Transparent electrodes based on ultrathin/ultra smooth Cu films produced through atomic layer deposition as new ITO-free organic light-emitting devices

Pengpeng Xiong, Yunfei Liu, Tao Ding, Ping Chen, Haoran Wang, Yu Duan

Index: 10.1016/j.orgel.2018.03.036

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Abstract

Ultrathin and continuous metallic films with mechanical robustness, high electrical conductivity and high elevated optical transparency are considered as ideal alternatives of the ITO electrodes. However, continuous and smooth morphology have not yet been simultaneously fully satisfied for making a rather thin metallic film. In this study, the atomic layer deposition (ALD) of copper ultrathin films was investigated at temperatures below 110 °C to obtain films used as transparent electrodes in flexible organic light-emitting devices with ITO-free electrode. During the process, ligand-exchange reaction of [Cu-(dmap)2] (dmap dimethylamino-2-propoxide) and diethyl zinc (Et2Zn) took place at the internal surface of the substrate at low temperatures. The data suggested that the deposition process at 110 °C followed an ideal self-limiting ALD fashion with a saturated growth rate of 0.022 nm/cycle. The resulting deposited Cu film with a thickness of 10 nm showed excellent surface morphology with a root-mean-square roughness value of 0.62 ± 0.03 nm. This led to favorable optical properties with a transparency of 74% at 550 nm wavelength and good conductivity with a sheet resistance of 10.2Ω sq−1. The use of the flexible ultrathin Cu films as anodes on Polyethylene terephthalate (PET) in OLEDs yielded higher luminance values when compared to OLEDs with traditional ITO anodes. Furthermore, the electrodes could be bent at less than 5 mm bending radius. The relevant device efficiency and superior luminance of the flexible ultrathin Cu anodes look promising as alternatives to ITO in flexible electronics.