Zhuo Xu

A 1,1′-vinylene-fused indacenodithiophene-based low bandgap polymer for efficient polymer solar cells

A 1,1′-vinylene-fused indacenodithiophene (IDTV) donor unit with 22 π-conjugated electrons was synthesized. A ladder-type D–A copolymer PIDTV-ffBT using IDTV as the donor unit and 5,6-difluorobenzothiadiazole (ffBT) as the acceptor unit was developed for application as a donor material in polymer solar cells (PSCs). Compared to other analogue polymers, PIDTV-ffBT possesses a two-dimensional conjugated multi-electron fused ring, excellent planarity and close π–π stacking, leading to a higher light harvesting coefficient, an enhanced charge carrier mobility of 0.032 cm2 V−1 s−1 and improved photovoltaic performance. The PSCs based on PIDTV-ffBT:PC71BM achieved a promising power conversion efficiency (PCE) of 7.3% with a high short-circuit current density (Jsc) of 17.1 mA cm−2. These results indicate that the introduction of the 1,1′-vinylene-fused system into IDTV for ladder-type polymers is an effective strategy to enhance the light absorption coefficient and improve charge carrier mobility for high efficiency PSCs.

Solution-Processable Organic Molecule for High-Performance Organic Solar Cells with Low Acceptor Content

A new planar D2-A-D1-A-D2 structured organic molecule with bithienyl benzodithiophene (BDT) as central donor unit D1 and fluorine-substituted benzothiadiazole (BTF) as acceptor unit and alkyl-dithiophene as end group and donor unit D2, BDT-BTF, was designed and synthesized for the application as donor material in organic solar cells (OSCs). BDT-BTF shows a broad absorption in visible region, suitable highest occupied molecular orbital energy level of -5.20 eV, and high hole mobility of 1.07 × 10(-2) cm(2)/(V s), benefitted from its high coplanarity and strong crystallinity. The OSCs based on BDT-BTF as donor (D) and PC71BM as acceptor (A) at a D/A weight ratio of 3:1 without any extra treatment exhibit high photovoltaic performance with Voc of 0.85 V, Jsc of 10.48 mA/cm(2), FF of 0.66, and PCE of 5.88%. The morphological study by transmission electron microscopy reveals that the blend of BDT-BTF and PC71BM (3:1, w/w) possesses an appropriate interpenetrating D/A network for the exciton separation and charge carrier transport, which agrees well with the good device performance. The optimized D/A weight ratio of 3:1 is the lowest acceptor content in the active layer reported so far for the high-performance OSCs, and the organic molecules with the molecular structure like BDT-BTF could be promising high-performance donor materials in solution-processable OSCs.

TPD-based polythiophene derivatives with higher Voc for polymer solar cells

New D–A copolymers, PTPD-DT and PTPD-DFDT, based on a thieno[3,4-c]pyrrole-4,6-dione (TPD) acceptor unit and 2,2′-bithiophene (DT) or 3,3′-difluoro-2,2′-bithiophene (DFDT) donor units, were designed and synthesized for application as donor materials in polymer solar cells (PSCs). A control polymer PTPD-DT with a similar structure but without fluorine substitution on the 2,2′-bithiophene (DT) unit was also synthesized for comparison. Compared with PTPD-DT, the polymer PTPD-DFDT with fluorine substitution on the DT unit shows a lower HOMO energy level of −5.55 eV, more broad absorption in the wavelength range from 300 to 700 nm, greater coplanarity and crystalline structure. The PSCs based on PTPD-DFDT/PC71BM demonstrated a power conversion efficiency of 5.52%, with a higher open-circuit voltage of 0.96 V. Furthermore, PTPD-DFDT exhibits a simpler molecular structure and easier synthesis steps, which is beneficial for mass production in future.