Excitation of Charge Transfer States and Low-Driving Force Triplet Exciton Dissociation at Planar Donor/Acceptor Interfaces

Here, we investigate charge transfer at archetypal planar heterojunction solar cells based, upon phthalocyanines as donors and C-60 or a perylene derivative as acceptors. We demonstrate the ability to measure photocurrent from direct charge transfer state excitation despite the intrinsically small interface area for bilayer systems. We then discuss the implications of triplet versus singlet excitons in these systems, and find that neither the low triplet energy nor low mobility of perylene acceptors with respect to C-60 are responsible for reduced performance, but rather the low exciton diffusion length and unfavorable absorption profile. Furthermore, we show that triplet and singlet excitons from the phthalocyanine donors are able to dissociate with equal efficiency, even though the driving force is 0.5 eV less, and in fact only about twice the background thermal energy. Therefore, hot charge transfer states are not required, and efficient exciton dissociation is driven by an internal electric field at the heterojunction from either an interface dipole or beneficial polarization effects.