Kibum Kim

Correlation between photoelectric and optical absorption spectra of thermally evaporated pentacene films

We have measured the spectral photoresponse of Al/pentacene Schottky junction photodiodes and optical absorption spectra of pentacene films thermally evaporated on glass. The photoelectric response exhibited the genuine highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) transition at 1.97 eV and interband absorption peaks at 2.3 and 2.5 eV. These peaks are also identified in the optical absorption spectra, but they are dominated by additional strong exciton peaks at 1.82 and 2.1 eV. By comparing these complementary measurements, we determine the HOMO–LUMO gap energy of 1.97 eV and the fundamental exciton binding energy of 0.15 eV for thin solid pentacene.

Optimum channel thickness in pentacene-based thin-film transistors

We report on the influence of pentacene channel thickness on the field-effect hole mobility in pentacene-based thin-film transistors (TFTs) that employ the top-contact mode for the source/drain electrodes. Our pentacene channel layers were deposited in the thickness range of 16–90 nm by thermal evaporation on 450 nm thick Al2O3+x dielectric films. The TFTs with increasingly thinner pentacene layers displayed correspondingly higher hole mobility, but an optimum thickness was determined to be about 30 nm because the TFTs with pentacene layers thinner than 30 nm exhibited high leakage current in the off-state bias regime. After a proper chemical treatment was performed onto the Al2O3+x gate dielectric, our optimized TFT with a 30 nm thick pentacene channel exhibited high mobility of ∼0.2 cm2/V s with an on/off current ratio of 105.

Fabrication of p-pentacene/n-Si organic photodiodes and characterization of their photoelectric properties

We have fabricated p-pentacene/n-Si organic photodiodes by thermal evaporation of pentacene films at 25 (RT), 60, and 80 °C. Our pentacene films exhibit the main absorption peak (highest occupied molecular orbital-lowest unoccupied molecular orbital gap transition) at 1.82 eV and photoelectric effects were clearly observed from our p-pentacene/n-Si diodes when illuminated by a monochromatic red light of 1.85 eV (670 nm). Excellent photoresponsivity of 2.6 A/W and the photoresponse time of less than 40 ns have been demonstrated. The photodiodes exhibited typical rectifying behavior in the dark and the leakage current increases with the deposition temperature.

Structural and optical properties of 6,13-pentacenequinone thin films

We report on the structural and optical properties of 6,13-pentacenequinone thin films deposited on n-Si substrates by thermal evaporation at room temperature. X-ray diffraction data indicate excellent crystallinity but the films have two crystalline phases mixed. Photoluminescence (PL) spectra exhibited double peaks at 1.97 and 2.09 eV while the corresponding (double) peaks, associated with the highest occupied molecular orbital–lowest unoccupied molecular orbital transition, were also observed from direct absorption spectra at 2.94 and 3.11 eV. The yellow PL band was quite intense under UV illumination; the intensity was comparable to that from Tris-(8-hydroxyquinoline) aluminum (Alq3) thin films. A large Stokes shift of ∼1eV found in our 6,13-pentacenequinone thin films suggests potential applications to UV-detectors and UV-detecting cards.

Raman and infrared spectroscopy of YBa2Cu3O7−δ–BaPbO3 composites

Abstract We present results of Raman and infrared spectroscopic studies of the composite system of superconducting YBa 2 Cu 3 O 7−δ and metallic BaPbO 3 , with the BaPbO 3 fraction ranging from 5 to 30 in weight. The Raman spectra of the YBa 2 Cu 3 O 7−δ grains in the composite system are typical of well-oxygenated YBa 2 Cu 3 O 7−δ bulk single crystals, even up to the grain boundaries, independent of the BaPbO 3 fraction. We have also performed infrared reflectivity measurements and have identified the major c -axis transverse-optical phonons of YBa 2 Cu 3 O 7−δ , consistent with the single-crystal data, confirming that the constituent YBa 2 Cu 3 O 7−δ crystallites are fully oxygenated. The absence of mode splitting suggests that the crystal symmetry of YBa 2 Cu 3 O 7−δ grains remains the same as that of bulk single crystals. No phonons indicative of other YBa 2 Cu 3 O 7 - or BaPbO 3 -related impurity phases, such as YBa 2 Cu 3 O 6 , Y 2 BaCuO 5 , and Ba 2 PbO 4 , have been detected in our Raman and infrared spectra. All these optical features clearly indicate that YBa 2 Cu 3 O 7−δ is well separated from BaPbO 3 and that BaPbO 3 is an excellent additive material for the synthesis of YBa 2 Cu 3 O 7−δ bulk materials.