Liquid Crystalline Polymers for Efficient Bilayer-Bulk-Heterojunction Solar Cells

The unique temperature-dependent solubility of liquid crystalline poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2- b ]thiophene) (PBTTT) enabled us to construct the first bilayer-bulk-heterojunction devices based on a PBTTT:PCBM (1:2)/MDMO-PPV:[70]PCBM (1:4) active layer. The bilayer device exhibited an extended optical absorption over the solar spectrum and concentration gradient that enhanced charge carrier transport. Postannealing the bilayer-bulk-heterojunction devices yielded a V oc of 0.59 V, J sc values of 10.1−10.7 mA/cm 2, an FF of 0.50, and PCE values of 3.0−3.2%, showing an increased J sc and PCE by a factor of 2 with respect to their single layer counterparts. This study suggests that the bilayered device structure has distinct advantages for the development of highly efficient polymer solar cells.