Bicontinuous network of electron donor-acceptor composites achieved by additive-free sequential deposition for efficient polymer solar cells

We report that sequential deposition of a highly crystalline polymer donor and a soluble fullerene acceptor leads to a well-defined interpenetrating network and enhanced power conversion efficiencies in bilayer polymer solar cells. Even without the use of solvent additives, layered thin films of poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2’; 5′,2’’; 5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM), as electron donor and acceptor materials, respectively, showed bicontinuous networks similar to those of a PffBT4T-2OD:PC₇₁BM bulk-heterojunction (BHJ) thin film processed with 1,8-diiodooctane (DIO) as a solvent additive. Transmission electron microscopy results confirmed the BHJ-like morphology of the bilayered PffBT4T-2OD/PC₇₁BM thin films. Bilayer solar cells fabricated without the DIO additive produced a power conversion efficiency of η ≈ 7.65%, which is even higher than that of a BHJ solar cell fabricated with the DIO additive (η ≈ 7.04%). These results demonstrate that a highly crystalline polymer donor and an electron-accepting small molecule can be a good combination for efficient bilayer polymer solar cells.