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Center for Organic Photonics and Electronics

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Solution doping of organic semiconductors using air-stable n-dopants

(Georgia Institute of Technology, 2012-02) Qi, Yabing ; Mohapatra, Swagat K. ; Kim, Sang Bok ; Barlow, Stephen ; Marder, Seth R. ; Kahn, Antoine

Solution-based n-doping of the polymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI₂OD-T₂)] and the small molecule 6,13-bis(tri(isopropyl)silylethynyl)pentacene (TIPS-pentacene) is realized with the air-stable dimers of rhodocene, [RhCp₂]₂, and ruthenium(pentamethylcyclopentdienyl)(1,3,5-triethylbenzene), [Cp*Ru(TEB)]₂. Fermi level shifts, measured by direct and inverse photoemission spectroscopy, and orders of magnitude increase in current density and film conductivity point to strong n-doping in both materials. The strong reducing power of these air-stable dopants is demonstrated through the n-doping of TIPS-pentacene, a material with low electron affinity (3.0 eV). Doping-induced reduction of the hopping transport activation energy indicates that the increase in film conductivity is due in part to the filling of deep gap states by carriers released by the dopants.
Remote doping of a pentacene transistor: Control of charge transfer by molecular-level engineering

(Georgia Institute of Technology, 2010-09) Zhao, Wei ; Qi, Yabing ; Sajoto, Tissa ; Barlow, Stephen ; Marder, Seth R. ; Kahn, Antoine

We demonstrate that holes from a p-doped N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) layer transfer to an adjacent pentacene film. The spatial separation of carriers from dopants, or remote doping, is demonstrated with a combination of photoemission spectroscopy and current-voltage measurements for a p-doped α-NPD/pentacene heterojunction. Increased conductivity of the pentacene film is observed in both nongated temperature-dependent conductivity and gated thin-film transistor measurements.
Fabrication of a blue M x N pixel organic light-emitting diode video display incorporating a thermally stable emitter

(Georgia Institute of Technology, 2009-04) Haldi, Andreas ; Kim, Jung B. ; Domercq, Benoit ; Kulkarni, Abhishek P. ; Barlow, Stephen ; Gifford, Angela P. ; Jenekhe, Samson A. ; Marder, Seth R. ; Kippelen, Bernard

A 7x11 pixel blue OLED display was fabricated using a patterned indium-tin-oxide (ITO) substrate. The fabrication process for an M x N pixel organic light-emitting diode (OLED) video display including an electrical insulating layer and a physical pixel separator layer is presented. An efficient and thermally stable blue fluorescent organic material, 6, 6'-bis((2-p-biphenyl)-4-phenylquinoline) (B2PPQ), was used in combination with an evaporated hole-transport small molecule with a high ionization potential.
Efficient all-polymer solar cells based on blend of tris(thienylenevinylene)-substituted polythiophene and poly[perylene diimide-alt-bis(dithienothiophene)]

(Georgia Institute of Technology, 2008-08-18) Tan, Zhan'ao ; Zhou, Erjun ; Zhan, Xiaowei ; Wang, Xiang ; Li, Yongfang ; Barlow, Stephen ; Marder, Seth R.

A narrow band-gap alternating copolymer of perylene diimide and bis(dithienothiophene) (2) and a polythiophene derivative substituted by a tris(thienylenevinylene) conjugated side chain (4) are used as acceptor and donor, respectively, in all-polymer solar cells (SCs). The optimized device based on the blend of 4 and 2 in the ratio 3:1 (w/w) gives a short circuit current (Jsc) of 5.02 mA cm⁻²and a power conversion efficiency of 1.48%, under simulated AM 1.5 illumination at 100 mW cm⁻¹. These values are among the highest values reported for all-polymer SCs
Substrate-dependent electronic structure of an organic heterojunction

(Georgia Institute of Technology, 2008-04) Zhao, Wei ; Salomon, Eric ; Zhang, Qing ; Barlow, Stephen ; Marder, Seth R. ; Kahn, Antoine

This work focuses on organic-organic heterojunctions (OOHs) formed between two small-molecule, low band-gap, semiconductors, tris{2,5-bis(3,5-bis-trifluoromethyl-phenyl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12 -hexaaza-triphenylene (THAP), and copper phthalocyanine (CuPc). The organic layers are deposited on various substrates, and the energy level alignment between them is investigated by ultraviolet photoemission spectroscopy. The electronic structure of the OOH is found to depend on the work function of the organic underlayer predeposited on the different substrates. The vacuum level offset between THAP and CuPc, which consists of the sum of the interface dipole and the molecular level shift, varies from 0.26 to 1.37 eV. The interface dipole between the two organic films linearly changes with the work function of the organic underlayer.
Broadband Z-scan characterization using a high spectral irradiance high quality supercontinuum

(Georgia Institute of Technology, 2008-02) Balu, Mihaela ; Padilha, Lazaro A. ; Hagan, David J. ; Van Stryland, Eric W. ; Yao, Sheng ; Belfield, Kevin D. ; Zheng, Shijun ; Barlow, Stephen ; Marder, Seth R.

