Enhancing the Photovoltaic Performance of a Benzo[c][1,2,5]thiadiazole-Based Polymer Donor via a Non-Fullerene Acceptor Pairing Strategy

Chlorination Enabling a Low-Cost Benzodithiophene-Based Wide-Bandgap Donor Polymer with an Efficiency of over 17

Three regioregular benzodithiophene-based donor-donor (D-D)-type polymers (PBDTT, PBDTT1Cl, and PBDTT2Cl) are designed, synthesized, and used as donor materials in organic solar cells (OSCs). Because of the weak intramolecular charge-transfer effect, these polymers exhibit large optical bandgaps (>2.0 eV). Among these three polymers, PBDTT1Cl exhibits more ordered and closer molecular stacking, and its devices demonstrate higher and more balanced charge mobilities and a longer charge-separated state lifetime. As a result of these comprehensive benefits, PBDTT1Cl-based OSCs give a very impressive power conversion efficiency (PCE) of 17.10% with a low nonradiative energy loss (0.19 eV). Moreover, PBDTT1Cl also possesses a low figure-of-merit value and good universality to match with different acceptors. This work provides a simply and efficient strategy to design low-cost high-performance polymer donor materials.

p-Tetrafluorophenylene Divinylene-Bridged Nonfullerene Acceptors as Binary Components or Additives for High-Efficiency Organic Solar Cells

In this study, we designed, synthesized, and characterized an A-D-A'-D-A-type indacenodithienothiophene (IDTT)-based molecular acceptor that exhibited a broader absorption range and higher lowest unoccupied molecular orbital energy level with a nearly comparable band gap compared to a well-known electron acceptor IT-M. The designed electron-deficient molecular acceptor FB-2IDTT-4Cl with a fluorinated benzene tether (FB), that is, p-tetrafluorophenylene divinylene, demonstrated long-wavelength absorption and high hole and electron charge mobility in the thin films blended with the electron donor PBDB-T for an inverted organic photovoltaic (OPV) binary device, resulting in a maximum power conversion efficiency (PCE) of 11.4%. Such a performance is comparably as high as that of the device with PBDB-T:IT-M, and particularly, it was 18.8% higher than that of the devices with ITIC-4Cl as the acceptor (PCE 9.1% ± 0.5%) and 24.9% higher than that of the devices with the thiophene-flanked benzothiadiazole-bridged acceptor CNDTBT-IDTT-FINCN (PCE 9.01% ± 0.13%). Furthermore, varying the illumination intensity from 200 to 2000 lux increased the J_sc and V_oc values as well as the FF values, thus leading to increased PCE levels. In addition, the best PCE of the PM6:Y6 device with 1% FB-2IDTT-4Cl as additives was 16.9%. Our stability test showed that the PM6:Y6 standard device efficiency downgraded very soon either at room temperature or under thermal-annealing conditions. However, with the addition of 1% FB-2IDTT-4Cl as additives, the device efficiency still can be maintained at 90-95% in 500 h at room temperature and 95% at 20 h and 85-95% in 45 h at an annealing temperature of 80 °C. These findings demonstrate FB-2IDTT-4Cl to be a promising candidate as an electron acceptor with a fluorinated π-bridging fused-ring design for OPV applications.