Synthesis of large band gap oxides and sulfides for Cu(In,Ga)Se₂ thin film solar cells by Atomic Layer Deposition (ALD) and Plasma Enhanced - ALD (PEALD)

This thesis focuses on the development of innovative and efficient materials for the fabrication of the buffer layer of Cu(In,Ga)Se₂ (CIGS) thin film solar cells. For the first time, In₂(S,O)₃ and Zn(O,S) thin films were synthesized by Plasma Enhanced Atomic Layer Deposition (PEALD) in order to substitute the conventional cadmium sulfide buffer layer. By creating reactive species, this deposition technique allows reactions which could not be possible using thermal ALD. The comparison of both methods allows the evaluation of their respective assets and constraints. For instance, In₂(S,O)₃ thin films could only be achieved using PEALD through exchange reaction mechanisms between oxygen radicals from the plasma and sulfur atoms of In₂S₃ growing film. In order to obtain CIGS/In₂(S,O)₃ solar cells with efficiencies of 11.9%, the initial deposition process was improved by correlating X-Ray Photoelectron Spectroscopy and Quadrupole Mass Spectrometry analyses. At the same time, the deposition temperature proved to have a crucial effect on CIGS/Zn(O,S)-ALD device optoelectronic properties and we evidenced the existence of two deposition temperature ranges, at T_dep < 160 deg. C and T_dep > 200 deg. C, where the performances are enhanced. In the low temperature range, the high performances were explained by specific Zn(O,S) properties, while at high temperature they are enhanced by favorable interdiffusion mechanisms at the CIGS/Zn(O,S) interface. Increasing the deposition temperature allowed the fabrication of CIGS/Zn(O,S) solar cells with efficiencies up to 15.6%. (author)