Letter pubs.acs.org/journal/apchd5
Sequential Deposition of CH3NH3PbI3 on Planar NiO Film for Efficient Planar Perovskite Solar Cells Long Hu,†,‡ Jun Peng,§ Weiwei Wang,§ Zhe Xia,†,‡ Jianyu Yuan,§ Jialing Lu,§ Xiaodong Huang,§ Wanli Ma,*,§ Huaibing Song,†,‡ Wei Chen,† Yi-Bing Cheng,†,∥ and Jiang Tang*,†,‡ †
Wuhan National Laboratory for Optoelectronics (WNLO) and ‡School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China § Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China ∥ Department of Materials Engineering, Monash University, Wellinton Road, Clayton, Victoria 3800, Australia ABSTRACT: The new emerging organometal trihalide perovskite holds great potential for high-efficiency, low-cost solar cells because of its high solar to electricity conversation efficiency (>16%) achieved within 4 years of research and its low-temperature solution processing. In this Letter we introduce NiO as the hole-collecting and -conducting layer in perovskite solar cells. Through a modified sequential deposition strategy, we successfully fabricated high-quality CH3NH3PbI3 onto a planar NiO layer and built a planar inverted ITO/NiO/CH3NH3PbI3/ PCBM/Al photovoltaic device. A device efficiency of 7.6% was achieved with an impressively high open-circuit voltage (Voc) of 1.05 V. Our study demonstrates the potential application of a deep work function NiO layer for perovskite solar cells. KEYWORDS: perovskite, solar cell, NiO, sequential deposition
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gradually attracting more attention because removal of the mesoscopic layer simplifies device preparation and reduces materials cost. Many planar device architectures, such as TiO2/ perovskite/spiro-MeOTAD,5 PEDOT:PSS/perovskite/C60,12,13 and ZnO/perovskite/spiro-MeOTAD,6 were reported, with the best efficiency achieving 15.7%,13 competing favorably with perovskite solar cells employing mesoscopic layers. One additional benefit, not yet fully demonstrated in perovskite solar cells but widely accepted in traditional thin-film photovoltaic fields, is the reduced heterojunction interface area in planar devices. For a heterojunction device, the reverse saturation current is proportional to the junction area; minimizing the interface area will minimize possible leakage channels, reduce reverse saturation current density, and improve device efficiency. In this Letter, we present the fabrication of planar CH3NH3PbI3 heterojunction solar cells. The device architecture is ITO/NiO/CH3NH3PbI3/PCBM/Al, where ITO stands for indium-doped tin oxide and PCBM stands for [6,6]-phenylC61-butyric acid methyl ester. NiO is a cubic p-type material with a large band gap widely applied in photovoltaic devices as a hole-conducting and -collecting layer;14,15 however, the successful exploration of NiO in perovskite solar cells has not yet been reported. We chose NiO over TiO2 because lightinduced desorption of surface-adsorbed oxygen on TiO2
igh-efficiency, low-cost solar cells are the everlasting pursuit of photovoltaic research. Very recently, organometal halide perovskite has attracted tremendous research attention because of its potential to realize this goal. First introduced in 2009 by Miyasaka’s group as the sensitizer for dye-sensitized solar cells (DSSCs) using liquid electrolyte,1 organometal halide perovskites gradually evolved to solid-state DSSCs by incorporating triarylamine derivative 2,2′,7,7′tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD)2 or other materials3,4 as the hole-transporting and -collecting layer and finally developed into the current form of a planar p−i−n heterojunction device configuration.5,6 State-of-the-art perovskite solar cells include sequentially deposited CH3NH3PbI3 to sensitize a mesoporous TiO2 electrode and achieved 14.1% certified device efficiency7 and also include a thermally evaporated CH3NH3PbI3−xClx absorber to build 15.4% planar heterojunction solar cells.5 This soaring conversion efficiency, coupled with their simple device fabrication process, enables the perovskite solar cell to be a very promising candidate for high-efficiency, low-cost photovoltaics.8,9 Similar to other absorber materials in solar cells, organometal trihalide perovskite is capable of efficient and balanced electron and hole transport. Diffusion lengths of more than 100 nm for CH3NH3PbI310 and more than 1000 nm for CH3NH3PbI3−xClx11 were reported from a femtosecond transient optical spectroscopy study and transient absorption and photoluminescence-quenching measurements, respectively. As a result, planar heterojunction device configurations are © XXXX American Chemical Society
