Influence of charge carrier extraction parameters on the performance of bulk heterojunction organic solar cells

An expression for the extraction coefficient (beta) of free charge carriers in bulk heterojunction (BHJ) organic solar cells (OSCs) is derived as a function of the effective carrier mobility, effective carrier concentration, active layer thickness, dielectric constant and built-in voltage. Also a relation between the extraction coefficient beta and fill factor (FF) is derived. The results show that FF increases when beta increases, which occurs due to increase in extraction parameters like effective mobility and dielectric constant, and decrease in light intensity and the active layer thickness. These results agree well with the experimental observations. This study provides an alternative way of optimizing the material parameters crucial in the synthesis of new materials in order to achieve a high charge carrier extraction efficiency and hence a high PCE in BHJ OSCs.