Low Hysteresis Perovskite Solar Cells Using an Electron-Beam

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Low Hysteresis Perovskite Solar Cells using e-beam Evaporated WO3-x Thin Film as Electron Transport Layer Fawad Ali, Ngoc Duy Pham, Lijuan Fan, Vincent Tiong, Kostya (Ken) Ostrikov, John Bell, Hongxia Wang, and Tuquabo Tesfamichael ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00547 • Publication Date (Web): 24 Jun 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Energy Materials

Low Hysteresis Perovskite Solar Cells using e-beam Evaporated WO3-x Thin Film as Electron Transport Layer Fawad Ali, Ngoc Duy Pham, Lijuan Fan, Vincent Tiong, Ken Ostrikov, John M. Bell, Hongxia Wang* and Tuquabo Tesfamichael*

School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology 2 George Street, Brisbane, 4000, QLD Australia *Corresponding author: Phone: +61-7-31381988 Fax: +61-7-31381516 Corresponding author email: [email protected] [email protected] ABSTRACT Perovskite solar cells utilize metal oxide thin films as electronic transport for high performance devices. These electronic transport metal oxides are generally processed at higher temperatures. In this research we report room temperature processed WO3-x thin film as electron transport layer for high performance and low hysteresis device. High oxygen deficient WO3-x film was deposited at room temperature using e-beam evaporation in high vacuum condition. For comparison, the amount of oxygen vacancies was reduced by postannealing of the as-deposited WO3-x films at 300 oC for 1 hour in air. XRD and Raman measurements showed no WO3-x characteristic peak of both the as-deposited and annealed films. From XPS and EPR, the as-deposited film shows large amount of oxygen vacancies compared to the post-annealed film. The bandgap of the post-annealed film increases due to reduced conductivity and thus a reduction in the device performance, mainly because of the low Voc and high current-voltage hysteresis in the forward and reverse scans. The perovskite solar cell device developed using the room temperature deposited electron transport WO3-x layer has shown low current-voltage hysteresis. This device achieved a power conversion efficiency of 10.3% and hysteresis index of 2.1%. This work demonstrates the feasibility of WO3-x film as electron transport layer for high efficiency perovskite solar cell with reduced hysteresis fabricated at low temperature using industrially viable e-beam evaporation method.

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Graphical Abstract

Keywords: WO3-x thin film, Oxygen vacancy, e-beam evaporation, hysteresis, perovskite solar cells 1. Introduction Perovskite solar cells (PSCs) using organo-metal lead halides perovskites as light absorber are at the centre of attention in the photovoltaic research community due to their low cost, ease of fabrication and higher power conversion efficiency (PCE). The low binding energy,1 optimal direct tunable bandgap of 1.2 1.6 eV, long diffusion length, long carrier life time 2-4 have made perovskite the most attractive material for optoelectronic devices including solar cells, light emitting diodes, etc. The efficiency of PSC has increased from 3.8% 5 in 2009 to over 22% in 2016 6-7 thanks to the advancement of material synthesis approaches and device architecture engineering. In a typical perovskite solar cell, the absorber layer is sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL). The photo-generated charge carriers in the perovskite are extracted through the charge selective ETL and HTL. Metal oxides such 2 ACS Paragon Plus Environment

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as TiO2 and SnO2 have been extensively used as electron transport layer for high performance PSCs.6-10 TiO2 is the most common ETL for PSCs. In order to get high crystallinity and conductivity, TiO2 need to be annealed at high temperature (450 °C).6, 11 Similarly, precursor solution of SnO2 requires post-annealing at temperature above 180 °C. The requirement for high annealing temperature not only adds complexity and cost in the manufacturing process of the device, but also halts further development of flexible PSCs and tandem solar cells. To overcome these issues, alternative metal oxide semiconducting materials have been investigated as electron transport material. Tungsten oxide (WO3) is n-type semiconducting material with high electron mobility (10-20 cm2V-1s-1) and wide bandgap energy (2.7-3.9 eV). It has high stability against moisture, low material cost and can be made at low temperature. The high electrical conductivity and comparable optical transmittance to TiO2, means WO3 is a potential ETL for perovskite solar cells. In spite of these extraordinary properties of WO3, compared to TiO2 and SnO2, there is much less reports on perovskite solar cells using WO3 based ETL. Currently, the PSC using WO3 as ETL has low energy conversion efficiency (less than 10%) and the device normally showed high hysteresis due to the imbalanced charge transport at the ETL/perovskite and perovskite/HTL interfaces. Wang et al. reported PSC using WOx as ETL processed at 150 °C. The device showed higher circuit-current density (Jsc) than the device using TiO2, but much lower open-circuit voltage (Voc) (0.71 eV), leading to lower power conversion efficiency (PCE).12 The lower Voc was explained by the inherent charge recombination in the WO3. In order to overcome this problem a hybrid ETL consisting of amorphous WOx coated TiOx (TiOx-WOx) processed at 150 °C was developed. Using this hybrid ETL an impressive PSC efficiency of 17.47% was observed.13 On the other hand, Nb doped WOx ETL processed at low temperature (120 °C) for flexible PSC with efficiencies of 13.14% was developed.14 These results suggest that WO3 has a great potential as low temperature processed ETL for PSCs.



