Designing Transparent Electrodes Materials Toward Broadband Light Trapping

Abstract

Transparent conducting oxides (TCOs) have been used for various optoelectronic applications, such as flat-panel displays and thin-film solar cells. Especially for a-Si thin-films solar cells, light-scattering capability of TCOs via surface texturing is one of the most important characteristics due to effective scattering enhances power-conversion efficiency. Among various TCOs, ZnO:Al has received strong attention because of its larger feature size by wet-chemical etching. Basically, textured morphology by wet etching depends on the nanostructure of TCOs due to the anisotropy of etching rates. However, nanostructural control of TCOs in mass manufacturing is pretty limited with consideration of electrical conductivity and high throughput. Therefore, surface texturing by simple etching for the given TCO nanostructures offers a great merit for the strategy of TCO development. In this thesis, the novel etching system by organic acid for the surface-textured ZnO:Al films is investigated. The Chap. 1 describes the general scientific context and the research field in which this thesis is included. First, a brief overview of the photovoltaic technologies and the current issues of the Si thin-film solar cells. Second, the TCOs are introduced and their use as a front electrode in Si thin-film solar cells is explained. Finally, the motivation and objectives of this work are summarized. In Chap. 2, an organic acid for the surface texturing of ZnO:Al is introduced as an alternative to conventional HCl. The texturing behavior by oxalic acid is investigated in terms of vertical roughness, lateral correlation length, and thickness change according to the crater evolution. Etching with oxalic acid results in superior light-scattering performance (by ~8% increase at λ = 1000 nm) with maintaining transparency and resistance, compared to etching with HCl. This fascinating behavior is understood by crater evolution with the difference in relative etching rates. Significantly, this simple and reproducible texturing tactic extends tunability for desirable TCO morphology, enabling efficient light trapping, and therefore appears potentially applicable for large-scale photovoltaic devices in industry. Lastly, all results and conclusion of the thesis are summarized in Chap. 3.