Polymer based bulk-heterojunction (BHJ) photovoltaic cells, based on blends of conjugated polymers and fullerenes, are one potential economically viable option for low cost renewable power generation. The most promising new conjugated polymer:fullerene active layers in organic photovoltaics now utilize low band-gap (LBG) donor-acceptor copolymers. Unfortunately, for most of those LBG devices, there is no further treatment available after film deposition to optimize the morphology. Moreover, replacing PCBM with bis-adduct fullerenes (i.e. ICBA) has been reported to significantly improve device performance in P3HT device. However, in LBG polymers system, replacing PCBM with ICBA results in very poor device performance. So we have completed small angle neutron scattering and neutron reflectometry experiments to study the impact of solvent annealing (SA) and solvent quality on the morphology the LBG/bis-fullerene (i.e. PCPDTBT/ICBA) BHJ photovoltaics. The results show that SA in a solvent which is selective for the LBG polymer results in a decrease in polymer ordering, which limits device performance. SA in a solvent vapor which has similar solubility for polymer and fullerene, results in a higher degree of polymer ordering and fullerene phase separation, which facilitates charge transport and increases power conversion efficiency (PCE) by 100%. The highest degree of polymer ordering along with large amount of fullerene phase separation is obtained by SA in a solvent vapor which is selective for the fullerene. The resultant morphology increases PCE by 190%. These results indicate that solvent annealing and solvent choice provides a unique tool to tune the morphology of LBG/ICBA BHJ system, providing both sufficient polymer ordering and formation of a ICBA pure phase to optimize the morphology of the active layer.

Control morphology of low band gap polymer: Bis-fullerene mixed heterojunctions in organic photovoltaics with solvent annealing

247th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.