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High-Performance, Air-Stable, Low-Temperature Processed Semitransparent Perovskite Solar Cells Enabled by Atomic Layer Deposition Chih-Yu Chang, Kuan-Ting Lee, Wen-Kuan Huang, Hao-Yi Siao, and Yu-Chia Chang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 02 Jul 2015 Downloaded from http://pubs.acs.org on July 3, 2015
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Chemistry of Materials
High-Performance, Air-Stable, Low-Temperature Processed Semitransparent Perovskite Solar Cells Enabled by Atomic Layer Deposition Chih-Yu Chang,* Kuan-Ting Lee, Wen-Kuan Huang, Hao-Yi Siao, and Yu-Chia Chang Department of Materials Science and Engineering, Feng Chia University, No. 100 Wenhwa Rd., Seatwen, Taichung, Taiwan 40724, R.O.C. ABSTRACT: We demonstrate high-performance, air-stable, low-temperature processed (≦100 °C) semitransparent (ST) perovskite solar cells (PSCs) by the applications of atomic layer deposition (ALD) technology to deposit ZnO and Al2O3 films as cathode buffer layer (CBL) and encapsulation layer, respectively. The application of ALD ZnO film as CBL in PSCs deliver several remarkable features, including fine tunability of the work function of the electrode, low deposition temperature (80 °C), high charge selectivity, good electron-transporting ability (filed-effect mobility = 16.1 cm2 V-1 s-1), and excellent film coverage. With these desired interfacial properties, the device with opaque Ag electrode delivers high power conversion efficiency (PCE) up to 16.5%, greatly outperforming the device with state-of-the-art CBL ZnO nanoparticles film (10.8%). For ST PSCs employing Ag nanowires as transparent top electrode, a remarkable PCE of 10.8% with a corresponding average visible transmittance (AVT) of 25.5% are demonstrated, which represents the highest PCE ever reported for ST PSCs with similar AVT. More significantly, the insufficient ambient stability of ST device is significantly improved by employing excellent gas-barrier performance of ALD Al2O3-based encapsulation layer, with an oxygen transmission rate of 1.9×10-3 cm3 m-2 day-1 and a water vapor transmittance rate of 9.0×10-4 g m-2 day-1.
Introduction Hybrid organic–inorganic halide perovskite solar cells based on methylammonium lead iodide (CH3NH3PbI3) have emerged as a promising next-generation photovoltaic technology due to their potential to be light weight, mechanical flexibility, semitransparent (ST), and manufactured in a cost-effective continuous printing process.1-7 Recent progress in the development of perovskite solar cells with a planar heterojunction structure have led to the rapid increase in the power conversion efficiency (PCE) over 19%,5 which is nearly on par with those of current commercial technologies. The significant advancement in PSCs performance is driven by the development of new perovskite materials combined with improvement of processing techniques for the perovskite layer.8-10 In addition to the perovskite materials, the effects of interface properties between the perovskite active layer and the electrodes (i.e., anode and cathode) have also been studied, and their crucial role on the overall device performance has been demonstrated.4-7 Ideally, the workfunction (WF) of the cathode and anode should be aligned with the energy of the photo-excited quasi-Fermi levels of active layer to ensure ohmic contact for maximizing achievable open-circuit voltage (Voc) and minimizing energy barrier for charge extraction without causing excessive interface recombination.4-7 A basic planar heterojunction PSC device is the sandwich of light absorbing perovskite active layer between the hole transport layer (HTL) and electron transport layer (ETL) that are in contact with their corresponding electrodes.1-3 Among various device architectures utilized in the study of planar heterojunction PSCs, the inverted architecture (device configuration: substrate/anode/HTL/ perovskite/ETL/cathode) has drawn much attention due to their low manufacturing temperature and potential for
