Fine-Tuning of Molecular Energy Level of Alternating Copolymers On the basis of [1,2,5]Thiadiazolo[3,4-g]quinoxaline Derivatives for Polymer Photovoltaics Yoonkyoo Lee and Won Ho Jo* WCU Hybrid Materials Program and Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea * S Supporting Information ABSTRACT: A series of low-bandgap alternating copolymers consisting of electron-accepting [1,2,5]thiadiazolo[3,4-g]- quinoxaline (TQ) derivatives and electron-donating fluorene or carbazole were synthesized via the Suzuki coupling reaction. For the purpose to fine-tune the molecular energy level of alternating copolymers and thus to improve charge transfer between polymers and PCBM, two different TQ derivatives substituted with strongly electron-donating butoxy group or weakly electron-donating thienyl group were synthesized and used as a building block of alternating copolymers. Copolymers with butoxy-substituted TQ have proper lowest unoccupied molecular orbital (LUMO) energy levels for effective charge dissociation between polymer and PCBM, whereas the LUMO levels of copolymers with thienyl-substituted TQ are too close to that of PCBM to be effective for charge dissociation. The power conversion efficiency was achieved up to 2.17%, which is the highest value among the TQ-based polymer solar cells, when the blend of copolymer with butoxy-substituted TQ and [6,6]- phenyl-C 71 -butyric acid methyl ester was used as an active layer material in bulk heterojunction solar cells. ■ INTRODUCTION In recent years, low-bandgap alternating conjugated copolymers based on the internal donor-acceptor (D-A) interaction have attracted great interest because their electronic properties can easily be tuned by a proper combination of D and A units, and their absorption ranges can be extended to longer wave- lengths. 1-7 Among a wide variety of acceptor units, [1,2,5]- thiadiazolo[3,4-g]quinoxaline (TQ) has been a promising building block for synthesis of low-bandgap conjugated polymers because of the strong electron-withdrawing property of four imine nitrogens in the TQ unit. Using TQ derivatives, several research groups have synthesized D-A type low- bandgap copolymers and reported the photovoltaic properties of those copolymers. 8-10 Among TQ derivatives, 6,7-diphenyl- [1,2,5]thiadiazolo[3,4-g]quinoxaline has been widely used as a building block for alternating conjugated copolymers. However, most of the alternating copolymers composed of TQ derivative and fluorene or carbazole have shown poor photovoltaic performance with a power conversion efficiency (PCE) lower than 1%. 11-13 This low efficiency can be attributed to the low- lying LUMO level of the copolymers (-3.9 to -4.0 eV) which is very close to that of PCBM (-4.0 eV). Since the LUMO- LUMO offset between the donor polymer and PCBM should be at least higher than 0.3 eV for effective charge dissociation, the small LUMO-LUMO offset between TQ-based polymers and PCBM may prevent effective charge separation, which causes a poor photovoltaic performance. In this sense, it is required to raise the LUMO level of TQ-based polymers in order to promote the charge transfer between donor polymer and PCBM and thus to achieve better photovoltaic perform- ance. One feasible approach to control the LUMO level of the donor-acceptor type alternating copolymer is to tune the energy level of acceptor unit in the copolymer. In general, the hybridized LUMO level of D-A type alternating copolymers is located nearly at the LUMO level of acceptor moiety, while the hybridized HOMO level is governed by the HOMO level of donor moiety. 14,15 In the present work, we have synthesized new TQ derivatives substituted with electron-donating thienyl or butoxy group to diminish the electron deficiency of TQ unit, thereby weakening the electron affinity of TQ unit. Since the electron-donating ability of butoxy group is stronger than thienyl group, it is expected that TQ with butoxy group (BOTQ) exhibits higher LUMO level than TQ with thienyl group (THTQ). When a fluorene or carbazole derivative was coupled to each of the two TQs to synthesize four alternating copolymers, it was clearly observed that the LUMO levels of the copolymers were fine-tuned by the LUMO levels of TQ units. One of BOTQ-based copolymers, poly(N-9′-heptade- canyl-2,7-carbazole-alt-6,7-dibutoxy-4,9-bis(4-hexylthien-2-yl)- [1,2,5]thiadiazolo[3,4-g]quinoxaline) (PCBOTQ), showed the best photovoltaic performance with a PCE of 2.17%, which is Received: December 23, 2011 Revised: March 19, 2012 Published: March 27, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 8379 dx.doi.org/10.1021/jp2124247 | J. Phys. Chem. C 2012, 116, 8379-8386