The concept of introducing an intermediate band to overcome the efficiency limit of single-bandgap solar cells was proposed by Luque and Martí in 1997. It is predicted that utilising the intermediate band for multi-photon absorption can significantly improve the photocurrent generation without accompanying output voltage loss. Amongst several approaches to develop an intermediate band solar cell, quantum dots have drawn much attention as intermediate band due to their three-dimensional quantum confinement and bandgap tunability. However, despite the effort expended so far, there still remains several major challenges that prevent the successful implementation of quantum dot intermediate band solar cells. The work reported in this thesis aims to provide solutions to the main challenges in implementing high-efficiency quantum dot solar cells. The work involves the design, epitaxial growth by molecular beam epitaxy, device processing, and characterisation of QDSCs. This thesis first investigates the influence of direct Si doping on InAs/GaAs quantum dot solar cells with AlAs cap layers. Si doping in QDs leads to state filling of the intermediate band, which is one of the key requirements for a high-efficiency intermediate band solar cell. Moreover, the introduction of moderate amount of Si dopants leads to passivation of defect states, and hence prolongs the carrier lifetime and increases the open-circuit voltage. Secondly, type-II InAs/GaAsSb quantum dot solar cells are studied. Increased photocurrent contribution from the quantum dot region is observed due to the prolonged carrier lifetime associated with the type-II band alignment. Lastly, different types and positions of quantum dot doping methods are investigated. The photoluminescence spectra indicate that using delta or modulation doping in quantum dots can reduce the degradation of crystal quality, and hence decrease the number of non-radiative recombination centres, when compared with using direct doping
