Microstructure and electro-optical properties of Cu–Ni co-doped AZO transparent conducting thin films by sol–gel method Xinglai Zhang • K. S. Hui • K. N. Hui • Y. R. Cho • Wei Zhou • Rajaram S. Mane • Ho-Hwan Chun Received: 22 September 2014 / Accepted: 15 November 2014 / Published online: 22 November 2014 Ó Springer Science+Business Media New York 2014 Abstract Cu–Ni co-doped Zn 1-x Al x O (AZO; Al/Zn = 1.5 at.%) films with fixed Ni concentration at 0.5 mol% and different Cu concentrations (0–2.0 % mole ratio) were synthesized on glass substrates using a sol–gel method. The effects of the Cu composition on the structure, electrical and optical properties were examined. X-ray diffraction (XRD) of the Cu–Ni co-doped AZO (CuNi:AZO) films revealed a polycrystalline ZnO phase with a hexagonal wurtzite structure. The stress variation of the CuNi:AZO films were analyzed from the XRD pattern. XPS spectra indicated the existence of two valence states of Cu atoms in Cu ? and Cu 2? states after N 2 /H 2 (95/5) forming gas heat treatment for CuNi:AZO films. Scanning electron micros- copy showed that all the films were smooth with a good packing density. The particle size was calculated by both XRD and SEM analysis, and the difference between them has been discussed in detail. Hall measurements indicated that the lowest resistivity of the CuNi:AZO film is approximately 1.16 9 10 -3 X cm at a 1.0 mol% Cu con- tent, which is one order of magnitude lower than that of AZO film (1.01 9 10 -3 X cm) and 43.9 % lower than that of Ni-doped AZO film (2.07 9 10 -3 X cm). All the films exhibited high transmittance in the visible region and showed sharp absorption edges in the UV region. The optical band gap shifted from 3.44 to 3.35 eV with increasing Cu content. This study provides a simple and efficient route for preparing low resistivity and high transparency CuNi:AZO films for optoelectronic applications. 1 Introduction Recently, a high transparent and low resistivity transparent conducting oxide (TCO) has been studied extensively for optoelectronic applications requiring low energy con- sumption, such as organic light-emitting diodes [1, 2], solar cell [3], and UV detector [4]. Among the TCOs reported, indium tin oxide [5], tin oxide [6], cadmium oxide [7], titanium dioxide [8], and zinc oxide [9–11] have been studied extensively for high-performance electrode appli- cations. In particular, zinc oxide has attracted considerable attention because of its direct wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [12, 13]. Generally, intrinsic ZnO thin films is an n-type semiconductor with a relative high resistivity because of its native defects, such as oxygen vacancies and zinc interstitials [14]. The conductivity of ZnO films can be improved substantially by doping with suitable metal X. Zhang  K. N. Hui (&)  Y. R. Cho Department of Materials Science and Engineering, Pusan National University, San 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea e-mail: bizhui@pusan.ac.kr K. S. Hui (&) Department of Mechanical Convergence Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea e-mail: kshui@hanyang.ac.kr W. Zhou Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen 361005, China R. S. Mane Centre for Nano-materials and Energy Devices, School of Physical Sciences, SRTM University, Nanded 431606, India H.-H. Chun Global Core Research Center for Ships and Offshore Plants (GCRC-SOP), Pusan National University, San 30 Jangjeon- dong, Geumjeong-gu, Busan 609-735, Republic of Korea 123 J Mater Sci: Mater Electron (2015) 26:1151–1158 DOI 10.1007/s10854-014-2519-5