Fundamental Insight of Electron Transport in Organic Electronic Applications

To develop molecular and polymer organic semiconductors with high charge-carrier mobilities in the semiconducting or metallic regime, a major challenge and opportunity lies with understanding the inherent charge-transport mechanism. In the absence of chemical and physical defects, the nature of charge transport depends on a subtle interplay between electronic and electron-vibration interactions. A significant portion of our work centers on determining molecular and crystal electronic structures and evaluating electronic polarization effects in the solid state. Key experimental allies in these determinations are ultraviolet photoelectron spectroscopy (UPS, both gas phase and solid state) and inverse photoelectron spectroscopy (IPES). Such experimental techniques are essential in the study of how the discrete molecular electronic structures transform to band structures in the solid state, and provide essential information concerning energy level alignment at interfaces. Our simulations assist with the assignment of electron transport levels - identification of which is oftentimes not straightforward. Our computational efforts provide for: the determination of the electronic and vibrational structures obtained in gas-phase UPS; the simulation of spectra based on theoretical densities of states to help identify transport levels in UPS/IPES data; and a check of the quality of theoretical calculations through a direct comparison with experiment.

Our work on organic crystals also allows us to estimate the electronic couplings between neighboring molecules and evaluate the importance of electron-vibration (electron-phonon) interactions on the coupling. Combining these results with polarization effects is allowing us to begin to construct a complete picture of charge-carrier mobility in organic solids.

A more recent interest in the group has focused on the influence of molecular dopants. Molecular doping of organic semiconductors offers a way to improve charge injection, charge transport, and charge separation in organic (opto)electronic devices. Doping is also important in the formation of highly conductive polymers used as both transport/injection layers and electrodes. Our theoretical work: develops methodology to accurately describe intermolecular charge transfer; understands experimental signatures of doping such as IR frequency shifts; investigates the fundamental roles of the intermolecular energy level offset and electronic coupling in charge transfer; and investigates the charge transport mechanism in doped polymers.

