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Indium Zinc Oxide Electron Transport Layer for High-Performance Planar Perovskite Solar Cells Liang Wang, Fengjing Liu, Xiaoyong Cai, Tingli Ma, and Chao Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08869 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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The Journal of Physical Chemistry

Indium Zinc Oxide Electron Transport Layer for Highperformance Planar Perovskite Solar Cells Liang Wang,a Fengjing Liu,ab Xiaoyong Cai,a Tingli Ma*c and Chao Jiang*a aCAS

Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for

Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China. E-mail: [email protected] bUniversity

of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, P. R. China

cGraduate

School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4

Hibikino, Wakamatsu , Kitakyushu, Fukuoka 808-0196 , Japan. E-mail: [email protected]

Abstract Besides the high-quality perovskite light absorbing layer, the electron and hole transport layers also play significant roles in achieving high-efficiency planar perovskite solar cells (PVSCs). In this study, a facile, environmentally-friendly, onestep spinning coating method is employed to fabricate high-quality indium zinc oxide (IZO) electron transport layer (ETL). Improvements in charge transport, conductivity and light transmittance of IZO ETL relative to that of TiO2-ETL should be responsible for the large short circuit current density of PVSCs. Using this optimized IZO ETL film, a high-power conversion efficiency (PCE) of 16.25% was achieved, which resulted to an absolute efficiency gain of 2.42% compared with TiO2-based PVSC (13.83%). A steady-state efficiency of 15.8% with negligible hysteresis was also demonstrated. Introduction Perovskite(PVK) solar cells (PVSCs), which are based on organic/inorganic hybrid 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

perovskite materials (typically CH3NH3PbI3), are used as light absorption layer and have triggered considerable attention since its first introduction as a sensitizer for photovoltaics by Kojima et al. in 2009.1 It has 3.8 % solar to electric power conversion efficiency (PCE), which enables it to achieve more than 20% PCE in the past few years.2-6 The outstanding advances are derived from their superior properties such as high absorption coefficient, excellent charge transport for both holes and electrons, tunable band gap, long diffusion lifetimes, low exciton binding energy, and various simple synthetic processes.7-14 The typical PVSC device consists of a transparent conductive substrate (e.g. FTO, ITO), an electron transport layer (ETL, e.g. TiO2, ZnO, SnO2, and PCBM),14-18 a perovskite light absorption layer, a hole transport layer (HTL, e.g. Spiro-OMeTAD, NiO, CuSCN),14, 19, 20 and a Au- or Ag-back electrode. To date, considerable research has focused on improving the crystallinity and uniformity of perovskite film by onestep or two-step solution processes, anti-solvent procedure process, thermal evaporation, and other methods.14,

21-26

In addition to high-quality perovskite light

absorption layer, the electron and hole transport layers also play a crucial role in improving the PCE of PVSCs. Among them, the ETL plays an important role in transporting electrons and also blocking holes. Although there are many types of materials used for ETL, TiO2 is still a promising choice and has been widely investigated in dye-sensitized solar cells; it has the best PCE of 14.5 %,27 whereas the PVSC has the PCE of 23.2% in PVSC.4 However, J-V hysteresis is known to widely exist in TiO2-based PVSCs especially in planar TiO2-based PVSCs because of defects and/or traps at the interface between TiO2 and PVK, as well as unbalanced electron and hole transport behavior.28-30 To eliminated the hysteresis, some efforts such as searching alternatives, and optimizing device structure and interface have

