Highly stable and efficient planar perovskite solar cells using ternary metal oxide electron transport layers

Highly stable and efficient planar perovskite solar cells using ternary metal oxide electron transport layers

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Journal of Power Sources xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Highly stable and efficient planar perovskite solar cells using ternary metal oxide electron transport layers M. Thambidurai a, b, Foo Shini a, b, c, P.C. Harikesh b, Nripan Mathews b, c, Cuong Dang a, b, * a

LUMINOUS! Centre of Excellence for Semiconductor Lighting and Displays, School of Electrical and Electronic Engineering, The Photonics Institute (TPI), Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore b Energy Research Institute @NTU (ERI@N), Research Techno Plaza, X-Frontier Block, Level 5, 50 Nanyang Drive, 637553, Singapore c School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Optimized ternary zinc-tin oxide (ZTO) as efficient ETL candidates. � Zn1Sn1Ox/SnO2 ETL based devices exhibited paramount PCE of 19.01%. � Extended stability with 90% retainment of initial PCE in champion device. � Zn1Sn1Ox/SnO2 device showed better charge transfer and reduced recombination.

A R T I C L E I N F O

A B S T R A C T

Keywords: Zinc tin oxide Electron transport layer Perovskite solar cells Long-term stability Photovoltaic performance

In planar perovskite solar cells, the electron transport layer (ETL) plays a vital role in effective extraction and transportation of photogenerated electrons from the perovskite layer to the cathode. Ternary metal oxides exhibit excellent potentials as ETLs since their electrical and optical properties are attunable through simple composi­ tional adjustments. In this paper, we demonstrate the use of solution-processed zinc oxide (ZnO) and zinc tin oxide (ZTO) films as highly efficient ETLs for perovskite solar cells. We observe poor compatibility between ZnO and perovskite which impedes device reproducibility, stability, and performance unlike ZTO ETL devices. Furthermore, we modify the ZTO/perovskite interface by introducing a thin passivating SnO2 interlayer. The Zn1Sn1Ox/SnO2 ETL device demonstrates paramount power conversion efficiency (PCE) of 19.01% with corre­ sponding short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) values of 21.93 mA cm 2, 1.10 V, and 78.82%. Moreover, the Zn1Sn1Ox/SnO2 ETL device displays superior stability, maintaining 90% of its initial PCE after 90 days in the absence of encapsulation at relative humidity of 30–40%. Enhancement in charge extraction, favourable energy alignment, and reduction in recombination sites greatly contribute to the optimal performance, stability, and reproducibility of the Zn1Sn1Ox/SnO2 ETL device.

* Corresponding author. LUMINOUS! Centre of Excellence for Semiconductor Lighting and Displays, School of Electrical and Electronic Engineering, The Photonics Institute, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore. E-mail address: [email protected] (C. Dang). https://doi.org/10.1016/j.jpowsour.2019.227362 Received 5 August 2019; Received in revised form 21 October 2019; Accepted 29 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: M. Thambidurai, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227362

