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

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 cm² 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%.