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Aluminum-Doped Zinc Oxide as Highly Stable Electron Collection Layer for Perovskite Solar Cells Xingyue Zhao,† Heping Shen,† Ye Zhang,† Xin Li,† Xiaochong Zhao,†,‡ Meiqian Tai,† Jingfeng Li,† Jianbao Li,†,§ Xin Li,*,∥ and Hong Lin*,† †

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 10084, China ‡ Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang 621907, China § Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, Hainan University, Haikou 570228, China ∥ Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361000, China S Supporting Information *

ABSTRACT: Although low-temperature, solution-processed zinc oxide (ZnO) has been widely adopted as the electron collection layer (ECL) in perovskite solar cells (PSCs) because of its simple synthesis and excellent electrical properties such as high charge mobility, the thermal stability of the perovskite films deposited atop ZnO layer remains as a major issue. Herein, we addressed this problem by employing aluminum-doped zinc oxide (AZO) as the ECL and obtained extraordinarily thermally stable perovskite layers. The improvement of the thermal stability was ascribed to diminish of the Lewis acid−base chemical reaction between perovskite and ECL. Notably, the outstanding transmittance and conductivity also render AZO layer as an ideal candidate for transparent conductive electrodes, which enables a simplified cell structure featuring glass/AZO/perovskite/SpiroOMeTAD/Au. Optimization of the perovskite layer leads to an excellent and repeatable photovoltaic performance, with the champion cell exhibiting an open-circuit voltage (Voc) of 0.94 V, a short-circuit current (Jsc) of 20.2 mA cm−2, a fill factor (FF) of 0.67, and an overall power conversion efficiency (PCE) of 12.6% under standard 1 sun illumination. It was also revealed by steady-state and time-resolved photoluminescence that the AZO/perovskite interface resulted in less quenching than that between perovskite and hole transport material. KEYWORDS: solar cell, perovskite, zinc oxide, aluminum doping, thermally stable

1. INTRODUCTION Organic−inorganic lead halide perovskite solar cells emerge as a new generation of photovoltaic technologies with easy solution fabrication since the pioneering work of Miyasaka,1 which also exhibited a sky-rocketing power efficiency that has exceeded 20%.2−5 In a typical device, the perovskite absorber layer, either with or without mesoporous scaffold, is sandwiched between the electron and hole transport layers (ETLs and HTLs, respectively), the most commonly employed being TiO2 and 2,2′,7,7′-tetrakis(N,N-dipmethoxyphenylamine)-9,90-spirobifluorene (Spiro-OMeTAD).6,7 However, TiO2 was reported to have a low charge transport mobility © 2016 American Chemical Society

and requires a high sintering temperature (usually above 450 °C) to obtain a high-quality crystallization that hinders the development of flexible devices on the plastic substrates.8 Some other metal oxides, such as ZnO9−11 and SnO2,12−14 which exhibit similar or even better optical and electronic properties than TiO2, are then considered to be candidates for ETL. Notably, ZnO has attracted considerable interests15 where a highly efficient planar heterojunction perovskite solar cell with Received: January 15, 2016 Accepted: March 10, 2016 Published: March 10, 2016 7826

