Numerical Study Of Organic Solar Cells

In recent years, the study of organic solar cells has attracted much interest and obtained great achievements due to their low cost, easy of fabrication, and procession temperatures which are suitable for fabrication onto lightweight plastic substrates. Comparing with the great achievement of experiment, the physical theory of the electric carrier transport in organic solar cells devices is insufficient, which greatly restricts to improve the performance of the organic solar device devices. Because the carrier transport in inorganic semiconductor is different from in organic semiconductor, it is greatly needed to build a theory model holding for organic semiconductor. Based on those problems, following work has been done.1. The single layer structure of "metal/organic layer/metal" is the basis form of organic solar cells. Although the efficiently is low, this structure has the advantage of simple fabrication and low cost. Base on the onsager theory of geminate recombination and the drift-diffusion theory, we developed numerical model for single layer organic solar cells. Calculation result show that the short-circuit current density will increase when increase the hole mobility while kept the electron mobility constant. The open-circuit voltage is nearly independent on carrier mobility .the efficiency exciton-generation rate ,band gap and the distance of electron and hole in excitons on the performance of the device are discussed.2 Bulk-heterojunction (BHJ) is a mixture of a donor and accepter material. In the BHJ the exciton dissociation rate is generally larger than that in heterojunctions. In the BHJ ,due to increased contact between donor and acceptor molecules, which enhances the energy conversion efficiency of the cell. The device physics of BHJ solar cells can be reduced to that of the single layer solar cells in which the LUMO and the HOMO of the organic layer are the LUMO of the donor and HOMO of the acceptor material respectively. Calculation results show that the dissociation rate can be increased and the recombination rate can be decreased by increasing the effective applied voltage. The dissociation rate will tends to zero with increasing effective applied voltage, and the photocurrent density will tends to saturate. For large effective applied voltage, the variation of photocurrent with temperature will be weak.3. A planar heteroj unction organic solar cell consists of a donor layer and an acceptor layer sandwiched between two electrodes .this structure characterized of low carrier recombination which is benefit for high efficiency. Meanwhile, the exciton dissociation-rate is small due to limited contact area between donor and acceptor molecules. Under the consideration of the optoelectronic specialty of the donor-acceptor interface, developed a model for planar heteroj unction organic solar cells, in which the carrier injection at the metal-organic contact is described by the thermionic theory of Scott and Malliaras. The calculation results show that the short-circuit quantum efficiency is determined by the competition between exciton dissociation and recombination. The model shows a logarithmic dependence of the open-circuit voltage on the incident intensity. This light intensity-dependent open-circuit voltage arises from the field required to produce a drift current that balances the current due to diffusion of carriers away from the interface.