<i>N</i>‑Octyl-2,7-dithia-5-azacyclopenta[<i>a</i>]pentalene-4,6-dione-Based Low Band Gap Polymers for Efficient Solar Cells

We report the synthesis, characterization, and solar cell properties of new donor–acceptor-type low band gap polymers (POBDTPD and PEBDTPD) that incorporate dialkoxybenzodithiophene (BDT) as the donor and <i>N</i>-octyl-2,7-dithia-5-azacyclopenta­[<i>a</i>]­pentalene-4,6-dione (DTPD) as the acceptor. The newly developed DTPD moiety was carefully designed to lower a band gap via strong interaction between donor–acceptor moieties and keep polymer energy levels deep. Remarkably, the DTPD acceptor moiety effectively widens the light absorption range of the polymers up to ∼900 nm while positioning their HOMO and LUMO levels in the optimal range, i.e., −5.3 and −4.0 eV, respectively, for high power conversion efficiencies (PCEs) as we intended. Solar cell devices were fabricated according to the structure ITO/PEDOT:PSS/photoactive (polymer:PC₇₀BM)/TiO₂/Al. The POBDTPD devices exhibited a PCE of 4.7% with a <i>V</i>_oc of 0.70 V, a <i>J</i>_sc of 10.6 mA/cm², and a FF of 0.64. The PEBDTPD devices yielded a higher PCE of 5.3% with a <i>V</i>_oc of 0.72 V, a <i>J</i>_sc of 13.5 mA/cm², and a FF of 0.54. AFM, TEM, and PL quenching measurements revealed that the high <i>J</i>_scs are a result of the appropriate morphology and efficient charge separation. In comparing the performances of the two polymer devices, the higher <i>J</i>_sc for the PEBDTPD device was attributed to its better nanoscale phase separation, smoother surface, and higher carrier mobility in the polymer:PC₇₀BM blend films. The higher FF for the POBDTPD device was ascribed to a good balance between the hole and electron mobilities. Overall, we demonstrate that the DTPD unit is a promising electron-accepting moiety to develop high performance low band gap polymers