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Reports: DNI655184-DNI6: Visualizing Molecular Organization and Energy Transport Dynamics at Organic Surfaces and Heterojunctions with Interface Specific Femtosecond Spectroscopy

Sean T. Roberts, PhD, University of Texas at Austin

Organic semiconductors (OSCs) are a unique class of petroleum-derived materials that blend many of the processing advantages of plastics with the electrical properties of semiconductors. In contrast to common semiconductor materials such as silicon and GaAs, OSCs can be readily processed from solution into highly-absorbing thin, conductive films, making OSCs attractive materials for use in a wide range of optoelectronic applications, including photovoltaic cells, light emitting diodes, and photodetectors. In each of these applications, the transfer of charge to and from OSCs is fundamental to device operation. The primary goals of our work carried out through this grant are to (1) develop a series of interface-specific, nonlinear spectroscopies that can be used to investigate OSC material interfaces, and (2) establish how the molecular organization of these regions controls their ability to donate and accept charge. Specifically, we have been using electronic sum frequency generation, a nonlinear process that selectively occurs at regions of a sample that experience a breakage of inversion symmetry, to probe the electronic density of states of buried OSC interfaces. Following the start of our funding this past January, we have achieved a series of key milestones for this research project which are described in detail below:

Construction of an Electronic Sum Frequency Generation (ESFG) Spectrometer:  We have successfully completed the construction of a spectrometer that can be used to obtain broadband ESFG spectra of thin film samples. Figure 1 illustrates the design of this instrument. In a broadband ESFG measurement, a small portion of the output of a Ti:sapphire amplifier is used to generate a supercontinuum excitation field that spectrally extends from ~400 – 750 nm. This field is mixed in a target sample with a portion of the 800 nm output from the Ti:sapphire amplifier, generating an ESFG field in the ultraviolet range that is detected using a spectrometer and a silicon CCD. Representative ESFG spectra obtained from organic thin film samples appears in Figures 2 and 3 below. Our spectrometer is designed such that we can rapidly switch between collecting of ESFG spectra reflected from a sample surface or transmitted through a sample. As we describe below, for thin films both transmitted and reflected ESFG signals can substantially aid in isolating the portion of the signal that originates from a specific buried interface of interest within a sample. We are currently working to implement a heterodyne detection scheme that can boost the strength of our collected ESFG signal by interfering it with a known reference field generated by refocusing the white light and 800 nm excitation fields onto a piece of GaAs after the sample. We expect that this will boost our ESFG signal strength by 10-100×, which will significantly aid in implementing time-resolved measurements that detect transient changes in the ESFG signal of a sample following photoexcitation by a femtosecond pump pulse.

              Figure 1: Experimental layout for ESFG measurements (WLG: white light generation; MM: magnetic-base mirror mount).

Characterization of the Interfacial Density of States of Copper Phthalocyanine Thin Films:  As an initial system for study, we have focused on using ESFG to characterize the interfacial density of states of copper Phthalocyanine (CuPc) thin films deposited on SiO2. CuPc is an exceptionally well-studied OSC, making it a fantastic model system for benchmarking our ESFG spectrometer. Figure 2 plots a comparison between the bulk absorption spectrum of a 40 nm thick CuPc film and its ESFG response. The ESFG spectrum is clearly shifted to higher energy. While this result in part reflects interference between ESFG signals emitted from the exposed CuPc:air and buried CuPc:SiO2 interface, a fit to our spectral data that accounts for this interference (see below) suggests that the CuPc HOMO-to-LUMO transition at the buried interface is spectrally narrowed and shifted to higher energy at the buried CuPc:SiO2 interface. Such narrowing and shifting of this transition may reflect both a decrease in excitonic coupling between CuPc molecules at the buried interface as well as the difference in dielectric constant between CuPc and SiO2. A manuscript describing these results is currently in preparation for submission to the Journal of Physical Chemistry Letters.

Figure 2: Comparison of ESFG and absorption spectra of a 40 nm CuPc film (structure inset).

In addition to our work on CuPc films, we have also started preparing thin films of squaraine dyes. These materials are strong absorbers in the visible and near-infrared spectral range, but it is unclear how the packing arrangements that these materials adopt at interfaces in electrical devices affects their ability to accept and transfer charge. Squaraine dyes for this work are currently being provided to us through a collaboration with Prof. Mark Thompson’s research group at the University of Southern California.

Implementation of a Thin Film Interference Model for Extracting Signals from Buried Interfaces:  A complication that must be accounted for when investigating thin films using ESFG is the fact that the film has two potentially active ESFG interfaces. Given that the thicknesses of the films that we investigate are on the order of ~50 - 100 nm, it is not possible to spatially isolate these signals. However, at a detector, these two signals can interfere with one another. As the thickness of the film is changed, the phase difference between the ESFG signals emitted from its top and bottom interfaces can change from destructive to constructive. This causes the amplitude of the ESFG signal to oscillate with film thickness, providing data that can be fit to isolate the ESFG response of the buried interface. Over the course of the past year, we have developed a software package that uses a thin film transfer matrix modeling approach to calculate the amplitude of the two driving fields that stimulate ESFG from the sample at each spatial position within the film. This code uses this information to calculate the emitted ESFG signal. By fitting how the strength of the emitted signal varies as a function of film thickness in both reflected and transmitted ESFG spectra, we can isolate the ESFG response of the buried interface of interest.

Figure 3: (A) Thin films can generate ESFG from both of their interfaces. (B) ESFG transmission and reflection spectra as a function of CuPc film thickness (SSP polarization). (C) Simulated CuPc ESFG spectra as a function of film thickness. (D) Comparison of simulated and experimental spectra.