Synthesis and Photovoltaic Performance of Low Band Gap Dithieno[3,2-b:2′,3′-d]pyrrole-based Conjugated Copolymers

In the polymer solar cells research field, dithieno[3,2-b:2',3'-d]pyrrole (DTP) is a good donor unit due to its good planarity when it is used as build donor-acceptor (D-A) polymer. In this work, three DTP based polymers (named P1, P2, P3) are synthesized by polymerizing DTP with DPP, DTBO and TQ co-monomer acceptor unit, respectively. These three polymers exhibit good solubility in common solvent such as dichlorobenzene. GPC results show that the polymers show moderate number-average molecular weights (M-n, 9 x 10(3) to 2.2 x 10(4)) and narrow PDI (1 - 2). UV-Vis absorption spectra in solution and thin film show that the polymers have a relatively narrow band gap (P1: 1.23 eV; P2: 1.51 eV; P3: 1.50 eV), which enhances the polymer absorption of sunlight, with P1 showing the widest absorption (extending UV-Vis absorption to nearly 1000 nm). The thermal property of these polymers is measured by thermogravimetric analysis (TGA), giving their T-d located at 313 - 410 degrees C. The highest occupied molecular orbital (HOMO) energy levels of the polymers are between -5.2 eV and -5.4 eV, as tested by CV method. Photovoltaic devices are fabricated with P1 - P3 as donor and PC71BM as acceptor in active layer. The solar cells J-V test results reveal that P1 has the highest PCE of 3.33% with J(SC) of 15.82 mA/cm(2), V-OC of 0.38 V when the mass ratio of P1 to PC71BM is kept at 1:3. The relative high PCE of P1 based PSCs devices is due to the wide light absorption, leading to high J(SC) of PSCs devices, which also can be confirmed by external quatum efficiency (EQE) test. For the polymers P2 and P3, with the mass ratio of polymer/PC71BM at 1:2, PCE of 1.20% for P2 and 1.37% of P3 are achieved respectively, and the relatively low PCE is caused by low short circuit current density (J(SC)) (P2: 9.70 mA/cm(2), P3: 9.21 mA/cm(2)) and the low open-circuit voltage (V-OC, only around 0.3 V). Transmission electron microscopy is also used to explore the polymer solar cells active layer morphology, which correlates closely with the donor and the acceptor aggregations and the phase separation.