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been employed.31-36 To the best of our knowledge, seeking new materials for improving the performance and reducing hysteresis is still an efficient approach. Indium-zinc oxide (IZO), a metal-oxide semiconductor, have been widely investigated as a channel material for thin-film field-effect transistors in display backplanes and other optoelectronic devices due to their high transparency, high mobility, and high conductivity. These advantages combined with simple preparation process are hopeful for low-cost, high-efficiency PVSCs with less hysteresis. In 2017, Dou et al. reported IZO as conductive layer in flexible PVSCs, which shows a PCE of 18.1%

in

MgF2/Willow

Glass/IZO/SnO2/FAMACs/Spiro-MeOTAD/MoOx/Al

construction.37 In this study, the IZO ETL with high transparency and high conductivity was prepared through facile, green solvent one-step method and applied to planer PVSC for the first time. The results showed that IZO has a deeper conduction band and higher electron transport than TiO2, which means IZO is beneficial for the collection of electron from perovskite to ETL, as well as FTO substrate. Furthermore, IZObased PVSC shows a 16.3% PCE, which is 30% improvement than that of TiO2based PVSC without hysteresis. In conclusion, this study provides an alternative for high-performance ETL in PVSC. Experimental Details Preparation of IZO precursor solution First, 128 mg (0.1M) indium (III) nitrate hydrate (J&K Chemicals, 99.9%) and 76mg (0.1M) zinc nitrate hexahydrate (J&K Chemicals, 99.9%) were dissolved in 4 mL purified water and stirred at room temperature for 4h to form a clear and transparent solution before use. Different precursor solutions were also prepared by adjusting the ratio of indium (III) nitrate hydrate and zinc nitrate hexahydrate. The TiO2 precursor solution for control was prepared according to previous reference.17 3 ACS Paragon Plus Environment


Preparation of PVSC In brief, patterned FTO substrates were coated with a TiO2 or IZO layer by spincoating the precursor solution at 3000 rpm for 30 s, followed by sintering at 500 °C for 30 min. The perovskite absorbers were prepared via a one-step spin-coating method according to previous report.16 First, PbI2 (462 mg, Sigma Aldrich) and CH3NH3PbI3 (159 mg, Xi’an Polymer Light Technology Corp) were dissolved in 1 mL of N,N-dimethylformamide (DMF) under stirring at 70 °C for 2 h. Then, 100 μL precursor solution was dropped on the FTO/ETL substrate, followed by a two-stage spin coating processes (1000 rpm for 10 s and 4000 rpm for 30 s). During the second stage, 200 μL of chlorobenzene was poured on the spinning substrate 15 s prior to the end of the program. Finally, the perovskite films were annealed at 100 oC for 10 min. The hole transfer layer (HTL) was then deposited by spin coating at 2000 rpm for 30 s; this layer was composed of 72 mg/L 2,2′,7,7′-tetrakis(N,N-di-4methoxyphenylamine) -9,9- spirobifluorene (Spiro-OMeTAD), 28.8 μL of 4-tertbutylpyridine, and 17.5 μL of 520 mg/mL lithium bis(trifluoromethylsulfonyl)imide in acetonitrile in 1 mL chlorobenzene. Finally, 100 nm of silver was thermally evaporated on top of the HTL to form a metal back electrode. The active area of PVSC was fixed at 0.08 cm2. The best ration of In and Zn in IZO have been optimized and demonstrated in Figure S1 and Table S1. Characterization UV-vis absorption spectra were recorded using a spectrophotometer (Lambda 950, USA). The phase composition and crystal structure were studied via X-ray diffraction (Smart Lab, Japan). The top-view and cross-section of the devices were observed via field emission scanning electron microscopy (Merlin, Germany). For preparing crosssectional sample, the FTO substrate with ETL and perovskite layers was put into the liquid nitrogen. The samples could be obtained by the freeze fracture method. The 4 ACS Paragon Plus Environment

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surface morphology of the active layer was characterized using atomic force microscopy (AFM, Multimode 3D, Veeco, USA) in tapping mode. Ultraviolet photoelectron spectroscopy characterizations were performed via monochromatized HeI

radiation

at

21.2

eV

(ESCALAB250XI,

USA).

