite solar cells Nikolai Tsvetkov, Anna Nikolskaia, Oleg Shevaleevskiy, Sergey Kozlov, Marina Vildanova, Byeong Cheul Moon, Jeung Ku Kang, Liudmila Larina PII: DOI: Reference:

S0169-4332(20)30422-0 https://doi.org/10.1016/j.apsusc.2020.145666 APSUSC 145666

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

26 November 2019 20 January 2020 5 February 2020

Please cite this article as: N. Tsvetkov, A. Nikolskaia, O. Shevaleevskiy, S. Kozlov, M. Vildanova, B. Cheul Moon, J. Ku Kang, L. Larina, TiO2/halide perovskite interface: the impact of surface state passivation on energy alignment and photovoltaic performance of perovskite solar cells, Applied Surface Science (2020), doi: https:// doi.org/10.1016/j.apsusc.2020.145666

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TiO2/halide perovskite interface: the impact of surface state passivation on energy alignment and photovoltaic performance of perovskite solar cells Nikolai Tsvetkova,b,*, Anna Nikolskaiab, Oleg Shevaleevskiyb, Sergey Kozlovb, Marina Vildanovab, Byeong Cheul Moona, Jeung Ku Kanga,*, and Liudmila Larinab,* a

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Department of Materials Science and

Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea b

Photovoltaic Laboratory, Emanuel Institute of Biochemical Physics, Russian Academy of Science, Moscow, Russia

* Corresponding Authors: [email protected], [email protected], [email protected] Keywords: Perovskite solar cells; electron transfer layer; interface electronic structure; surface passivation; photoelectron spectroscopy

Abstract In perovskite solar cells (PSCs) trap-assisted recombination is the dominant mechanism limiting the cell performance. Here to overcome this challenge we apply the band gap tuning and surface passivation strategy for reducing the recombination losses at the interface between the TiO2 layer and perovskite absorber. The TiO2 surface was modified by SO42- anions and Cd2+ cations using a low-cost chemical solution technique. X-ray photoelectron spectroscopy analysis revealed that the chemical modification of the TiO2 surface leads to the downward shift of the valence band maximum pointing out the increased conduction electron density and resulting in minimizing the barrier losses at the interface. Electrochemical impedance measurements proved that the modification of the TiO2 surface significantly decreases the charge transfer resistance at the interface with perovskite light absorber. As a result, open circuit voltage and fill factor parameters of the PSCs were enhanced and the hysteresis is decreased. Moreover, the surface passivation significantly improves the air stability of PSCs indicating that the stability of the whole device critically depends on the TiO2/perovskite interface structure. Thus, our study provides valuable guideline toward the designing of hetero-interfaces to enhance both the power conversion efficiency and stability of PSCs.

1. Introduction Organic-inorganic metal halide perovskite solar cells (PSCs) attract much research interest since they are promising for low-cost mass production photovoltaic technologies[1,2]. A record power conversion efficiencies (PCEs) of PSCs have now exceeded 25% making them a serious alternative to the conventional silicon-based solar cells[3]. The PSC architecture typically comprises a perovskite (CH 3NH3PbI3) material, sandwiched between a mesoscopic layer of TiO 2 nanoparticles on a conductive glass substrate, a holeconductive layer and a metallic counter electrode[4]. The charge carriers are generated within perovskite light absorber and are transferred toward the contacts through the interfaces of the perovskite/electron transfer layer (ETL) and perovskite/hole transfer layer (HTL). The energetic losses at both interfaces

