Solution-processed photovoltaics with advanced characterization and analysis
During the last few decades, numerous promising solar cell concepts, ranging from single-crystal silicon to thin-film technologies, have been developed and are being researched intensely by a growing number of scientific groups and companies. Thin-film kesterite Cu2ZnSn(S,Se)4 (CZTS) photovoltaic technology, in which the indium in Cu(In,Ga)(S,Se)2 (CIGS) is replaced with more abundant and less expensive zinc and tin, has emerged as a potential absorber material in next generation thin film solar cells. Despite the recent demonstration of solution-processed CZTS devices over 11% power conversion efficiency, the development of CZTS as an absorber material is still behind in terms of both fundamental understanding of the material system and in the capability to precisely control the material properties for device fabrication, as compared with those of CIGS and CdTe. This dissertation targets the three key areas in this field: (1) Defect characterization and understanding in order to recover Voc loss; (2) Phase stability and processing control to produce a purer absorber material and (3) Solution-processing with environmentally friendly solvents for large-scale production. We start by exploring various precursor systems (hydrazine, benign organic solvents and nanoparticles) and have successfully processed CZTS from a molecular solution in a benign solvent system. A single component precursor has also been developed and proved to offer more precise phase and composition control. Lastly, using electrical and optical characterization, we have conducted detailed investigations on the bulk and the interface defects that govern the carrier recombination and the resulting device characteristics. They reveal the effects of the anions in CZTS on the defect concentration and on voltage losses of the solar cells.