Homogeneous bulk heterojunction networks via surface energy matching at polymer/fullerene interfaces

This work presents homogeneous bulk heterojunction (BHJ) networks between a push-pull π-conjugated polymer donor, poly(2,7-(5,5,10,10-tetrakis(2- ethylhexyl)-5,10-dihydroindeno[2,1-a]indene)-alt-5,5-(4,7-bis(2,2-bithien-5-yl) -2,2-dimethyl-2H-benzimidazole)) (PININE-BBTMBI) and a fullerene derivative acceptor, [6,6]-phenyl C ₇₁-butyric acid methyl ester (PC ₇₁BM). By introducing a bi-thiophene unit between the electron-rich and electron-deficient moieties in a repeating unit, the surface energy of PININE-BBTMBI increased to that of PC ₇₁BM, enhancing its miscibility in PININE-BBTMBI/PC ₇₁BM blends. Direct evidence obtained via atomic force microscopy and the performance of BHJ solar cells indicate that the introduction of the bi-thiophene unit leads to homogeneous BHJ networks and high-performance BHJ solar cells by matching the surface energies at the polymer/fullerene interfaces.