represent one of the largest limiting factors suppressing charge carrier collection and transport efficiencies. Recent works demonstrated that the photo-generated charges from perovskite light absorber are accumulated at the interfaces due to slow charge extraction rate hindering efficient charge transfer [5–7]. It was shown that the existence of trap states at the surface of TiO 2 ETL results in high recombination rates of photogenerated electron-hole pairs, which reduces the fill factor (FF) and the open circuit voltage (V OC) limiting the efficiency of PSCs [8,9]. Thus, the interfacial engineering approach plays a critical role in the enhancement of the performance of PSCs. Passivation of the TiO2 surface defects (Ti3+ sites and oxygen vacancies) is a possible way to suppress the photo-generated charge carrier recombination and to improve the electron extraction rates. For example, TiO2 doping can be an efficient tool for enhancement of the charge transfer across the ETL[10,11]. At the same time, many efforts were directed toward the passivation of trap state defects at perovskite/ETL and HTL/perovskite interfaces where trap-assisted recombination is the dominant mechanism limiting the cell performance [12–14]. Recently studies on the insertion of interlayers including CsBr, SnSb, BaTiO3 or zwitterionic compounds, which can prevent back electron recombination from TiO 2 to perovskite were reported [15–20]. It is known that to obtain high PCEs the proper energy level alignment at the perovskite/interlayer/ETL interfaces is required. It is a key factor for decreasing carrier recombination at the interfaces. Thus, the optimization of energy level alignment at the interfaces seems to be critical for improving photovoltaic (PV) parameters of PSCs. Ma et al. reported that introducing of a CdS layer in planar-type PSCs between TiO2 and perovskite, using the microwave-assisted hydrothermal deposition method, significantly enhanced air stability and suppressed recombination between the trapped electrons and perovskite[21]. However, the reported relatively thick interlayer films possess a serious disadvantage. The interlayer films absorb a substantial part of the incident light thus decreasing the density of charges generated within the perovskite layer and limiting the short-wavelength response of PSCs. Therefore, to achieve high-performance PSCs, it is important to explore alternative deposition methods capable of fabrication of ultra-thin interlayers that do not affect the light absorption characteristics of ETL but enables the interface passivation. At the same time, the other studies have shown that CdS quantum dots can be utilized as an efficient mediator between light absorber and hole transfer material [22]. Thus, somehow CdS at the same time was shown to be applied as either ETL or HTL sides of perovskite light absorber. This points out that still there is a need for clarification of the electronic structure of CdS with the respect to perovskite to enable the most efficient configuration for the extraction of photo-generated charge. CdS and related n-type direct band gap (2.4–2.5 eV) semiconductors possessing high electron transport ability are widely used as the state-of-the-art buffer layers in thin film CIGS solar cells[23]. The buffer layers for CIGS solar cells are usually formed using chemical solution technique (chemical bath deposition, CBD). Recently, Cd2+ wet treatment of CIGS absorber has been exploited to increase the efficiency of Cd-free CIGS solar cells [24]. For PSCs, the tailoring of the chemical state of the TiO 2 surface and electronic structure of perovskite/ETL interface by chemical solution pre-treatments can be also a useful approach to fabricate highquality interfaces. Here, for the first time, we implemented a novel solution for reducing the recombination

losses at the TiO2/perovskite interface by chemical passivation of the TiO2 ETL surface using the chemical solution technique (Fig. 1a). We investigated the influence of the passivation of the TiO2 surface on the electronic structure of the perovskite/ETL interface, and as a consequence, on the PV parameters of PSCs. We have found that the passivation of the TiO 2 surface with Cd2+ cations and SO42- anions results in the downward shift of valence band maximum (VBM) with the respect to the Fermi level indicating the increased electron density at the interface with the perovskite which in turn positively affect the VOC and FF of the PSCs. Thus, the power conversion efficiency of the fabricated PSCs was increased from 16.0% to 17.6%. Moreover, we demonstrated that the proposed passivation of TiO 2 surface defects leads to enhance air stability of PSCs.

