Benzobis(silolothiophene)-Based Low Bandgap Polymers for Efficient Polymer Solar Cells

Two new low bandgap copolymers that contain a thiophene-phenylene-thiophene fused ring in which the linked carbon atoms are replaced by silicon atoms were designed and synthesized. A facile synthetic method was developed to synthesize the benzobis(silolothiophene) donor unit in high yield. 2-Bromothiophene was lithiated by LDA and subsequently quenched with trimethylsilyl chloride, yielding colorless oil. Further lithiation of 2 with LDA and then reaction with 2-isopropoxy-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolane afforded 3 in 52% yields. After lithiation of 5 by n-BuLi, cyclization with dichlorodialkylsilane was performed to get the disilole 6 in 76% yields. Low band gap polymers PBSTBT and PBSTDTBT were obtained though a Stille reaction between monomer and 4,7-dibromo-2,1,3-benzothiadiazole or 4,7-bis(2-bromo-5-thienyl)-2,1,3- benzothiadiazole, using Pd(PPh₃)₄ as catalyst in toluene. The AFM images of the blended films containing both polymer and PC₇₁BM showed rather smooth morphology with small phase segregation.