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Highly Efficient Amorphous Zn2SnO4 ElectronSelective Layers Yielding over 20% Efficiency in FAMAPbI3‑Based Planar Solar Cells Downloaded via HONG KONG UNIV SCIENCE TECHLGY on September 17, 2018 at 21:22:07 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Kyungeun Jung,†,‡ Jeongwon Lee,‡ Chan Im,† Junghwan Do,† Joosun Kim,§ Weon-Sik Chae,∥ and Man-Jong Lee*,†,‡ †
Department of Chemistry, Konkuk University, Seoul 143-701, Republic of Korea Department of Advanced Technology Fusion, Konkuk University, Seoul 143-701, Republic of Korea § High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea ∥ Analysis Research Division, Daegu Center, Korea Basic Science Institute, Daegu 702-701, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Amorphous Zn2SnO4 (am-ZTO) films with extreme surface uniformity, high electron mobility, and fewer charge traps were successfully developed by controlling the concentrations of 2methoxyethanol solutions containing the 2:1 stoichiometric ratio of Zn to Sn. For the first time, we demonstrate that solution-processed amZTO thin films are highly efficient as an electron-selective layer (ESL) for mixed perovskite solar cells (PSCs). When am-ZTO ESLs were combined with bandgap-tuned FAMAPbI3 perovskites, a champion efficiency of 20.02% was achieved. In addition, devices based on am-ZTO showed a statistical reproducibility of 18.38 ± 0.61% compared to 15.85 ± 1.02% of the TiO2-based counterparts. This high efficiency is achieved by the significant increase in both the short-circuit current and opencircuit voltage owing to improved charge transport/extraction and recombination. Moreover, am-ZTO ESL-based devices show improved stability and reduced hysteresis, which is a promising result for future PSC research. electron mobility (0.1−1 cm2 V−1 S−1).5 Indeed, low electron mobility causes insufficient charge separation at the ESL/ perovskite interface.6−10 Furthermore, TiO2 shows low photonic instability under ultraviolet (UV) illumination.5,11 To solve this problem, an additional layer7,10,12 and proper dopants3 on the compact TiO2 have to be added to the compensate for electron extraction because hole extraction is more efficient than electron extraction in PSCs.12,13 Several alternative semiconductor metal oxides have been reported to complement the disadvantages of TiO2. The most highlighted ESL is SnO2 films because they achieve a certified PCE of >20% based on mesoporous-free planar devices,14 which is expected to further increase when combined with mixed perovskites. Semiconducting Zn2SnO4 (ZTO) is another promising metal oxide with excellent optical and electronic properties, which are actively applied in thin-film
O
rganic−inorganic lead halide perovskite solar cells (PSCs) have shown unique enhancement in the power conversion efficiency (PCE) of up to 22.1%1 and are expected to be a strong alternative to Si-based solar cells owing to their outstanding performance and costeffectiveness. Among the elemental films in a PSC architecture, the electron-selective layers (ESLs) play a seminal role because they critically determine the separation and extraction of photogenerated charge carriers from perovskite to ESL. An ideal ESL should have no interface recombination and charge injection/transport losses. Thus, developing reliable and easily scalable methods to synthesize ESLs with an optimized surface roughness and controlled optoelectronic properties is needed to achieve highly efficient PSCs. Conventionally, the sol−gel processed TiO2 is widely adopted as ESLs.2,3 However, pristine TiO2 has several disadvantages in its reproducibility and performance; poor reproducibility stems from the considerable inherent deep and mid-bandgap traps in solution-processedTiO2, inducing charge accumulation at the selective layer and interfacial charge recombination.4,5 Low performance is attributed to low conductivity (∼1.1 × 10−5 S cm−1) and © XXXX American Chemical Society
Received: August 16, 2018 Accepted: September 13, 2018
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DOI: 10.1021/acsenergylett.8b01501 ACS Energy Lett. 2018, 3, 2410−2417
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Cite This: ACS Energy Lett. 2018, 3, 2410−2417
Letter
ACS Energy Letters
Figure 1. (a) XRD patterns of am-ZTO films formed at various precursor concentrations (0.15, 0.30, 0.45, 0.75, and 1.05 M), (b) GISAXS pattern of ZTO films on Si substrate, and (c) HRTEM image of ZTO films with a SAED ring pattern in the inset. Core-level XPS spectra of the ZTO thin films: (d) XPS narrow scan Zn 2p3/2 spectra, (e) XPS narrow scan Sn 3d5/2 spectra, and (f) XPS narrow scan O 1s spectra.
