Origin of the tunable open circuit voltage in ternary blend bulk heterojunction organic solar cells

Abstract

Ternary blend bulk heterojunction organic solar cells comprising two polythiophene polymer donors or two fullerene acceptors are shown to have unique electronic properties. Measurements of the photocurrent spectral response and the open circuit voltage show that the HOMO and LUMO levels change continuously with composition. However, optical absorption of the exciton states retains the individual molecular properties of the two materials across the blend composition. This difference is attributed to the highly localized nature of the exciton which is contained within a molecule, and the more delocalized nature of electrons and holes that extend over several molecules and hence reflect the average composition. Even at low alloy concentration, optical excitation of the low energy polymer creates mobile electrons and holes with high efficiency, showing that excitons reach the interface even in systems below the percolation threshold. The combination of molecular excitations that can harvest a wide range of photon energies, and electronic states that can adjust the open circuit voltage are highly advantageous for efficient solar cells.