Investigation of the front contact grid on GaAs wafers as a cap of III-V/Si tandem solar cells by low cost processes

Abstract

Many different solar cell technologies have been developed over the last years, all aiming to improve efficiency in energy conversion, while reducing material expense and cost of production, which is difficult task to fulfil. In order to gain this goal, the SiTaSol project explores to increase the efficiency of inexpensive c-Si solar cells up to 30% by integrating III-V semiconductors as top absorbers. The aim of this thesis is to investigate the metallization of front contact solar cells by low cost deposition methods like inkjet and spin-coating. The copper nanoparticles ink was printed on different substrate such as glass, GaAs and Si. Afterwards were examined the sintering of printed copper nanoparticles ink by using a laser as a high speeds sintering technique. The resistivity of sintered structures was measured and plotted to determine the sheet resistance and contact resistance in the GaAs and Si substrate. Therefore, this work presents experimental investigation of the sintering processing window of pico-second pulsed Nd-YAG laser. The dependence of processing window on Cu layer thickness, different substrates and laser exposure duration has also been studied. This study indicated that, the higher thickness of printed copper NPs enabled to obtain the lower contact resistance and the sintering process under higher laser power with higher exposure time (lower scan speed) decreased the resistivity obviously and caused superior outcomes. In addition, the behavior of pure Copper and Silver, which were deposited with electron-beam physical vapor deposition (EBPVD) technique on GaAs was surveyed to demonstrate the possibility of ohmic contact of copper and silver on two types of GaAs substrate. Furthermore, to improve the contact resistance and adhesion of copper some post processing and surface treatment were done. Various measurements were shown, the lowest contact resistance was gained between pure deposited copper and high doped GaAs (Azur) without post annealing. Also it was indicated that, the copper is the better candidate for these types of GaAs wafers to obtain the ohmic contact with the low resistivity.