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Although various methods have been reported for synthesis of WO3 thin films including evaporation,15-16 sol-gel,17 sputtering

18-21

and chemical vapour deposition,22 the WO3 for

PSCs application has mostly been produced through solution based spin-coating processing method.13-14 The problem with the solution based method is that a post annealing treatment is required to remove the solvent and to decompose the precursors into WO3. Also uniformity of the film quality by spin coating over large area is another issue. In addition, a large hysteresis is often reported with WO3 based PSC, which is caused by imbalanced charge extraction at the interface of perovskite/ETL and perovskite/HTL. In the literature a small amount of oxygen vacancies can increase the electrical conductivity of WO3.23 Different studies have reported that creating oxygen vacancies in WO3 can affect the conductivity, crystallinity and charge transport properties of the material.20, 24-26 These vacancies also cause defect band which help in increasing the conductivity by reducing the bandgap

25, 27

as

demonstrated in our previous work of sputtering-deposited SnO2 films and in the work by others. Liao et al. have showed that the conductivity of WO3 decreases with decreasing the amount of oxygen vacancies.28 Similarly for photoelectrical conversion an improved photoelectric conversion efficiency was attributed to the oxygen vacancies in the WO3 thin film.29 Usually oxygen vacancies are produced by doping or post-thermal treatments in limited oxygen atmosphere which increase the processing cost and also complicated the process. A simple one step method for controlling oxygen vacancies is desired which helps to speed up the process of device fabrication at lower cost. In this study we report a room temperature deposited WO3-x thin film by electron-beam evaporation in high vacuum. Electron beam (e-beam) evaporation is a versatile and robust technique for deposition of uniform metal oxide films over a large surface area with good control over film quality and composition. Also, oxygen vacancies can be created in the WO3 4 ACS Paragon Plus Environment

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through control of oxygen environment which can be beneficial in device performance improvement. The WO3-x exhibited high optical and electrical properties, large oxygen vacancies with wide bandgap and high electron mobility. For comparison we also annealed the as-deposited film at 300 °C to reduce the oxygen vacancies and compare their performance in perovskite solar cells. The as-deposited WO3-x as ETL has demonstrated a much higher Voc and FF compared to the annealed WO3-x film, leading to energy conversion efficiency over 10% under AM1.5 one sun illumination. Most importantly, the current-voltage hysteresis of the as-deposited WO3-x film was almost eliminated compared to the annealed sample. 3. Results and discussion: Figure 1a shows spectral transmittance of the as-deposited and post-annealed WO3-x thin films. The films are highly transparent in the visible and near infrared wavelength and their transmittance sharply drops in the ultraviolet spectral wavelength. The transmittance of the post-annealed film at 300 °C decreases slightly and its absorption edge shifts towards a shorter wavelength suggesting increased bandgap.33 The weighted optical transmittance of both films in the wavelength range of 400-1100 nm is above 75%. The bandgap energy of the films was calculated using the relation for an indirect band material of WO3-x: (αhν)1/2 = A(hν-Eg) where α is the absorption coefficient, A is the band edge parameter, h is the Plank constant, and ν is the frequency of light. The plot produced from this relation is known as the Tauc plot. The Eg of the films is then calculated by extrapolating the linear region of the Tauc plot to zero. The bandgap for the as-deposited WO3-x is 3.84 eV, which increased to 3.91 eV after annealing at 300 °C in air as shown in Figure 1b and Table 1. The larger values of the bandgap energies observed in this study are thought to be the result of quantum size effect 5 ACS Paragon Plus Environment


due to small crystal sizes and amorphous nature of WO3-x.34-35 In our previous studies we have observed that the bandgap energy of MoO3-x

