Multi-length Scale Morphology for Low Band Gap Polymer Photovoltaics

摘要

Low band gap conjugated polymer bulk heterojunction solar cells have attracted much attention recently due to their promising application in fabricating high efficiency plastic solar cells . For many low band gap polymers, their device processing requires small amount of chemical additives to produce ideal phase separation. It has been shown that the chemical additive, usually a low vapor pressure solvent, could kinetically drive conjugated polymer to form crystals while keeping modified fullerenes, commonly used acceptor materials in the solubilized state. Further evaporation of additive molecules finally deposits the active film. While polymer crystallization is well examined by grazing incidence wide angle x-ray scattering technique, the morphology detail, produced by this two-step film deposition process (the first step is the crystallization of low band gap polymers in a bad solvent due to evaporation of good solvent; a second step is the evaporation of additives, making the final BHJ film), requires more effort to understand. In this work, we used two low band gap polymers, blended with modified fullerene derivatives, to study their morphology details produced by this additive driven morphology evolution. We observed that a multi-length scale morphology or hierarchical morphology exists in the final BHJ film, which gives good performance in solar cell devices.