Along with the ongoing research and industry development to reduce the cost of conventional PV devices such as Si-based solar cells, significant research efforts have been focused on exploring new concepts and approaches for high efficiency III-V compound semiconductor solar cells, especially through the fast emerging nanotechnology to exploit the unique properties of nanostructures such as self-assembled quantum dots (QDs). By incorporating self-assembled QDs into the intrinsic region of a standard p-i-n solar cell structure during epitaxial growth, photons in the solar spectrum with energy lower than the energy gap of the host material can be absorbed by the QD layers, leading to an extended photoresponse into longer wavelengths and hence larger photocurrent. In addition, the size and composition of the QDs can be varied and thereby allowing the bandgap to be tuned for absorption in different regime of the solar spectrum. However, due to the small QD absorption cross section, the increase of photocurrent in Quantum Dot Solar Cells (QDSCs) is not significant and always accompanied with some reduction in other device characteristics such as the open circuit voltage and fill factor.
To understand the fundamental process of QDSC operation, in this thesis self-assembled In0.5Ga0.5As/GaAs QDSCs have been designed, fabricated, characterized and investigated in comparison with conventional GaAs p-i-n solar cells. Different approaches to further enhance the device performance have been also proposed and investigated, such as QDSCs with more number of QD layers to increase light absorption, modulation doping of QD barrier layers to partially occupy the QD states to improve carrier extraction and transport, as well as postgrowth thermal annealing and plasmonic light trapping.