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Solvent Toolkit for Electrochemical Characterization of Hybrid Perovskite Films Mehedhi Hasan,† Swaminathan Venkatesan,† Dmitry Lyashenko,‡ Jason D. Slinker,§ and Alex Zakhidov*,†,‡ †

Materials Science, Engineering, and Commercialization, Texas State University, San Marcos, Texas 78666, United States Department of Physics, Texas State University, San Marcos, Texas 78666, United States § Department of Physics, The University of Texas at Dallas, 800 W. Campbell Road, PHY 36, Richardson, Texas 75080-3021, United States ‡

S Supporting Information *

ABSTRACT: Organohalide lead (hybrid) perovskites have emerged as competitive semiconducting materials for photovoltaic devices due to their high performance and low cost. To further the understanding and optimization of these materials, solution-based methods for interrogating and modifying perovskite thin films are needed. In this work, we report a hydrofluoroether (HFE) solvent-based electrolyte for electrochemical processing and characterization of organic−inorganic trihalide lead perovskite thin films. Organic perovskite films are soluble in most of the polar organic solvents, and thus until now, they were not considered suitable for electrochemical processing. We have enabled electrochemical characterization and demonstrated a processing toolset for these materials utilizing highly fluorinated electrolytes based on a HFE solvent. Our results show that chemically orthogonal electrolytes based on HFE solvents do not dissolve organic perovskite films and thus allow electrochemical characterization of the electronic structure, investigation of charge transport properties, and potential electrochemical doping of the films with in situ diagnostic capabilities.

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amount of salt and support electrochemical measurements. In this manner, it should function as an “orthogonal solvent,” exhibiting solution behavior distinct from conventional polar and nonpolar solvents. Unfortunately, most solvents that have high dielectric constants dissolve either the perovskite material or its components. In addition to these properties, hydrogenbond acidity, basicity, and dispersion forces should be considered, among other parameters.9 Fortunately, it has been demonstrated that chemically orthogonal processing of nonfluorinated organic films, both polar and nonpolar, can be accomplished using highly fluorinated solvents such as hydrofluoroethers (HFEs).10 Previous HFE treatments of various organic films did not cause any dissolution, cracking, delamination, or other unfavorable short-term or long-term physical or chemical damage. This was true even under extreme conditions, such as prolonged solvent exposure or elevated temperature processing.11−13 Moreover, one can use HFE solvents in concert with a small addition of cosolvents to make functional electrolytes, as

ver the past few years, hybrid perovskites have been materials of extensive interest because of their rapid progress in optoelectronic devices.1 In addition to outstanding optoelectronic properties, extreme interest in perovskites is attributed to the ease of processing and low materials cost.2 The simplicity of solution-based processing of perovskite materials has resulted in quick adoption in solar cells, thin film transistors (TFT),3 light emitting diodes (LED),4 light emitting field effect transistors,5 lasers,6 and more. While the properties of intrinsic perovskite materials are impressive, the delicate nature of these materials limits their ability to be precisely patterned or modified to tailor their structure and optoelectronic properties for specific applications. Furthermore, solution-based measurements of these films by electrochemistry can reveal fundamental aspects of these materials in solid-state films, such as emergent energy levels and charge transfer rates. However, to date, such solution-based processing and measurements of solid-state perovskite materials have been limited,7,8 as these materials are susceptible to dissolution in polar solvents, and nonpolar organic solvents do not support ionic transport for electrochemistry and ionic modification of films. Alternatively, an ideal fluid for processing and interrogation should not dissolve the perovskite. Furthermore, as an electrolyte, it should possess a high dielectric constant to dissolve a sufficient © XXXX American Chemical Society

Received: July 18, 2017 Accepted: August 18, 2017 Published: August 18, 2017 A

DOI: 10.1021/acs.analchem.7b02800 Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry