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1. Introduction

perovskite device [11]. Likewise, Li et al. investigated the plausibility of Ti–Fe–O ternary oxides as efficient ETLs and demonstrated reduction in surface recombination and favourable charge extraction properties whereby Ti0.5Fe0.5Ox based perovskite device attained the highest effi­ ciency of 14.7% [28]. Pang et al. demonstrated PCE of 17.21% when Zn2Ti3O8 nanoparticles were utilized in perovskite solar cells [29]. In perovskite solar cell application, Zn2SnO4 ternary metal oxide shows great potential as a promising ETL candidate [30–33]. With a wide band gap of 3.7 eV, Zn2SnO4 is a n type semiconductor with high electron mobility of 10–15 cm2 V–1S 1 and low reflective index of 1.37 [27,32]. In addition, Zn2SnO4 shows good chemical stability in both acid/base and ambient conditions [34]. For instance, Oh et al. used Zn2SnO4 nanoparticles in perovskite solar cells and reported an approximate PCE of 7% [35]. Liu et al. also showed superior performance of 14.5% when Zn2SnO4 ETL was employed, instead of the conventional TiO2 ETL of 13.01% [34]. Wu et al. demonstrated good PCE whereby its champion cell attained PCE of 16.38%. Similarly, high performance of 17.21% arising from the favourable intrinsic photoelectrical characteristics of Zn2SnO4 was also reported [36]. In this paper, we examine the use of various ternary zinc-tin oxide (ZTO) as plausible compact ETL candidates for efficient extraction of electrons from the adjacent perovskite layer in solar cell application. Using simple solution-based techniques, different binary and ternary metal oxide layers were fabricated as the underlying electron trans­ porting layer (ETL) in which their chemical, optical, and photovoltaic characterizations were investigated. It was discovered that general performance and stability of the ZnO ETL based devices was poor due to the presence of unfavourable chemical reactions between ZnO/perov­ skite interface. With the addition of tin, we found that both efficiency and device stability improved significantly, with the best photovoltaic performance attained when the molar ratio of Zn to Sn was 1:1. To prevent direct contact between ZnO and perovskite, passivation at the ZTO/perovskite interface was employed using a thin layer of SnO2. Benefitting from the superior electron mobility and passivating char­ acter of SnO2, dominant PCE of 19.01% was attained when Zn1Sn1Ox/ SnO2 bilayer ETL was utilized. Remarkable device stability was also observed in which 90% of its initial PCE was retained despite storage in the absence of encapsulation for 90 days. In essence, this study suggests the favorability of ZTO as a prime ETL, an alternative to the current ETL selections, due to its excellent intrinsic properties for high performance in perovskite solar cell applications. By using the Zn1Sn1Ox/SnO2 bilayer ETL, enhancements in photovoltaic parameters, reproducibility, stabil­ ity, charge transfer, and also reduced carrier recombination were demonstrated in this study.

Since their initial introduction in 2009, organic-inorganic metal halide perovskite solar cells continue to excite and appeal to many solar activists as a viable replacement to existing photovoltaics such as silicon solar cells. This is because perovskite possesses several unprecedented qualities such as a high absorption coefficient, long charge carrier diffusion lengths, a tunable band gap, low exciton binding energy, and the ability to utilize relatively cost-effective fabrication methods [1–6]. Within a short span of 10 years, the power conversion efficiency (PCE) of perovskite based solar cells escalated from an unassuming 3.8% to the latest certified PCE of 23.7% in 2018 [7–9]. In a typical n-i-p configuration, the n-type material acts as the elec­ tron transporting layer (ETL) to effectively extract as well as transport photogenerated electrons from the perovskite absorbing film to the conducting fluorine-doped tin oxide (FTO) substrate. Similarly, photo­ generated holes are selectively transported by the hole-transporting layer (HTL) before collection at the back electrode such as gold or sil­ ver [10,11]. With the intention of achieving higher PCEs, both meso­ porous and planar transporting layers have been explored although the former usually produces higher PCE compared to the latter. By allowing the perovskite absorbing material to infiltrate the mesoporous ETL, the mesoporous structure is believed to greatly enhance electron extraction and hence the overall conversion efficiency. That being said, develop­ ment of highly efficient planar perovskite solar cells remains preferable due to their ease in commercialization while the fabrication of meso­ porous layers is much more complicated and costly [12–14]. Unlike its counterpart, effective charge extraction using planar ETLs is more difficult due to their limited surface area. As such, the intrinsic prop­ erties of planar ETLs are required to be more pivotal and critical. Apart from the well-aligned energy level between the absorbing layer and ETL, the transporting material should also have high electron mobility so as to favour electron injection and rapid electron transportation [12,15]. Moreover, planar ETLs should be compact and free of pinholes to pre­ vent shunting and possible charge leakages. Multiple transporting layers are often employed to mitigate possible losses in device performances due to shunting pathways. As a matter of fact, conventional perovskite devices also utilize two electron transporting layers: the compact tita­ nium dioxide (c-TiO2) and the mesoporous TiO2 layers [16–18]. The most frequently employed ETL remains as c-TiO2 owing to its favourable energy levels, promising optical properties, and assuring electrical characteristics. Yet, planar TiO2-based perovskite solar cells often exhibit severe hysteresis which is believed to be caused by the presence of defects and traps at the c-TiO2 and perovskite interface. Furthermore, c-TiO2 has a relatively low electron mobility of 0.1–10 cm2V 1s 1 [19]. Its poor electron mobility not only limits effective transportation but also increases the probability of charge recombination. As such, researchers explore other n-type materials such as, ZnO, Al2O3, and SnO2 [5,20–22]. The employment of ZnO as the n-type ETL shows promising properties owing to its appropriate wide band gap of 3.2 eV, high transparency, and high bulk mobility of 200–300 cm2 V 1 S 1 [19,23,24]. Furthermore, Wang et al. showed the possibility of achieving relatively high PCE of 16.25% using indium-doped ZnO with negligible hysteresis [25]. Yet, ZnO ETLs for solar cell application are less prevalent. This could be due to the chemical instability of ZnO and thermally induced degradation at the ZnO/perovskite interface [26]. In comparison to the more widely utilized binary oxides, ternary oxides offer greater degree of freedom by which various properties of ternary metal oxides materials can be tuned via simple adjustment in their chemical stoichiometry. Recently, ternary metal oxides have gained popularity as a competent electron transporting layer (ETL). For example, Shin et al. utilized BaSnO3 ternary metal oxides in dyesensitized solar cells and reported efficiency of 6.2% [27]. By exploring various Ti–Zn ternary systems, Yin et al. showed high PCE of 15.10% when a compact TiZn2O4 ETL film was employed in the

