Article pubs.acs.org/cm
Thermochromism, Franck−Condon Analysis and Interfacial Dynamics of a Donor−Acceptor Copolymer with a Low Band Gap Matthew E. Reish,†,‡ Gregory S. Huff,†,‡ Wonho Lee,§ Mohammad Afsar Uddin,§ Alex J. Barker,†,⊥ Joseph K. Gallaher,†,⊥ Justin M. Hodgkiss,†,⊥ Han Young Woo,§ and Keith C. Gordon*,†,‡ †
The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6140, New Zealand Department of Chemistry, University of Otago, Dunedin 9054, New Zealand § Department of Cogno-Mechatronics Engineering, Pusan National University, Miryang 627-706, Republic of Korea ⊥ School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington 6140, New Zealand ‡
S Supporting Information *
ABSTRACT: The electronic properties of the donor−acceptor (DA) polymer poly{5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c]-1,2,5-thiadiazole} (PTBT) have been investigated using spectroscopic and computational techniques. Electronic absorption and emission spectra reveal the presence of an ordered and a disordered phase in solution. Franck−Condon modeling of the ordered phase yields Huang−Rhys factors of 0.55 (20 °C) and 0.51 (−180 °C), indicating little structural distortion between ground and excited state. DFT calculations with resonance Raman spectroscopy are consistent with a lowest energy excited state that is electronically delocalized and has little charge-transfer character, unexpected for a copolymer with a low bandgap (∼1.8 eV). Transient absorption spectroscopy of PTBT:fullerene blends reveals near-unity internal charge-transfer yields in both ordered and disordered film morphologies. In the disordered blend, charge transfer is complete within the laser pulse (100 fs), whereas the ordered blend also features a slower phase due to exciton diffusion in the phase separated morphology. In the ordered blend, the spectra and dynamics of charge transfer reveal that excitons and charges promptly occupy delocalized states on extended polymer chains. The pervasive use of donor−acceptor structures in polymer devices makes understanding the interplay of morphology and electronic structure of these polymers essential and here a spectroscopic and computational investigation gives an extensive picture of the electronic properties and their effect on charge dynamics in a DA polymer.
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the excited state.14 Additionally, asymmetry of the electronic absorption band is predicted for 1D semiconductors and is a consequence of a broad density of states in the conduction band with absorption into k ≠ 0 states possible.15,16 In this study we investigate the properties of PTBT (Figure 1), a recently synthesized DA copolymer that exhibits two-phase thermochromism in solution, distinct vibronic structure, and
INTRODUCTION The nature of delocalization of the π-electrons in semiconducting polymers has been a focal point of research for several decades, but the understanding of π-electron delocalization is still evolving.1−3 Recently donor−acceptor (DA) copolymers have been utilized in polymer photovoltaics to increase absorption of the solar spectrum, but the description of conjugation is further complicated in these polymers.4−6 The charge-transfer (CT) type electronic excitation in these polymers is seemingly incompatible with extended conjugation.6,7 However, efficient charge separation on a short time scale (
