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Abstract
Organic solar cells are a promising future energy technology which could overcome the challenge of rising global energy demand. This technology represents a cost-effective alternative to silicon based photovoltaics, due to its potential low fabrication cost and high processing speeds. Organic bulk heterojunction (BHJ) solar cells have attracted large attention in the last decade. The donor and acceptor morphology, which includes the interfaces and blend networks of donor and acceptor, is the key factor to determine the efficiency of BHJ solar cells.
This thesis presents the research results on device optimisation of the BHJ solar cells, which can be categorized into three parts. The first part introduces background information for this study, including operation mechanism, a systematic literature review of BHJ solar cell morphology control and a brief review of ternary solar cells. The second part focuses on the optimisation of the fabrication process of the electron and hole buffer layers. This reveals that the contact between the ZnO layer and active layer is crucial factor in determining device performance. The last part investigates the effects of the donor and acceptor morphology by adding solvent additives, varying the blend composition and using the ternary concept. The polymer phase separation and vertical phase composition were used to investigate the evolution of donor and acceptor morphology. We demonstrated that changing the donor and acceptor morphology influences light harvesting, exciton separation, charge carrier transportation and collection, which has a profound impact on the power conversion efficiency of the BHJ solar cells. The relationship between the crystallinity of the polymer and vertical phase separation were also investigated. These results suggest that the donor and acceptor morphology is a simple and effective way to improve the performance of BHJ solar cells.