Control of morphology and function of low band gap polymer-bis-fullerene mixed heterojunctions in organic photovoltaics with selective solvent vapor annealing

Replacing PCBM with a bis-adduct fullerene (i.e. ICBA) has been reported to significantly improve the open circuit voltage (V-OC) and power conversion efficiency (PCE) in P3HT bulk heterojunctions. However, for the most promising low band-gap polymer (LBP) systems, replacing PCBM with ICBA results in very poor short-circuit current (J(SC)) and PCE although the V-OC is significantly improved. Therefore, in this work, we have completed small angle neutron scattering and neutron reflectometry experiments to study the impact of post-deposition solvent annealing (SA) with control of solvent quality on the morphology and performance of LBP-bis-fullerene BHJ photovoltaics. The results show that SA in a solvent that is selective for the LBP results in a depletion of bis-fullerene near the air surface, which limits device performance. SA in a solvent vapor which has similar solubility for polymer and bis-fullerene results in a higher degree of polymer ordering, bis-fullerene phase separation, and segregation of the bis-fullerene to the air surface, which facilitates charge transport and increases power conversion efficiency (PCE) by 100%. The highest degree of polymer ordering combined with significant bis-fullerene phase separation and segregation of bis-fullerene to the air surface is obtained by SA in a solvent vapor that is selective for the bis-fulterene. The resultant morphology increases PCE by 190%. These results indicate that solvent annealing with judicious solvent choice provides a unique tool to tune the morphology of LBP-bis-fullerene BHJ system, providing sufficient polymer ordering, formation of a bis-fullerene pure phase, and segregation of bis-fullerene to the air surface to optimize the morphology of the active layer. Moreover, this process is broadly applicable to improving current "disappointing" LBP-bis-fullerene systems to optimize their morphology and OPV performance post-deposition, including higher V-OC and power conversion efficiency.