Korea b S Supporting Information P olymer solar cells (PSCs) have attracted wide attention in recent years due to their low-cost solution fabrication process, light weight, large area, and flexible panels as well as potential contribution to clean and renewable energy. 1 Among the various types of polymer-based bulk heterojunction (BHJ) photovoltaic devices, the most efficient cells, with a power conversion efficiency (PCE) of ∼3-6%, are fabricated using a blend of poly(3-hexylthiophene) (P3HT) (electron-donor p-type) and PCBM (electron-acceptor n-type) as the active layer. 2 However, P3HT only harvests photons with wavelengths below 650 nm, while the majority of the energy from solar photons has a longer wavelength around 700 nm. 2e Therefore, polymer materials with low band gaps are needed to harvest the longer wavelength solar photons, particularly in the NIR region. Various design strategies have been pursued to fulfill this requirement. One popular approach introduced by Havinga et al. 3 in macromolecular systems is to synthesize copolymers containing alternating electron-rich donor (D) and electron-poor acceptor (A) monomeric units on a conjugated molecular backbone. A great deal of attention has been paid to D-A conjugated polymers whose optical and electronic properties could be tunable through intramolecular charge transfer (ICT) from the D to the A with a better power conversion efficiency (up to around 5-6%). 4,9c In the design of D-A conjugated polymer, a useful strategy is to introduce a monomer unit with quinoidal character into the conjugated system, which can efficiently reduce the band gap and enhance π-π stacking. Fused thiophene ring systems are wellknown to stabilize the quinoidal structure. 5 Recently, solar cell devices with efficiencies greater than 7% have been demonstrated using fused thiophene (thieno[3,4-b]thiophene) conjugated polymers through a systematic tuning of the band gap, absorption, and relevant device parameters. 1b,9 In our search for new electron-rich monomers and taking into account these recent results, we became interested in the dithieno[3,2-b:2 0 ,3 0 -d]thiophene (DTT) unit, 6a,6b an important building block for a wide variety of functional organic materials. The planarity and S-S interaction of the fused DTT structure promotes highly ordered π-stacking 7d,8 and high hole mobility, 8a which are predictors for high charge transport in devices. 8b Several groups have reported the synthesis of DTT derivatives for applications in organic thin film transistors (OTFTs). 7 It is interesting to note that despite all of these promising features, to the best of our knowledge, there have been quite a few reports on the photovoltaic properties of DTT-containing D-A type copolymers. 10 Xiaowei Zhan and co-workers 10a,10d reported polymers consisting of alternating perylenediimides (PDI)-dithienothiophene (DTT) unit as an acceptor and bis(thienylvinylene)-substituted polythiophene as a donor with PCE ∼ 1% under simulated AM1.5, 100 mW/cm 2 conditions. Moreover, this group also re...
