We are an ultrafast spectroscopy group specializing in the
development and application of two-dimensional infrared (2D IR) spectroscopy to
study chemical processes in functional organic electronic materials. Ultrafast infrared spectroscopy is combined
with electrochemical techniques to elucidate the structure and dynamics of
charged defects and their involvement in electron transfer, charge carrier
diffusion, and bimolecular charge recombination in emerging photovoltaic
materials. Ultrafast and microsecond
time scale infrared methods are also utilized to study the influence of the
dielectric properties of materials on bimolecular charge recombination and
charge transport processes.
Defects in Organic Electronics
The modern electronic age was facilitated by the ability to
control defects in silicon, which enabled the transport and recombination
behavior of charge carriers to be tuned.
Emerging applications in inexpensive photovoltaics, lighting and display
technologies call for flexible electronic materials for which the rigidity of
silicon is not well suited. Organic
electronics promise to fill this niche – but only if defects in these materials
can be similarly controlled. To date, clear
pathways to develop control strategies have not emerged because few techniques
exist that can examine defects in organic materials with structural
The principle objective of Professor Asbury’s research
program is to explicate the mechanisms by which the molecular structures of defects
in organic electronic materials determine their charge transport, trapping, and
recombination characteristics. We do
this by combining multi-dimensional spectroscopy with electrochemistry. Our research is guided by the hypothesis that
the molecular vibrations of organic electronic materials, whose frequency and
dynamics depend sensitively on their structures, can be used to examine the
structures of charged defects through their vibrational spectra.
2D IR Spectroelectrochemistry
The essential elements of the 2D IR spectroelectrochemical
technique are highlighted in Fig. 1. Charged
defects are populated by shifting the Fermi energy of the material with an
applied potential, V (step 1). One of the electrodes is transparent in the
infrared allowing one- and two-dimensional spectra of the defect vibrations to
be recorded (step 2). The vibrational
dynamics and two-dimensional line shapes of charged defects are compared to the
corresponding features of the neutral materials for which structural
assignments are known. In this way, the
vibrational assignments of the pristine materials can be mapped directly onto
the vibrational features of the defects – thus facilitating their structural
Figure 1. Scheme
outlining the essential elements of the 2D IR spectroelectrochemical
method. Charge defects are populated by
applying a voltage to the electrochemical cell.
The corresponding changes of the 1D and 2D IR spectra provide a means to
elucidate the structures of the charged defects.
The Asbury group uses the 2D IR spectroelectrochemical
methods to examine defects in a variety of emerging organic photovoltaic
materials including those depicted in Figs. 2 and 3. The efficiencies of many organic solar cells
based on polymeric materials (Fig. 2A) are reduced by bimolecular charge
recombination (Fig. 2B, step 1) because this process
occurs on a similar time scale as charge percolation to the electrodes to make
photocurrent (Fig. 2B, step 2). We examine a variety of polymeric materials
to understand how the molecular structures and morphologies of the polymer
blends influence the charge carrier dynamics and defect structures.
Figure 2. Examples of
electron donating polymers and electron accepting molecules in which charged
defects are examined in organic photovoltaic materials.
Solar Cells composed of colloidal quantum dots (CQD)
such as CdSe, CdTe, PbS, or PbSe (Fig. 3) may enable the absorption spectrum of
inexpensive photovoltaic materials to be extended into the near- and mid-IR
spectral regions to fully utilize the solar spectrum for electrical power
generation. The efficiencies of these
devices are limited by the presence of charged defects at the quantum dot –
interfaces which decrease the photovoltage and photocurrent of the
devices. We examine a variety of ligands
to understand how their molecular structures and surface chemistry influence
the type and density of defects.
Figure 3. Schematic diagram of a colloidal quantum dot
solar cell. The quantum dots are covered
by a monolayer of ligands (L). Defects at
the quantum dot – ligand interfaces reduce the efficiency of CQD solar cells.
A variety of vibrational modes are used to probe structures
in the materials including C-H, C=O, CºN, C=N, O-H, N-H, S-H, and
P=O stretch and bend modes with vibrational frequencies ranging from 3000 cm-1
to 1000 cm-1.
The approach of the Asbury group is very interdisciplinary
and includes collaboration with groups in various engineering fields as well as
within the Chemistry department. The
general areas of chemical study include physical, analytical, physical organic
and materials/ polymer chemistry.