2. Experiments Preparation of ZTO compact layer: In our study, ZTO films were prepared by dissolving 0.1 M of zinc acetate dihydrate (Zn(CH3COO)2 ⋅ 2H2O; Sigma Aldrich) and 0.05 M, 0.1 M, or 0.15 M of tin(II) chloride dihydrate (SnCl2 ⋅ 2H2O; Sigma Aldrich) in 2-methoxyethanol (5 mL) and acetylacetone (100 μL). The molar ratio of zinc acetate dihydrate and tin (II) chloride dihydrate was varied 1:0.5, 1:1, and 1:1.5. The precursor solution was stirred at 60 � C for 3 h and solution was spin coated onto FTO substrates at 3000 rpm (500 rpm acceleration) for 30 s. Then the coated films was annealed at 500 � C for 1 h. ZnO films were prepared without tin(II) chloride dihydrate by the same procedure as that used for ZTO layer. Preparation of SnO2 layer: The SnO2 solution was prepared by dissolving tin (II) chloride dihydrate (0.1 M) in absolute ethanol. The SnO2 solution was deposited through a two-step spin coating program; first at 1500 rpm for 10 s and then at 5000 rpm for 10 s. The freshly spun SnO2 films were then annealed at 200 � C for 1 h and transferred into a N2 filled glove box. Device fabrication: FTO glass substrates (Nippon sheet glass Co., Ltd., Japan) were washed in detergent before sequential sonication in 2