use in plastic flexible devices.9-13 A typically inverted PSC comprises substrate/indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS)/peorvskite/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)/cathode.9-12 To ensure efficient electron extraction, low WF metals such as Al (~4.3 eV) or Ca (~2.9 eV) are generally used as cathode electrodes.10-12 These low-work-function metals, however, are vulnerable to oxidation in ambient air, leading to insufficient device stability. Although the use of more environmental stable high WF metals such as Au (~5.1 eV) or Ag (~4.7 eV) as cathode is highly desirable, the mismatch between the WF of these metals and the lowest unoccupied molecular orbital (LUMO) level of PC61BM (~4.2 eV) results in low device performance.14-16 To circumvent this problem, an effective approach is to insert additional cathode buffer layer (CBL) between PC61BM layer and the cathode.14-16 Several CBL materials, including metal oxides,11,13 bathocuproine (BCP),10,17 LiF,15 polyelectrolyte,14,18, perylene-diimide derivative,16 aminofunctionalized polymer,19 or bis-C60 surfactant,9 have been applied to improve the contact properties and consequently enhance the device performance. Among various CBL materials, n-type metal oxide semiconductors (e.g. TiOx or ZnO) possess several desired properties for PSCs, including low cost, high charge carrier mobility, and excellent stability. The effectiveness of using sol-gel derived metal oxides as CBL in PSCs has been successfully demonstrated in the regular architecture device (configuration: substrate/cathode/TiOx or ZnO/perovskite/HTL/ anode).1-3,11 Nevertheless, their applicability in the inverted PSCs remains challenging due to the fact that the formation of these CBLs generally requires an annealing step at relatively high temperature (≧400 °C) to ensure high film quality.16,20,21 This high-temperature processing requirement also obstructs the road towards the develop-
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Chemistry of Materials
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ment of flexible solar cells and high-performance multijunction tandem solar cells.16 Despite the fact that recent efforts have been devoted to developing relatively lowtemperature processing of metal oxides nanoparticles (NPs) film as CBLs in PSCs, the resulting device performance is considerably inferior to those of the devices with high-temperature processed metal oxide CBLs.11,22 In addition, it is of particular importance to deposit a uniform and pinhole-free CBL in the inverted structure PSCs since perovskite film prepared by the solution deposition routes (e.g. one-step precursor deposition or two-step sequential deposition) usually possesses incomplete surface coverage with relative high surface roughness (root-mean-square (rms) roughness ~16 nm), which can lead to electrical shunting paths and thus deteriorating the device performance.10,15,16 On the other hand, semitransparent (ST) solar cells that can harvest solar energy to generate electricity without obscuring the view have created a new market for the development of novel value-added products (e.g. power generating windows).23-26 The unique advantages of PSCs makes it a promising candidate for ST solar cells applications.27-34 To ensure good optical transmittance of ST devices, some promising strategies to manipulate perovskite layer have been reported (e.g. thickness control, microstructure formation).28-31 In addition, much research in this field has focused on the development of top transparent electrodes while the bottom ones typically employ ITO electrodes.32,34 Recently, Ag nanowires (NWs) have been studied as a promising material to serve as the top electrode in ST solar cells, thanks to their several unique advantages such as solution processability, high electrical conductivity (~15 ohm sq−1), good optical transmittance (>80%), and mechanical flexibility.34-38 The successful implementations of Ag NWs in polymer solar cells, dyesensitized solar cells, and silicon solar cells have already been demonstrated.36-38 However, their applications in PSCs is majorly impeded by high susceptibility of active layer to solution-based Ag NWs procedure,34 and a straightforward approach is to deposit a dense buffer layer such as metal oxide thin films on top of active layer to prevent direct contact between active layer and Ag NWs. Another issue that should be addressed is the inferior long-term stability of Ag NWs. The Ag NWs can undergo oxidation or sulfurization upon exposure to ambient air, leading to the deterioration in electrical conductivity.39-41 Therefore, the incorporation of an encapsulation layer with low gas-permeability is crucial to ensure long-term stable PSCs with Ag NWs electrode. This study aims to overcome the above-mentioned challenges to achieve efficient and air-stable ST PSCs by employing a multipronged approach based on atomic layer deposition (ALD), taking advantages of ALD’s unique capabilities such as low deposition temperature, excellent film conformality, high uniformity over large area substrates, and precise thickness control.42,43 The viability of ALD in high-throughput roll-to-roll compatible process has also been demonstrated recently.44-46 Con-