of 15.85 ± 1.02%). Notably, the devices with am-ZTO ESLs indicate significantly improved short-circuit current (Jsc) and open-circuit voltage (Voc) owing to the highly conformal/ uniform surfaces, fewer deep and mid-bandgap traps, and superior conductivity/mobility of am-ZTO ESLs. We demonstrate for the first time that am-ZTO is a promising ESL for highly efficient PSCs owing to improved stability and reduced J−V hysteresis. Am-ZTO films were synthesized by a simple solution process using zinc acetate and tin acetate. First, 2methoxyethanol solutions were prepared by dissolving zinc acetate and tin acetate while maintaining the 2:1 molar ratio of zinc to tin. Extremely uniform ZTO films with a controlled thickness can be achieved by the precise control of concentrations of a precursor solution from 0.15 to 1.05 M, which is one of the most important factors determining the charge transport and extraction from perovskites to the ESLs.26 Figure 1a shows the X-ray diffraction (XRD) patterns of amZTO films formed by precursors at various concentrations (0.15, 0.30, 0.45, 0.75, and 1.05 M). Like the previous reports, a very small and broad peak at 34° was detected at these concentrations.27−29 In films formed by precursors at 0.15 and 0.30 M, the peak was very faint. As the concentrations increased above 0.30 M, however, the peak at 34.2° became visible. To confirm the amorphous nature of ZTO films, grazing incidence X-ray diffraction (GIXRD) analysis was performed with a fixed incident angle (ω) as shown in Figure S1 in the Supporting Information. Although ω was changed up to 1°, no peaks were found indicating a typical amorphous structure. We also performed the grazing incidence small angle X-ray scattering (GISAXS) measurement (Figure 1b) of ZTO
transistors (TFT),15,16 solar cells,17,18,23,24 and Li-ion batteries.19,20 An n-type ZTO has a higher electron mobility (10− 15 cm2 V−1 S−1), a higher electrical conductivity, a broad optical bandgap of 3.8 eV, and a comparatively low refractive index of 2.0.21,22 Several studies have reported on crystalline ZTO ESLs. Wu et al. reported planar perovskite devices based on a compact ZTO ESL to achieve a PCE of 16.0%.23 Sha et al. reported a PCE of 17.21% using mesoporous ZTO layer-based PSCs.18 Using a well-dispersed crystalline ZTO colloidal solution, Shin et al. reported the highly transparent PEN/ITO/ ZTO substrate that showed high quantum efficiency and a PCE of 15.3%.24 In all studies on ZTO, only crystalline ZTO has been tested. However, the amorphous (am) ZTO has several advantages over crystalline oxides, such as high fieldeffect mobility, good uniformity, transparency in visible light, and simple sol−gel-based solution processing.15,25 Therefore, am-ZTO is expected to be one of the most popular ESLs to replace TiO2, although it has not been studied. Herein, using am-ZTO as an ESL for the first time, we report highly efficient FAMAPbI3-based planar PSCs with improved stability and reduced hysteresis. We synthesized a series of am-ZTO ESLs through a spin-coating process of a precursor solution with various concentrations and describe the properties that make am-ZTO so remarkable as an ESL in mixed FAMAPbI3 perovskite PSCs. To provide statistically significant reproducibility of the results, more than 40 devices were tested in each case. The champion perovskite device based on the am-ZTO ESL yields a PCE of 20.02% with an average and standard deviation of 18.38 ± 0.61%, which is significantly higher than the same cells with the TiO 2 counterparts (the champion PCE of 17.59% with the average 2411
DOI: 10.1021/acsenergylett.8b01501 ACS Energy Lett. 2018, 3, 2410−2417
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ACS Energy Letters
Figure 2. Cross-sectional FE-SEM images of (a) am-ZTO and (b) TiO2-based devices, (c) PCE of a champion device (inset: PV parameters), (d) the external quantum efficiency (EQE) of two cells, (e) MPPT of an am-ZTO-based cell, and (f−i) variations in Voc, Jsc, FF, and PCE of two cells.