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acetone, ethanol, and isopropyl alcohol for 30 min each. After 20 min of UV treatment, the compact ZTO layers were spin-casted onto the sur­ faces of the dried FTO substrates and heated to 500 � C for an hour. To prepare the bilayer ETLs, UV treatment was carried out before the deposition of the thin SnO2 layer onto the surfaces of the as-prepared samples and annealed at 200 � C for an hour. The triple-cation perov­ skite solution was prepared using 28 mg of MABr (Dyesol), 101 mg of PbBr (Tokyo Chemical Industry), 215 mg of FAI (Dyesol), 633.8 mg of PbI2 (Tokyo Chemical Industry), dissolved in 800 mL anhydrous DMF (Sigma Aldrich) and 200 mL DMSO (Sigma Aldrich). The as-prepared solution was then left to dissolve before 52.6 μL of CsI (1.5 M) stock solution was added to the solution and stirred. Two-step spin coating procedure of the perovskite solution was conducted: (1) 1000 rpm for 10 s and (2) 6000 rpm for 22 s. With 7s remaining in the second step, 100 μL of chlorobenzene was drop casted onto the spinning substrates. Subsequently, the perovskite films were annealed at 100 � C for an hour. di-4-methoxyphenylamine)-9,9The 2,20 ,7,70 -tetrakis(N,Nspirobifluoren (Spiro-OMeTAD; Sigma Aldrich) layer was spin-casted at 4000 rpm for 30 s, in which 70 mg of Spiro-OMeTAD was dissolved in 1 mL of chlorobenzene. After that 28, 16.94, and 35 μL of 4-tert-butyl­ pyridine (Sigma Aldrich), bis(trifluoromethylsulfonyl)amine lithium salt (520 mg/mL in acetonitrile; Sigma Aldrich), and FK209 of Co(III) TFSI Salt (18.8 mg/50 μL in acetonitrile; Sigma Aldrich) were added to the mixture. Finally, gold electrodes were evaporated onto the sample to form the back metal contact. The thicknesses of the metal oxide films are 31, 35, 40, 46 and 40 nm for ZnO, Z1T0.5O, Z1T1O, and Z1T1.5O, and SnO2, respectively. The film thicknesses of perovskite and spiroOMeTAD are 480 and 200 nm. Characterization: Feld emission scanning electron microscopy (FESEM; JEJOL JSM-7600F) was utilized to analyze the cross-sectional morphologies of the active absorber surfaces. X-ray diffraction (XRD; Bruker D8 Advance XRD), X-ray photoelectron spectroscopy (XPS; Kratos AXIS Supra XPS), and ultraviolet photoelectron spectroscopy (UPS; Kratos AXIS Supra) characterized the structural and chemical properties of the samples. Using the Keithley 2612A source meter, cur­ rent density against voltage (J-V) plots were recorded under one-sun AM 1.5G (100 mW/cm2), with its light intensity calibrated against the traditional silicon solar cell. PVE300 (Bentham) was utilized to evaluate the incident photon-to-current conversion (IPCE) of the devices. UV-VIS spectrophotometer (Shimadzu, UV-1800) and spectrofluorophotometer (Shimadzu, RF-5301PC) were employed to record the absorption, transmittance spectra and photoluminescence (PL) spectra, respectively. Conducted at a DC voltage of 800 mV, impedance spectroscopy (Autolab PGSTAT302 N) was conducted under dark condition with applied perturbation of frequencies from 10 Hz to 1 MHz.

