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 specificity.
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 elucidation.
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 – ligand (L) 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 (CQD) 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.