Investigations on wet and dry laser assisted texturing on flexible polymeric substrates and their suitability towards photovoltaic applications

Abstract

The passive substrate type architecture of photovoltaic device bestows flexibility in selection of substrate such as flexible, transparent and semitransparent substrates. The flexible substrates, polyethylene terephthalate (PET) and polyimide (PI) polymeric foils are widely used in flexible solar cell fabrication due to thermal, mechanical stability and lifetime reliability. Although flexible substrate based thin film solar cells offer reliable performance, achieving effective utilization of incident sunlight remains key challenge. The utilization of incident sunlight can be enhanced by means of engineering the homogeneous surface pattern and fine tuning of surface characteristics of flexible polymeric substrates.In this work, a correlation between three different types of laser-assisted texturing techniques and their influence on absorption of copper indium gallium di-selenide (CIGS) layer were investigated. Laser texturing has been of interest in the recent years due to its ability of selectively texturing the heat sensitive polymeric substrates. The flexible polymeric substrates are textured using nanosecond Nd3+: YAG (Neodymium-doped Yttrium Aluminum Garnet) laser with three different harmonics i.e. primary (1064 nm), secondary (532 nm) and tertiary (355 nm) harmonics. In order to reduce the experimental complexities, the detailed simulations were carried out to determine the temperature profile of the flexible substrate at dry and wet conditions. Using simulation results, the crystallization and ablation regions were outlined with varying laser parameters.The influencing parameters such as laser wavelength, fluence, and texture environment were varied systematically for texturing techniques such as (i) Laser-induced Self-assembled Patterning (LISP), (ii) Laser-assisted Wet Texturing (LAWT), and (iii) Laser Line Beam (LLB) Texturing techniques. In all three techniques, laser wavelength 355 nm found to be suitable for texturing of PET and PI substrate in dry and Wet condition. The LISP process produced rippled structure and improved UV-Vis absorption by 40 %. The LAWT technique generated triangular prism of aspect ratio 2:3 and enhanced absorption by 50 %. The LLB patterning revealed large area surface patterning with increased absorption by 65 %. These enhancements were observed in comparison to parent substrates selected for LISP, LAWT and LLB texturing techniques. The influence of textures on absorber CIGS layer was observed to improve the absorption and I-V characteristics by 10 %. The results reported in the thesis are of great importance for developing highly efficient photovoltaic devices.