Uncovering Structure-Function Relationships at Heterojunction Interfaces in Molecular Photovoltaic Materials
　

Our research interests are focused on developing hybrid spectroscopic and electrochemical tools for uncovering structure-function relationships at heterojunction interfaces in photovoltaic  materials. In molecular optoelectronics applications, interfaces govern important electronic processes, such as energy and charge transfer, that ultimately determine device functionality and efficiency. Reliable control over interfacial processes is only possible if an accurate molecular level picture of interfaces exists. To address this issue, we will develop powerful spectroscopic and electrochemical approaches to correlate the chemical structure, conformation and morphology to efficacies of energy and charge transfer at interfaces of state-of-the-art materials systems for photovoltaic, light-emitting diode and field-effect transistor applications. The ultimate goal of this research is to aid in the rational design of materials strategies for improved device efficiencies and lifetimes.

Structure-function relationships at molecule-metal electrode interfaces. Molecule-metal interfaces are ubiquitous in optoelectronic devices and play a major role in determining device efficiency and functionality. The overarching goal of this research project is to ascertain the role of interfacial chemical interactions on charge injection and transport yields. In addition we will identify chemical reactions leading to the formation of defect species (traps) that alter device performance over time and repeated voltage cycling. Raman spectroscopy and imaging will be the primary techniques used to study molecule-electrode interfacial properties in conjunction with traditional device characterization techniques such as current-voltage (I-V) and capacitance measurements. Furthermore, because metal electrodes are most often vapor deposited, metal nanoparticles (NPs) of various sizes exismages as a function applied device voltage will reveal preferential charge injection behavior perhaps due to orientational aspects of the molecules at the molecular interface which are known to promote charge transfer interactions. We will take advantage of large local field enhancements arising from NP plasmon resonance to specifically probe the molecular species at these interfaces. These voltage-dependent surface-enhanced Raman (SERS) experiments will provide a rich information content and specific structural features can be directly correlated to electronic properties in a functioning device structure.

Using conformational control to understand and optimize charge generation yields in molecular photovoltaic systems. Maximizing charge separation in donor (D) and acceptor (A) materials is crucial for improving photovoltaic power conversion efficiencies and remains a key challenge for molecular-based light energy conversion applications. Morphology-dependent charge separation yields will be studied in prototypical conjugated polymer/fullerene (D/A, respectively) and representative surrogate D/A materials, which are among the most promising molecular-based photovoltaic strategies to date. It has been shown in many D/A materials that both photoinduced electron transfer (eT) and resonance energy transfer (RET) can contribute to charge generation and both processes also compete with rapid energy relaxation (funneling) within the polymer excited states. This research will unravel these complex kinetics and heterogeneity at the single molecule level by taking advantage of the different D/A distance and geometry and polymer chain conformation of eT, RET, and funneling and ultimately learn the ‘rules’ for optimizing these processes independently.  Charge generation will be studied by time-resolved spectroscopy and resonance Raman spectroscopy (RRS) in polymer/fullerene thin films, nanoparticles and at the single molecule level.  This research is expected to have a significant impact in understanding the role of structural/conformational factors in regulating energy- and charge transfer at heterojunction interfaces in photovoltaic devices and can serve as a tool for materials chemists to develop synthetic methodologies to control eT/RET branching ratios by controlling structural and conformational properties.