A systematic evaluation of triisopropylsilylethynyl-substituted thienyl side chain effects on series of benzo[1,2-b:4,5-b′]dithiophene based polymer donors and their photovoltaic performances

Abstract

In this report three new donor copolymers based on alternate octyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate as acceptor and benzo[1,2-b:4,5-b′]dithiophene (BDT) derivatives having triisopropylethynylsilane (TIPS), triisopropyl((thiophen-2-yl)ethynyl)silane (TIPS-Th) and triisopropyl((3-octylthiophen-2-yl)ethynyl)silane (TIPS-ThC8) side chains as donors, (respectively named as P1, P2 and P3) are synthesized to assess whether insertion of thiophene or alkyl thiophene units between TIPS side chains and BDT backbones can improve their properties and photovoltaic performance. Indeed, the modification of TIPS side chains on BDT backbone found to distinctly affect their opto-electrochemical properties, molecular orientations and nanoscale morphologies in resultant polymers. Among the polymers, optimized organic solar cells (OSCs) of P1:PC71BM exhibited best power conversion efficiency (PCE) of 6.55% with a very high VOC of 1.02 V, thereby outperforming P2 and P3 (PCE of 4.72% and 5.47%, respectively). The detailed characterization of optimized blends revealed that this superior device performance of P1:PC71BM blend films attributed to their compact and “face-on” molecular orientation, high hole mobility and superior nanoscale morphology, which minimized voltage loss and leading to an efficient charge transfer with low recombination losses in OCSs. In contrast, lower performances of P2 and P3 were mainly originating from their decreased solubilities caused by the insertion of TIPS-Th or TIPS-ThC8, thereby negatively affecting their molecular weights, morphology and charge carrier mobilities. Thus, these results provide a valuable guideline for designing polymers based on 2-TIPSTh substituted BDT units.