Effects of Thieno[3,2-b]thiophene Number on Narrow-Bandgap Fused-Ring Electron Acceptors

We synthesize and compare four near-infrared absorbing fused-ring electron acceptors named as n TTIC(n=2, 3, 4, and 5), based on different number of thieno[3,2-b]thiophene(TT) unit as the electron-donating core. With increasing the TT unit, absorption spectrum of the TTIC series red shifts, and the highest occupied molecular orbital(HOMO) upshifts notably. It is worth noting that 4TTIC and 5TTIC exhibit absorption edges approaching 1100 nm, which is the photoresponse limit of solar cells based on crystal silicon. When the TTIC series acceptors are blended with polymer donor PM6, the binary-blend organic solar cells based on 3TTIC show the best power conversion efficiency(PCE) of 13.1%. In contrast, 2TTIC-based devices exhibit relatively lower PCE of 8.32%, mainly caused by the larger energy loss and blue-shifted absorption. Due to insufficient driving force of charge separation caused by very high HOMO, 4TTIC and 5TTIC show poor PCEs lower than 3%.