Self-organizing mesomorphic diketopyrrolopyrrole derivatives for efficient solution-processed organic solar cells

We describe the synthesis, self-organization, and photovoltaic properties of diketopyrrolopyrrole (DPP)-based mesomorphic small-molecule materials, 3,6-bis{5-(4-alkylphenyl)thiophen-2-yl}-2,5-di(2-ethylhexyl)-pyrrolo[3,4-c] pyrrole-1,4-diones (DPP-TP6 and DPP-TP12), which have strong visible absorption characteristics. The effects of varying terminal alkyl chains on the self-assembling properties and photovoltaic device performances have been studied. With the appropriate ratio of the lengths of the alkyl chains to its rigid DPP core, DPP-TP6 exhibits liquid-crystalline properties and forms well-developed nanostructured bulk heterojunction (BHJ) architectures with a fullerene derivative (PC₇₁BM). Organic solar cells (OSCs) employing BHJ DPP-TP6:PC₇₁BM films show a power conversion efficiency as high as 4.3% with a high open-circuit voltage of 0.93 V and a fill factor of 0.55. These results demonstrate that liquid-crystalline organization to direct molecular self-assembly is a promising strategy for fabricating high-performance solution-processed small-molecule OSCs.