Optoelectronic analytical modeling and performance evaluation of bulk heterojunction organic solar cells under oblique incidence

Abstract

In this thesis, we analyze the effect of incidence angle on the performance of bulk heterojunction organic solar cells. In this regard, we present an analytical optoelectronic model for describing the current-voltage characteristics of bulk heterojunction organic solar cells at oblique solar irradiation. Firstly, a closed form general expression of the optical generation rate in the active layer is derived for oblique incidence employing transfer matrix formalism. The derived expression is then incorporated with classical drift-diffusion transport equation and continuity equation of charge carriers to arrive at a unified expression of voltage dependent current density combining optical and electrical parameters. The model is capable of determining accurately the optical absorption in the active layer as well as predicting the external quantum efficiency for different angles of incidence, which is verified by comparison with published numerical and experimental results. Subsequently, we investigate the effect of incidence angle on the solar cell performance parameters of interest and show that maximum efficiency might be achieved at an oblique angle of incidence rather than at normal incidence. The angle at which the maximum efficiency occurs is found to be active layer thickness dependent. In this regard, we also report the optimum angles at which the efficiency of these cells would maximize for typical blend thicknesses. For widely used P3HT:PCBM blend, maximum efficiency occurs for 100 nm thickness and the corresponding angle of incidence is 34 degree. Lastly, we compare the angular responses of organic solar cells realized with different donor materials.