Engineered Type-II Heterostructures for High Efficiency Solar Cells.

Abstract

The third-generation solar cell technologies are aiming to achieve substantially higher efficiency over the Shockley-Queisser limit of a single junction solar cell while maintaining low fabrication cost per area in order to become cost competitive with coal fuel. The demanding of a breakthrough in efficiency leads the research to the development of photovoltaic devices with a new concept of fundamental operation other than the structure of single junction solar cells and to the employment of various materials and nanostructures to the devices. The purpose of the projects in this dissertation to present nanostructures with type-II heterointerface that can provide additional possibilities to certain types of third generation solar cells for achieving an efficiency close to the theoretical maximum limit. First, the electronic structure and optical properties of type-II GaAsBi/GaAsN superlattices with an effective lattice constant match to GaAs are studied based on the 8-band k.p method and Self-consistent Schrödinger-Poisson equation. Results show that this heterostructure can provide a new material system with a range of effective bandgap of 0.89eV - 1.32eV, which is a spectral range of high importance for the multi-junction solar cells, for Bi,N composition of less than 5% and period thickness of up to 100A. Second, the electronic structure, optical properties, and thermal carrier capture and escape mechanisms of the type-II GaSb/GaAs self-assembled quantum dots (QDs) are studied theoretically and experimentally to examine the advantages of the heterostructure for the intermediate band solar cells (IBSCs). The electronic structures of the GaSb QDs calculated based on the VFF and the 8-band k.p model show that there is substantially closer spacing between the states because of the large hole effective mass, and 16 holes can fill a QD completely. Admittance spectroscopy measurements show that the thermal emission rate is significantly lower than prior reports for InAs/GaAs type-I QDs, and is estimated to be lower than the optical generation rates in an IBSC under solar concentration. Also, the fabricated GaSb/GaAs QD-IBSCs demonstrate the sub-bandgap response occurred through sub-bandgap photon absorption via the QD confined states and the enhanced the short circuit current compared to a control sample without the QDs.