Efficient Conventional- and Inverted-Type Photovoltaic Cells Using a Planar Alternating Polythiophene Copolymer

A low-band-gap alternating copolymer, poly{5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c]-1,2,5-thiadiazole} (PTBT), was synthesized and investigated for photovoltaic applications. PTBT showed a minimized torsion angle in its main backbone owing to the introduction of solubilizing octyloxy groups on the electron-poor benzothiadiazole unit, thereby resulting in pronounced intermolecular ordering and a deep level of the HOMO (-5.41 eV). By blending PTBT with [6,6]phenyl-C61-butyric acid methyl ester (PC61BM), highly promising performance was achieved with power-conversion efficiencies (PCEs) of 5.9 and 5.3?% for the conventional and inverted devices, respectively, under air mass 1.5 global (AM 1.5G, 100 mW?cm-2) illumination. The open-circuit voltage (VOC0.850.87 V) is one of the highest values reported thus far for thiophene-based polymers (e.g., poly(3-hexylthiophene) VOC0.6 V). The inverted device also achieved a remarkable PCE compared to other devices based on low-band-gap polymers. Ideal film morphology with bicontinuous percolation pathways was expected from the atomic force microscopy (AFM) images, space-charge-limited current (SCLC) mobility, and selected-area electron-diffraction (SAED) measurements. This molecular design strategy is useful for achieving simple, processable, and planar donoracceptor (DA)-type low-band-gap polymers with a deep HOMO for applications in photovoltaic cells.