Organic-inorganic hybrid perovskite materials for flexible energy devices

Abstract

Photovoltaic (PV) devices, which directly converts the photons to electricity, can be regarded as one of the promising solution for increasing energy demands and concerns environmental pollutions induced by fossil fuels. Organic-inorganic hybrid perovskite solar cells (PSCs) have received considerable attention within few years, due to their excellent electrical and optical properties. Their performance has rapidly improved reaching efficiency as high as certified value of 20.1%. However, to reduce the cost for production of photovoltaics and give direction towards potential application such as wearable electronic textiles, bendable display devices, and portable electronic chargers, fabricating them onto flexible substrate should be realized. So far, most of flexible PSCs (FPSCs) have been prepared on a plastic substrate. However, there is limitation for calcination of TiO2 electrode, which is most often used for electron transport materials because it is still holding the efficiency record as well as mesoporous TiO2 layer can solve the hysteresis problem of perovskite solar cells, at high temperature onto plastic based substrate due to their thermal instability. Metal substrate such as Ti, and stainless steel can be considered as viable alternative to plastic substrate. Metallic foils can provide several advantages such as superior mechanical and thermal stability. Furthermore, they have greater electrical conductivity and low materials cost. Although metal substrates offer promising properties, they have the key obstacle to fabricate with conventional metal top electrode due to their opacity. Since the light cannot penetrate though metal substrate, metal top electrodes should be replaced with transparent conducing materials. My thesis presents three different approaches that are aimed at contributing to the development of transparent and conductive top electrodes for integrating FPSCs into metal substrate. At first, I present silver thin film as a semitransparent top electrode in metal based FPSCs. Since the thickness of the Ag layer controls the number of photons penetrating into the active layer and the collecting ability, optimizing the thickness of Ag layer is needed. I have varied the thickness of the Ag layer, the performance and properties are addressed this part. Second, I report indium tin oxide (ITO) for top electrode in a PSCs. ITO has been used as a transparent conducting electrode for photovoltaic devices due to its advantages such as low electrical resistivity, structural uniformity, and high transparency compared with those of other promising transparent conducting materials. Although ITO has superior characteristics, its inherent brittleness has limited its use in flexible electrode applications. I suggest that the insertion of an ultra-thin Ag layer (1-3 nm) by thermal evaporation between spiro-MeOTAD and ITO may be a solution to enhance the performance and brittleness issue in devices. Third, I introduce silver nanowires (AgNWs) for top electrode deposited using spray coating methods. Fully solution based indium-free flexible photovoltaics, which has advantages in terms of price and process, have potentials to be made scalable commercial production. Moreover, efficient fiber-shaped perovskite photovoltaics with AgNWs as top electrode would be addressed for future electronic systems. The processes were all fully solution-based, and are therefore feasible for large-scale industrial production. These strategy in my thesis expected to provide important insight into design of next generation photovoltaic cells.