Fabrication and modification of defects in Cu2ZnSnSe4 single crystals and thin films

Birkmire, Robert W.Cu2ZnSn(S,Se)4 solar cells have achieved the highest efficiency among non-toxic earth-abundant thin film solar materials, however low voltage in solar cell devices caused by band tailing and high levels of poorly understood defects bottleneck commercial relevance. Understanding and controlling the complex defect chemistry of the quaternary material is further inhibited by complexities in thin film materials such as grain boundaries, secondary phases and non-homogenous regions. In this work, single crystals are grown and used as a model system to advance the understanding of the defects controlling the electronic properties of Cu2ZnSnSe4. New methods of bulk crystal growth are developed without the use of an external flux agent leading to the growth of millimeter sized single crystals. The optoelectronic properties of both the surface and bulk are evaluated and controlled to reduce recombination and improve solar cell open circuit voltage. Passivation methods are developed for the surface, and new insights into the optimal bulk stoichiometry for solar cells are demonstrated. The effects of Na doping on sub-bandgap defects are shown, and the solubility limits of Na doping into the bulk CZTSe lattice is measured. Defect populations are further manipulated by annealing where the temperature and cool-down rate are controlled, which are shown to change sub-Eg defects critical for band-tailing. Single crystal Cu2ZnSnSe4 devices are demonstrated for the first time, and based on optimized methods for improving surface and bulk defects, a 7.8% efficient single crystal is demonstrated with a voltage equivalent to record thin film device.University of Delaware, Department of Materials Science and EngineeringPh.D