Gary P. Kushto

Transparent conducting aluminum-doped zinc oxide thin films for organic light-emitting devices

Aluminum-doped zinc oxide (AZO) thin films (∼3000 A) with low electrical resistivity and high optical transparency have been grown by pulsed-laser deposition on glass substrates without a postdeposition anneal. Films were deposited at substrate temperatures ranging from room temperature to 400 °C in O2 partial pressures ranging from 0.1 to 50 mTorr. For 3000-A-thick AZO films grown at room temperature in an oxygen pressure of 5 mTorr, the electrical resistivity was 8.7×10−4 Ω cm and the average optical transmittance was 86% in the visible range (400–700 nm). For 3000-A-thick AZO films deposited at 200 °C in 5 mTorr of oxygen, the resistivity was 3.8×10−4 Ω cm and the average optical transmittance in the visible range was 91%. AZO films grown at 200 °C were used as an anode contact for organic light-emitting diodes. The external quantum efficiency measured from these devices was about 0.3% at a current density of 100 A/m2.

Effect of film thickness on the properties of indium tin oxide thin films

Transparent conducting indium tin oxide (ITO) thin films (40–870 nm) were grown by pulsed laser deposition on amorphous substrates and the structural, electrical, and optical properties of these films were investigated. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 2 J/cm2, at substrate temperature of 300 °C and 10 mTorr of oxygen pressure. For ITO films (30–400 nm thickness) deposited at 300 °C in 10 mTorr of oxygen, a resistivity of 1.8–2.5×10−4 Ω cm was observed and the average transmission in the visible range (400–700 nm) was about 85%–90%. The Hall mobility and carrier density for ITO films (40–870 nm thickness) were observed to be in the range of 24–27 cm2/V s and 8–13×1020 cm−3, respectively. The ITO films have been used as the anode contact in organic light emitting diodes and the effect of ITO film thickness on the device performance has been studied. The optimum thickness of the ITO anode for the maximum device efficiency was observed to be about 60–100 nm....

Indium tin oxide thin films grown on flexible plastic substrates by pulsed-laser deposition for organic light-emitting diodes

Transparent conducting indium tin oxide (ITO) thin films were grown by pulsed laser deposition (PLD) on flexible polyethylene terephthalate (PET) substrates. The structural, electrical, and optical properties of these films were investigated as a function of substrate deposition temperature and background gas pressure. ITO films (200 nm thick), deposited by PLD on PET at 25 °C and 45 mTorr of oxygen, exhibit high optical transparency (∼87%) in the visible (400–700 nm) with a low electrical resistivity of 7×10−4 Ω cm. ITO films grown by PLD on PET were used as the anode contact in organic light-emitting devices. A luminous power efficiency of ∼1.6 lm/W was achieved at 100 cd/m2, slightly higher than that (∼1.5 lm/W) measured for the control device based on a sputter-deposited ITO on glass.

Transparent conducting Zr-doped In2O3 thin films for organic light-emitting diodes

Zirconium-doped indium oxide (ZIO) thin films (∼2000 A thick) have been deposited by pulsed-laser deposition on glass substrates without a postdeposition anneal. The structural, electrical and optical properties of these films have been investigated as a function of substrate temperature and oxygen partial pressure during deposition. Films were deposited at substrate temperatures ranging from 25 °C to 400 °C in O2 partial pressures ranging from 0.1 to 50 mTorr. The films (∼2000 A thick) deposited at 200 °C in 25 mTorr of oxygen show electrical resistivities as low as 2.5×10−4 Ω cm, an average visible transmittance of 89%, and an optical band gap of 4.1 eV. The ZIO films were used as a transparent anode contact in organic light emitting diodes and the device performance was studied. The external quantum efficiency measured from these devices was about 0.9% at a current density of 100 A/m2.

Flexible organic photovoltaics using conducting polymer electrodes

Single heterojunction, small-molecule organic photovoltaic devices (OPVs) have been prepared on fully flexible thermoplastic substrates using prepatterned conducting polymer electrodes(∼450Ω∕◻). OPVs were fabricated via sequential vacuum vapor deposition of layers of the organic electron donating/hole transporting material: N,N′-(α-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine and the electron accepting/transporting material: C60. The resulting photovoltaic cells exhibit white-light power conversion efficiencies of 1% (AM1.5, 97mW∕cm2), virtually identical to those fabricated on prepatterned tin-doped indium oxide/glass substrates.

