Does Conjugation Help Exciton Dissociation? A Study on Poly(p‐phenylene)s in Planar Heterojunctions with C60 or TNF

The process of exciton dissociation in organic semiconductors is the key step that needs to be understood for the realization of efficient organic solar cells.[1] A systematic optimization of solar cells requires a well-founded and detailed understanding of the mechanism of exciton dissociation and its dependence on material parameters. Consequently, it is being investigated intensely.[2–11] There is widespread agreement on the elementary steps that accompany charge carrier generation at an organic molecular heterojunction.[1,12] Light absorption creates an excited state on one molecule. This is followed by electron transfer to a neighboring molecule, which results in the formation of a more or less strongly bound metastable intermolecular state. This excited state has a strong charge-transfer character and will eventually break up into free charge carriers or lead to recombination. The central question, therefore, is how to overcome the Coulombic binding energy that prevents the final separation of the opposite charges. There are studies implying that dissociation may be favored by a higher hole mobility,[13–15] which is typically associated with a larger conjugation length and a low degree of energetic disorder.[4] On the other hand, there are also reports that in certain circumstances an increased disorder or a lower hole mobility may be preferential.[16,17] Using a set of poly(p-phenylene)s (PPPs) as systematic model compounds, we have investigated how the conjugation length of the chromophore affects the process of charge carrier dissociation at a molecular heterojunction. In field-dependent photocurrent measurements we find the photocurrent yield to saturate at 100% at electric fields between 5 × 104 V cm−1 and 1 × 106 V cm−1. Importantly, the saturation field that is required decreases with increasing conjugation length, implying that the excited state delocalization is of crucial importance for the yield. We conclude that the rate-limiting step of the photogeneration of charges is the formation of a loosely bound (“hot”) electron–hole pair that can either be dissociated at moderate field or relax back to a tightly bound electron–hole pair.