ascribed to the oxygen deficiency and surface oxygen and hydroxyl groups [40]. Fig. 1a–c shows the XPS spectra of the prepared Z1T0.5O, Z1T1O, and Z1T1.5O films. The Zn2p, Sn3d, and O1s peaks found in the ternary oxide films are all shifted toward lower binding energies by which increasing the tin content resulted in greater shifts in peak posi­ tions as seen in the spectral trend from Z1T0.5O to Z1T1.5O. The XPS spectra of SnO2 film is shown in Fig. S3. XPS spectra of tin shows a peak doublet representing the Sn3d5/2 and Sn3d3/2 at binding energies of 487.3 eV and 495.7 eV, respectively. The binding energy difference of 8.4 eV between the Sn3d5/2 and Sn3d3/2 indicates that the Sn4þ state is present [37]. Ultraviolet photoelectron spectroscopy (UPS) measure­ ments were carried out to study the electronic structures and energy levels of the binary and ternary oxide films. The low binding energy (Eonset) and high binding energy cutoff (Ecutoff) of the UPS spectra of binary and ternary oxide films are shown in Fig. 1d–f & S4. The valence band maximum (VBM) energy levels of ZnO, Z1T0.5O, Z1T1O, Z1T1.5O, and SnO2 films are 7.76, 7.70, 7.67, 7.64 and 7.88 eV, respec­ tively, calculated from the equation VBM ¼ ½21:22 ðEcutoff Eonset Þ� [41]. The optical band gaps of the binary and ternary oxide films are determined from the plot of (αhυ)2 as a function of photon energy (hυ) as shown in Fig. S5 &S6. The calculated band gap of ZnO, Z1T0.5O, Z1T1O, Z1T1.5O, and SnO2 films are 3.45, 3.54, 3.60, 3.67 and 3.80 eV, respec­ tively. The conduction band minimum (CBM) of the binary and ternary oxides are calculated based on EVBM and band gaps. The CBM of ZnO, Z1T0.5O, Z1T1O, Z1T1.5O, and SnO2 are 4.31, 4.16, 4.07, 3.97 and 4.08 eV, respectively. Fig. 2a provides a simple illustration of the electron and hole extraction procedure and the corresponding energy levels of each layer in a complete perovskite solar device. Displaying well-aligned valence band and conduction band energy levels between the as-prepared binary and ternary oxide ETLs and adjacent perovskite film, efficient electron extraction and effective blocking of hole transference are expected. The device architecture of the complete device is shown in Fig. 2b in which the compact electron transporting film was spun on the surface of the cleaned fluorine-doped tin oxide (FTO) substrate before annealing for an hour. A light absorbing layer was then deposited using the two-step antisolvent method followed by further heat treatment at 100 � C for effective nucleation and growth of the triple cation perovskite film. To allow effective extraction of electron-hole pairs, spiroOMeTAD was then coated on top of the perovskite absorber layer to act as an efficient holetransporting layer (HTL). Finally, a thin layer of gold was thermally evaporated as the top electrode to form the complete solar device. To understand the effect and suitability of the ternary films as effective ETLs, optimization of the ZTO films was carried out by varying the amount of tin into a fixed concentration of zinc containing precursor solution. Specifically, the photovoltaic performances of the ZnO, Z1T0.5O, Z1T1O, and Z1T1.5O ETL based devices were measured under AM1.5G simulated irradiation with 100 mW/cm2 illumination. Fig. 2c shows the current density voltage (J V) characteristics of the binary and ternary oxide based perovskite devices and their corresponding photovoltaic parameters are summarized in Table 1. In essence, the ternary ZTO ETL based devices showed superior device performance compared to the ZnO ETL based device whereby increasing the tin content increases the device efficiency in which the highest PCE of 16.41% was achieved when the tin content equal to the zinc content. The champion Z1T1O based device obtained corresponding open circuit voltage (Voc) of 1.05 V, short circuit current density (Jsc) of 21.18 mA cm 2, and fill factor (FF) of 74.0%. In contrast, the ZnO device has a lower PCE of 10.77% with Voc of 0.96 V, Jsc of 18.37 mA cm 2 and FF of 60.99%. However, photovoltaic performance declined when too much tin was added to the ZTO film. The incident photon-to-current conversion efficiency (IPCE) of Z1T1O based perovskite solar cells were measured, as shown in Fig. 2d. The integrated Jsc values calculated from the IPCE spectra was 19.82 mA cm 2 for the Z1T1O device, which is in good agreement with the measured values of Jsc. We further measured the stabilized power output for the ZnO and Z1T1O based devices under

3. Results and discussion Binary and ternary oxide layers were fabricated as the underlying electron transporting layer (ETL) responsible for efficient extraction of electrons from the adjacent perovskite layer. Specifically, binary oxides of ZnO and SnO2 along with ternary oxides whose nominal compositions are Zn1Sn0.5Ox, Zn1Sn1Ox, and Zn1Sn1.5Ox termed as Z1T0.5O, Z1T1O, and Z1T1.5O films were synthesized, respectively. Fig. S1 shows the x-ray diffraction (XRD) patterns of the binary (ZnO, SnO2) and ternary oxide (Z1T0.5O, Z1T1O, and Z1T1.5O) ETLs. With no noticeable peak present in the XRD patterns, all films were amorphous structures with no orien­ tation. To verify the successful fabrication of the as-prepared films, the surface chemical states and compositions were investigated using x-ray photoelectron spectroscopy (XPS). In good agreement with other re­ ports, Fig. S2a presents the typical XPS data of Zn2p3/2 and Zn2p1/2 peaks located at approximately 1022.3 eV and 1045.3 eV, with a 23.0 eV binding energy difference between the two peaks, respectively. Hence, we can conclude that the oxidation state of the Zn ions is Zn2þ [37–39]. The O1s spectra represented by Fig. S2b can be further deconvoluted into two Gaussian peaks. The peaks at 531.1 eV and 532.5 eV can be 3

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Fig. 1. XPS spectra of ZTO films with different tin concentrations (a) Zn 2P, (b) Sn 3d and (c) O 1S. UPS spectra of (d) Z1T0.5O, (e) Z1T1O, and (f) Z1T1.5O films.

