Electronic structure of the poly(3-hexylthiophene):indene-C₆₀ bisadduct bulk heterojunction

The electronic structure of the poly(3-hexylthiophene):indene-C ₆₀ bisadduct (P3HT:ICBA) blend is investigated with direct and inverse photoemission spectroscopy. The energy gap between the lowest unoccupied molecular orbital (LUMO) of ICBA and the highest occupied molecular orbital (HOMO) of P3HT, which is central to the performance of photovoltaic cells made with this blend, is found equal to 1.68 eV. This value is larger than the difference between the ionization energy of the donor and the electron affinity of the acceptor, implying the existence of an interface dipole barrier between these two materials. Complementary contact potential difference measurements are done with a Kelvin probe in the dark on similar donor/acceptor pairs, e.g., P3HT:C₆₀ and copper phthalocyanine (CuPc):C₆₀, and confirm the existence of a dipole at these interfaces. It is estimated that a 0.3-0.4 eV ground state dipole exists at the P3HT:ICBA interface.