films on a Si substrate. The GISAXS pattern shows the ZTO films have a perfect amorphous structure with no orientation. Furthermore, high-resolution electron microscopy (HRTEM) studies were performed as shown in Figures 1c and S2, which show typical selective area electron diffraction (SAED) ring patterns with no nanoscale crystallinity. To confirm the chemical bonding state of am-ZTO, X-ray photoelectron spectroscopy (XPS) was applied. Panels d, e, and f of Figure 1 show the XPS spectra of Zn, Sn, and O ions in am-ZTO films, respectively. Here, a Zn 2p3/2 peak observed at 1021.3 eV (Figure 1d) indicates the Zn−O bond, and a Sn 3d5/2 peak found at 486.1 eV (Figure 1e) shows the Sn−O bond.25 The O 1s spectrum has two peaks at 529.8 and 531.4 eV (Figure 1f). In the work of Yunlong et al., the O 1s XPS spectrum of amorphous ZTO was divided into three peaks at 530, 531, and 532 eV.29 In our am-ZTO films, the O 1s peak at 529.8 eV is attributed to O2− ions completely coordinated with Zn and Sn atoms, and the peak at 531.4 eV is ascribed to O2− ions in the vacant oxygen of ZTO. In our am-ZTO synthesized by acetate precursors, there is no O 1s peak at 532 eV. Using density functional theory for local density approximation, Körner et al.30 showed that the deep and midlevel defects in ZTO above the valence band, whether it is crystalline or amorphous,
originate from undercoordinated oxygen atoms. In such ZTO films containing considerable traps, O−H bonds may be inevitably created in an air atmosphere. Accordingly, because our am-ZTO films do not show any peaks originating from O− H bonds, there may be a few deep-level traps, which should be studied further. For device fabrication, a bandgap-tuned FAMAPbI3 perovskite31,32 with a single α-FAPbI3 phase containing a small amount of PbI2 (Figure S3) was selected for bandgap alignment. The detailed synthesis method is presented in the Supporting Information. To compare the effect of different ESLs on photovoltaic (PV) properties, we synthesized FAMAPbI3-based planar solar cells with different ESLs and a cell structure of FTO/ESL (50 nm)/FAMAPbI3 (400 nm)/ Spiro-MeOTAD (100 nm)/Au. Panels a and b of Figure 2 show the cross-sectional field-emission scanning electron microscopy (FE-SEM) images of devices based on am-ZTO and TiO2, respectively. The SEM images (Figure S4) of amZTO-coated FTO glasses display the thickness of am-ZTO synthesized at different precursor concentrations. The thickness gradually increases from 20 to 80 nm as the concentration increases from 0.15 to 1.05 M. To optimize the thickness of am-ZTO films, the PV performance of devices was evaluated 2412
DOI: 10.1021/acsenergylett.8b01501 ACS Energy Lett. 2018, 3, 2410−2417
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ACS Energy Letters
Figure 3. (a and b) RQs of two ESLs deposited on FTO glasses, (c) I−V curves measured in dark conditions using electron-only devices with a structure of FTO/ESL/Au (inset), and (d) variations in the SCLC for am-ZTO and TiO2 ESLs with a structure of FTO/ESL/Au (inset).
