title: Efficient conventional- and inverted-type photovoltaic cells using a planar alternating polythiophene copolymer.
abstract: 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 (PC(61)BM), 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 (V(OC) ≈ 0.85-0.87 V) is one of the highest values reported thus far for thiophene-based polymers (e.g., poly(3-hexylthiophene) V(OC) ≈ 0.6 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 donor-acceptor (D-A)-type low-band-gap polymers with a deep HOMO for applications in photovoltaic cells.
authors.affiliation: Department of Nanofusion Technology (BK21) and Cogno-Mechatronics Engineering (WCU), Pusan National University, Miryang 627-706, Republic of Korea.
authors.forename: Wonho
authors.initials: W
authors.lastname: Lee
authors.forename: Hyosung
authors.initials: H
authors.lastname: Choi
authors.forename: Sungu
authors.initials: S
authors.lastname: Hwang
authors.forename: Jin Young
authors.initials: JY
authors.lastname: Kim
authors.forename: Han Young
authors.initials: HY
authors.lastname: Woo