Role of Second Polymer Donor in Polymer–Fullerene Ternary Blend Solar Cells Evaluated by Photoconductive Atomic Force Microscopy

The introduction of a third component to a donor–acceptor blend binary device can boost the performance of organic solar cells. This study investigated the role of a second polymer donor (D2, the third component) in the operation of polymer–fullerene blend solar cells in terms of the spatial distribution of D2 in the ternary active layer, which was visualized using photoconductive atomic force microscopy (PC-AFM). Ternary devices that consisted of a low-bandgap polymer, poly­[(4,4-bis­(2-ethylhexyl)-dithieno­[3,2-b:2′,3′-d]­silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7 diyl] (PSBTBT), serving as D2, a wide-bandgap polymer donor, poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)-benzo­[1,2-b:4,5-b′]­dithiophene))-alt-(2,2-ethyl-3­(or4)-carboxylate-thiophene)] (PTO2), and a fullerene acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), utilized as host materials, were prepared at different PSBTBT loading amounts. The photocurrent images obtained using PC-AFM with selective illumination of PSBTBT and nonselective white-light illumination were compared to distinguish the photocurrents of PSBTBT from those of all the components. The comparison indicated that the photogenerated hole transport channels of PSBTBT were formed without a distinct phase separation from PTO2. The functional nanomorphology visualized by PC-AFM provides insights into the evolution of the open-circuit voltage and fill factor of the PSBTBT:PTO2:PCBM ternary device due to the increased PSBTBT loading.