Eung-Gun Kim

Tuning the Charge-Transport Parameters of Perylene Diimide Single Crystals via End and/or Core Functionalization: A Density Functional Theory Investigation

Perylene tetracarboxylic diimide (PTCDI) derivatives stand out as one of the most investigated families of air-stable n-type organic semiconductors for organic thin-film transistors. Here, we use density functional theory to illustrate how it is possible to control the charge-transport parameters of PTCDIs as a function of the type, number, and positions of the substituents. Specifically, two strategies of functionalization related to core and end substitutions are investigated. While end-substituted PTCDIs present the same functional molecular backbone, their molecular packing in the crystal significantly varies; as a consequence, this series of derivatives constitutes an ideal test bed to evaluate the models that describe charge-transport in organic semiconductors. Our results indicate that large bandwidths along with small effective masses can be obtained with the insertion of appropriate substituents on the nitrogens, in particular halogenated aromatic groups.

Prediction of Remarkable Ambipolar Charge-Transport Characteristics in Organic Mixed-Stack Charge-Transfer Crystals

We have used density functional theory calculations and mixed quantum/classical dynamics simulations to study the electronic structure and charge-transport properties of three representative mixed-stack charge-transfer crystals, DBTTF-TCNQ, DMQtT-F(4)TCNQ, and STB-F(4)TCNQ. The compounds are characterized by very small effective masses and modest electron-phonon couplings for both holes and electrons. The hole and electron transport characteristics are found to be very similar along the stacking directions; for example, in the DMQtT-F(4)TCNQ crystal, the hole and electron effective masses are as small as 0.20 and 0.26 m(0), respectively. This similarity arises from the fact that the electronic couplings of both hole and electron are controlled by the same superexchange mechanism. Remarkable ambipolar charge-transport properties are predicted for all three crystals. Our calculations thus provide strong indications that mixed-stack donor-acceptor materials represent a class of systems with high potential in organic electronics.

Electronic evolution of poly(3,4-ethylenedioxythiophene) (PEDOT): from the isolated chain to the pristine and heavily doped crystals.

Poly(3,4-ethylenedioxythiophene) (PEDOT) is the prototypical conjugated polymer used in the doped state as the hole injection/transport layer in organic (opto)electronic devices. Numerous experimental studies have been successful only in drawing a partial microscopic picture of PEDOT due to its complex morphology, which has also hampered application of theoretical approaches. Using density functional theory methods, combined with refined structural models built upon crystallographic data of PEDOT and other substituted polythiophenes, our work seeks to establish a comprehensive understanding of the electronic and geometric structures of PEDOT, as an isolated chain and in the pristine and doped bulk phases. We find that ethylenedioxy substitution planarizes the polythiophene backbone but the experimentally observed bandgap reduction is caused mainly by a stronger destabilization of the valence band than the conduction band via donor-type substitution. The calculated crystal of pristine PEDOT has a monoclinic lamellar structure consisting of inclined pi-stacks. The impact of interchain interactions on the charge carrier effective masses is greater than that of the ethylenedioxy substitution and leads to the reversal of the relative masses; the electrons are lighter than the holes in the pristine crystal. The small interchain electron effective mass is comparable to the hole effective masses found in high mobility organic crystals. Tosylic acid-doped PEDOT (PEDOT:Tos), which is receiving renewed interest as an anode material to replace indium tin oxide, is calculated to be a two-dimensional-like metal. The PEDOT:Tos crystal is found to have an embedded mirror plane in the tosylate monolayer that is sandwiched between PEDOT stacks, and thus to have twice the size of the unit cell proposed earlier. Doping is seen to remove the intrastack inclination of the PEDOT chains.

Use of a High Electron-Affinity Molybdenum Dithiolene Complex to p-Dope Hole-Transport Layers

Experimental and theoretical results are presented on the electronic structure of molybdenum tris[1,2-bis(trifluoromethyl) ethane-1,2-dithiolene] (Mo(tfd)(3)), a high electron-affinity organometallic complex that constitutes a promising candidate as a p-dopant for organic molecular semiconductors. The electron affinity of the compound, determined via inverse photoemission spectroscopy, is 5.6 eV, which is 0.4 eV larger than that of the commonly used p-dopant F(4)-TCNQ. The LUMO level of Mo(tfd)(3) is calculated to be delocalized over the whole molecule, which is expected to lead to low pinning potential. Efficient p-doping of a standard hole transport material (alpha-NPD) is demonstrated via measurements of Fermi level shifts and enhanced conductivity in alpha-NPD:1% Mo(tfd)(3). Rutherford backscattering measurements show good stability of the three-dimensional Mo(tfd)(3) molecule in the host matrix with respect to diffusion.