Modeling of bulk and bilayer organic heterojunction solar cells

Solar cells require symmetry-breaking features such as built-in electrostatic fields and/or effective fields. We examine an organic heterojunction structure with no built-in field and explore the origins of its current-voltage characteristics and open circuit voltage (VOC). Two behaviors are found: (1) VOC=VI+m[(HOMO(D) - (LUMO(A)] where m≈1, the intercept (VI) is determined by interface recombination kinetics, HOMO(D) is the donor highest occupied molecular orbital, and LUMO(A) is the acceptor lowest unoccupied molecular orbital; (2) if interface recombination is suppressed, VOC is controlled by bulk/contact recombination and is not dependent upon HOMO(D) - LUMO(A).