2. Experimental TiO2 deposition and surface passivation. Solaronix glasses with a size of 2.5 × 2.5 cm with fluorine-doped tin oxide (FTO) conductive coating were used as substrates for PSC fabrication. The 0.15 M titanium disopropoxide dis(acetylacetonate) solution in 1-butanol was spin-coated at 2000 rpm for 60 sec on the substrates following by drying at 125 oC for 10 min. Mesoporous TiO2 layers with a thickness of 200 nm were prepared by the previously known technique [25,26]. The passivation was fulfilled using CBD method from a mixture of 0.0015 M aqueous solution of cadmium sulfate (source of Cd2+) and 0.05 M aqueous solution of thiourea (source of S2-). Ammonium hydroxide was used as a complexing agent. The pH of the bath solution was adjusted to a value of 11.0 by using ammonium hydroxide. The TiO2 ETLs were dipped into the bath at 40oC and the deposition time was varied from 1 to 2 min. After the deposition, the substrates were rinsed in deionized water with ultrasonic irradiation and dried in argon flow. Fabrication of perovskite solar cell. The perovskite deposition on the substrate was conducted inside the nitrogenfilled glovebox with a moisture level of 10-15 ppm. We used the triple cation perovskite with a nominal composition of Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3. The 1.5 M perovskite solution was prepared by dissolving in DMF: Dimethyl sulfoxide (DMSO) mixture (4:1 v/v) with a stoichiometric amount of FAI, MABr, PbI2 , and PbBr2. Finally, the appropriate amount of 1.5M solution of CsI in DMSO was added to the perovskite solution. After overnight stirring, the solution was deposited on the TiO2 layer by spin coating at 500 rpm for 10 sec and 6000 rpm for 25 sec sequentially. 250 μl of chlorobenzene solution was dropped at the film surface after 17 sec spinning at 6000 rpm. Then, the samples were annealed at 100 °C for 25 minutes. As regards to a hole transporting material, the SpiroMeOTAD solution was deposited by spin-coating at 4000 rpm for 30 seconds. The solution was prepared by dissolving 72.3 mg of Spiro-MeOTAD powder into the mixed solution of 28.8 µl 4-tert-butyl pyridine, 17.7 µl stock solution with 52 mg of Li-TFSI in 100 µl acetonitrile, and 1 ml of chlorobenzene. Finally, the 50 nm-thick gold layer was deposited on HTL as the back electrode of a solar cell using the thermal evaporator (a deposition rate of 3-4 Å/s). Characterizations. The structures of the samples were characterized using the double Cs-corrected transmission electron microscope Titan3 G2 60-300, Jeol. The morphology of the samples was investigated using field-emission 3

scanning electronic microscopy (Magellan400, FEI). The Fourier-transform infrared (FTIR) spectra were collected using the FT-TR-6100 instrument (JASCO). The absorption spectra of TiO2 ETLs were analyzed by a double beam Shimadzu UV 3600 UV-VIS-NIR spectrophotometer. Time-resolved photoluminescence (TRPL) measurements were conducted using the time-correlated single-photon counting method using a fluorescence lifetime spectrometer (FL920, Edinburgh Instruments). The PV characteristics of solar cells were measured under simulated AM 1.5G sunlight at 100 mWcm-2 irradiance. The power of the lamp was calibrated using the HS Technologies calibrated silicon reference cell PECSI01. The PV performance was measured using the Oriel solar simulator with the IviumStat potentiometer. The parameters were measured under ambient conditions using the aperture mask with an active area of 0.105 cm2. I-V curves were recorded with a scan rate of 0.02 mV/s. The electrochemical impedance spectroscopy analyses were performed under open circuit condition and 100 mW/cm2 illumination in the frequency range of 1061 Hz using the IVIUM potentiometer, and the amplitude of the modulated voltage was 10 mV. The surface chemical states and electronic structures of the samples were identified using a K-alpha spectrometer (Thermo Scientific) equipped with monochromatic Al Kα source. Calibration of the spectra was performed by referring to the C 1s peak (C-C bond) at 285.0 eV. The VBM positions were estimated by extrapolation of the leading edge in valence band spectra to the baseline.

3. Results and discussion Passivation of TiO2 surface defects. Figure 1a shows the structure of PSC with the passivated interface between TiO2 and perovskite light absorber. The typical cross-sectional scanning electron microscopy (SEM) image of the fabricated device is given in Figure 1b. The purpose of our study is the passivation of interface between ETL and light absorber yielding the modification of its electronic structure required for the promotion of electron injection but hinders the hole transfer and, thus, the charge recombination. An optimal electronic structure of PSC is schematically depicted in Figure 1c. The FTO substrates with deposited mesoporous TiO2 layers were employed as the ETLs and treated using the CBD method. The treatment was performed with an aqueous solution consisting of thiourea, ammonium hydroxide, and cadmium sulfate. ETLs treated during 1, 1.5 and 2 min are labeled as Cd1, Cd1.5, and Cd2, respectively. Chemical reactions that occurred during the CBD process are listed in Supporting Information (Section 2.1). To identify the modification of the chemical state and electronic surface structure of the TiO2 ETL induced by chemical treatment, X-ray photoelectron spectroscopy (XPS) was used. XPS spectra near the valence edge were taken for all samples and shown in Figure 1d. We found that wet treatment leads to the downward VBM shifts of by about 0.2 eV for Cd1 and Cd1.5 samples, as compared to the pristine TiO2 ETL. (Figure 1d). The downward VBM shift of the TiO2 subjected to wet chemical treatment is favorable for the prevention of hole recombination at the TiO2/perovskite interface (Figure 1c) as further discussed below. Moreover, the downward shift of the VBM with respect to the Fermi level indicates the increased in the electron density at the surface of passivated TiO2. In contrast, the increase of the CBD time up to 2 min leads to a significant upward VBM shift of around 1.5 eV. Such a large shift can be attributed to the different chemical nature of the surface of the Cd2 sample compared to those of the Cd1