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sidering the unique capabilities afforded by ALD—high quality film and large area/large batch deposition with low deposition temperature, we envision that ALD can potentially offer a manufacturing-friendly and costeffective solution to the development of the PSC technology. Despite these attractive advantages, only very few studies have dedicated so far to integrating ALD-growth thin films into PSCs, and only moderate PCEs are achieved (99.5%) was purchased from Solenne. Unless otherwise stated, all chemicals were purchased from Sigma-Aldrich and used as received. Synthesis of Ag NWs: The Ag NWs were synthesized by the polyol method according to the literature with slight modifications to the reaction procedures.35 For a typical synthesis, a mixture of 0.334 g polyvinylpyrrolidone (PVP), 0.005 g potassium bromide, and 20 mL ethylene glycol (EG) is heated at 175 °C in a pear shaped three-necked flask for 100 min with constant magnetic stirring (∼800 rpm). After attaining a stable solution temperature, finely ground silver chloride (0.025 g) was added to the flask for initial nucleation of the silver seeds. After about 10 min, 0.110 g of silver nitrate and 10 mL EG was then added to the reaction mixture for more than 25 min. The flask was then heated for additional 50 min and then allowed to settle at room temperature for 2 hr. The cooled-down solution was then centrifuged twice at 3000 rpm for 40 min to remove EG, PVP, and redundant impurities. Then, the supernatant was centrifuged twice at 6000 rpm for 30 min to precipitate the wires. Finally, the precipitate of Ag NWs was dispersed in 5 mL of anhydrous methanol. Synthesis of ZnO NPs: ZnO NPs were synthesized by a solution-precipitation process according to literature procedures.6 Briefly, zinc acetate dihydrate (2.95 g) was dissolved in methanol (125 mL) at room temperature. A potassium hydroxide solution (1.48 g in 65 mL methanol)
was then added dropwise within 30 min and stirred for 3 hr at 65 °C. The cooled-down solution was then decanted and the precipitate washed twice with ethyl acetate and ethanol. Afterward, ethanol was added to disperse the precipitates and produce ZnO NPs solution. Atomic layer deposition: The precursors used in ALD were as follows: ZnEt2 and H2O for ZnO; trimethylaluminum (TMA) and H2O for Al2O3. The deposition temperature was kept at 80 °C for ZnO and 100 °C for Al2O3. The process pressure was 0.1 Torr with high-purity N2 (flow rate = 20 sccm) as the carrier and purging gas. The ALD cycle settings were as follows: (1) ZnO: 0.05 sec pulse of ZnEt2, 25 sec soak, 15 sec purge, 0.02 sec pulse of H2O, 30 sec purge; (2) Al2O3: 0.03 sec pulse of TMA, 2 sec soak, 5 sec purge, 0.02 sec pulse of H2O, 5 sec purge. Solar cell fabrication: ITO-coated substrates were cleaned stepwise in detergent, water, acetone, and isopropyl alcohol under ultrasonication for 20 min each and subsequently pretreated by UV-ozone for 60 min. PEDOT:PSS layer (25 nm) was spin-coated on an ITO surface and then annealed at 120 °C for 15 min. The CH3NH3PbI3 perovskite layer (~220 nm) was prepared following two-step solution deposition. The lead iodide (PbI2) and methyl ammonium iodide (MAI) were dissolved into dimethylformamide (DMF) and 2-propanol with concentrations of 450 mg ml-1 for PbI2 and 40 mg ml-1 for MAI, respectively. Both solutions and substrates were heated at 100 °C for 10 min before being used. The PbI2 solution was spun on preheated substrate (5000 rpm for 40 sec) and then annealed at 70 °C for 10 min. The MAI solution was then spun on top of dried PbI2 film (6000 rpm for 30 sec), followed by annealing at 100 °C for 2 hr. PC61BM layer (60 nm) was then casted by spin-coating a solution (15 mg mL-1 PC61BM in chloroform) on top of the formed perovskite layers (1000 rpm for 60 sec). Afterward, ZnO CBL was deposited from either ZnO NPs solution (10 mg mL-1 in ethanol) or ALD. The Ag layer (150 nm) was then deposited from either thermal evaporator under high vacuum (
