Date of Award
Doctor of Philosophy (PhD)
Colm T. Whelan
The incorporation of alkali metal has contributed tremendously in a bid to realize greater than 20% efficient Cu(In,Ga)Se2(CIGS) solar cells. Achieving high efficiency is one key parameter for the success of a photovoltaic technology but so is its long-term stability. In this thesis, the relationship between the performance of alkali treated Cu(In,Ga)Se2 solar cells and their physicochemical, electronic and structural properties are explored through a comparative study between standard devices and alkali (K, Rb) treated devices. The alkali treated devices tend to have a lower concentration of Ev+0.98 eV trap, higher majority carrier concentration and improved minority carrier lifetime, contributing to the experimentally observed improvement in open circuit voltage. Critical changes in alkali elemental profile occur throughout the film, while no other major physicochemical or structural properties are modified. Furthermore, we explored the long-term stability of CIGS solar cells due to damp heat treatment. We specifically study the influence on the molybdenum back contact and the CIGS absorber layer itself, with an emphasis on the role played by sodium. Molybdenum thin films showed drastic micro-structural, surface morphology, electrical and optical properties deterioration leading to the degradation of solar cell performance. In the case of bare CIGS thin films, we observed surface oxidation and degraded electronic properties, also leading to degradation of the solar cell performance. In both cases, alkali migration is responsible for the most part of the degradation, along with surface oxidation.
"Effect of Alkali on the Efficiency and Reliability of Cu(In,Ga)Se2 Solar Cells"
(2019). Doctor of Philosophy (PhD), dissertation, Physics, Old Dominion University, DOI: 10.25777/q1y5-zw05