and Cd1.5. The observed valence band structure and VBM position indicate the presence of the CdS layer on the TiO2 surface in the Cd2 sample [27]. Moreover, a positive shift in the binding energy of the Ti 2p core-level was observed upon the treatment indicating the formation of new chemical bonds on the TiO 2 surface (Figure 1e). Alteration of chemical state can be the sign of the surface dipole formation which is beneficial for charge extraction at the TiO2/perovskite interface[17]. The representative scheme of energy band alignment at the TiO2-perovskite interfaces for the Cd1.5 and Cd2 cells are constructed and given in Figures 6a,b.

Fig. 1. (a) The schematics of PSC with passivated mp-TiO2 ETL. (b) Cross-sectional SEM image of the typical PSC. (c) Schematic energy band diagram of PSC with optimal energy structure for efficient charge transfer. (d) Valence band and (e) Ti 2p core-level XPS spectra of bare TiO2 ETL and ETLs subjected to CBD for different times. The binding energy scale is referred to the Fermi level. (f) Absorption spectra of the pristine and passivated TiO2 ETLs

The absorption UV-vis spectra of a representative set of TiO2 ETLs are shown in Figure 1f. The passivation of the TiO2 ETLs during 1 and 1.5 min (Cd1 and Cd1.5 samples) does not affect the optical absorption of TiO2 films. At the same time, we found the long-wavelength shift of the absorption edge and increase absorption in the visible region for TiO2 ETL subjected to the CBD for 2 min. To clarify the changes in the surface chemical composition of the TiO2 films induced by CBD treatment, the XPS spectra of the S 2p and Cd 3d core-levels were recorded. Given the conditions of the CBD process such as the relatively low reaction temperature of 400 oC and the pH of solution, it is notable that Cd or S atoms could not be incorporated at the interstitial sites as TiO2 is insoluble in the CBD solution, so that Cd and S atoms are deposited at the surface of TiO2 nanoparticles. Comparative S 2p and Cd 3d core-level spectra of the films subjected to CBD for different periods are shown in Figure 2 a,b. The increase in the deposition time yields the increase of the intensity of S 2p and Cd 3d core-level emission, indicating a larger amount of the Cd and S elements on the TiO2 surface. In the S 2p spectrum (Figure 2a), the broad and asymmetric emissions were 5

observed only at around 169 eV for the both Cd1 and Cd1.5 samples, which can be univocally attributed to the S 2p emission from SO42- anions. However, the additional intense S 2p signal is appeared at around 162 eV for the Cd2 sample. This emission can be identified with alteration in the electronic structure pointing out the presence of the CdS [27]. The spin-orbital splitting of Cd 3d3/2 - 3d5/2 is observed for all samples under study (Figure 2b). For Cd1 and Cd1.5 samples, relatively weak Cd 3d peaks centred at around 405.8 and 412.5 eV were recorded. However, for the Cd2 sample, the emission intensity of Cd 3d core-level was increased more than twice. Moreover, the Cd 3d peak is becoming asymmetric (Figure S2, Supporting Information) indicating the alteration in the chemical environment of Cd. However, the identification of the difference in the chemical composition of samples using the Cd 3d corelevel is not possible, because the binding energy shift between Cd 3d emissions of samples under study was too small to be resolved [27]. The FTIR measurements were used to clarify the chemical bond formation on the TiO2 surface before and after wet chemical treatment. Figure 2c shows the FTIR spectra taken for the bare and treated TiO2 films. With the increase in the treatment time, the formation of the intense absorption peaks at around 1070 and 1180 cm-1 was observed, which match well to the SO42-absorption band [28]. The sample treated for the 2 min shows additional features at around 600 and 660 cm-1 which can be attributed to Cd-S absorption bands [28]. Thus, XPS, UV-vis and FTIR analysis revealed that the two different types of passivation are realized. With short treatment time, the TiO2 surface modification by SO42- anions and Cd2+ cations is realized which does not significantly affect the light absorption of ETL (Figure 1f). The obtained results are in good agreement with previous publications, where it was demonstrated the possibility of the TiO2 surface modification with SO42- anions for enhancement of the TiO2 photocatalytic activity [29,30]. With the further increase in the deposition time, the CdS layer is formed at the TiO2 surface as expected under the CBD process. The chemical reactions of the formation of the CdS layer are listed in Supporting Information (Section 2.1).

