Development of nickel oxide thin films using hollow cathode gas flow sputtering for perovskite solar cells

Climate change poses a serious threat to the future of humanity which calls upon an urgent need for sustainable energy solutions. Solar energy is a viable option as our society struggles to transition away from fossil fuel dependency. Perovskite solar cells (PSCs) have emerged as a promising technology for photovoltaic (PV) applications in the last decade. The metal halide perovskites (MHPs) have excellent opto−electronic properties resulting in high efficiency in solar cells and ease of production additionally. Long−term stability is one of the key challenges that impede the successful commercialization of PSCs. There is a need for the development of materials for PSCs that not only address the stability concern but at the same time are scalable and mass-producible. This dissertation explores the development of nickel oxide (NiOx) thin films using hollow cathode gas flow sputtering (GFS) for their application in PSCs. NiOx is considered a suitable inorganic hole transport layer (HTL) owing to its high hole mobility, excellent stability, and favorable energy level alignment with MHP materials. Moreover, NiOx is a versatile material used in various fields such as optoelectronics, spintronics, electrochemistry, and catalysis which is prepared with a wide range of deposition methods. The GFS technique offers a unique approach for the sputter deposition of high−quality NiOx thin films with controlled properties. This technique is distinct and has numerous advantages compared to conventional sputtering methods. In this work, the optimization of deposition parameters, characterization of the NiOx thin films, and their integration into single−junction PSCs are covered. The NiOx films are sputtered at low temperatures (100 ºC) for various oxygen partial pressures using a metallic Ni target. Additionally, NiOx films with Cu incorporation (NiOx:Cu) are prepared with Cu concentarations ranging from 1 and 9 at.% Cu. The properties of NiOx thin films are thoroughly examined, comprising a detailed analysis of their elemental composition, optoelectronic characteristics, structural features, and surface morphology. The NiOx and NiOx:Cu-based PSCs devices are benchmarked against those based on the state‐of‐the‐art self‐assembled monolayers (SAM) ([2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (also known as MeO−2PACz). The best−performing NiOx PSC achieves a photoconversion efficiency (η) of ≈16% without a passivation layer at the HTL interface and demonstrates better operational stability compared to the SAM device. According to ISOS−L−2I testing, the NiOx with 4.6 at.% Cu demonstrated exceptional operational stability over 240 hours. An optimal Cu doping level of 1.8 at.% was identified for maximizing the η of the PSCs. Additionally, Mg and Zn were also recognized as promising dopants for NiOx in PSC applications. This research advances the field by emphasizing the potential of the hollow cathode GFS technique in improving PSCs, thereby potentially accelerating their commercialization and supporting the transition to sustainable solar energy solutions.