Improving Efficiency of Organic Solar Cells by Restricting the Rotation of Side Chain on Small Molecule Acceptor

Abstract

Herein, three carbazole‐based small molecule acceptors (SMAs), named 4TC‐4F‐C8C8, 4TC‐4F‐C6C8, and 4TC‐4F‐C16, are synthesized to study the influence of side chain conformation on SMAs. The three acceptors exhibit similar optical and electrochemical properties, but different crystallization properties. 4TC‐4F‐C16 shows a high crystallinity due to the small steric hindrance of linear n‐hexadecyl (C16) side chain. The large steric hindrance and free rotation for the 2‐hexydecyl (C6C8) side chain seriously disturb the molecular packing and result in a low crystallinity for 4TC‐4F‐C6C8. Despite the large steric hindrance for the 1‐octylnonyl (C8C8) side chain, 4TC‐4F‐C8C8 shows a moderate crystallinity due to the large torsion barrier restricting the rotation of C8C8 side chain. Attributing to the ideal morphology and better crystalline ordering in blend film, organic solar cell based on poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene‐alt‐N‐(2‐hexyldecyl)‐5′5‐bis[3‐(decylthio)thiophene‐2‐yl]‐2′2‐bithiophene‐3′3‐dicarboximide] (PBTIBDTT):4TC‐4F‐C8C8 displays a power conversion efficiency of 12.06%, higher than PBTIBDTT:4TC‐4F‐C6C8 (2.38%)‐ and PBTIBDTT:4TC‐4F‐C16 (9.53%)‐based devices. The work indicates that controlling the conformation of bulky side chain can tune the molecular packing of SMAs and the morphology of blend film, providing a new insight into the molecular design of SMAs.