New High‐Bandgap 8,10‐Dihydro‐9H‐Bistieno[2′,3′:7.8;3″,2″:5.6]Naphtho[2,3‐d] Imidazole‐9‐One‐Based Donor–Acceptor Copolymers for Nonfullerene Polymer Solar Cells

Three D–A conjugated copolymers based on the same 8,10‐dihydro‐9H‐bisthieno[2′,3′:7.8;3″,2″:5.6]naphtho[2,3‐d]imidazol‐9‐one (DTNIA) acceptor unit and different donor units, i.e., 2‐dodecylbenzo[1,2‐b:3,4‐b′:6,5‐b″]trithiophene (3TB) (P1), 5,6‐dioctylnaphtho[2,1‐b:3,4‐b′]dithiophene (DTN) (P2), and 4,5‐diundecylbenzo[2,1‐b:3,4‐b′]dithiophene (DTB) (P3), are formulated and synthesized. All the copolymers exhibit deep highest occupied molecular energy levels of −5.43, −5.50, and −5.51 eV for P1, P2, and P3, respectively, and show an optical bandgap of 2.18, 2.12, and 2.11 eV, for P1, P2, and P3, respectively. These copolymers are used as donors for the construction of polymer solar cells combining ITIC‐m as an electron acceptor. The optimized polymer solar cells based on P1:ITIC‐m, P2:ITIC‐m, and P3:ITIC‐m realize overall power conversion efficiency of ≈9.62%, 12.84%, and 11.80%, respectively. The greater value of open circuit voltage for P2 and P3 relative to P1 may be due to the deeper highest occupied molecular orbital energy level of P2 and P3 as compared to P1. The highest power conversion efficiency for the P2‐based polymer solar cells may be originated from the denser π–π stacking distance and relatively improved crystallinity, which are advantageous for balanced charge transport, resulting in a comparatively high fill factor and short circuit current. The polymer solar cell based on P2:ITIC‐m shows the highest power conversion efficiency (12.84%) and the P1:ITIC‐m counterpart exhibits the lowest power conversion efficiency (9.62%).