New Insights into the Correlation between Morphology, Excited State Dynamics, and Device Performance of Small Molecule Organic Solar Cells

Morphology plays a vital role on the performance of organic photovoltaics. However, our understanding of the morphology-performance relationships for organic photovoltaics remains lacking. Specifically, it is still an open question why some bulk-heterojunction blends exhibit electric field dependent J-V curves, while others do not. Through detailed fs-s transient absorption spectroscopy and morphology studies on the representative bulk-heterojunction type small molecule (SM) donor system, a picture of different J-V behaviors from morphology aspects and excited dynamics is revealed. Our findings reveal that amorphous morphology in the lack of percolated pathways leads to the formation of strongly bound charge transfer states (CTSs), which accounts for about one third of the photoexcited species. Therefore, field-dependent J-V curves are obtained as these CTSs mainly undergo geminate recombination or function as interfacial traps for nongeminate recombination. On the other hand, the CTSs are totally suppressed after post-treatment owning to the formation of bicontinuous morphology, which results in very high efficiencies from exciton generation, diffusion, dissociation to charge extraction, thus contributes to field-independent J-V characteristics. The insights gained in this work provide the effective guidelines to further optimize the performance of bulk-heterojunction type SM-organic photovoltaics through judicious morphology control and engineering.