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.

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We designed and synthetized a new poly{4,8-bis ((2-ethylhexyl)The optical bandgap of PTTBDT-FTT was 1.55 eV. The energy levels of the highest occupied and lowest unoccupied molecular orbitals of PTTBDT-FTT were −5.31 and −3.73 eV, respectively. Two-dimensional grazing-incidence X-ray scattering measurements showed that the film's PTTBDT-FTT chains are predominantly arranged with a face-on orientation with respect to the substrate, with strong π−π stacking. An organic thin-film transistor fabricated using PTTBDT-FTT as the active semiconductor showed high hole mobility of 2.1 × 10 −2 cm 2 /(V•s). Single-junction bulk heterojunction photovoltaic cells with the configuration ITO/PEDOT:PSS/PTTBDT-FTT:PC 71 BM/Ca/Al were fabricated, which showed a maximum power conversion efficiency (PCE) of 7.44%. Inverted photovoltaic cells with the structure ITO/PEIE/PTTBDT-FTT:PC 71 BM/MoO 3 /Ag were also fabricated, with a maximum PCE of 7.71%. A tandem photovoltaic device comprising the inverted PTTBDT-FTT:PC 71 BM cell and a P3HT:ICBA-based cell as the top and bottom cell components, respectively, showed a maximum PCE of 8.66%. This work demonstrated that the newly developed PTTBDT-FTT polymer was very promising for applications in both single and tandem solar cells. Furthermore, this work highlighted the fact that an extended π-system in the electron-donor moiety in low bandgap polymers is crucial for improving polymer solar cells.

Two semiconducting conjugated polymers were synthesized via Stille polymerization. The structures combined unsubstituted or (triisopropylsilyl)ethynyl (TIPS)-substituted 2,6-bis(trimethylstannyl)benzo[1,2-b:4.5-b']dithiophene (BDT) as a donor unit and benzotriazole with a symmetrically branched alkyl side chain (DTBTz) as an acceptor unit. We investigated the effects of the different BDT moieties on the optical, electrochemical, and photovoltaic properties of the polymers and the film crystallinities and carrier mobilities. The optical-band-gap energies were measured to be 1.97 and 1.95 eV for PBDT-DTBTz and PTIPSBDT-DTBTz, respectively. Bulk heterojunction photovoltaic devices were fabricated and power conversion efficiencies of 5.5% and 2.9% were found for the PTIPSBDT-DTBTz- and PBDT-DTBTz-based devices, respectively. This difference was explained by the more optimal morphology and higher carrier mobility in the PTIPSBDT-DTBTz-based devices. This work demonstrates that, under the appropriate processing conditions, TIPS groups can change the molecular ordering and lower the highest occupied molecular orbital level, providing the potential for improved solar cell performance.

Solution-processable semiconducting copolymers, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and poly [4,8-bis(2-ethylhexyl-2-thenyl)-benzo[1,2-b:4,5-b′]dithiophene-alt-5,5′-(4′,7′-di-2thienyl-2′,1′,3′-benzothiadiazole)] (PBDTDTBT), and their pyrene-containing terpolymers were synthesized using Suzuki or Stille coupling. Pyrene units were introduced to improve the chargetransporting abilities of the polymers. The resulting polymers were found to be soluble in common organic solvents and formed smooth and uniform spin-coated thin films. They also exhibited good thermal stability and lost <5% of their weight upon heating to ∼350 °C. Solution-processed field-effect transistors fabricated using these polymers showed p-type organic thin-film transistor characteristics. The pyrene-containing terpolymers showed higher field-effect mobilities than their corresponding parent polymers, and their mobility increased with increasing pyrene content. Furthermore, they had lower HOMO energy levels than the corresponding PCDTBT or PBDTDTBT polymers. Bulk heterojunction solar cells with an ITO/PEDOT:PSS/ polymer:PC 71 BM/Ca/Al configuration fabricated using the pyrene-containing polymers had higher power conversion efficiencies than those using the corresponding parent polymers.

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Two donor–acceptor (D–A) copolymers, based on the donor unit TIPS substituted benzodithiophene (TIPSBDT) and the acceptor quinoxaline-based units with or without fluorine substitution (PTIPSBDT-DTQX and PTIPSBDT-DFDTQX), were designed and synthesized as a donor material for bulk-heterojunction (BHJ) photovoltaic cells. The introduction of F atoms with high electron affinity to be quinoxailine moieties is effective in further lowering both the HOMO and LUMO energy levels of PTIPSBDT-DFDTQX to attain higher open-circuit voltage (V oc). Single junction photovoltaic cells were fabricated, and the polymers:PC71BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In a single layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 2–6%. The solution process inverted tandem photovoltaic cells, in which two photovoltaic cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer processed from solution with the use of BHJ materials comprising semiconducting polymers and fullerene derivatives. We first report here on the design of PTIPSBDT-DFDTQX equivalent poly(3-hexylthiophene), the current medium band gap polymer of choice, which thus is a viable candidate for use in the highly efficient bottom layer in inverted tandem cells.

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