Investigation of charge transfer between donor and acceptor for small-molecule organic solar cells by scanning tunneling microscopy and ultrafast transient absorption spectroscopy

Small-molecule organic solar cell is a category of clean energy potential device since charge transfers between donor and acceptor. The morphologies, co-assembly behavior, interaction sites, and charge transfer of BTID-nF (n = 1, 2)/PC₇₁BM donor–acceptor system in the active layer of organic solar cell have been studied employing scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), density functional theory (DFT) calculations, and transient absorption (TA) spectroscopy. The results show that BTID-1F and BTID-2F form bright strip structures, whereas BTID-nF (n = 1, 2)/PC₇₁BM form ridge-like structures with each complex composed of one BTID-nF (n = 1, 2) molecule and four PC₇₁BM molecules which adsorbed around the BTID-nF (n = 1, 2) molecule by S···π interaction. With the assistance of S···π interaction between BTID-nF (n = 1, 2) and PC₇₁BM, BTID-nF (n = 1, 2)/PC₇₁BM co-assembled ridge-like structures are more stable than the BTID-nF (n = 1, 2) ridge structures. To investigate the charge transfer of BTID-nF (n = 1, 2)/PC₇₁BM system, STS measurements, DFT calculation, and TA spectroscopy are further performed. The results show that charge transfer occurs in BTID-nF (n = 1, 2)/PC₇₁BM system with the electron transferring from BTID-nF (n = 1, 2) molecules to PC₇₁BM.