Technical Note

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demonstrated with lithium-based salts having fluorinated organic anions.14,15 HFE-based electrolytes are promising for improving lithium ion battery safety and performance due to their nonflammability, high voltage stability window, lower gas decomposition, modest viscosity, low freezing temperature, high vapor pressure, and low surface tension. In addition, HFEs are green solvents with low toxicity that do not disturb the ozone. As such, these solvents are thus candidates for replacement of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and perfluorocarbons (PFCs).16 In this report, we demonstrate the selection and optimization of a solvent based on an HFE for electrochemical characterization of perovskite films, particularly methylammonium lead iodide perovskite (CH3NH3PbI3). A series of solvents were investigated to balance low solubility with high dielectric behavior, revealing the superiority of an HFE approach. A cosolvent system was then developed to create an electrolyte that optimized lithium salt solubility and conductivity. This system was then used to electrochemically characterize a CH3NH3PbI3 perovskite film. Finally, we tested the impact of this solution electrolyte exposure on CH3NH3PbI3 photovoltaic device performance to see if this approach maintains high photovoltaic performance.

RESULTS AND DISCUSSION Organic−inorganic trihalide perovskite films are typically deposited from a solution that consists of an organic salt, such as methylammonium halide (MAX, here X = Cl, Br, I), and an inorganic salt, such as lead halide (PbX2). Between these two salts, MAX is generally the most soluble component of the perovskite; hence, it is necessary to ensure that the electrolyte to be used does not dissolve MAX. To determine an appropriate solvent, we investigated the solubility of MAI (a popularly used and highly soluble representative of methylammonium halide) in different classes of solvents. Table 1 Table 1. List of Maximum Solubility of MAI in Different Polar, Nonpolar, and Fluorous Solvents along with Dielectric Constant

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EXPERIMENTAL SECTION Glass substrates with prepatterned indium tin oxide (ITO) electrodes (90 nm) were purchased from Lianyungang Liaison Quartz Co., Ltd. Substrates were cleaned for 20 min in an ultrasonic bath with 5 wt % of Deconex OP121 detergent in DI water solution. After the ultrasonic bath, ITO substrates were rinsed with DI water, dried at 200 °C for 10 min on a hot plate, and treated with oxygen plasma (Harrick Plasma, Pdc-32G) for 5 min. Perovskite film fabrication followed a protocol published elsewhere.17 In short, CH3NH3I (Lumtec) and lead acetate (Pb(Ac)2, Sigma-Aldrich) were dissolved in anhydrous N,Ndimethylformamide at a 3:1 molar ratio with final concentrations of 40 wt %. Perovskite films of c.a. 450 nm were prepared by spin-coating the perovskite solution at 2000 rpm in a nitrogen-filled glovebox. After spin-coating, the films were annealed at 100 °C for 5 min. Electrochemical measurements were performed using a CH Instruments CHI7014E potentiostat under Ar atmosphere. A three-electrode configuration with lithium metal as reference and counter electrode was used. HFE solvent was donated by Orthogonal Inc. All other solvents and bis(trifluoromethane) sulfonimide lithium (LiTFSI) salt were purchased from Sigma-Aldrich. For solar cell device fabrication, a conducting polymer hole transport layer (PEDOT:PSS Hereaus 4083) was spin coated onto precleaned ITO/glass substrates at 6000 rpm and then annealed at 150 °C for 10 min, resulting in formation of an ∼30 nm thick film. Subsequently, perovskite films were deposited following the protocol described above. Devices were then transferred to a Trovato 300C vacuum thermal evaporator where 30 nm of buckminsterfullerene (C60, Sigma-Aldrich) (deposition rate: 0.1 nm per second), 5 nm of bathocuproine (Sigma-Aldrich) (deposition rate: 0.05 nm per second), and 100 nm of Ag metal (Kurt J. Lesker) (deposition rate: 0.2 nm per second) were deposited at a base pressure of 5 × 10−7 Torr. Samples were encapsulated with recessed glass caps using UV curable epoxy resin (Ossila E131) in the glovebox and removed to air where characterization took place. Solar cell devices were tested by using an Oriel ABA solar simulator and a Keithley 2400 source measure unit.

solvent category

solvent

MAI solubility (M)

dielectric constant

polar polar polar polar fluorous fluorous nonpolar nonpolar

IPA DMF water acetonitrileb HFE 7100 HFE 7300 diethyl carbonate toluene

0.45 6.3 12 0.75