The effect of the length of alkyl side-chains on the molecular aggregation and photovoltaic performance of the isoindigo-based polymers

Abstract

To investigate the length of the alkyl side-chains on the photovoltaic performance, three isoindigo-based polymers with different-length side chains on the thiophene-benzene-thiophene (TBT) (electron-donating) moiety and isoindigo (electron-withdrawing) moiety have been designed and synthesized, in which the total carbon atom numbers of the side chains in a constitutional repeating unit are completely identical. The results indicate that the polymer with longer branched side chains on the TBT units and shorter linear side chains on the isoindigo units shows stronger interchain interaction and light-harvesting capacity in 1,2-dichlorobenzene solution and thin film, lower band gaps, stronger π−π stacking interaction, more appropriate microphase separation with PC71BM, higher hole mobility and better photovoltaic performance. As a result, the bulk heterojunction (BHJ) device based on PTBTOD-IDB with 2-octyldodecyl on the TBT moiety and n-butyl on the isoindigo moiety exhibits a power conversion efficiency (PCE) of 5.29%, which is almost eight times as high as that of PTBTEH-IDHD with 2-ethylhexyl side chain on TBT moiety and 2-hexyldecyl side chain on isoindigo moiety.