Fig. 2. (a) Energy level alignment diagram of the planar perovskite solar cell. (b) Schematic diagram of the as-prepared perovskite solar device. (c) Current densityvoltage (J–V) curves of the ZnO and ZTO based perovskite solar cells. (d) IPCE spectra of the Z1T1O based device. (e) The steady state efficiency of pristine ZnO and Z1T1O based devices measured at maximum power output. (f) Statistical distribution of power conversion efficiency (PCE) with different ETLs.

constant voltage bias at maximum power point tracking (0.71 V for ZnO, 0.80 V for Z1T1O) as shown in Fig. 2e. The stabilized PCEs of 8.91% and 15.09% were obtained for the ZnO and Z1T1O based devices, respec­ tively. Fig. 2f shows the statistical distribution of PCE with different ETLs. The average PCEs of the ZnO, Z1T0.5O, Z1T1O and Z1T1.5O based devices were 9.07%, 13.18%, 15.57% and 14.93%, respectively. In addition, stability tests were also conducted in a controlled atmosphere

(40% relative humidity and 25 � C temperature) without encapsulation. The stability of the Z1T1O based device was significantly improved in comparison to the pristine ZnO based device. As shown in Fig. S7, the Z1T1O device retained 99% of its initial PCE after 24 h. By contrast, fast degradation was observed for the ZnO device in which PCE quickly declined to 61% of initial PCE within 24 h. The instability can be explained by the incompatibility between the perovskite layer and ZnO 4

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observed. To evaluate the influence of the SnO2 interlayer at the binary or ternary oxide/perovskite interface, devices having general configu­ rations of FTO/binary or ternary oxide/SnO2/perovskite/spiro-OMe­ TAD/Au were fabricated and tested. Fig. 3b & S8 shows the photocurrent density-voltage (J-V) curves of fabricated devices measured under simulated solar light. As expected and further sum­ marized in Table 1, overall photovoltaic performances were signifi­ cantly enhanced through the addition of a SnO2 interlayer. Following a similar trend as the single layer devices, the photovoltaic performance of the bilayer ETL based devices demonstrated enhancement in the ternary ZTO compared to the binary compound, with the Z1T1O/SnO2 bilayer ETL based device presenting the highest PCE. The control ZnO/SnO2 bilayer ETL based device showed PCE of 17.54% with Voc, Jsc, and FF of 1.08 V, 20.96 mA cm 2, and 76.98%, respectively. Remarkably, the champion Z1T1O/SnO2 bilayer ETL based device showed PCE of 19.01%, resulting from corresponding Voc of 1.10 V, Jsc of 21.93 mA cm 2 and FF of 78.82%. In comparison, the pristine SnO2 device (Fig. S9) attained lower PCE of 16.90%, Voc of 1.08 V, Jsc of 20.89 mA cm 2, and FF of 74.59%. As seen in Fig. S10, hysteresis exist in our study in which all ETL compositions displayed notable amount of hysteresis although hystere­ sis in the Z1T1O/SnO2 ETL device is smaller than that of ZnO/SnO2 ETL based device. Using the equation of Hysteresis Index ðHIÞ ¼

Table 1 Photovoltaic parameters of perovskite devices with different ETLS. ETLs ZnO Z1T0.5O Z1T1O Z1T1.5O ZnO/SnO2 Z1T0.5O/SnO2 Z1T1O/SnO2 Z1T1.5O/SnO2 SnO2

Voc

Jsc 2

FF

PCE

[V]

[mA cm ]

[%]

[%]

