Synthesis and Photovoltaic Properties of 2D π-Conjugated Polymers Based on Alkylbenzothiophene Substituted Benzodithiophene Donor Unit with Titanium Sub-Oxide (TiOX) as an Interlayer in the Bulk Heterojunction Device Structure

Abstract

A newbenzo[1,2-b:4,5-b’]dithiophene (BDT) derivative with conjugated side chain substituent, 5-ethylhexylbenzothiophene group, was synthesized as donor unit (D) and copolymerized with two acceptor units (A), 3,3′-diethylhexyl-3,4-propylenedioxythienyl-bisbenzothiadiazole and 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP), respectively, using Stille coupling reaction to afford two new copolymers, P1 and P2. Low bandgaps were successfully accomplished for P1 (1.64 eV) and P2 (1.37 eV) and were attributed to the strong intramolecular charge transfer within the D–A alternating structure. Both P1 and P2 exhibited highest occupied molecular orbital (HOMO) energy levels that were deeper due to the conjugated benzothiophene substituent. The photovoltaic performances of the synthesized polymers were investigated using the device configuration ITO/PEDOT:PSS/polymer:PC71BM/(with or without)TiOx/Al. We employed titanium sub-oxide (TiOx) as an effective interlayer in the bulk heterojunction device structure. The subsequent photovoltaic performances showed high open-circuitvoltages (Voc) ranging from 0.78 to 0.81 V, whereas the power conversion efficiencies (PCEs) depended strongly on the blend morphologies. The polymer solar cell devices based on the blend of P2 and PC71BM gave the best photovoltaic performance among the series, with a Voc of 0.78 V, short-circuit current density (Jsc) of 5.69 mA/cm2, fill factor (FF) of 58.09 % and PCE of 2.60 %.