ACS Applied Materials & Interfaces 
Highly Conductive and Visibly Transparent p-Type CuCrO2 Films by Ultrasonic Spray Pyrolysis


The development of high-performing p-type transparent conducting oxides will enable immense progress in the fabrication of optoelectronic devices including invisible electronics and all-oxide power electronics. While n-type transparent electrodes have already reached widespread industrial production, the lack of p-type counterparts with comparable transparency and conductivity has created a bottleneck for the development of next-generation optoelectronic devices. In this work, we present the fabrication of delafossite copper chromium oxide p-type transparent electrodes with outstanding optical and electrical properties.



These layers were deposited using ultrasonic spray pyrolysis, a wet chemical method that is fast, simple, and scalable. Through careful screening of the deposition conditions, highly crystalline, dense, and smooth CuCrO2 coatings were obtained. A detailed investigation of the role played by the deposition temperature and the cation ratio enabled the properties of the prepared layers to be reliably tuned, as verified using X-ray diffraction, X-ray photoelectron spectroscopy,optical spectroscopy, Hall effect measurements, and electron and atomic force microscopies. We demonstrate record conductivities for solution-processed CuCrO2, exceeding 100 S cm−1, and we also obtained the highest value for two separate figures of merit for p-type transparent conducting oxides. These performances position solution-deposited CuCrO2 as the leading p-type transparent-conducting oxide currently available.

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KEYWORDS: transparent electrodes; p-type oxide; Delafossite; Thin films; Optoelectronics


Materials. Copper(II) acetylacetonate (Cu (acac)2, 99.9%) and chromium(III) acetylacetonate (Cr (acac)3, 99.7%) were purchased from Sigma-Aldrich. Methanol, acetone, and isopropanol (IPA) were obtained from Univar. All chemicals were used without further purification.

Film Deposition. CuCrO2 thin films were deposited from sol−gel precursors using the spray pyrolysis technique. A precursor solution consisting of 20 mM of Cu(II) and 20 mM of Cr(III) acetylacetonate in methanol was prepared and stirred for 2 h. For non-stoichiometric compositions, suitable precursor combinations (Cu:Cr molar ratio ranging from 30:70 to 70:30) were prepared in methanol, still  maintaining an overall concentration of [Cu + Cr] = 10 mM. Borosilicate glass and silicon substrates were ultrasonically cleaned in acetone and IPA for 10 min each and then dried with a nitrogen stream.


Deposition of CuCrO2 films was carried out with a Nadetech ND-SP ultrasonic spray coater equipped with a 40 kHz nozzle mounted on a gantry for the X−Y movements controlled by software and positioned above a Harry Gestigkeit Titan hot plate as the heat source. The substrates were positioned on the hotplate, heated to the desired deposition temperature, and let stabilize for 15 min prior to deposition. The optimized spraying parameters are as follows: 120 mm vertical distance between the nozzle and substrates, 10 mm/min spraying speed, 3 mL/min precursor flow rate, and nitrogen as the shaping gas for the spray mist. To control the film thickness, the spray nozzle was moved multiple times back and forth over the substrates on the hotplate.