A broad outline of research interests

Summary of research interests

Our group explores light-matter and intermolecular interactions in organic optical and optoelectronic materials. We also develop and characterize novel naturally-derived sustainable organic materials, develop novel experimental techniques for characterization of charge and energy transfer at nanoscales, and utilize optical probes in entomology.

In particular, we are interested in:

  1. understanding basic physics of exciton and photogenerated charge carrier dynamics in organic semiconductors
  2. utilizing photophysical and electronic properties of individual molecules in studies of complex environments such as donor-acceptor heterojunctions
  3. developing novel high-performance optical materials for electronic and photonic applications and characterization techniques
  4. exploring applications of molecular photophysics in interdisciplinary research such as insect behavior modification using visual signals

Experimental set-ups in our lab utilize time-resolved spectroscopy and microscopy, time-resolved and cw photoconductivity techniques, single-molecule fluorescence microscopy, and optical tweezers combined with spectroscopy.

Motivation

Organic (opto)electronic materials have been extensively studied due to their low cost, opportunities to create solution processable devices on flexible substrates, and tunable properties. Applications of organic (opto)electronic materials include xerography, thin-film transistors, light-emitting diodes, solar cells, photorefractive devices, and many others [1]. By slight synthetic modifications or doping, it is possible to vary optical properties (such as absorption and fluorescence spectra), thermal and structural properties (such as phase transition temperatures and a type of packing in a crystallographic unit cell), and electronic properties (such as charge carrier mobility) of organic materials and therefore, tailor them for specific applications. In spite of many demonstrated and commercialized applications of organic materials, a number of issues, both fundamental and applied, remain. For example, basic physics of light-induced charge carrier generation and subsequent transport and extraction, the processes that lay foundation for most of the applications of organic optical materials, is still not completely understood. On the applied side, it is often challenging to make a series of organic thin films with exactly reproducible properties, especially if large-area devices are desired. Indeed, the dependence of the thin film structure on the fabrication methods and conditions and the relationship between the structure and optical and electronic properties of the film are not straightforward. Therefore, systematic comprehensive studies are needed to reveal the physical nature of all processes contributing to the device performance and understand structure-property relationships [2-6].

As many photophysical processes rely on the nanoscale morphology and local nanoenvironment, studies of photophysics on the molecular level using single-molecule spectroscopy represent a powerful tool for better understanding of physics behind light-matter and intermolecular interactions in organic devices [7]. The physical description of the stochastic behavior of single molecules is still incomplete and presents exciting opportunities for both experimental and computational studies.


References
[1] O. Ostroverkhova, “Organic Optoelectronic Materials: Mechanisms and Applications,” Chemical Reviews 116 , 13279-13412 (2016).
[2]K. Paudel, B. Johnson, M. Thieme, M. Haley, M. M. Payne, J. E. Anthony, and O. Ostroverkhova, "Enhanced charge photogeneration promoted by crystallinity in small-molecule donor-acceptor bulk heterojunctions," Applied Physics Letters 105, 043301 (2014).
[3] K. Paudel, B. Johnson, A. Neunzert, M. Thieme, B. Purushothaman, M. M. Payne, J. E. Anthony, and O. Ostroverkhova, "Small-molecule bulk heterojunctions: distinguishing between effects of energy offsets and molecular packing on optoelectronic properties," Journal of Physical Chemistry C 117(2), 24752-24760 (2013).
[4]M. J. Kendrick, A. Neunzert, M. M. Payne, B. Purushothaman, B. D. Rose, J. E. Anthony, M. M. Haley, and O. Ostroverkhova, "Formation of the donor-acceptor charge-transfer exciton and its contribution to charge photogeneration and recombination in small-molecule bulk heterojunctions," Journal of Physical Chemistry C 116, 18108-18116 (2012).
[5]K. R. Rajesh, K. Paudel, B. Johnson, R. Hallani, J. E. Anthony, and O. Ostroverkhova, "Design of organic ternary blends and small-molecule bulk heterojunctions: photophysical considerations," Journal of Photonics for Energy 5, 057208 (2015).
[6]B. Johnson, M. J. Kendrick, and O. Ostroverkhova, "Charge carrier dynamics in organic semiconductors and their donor-acceptor composites: numerical modeling of time-resolved photocurrent," Journal of Applied Physics 114, 094508 (2013).
[7]W. E. B. Shepherd, R. Grollman, A. Robertson, K. Paudel, R. Hallani, M. A. Loth, J. E. Anthony, and O. Ostroverkhova, "Single-molecule imaging of organic semiconductors: toward nanoscale insights into photophysics and molecular packing," Chemical Physics Letters 629, 29-35 (2015).