Research highlights:
  • Effect Of Electronic Polarization On Charge-Transport Parameters In Molecular Organic Semiconductors, Valeev, EF; Coropceanu, V; da Silva, DA; Salman, S; Bredas, JL, Journal Of The American Chemical Society, 128, 9882JA061827H-9886 [2006]
  • Transport Properties In The Rubrene Crystal: Electronic Coupling And Vibrational Reorganization Energy, da Silva, DA; Kim, EG; Bredas, JL, Advanced Materials, 17, 1072[2005]
Recent papers on charge transfer:
  • Charge Transfer in Molecular Complexes with 2,3,5,6-Tetrafluoro- 7,7,8,8-tetracyanoquinodimethane (F4-TCNQ): A Density Functional Theory Study, Lingyun Zhu, Eung-Gun Kim, Yuanping Yi, and Jean-Luc Brédas, Chemistry Of Materials, 23, 5149-5159[2011]
  • Charge Transport In Oligo Phenylene And Phenylene-Thiophene Nanofibers, Kjelstrup-Hansen, J; Norton, JE; da Silva, DA; Bredas, JL; Rubahn, HG, Organic Electronics, 10, 1228-1234 [2009]
  • Influence Of Intermolecular Vibrations On The Electronic Coupling In Organic Semiconductors: The Case Of Anthracene And Perfluoropentacene, Martinelli, NG; Olivier, Y; Athanasopoulos, S; Delgado, MCR; Pigg, KR; da Silva, DA; Sanchez-Carrera, RS; Venuti, E; Della Valle, RG; Bredas, JL; Beljonne, D; Cornil, J, Chemphyschem, 10, 2265-2273 [2009]
  • Electronic Structure And Charge-Transport Parameters Of Functionalized Tetracene Crystals: Impact Of Partial Fluorination And Alkyl Or Alkoxy Derivatization, Salman, S; Delgado, MCR; Coropceanu, V; Bredas, JL, Chemistry Of Materials, 21, 3593-3601 [2009]
  • Impact Of Perfluorination On The Charge-Transport Parameters Of Oligoacene Crystals,Delgado, MCR; Pigg, KR; Filho, DADS; Gruhn, NE; Sakamoto, Y; Suzuki, T; Osuna, RM; Casado, J; Hernandez, V; Navarrete, JTL; Martinelli, NG; Cornil, J; Sanchez-Carrera, RS; Coropceanu, V; Bredas, JL, Journal Of The American Chemical Society, 131, 1502-1512[2009]
  • The Interaction Of Charge Carriers With Lattice Vibrations In Organic Molecular Semiconductors: Naphthalene As A Case Study, Coropceanu, V; Sanchez-Carrera, RS; Paramonov, P; Day, GM; Bredas, JL, Journal Of Physical Chemistry C, 113, 4679-4686 [2009]
  • Charge-Transport Properties Of The 1,4-Diiodobenzene Crystal: A Quantum-Mechanical Study, Sanchez-Carrera, RS; Coropceanu, V; Kim, EG; Bredas, JL, Chemistry Of Materials, 20,5832-5838 [2008]
  • Simulating Charge Transport In Tris(8-Hydroxyquinoline) Aluminium (Alq3), Kwiatkowski, JJ; Nelson, J; Li, H; Bredas, JL; Wenzel, W; Lennartz, C, Physical Chemistry Chemical Physics,10, 1852-1858 [2008]
  • Experimental And Theoretical Study Of Temperature Dependent Exciton Delocalization And Relaxation In Anthracene Thin Films, Ahn, TS; Muller, AM; Al-Kaysi, RO; Spano, FC; Norton, JE; Beljonne, D; Bredas, JL; Bardeen, CJ, Journal Of Chemical Physics, 128, 54505[2008]
  • Modeling Electron And Hole Transport In Fluoroarene-Oligothiopene Semiconductors: Investigation Of Geometric And Electronic Structure Properties, Koh, SE; Risko, C; da Silva, DA; Kwon, O; Facchetti, A; Bredas, JL; Marks, TJ; Ratner, MA, Advanced Functional Materials,18, 332-340 [2008]
Recent papers on doping:
  • Use Of A High Electron-Affinity Molybdenum Dithiolene Complex To P-Dope Hole-Transport Layers, Qi, YB; Sajoto, T; Barlow, S; Kim, EG; Bredas, JL; Marder, SR; Kahn, A,Journal Of The American Chemical Society, 131, 12530 [2009]
  • Doping Molecular Wires, Heimel, G; Zojer, E; Romaner, L; Bredas, JL; Stellacci, F, Nano Letters, 9, 2559-2564 [2009]
  • Electronic Evolution Of Poly(3,4-Ethylenedioxythiophene) (Pedot): From The Isolated Chain To The Pristine And Heavily Doped Crystals, Kim, EG; Bredas, JL, Journal Of The American Chemical Society, 130, 16880-16889 [2008]
  • Molecular n-Type Doping Of 1,4,5,8-Naphthalene Tetracarboxylic Dianhydride By Pyronin B Studied Using Direct And Inverse Photoelectron Spectroscopies, Chan, CK; Kim, EG; Bredas, JL; Kahn, A, Advanced Functional Materials, 16, 831-837 [2006]
  • Colorless Molecular Dopants For Low-Operating-Voltage Nematic Liquid Crystals,Haddock, JN; Kippelen, B; Mathine, DL; Schmidtke, J; Kumaraswamy, S; Marder, SR; Kwon, O; Zojer, E; Bredas, JL; Marder, SR, Molecular Crystals And Liquid Crystals, 428, 17-32[2005]
Recent papers on photoelectron spectroscopy:
  • Electronic And Vibronic Interactions At Weakly Bound Organic Interfaces: The Case Of Pentacene On Graphite, Paramonov, PB; Coropceanu, V; Bredas, JL, Physical Review B, 78,41403 [2008]
  • Charge Transport Parameters Of The Pentathienoacene Crystal, Kim, EG; Coropceanu, V; Gruhn, NE; Sanchez-Carrera, RS; Snoeberger, R; Matzger, AJ; Bredas, JL, Journal Of The American Chemical Society, 129, 13072-13081 [2007]
  • Photoelectron Spectroscopic Study Of The Electronic Band Structure Of Polyfluorene And Fluorene-Arylamine Copolymers At Interfaces, Hwang, J; Kim, EG; Liu, J; Bredas, JL; Duggal, A; Kahn, A, Journal Of Physical Chemistry C, 111, 1378-1384 [2007]
  • Vibronic Coupling In The Ground And Excited States Of Oligoacene Cations, Sanchez-Carrera, RS; Coropceanu, V; da Silva, DA; Friedlein, R; Osikowicz, W; Murdey, R; Suess, C; Salaneck, WR; Bredas, JL, Journal Of Physical Chemistry B, 110, 18904-18911 [2006]
  • Probing Charge Transport In Pi-Stacked Fluorene-Based Systems, Coropceanu, V; Nakano, T; Gruhn, NE; Kwon, O; Yade, T; Katsukawa, K; Bredas, JL, Journal Of Physical Chemistry B, 110, 9482-9487 [2006]
  • Molecular n-Type Doping Of 1,4,5,8-Naphthalene Tetracarboxylic Dianhydride By Pyronin B Studied Using Direct And Inverse Photoelectron Spectroscopies, Chan, CK; Kim, EG; Bredas, JL; Kahn, A, Advanced Functional Materials, 16, 831-837 [2006]
  • Vibronic Coupling In Organic Semiconductors: The Case Of Fused Polycyclic Benzene-Thiophene Structures, Coropceanu, V; Kwon, O; Wex, B; Kaafarani, BR; Gruhn, NE; Durivage, JC; Neckers, DC; Bredas, JL, Chemistry-A European Journal, 12, 2073-2080 [2006]
  • Electron Affinities Of 1,1-Diaryl-2,3,4,5-Tetraphenyisiloles: Direct Measurements And Comparison With Experimental And Theoretical Estimates, Zhan, XW; Risko, C; Amy, F; Chan, C; Zhao, W; Barlow, S; Kahn, A; Bredas, JL; Marder, SR, Journal Of The American Chemical Society, 127, 9021-9029 [2005]