Fig. 2. XPS spectra of (a) S 2p and (b) Cd 3d core - levels for Cd1, Cd1.5, and Cd2 ETLs. (c) FTIR spectra of pristine and passivated TiO2 layers. (d) The TRPL decay curves of perovskite PL peaks recorded for the FTO/TiO2/perovskite/HTL sample with pristine and passivated TiO2 layers.

To evaluate the stability of the modified TiO2 surface, all samples under investigation were rinsed several times in deionized water and DMF solution and then were dried in argon flow. We observed that the emission intensities of the S 2p and Cd 3d core-levels remained almost unchanged after washing confirming the chemical stability of the treated samples (Figure S3, Supporting Information). Finally, we compared the efficiency of interfacial charge injection from the perovskite to the pristine and passivated ETLs using the TRPL measurements. Figure 2d shows the TRPL decay curves of PL peaks for the perovskite at 775 nm upon laser excitation. We have found that TiO2 passivation (Cd1 and Cd1.5 samples) leads to the faster TRPL signal decays compared to that in pristine TiO2. This result implies that the chemical treatment is favorable for enabling fast and efficient electron injection at the TiO2/perovskite interface [31]. However, the increase in the treatment time, which yields the CdS layer formation on the TiO2 surface, leads to an increase in the decay time. This result indicates slower electron extraction in Cd2 sample. XPS and energy dispersive X-ray (EDS) analyses of the samples revealed that the concentration of the Cd more than twice exceeds that of S for Cd1 and Cd1.5 sample and is almost equal to the S concentration for Cd2 sample. Taking into account the existence of the hydroxyl groups at the TiO2 surface (Figure S1, Supporting Information) the interaction of Cd2+ ions with hydroxyl groups at the TiO2 surface is suggested and can be expressed as follow:

 Ti  OH

 Cd 2    Ti  OCd





H

(1)

In the early stage, this reaction can compete with the CdS formation process at the TiO2/perovskite interface. It is well known that the passivation of the OH- groups at the TiO2 surface is beneficial for the improvement of the stability of the TiO2/perovskite interface since those groups may react with perovskite halide light absorber under illumination[32]. Moreover, the replacement of monovalent hydrogen atom with bivalent cadmium results in the formation of the donor defect which can donate the electron to the conduction band thus leading to the upward Fermi level shift with respect to valence band as it was observed by XPS (Figure 1d). At the same time, the anchoring of SO42- groups to the TiO2 surface is taking place via attachment of SO42- groups with two surface titanium atoms [33] thus the termination of the TiO2 surface is changing from Ti-O bonds to either Cd-O or Ti-SO4 bonds. It should be noted that the main reason for fast degradation at the TiO2/perovskite interface is the formation of the oxygen vacancies under illumination and the chemical reaction of the released oxygen atoms with perovskite halide [34]. The concentration of oxygen vacancies is typically higher in the areas near structural defects or particle interconnections [35,36]. At the same time, these areas can be the active sites for interactions such as Cd2+ deposition or SO42- group anchoring. Given aforementioned, we suggested that the passivation of the defect states at the TiO2 surface is of primary importance for hindering the chemical reaction between TiO2 and perovskite under illumination and, thus, for designing the stable TiO2/perovskite interfaces.

7

Effect of the surface passivation on the TiO2 morphology. Further investigations of TiO2 samples subjected to CBD for the different times were performed using high- resolution transmission electron microscopy (HRTEM). Figure 3a shows the HRTEM image of the TiO2 nanoparticles from the Cd1.5 sample. For both Cd1 and Cd1.5 samples, we did not detect any surface layer at the TiO2 particle edges. However, we observed the ragged edges on the surface of TiO2 particles (Figure 3a) which can be due to the chemical modification of the TiO2 surface by SO42- anions and Cd2+ cations as it was discussed above. Despite the fact that we did not observe any surface layers for the Cd1 and Cd1.5 samples, the elemental mapping obtained by EDS revealed the presence of the Cd and S in both samples (Figures 3b-e). An example of EDS spectra obtained from the surface of treated TiO2 samples are shown in Supporting Information (Figure S4) In contrast to Cd1 and Cd1.5 samples, a few nm thick film is clearly visible at the surface of the TiO2 nanoparticle of Cd2 ETL (Figure 3f). The scanning electron microscopy (SEM) images of the bare and passivated TiO2 ETLs are shown in Figures 3g-j. For the Cd1 and Cd1.5 samples, the morphology of the TiO2 ETL remains almost the same as of bare TiO2. With the increasing the CBD time up to 2 min the individual particles form agglomerates on the surface as depicted in Figure 3j. The formation of the agglomerates was attributed to the existence of the thin CdS layer at the TiO2, which is clearly visible in the STEM image in Figure 3f.

