Donor Halogenation Effects on Electronic Structures and Electron Process Rates of Donor/C60 Heterojunction Interface: A Theoretical Study on FnZnPc (n = 0, 4, 8, 16) and ClnSubPc (n = 0, 6)

Authors:
  • RBRui-Rong Bai
    Rui-Rong Bai
  • YWYou-Zhi Wu
    You-Zhi Wu
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Abstract
Molecular engineering is significantly important for developing electron donor and acceptor materials of active layers in organic photovoltaic (OPV). The OPVs based on halogenated donors frequently produced high power conversion efficien-cies. Here, based upon density functional theory calculations with optimally tuned range separation parameters and solid polarization effects, we studied the effects of donor halogenation on molecular geometries, electronic structures, excita-tion and spectroscopic properties for FnZnPc (n=0,4,8,16) and ClnSubPc (n=0,6) monomers and the complexes with C60, as well as the photo-induced direct charge transfer (CT), exciton dissociation (ED) and charge recombination (CR) processes that were described by rate constants calculated using Marcus theory. The tiny differences of molecular orbital energy gap, excitation and spectroscopic properties of FnZnPc (n=0,4,8,16) and ClnSubPc (n=0,6) monomers suggest halogenation cannot effectively tune electronic and optical gap, but the significant decrease of molecular orbital energies support halo-genation causes remarkable influence on the energy level alignment at heterojunction interfaces. The halogenation also enhances intermolecular binding energies between C60 and donors, and increases the CT excitation energies of donor/C60 complexes, which are favorable for improving open circuit voltage. Furthermore, for FnZnPc/C60 (n=0,4,8,16) and SubPc/C60 (n=0,6) complexes, the CR rates dramatically decrease (several orders) with increasing of halogen atoms (ex-cept for F16ZnPc/C60), meaning suppression of CR processes by halogenation. As to the special case of F16ZnPc/C60, it un-derlines the importance fluorination degree in molecular design of donor materials. This study provides a theoretical understanding the halogenation effects of donors in OPVs, and may be helpful to molecular design for electron donor materials.
Funding Information
  • National Natural Science Foundation of China (11164016)
  • HongLiu First-class disciplines Development Pro-gram of Lanzhou University of Technology

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