A dithieno[3,2-b:2′,3′-d]pyrrole based, NIR absorbing, solution processable, small molecule donor for efficient bulk heterojunction solar cells

Abstract

A novel, NIR absorbing organic small molecular donor material denoted as ICT3 with an A–D–D–D–A architecture having dithieno[3,2-b:2′,3′-d]pyrrole (DTP) and butylrhodanine as donor and acceptor moieties, respectively, is synthesized and its thermal, photophysical, electrochemical and photovoltaic properties are explored. ICT3 has excellent stability over a broad range of temperatures with a decomposition temperature (T_d corresponds to 5% weight loss) of 372 °C, soluble in most common organic solvents (solubility up to 30 mg mL⁻¹) and suitable for solution processing during device fabrication. ICT3 has broad (520–820 nm) and intense visible region absorption (molar excitation coefficient is 1.69 × 10⁵ mol⁻¹ cm⁻¹) and has suitable HOMO and LUMO energy levels with the [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) acceptor for efficient exciton dissociation and charge transfer. Bulk heterojunction solar cells (BHJSCs) with an indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/ICT3:PC₇₁BM/poly(9,9-bis(3′-(N,N-dimethylamino)propyl)fluorene-2,7-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl) (PFN)/aluminium (Al) structure are fabricated and the BHJSCs with the active layer as cast from chloroform solution displayed a power conversion efficiency (PCE) of 3.04% (J_SC = 8.22 mA cm⁻², V_OC = 0.86 V and FF = 0.43). Annealing the active layer significantly improved the PCE of these BHJSCs. While thermal annealing of the active layer improved the PCE of the BHJSCs to 4.94%, thermal followed by solvent vapour annealing enhanced the PCE to 6.53%. X-ray diffraction and atomic force microscopy analyses are carried out on the active layer and these results revealed that annealing treatment improves the crystallinity and nanoscale morphology of the active layer, enriches the device exciton generation and dissociation efficiency, charge transport and collection efficiency and reduces carrier recombination. The observed higher PCE (6.53%) of the BHJSCs having ICT3 with a DTP donor moiety broadens the scope to develop new, efficient DTP based small molecular donor materials for BHJSCs.