Fig. 3. (a) HRTEM image of TiO2 (Cd1.5 sample) nanoparticles. (b) STEM image of the CdS1.5 sample particles along with elemental mapping for (c) Ti, (d) Cd, and (e) S. (f) The STEM image of TiO2 nanoparticles (Cd2) sample, where the surface layer can be clearly seen. SEM plane images of the (g) bare TiO2, (h) Cd1, (l) Cd1.5, and (j) Cd2 samples. Performance of PSCs: effect of the passivation of TiO2/perovskite interface. Several sets of PSCs based on the mesoporous TiO2 were fabricated and their power conversion efficiencies were compared. It is notable that the PV parameters of the devices have been reproduced on 15–20 cells for each type of PSC and there is no significant difference was observed in the current-voltage (J-V) curves recorded with different scan rates. The J-V curves of best-performing devices are shown in Figure 4a and the corresponding PV parameters of PSCs are summarized in Figure 4b and Table 1. The statistics of the device performance is given in Supporting information (Figure S5). The improvement in the efficiency from 16.0% obtained for reference device up to 17.6% for the PSCs based on Cd1.5 ETL is due to the enhancements of VOC and FF. The corresponding external quantum efficiency spectra of PSCs with

pristine and CBD treated ETLs are given in Figure S6. The increases are attributed to the successful modification of the electronic structure of the TiO2 surface with SO42- anions and Cd2+ cations. To clarify the origin of the increase of PSC performance, the effect of passivation of the TiO2 surface on the interface electronic structure was investigated.

ETL

Jsc, mA/cm2

Voc/V

FF

Efficiency

TiO2

23.0

1.02

68%

16.0%

Cd1

23.2

1.03

69%

16.6%

Cd1.5

23.3

1.05

72%

17.6%

Cd2

20.8

0.99

67%

14.4%

Table 1. Photovoltaic parameters of PSCs with bare TiO2 ETL and ETLs modified using the CBD process. The XPS valence band spectra (Figure 1d) revealed that the passivation yields the downward VBM shifts of 0.2 eV for both the Cd1 and Cd1.5 samples, as compared to VBM of the bare TiO2. The downward shift of VBM results in the increase of electron density at the surface of TiO2 leading to an increase the electronic conductivity at the interface. At the same time, the downward shift of the VBM yields the formation of the potential barrier at the perovskite/TiO2 interface for hole transport from the perovskite absorber (Figure 1c). The existence of a blocking barrier is critical for the junction characteristics due to preventing the electron-hole recombination at the interface. Thus, the downward shift of the valence band edge of Cd1.5 ETL results in an increase of the VOC and FF (Figure 4a). The hysteresis behaviour of fabricated PSCs are shown in Figures 4c-d. PSC with passivated TiO2 ETLs have significantly lower hysteresis (less than 7%) at reverse and forward scans compared to the device with pristine TiO2 ETLs (more than 20%). The obtained results indicate a low trap site concentration at the interface [37] which can be due to lower defect concentration at the surface of the passivated TiO2 . The lowering of the defect concentration at the surface of passivated ETLs results in the more efficient electron transfer in PSCs as was predicted from TRPL measurements (Figure 2d).

9

Fig. 4. (a) Representative J–V characteristics of PSCs fabricated with bare and CBD-treated ETLs. (b) Histogram of the PV parameters of the fabricated PSCs. Scan-direction-dependent J–V curves of PSCs with (c) pristine TiO2 and (d) Cd1.5 ETLs.

