Designs for Donor-Acceptor Copolymer-Based Double Heterojunction Solar Cells

Self-consistent field calculations were used to examine a design motif for organic double heterojunction solar cell materials. They are a specific type of cascade heterojunction designed to increase cell voltage without sacrificing current and consist of fully conjugated block polymers. The design employs three sections: a p-type section, an n-type section, and a third section called the bridge. The energy alignment between sections is important to optimal device function, and a motif based on electron-donating and electron-accepting subunits was evaluated. If the energetic offset between p-type and n-type sections is greater than the exciton binding energy and if the bridge is formed using the scheme presented, a nearly ideal energetic alignment is obtained. In addition, calculations on the excited states of the system were performed to illustrate the relationship between bridge length and the magnitude of charge carrier recombination currents. An order of magnitude decrease relative to the corresponding diblock polymer can be expected for bridge lengths of >6 repeat units. Taken in sum, these results offer concrete guidelines for the development of synthetic targets in this promising class of materials.