We generate a high-spectral-irradiance, high-quality continuum by weakly focusing femtosecond pulses in Kr gas. We use this continuum as a source for rapid Z-scan measurements of the degenerate nonlinear absorption spectrum and the associated dispersion of the nonlinear refraction in optical materials throughout the visible. We measure the degenerate two-photon absorption spectra and the dispersion of the nonlinear refractive index, n 2 , of two well-characterized semiconductors (ZnSe and ZnS ) as reference samples for our method, along with dilute solutions of organic materials. The latter materials demonstrate application of the technique to samples with lower nonlinearities.
Commensurate growth and diminishing substrate influence in a multilayer film of a tris(thieno)hexaazatriphenylene derivative on Au(111) studied by scanning tunneling microscopy

(Georgia Institute of Technology, 2008-02) Ha, Sieu D. ; Zhang, Qing ; Barlow, Stephen ; Marder, Seth R. ; Kahn, Antoine

Layer-by-layer growth of the electron-transport material tris 2,5-bis 3,5-bis-trifluoromethyl-phenyl -thieno 3,4-b,h,n -1,4,5,8,9,12-hexaazatriphenylene THAP on Au 111 is probed by scanning tunneling microscopy STM . A relative of discotic liquid crystalline molecules, THAP is shown to grow in commensurate ordered planes from the first to fourth monolayers. The four monolayers all show a concordant ordered structure in which the molecules arrange parallel to the substrate in a hexagonal close-packed lattice with a herringbone pattern defined by alternating rows of molecules with antiparallel orientation. The unit cell is rectangular with two molecules per cell and is nearly equivalent for each layer. The spatial broadening of the local density of states due to the metallic substrate is appreciably diminished in upper layers, as expected and as evidenced by the localization of states seen in STM. There is good agreement between the highest occupied molecular orbital obtained in density functional theory calculations for a single molecule and STM images of the upper layers, in accord with the localized nature of electronic states on molecules under minimal substrate influence.
Incorporation of cobaltocene as an n-dopant in organic molecular films

(Georgia Institute of Technology, 2007-07-01) Chan, Calvin K. ; Kahn, Antoine ; Zhang, Qing ; Barlow, Stephen ; Marder, Seth R.

Electrical or chemical doping of molecular films is an efficient means of improving and controlling charge injection and carrier transport in organic devices. Recent work demonstrated that bis(cyclopentadienyl)cobalt(II) (cobaltocene, CoCp₂) efficiently dopes a tris(thieno)hexaazatriphenylene (THAP) derivative, as shown by a 0.56 eV shift of the Fermi level toward the empty states and an increase of current density by a factor of 10³ over undoped THAP devices. In this work, a combination of x-ray photoemission spectroscopy and Rutherford backscattering is used to elucidate the details of dopant incorporation into bulk films. Cobaltocene is observed to codeposit into the THAP matrix in a controllable manner, with preferential adsorption of the dopant onto the surface of the host film. In the case of CoCp₂-doped tris(8-hydroxy-quinolinato) aluminum (Alq₃) films, negligible amounts of the dopant are found in the bulk matrix and on the film surface, resulting in minimal improvements in the electrical characteristics of doped Alq₃ films. The process of CoCp₂ adsorption onto a film surface or the evolving surface of a growing film which leads to dopant incorporation is likely assisted by charge transfer from cobaltocene to the host material, resulting in ion pairing between the dopant and host.
Strong two-photon absorption at telecommunications wavelengths in nickel bis(dithiolene) complexes

(Georgia Institute of Technology, 2007-03) Cho, Jian-Yang ; Barlow, Stephen ; Marder, Seth R. ; Fu, Jie ; Padilha, Lazaro A. ; Van Stryland, Eric W. ; Hagan, David J. ; Bishop, Maximilienne

The two-photon absorption spectrum of a nickel bis(dithiolene) complex with extended conjugation and π-donor substitution is measured by using Z-scan and pump–probe techniques with femtosecond pulses over the spectral range from 1.20to1.58 μm , which includes much of the telecommunications range. The peak two-photon cross section of over 5000 GM (1 GM=10 −50 cm ⁴ sphoton ⁻¹ molecule ⁻¹ ) occurs at ∼1.24 μm , with significant two-photon absorption (>440 GM) throughout the spectral range examined.
65 nm feature sizes using visible wavelength 3-D multiphoton lithography

(Georgia Institute of Technology, 2007-03) Haske, Wojciech ; Chen, Vincent W. ; Hales, Joel M. ; Dong, Wenting ; Barlow, Stephen ; Marder, Seth R. ; Perry, Joseph W.

Nanoscale features as small as 65 ± 5 nm have been formed reproducibly by using 520 nm femtosecond pulsed excitation of a 4,4'- bis(di-n-butylamino)biphenyl chromophore to initiate crosslinking in a triacrylate blend. Dosimetry studies of the photoinduced polymerization were performed on chromophores with sizable two-photon absorption crosssections at 520 and 730 nm. These studies show that sub-diffraction limited line widths are obtained in both cases with the lines written at 520 nm being smaller. Three-dimensional multiphoton lithography at 520 nm has been used to fabricate polymeric woodpile photonic crystal structures that show stop bands in the near-infrared spectral region.