Spectroscopic and theoretical studies on the structural, electronic, and optical properties of zinc octaethylporphyrin/C60 co-deposited films.

Abstract

We have examined the structural, electronic, and optical properties of zinc-octaethylporphyrin [Zn(OEP)]/C60 co-deposited films to elucidate the donor (D)-acceptor (A) interactions at the D/A interface of heterojunction organic solar cells (OSCs), using Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence (PL) spectroscopy in combination with first-principles and semi-empirical calculations. The FT-IR and XRD results indicated that Zn(OEP) and C60 were mixed with each other at the molecular level in the co-deposited film. The theoretical calculations suggested that in the interfacial region, it is energetically preferable for the C60 molecule to face the center of the planar structure of Zn(OEP) at a distance of 2.8 Å rather than the edge of the structure at a distance of 5.0 Å. After consideration of the C60 solvent effects, this coordination model for C60-Zn(OEP) adequately explained the line shift of the UV-vis peaks with respect to the proportion of C60 in the co-deposited films. A comparison of the energy level diagrams of Zn(OEP) before and after the interaction with C60 revealed that the LUMO, HOMO, and HOMO-1 were significantly affected by the interaction with C60. In particular, the HOMO-1 wave function became spread over a portion of C60, although the charge transfer between Zn(OEP) and C60 was almost negligible. Since no PL peaks (S1 → S0) from the excited Soret band of Zn(OEP) were observed for the Zn(OEP)/C60 co-deposited films, the D/A mixing layers played a crucial role in completely dissolving the photogenerated excitons to electrons-hole pairs that cause the short-circuit current, which is relevant to improving the energy conversion efficiency of OSCs.