Polymer solar cells (PSCs) based on bulk heterojunction (BHJ) structures that consist of polymeric donors and fullerene-based acceptors are attracting considerable attention because of their fascinating potential for low-cost, large-area production through solution processing. Because donor-acceptor (D-A) conjugated polymers possess high light-harvesting aptitude, easily tunable optical/electronic properties, and ambipolar charge transporting features, they have been applied extensively to BHJ photovoltaics in the past few years. Specifically, the D-A design concept is the most effective strategy toward harvesting more photons by lowering optical bandgaps while retaining suitable electronic energy levels. In recent years, several efficient D-A polymers have displayed promising potentialities for photovoltaic applications. For instance, electron-deficient units derived from 2,1,3-benzothiadiazole (BT), 2,1,3-benzoxadiazole (BO), 3,6-diaryl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP), and thieno[3,4-b]thiophene-2-carboxylate moieties, when conjugated with various electron-donating units, such as fluorene, carbazole, dithienosilole, and benzodithiophene, have demonstrated distinguished PCEs of up to 7% after systematic optimization. The electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) moiety exhibits a symmetric, rigidly fused, coplanar structure and strong electron-withdrawing properties, which make it a potential system for increasing intramolecular/intermolecular interactions, reducing optical bandgaps, and/or lowering highest occupied molecular orbital (HOMO) energy levels when incorporated into polymeric backbones. Very recently, low-bandgap D-A polymers, comprising TPD and benzodithiophene moieties, were prepared that featured relatively low-lying HOMO energy levels (-5.43 to -5.57 eV), high open-circuit voltages (Voc, up to 0.89 V), and prominent PCEs (4-6%). When designing new efficient D-A polymers, it is important to select suitable electron donors as well as acceptors by taking into consideration their spectral absorption ranges and electronic energy levels. Furthermore, the development of new D-A polymers exhibiting crystalline characteristics can lead to significantly enhanced charge transporting mobilities in the active layers of the devices. Several p-type conjugated polymers containing the symmetric bi(dodecyl)thiophene unit exhibit crystalline characteristics and high hole mobilities when used in organic field effect transisitors. Considering not only the electronic energy level but also the hole mobility, we prepared a newD-Apolymer, PBTTPD,wherein the electronwithdrawing TPD unit was conjugated with the symmetrical electron-donating bi(dodecyl)thiophene unit to provide crystalline characteristics and a low-lying HOMO energy level, resulting from the combination of the rigidly fused TPD moiety and the symmetrical bi(dodecyl)thiopheneunits.Becauseof these desirable features, we expected PBTTPD to exhibit good hole mobility and high values of Voc when used in photovoltaic applications. We prepared PBTTPD (Scheme 1) through Stille polymerization of themonomers 1,3-dibromo-5-ethylhexylthieno[3,4-c]pyrrole-4,6dione (M1) and 4,40-didocecyl-5,50-bis(trimethylstannyl)-2,20-bithiophene (M2) using tris(dibenzylideneacetone)dipalladium/tri(o-tolyl)phosphine [Pd2dba3/P(o-tolyl)3] as the catalyst. The branched 2-ethylhexyl chain of theTPDmoietywas present to promote the solubility of the polymer. The number-average molecular weight of PBTTPD was 9.7 kg mol, with a polydispersity of 1.4, as determined through gel permeation chromatography (GPC) using chloroform as the eluent. This polymer was readily soluble in hot chlorinated solvents, namely chloroform, chlorobenzene, and dichlorobenzene. PBTTPD exhibited good thermal stability, with its decomposition temperature (Td) greater than 400 C, as measured using thermogravimetric analysis (TGA; Supporting Information, Figure S1). We investigated the thermal behavior of this polymer using differential scanning calorimetry (DSC), which revealed two distinct peaks;a melting point at 297 C and a crystallization point at 268 C;but no glass transition (Supporting Information, Figure S2). The presence of the pronounced crystallization peak indicated that PBTTPD had a strong inclination to crystallize, which we further confirmed from its film’s grazing-incidence X-ray diffraction pattern (Figure 1a). In Figure 1a, the (100), (200), and (300) diffraction peaks for PBTTPD are at 3.4 , 6.8 , and 10.2 , respectively, indicating a highly ordered structure along with a d-spacing of 26 A that is ascribable to the interchain distance separated by the alkyl side chains; the broad feature at 24.6 , corresponding to a short distance of 3.6 A, is assigned to the facialπ-π stacking between polymeric backbones; such high crystallinity suggested that PBTTPD would exhibit good carrier mobility when applied in PSCs. Figure 1b presents absorption spectra of PBTTPD in dilute chloroform solution and in the solid state. In solution, the polymer exhibited an absorption signal at 468 nm, which we assign to internal charge transfer between the TPD acceptor and the bithiophene donor. The absorption maximum of the solid state polymer appeared at 572 nm;a significant red shift of 104 nm relative to that in solution, indicating that considerably strong intermolecular interactions existed in the solid film. Additionally, a vibronic shoulder at 628 nm implies an ordered arrangement of PBTTPD in the solid film, with strong π-π stacking between the polymeric backbones, a feature that also appears in regioregular poly(3-hexylthiophene). The optical bandgap of PBTTPD, estimated from the onset of absorption in the solid film, was 1.82 eV; this value is less than that of P3HT (1.91 eV) because of the presence of the electron-accepting TPD moiety in the polymer main chain; accordingly, we expected that PBTTPD would harvest more photons relative to P3HT. Figure S3 presents the redox behavior of the polymer (see Supporting Information). On the basis of the onset potentials, we Scheme 1. Synthetic Route and Structure of PBTTPD