0.96 0.99 1.05 1.03 1.08 1.10 1.10 1.10 1.08

18.37 19.74 21.18 20.82 20.96 21.41 21.93 21.22 20.89

60.99 72.90 74.00 74.63 76.98 77.27 78.82 76.87 74.59

10.77 14.34 16.41 16.00 17.54 18.23 19.01 18.10 16.90

ETL [42,43]. Henceforth, the as-fabricated ternary films appear as convincing and effective ETL candidates for efficient perovskite solar devices, particularly the Z1T1O ETL film. As mentioned above, unfavourable reactions at the ZnO/perovskite interface often result in reduced photovoltaic performance and stability of the perovskite solar devices. To prevent direct contact of the ZnO film with the perovskite layer, we further explored the use of SnO2 interlayer between the ZTO and the perovskite layers to form a bilayer ETL structure. Moreover, the bilayer ETL impedes formation of pinholes and penetration of the perovskite material, preventing the possibility of current leakage issues. With the simple insertion of a thin SnO2 layer as seen in the cross-section image represented by Fig. 3a, enhancement in photovoltaic parameters, reproducibility, stability, and charge trans­ ference, together with suppression of carrier recombination were

PCE ðReverseÞ PCE ðForwardÞ , PCE ðReverseÞ

HI obtained from the Z1T1O/SnO2 ETL based

device is approximately 0.116 while HI of the ZnO/SnO2 ETL based device is larger of 0.193, as seen in Table S1. This signifies that the degree of hysteresis is less severe in the Z1T1O/SnO2 ETL based device, although still present. The IPCE spectra of ZnO/SnO2 and Z1T1O/SnO2 bilayer ETL based devices are shown in Fig. 3c. In good agreement with

Fig. 3. (a) Cross-sectional scanning electron microscopy (SEM) images of Z1T1O/SnO2 bilayer ETL based perovskite solar cells. (b) Current density-voltage (J–V) curves of the ZnO/SnO2 and Z1T1O/SnO2 bilayer ETL based perovskite solar cells. (c) IPCE spectra of ZnO/SnO2 and Z1T1O/SnO2 bilayer ETL based device. (d) The maximum power point tracking (MPPT) of the bilayer ETL based perovskite solar cells based on (d) ZnO/SnO2 and (e) Z1T1O/SnO2. Statistical distribution of (f) Voc, (g) Jsc, (h) FF, and (i) PCE of devices with different ETLs. The statistical data were obtained from 20 cells for each molar ratio. 5

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the measured Jsc values, the integrated Jsc values were calculated from IPCE to be 19.68 mA cm 2 and 20.76 mA cm 2 for the ZnO/SnO2 and Z1T1O/SnO2 bilayer ETL based devices, respectively. Maximum power point tracking (MPPT) was also performed to evaluate the stabilized power output (SOP). As shown in Fig. 3d, the ZnO/SnO2 ETL based device demonstrated Jsc of 18.80 mA cm 2 and PCE of 15.93% at voltage bias of 0.84 V. On the other hand, perovskite solar cells with Z1T1O/ SnO2 (Fig. 3e) display Jsc of 20.12 mA cm 2 and PCE of 17.94% at voltage bias of 0.89 V. Compared to the results in Fig. 2e, the SnO2 interface layer displayed critical importance to the prolonged device stability. To evaluate the device reproducibility with the binary and ternary metal oxide/SnO2 ETLs, 20 solar cells of each molar ratio were fabri­ cated and recorded, as seen in Fig. 3f–i. In good agreement with the J-V results above, average photovoltaic parameters remained superior for the Z1T1O/SnO2 ETL samples compared to other configurations. The average PCE of ZnO/SnO2, Z1T0.5O/SnO2, Z1T1O/SnO2, and Z1T1.5O/ SnO2 were 16.74%, 17.23%, 18.18%, and 17.12%, respectively. As seen in Fig. S11, statistical data was also obtained for the SnO2 based devices in which an average PCE of 15.90% was obtained. Effectively, statistics analysis affirmed the consistency and high performance of the bilayer ETL based devices compared to the single layered ternary oxide based device (Fig. 2f). Long-term stability of the bilayer ETL based devices stored in a dark and controlled atmosphere (40% relative humidity and 25 � C) for 90 days was investigated. As seen in Fig. 4a–d, initial PCE of ZnO/SnO2 ETL based device retained 80% PCE after 90 days. In contrast, the Z1T1O/ SnO2 ETL based devices displayed superior stability with 90% of their initial PCE even in the absence of encapsulation. In addition, statistical distributions collected for the long-term stability of ZnO/SnO2 ETL based devices, as well as Z1T1O/SnO2 ETL based devices, are recorded in Fig. S12&S13 for five separate devices. It is important to note that bilayer ETL based devices displayed exceptional stability compared to the single layer ETL devices mentioned previously. Regardless of the underlying composition of the first layer in the bilayer ETL structure, it is evident that the utilization of the second layer enhances not only the photovoltaic performance of the solar devices but also their long-term