Received: January 9, 2014
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dx.doi.org/10.1021/ph5000067 | ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
pressure of 1 × 10−6 mbar through a shadow mask. The active device area was 7.25 mm2. Measurement and Characterization. The current density−voltage characteristics of the devices were measured using a Keithley 2400 (I−V) digital source meter under a simulated AM 1.5G solar irradiation at 100 mW/cm 2 (Newport, AAA solar simulator, 94023A-U) inside the glovebox. Ultraviolet photoemission spectroscopy (UPS) measurement was carried out in a Kratos AXIS Ultra-DLD ultra-high-vacuum photoemission spectroscopy system with an Al Kα radiation source. Tapping mode atomic force microscopy (AFM) was performed using a Veeco multimode V instrument. Scanning electron microscopy (SEM) images were obtained using a FEI Nova NanoSEM 450. X-ray diffraction (XRD) results were acquired using a Phillips X’Pert PRO. Photoluminescence spectra were obtained by using an automated spectrofluorometer (FluoMax, Horiba Jobin-Yvon). NiO layer thickness was obtained by deposition of a NiO layer onto a Si wafer and measured by ellipsometry (J. A. Woollam Corporation, AlphaSE).
severely degraded perovskite solar cell performance when they were subjected to long-term sunlight exposure. 16 The advantages of NiO over previously explored PEDOT:PSS are its inertness without corrosion to ITO substrates and its deeper work function, which would promise higher open-circuit voltages (Voc). In addition, this new inverse device architecture (from the viewpoint of traditional DSSCs), with holes flowing to the ITO substrate instead of the traditional device involving TiO2, where electrons flow to the ITO substrate, will expand the choice of selection for the integration of perovskite solar cells with silicon or copper indium gallium selenide (CIGS) solar cells to build tandem devices and target >40% device efficiency.8 Using an improved sequential deposition strategy, we successfully deposited high-quality CH3NH3PbI3 film onto NiO film and obtained a solar conversion efficiency of 7.6% from this inverse planar heterojunction device. Moreover, due to the deep work function of the NiO layer (EF = −5.26 eV), a high open-circuit voltage (Voc) of 1.05 eV was obtained, showcasing the promise of using NiO as a hole-collecting layer in perovskite solar cells.
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EXPERIMENTAL SECTION NiO Film and CH3NH3I Fabrication. We first prepared NiO precursor solutions by adding nickel acetate tetrahydrate (Sinopharm Chemical reagent Co. Ltd., >98%) and monoethanolamine (Alfa) in methoxyethanol (Tokyo Chemical Industry Co. Ltd., >99%) and stirring overnight. Precursor solutions with nickel acetate weight percentages of 1%, 2%, 5%, 10%, and 15% were prepared to fabricate NiO film of different thicknesses. Prepatterned ITO glass substrates were cleaned by sequential ultrasonic treatment in acetone, detergent, acetone, isopropyl alcohol, and acetone and then were treated with UVozone (Seabiscuit-trade) for 30 min. Nickel acetate solution was spin-coated at 4000 rpm for 30 s on the ITO substrates, then annealed on a hot plate at 150 °C for 5 min, 250 °C for 10 min, and 350 °C for 60 min, all in air. When naturally cooled to room temperature, the NiO film was subjected to UVO treatment in a UV-ozone cleaner for 10 min. Methylamine (33 wt % in ethanol; Alfa) and hydroiodic acid (57 wt % in water; Alfa) were stirred at 0 °C for 2 h. After the reaction, the solvent in solution was evaporated with a rotary evaporator. A white powder, methylammonium iodide, was obtained and subsequently dissolved in ethanol and precipitated with diethyl ether. This process was repeated three times, and finally the products were dried at 60 °C in a vacuum for 24 h. A 300 mg/ mL PbI2 solution was prepared by dissolved PbI2 in anhydrous N,N-dimethylformamide (DMF; Alfa) at 60 °C and stirred for 12 h inside a nitrogen-filled glovebox with oxygen and moisture levels of
