Interfacial Electronic Structures In Organic Photovoltaic Cells

The problems of energy sources crisis and environmental pollution have become more and more serious with respect to the development of modern society. Photovoltaic (PV) cells are now attracting more and more research interests as the renewable and clean energy sources. Small molecular weight organic materials and conjugated polymers are excellent candidates for the use in photovoltaics because of their unique features such as light weight, low cost, mechanical flexibility, large area fabrication, and so on. These advantages offer the potential for photovoltaic industry to reduce the manufacturing cost. However, there are still some problems such as immature theory, low efficiency, short lifetime and poor stability, which have to be solved before they are achievable for commercial application.In this thesis, the emphasis focuses on the study of small molecular organic solar cells based on copper phthalocyanine-fullerene (CuPc/C₆₀ or CuPc/C₇₀) systems. The influences of the electronic structures of donor-acceptor (D-A) interfaces and the thermal annealing treatment on the performance of OPVs have been systematically investigated.(1) The effect of D-A electronic structures on the organic photovoltaic cellsUnderstanding the electronic structures of organic D-A heterojunction is of pronounced importance for the OPV optimization. For example, open-circuit voltage (Voc) is critical to the cell performance. It is generally believed that the energy level offset between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor (EDHOMO-EALUMO) directly determines the value of Voc. Here, the detailed electronic suructures of a planar C₇₀/CuPc bilayer and a mixed C₇₀:CuPc BHJ have been studied via in situ photoemission spectroscopy. The energy offset is found to increase from 0.55 eV in the bilayer structure to 0.8 eV in the mixed bulk heterojunction.(2) Thermal annealing-induced vertical phase separation in bulk D-A heterojunctionThermal annealing is extensively employed to modify interfacial morphology and molecular arrangements. The D-A interpenetrating networks morphology formed by annealing treatment govern the balance between exciton dissociation and charge transport property. The effect of thermal annealing treatment on the BHJ morphology change in organic photovoltaic cells was studied by photoemission spectroscopy. The results reveal that vertical phase separation upon annealing occurs in CuPc:C₆₀ and CuPc:C₇₀ BHJ layers with inhomogeneous concentration distribution in profile, varying from CuPc-rich near the air surface to C₆₀ (or C₇₀)-rich adjacent to the substrate interface. The morphology variation is associated with the difference in surface energy of CuPc and C₆₀ (or C₇₀), leading to the accumulation of CuPc to the interface with air to lower the overall energy of the free surface.(3) The effect of an exciton blocking layer (EBL) on OPVsBy inserting different materials as an EBL between organic layer and metal electrode, the performance of organic photovoltaic cell was significantly improved. Many studies suggest that EBL can block the exciton diffusion to the cathode, leading to the reduction of exciton quenching at the cathode interface. the effect of EBL on the cell performance was studied according to the energy level alignment.