33

thin films decreases after annealing in

vacuum. This is because the donor orbitals overlap with the increase of oxygen vacancies. When the concentration of oxygen vacancies is high, the defect band broadens to an extent that the gap between the conduction band and defect band disappeared and the band gap reduced.25

Figure 1. (a) Transmittance spectra, (b) (αhν)1/2 vs hν plot, (c) He-I UPS spectra, inset in fig c is the fermi-edge region and (d) band energy alignment of ETLs with perovskite light absorbing material, for the room temperature deposited and post-annealed WO3-x thin film samples. The valence band maximum (Evb) and conduction band minimum of the WO3-x (annealed and as-deposited) films were determined by a combined measurement of UPS and UV-visible. The fermi level was estimated to be –5.12 eV for both the as-deposited and annealed WO3-x. 6 ACS Paragon Plus Environment

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The EVB of the films is –8.22 eV which is determined by subtracting the VBM (fermi edge which is 3.1 eV as shown in the inset in Figure 1c) from the fermi level EF (–5.12 eV): EVB = EF – VBM= –5.12 eV – 3.1 eV = –8.22 eV. The conduction band position was then calculated by adding the bandgap energy value to the valance band and we get ECB = –4.38 eV for the room temperature deposited WO3-x thin film and ECB = –4.31 eV for the post-annealed film. Eg was calculated from the Tauc plot as shown in Figure 1b. The schematic diagram of the conduction band positions of WO3-x thin films and perovskite are shown in Figure 1d. The deeper conduction band position of the asdeposited WO3-x film relative to the perovskite creates a favourable energy alignment for faster electron injection from the conduction band of perovskite to the conduction band of the WO3-x ETL.36 The chemical composition of the WO3-x thin film was investigated by XPS. Figure (S1) contains the survey spectra of the as-deposited and post-annealed samples. The corresponding high resolution W 4f, O 1s and C 1s core level spectra are shown in Figure 2. In both samples the W 4f core level consists of a single component at 36.0 eV (see Figure 2a) which is in agreement with the peak positions reported for tungsten oxide.16, 37 The corresponding O 1s can be seen in the Figure 2b at 531.0 eV. The O 1s core level also has peaks at 531.9 eV and 533.1 eV which have been ascribed to adventitious O-C and adsorbed H2O, respectively. The ratio of oxygen to tungsten for each sample was calculated based on the XPS. The roomtemperature sample was found to be sub-stoichiometric with a O:W ratio of 2.79 ± 0.06, whereas the sample annealed at 300 °C is nearly stoichiometric WO3 with a O:W ratio of 2.91 ± 0.06 as shown in Table 1. In contrast, despite the non-stoichiometric composition of WO3-x, we find that the data can be fitted with peaks of the same line-shape and FWHM indicating the presence of only the W(VI) state (see Figure 2(a)). 7 ACS Paragon Plus Environment


Figure 2. High resolution XPS spectra of as-deposited and annealed WO3-x thin films shows (a) W 4f, (b) O 1s, (c) C 1s core levels with fits to the spectral envelopes. Electron paramagnetic resonance (EPR) was carried out to measure the oxygen vacancies of the as-deposited and post-annealed samples as shown in Figure 3a. The g-value (2.004) calculated (calculation shown in supporting information) from the resonance magnetic field is almost equal to the g-value (2.008) of a free electron38 which corresponds to oxygen vacancy. It is known that the peak intensity increased with increasing content of oxygen vacancies.24, 33 The higher peak intensity of the as-deposited sample than the annealed sample supports the XPS result that more oxygen vacancies are present in the sample deposited at room temperature.


