Polymer solar cells (PSCs) have attracted much attention because of their potential application in flexible, light-weight, and low-cost large-area devices through roll-to-roll printing. [1] The bulk heterojunction PSCs showed advanced features in realizing high efficiencies and solution-processible devices. The active layer in this kind of device consists of an interpenetrating network formed by an electron-donor material blended with an electron-acceptor material. [2, 3] Typically, conjugated polymers [4] are used as electron donors and fullerene derivatives are used as the electron acceptors [5] in the PSCs. Recently, power conversion efficiencies (PCEs) of 6-7 % have been realized by using new conjugated polymer donors [6][7][8] or new fullerene-derived acceptors.[9] Short circuit current density (J sc ), open circuit voltage (V oc ), and fill factors (FF) are key parameters for a PSC device, because the PCE of the device is proportional to the values of the three parameters. To broaden the response wavelength range of a PSC device by using conjugated side chains [4c, 10, 11] or narrowband-gap conjugated polymers [12][13][14][15][16][17][18][19] is an effective way to realize high J sc values. Conjugated polymers with lower HOMO levels are helpful in realizing high V oc and PCE values, as the V oc value of PSCs is directly proportional to the offset between the HOMO level of electron donor and the LUMO level of electron acceptor. [20] PSiFDTBT, [13] PFDTBT, [21] and PCDTBT [14] are three excellent examples for this concept. Consequently, by using conjugated polymers with lower HOMO levels and also narrow band gaps, high PCEs were realized in different families of conjugated polymers. [6][7][8] Conjugated polymers based on benzo[1,2-b:4,5-b']dithiophene (BDT) units have attracted interest as electron donors in the PSC field in recent years, since the report of Hou and Yang et al. on the synthesis and photovoltaic properties of a series of copolymers based on BDT.[22] Many copolymers of BDT with different conjugated units, such as thieno [3,4-b]thiophene (TT), [6,23,24] 4,7-dithiophene-2-yl-2,1,3-benzothiadiazole (DTBT), [7a] N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD), [15] and bithiazole, [25] etc. were synthesized, and the copolymers showed promising photovoltaic properties. In these BDT-based polymers, the alternative copolymers of BDT and TT, namely PBDTTTs, are an important family of photovoltaic materials. For additional improvements in the photovoltaic performance of the PBDTTTs, structural modifications brought about by using different substituents on BDT, or the copolymerized moieties is of great importance. For example, Liang et al. introduced a fluorine atom into the TT unit of the PBDTTTs, and the HOMO level of the resulting polymer was successfully lowered by approximately 0.12 eV, and thus a higher V oc value was achieved, resulting in a great improvement of PCE.[26] Hou et al. optimized PBDTTTs further by replacing the alkoxycarbonyl group on the TT unit with the alkylcarbonyl groups.[24]...

We develop an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithieno[3,2-b]thiophene core and thienyl side-chains for organic solar cells (OSCs). Relative to its counterpart with phenyl side-chains (ITIC), ITIC-Th shows lower energy levels (ITIC-Th: HOMO = -5.66 eV, LUMO = -3.93 eV; ITIC: HOMO = -5.48 eV, LUMO = -3.83 eV) due to the σ-inductive effect of thienyl side-chains, which can match with high-performance narrow-band-gap polymer donors and wide-band-gap polymer donors. ITIC-Th has higher electron mobility (6.1 × 10(-4) cm(2) V(-1) s(-1)) than ITIC (2.6 × 10(-4) cm(2) V(-1) s(-1)) due to enhanced intermolecular interaction induced by sulfur-sulfur interaction. We fabricate OSCs by blending ITIC-Th acceptor with two different low-band-gap and wide-band-gap polymer donors. In one case, a power conversion efficiency of 9.6% was observed, which rivals some of the highest efficiencies for single junction OSCs based on fullerene acceptors.