Zinc Oxide: A Fascinating Material for Photovoltaic Applications

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Zinc oxide (ZnO), an attractive functional material having fascinating properties like large band gap (~3.37 eV), large exciton binding energy (~60 meV), high transparency, high thermal, mechanical and chemical stability, easy tailoring of structural, optical and electrical properties, has drawn a lot of attention for its optoelectronic applications including energy harvesting. Some of the promising applications are solar cells, ultraviolet light emitting diodes, photodiodes, ultraviolet lasers, high-temperature electronics, and spintronics devices. ZnO is a very versatile material vindicating itself with different access such as nanostructures, epitaxial structures, composite, and thin films. The ZnO nanostructures exist in various shapes and sizes including 0-D (nanoparticles), 1-D (nanowires, nanorods), 2-D (nanopetals, sheets), and 3-D (nanoflowers, tetrapods) structures with its tunable band gap energy, nature malleable behavior, and its potential application in optoelectronic devices. The ZnO being naturally an n-type inorganic semiconductor has been used in various types of solar cells such as conventional Si wafer solar cells, thin film solar cells, organic solar cells (OPVs), dye-sensitized solar cells (DSSCs), perovskite solar cells, hybrid solar cells (HSCs), and in several organic/inorganic as well as inorganic/inorganic heterojunction solar cell concepts. The ZnO acts as electron transport material, thereby it plays a major role in all the emerging third-generation PV devices. The ZnO thin films have manifold properties to make it interesting in photovoltaic applications. The ZnO thin film, owing to its easy synthesis and simple deposition techniques, reliability, cost effectiveness, non-toxicity, high stability, and good optoelectronic properties, has been studied extensively in several PV devices including the conventional silicon wafer-based solar cells as an antireflection and surface passivation layer. Here, a short review on ZnO nanostructures and thin films is presented in the perspective of their photovoltaic applications in different roles which include, as capping layer, electron selective layer, window layer, buffer layer, antireflection and passivation layer, as well as active layer for different types of solar cells. A brief overview of the synthesis methods of ZnO nanostructures and different deposition techniques of ZnO thin films via physical methods, cost-effective chemical routes and green methods is discussed. A brief discussion on the structural, optical, and electrical properties of the ZnO nanostructures and thin films is also included which are important for their PV applications. Finally, the chapter briefly outlines the different types of solar cells’ structures employing ZnO (nanostructures and thin films) in different roles, progress so far, their state-of-art-performance, and the challenges associated with different ZnO-based photovoltaic devices are critically discussed. At last, chapter closes with a summary including a remark indicating the future prospects of ZnO-based PV devices.