Transparent Conductive Oxide (TCO) bilayer designs for thin film Si-based solar cells

Abstract

Photovoltaic solar energy is one of the most powerful renewable sources, playing a key role in transitioning the global energy sector from fossil fuels to zero-carbon emissions. The second generation of photovoltaic technology, such as thin-film silicon-based devices, offers potential for significant performance improvements. Among the layers that can be optimized, the front contact, composed of transparent conductive oxide (TCO) materials, is crucial. As the first layer to encounter incident light, TCO has to meet the high transparency requirement. Additionally, it has to offer high conductivity to transport carriers from the absorber layers to the metal contact. However, there is a trade-off between transparency and conductivity—improving one often compromises the other. Instead of seeking a single layer that balances these properties, an alternative approach involves the use of bilayers. Bilayers consist of two TCOs: one optimized for transparency and the other for conductivity. This thesis study focus on the design of bilayer configurations using two distinct materials, one optimized for transparency and the other for conductivity. Those designs aim to overcome the limitations of single-layer TCOs and achieve superior performance. Indium Cerium Oxide (ICO) and Hydrogenated Indium Oxide (IOH) were selected as conductive materials due to their excellent electrical properties, while intrinsic Zinc Oxide (i-ZnO), known for its high transparency, was used as the transparent layer. Tin Oxide (SnOx), a promising alternative, was also explored and integrated into bilayer configurations. All depositions were performed using Radio Frequency magnetron sputtering. The design process began by optimizing deposition conditions for each material. For ICO, power and process pressure were the examined variables, while for IOH, power and partial water pressure were studied. For SnOx, power and gas composition during deposition were evaluated. After determining the optimal deposition parameters based on opto-electrical properties examination, the following bilayer combinations were produced: IOH/i-ZnO, ICO/i-ZnO, and IOH/SnOx. In these bilayer depositions, the conductive material (IOH or ICO) was deposited first, followed by the transparent layer (i-ZnO or SnOx). The bilayer configurations exhibited superior opto-electrical properties compared to single layers. Specifically, the bilayers maintained high transparency with carrier densities of 1-2×1019 cm−3, minimizing parasitic absorption in the near-infrared region of the solar spectrum. Mobility values of 50-60 cm2/Vs ensured excellent lateral conductivity. These results demonstrated that bilayers preserve the best attribute of each composed material and the combination is a superior option for silicon-based solar cells. The initial depositions were conducted on flat glass substrates. Since silicon-based solar cells use textured substrates, the same bilayer configurations were also deposited on textured surfaces. The results showed that mobility values remained consistent at 50-60 cm2/Vs, though free carrier density was increased to 7-9×1019 cm−3 . However, those bilayers still outperformed single layers, and with further optimization, the issue of reduced free carrier density can be addressed.