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Figure 3. (a) EPR spectra and (b) conductivity and resistivity of WO3-x thin films deposited at room temperature (RT) and annealed at 300 °C. Table 1. Oxygen to tungsten ratio and electronic properties of RT deposited and postannealed WO3-x thin films. ETL

O/W

Conductivity (S/cm)

Bandgap (Eg) (eV)

ECB (eV)

WO3-x RT

2.79

0.034

3.84

– 4.31

WO3-x 300 oC

2.91

0.012

3.91

– 4.38

The relative number of spins (Ns) of unpaired electrons (in other words the oxygen vacancy) participating in the resonance is calculated by using the formula below 39; Ns α I (ΔH)2 In this equation, I is the intensity, and ΔH is the width of the EPR line. From this equation the resultant Ns value calculated for the as deposited WO3-x is 1.53 x 108 which is much higher compared to annealed WOx (Ns = 3.6 x 107). This result is in good agreement with the XPS result, which shows a decrease in oxygen vacancy after annealing at 300 oC. Also, a broad EPR peak is observed for amorphous materials while a sharp peak is observed in crystalline materials.40 The peaks in Figure 3a support the amorphous nature of the as-deposited and annealed WO3-x thin films. In order to see the effect of oxygen vacancies on the electrical properties of WO3-x thin films, four-point-probe measurement were carried out. The resistivity of the as-deposited WO3-x film increased significantly after post-annealing the film at 300 °C in air as shown in Figure 3b, leading to reduced conductivity (Table S1). The reduction in conductivity of WO3-x thin films is due to the reduction in the amount of oxygen vacancies as reported in the literature.41



PSC with a structure of FTO/WO3/CH3NH3PbI3/Spiro-OMeTAD/Au were fabricated using 60 nm thick WO3-x film as ETL as shown in schematic in Figure 4a. The cross-sectional SEM image of the device is shown in Figure 4b. The J-V curve for the PSC using the 3003-x-RT and WO3-x-300 °C film at both reverse and forward scan is shown in Figure 4c. The active area of the devices was 0.1256 cm2 which was controlled with a black masked for the J-V measurement. The PCE of the device using the as-deposited WO3-x-RT ETL is 10.3 % for reverse scan and 10.08% for forward scan. Both the reverse and forward scans have the same Jsc value of 18 mA/cm2, while the Voc is slightly reduced from 0.87 to 0.86 V, from reverse scan to forward scan, respectively (see Figure 4c). The fill factor (FF) of the device based on WO3-x –RT is 65.5 % and 64.5 % for the reverse and forward scans, respectively. Statistic of the photovoltaic performance is shown in Figure S3. It is noted that the device using the asdeposited WO3-x-RT film has very little current-voltage hysteresis in the reverse and forward scans. In contrast the device with post-annealed WO3-x-300 °C film shows a reduced performance mostly because of much lower Voc and higher current-voltage hysteresis in the forward scan compared to the reverse scan measurements. We need to emphasize that although the power conversion efficiency of the PSCs in this work is much lower than the high efficiency PSCs reported in literature, which normally used SnO2 or TiO2 as electron transport layer (ETL) and has better energy alignment between the ETL and the perovskite film, leading to high Voc of around 1.1 V.42 However preparation of SnO2 or TiO2 based ETL requires high temperature annealing. The efficiency of the PSCs using the as-deposited WO3x-RT

film in this work is already one of the best according to literature on PSC using only

tungsten oxide as ETL.12,

43-44

The series resistance (Rs) and shunt resistance (Rct) of both

devices were calculated based on the J-V plot (Shown in Table S2). We have found the WO3x

-RT based device has lower series resistance (Rs = 6.1 Ωcm2) and larger Voc (Voc = 0.87 V)

compared to the WO3-x -300 ˚C (Rs = 7.4 Ωcm2, Voc = 0.82 V in reverse scan and Voc = 0.66


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for forward scan). This explains the higher FF of the WO3-x -RT based device than the WO3-x -300 ˚C based device. Figure 4d, shows the external quantum efficiency (EQE) for the best performing devices using WO3-x-RT and WO3-x-300 °C films. About 80 % of EQE spectrum is shown in the range from 400 to 760 nm for the WO3-x-RT based PSC. The integrated Jsc value obtained by IPCE for device using WO3-x-RT (18.1%) is in good agreement with the experimental value, whereas the Jsc obtained from IPCE (17 mA/cm2) for the device using WO3-x-300 °C is much lower than the experimental value (18.7 mA/cm2).

Figure 4. a) Schematic diagram of perovskite solar cell, b) Cross-sectional SEM image of actual perovskite solar cells device, c) Current-voltage (J-V) curve of PSC at both reverse (Rev) and forward (Fw) scan, and d) external quantum efficiency (EQE) of perovskite solar cell for both the WO3-x-RT and WO3-x-300 °C thin films.

Table 2. Reverse and forward scan photovoltaic I-V parameters of PSCs using the asdeposited and post-annealed WOx-3 films as ETL.



