Extracting the Density of States of Copper Phthalocyanine at the SiO₂ Interface with Electronic Sum Frequency Generation

Abstract

Organic semiconductors (OSCs) constitute an attractive platform for optoelectronics design due to the ease of their processability and chemically tunable properties. Incorporating OSCs into electrical circuits requires forming junctions between them and other materials, yet the change in dielectric properties about these junctions can strongly perturb the electronic structure of the OSC. Here we adapt an interface-selective optical technique, electronic sum frequency generation (ESFG), to the study of a model OSC thin-film system, copper phthalocyanine (CuPc) deposited on SiO2. We find that by modeling the thickness dependence of our measured spectra, we can identify changes in CuPc's electronic density of states at both its buried interface with SiO2 and air-exposed surface. Our work demonstrates that ESFG can be used to noninvasively probe the interfacial electronic structure of optically thick OSC films, indicating that it can be used for the study of OSC-based optoelectronics in situ.

Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues

“…Most importantly, the 2D aspects of the IR-Vis, Vis-IR and Vis-Vis DR-SFG that can be used to study the vibrational-electronic (ground state vibration), electronic-vibrational (electronic vibration), and electronic-electronic couplings have yet to be explored and utilized in studying molecular systems on surfaces and at interfaces, such as optoelectronic thin films, etc. [126].…”

Using Electronic Sum-Frequency Generation to Analyze the Interfacial Structure of Singlet Fission-Capable Perylenediimide Thin Films

“…Because of this selection rule, ESFG provides spectral information analogous to an interfacial absorption spectrum. Moreover, the dependence of the polarization of the emitted ESFG field on that of the excitation fields reports the hyperpolarizability of molecules at buried interfaces, which can be decomposed into a product of the molecules’ transition dipole moment and polarizability. ,,, If the mapping of these parameters onto the molecular frame is known, it is possible to determine from ESFG spectra the spatial orientation of molecules at buried interfaces.…”

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

“…Despite its potential, there are limited examples of eSFG being utilized in the literature. − None of these existing studies have investigated the electronic structure of nanomaterials despite the importance of their surfaces. A recent SFG study probed the ligand vibrational modes on nanoparticles surfaces in the presence of a pulse resonant with the QD exciton transitions but was not able to extract any electronic structure information beyond the first exciton .…”