Stability of PSCs: effect of the passivation of TiO2/perovskite interface. The measurements of PSC stability revealed that the passivation of the TiO2 ETL surface significantly slows down the degradation of device performance under 1.5 AM illumination (Figure 5). The device with pristine TiO2 ETL shown more than 60% performance loss during the first 60 hours of illumination. However, the PSCs with passivated TiO 2/perovskite interfaces were found to be much more stable. The loss of energy conversion efficiency was found to be only around 10-20% for the first 60 hours of illumination for solar cells based on Cd1.5 and Cd2 ETLs. The similarity in efficiency decay kinetics for solar cells based on Cd1 and Cd1.5 ETLs points out the participation of the same process governing the degradation of device performances. The loss of performance mainly observed during the first 10 hours of illumination, can be attributed to the unavoidable degradation of organic HTL and increase of the thermal losses due to cell heating. Enhanced stability of PSCs with passivated ETLs can be attributed to the alteration of the surface chemical composition of the ETL hindering illumination-induced chemical reactions between halide perovskite and TiO2. It is shown that the illumination induces the formation of the oxygen vacancies at the TiO2 surface releasing the oxygen atoms which can react with perovskite [34,38]. Surface passivation of the TiO2 layer with Cd2+ cations and SO42anions can hinder the activation of the aforementioned processes as it was discussed above. The experimentally observed evolution of performance of the PSCs with passivated ETLs provides an excellent illustration of our suggestions. The best stability is shown by solar cells based on Cd2 ETLs. The cell efficiency falls by 10-20% to an almost steady-state value for the first 10 hours of illumination. The observed high stability can be assigned to introducing a few nm-thick CdS layer at the TiO2 ETL/perovskite interface leading to the blocking of the pathway for reactions between perovskite absorber and ETL. At the same time, performance decay of the reference

cell is almost linear during first 30 hours of illumination suggesting the lack of any limitation of the light-induced reactions between halide perovskite and TiO2. Thus, our results revealed that the instability of PSCs is related to the processes initiated at the TiO2/perovskite interface, and further efforts on the TiO2/perovskite interface engineering are required to obtain stable PSCs.

Fig. 5. Normalized efficiency of PSCs with the pristine and passivated TiO2 ETLs under 1.5 AM illumination and ambient conditions, where all the solar cells were continuously illumined during 60 hours in ambient conditions. The efficiency measurements results are indicated with symbols in the plot and lines are drawn to guide the eye.

Electronic structure at TiO2/Perovskite interface: effect of CdS interlayer. The observed decrease of the PSC performance by introducing a relatively thin CdS layer between TiO2 ETL and perovskite absorber can be explained by the formation of a potential barrier for electron transport from the perovskite material. We have constructed the energy band diagram (Figure 6b) of the TiO2-CdS-perovskite interfaces based on the XPS valence band spectra and band gap values estimated using absorption spectroscopy (Figure S7, Supporting Information). The large spike-type conduction band offset (CBO) of 0.5 eV and a cliff of -0.7 eV were deduced at the perovskite/CdS and at CdS/TiO2 interfaces, respectively. Previous research on the subject proved that spike-type CBO in a range of 0 to 0.3 eV is most suitable to maximize VOC while maintaining decreases in current density (JSC) at a similar level[39]. Therefore, the obtained unfavourable for charge transfer interface electronic structure leads to a remarkable decrease in the efficiency of charge extraction from the perovskite absorber to Cd2 ETL. At the same time, the electronic configuration at the interface between Cd1.5 ETL and perovskite (Figure 6a) is suitable for fast electron injection as well as the hole repulsion enabling the efficient interface charge transfer and improved performance of PSCs. Electrochemical impedance spectroscopy. Finally, electrochemical impedance spectroscopy (EIS) was used to characterize the charge transfer and carrier recombination dynamics in PSCs. Figure 6c shows the Nyquist plots of PSCs with bare TiO2 ETL and ETLs - modified by SO42- and Cd2+ and the equivalent circuit used for fitting the impedance data[40,41]. Two semicircles in the EIS spectra were observed. The semicircle in the high-frequency range is assigned to the charge transfer resistance at the ETL/perovskite and perovskite/HTL interfaces, while the second semicircle in the low-frequency range is ascribed to the charge transfer within the TiO2 layer. In the equivalent 11

