Analysis of a device model for organic pseudo-bilayer solar cells

Abstract

Polymer solar cells fabricated through sequential-solution-processing of donor and acceptor materials consist of partially intermixed heterojunctions and are therefore termed as organic “pseudo-bilayer” solar cells. A steady-state model for such organic pseudo-bilayer solar cells, which generalizes the bulk-heterojunction and the bilayer solar cell models, is derived based on a donor-blend-acceptor structure for the active layer. In short, the model considers intrinsic solar cell physics such as optical generation, charge carrier transport, generation, and recombination, as well as the space charge effect. Parameter fitting and model validation are carried out with current-voltage measurements from in-house fabricated pseudo-bilayer solar cells. The model shows that even with a large exciton diffusion length, a pure large-molecule bilayer cell is unable to produce the photocurrents seen in the measurements. This confirms the experimental observation that an intermixed region, instead of a well-defined interface, exists between the donor and acceptor phases in pseudo-bilayer solar cells. In addition, the effect of blend region thickness on device performance is explored. Local features, such as the electric field and charge carrier concentrations, are also discussed.