University of North Carolina at Chapel Hill, B.S. Chemistry 2006
University of Florida, Ph.D. Chemistry 2011
Our research interests are centered around a common theme of interfacial interactions and energetics, with an application oriented approach targeting the development and understanding of interfaces in organic electronics, perovskite based solar cells, organic-inorganic composites, and thermoelectrics. In all of these application areas the various heterointerfaces play a major role in shaping the material properties or device performance.
In organic electronic devices, such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), and organic field effect transistors (OFETs), the various interfaces present have a major influence on device performance. These interfaces include the organic-organic interface, which for example plays a critical role in charge separation in OPVs, as well as the organic-inorganic interface, where charge injection or extraction occurs. The energetic landscape at these interfaces, which is determined both by intermolecular interactions and interfacial molecular conformations, can drastically influence charge transport or charge separation dynamics. To ultimately improve device level properties through obtaining a fundamental scientific understanding we aim to controllably tune interfacial properties, use various analytical techniques to probe the interface, and test the impact on device performance.
In any solar cell technology, such as the recently developed methyl ammonium lead iodide (CH3NH3)PbI3 perovskite system, the interface between the active layer and electrodes must be optimized to minimize performance losses. For example, low energy defect states at the surface can serve as recombination centers while improper energy level alignment can result in barriers to charge collection or energetic losses during charge collection, all of which reduce the power conversion efficiency of the solar cell. We seek to generate a better understanding of these interfaces and how they impact device performance through utilizing various optical and photoelectron spectroscopies to probe these interfaces. Through tuning interfacial properties and comparing with solar cell performance we will be able to draw relationships between surface chemistry, interfacial energetics, and solar cell performance.
Organic-inorganic composites provide promising opportunities for a large suite of applications, including materials with novel combinations of electronic, optical, and mechanical properties. To control these properties the surface chemistry of the inorganic-organic interface can be tuned. For example, in a polymer-nanoparticle blend the surface chemistry of the nanoparticle may be altered such that it interacts more strongly with the polymer and thus nanoparticle aggregation will be reduced. Furthermore, the interfacial energetics at the polymer-nanoparticle blend may be adjusted such that charges move more freely between polymer and nanoparticle. Here we aim to understand these interfacial interactions and how they effect material properties, followed by using these interfaces to create composite materials with improved properties (such as transparent electrodes or thermoelectrics).
A major tool and analytical focus of the group is the application of various photoelectron spectroscopies to thoroughly probe the nature of the various interfaces highlighted above. These include ultraviolet photoelectron spectroscopy to probe the valence band or ionization potentials, X-ray photoelectron spectroscopy to probe the chemical nature of the interface, and inverse photoelectron spectroscopy to probe the conduction band or electron affinities.
Analytical and Materials Chemistry
Liang, Z.; Graham, K. R.; ACS Appl. Mater. Interfaces 2015, 7, 21652. “Surface Modification of Silver Nanowires for Morphology and Processing Control in Composite Transparent Electrodes”
Graham, K. R.; Cabanetos, C.; Jahnke, J. P.; Idso, M. N.; Laban, A. E.; Ngongang Ndjawa, G. O.; Vandewal, K.; Heumueller, T.; Salleo, A.; Chmelka, B. F.; Amassian, A.; Beaujuge, P. M.; McGehee, M. D. J. Am. Chem. Soc. 2014, 136, 9608-9618. “Importance of the Donor:Fullerene Intermolecular Arrangement for High-Efficiency Organic Photovoltaics”
Vandewal, K.; Albrecht, S.; Hoke, E. T.; Graham, K. R.; Widmer, J.; Douglas, J. D.; Schubert, M.; Mateker, W. R.; Bloking, J. T.; Burkhard, G. F.; Sellinger, A.; Fréchet, J. M. J.; Amassian, A.; Riede, M. K.; McGehee, M. D.; Neher, D.; Salleo, A. Nature Mater. 2014, 13, 63-68. “Efficient charge generation by relaxed charge-transfer states at organic interfaces”
Graham, K. R.; Erwin, P.; Nordlund, D.; Vandewal, K.; Li, R.; Ngongang Ndjawa, G. O.; Hoke, E. T.; Salleo, A.; Thompson, M. E.; McGehee, M. D.; Amassian. A. Adv. Mater. 2014, 25, 6076-6082. “Reevaluating the Role of Sterics and Electronic Coupling in Determining the Open Circuit Voltage of Organic Solar Cells”
Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas, J. D.; Collins, B. A.; Tumbleston, J. R.; Graham, K. R.; Amassian, A.; Ade, H.; Fréchet, J. M. J.; Toney, M. F.; and McGehee, M. D. Adv. Energy Mater. 2013, 3 (3), 364-374. “The Importance of Fullerene Percolation in the Mixed Regions of Polymer-Fullerene Bulk Heterojunction Solar Cells”
Graham, K. R.; Stalder, R.; Patel, D.; Salazar, D. H.; Reynolds, J. R. ACS Appl. Mater. Interfaces. 2013, 5 (1), 63. “Morphology Control in Molecular Bulk-Heterojunction Photovoltaic Cells Through Tailor-Made Additives”
Graham, K. R.; Stalder, R.; Mei, J.; Wieruszewski, P.; Hartel, M. J.; Mei, J.; So, F.; Reynolds, J. R. Adv. Funct. Mater. 2012, 22, 4801. “Improved Performance of Molecular Bulk-Heterojunction Photovoltaic Cells through Predictable Selection of Solvent Additives.”
Patel, D. G.; Graham, K. R.; Reynolds, J. R. J. Mater. Chem. 2012, 22(7), 3004. “Diels-Alder Crosslinking for Dye Segregation and Multi-Layer Structures in Conjugated Polymer Devices: Towards Improvement of Device Efficiency”
Graham, K. R.; Yang, Y.; Sommer, J. R.; Shelton, A.; Schanze, K. S.; Xue, J.; Reynolds, J. R.; Chem. Mater. 2011, 23 (24), 5305. “Extended π-Conjugated Pt-Porphyrins for use in Near-Infrared Emitting Organic Light Emitting Diodes.”
Ellinger, S. E.; Graham, K. R.; Shi, P.; Farley, R. T.; Steckler, T. T.; Brookins, R. N.; Taranekar, P.; Mei, J.; Padilha, L. A.; Ensley, T. R.; Hu, H.; Webster, S.; Hagan, D. J.; Van Stryland, E. W.; Schanze, K. S.; Reynolds, J. R. Chem. Mater. 2011, 23 (17), 3805 “Donor-acceptor-donor based p-conjugated oligomers for near-IR emission”
Koldemir, U.; Graham, K. R.; Salazar, D. H.; McCarley, T.; Reynolds, J. R. J. Mater. Chem. 2011, 21, 6480. “Electron Rich APFO Polymer with Dual Electrochromism and Electroluminescence”
Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R. Org. Lett. 2010, 12 (4), 660. “Synthesis of isoindigo-based oligothiophenes for molecular bulk heterojunction solar cells”
Mortimer, R. J.; Graham, K. R.; Grenier, C. R. G.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2009, 1, 2269. “Influence of the Film Thickness and Morphology on the Colorimetric Properties of Spray-Coated Electrochromic Disubstituted 3,4-Propylenedioxythiophene Polymers