Relating nanoscale chemistry to electrical properties for high-efficiency Cu(In,Ga)Se2 solar cells

Abstract

Cu(In,Ga)Se2 solar cells demonstrate better efficiencies than poly-crystalline Si and all other thin films technologies. Yet most of the gains in efficiency have been empirical and further fundamental understanding is necessary for continued improvement. This thesis will use advanced characterization techniques to show the unique self-compensating behavior along with alkali impurity incorporation as key reasons for the technology’s advancement. Atom probe tomography and scanning transmission electron microscopy are utilized to relate nanoscale chemistry to band structure. High point-defect densities (VCu, InCu, NaCu, KCu) (~1021 cm-3) are shown to lead to variable stoichiometry. Comparable defect densities in traditional semiconductors such as Si and GaAs would be deleterious. However, for Cu(In,Ga)Se2 this is not the case. Point-defects at grain boundaries and the p-n junction are related to pronounced downward shifts in the valence band maximum (~100 meV) and conduction band minimum (~30 meV) as well as n-type doping; which is ultimately predicted to be instrumental in the reduction of recombination and largely responsible for increased electrical performance.