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Scan direction

Jsc (mA/cm2)

Voc (V)

FF

Efficiency (%)

WO3-x -RT

Reverse

18

0.87

65

10.3

WO3-x -RT

Forward

18

0.86

63

10.08

WO3-x -300 oC

Reverse

18.7

0.82

60

9.3

WO3-x -300 oC

Forward

18.7

0.66

59

7.5

ETL

Clearly the hysteresis increases after the as-deposited film is annealed at 300 °C in air. The bandgap of the as-deposited WO3-x increases from 3.84 eV to 3.91 eV after annealing, which means that the conduction band of the annealed film gets closer to the conduction band of the perovskite as shown in Figure 1b. There is not much difference in the Jsc of the reverse and forward measurements using the annealed WO3-x-300 °C. However, the Voc of the device is affected by scan direction dramatically as shown in Table 2 with larger value (0.82 V) in the reverse scan compared to the forward (0.66 V) scan. As shown in the XPS results, the asdeposited WO3-x is oxygen deficient and by annealing in air at 300 °C reduces the oxygen vacancies. Oxygen vacancies contribute free electrons to the conduction band and increase the film conductivity and electron transport properties.45 As shown in Figure 3b the electrical resistivity of the tungsten oxide film is increased due to reduced oxygen vacancies in the film.26 Also, it is reported by Gillet et al. that the defect band caused by oxygen vacancies improves the conductivity by reducing the bandgap.25 The increase in conductivity improves the electron extraction efficiency which contributes to lower hysteresis in the as-deposited WO3-x based PSC

46

(as shown in Table 2). Our results indicated that increased oxygen

vacancies in WO3-x thin films have proven benefit for enhanced PSC device performance and reduction in current-voltage hysteresis.


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Figure 5. (a) PL spectra and (b) Nyquist plots of PSCs under light using as-deposited and post-annealed WO3-x ETL. For comparison the PL of the perovskite absorber is also shown.

In order to further elucidate the effect of oxygen vacancies on charge transfer efficiency at the interface between perovskite and WO3-x ETLs, steady-state photoluminescence was conducted

for

glass/perovskite,

glass/WO3-x-300°C/perovskite

and

glass/WO3-x-

RT/perovskite. As shown in Figure 5a a photoemission peak at 780 nm is observed for all the samples which is originated from the perovskite material. When the perovskite absorbing layer is interfaced with the WO3-x, a clear quenching of the PL is observed. It is known that a decrease of PL intensity can be caused by non-radiative recombination and/or interfacial charge transfer, leading to reduction of electron-hole pairs which can release photon when recombines. Since all the perovskite films were made under the same condition and we did not observe noticeable morphological change of the film, we assume the properties of the perovskite should be largely the same. Therefore the contribution of non-radiative recombination on the PL should be the same as well. We have found that the reduction of the PL intensity is consistent with enlarged energy offset between the conduction band of the perovskite and the conduction band of the WO3-x which provides the driving force for charge injection from the perovskite to the adjacent WO3-x. As shown in Figure 1d, the driving force for charge injection between the perovskite and the WO3-x -RT is 0.48 eV, which is 70 mV 13 ACS Paragon Plus Environment


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higher than the driving force for charge transfer between the perovskite and WO3-x -300 ˚C. Therefore we believe the decrease of PL should be mainly due to interfacial charge transfer between the perovskite and the WO3-x based ETL. Hysteresis in a perovskite solar cell is caused by charge accumulation at the interface between perovskite and electron transport materials. To investigate the carrier recombination resistance of PSC using different WO3-x based ETL, impedance spectroscopy at open circuit voltage under 1 sun illumination was carried out. Figure 5b shows Nyquist plot of PSC using the WO3-x-RT and WO3-x-300 °C. The equivalent circuit of the Nyquist plot is shown in the inset of Figure 5b. The series resistance is represented by Rs in the equivalent circuit. The geometric capacitance of bulk material and surface, which reflects ion accumulation at the perovskite interface are represented by Cg and Cs, respectively. The sum of the resistive components, R1 and R2 is associated with the recombination resistance (Rrec) at the interface of WO3-x/perovskite. The fitted results from experimental date are shown in Table 3. Table 3. Extracted EIS parameters of perovskite solar cells measured under 1 sun illumination at open circuit voltage. Rs ETLs

( cm2)

R3

Cg (F/cm2)

( cm2)

Cs (F/cm2 )

R1 ( cm2)

