coatings Review Transparent p-Type Semiconductors: Copper-Based Oxides and Oxychalcogenides Nengduo Zhang 1,2,†, Jian Sun 3,† and Hao Gong 1,* 1 Department of Materials Science and Engineering, National University of Singapore, Singapore 117576, Singapore; [email protected] 2 NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 117456, Singapore 3 Faculty of Materials Science and Chemistry, China University of Geoscience, Wuhan 430074, China; [email protected] * Correspondence: [email protected] † These authors contributed equally to this work. Received: 31 January 2019; Accepted: 16 February 2019; Published: 20 February 2019 Abstract: While p-type transparent conducting materials (TCMs) are crucial for many optoelectronic applications, their performance is still not satisfactory. This has impeded the development of many devices such as photovoltaics, sensors, and transparent electronics. Among the various p-type TCMs proposed so far, Cu-based oxides and oxychalcogenides have demonstrated promising results in terms of their optical and electrical properties. Hence, they are the focus of this current review. Their basic material properties, including their crystal structures, conduction mechanisms, and electronic structures will be covered, as well as their device applications. Also, the development of performance enhancement strategies including doping/co-doping, annealing, and other innovative ways to improve conductivity will be discussed in detail. Keywords: p-type semiconductors; transparent oxides; delafossite; oxychalcogenide 1. Introduction Transparent conducting oxides (TCOs) possess high electrical conductivity and good optical transparency in the visible light range, and they have been intensively studied over the past few decades due to their important roles in electronic industries including photovoltaic cells (PV cells), touch screen displays, solid-state sensors, organic light-emitting diodes (OLEDs), and liquid crystal displays [1–3]. Currently, in the electronics industry, many different materials, especially impurity-doped materials, are used as TCOs for the applications. Three major materials are In2O3 doped with Sn (ITO), ZnO doped with Al (AZO), Fluor tin oxide or SnO2–F (FTO), and SnO2 doped with Sb (ATO) [4,5], Among them, ITO, with its great electrical conductivity around 1000 S·cm−1 and optical transparency greater than 80%, has a market share higher than 97% [6,7]. However, the aforementioned materials all belong to n-type TCOs, which show n-type conductivity. The popularity of n-type TCOs arises from their preferable electronic properties [8]. For n-type TCOs, the electrons as charge carriers move in the conduction band minimum (CBM). The CBM is largely formed by the spatially spread metal-s orbitals, resulting in a well-dispersed CBM and high electron mobility. In addition, the low formation energy of native intrinsic defects induces a high electron concentration and stable n-type electrical conductivity after impurity doping [9]. On the contrary, the development of high-performance p-type TCOs is very challenging [10,11], and it remains a grand challenge for researchers to solve. Indeed, it is very difficult to find p-type TCO with high conductivity due to the inherent low mobility of holes in the oxides. 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