Conjugated donor (D)-π-acceptor (A) copolymers, PBDT-TPD, PBDT-ttTPD, PBDTT-TPD, and PBDTT-ttTPD, based on a benzodithiophene (BDT) donor unit and thieno [3,4-c]pyrrole-4,6(5H)-dione (TPD) acceptor unit were designed and synthesized with different π bridges via Pd-catalyzed Stille-coupling. The π bridges between BDT and TPD were thiophene in PBDT-TPD and PBDTT-TPD, and 6-alkylthieno[3,2-b]thiophene in PBDTttTPD and PBDTT-ttTPD. The effects of the π bridges on the optical, electrochemical, and photovoltaic properties of the polymers were investigated, in addition to the film crystallinities and carrier mobilities. Copolymers with the 6-alkylthieno[3,2-b]thiophene π-bridge exhibited high crystallinity and hole mobility. Improved Jsc and FF were obtained to increase the overall power conversion efficiencies (PCE) in inverted single organic photovoltaic cells. A PCE of 6.81% was achieved from the inverted single device fabricated using the PBDTT-ttTPD:PC 71 BM blend film with 3 vol% 1,8-diiodooctane. A tandem photovoltaic device comprising the inverted PBDTT-ttTPD cell and a PTB7-based cell as the bottom and top cell components, respectively, showed a maximum PCE of 9.35% with a Voc of 1.58 V, Jsc of 8.00 mA/cm 2 , and FF of 74% under AM 1.5 G illumination at 100 mW/cm 2 . The obtained PCE of the bottom cell and FF of the tandem cell are, to the best of our knowledge, the highest reported to date for a tandem OPV device. This work demonstrates that PBDTT-ttTPD may be very promising in applications in tandem solar cells. Furthermore, 6-alkylthieno[3,2-b]thiophene π-bridge systems in medium bandgap polymers can improve the performance of tandem organic photovoltaic cells.

We synthesized a donor polymer of bis(2ethylhexyl)thiophene-substituted benzodithiophene (BDT-Th) and 1,3-bis(2-ethylhexyl)-5,7-di(thiophene-2-yl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione, for which the BDT-Th unit includes chlorine and sulfur-bridged 2ethylhexyl in the thiophene side group. When compared with PBDB-TF, which includes fluorine and 2-ethylhexyl in BDT-Th, PBDB-TSCl shows more efficient exciton dissociation and charge generation, which is probably because large dipole moment changes from ground to excited states lead to reduced exciton binding energy. Consequently, despite a small donor−acceptor interface in the bulk heterojunction (BHJ) film, PBDB-TSCl achieves higher photovoltaic performance than PBDB-TF under various light intensities; PBDB-TSCl achieved higher efficiency (13.13%) than the 12.12% of PBDB-TF under 1 sun illumination. Moreover, PBDB-TSCl showed the highest efficiency of 21.53% with fill factor (FF) of 76.29% under a 500 lx fluorescence lamp, whereas PBDB-TF has lower efficiency of 15.57% with FF of 65.25%. Furthermore, the PBDB-TSCl device shows improved thermal stability due to the more stabilized morphology of its BHJ film.

Optimization and analysis of conjugated polymer side chains for high‐performance organic photovoltaic cells (OPVs) reveal a critical relationship between the chemical structure of the side chains and photovoltaic properties of polymer‐based bulk heterojunction OPVs. In particular, the impact of the alkyl side chain length on the π‐bridging (thienothiophene, TT) unit is considered by designing and synthesizing a series of benzodithiophene derivatives (BDT(T)) and thieno[3,2‐b]thiophene‐π‐bridged thieno[3,4‐c]pyrrole‐4,6(5H)‐dione (ttTPD) alternating copolymers, PBDT(T)‐(R2)ttTPD, with alkyl chains of varying length on the TT unit. Using a combination of 2D X‐ray diffraction, Raman spectroscopy, and electrical device characterization, it is elucidated in detail how these subtle changes to the chemical structure affect the molecular conformation, thin film molecular packing, blend film morphology, optoelectronic properties, and hence overall photovoltaic performance. For copolymers employing both the alkoxy or alkylthienyl‐substituted BDT motifs, it is found that octyl side chains on TT unit yield the maximum degree of molecular backbone coplanarity and result in the highest quality of molecular packing and optimized hole mobility. Inverted devices fabricated using this PBDTT‐8ttTPD: polymer/[6,6]‐phenyl‐C71‐butylic acid methyl ester active layer show a maximum power conversion efficiency (PCE) of 8.7% with large area cells (0.64 cm2) maintaining a PCE of 7.5%.

We have synthesized a series of conjugated D-p-A copolymers, PT-ttTPD and PBT-ttTPD, based on a (5-hexyltridecyl)-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione (ttTPD) acceptor unit in order to develop better photovoltaic polymers based on the TPD moiety: an e-branched alkyl side chain on the TPD unit was coupled with 6-alkyl-thieno[3,2-b]thiophene (tt) p-bridge molecules. The Stille polymerization of the brominated ttTPD and stannylated simple thiophene (T) finally gave a promising PT-ttTPD polymer showing well-ordered inter-chain orientation in the BHJ active layer. PT-ttTPD-based OPVs exhibited a highest power conversion efficiency (PCE) of 9.21% (V OC = 0.86 V, J SC = 15.30 mA cm À2 , FF = 70%).