Effect of axial halogen substitution on the performance of subphthalocyanine based organic photovoltaic cells

Abstract

a b s t r a c tTo probe the influence of molecular dipole on the open circuit voltage (V OC ) of molecular heterojunction organic solar cells, we study axially fluorinated boron subphthalocyanine/ fullerene (SubPc-F/C 60 ) junctions. These exhibit an open-circuit voltage V OC = 1.00 V, a value closer to the HOMO-LUMO offset at the donor-acceptor interface = 1.69 eV than the V OC = 1.06 V measured for junctions between the archetypal chlorinated SubPc and C 60 , with corresponding HOMO-LUMO offset = 1.84 eV. Aside from the axial halogen substitution, the two compounds exhibit similar molecular structure and optical absorption. The energy levels and structure of the heteromolecular polaron pair are calculated, and the ideal organic diode model for SubPc-Cl is modified accordingly, successfully reproducing the experimental SubPc-F device characteristics. The reproducible difference in V OC is attributed to the different electric dipole strength between SubPc-F and SubPc-Cl and its influence on polaron pair dynamics at the heterojunction.

Predictive Simulations for Tuning Electronic and Optical Properties of SubPc Derivatives

“…These may be physical but only observable in single crystals or they are an artifact of the limited number of k-points used. Either way, the lowest-energy absorption peak position aligns well with experiment with a difference of 75 meV for B-Cl and 91 meV for B-F. 32 The lowest-energy peak positions also agree well with the extinction-coefficient measurements of Fulford et al for B-Cl dissolved in toluene. The maximum difference between B-Cl, B-F, and B-Br is 31 meV.…”

“…Finally, we simulate the I-V curves for the solar cell of Ref. 32 using the methods contained therein.…”

Hexachlorinated Boron(III) Subphthalocyanine as Acceptor for Organic Photovoltaics: A Brief Overview

“…In any heterojunction type, the charge transfer across the D/A interface could also be influenced by the dipole moment of a molecule of one or both components. It bound to occur in the pyramidal subphthalocyanine molecules bearing axially attached halogen (Figure 1) and can further be tuned by the axial/peripheral substitution [4,26,[56][57][58][59]. Mutual orientation of the acceptor/donor molecules at the interface affects both the dissociation distance and the local electric field during the charge transfer and separation, thus modifying the resulting V oc of the device [59].…”

“…It bound to occur in the pyramidal subphthalocyanine molecules bearing axially attached halogen (Figure 1) and can further be tuned by the axial/peripheral substitution [4,26,[56][57][58][59]. Mutual orientation of the acceptor/donor molecules at the interface affects both the dissociation distance and the local electric field during the charge transfer and separation, thus modifying the resulting V oc of the device [59]. Morris et al [59] experimentally investigated and modeled the characteristics of PHJ-based photovoltaic cells with two subphthalocyanines containing either chlorine or fluorine extraligand paired with the C 60 acceptor.…”

“…Mutual orientation of the acceptor/donor molecules at the interface affects both the dissociation distance and the local electric field during the charge transfer and separation, thus modifying the resulting V oc of the device [59]. Morris et al [59] experimentally investigated and modeled the characteristics of PHJ-based photovoltaic cells with two subphthalocyanines containing either chlorine or fluorine extraligand paired with the C 60 acceptor. These donor molecules have nearly identical structure, except for a permanent electrical dipole, which allows the analysis of the variations in V oc in terms of D/A separation width, polaron pair binding energy, and dipole orientation, other morphological factors being neglected.…”