Laser processing of nanocrystalline TiO2 films for dye-sensitized solar cells

Pulsed-laser deposition and laser direct-write have been applied to deposit dense (30nm thick) and porous nanocrystalline TiO2 (nc-TiO2, 5–20μm thick) layers incorporated in dye-sensitized solar cells. Laser direct-write is a laser-induced forward transfer technique that enables the fabrication of conformal structures containing metals, ceramics, polymers, and composites on rigid and flexible substrates without the use of masks or additional patterning steps. A pulsed UV laser (355nm) was used to forward transfer a suspension of TiO2(P25) nanopowder onto a F-doped SnO2 coated glass substrate. In this letter we demonstrate the use of laser transfer techniques to produce porous nc-TiO2 films required for dye-sensitized solar cells. The dye solar cells fabricated with the laser processed TiO2 layers on glass showed a power conversion efficiency of ∼4.3% under an illumination of 10mW∕cm2.

Efficient, single-layer molecular organic light-emitting diodes

The authors demonstrate efficient molecular organic light-emitting diodes that use direct hole injection from poly(3,4-ethylene-dioxythio-phene):poly(styrene-sulfonate) into a single layer of tris(8-hydroxyquinoline) aluminum (III) for carrier transport and electroluminescence. Single-layer devices have a lower operating bias and higher luminous power efficiency than conventional bilayer devices with a 4,4-bis[N-1-napthyl-N-phenyl-amino]biphenyl hole transport layer. The current density-voltage characteristics of single-layer devices follow Schottky-Richardson behavior and are consistent with an Ohmic contact at the anode.

Infrared spectra of HC[triple bond]C-MH and M-eta2-(C2H2) from reactions of laser-ablated group- 4 transition-metal atoms with acetylene.

Reactions of laser-ablated group 4 transition-metal atoms with acetylene have been carried out. The ethynyl metal hydrides (HC[triple bond]C-MH) and corresponding pi complexes (M-eta(2)-(C2H2)) are identified in the matrix infrared spectra. The observed M-H and C-M stretching absorptions show that oxidative C-H insertion readily occurs during codeposition and photolysis afterward. The absorptions from the pi complex, on the other hand, are relatively weak in the original deposition spectrum but increase dramatically in the process of annealing. The vinylidene complex, another plausible product, is not identified in this study. The observed spectra and DFT calculations both show that the back-donations from the group 4 metals to the antibonding pi* orbital of C2H2 are extensive such that the group 4 metals form unusually strong pi complexes. Thus, it is the formation of two Ti-C bonds in the group 4 systems than leads to the stronger bonding than that in the group 8 systems. While bonds form, the Ti atom is weakly bound to C2H2, and we still refer to it as a pi complex. Evidence of relativistic effects is also observed in frequency trends for the Ti, Zr, and Hf products.

An infrared spectroscopic and density functional investigation of dinitrogen activation by group IV metal atoms

Reactions of laser-ablated Ti, Zr, and Hf atoms with N2 in excess argon and excess dinitrogen have produced the MN and (N2)xMN molecules in addition to M(μ–N)2M with no dinitrogen bond. Evidence is presented for simple η1–N2 and higher complexes for each metal. The observation of cyclic Ti(N2) and Zr(N2) molecules with N–N stretching frequencies at 1125.9 and 1022.8 cm−1, respectively, indicate significant activation of the dinitrogen bond. The identification of product molecules is based on isotopic substitution and the results of density functional theory frequency calculations.

INFRARED SPECTRUM OF CCH+ IN SOLID ARGON AND NEON

Laser-ablation of over ten different transition, lanthanide, and actinide metals with concurrent codeposition of acetylene/argon samples at 7 K produced metal independent absorptions for CCH, CCH−, C4H, and C4H2, in agreement with previous matrix isolation work, and a sharp new 1820.4 cm−1 band. Isotopic substitution showed this band to be due to a largely C–C stretching mode of a species with one H and two inequivalent carbon atoms. The same species were observed in solid neon samples at 4 K, and the neon matrix counterpart of the new band was found at 1832.2 cm−1. When CO2 was added to serve as an electron trap, the yield of CCH− at 1772.8 cm−1 decreased and the 1832.2 cm−1 band increased relative to CCH at 1837.9, 1835.0 cm−1. Quantum chemical calculations at the coupled-cluster and density functional levels predict the C–C stretching mode of CCH+ between this mode for CCH and CCH− and support assignment of this new infrared absorption to the CCH+ cation in solid argon and neon.