The

steady-state

photoluminescence (PL) and time-resolved photoluminescence (TRPL) were measured using NanoLOG (TCSPC, USA) excited wavelength at 660 nm and emission wavelength at 780 nm. The current density-voltage curves of PSCs were measured using a Keithley digital source meter (Keithley 2400, USA) under a solar simulator simulating the AM 1.5 spectrum (100 mW cm-2, Class AAA, Oriel, United States). External quantum efficiency (EQE) measurements were carried out on an EQE measurement setup (Newport, Oriel IQE-200, USA). Results and Discussion

Figure 1 (A-C) In 3d, Zn 2p and O 1s XPS spectra of IZO. (D) UV-vis spectra of IZO and TiO2 films. The inset is Ahv1/2 vs. photo energy plot of the IZO and TiO2 films. (E) UPS photoemission spectra of IZO and TiO2 films at (left) low kinetic energy region (SECO region), (right) the low binding energy region (valence band region). (F) Band diagram of perovskite solar cell. 5 ACS Paragon Plus Environment


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XPS measurement was applied to identify the chemical composition of IZO, which was prepared via by solution method. Figure 1(A-C) shows the XPS spectra of In, Zn, and O ions for IZO on FTO substrate through annealing process. The binding energy was analyzed by examining the core level spectra of In 3d (443.9 eV and 451.4 eV), Zn 2p (1021.5 eV and 1044.5 eV), and O 1s (529.4 eV and 531.4 eV), as marked in Figure. The existence of strong In, Zn, and O peaks indicated that the sample is a mixture of In2O3 and ZnO. The results were identified by XRD and EDX measurement (Figure S2 and S3) and also agreed with the previously report.38,

39

Figure 1D shows UV-vis spectra of IZO and TiO2 films on FTO substrate. The estimated optical bandgap of IZO and TiO2 films are shown in the inset of Figure 1D, which are 2.78 eV and 3.06 eV, respectively. The valence band energy and work function were measured via ultraviolet photoelectron spectroscopy (UPS). According to the UPS spectra (Figure 1E), the work functions (WS) were 3.81 eV and 3.75 eV obtained from secondary electron cut off (SECO region), and the valence band maximum (VBM) were 3.16 eV and 3.51 eV derived from the valence band region or HOMO region for IZO and TiO2, respectively. Combined with band gap derived from Figure 2D inset, the conduction band (ECB) of IZO and TiO2 were -4.19 eV and -4.20 eV, respectively. The detail calculations of the WS, VBM and ECB are provided in Figure S4. Figure 2F shows the possible band diagram of perovskite solar cells based on different ETLs. When compared to TiO2, IZO have a similar position of ECB, which is more close to the ECB of perovskite. This means that IZO shows well aligns with that of perovskite, possible achieving a barrier-free energetic configuration and realizing faster electron injection and transport from perovskite to ETL. Expect energy level matching, the conductivity is also important for electron transfer from IZO to FTO substrate. By linear sweep voltammetry study, we found that IZO has larger slope than that of TiO2, which means that IZO has better conductivity (Figure S5). 6 ACS Paragon Plus Environment

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Furthermore, we also observed that IZO has better optical performance than that of TiO2. The higher transmittance is favorable to minimize optical loss (Figure S6). In conclusion, above mentioned properties of IZO are beneficial for further improving PVSC device performance than that of TiO2. Figure 2A and 2B shows the surface SEM images of IZO and TiO2 on FTO substrate and cross section images as inset. As shown in Figures 2A and 2B, FTO surface profile is clearly visible through IZO or TiO2 films. Compared with cross section images in particular, IZO film shows better shape preservation than that of TiO2 films (Figure S7). According to previous report, this characteristic is beneficial for electron transport to further improve the performance of PVSCs.40 AFM images (Figures 2C and 2D) also identify the shape preservation of IZO than TiO2. Figures 2E and 2F shows the SEM images of perovskite films on two ETL substrates prepared via one-step spin coating method. Similar profile and thickness can provide credible data for comparing performance variation in PCE based on IZO and TiO2.

Figure 2. (A-D) SEM and AFM images of ETL on FTO substrate and (E-F) SEM images of perovskite on ETL substrate.