WO3-x -RT

1.54

3.8 × 10-7

6.8

0.0005

11.3

WO3-x -300 ºC

1.55

3.8 × 10-7

2.8

0.002

8.7

As shown in Table 3, the recombination resistance i.e. R1 + R3 for PSC using the as-deposited (Rrec= 18.1 ( cm2)) WO3-x is almost 50% higher than the PSC using the annealed WO3-x (11.5 ( cm2)). This higher recombination resistance can be associated with the higher conductivity of the as-deposited WO3-x thin film. Also, the high recombination resistance at 14 ACS Paragon Plus Environment

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the ETL/perovskite interface might be the reason for low hysteresis.47-48 The charge recombination at the ETL/perovskite interface is mitigated by the improved charge injection (Cs of RT WO3-x is 5 x 10-4 F/cm2 and 2 x 10-3 F/cm2 for annealed WO3-x). The higher conductivity and lower conduction band position of the as-deposited WO3-x could be the driving force for better charge extraction from the perovskite material. Hysteresis index (HI) of the PSC which is defined by the following equation: HI = (PCErev – PCEfw)/PCErev HI was calculated for both the as-deposited and post-annealed WO3-x samples. The calculated HI using the WO3-x-RT ETL is 2.1% which is much lower than the WO3-x-300 °C ETL (25%). The higher conductivity and charge transport property of the as-deposited WO3-x with oxygen vacancies governs superior device performance with low J-V hysteresis. 4. Conclusion We have deposited oxygen deficient WO3-x thin film at room temperature using e-beam evaporation in high vacuum for perovskite solar cells. The oxygen vacancy of the WO3-x film was reduced to nearly stoichiometric composition after annealing the as-deposited film at 300 ºC

for 1 hour in air. Results show that the oxygen vacancies improve the film conductivity

and thereby enhance the Voc of the device. Oxygen vacancies also play a critical role in enhancing the photo-conversion efficiency and reducing the hysteresis by suppressing the recombination of photo-generated carriers. A PCE of 10.3% with minimum current-voltage hysteresis index of 2.1% were achieved from the PSC device using the room temperature deposited WO3-x film as ETL. The higher conductivity and charge transport property of the room temperature WO3-x with larger oxygen vacancies governs superior PSC device performance. It is also believed that this room temperature e-beam evaporatedWO3-x thin film as ETL can be a promising approach in developing flexible and tandem solar cells. 15 ACS Paragon Plus Environment


2. EXPERIMENTAL SECTION The materials used for experiment were purchased from Sigma-Aldrich and used as received, unless otherwise stated. For the preparation of Methylammonium lead triiodide (MAPbI3) perovskite films a Lewis acid-base adduct approach was used, details of which are described in the previous reports.10, 30-31 In Brief, a mixture of PbI2 and methylammonium iodide (MAI) (Dyesol), using 461 mg and 159 mg each respectively, was dissolved in 78 mg of dimethyl sulfoxide (DMSO) and 650 mg of dimethyl formamide (DMF) at room temperature, for the preparation of MAPbI3 perovskite precursor solution. The MAPbI3 solution was then filtered using syringe filter (pore size: 0.22 µm) prior to use for deposition of film. Spiro-OMeTAD based HTM solution was prepared by using 72.3 mg of 2,2’,7,7’-Tetrakis-(N,N-di-4methoxyphenylamino)-9,9’-spirobifluorene (Spiro-OMeTAD) (Borun New Material), 28.8 µL of 4-tert-butylpyridine, and 17.5 µL of Bis(trifluoromethane)sulfonimide lithium (Li-TFSI) solution (720 mg of Li-TFSI in acetonitrile) in 1 mL of chlorobenzene.

Device fabrication Solar cells were fabricated on fluorine-doped tin oxide (FTO) coated glass (Nippon Electric Glass, 15 /) as substrate. The substrate was patterned through partial removal of FTO via etching using 35.5 wt% HCl and zinc powder. Then a 5% Decon-90 detergent and a mixture of acetone, isopropanol and ethanol were used to clean the substrate for 20 mins in an ultrasonic bath, respectively. Prior to use, the substrate was treated with ultraviolet for 30 mins to fully remove organic solvent residuals. WO3-x thin films were developed using

electron beam evaporation technique (PVD 75 Kurt J. Lesker) in high vacuum (

Low Hysteresis Perovskite Solar Cells Using an Electron-Beam

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