Role of defects and microstructure on the electrical properties of solution-processed Al-doped ZnO transparent conductin
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Role of defects and microstructure on the electrical properties of solution‑processed Al‑doped ZnO transparent conducting films Anurag Kumar1 · Imteyaz Ahmad1 Received: 13 February 2020 / Accepted: 27 June 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract A cheaper, non-vacuum-based routes are required for the large-scale implementation of TCOs in solar cells and LEDs. Generally, solution-based processing routes result in lower transparency and greater resistivity. To achieve electrical conductivity and transparency via solution processing route, greater insight into the effect of processing and microstructure/defects on the electrical and optical properties is needed. In this work, Al-doped ZnO films were deposited on glass substrates by sol–gel spin coating route. The formation of wurtzite structure was confirmed, and the crystallite size of the films was estimated by X-ray diffraction (XRD) pattern analysis. Greater than 85% transparency in the films was obtained in visible and near IR regime as examined by UV–Vis spectroscopy. An increase in the band gap was observed with increasing Al concentration from 0 to 3 at.%. The electrical properties were evaluated by the Hall measurement, and obtained resistivity was in the order of 10–2 Ωcm. The presence of defect states and their co-relation with the electrical properties were investigated by photoluminescence spectroscopy and X-ray photoelectron spectroscopy. Keywords Transparent conducting oxides · Thin films · Spin coating · Aluminium-doped zinc oxide (AZO) · XPS · UV– Vis · Photoluminescence
1 Introduction Transparent conducting oxides (TCOs) find applications in a wide range of optoelectronic devices, including liquid crystal displays, touch-screens, and photovoltaic cells, usually as electrodes [1–6]. The electrical and optical properties of the TCOs depend upon the presence of dopants and extrinsic/intrinsic defects. The dopants and intra-band donor states close to the band edges increase the carrier concentration close to the conduction band. So far, the most popular TCO materials are indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) with ITO having the lower resistivity (~ 10–4 Ωcm) and a transparency > 85% [7, 8]. However, indium is costly (~ US $745/kg) and toxic [9].
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00339-020-03767-0) contains supplementary material, which is available to authorized users. * Imteyaz Ahmad [email protected] 1
Department of Ceramic Engineering, Indian Institute of Technology (BHU), Varanasi 221005, India
Therefore, earth-abundant, cheaper, and nontoxic replacement is required. There are several alternative TCOs, which have shown promise in recent years, including F-, Sb-doped SnO [10–13] and Al-, Ga-, In-, and B-doped ZnO [14–18]. ZnO has a band gap of around 3.37 eV and a high exciton binding energy of 60 meV [19]. Owning to its larger band gap, ZnO is transparent in the near-UV and visible range of the electromagnetic
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