High-Performance All-Polymer Solar Cells Based on Face-on Stacked Polymer Blends with Low Interfacial Tension

We report highly-efficient all-polymer solar cells with power conversion efficiencies of over 4.5{\%} by highly-intermixed blends of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-$b$:4,5-$b$']dithiophene-\textit{alt}-3-fluorothieno[3,4-$b$]thiophene-2-carboxylate] (PTB7-Th) donor and poly[[$N,N'$-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-\textit{alt}-5,5'-(2,2'-bithiophene)] P(NDI2OD-T2) acceptor polymers. The low interfacial tension and the face-on $\pi $-$\pi $ stackings of the all-polymer blends afforded desired nanophase morphology, which facilitates efficient charge transport from active layer to each electrode. In addition, the incorporation of 1,8-diiodooctane additives was able to tune the degree of crystallinity and orientation of P(NDI2OD-T2) acceptors, resulting in remarkable enhancement of electron mobility, external quantum efficiency and $J_{SC}$ values.