Abstract
Encapsulation of photoactive organic molecules inside single-walled carbon nanotubes (SWNTs) appears to be of great interest in terms of high power conversion efficiency and long-term stability for a commercial application of organic solar cells (OSCs). In this paper, we report a charge transfer (CT) evidence in donor–acceptor SWNTs filled with Sexithiophene oligomers (6T) by Raman spectroscopy. To compute the optimal diameter and demonstrate the most stable structure of the hybrid systems with either a single 6T molecule encapsulated into SWNTs (6T@SWNTs), or two 6T chains encapsulated (6T-6T@SWNTs), we have performed structural geometry optimization on the hybrid encapsulated systems using a convenient Lennard–Jones (LJ) expression of the van der Waals (vdW) intermolecular potential. Combining the density functional theory (DFT), molecular mechanics, bond polarizability model, and the spectral moment method (SMM), we computed the polarized nonresonant Raman spectra of 6T molecule and SWNTs (metallic and semiconducting) before and after encapsulation. The influence of the encapsulation on the Raman-active modes of the 6T molecule and those of the nanotube (radial breathing modes and tangential modes) are analyzed. In particular, significant changes observed in the G-band wavenumber. The possibility (or not) of an eventual CT between the 6T oligomer and the nanotube in both hybrid systems (6T@SWNTs and 6T-6T@SWNTs) is discussed. We show that there is a dependence of the CT with respect to the diameter of SWNTs, the CT vanish with increasing diameter of the nanotubes. Our finding of CT behavior in the filled SWNTs with respect to SWNT diameter will provide a useful guidance for enhancing the performance of OSCs by SWNTs.