DOI: 10.1021/acsami.6b00520 ACS Appl. Mater. Interfaces 2016, 8, 7826−7833

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic view of the device structure. (b) Energy level diagram of the device. ZnO nanoparticles were prepared according to the procedures shown in the literature.15 CH3NH3I was prepared using a method similar to a previously published work.23 In brief, CH3NH3I was synthesized by reacting CH3NH2 with HI at 0 °C for 2 h under vigorous stirring, after which the precipitate was recovered by evaporation at 100 °C. The product, CH3NH3I, was then recrystallized from ethanol and washed with diethyl ether three times and dried at 60 °C under vacuum before use. The precursor solution of perovskite was composed of CH3NH3I and PbI2 with a molar ratio of 1:1. The hole-transporting material solution was prepared by dissolving 72.3 mg of spiro-OMeTAD, 17.5 μL of 520 mg/mL bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) in acetonitrile, and 28.8 μL of 4-tertbutylpyridine (TBP) in 1 mL of anhydrous chlorobenzene. 2.2. Device Preparation. AZO-coated glass was etched with diluted hydrochloric acid and then ultrasonically cleaned with detergent, deionized water, ethanol, acetone, and isopropanol, separately. Before the fabrication of solar cells, the cleaned AZO glasses were then treated with ultraviolet/O3 for 15 min. To deposit perovskite films, the precursor solution of CH3NH3PbI3 was first dropped onto the AZO substrate and then spun at 5000 rpm; 4 s after this, chlorobenzene was quickly dropped onto the center of the substrate. The spinning was stopped when the color of the substrate changed to transparent, followed drying at 100 °C for 10 min. The HTL was then deposited by spin-coating at 4000 rpm for 45 s, and a thin gold electrode was evaporated on the HTL. 2.3. Film and Device Characterization. The morphological information was observed with a field emission SEM (Zeiss LEO1530) and a Nanonavi SPA400 AFM unit. The absorption and transmission spectra were measured by a PerkinElmer Lambda 950 spectrophotometer employing the integrating sphere module, by which the monochromatic light illuminates through the glass side of the samples. The crystal structure was examined on a Bruker D8 Advance X-ray diffractometer (XRD) using Cu Kα radiation (1.5406 Å) operating at 40 kV and 40 mA. Hall measurements were conducted via an accent Hl5500 Hall Measurement System. The work function was measured by PESA under constant dry air flow using a Riken Keiki AC-2 spectrometer. The X-ray photoelectron spectroscopy (XPS) spectra were performed on X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher SCIENTIFIC INC., USA) with Al Kα radiation (hν = 1486.6 eV) as source. The zeta potential of samples was measured by a zeta potential analyzer (CD-7020, Colloidal Dynamics, USA). Sodium hydroxide (NaOH, 1 mol/L) and hydrochloric acid (HCl, 1 mol/L) were used to adjust pH values. Steady-state and time-resolved photoluminescence (PL) decay spectra were obtained with a FLS920 transient optical spectrometer. Steadystate PL measurements were measured by exciting the sample with a monochromatic xenon lamp source (central wavelength λexc = 460 nm). The sample was kept at a 45° angle to the excitation beam. On the collection line, a long pass filter cutting at 495 nm was used in order to filter the excitation light. Time-resolved PL measurements were measured by a time-correlated single photon counting (TCSPC) system. Samples were photoexcited using a 405 nm laser beam (EPL405), pulsed at frequencies between 10 and 20 MHz, with a pulse

an efficiency of 15% was fabricated on the basis of the lowtemperature processed ZnO nanoparticles. Recently, a hysteresis-free mesostructured perovskite solar cell based on synergistically improved ZnO nanorod arrays with an efficiency up to 16.1% was also obtained.16 Despite efficiency comparable with that based on TiO2, it has also been observed that severe recombination exists at the ZnO/perovskite interface, and the perovskite layer deposited atop ZnO tends to decompose at a relative low temperature leading to poor device stability.17 Strategies for addressing the above-mentioned problems can be classified into three kinds, including high-temperature exclusion of excessive OH− groups and residual chemicals on the ZnO surface,18 deposition of polymers between the perovskite and ZnO layers to avoid direct interaction,19 and aluminum doping to improve its interface property.20 In this study, aluminum-doped zinc oxide (AZO) was employed as the ECL for which we expect the thermal decomposition of the perovskite film to be inhibited. More importantly, its good conductivity (sheet resistance of ∼7 Ohm/□) makes it possible to serve as a good transparent conductive substrate. In this way, we could significantly simplify the perovskite cell structure into a hole-blocking layer-free device featured as glass/AZO/perovskite/Spiro-OMeTAD/Au, which is illustrated in Figure 1a. This concept of constructing devices has been already realized on the basis of ITO17 and FTO21 substrates in the previous reports. However, compared to ITO which is expensive and FTO which is not easy to etch, AZO possesses the advantages including being low-cost, composed of abundant elements, and easy to etch. In addition, high transmittance in near-infrared region of AZO-based perovskite solar cell bears more potential for tandem applications. The perovskite layer was directly deposited on top of AZO using a modified one-step spin-coating method,22 and then covered by Spiro-OMeTAD. The energy level diagram is also shown in Figure 1. As illustrated in Figure S1, the work function of AZO was measured at around 4.6 eV by PESA (photoelectron spectroscopy in air), which enables a good charge transfer between perovskite and AZO and thus a good collection efficiency. More importantly, we also investigated the thermal stability of the perovskite layer on top of AZO by comparing with that deposited on ZnO.

2. EXPERIMENTAL SECTION 2.1. Materials. Unless specified, all the materials used were purchased from either Alfa Aesar or Sigma-Aldrich. Spiro-OMeTAD was purchased from Borun Chemical Co., Ltd. (Ningbo, China). AZO conducting glass (ZnO/Al2O3 = 98:2 wt %) was purchased from Kaivo Optoelectronic Technology Co., Ltd. (Zhuhai, China). 7827

DOI: 10.1021/acsami.6b00520 ACS Appl. Mater. Interfaces 2016, 8, 7826−7833

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) UV−vis spectrum of the AZO film on the glass substrate. (The inset shows a digital photo of the AZO substrate). (b) Electrical properties of the AZO film obtained by the Hall effect measurement. (c) Topography and (d) 3D AFM view of the AZO film; surface SEM images of (e) the AZO film and (f) the perovskite film grown on the AZO substrate. duration of

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