Rational Design of Ternary-Phase Polymer Solar Cells by Controlling Polymer Phase Separation

In this article, we report a novel route to control the ternary-phase morphology of the active layer of polymer solar cells (PSCs). Two typical polymers with complementary absorption ranges, i.e. poly­(3-hexylthiophene) (P3HT) and poly­[(4,4′-bis­(2-ethylhexyl)­dithienol­[3,2-<i>b</i>:2′,3′-<i>d</i>]­silole)-2,6-diyl-<i>alt</i>-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT), are selected to obtain ternary phase system by blending with (6,6)-phenyl-C71 butyric acid methyl ester (PC₇₁BM). A more than three times increase of power conversion efficiency is observed by tuning the morphologies of ternary phase with high second polymer loading. Different from the traditional disordered intermixing morphologies, the existence of submicrometer scale domains of polymer-rich phases are observed for P3HT and PSBTBT, respectively. The measurements of photoluminescence quenching demonstrate that with the morphology varying from intermixed to hierarchical morphology, the interactions between two polymers changing from charge transfer (CT) to fluorescence resonant energy transfer (FRET); at the same time charge transfer mainly occurs at polymers and PC₇₁BM interfaces. The photophysical process here is different from previous reports. A model named hierarchical interpenetrating networks model (HINM) is proposed to describe the optimal active layer of ternary-phase PSCs. Further Kelvin probe force microscopy (KPFM) results demonstrate the reason for our relatively low efficiency is limited by PSBTBT charge transport in blend matrix. We believe that this novel route for controlling morphology could be further optimized and would provide new thoughts and opportunities in the area of PSCs