D‐A Polymer with a Donor Backbone ‐ Acceptor‐side‐chain Structure for Organic Solar Cells

Abstract We report the design, synthesis, and properties of a novel type of donor (D)‐acceptor (A) polymer, poly(3‐(([2,2′:5′,2′′‐terthiophen]‐3‐yl‐5,5“‐diyl)methylene)‐1‐(2‐octyldodecyl)indolin‐2‐one) (PTIBT), with a donor backbone and acceptor side chains (Type II D‐A polymer) as donor for organic solar cells (OSCs) as opposed to the conventional D‐A polymers having both donor and acceptor units on backbone (Type I D‐A polymers). PTIBT having a backbone consisting of thiophene donor units and side chains containing indolin‐2‐one acceptor units was synthesized very conveniently in three steps. This polymer has a high dielectric constant of 7.70, which is beneficial for the exciton diffusion and dissociation in the active blend layer in an OSC. In addition, PTIBT was found to have a low‐lying HOMO energy level of −5.41 eV and a wide band gap of 1.80 eV in comparison to its counterpart Type I D‐A polymer. In organic thin film transistors (OTFTs), PTIBT showed typical p‐type semiconductor performance with hole mobilities of up to 1.81×10 −2 cm 2 V −1 s −1 . When PTIBT and ITIC were used as donor and acceptor to form a blend active layer, the best OSC device showed a J SC of 15.19 mAcm −2 , a V OC of 0.66 V, and a fill factor of 0.57, resulting in a power conversion efficiency (PCE) of up to 5.72%.