TiO2-based devices were 18.38 ± 0.61% and 15.85 ± 1.02%, showing improved average PCEs and enhanced reproducibility. Because the contact between the perovskite and ESL is responsible for the speed of charge extraction, the surface roughness of ESLs plays an important role in PV performance.35 Thus, we characterized the surface quality of each ESL using atomic force microscopy (AFM). Figure S6 shows the AFM images and the corresponding root-mean-square (RMS) roughness (RQ), where the RQ of pristine FTO is 49.1 nm and the RQs of am-ZTO ESLs are in the range of 30.3−57.1 nm depending on the processing conditions. The RQs of the two ESLs deposited on FTO glasses are 35.4 nm (am-ZTO) and 45.9 nm (TiO2), showing a considerable difference in surface roughness (Figure 3a,b). Considering RQ (49.1 nm) of bare FTO, the deposition of TiO2 ESL does not enhance surface roughness. On the contrary, the formation of am-ZTO significantly reduces the RQ from 49.1 to 35.4 nm, which is beneficial to the electron extraction from perovskite to the anode. Although am-ZTO ESLs synthesized from solutions with concentrations of 1.05 M show a minimum RQ, their performance is not the highest because of the thick ESL, which provides a longer distance for charge carrier transport. Another possible reason for improved performance is the enhancement of the electron conductivity and mobility of am-ZTO ESLs. To evaluate the conductivity, we performed I−V measurements in dark conditions using electron-only devices with a structure of FTO/ESL/Au (Figure 3c). The conductivity of am-ZTO and TiO2 ESLs was calculated using the equation σ = d/AR, where σ is the direct current conductivity, A the active area, R the resistance, and d the thickness of each ESL. The σ values of am-ZTO and TiO2 ESLs are 1.1 × 10−2 and 8.8 × 10−3 S cm−1, respectively, showing a significant difference. We also evaluated
with am-ZTO ESLs with different thicknesses. Figure S5 shows the variations in PV parameters and PCEs of 40 devices for each concentration value. The PCE shows a peak in devices synthesized from precursors at a concentration of 0.45 M and then decreases slightly as the concentration exceeds 0.45 M. This implies that the thickness of am-ZTO should be carefully adjusted because a thicker ESL generally shows a longer distance for a charge carrier to go through, which raises the internal series resistance and the possibility of recombination.33,34 In this study, we set the optimal concentration of the solution at 0.45 M, which produces am-ZTO films of 51.6 nm. We also compared the PV performance of two champion devices based on am-ZTO and TiO2 ESLs (Figure 2c), where the processing of two cells was identical except for ESLs. The champion cell based on am-ZTO ESLs achieved a PCE of 20.02%, where Jsc = 24.72 mA/cm2, Voc = 1.036 V, and FF = 78.15% (inset of Figure 2c), whereas the champion device based on TiO2 showed a PCE of 17.59%, where Jsc = 23.35 mA/cm2, Voc = 0.992 V, and FF = 75.97%. Both Jsc and Voc increased in am-ZTO-based cells. Next, the external quantum efficiency (EQE) of two cells was characterized (Figure 2d). The am-ZTO yields higher EQEs in the frequency range of 500−800 nm. Using the EQE curves, the Jsc values were calculated as 23.29 (am-ZTO) and 22.43 (TiO2) mA/cm2, respectively, which show slight differences from the values measured by the J−V curve. The maximum power point tracking (MPPT) of an am-ZTO-based cell measured in air (20 °C, 17% efficiency. Nano Energy 2017, 32, 187−194. (14) Jiang, Q.; Zhang, X.; You, J. SnO2: A Wonderful Electron Transport Layer for Perovskite Solar Cells. Small 2018, 14, 1801154. (15) Kim, C. H.; Rim, Y. S.; Kim, H. J. Chemical Stability and Electrical Performance of Dual-active-layered Zinc-tin-oxide/indiumgallium-zinc-oxide Thin-film Transistors Using a Solution Process. ACS Appl. Mater. Interfaces 2013, 5 (13), 6108−6112. (16) Shijeesh, M. R.; Saritha, A. C.; Jayaraj, M. K. Investigations on the Reasons for Degradation of Zinc Tin Oxide Thin Film Transistor on Exposure to Air. Mater. Sci. Semicond. Process. 2018, 74, 116−121. (17) Oh, L. S.; Kim, D. H.; Lee, J. A.; Shin, S. S.; Lee, J.-W.; Park, I. J.; Ko, M. J.; Park, N.-G.; Pyo, S. G.; Hong, K. S.; et al. Zn2SnO4based Photoelectrodes for Organolead Halide Perovskite Solar Cells. J. Phys. Chem. C 2014, 118 (40), 22991−22994. (18) Bao, S.; Wu, J.; He, X.; Tu, Y.; Wang, S.; Huang, M.; Lan, Z. Mesoporous Zn2SnO4 as Effective Electron Transport Materials for High-performance Perovskite Solar Cells. Electrochim. Acta 2017, 251, 307−315. (19) Kim, K.; Annamalai, A.; Park, S. H.; Kwon, T. H.; Pyeon, M. W.; Lee, M.-J. Preparation and Electrochemical Properties of Surfacecharge-modified Zn2SnO4 Nanoparticles as Anodes for lithium-ion Batteries. Electrochim. Acta 2012, 76, 192−200.
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