The upcoming chapter explores zinc oxide (ZnO) thin films, examining their crystalline and optical properties. It discusses various deposition techniques for creating thin films, defined as layers from nanometers to micrometers thick, used in electronics, optics, and coatings. Thin film deposition methods, crucial for controlling film thickness and properties, include physical vapor deposition (PVD), sputtering, thermal evaporation, electron beam evaporation, and pulsed laser deposition (PLD). These techniques are vital in electronics, optics, and materials research. The chapter also covers ZnO thin films’ applications, emphasizing their photocatalytic abilities. ZnO has a large bandgap energy (3.37 eV), making it effective in UV light absorption and photocatalytic reactions. Upon UV photon absorption, ZnO generates electron-hole pairs, leading to efficient redox processes. ZnO’s chemical stability ensures lasting photocatalytic activity, resisting degradation under harsh conditions. It can be formed into nanostructures like nanoparticles, nanowires, and nanosheets, increasing surface area and catalytic efficiency. ZnO generates reactive oxygen species (ROS) like hydroxyl radicals, superoxide radicals, and hydrogen peroxide during photocatalysis. These ROS degrade organic pollutants, functionalizing substrates to break down fertilizers, pesticides, pharmaceuticals, nitrates, and pollutant gases such as nitrogen oxides and benzene compounds.

The substrate plays a critical yet often underappreciated role in determining the performance, stability and manufacturability of photovoltaic devices. While conventional glass and polymer films have enabled the rapid development of solar technologies, emerging applications such as building-integrated photovoltaics, wearable systems and large-area conformal devices demand the use of non-conventional substrates, including ceramics, metals, paper, textiles and elastomeric materials. This review provides a comprehensive analysis of the current state of the art of non-conventional substrates for photovoltaic technologies, with particular emphasis on the interplay between material properties, surface chemistry and deposition processes. These substrates introduce distinct mechanical, thermal and interfacial constraints that fundamentally alter thin-film growth, defect formation and device reliability. Key challenges such as porosity, roughness, thermal transport limitations and outgassing are discussed in relation to nucleation, film continuity and interfacial stability. The role of substrate-dependent effects in both chemical and physical deposition techniques is critically examined, highlighting cases where conventional processing approaches are insufficient. Representative device demonstrations are analyzed to illustrate how substrate selection influences performance and integration strategies across different photovoltaic platforms. Finally, common limitations and emerging opportunities are identified, emphasizing the need for the co-design of substrates, materials and processing routes. This work establishes a unified framework to guide the development of next-generation photovoltaic devices on unconventional substrates.