circuit, RS corresponds to the series resistance of the cell, which can be estimated from the high-frequency intercept of the left semicircle with the x-axis. RTiO2 and CPETiO2 refer to the charge transfer resistance and the capacitive constant phase element (CPE) of the TiO2 layer, respectively. RCT and CPECT represent the charge transfer resistance and the capacitance at the ETL/perovskite and perovskite/HTL interfaces, respectively. We find that the values of R S are similar for PSCs with and without CBD treatment, indicating that the difference in device performances can be attributed to the variations in RCT and RTiO2. Indeed, RCT reflects the charge transfer resistance at the interface that primarily affects the VOC of PSCs. Since the cells have been fabricated using the identical perovskite/Spiro-MeOTAD interfaces, the differences in the EIS spectra can be caused by the change in RCT at the TiO2/perovskite interface. We find that the RCT value decreases for Cd1 and CdS1.5 ETLs but increases for PSCs with Cd2 ETLs resulting in some decrease in VOC. It is notable that not only RCT value is increasing in the Cd2 sample but also an additional feature is observed in the impedance spectra at low frequencies (Figure 6 c). The formation of this feature can be attributed to the suppression of the charge transfer across the CdS layer.

Fig. 6. (a) The scheme of an electronic structure of the TiO2-perovskite interfaces for the Cd1.5 sample. (b) The scheme of an electronic structure of the TiO2/perovskite interfaces for the Cd2 sample. The binding energy scale is

referred to the Fermi level. (c) The electrochemical impedance spectra of PSCs obtained under AM 1.5G illumination and open-circuit conditions.

4. Conclusion In summary, we introduce a novel solution for reducing the recombination losses in PSCs by passivation of the TiO2 surface with SO42-anions and Cd2+ cations using low-cost chemical solution technology for enabling robust interface between TiO2 and perovskite. Thus, the power conversion efficiency of the fabricated PSCs was improved from 16.0 % to 17.6%. XPS analysis revealed that chemical modification of the TiO2 surface by Cd2+ cations and SO42- anions using short time CBD leads to a downward shift of its VBM with the respect to Fermi level indicating the increase in the electron concentration at the interface. As a result, the attained electronic structure at the ETL/perovskite interface provides effective suppression of the electron-hole recombination leading to the significant decrease of the charge transfer resistance at the interface. Moreover, we demonstrated that the passivation of the TiO2 surface enhances the air stability of PSCs. In contrast, the increase of the CBD time leads to the growth of a few nm thick CdS film at the TiO2 surface that was found to be detrimental for the PSC performance. XPS analysis revealed a spike CBO of around 0.5 eV at the perovskite/CdS interface that decreased the efficiency of the charge extraction from the perovskite absorber significantly decreasing the photocurrent density. EIS confirmed that the TiO2 surface modification by Cd2+ cations and SO42- anions decrease the charge transfer resistance while upon CdS layer formation the interface resistance is significantly increased. Finally, our results show the importance of the passivation and bandgap tuning approach toward designing the interfaces to enhance both the power conversion efficiency and stability of PSCs.

Acknowledgments This research was mainly supported by the Global Frontier R&D Program on the Center for Hybrid Interface Materials (2013M3A6B1078884) funded by the Ministry of Science, ICT & Future Planning, and also supported by the Russian Foundation for Basic Research (grant No. 19-08-01042) and the National Research Foundation of Korea (2015H1D3A1062265, 2019M3E6A1104196).

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Highlights    

The recombination losses in PSCs were reduced by passivation of the TiO2/perovskite with SO42anions and Cd2+ cations using low-cost chemical solution technique Passivation does not affect the light collection efficiency of the PSCs Passivation leads to a downward shift of the TiO2 valence band at the interface as it was proved by XPS Favorable alteration of the electronic structure at the TiO2/perovskite interface: provides effective suppression of the electron-hole recombination leads to the significant decrease of the charge transfer resistance at the interface increases the power conversion efficiency of the PSCs from 16.0 % to 17.6% enhances the air-stability of PSCs

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Credit author statement: Nikolai Tsetkov: Conceptualization, Methodology, Experiment, Writing- Original draft preparation; Anna Nikolskaia: Methodology, Experiment; Oleg Shevaleevskiy: Methodology, Writing- Reviewing and Editing; Sergey Kozlov: Methodology, Experiment; Marina Vildanova: Methodology, Experiment; Byeong Cheul Moon: Methodology, Experiment; Jeung Ku Kang: Supervision, Writing- Reviewing and Editing; Liudmila Larina: Supervision, Writing- Reviewing and Editing.

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