Enhancing the Performance and Stability of Electrode Materials for Photovoltaic Devices

Abstract

Toward high efficiency and low cost as the ultimate goal of solar cells, the electrode materials have been intensively researched due to its important roles in photo-conversion efficiency. In case of transparent contact electrode, ZnO-based transparent conducting oxides (TCOs) have received much attention due to its excellent light-scattering performance and abundance. However, the stability in humid environment is still an unsolved problem, which prevents ZnO-based TCOs from actual applications in the solar cells requiring long-term stability. Therefore, full understanding of the underlined mechanisms on TCO degradation in harsh environments is critical for reliability. In this thesis, the stability and degradation mechanisms of ZnO-based TCOs are examined under the humid and hot atmosphere. 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 advantages of the thin-film silicon solar cells are given. Second, the TCO materials are introduced and their use as a transparent contact electrode in solar cell is explained. Finally, the motivation and objectives of this work are summarized. In Chap. 2, the degradation mechanism of transparent conducting film is examined in detail. A complete understanding of the reliability for ZnO based transparent conducting oxides is essential for actual applications in photovoltaic devices or displays requiring long-term stability. The stability and degradation mechanisms under a controlled damp-heat environment (humid and hot atmosphere) for sputter-deposited aluminum-doped zinc oxide (ZnO:Al) thin films were quantitatively studied. The continuous degradations of carrier concentration and mobility with the Fermi-level shift were observed, and these behaviors were resolved by separating the changes in the carrier-transport characteristics of the intragrain and grain boundary. By correlating the temperature dependence of electrical characteristics with x-ray photoelectron spectroscopy, the degradation is well explained by the increase of chemisorbed OH- in the grain boundaries. Lastly, all results and conclusion of the thesis are summarized in Chap. 3.