stability. Optimizing the composition of the ternary layer produced the Z1T1O/SnO2 ETL with the finest performance and stability. With the addition of a SnO2 layer to from a bilayer ETL structure, transportation of electrons is expected to improve due to the high electron mobility and passivating properties observed in SnO2 films [44]. Fig. 5a shows the absorption spectra of perovskite on glass, perovskite on various bilayer ETLs in which slight blue shift of the perovskite films on bilayer ETLs were observed compared to the pristine perovskite film on glass. To investigate the charge transportation and recombination properties of the bilayer ETL based devices, steady-state photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) characterizations were carried out. Fig. 5b shows the PL spectra of perovskite on glass alone and perovskite on ZnO/SnO2, Z1T0.5O/SnO2, Z1T1O/SnO2, and Z1T1.5O/SnO2 ETLs excited at a wavelength of 580 nm. Although, possessing a similar characteristic peak at 763 nm, it is clear that the PL intensity of Z1T1O/SnO2/perovskite on glass is much smaller compared to the rest of its counterparts. The highly quenched PL intensity indicates superior charge extraction and transportation of the Z1T1O/SnO2 ETL compared to other bilayer ETL configurations. In addition, electrochemical impedance spectroscopy measurements were conducted at a forward bias of 800 mV in dark condition, Fig. 5c rep­ resents the Nyquist plots of the ZnO/SnO2, Z1T0.5O/SnO2, Z1T1O/SnO2, and Z1T1.5O/SnO2 ETLs comprising of both the experimental and fitted data indicated by the symbol and line, respectively. Normally, there are two arcs in the Nyquist plot; the first arc at high frequency region is related to the contact resistance of the interface and the second arc at lower frequencies corresponds to the recombination resistance and chemical capacitance of a device [17,45–48]. Recorded in Table S2 and Fig. S14, the series resistance (Rs) and recombination resistance (Rrec) were obtained along with its equivalent circuit model (Fig. S15). As seen in Table S2, the recombination resistance (Rrec) of the Z1T1O/SnO2 ETL device is significantly larger than that of the other bilayer ETL based devices. 4. Conclusions In summary, this paper systematically investigated the plausibility of

Fig. 4. Long-term stability of bilayer ETL based devices stored in a controlled environment (40% relative humidity and 25 � C) for 90 days without encapsulation, specifically (a) Voc, (b) Jsc, (c) FF and (d) PCE. 6

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Fig. 5. (a) Optical absorption and (b) photoluminescence spectra of the perovskite coated on ZnO/SnO2, Z1T0.5O/SnO2, Z1T1O/SnO2, and Z1T1O/SnO2 film. (c) Nyquist plots of ZnO/SnO2, Z1T0.5O/SnO2, Z1T1O/SnO2, and Z1T1O/SnO2 bilayer ETL based devices.

ternary metal oxides, specifically ZTO, as efficient ETL candidates for planar perovskite solar cell applications as mentioned in Table S3. Poor chemical stability at the ZnO/perovskite interface has proven to be deleterious to the device performance and stability. With the usage of ZTO ETL, improved PCE and device stability were achieved with the Z1T1O ETL device showing highest PCE of 16.41%. By implementing the bilayer ETL configuration, paramount PCE of 19.01% and an average PCE of 18.18% were obtained for the Z1T1O/SnO2 ETL based device. Apart from the superior long-term stability in which 90% of its initial PCE was maintained even after 90 days, enhanced electron trans­ portation, improved consistency, and reduced charge recombination were also demonstrated. Our work suggests the viability of Z1T1O ternary metal oxides as effective ETL candidates and facile interfacial engineering as a practical approach to solve the incompatibility at the ZTO/perovskite interface with improved stability, performance, repro­ ducibility of the perovskite photovoltaic device.

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