Narrow-Band-Gap Conjugated Polymers Based on 2,7-Dioctyl-Substituted Dibenzo[a,c]phenazine Derivatives for Polymer Solar Cells

A series of new 2,7-dioctyl-substituted dibenzo­[a,c]­phenazine (BPz) derivatives were designed and synthesized as electron-deficient units, which were copolymerized with an electron-rich indacenodithiophene (IDT) to construct narrow-band-gap copolymers PIDT–OHBPz, PIDT–OFBPz, and PIDT–OBPQ via Stille polycondensation. The 2,7-dioctyl substituents enhanced solubility and offered a new approach for developing various BPz derivatives. All copolymers showed high hole mobilities above 0.01 cm² V^–1 s^–1 as measured by field effect transistors. The best performance of polymer solar cell was achieved based on PIDT–OFBPz with inverted device structure of ITO/ZnO/PFN/polymer:PC₆₁BM/MoO₃/Al, which showed an open-circuit voltage (V_oc) of 0.97 V, a short-circuit current (J_sc) of 8.96 mA/cm², and a fill factor (FF) of 58.99%, leading to a high power conversion efficiency (PCE) of 5.13%. These results indicate that 2,7-alkyl-substituted BPz derivatives can be used as an excellent electron-deficient building block for the construction of high-performance organic electronic materials.