Syntheses and optical, electrochemical, and photovoltaic properties of polymers with 6‐(2‐thienyl)‐4H‐thieno[2,3‐b]indole with a variety of electron‐deficient units

Conjugated polymers were synthesized and used for polymer solar cells with new electron‐rich units, 6‐(2‐thienyl)‐4H‐thieno[2,3‐b]indole (2‐TTI). 2‐TTI was coupled with electron‐pulling units, including benzothiadiazole and benzimidazole derivatives, to provide push–pull types of conjugated polymers (poly(8‐(heptadecan‐9‐yl)‐6‐(thiophen‐2‐yl)‐8H‐thieno[2,3‐b]indole)‐alt‐(2‐methyl‐5,6‐bis(octyloxy)‐4,7‐di(thiophen‐2‐yl)‐2‐(trifluoromethyl)‐2H‐benzimidazole) (PTTIDOCF3), poly(8‐(heptadecan‐9‐yl)‐6‐(thiophen‐2‐yl)‐8H‐thieno[2,3‐b]indole)‐alt‐(5,6‐bis(octyloxy)‐4,7‐di(thiophen‐2‐yl)‐2,1,3‐benzothiadiazole) (PTTIDOBT), poly(8‐(heptadecan‐9‐yl)‐6‐(thiophen‐2‐yl)‐8H‐thieno[2,3‐b]indole)‐alt‐(2,2‐dimethyl‐4,7‐di(thiophen‐2‐yl)‐2H‐benzimidazole) (PTTIMBI), and poly(8‐(heptadecan‐9‐yl)‐6‐(thiophen‐2‐yl)‐8H‐thieno[2,3‐b]indole)‐alt‐(2,2‐dimethyl‐5,6‐bis(octyloxy)‐4,7‐di(thiophen‐2‐yl)‐2H‐benzimidazole) (PTTIDOMBI)). The synthesized conjugated polymers provided deep highest occupied molecular orbital energy levels for higher open‐circuit voltages (VOC). The device composed of PTTIDOMBI and [6,6]‐Phenyl C71 butyric acid methyl ester PC71BM (1:2) with chloronaphthalene additive showed a VOC of 0.72 V, a short‐circuit current (JSC) of 9.16 mA/cm2, and a fill factor of 0.43; this gave a power conversion efficiency (PCE) of 2.84%. The PTTIDOMBI provided better morphology for enhanced charge transport, and this led to the higher JSC and PCE of the organic solar cells.