Origin of the boosted exciton separation at fullerene molecule modified poly(3-hexylthiophene)/ZnO interfaces

In this work, we investigate the multifaceted properties at the poly(3-hexylthiophene) (P3HT)/ZnO interface functionalized by a self-assembled monolayer (SAM) of the fullerene derivative C60 pyrrolidine tris-acid (C60-PTA), using a model sandwich structure which allows better mechanistic understanding in comparison to using a more complicated bulk heterojunction (BHJ). Results show that the C60-PTA modification induces fibril-like P3HT nanodomains at the interfaces which were not seen or reported before, yet the increase of work function on the modified ZnO surface is not pronounced. Besides, the quenching efficiency of the P3HT photoluminescence of the modified sample is nearly 7-fold higher than that of the unmodified one, leading to a more efficient exciton separation after the C60-PTA modification. Thus, we believe that the change in P3HT morphology at the interface might have contributed significantly to the substantially improved exciton separation efficiency observed, which was neglected in similar fullerene modified conjugated polymer/metal oxide systems previously. We further investigate the charge transport and recombination as well as the photovoltaic (PV) response in the C60-PTA modified BHJ sample based on vertical ZnO nanorod arrays (NRAs). Results indicate that the mechanistic understanding underlying the model sandwich structure is also applicable in the buried BHJ interfaces. The present study provides a new insight into the origin of the boosted exciton separation in the conjugated polymer/metal oxide heterojunction modified by fullerene molecules, which might be useful for rationally designing three-dimensional organic-inorganic heterojunctions with enhanced splitting capability of photo-excited excitons from the aspect of interfacial morphologies.