Figure 3. (A) steady-state and (B) time-resolved photoluminescence spectra of MAPbI3 films on different substrates. To further understand the effect of IZO and TiO2 on charge dynamics, we measured room temperature steady-state photoluminescence (PL) and time-resolved photoluminescence spectra (TRPL) of representative samples based on the structure of glass/ETL/PVK, with excitation at 660 nm. Three samples show a similar PL emission peak of around 780 nm in PL spectra, which is assigned to the MAPbI3 emission peak as shown in Figure 3A. The PL quenched obviously for IZO/PVK, indicating that IZO layer is more beneficial for the collection and transporting of electrons than that of TiO2. Furthermore, the TRPL further indicated the process of charge separation and transport process from MAPbI3 to ETL (Figure 3b). By fitting the data, we obtained the average lifetime of the charge carriers in those samples. The lifetime of GLASS/PVK, GLASS/TiO2/PVK and GLASS/IZO/PVK are 133.63 ns, 90.11 ns and 41.31 ns, respectively. The short lifetime characteristic suggested fastcharge separation and transfer process, as confirmed by steady-state PL. IZO with fast-charge dynamics process combined with high-light utilization and conductivity is a promising alternative as high efficiency ETL in PVSC device.


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Figure 4 (A) J-V curves of the PVSCs under 1 sun illumination (100 mW cm-2) based on IZO and TiO2 ETL and (B) their corresponding EQE spectra. (C) J-V curves of the IZO-based PVSC at different scan directions. (D) Stabilized electric power output and the photocurrent density for the best performing IZO-based PVSC shown in panel A. The photovoltaic performances of PVSCs based on IZO and TiO2 ETL were measured under 1 sun illumination and shown in Figure 4A. The detail parameters are also summarized in Figure 4A as an inset. The IZO-based PVSC parameters of open-circuit photo-voltage (Voc ), photocurrent density (Jsc), and fill factor (FF) are 1.05 V, 23.36 mA cm-2, and 0.663, respectively, yielding a PCE of 16.25%, which is higher than that of TiO2(13.83 %). Comparing with the parameters, the Jsc improved significantly from 18.68 mA cm-2 to 23.36 mA cm-2, which should be the main reason for high-efficiency IZO-based PVSC. Improved Jsc should be attributed to better light utilization and highly efficient charge collection and transport. The EQE of two representative PVSC devices are shown in Figure 4b. The integrated Jsc from the EQE measurement agrees with the Jsc measured from the J -V test. This result indicated the facticity of the Jsc from the J-V measurement, as well as the matching spectrum between different light sources. To observe the hysteresis behavior of IZO9 ACS Paragon Plus Environment


based PVSK, the J-V measurement under different scan directions are shown in Figure 4C. The results show negligible hysteresis, which may be explained by the high-quality IZO, perovskite and interface between them, as well as efficient charge transport. A 15.8% stabilized PCE output with a photocurrent density of 19.8 mA cm−2 (Figure 4D), was detected for IZO-based PVSC in panel A, further indicating that the PCE from J-V scanning conditions are near actual device performance. The results of PCE decay show that the IZO-based PVSCs have similar stability to that of TiO2-based devices when they are stored in the glove box. While the IZO-based devices show slightly better stabilities compared with the TiO2-based devices under the ambient conditions. (Figure S8) Conclusions In conclusion, IZO prepared by a facile process was achieved and applied to PVSC as ETL for the first time. The research results show that the IZO is a more appropriate energy structure, has better light utilization, and has faster chargetransport characterization than that of traditional TiO2. The best PCE of 16.25% was obtained via IZO-based PVSC and improved by 2.42 % than that of TiO2-based ones (13.83%). A steady-state efficiency of 15.8% with negligible hysteresis was also observed. This study paved a new alternative for developing high-performance ETL and PVSC devices. Supporting Information SEM images (Figure S1), J-V parameters (Table S1), XRD patterns (Figure S2), EDX images (Figure S3), Primary UPS spectra (Figure S4), Linear sweep voltammetry curves (Figure S5), Transmittance spectra (Figure S6), AFM images (Figure S7) and Stability of PVSCs (Figure S8). Acknowledgement This work was supported by the National Natural Science Foundation of China 10 ACS Paragon Plus Environment

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