Electronic structure and carrier dynamics at organic/organic or organic/inorganic semiconductor interfaces

Abstract

Delayed Triplet-State Formation through Hybrid Charge Transfer Exciton at Copper Phthalocyanine /GaAs Heterojunction

Light absorption in organic molecules on an inorganic substrate and subsequent electron transfer to the substrate create so-called hybrid charge transfer exciton (HCTE). The relaxation process of the HCTE states largely determines charge separation efficiency or optoelectronic device performance. Here, the study on energy and time-dispersive behavior of photoelectrons at the hybrid interface of copper phthalocyanine (CuPc)/p-GaAs(001) upon light excitation of GaAs reveals a clear pathway for HCTE relaxation and delayed triplet formation. According to the ground-state energy level alignment at the interface, CuPc/p-GaAs(001) shows initially fast hole injection from GaAs to CuPc. Thus, the electrons in GaAs and holes in CuPc form an unusual HCTE state manifold. Subsequent electron transfer from GaAs to CuPc generates the formation of the triplet state in CuPc with a few ps delay. Such two-step charge transfer causes delayed triplet state formation without singlet excitation and subsequent intersystem crossing within the CuPc molecules.

Change of Molecular Orientation using Copper Iodide and Its Influence on Exciton Dynamics at Metal Phthalocyanine/Fullerene Interface for Organic Photovoltaics

To achieve a highly efficient organic solar cell (OPVs), control of molecular orientation is prime important. While molecular orientation within a film influences charge conduction, relative configuration at donor/accepter interface determines energy level alignment as well as electron-hole separation. Here, we report the change in molecular orientation of planar-shape donor molecules, metal phthalocyanine (CuPc or ZnPc) on ITO by adding CuI as templating layer. Upon ultraviolet photoelectron spectroscopy studies, CuPc reveals a noticeable change in ionization potential ($\deltaIP = ~0.3 eV$) on CuI, which represents the strong change in molecular orientation. However, energy levels of ZnPc molecules are scarcely changed, that is, CuI has little influence on molecular orientation of ZnPc. Such difference in molecular orientation is confirmed additionally by near edge X-ray absorption fine structure spectroscopy. To explain the templating effect of CuI, we demonstrate their difference in molecular orientation by dipole-assisted heteroepitaxial growth by considering the interaction between CuI and MPc molecules depending on the TM ions (Cu, Zn). Furthermore, using time-resolved two-photon photoemission spectroscopy, the influence of molecular orientation on exciton dynamics at $MPc/C_{60}$ interface is studied in detail. On CuI layer, “lying-down” CuPc molecules show CT characteristics at the $CuPc/C_{60}$ interface, while ZnPc molecules only show long-lived S1 peak even at the $ZnPc/C_{60}$ interface. Thus, it is verified that well-oriented $CuPc/C_{60}$ interface induces CT exciton by strong $\pi-\pi$ overlap between donor and acceptor. Finally, we discuss the influence of molecular orientation on the enhanced carrier transport and overall solar cell efficiency by measuring device performance.