Deep-Level Optical Spectroscopy Investigation of Trap Levels in Tris(8-Hydroxyquinoline) Aluminum
- PDF / 511,354 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 39 Downloads / 200 Views
0965-S09-21
Deep-Level Optical Spectroscopy Investigation of Trap Levels in Tris(8-Hydroxyquinoline) Aluminum Yoshitaka Nakano, Koji Noda, Hisayoshi Fujikawa, Takeshi Morikawa, and Takeshi Ohwaki TOYOTA Central Research and Development Laboratories, Inc., Nagakute, Aichi, 480-1192, Japan
ABSTRACT We have investigated band gap states in tris(8-hydroxyquinoline) aluminum (Alq3) with and without quinacridone (Qd) doping on fabricated indium-tin-oxide (ITO)/Alq3(:Qd)/LiF/Al devices by using a deep-level optical spectroscopy (DLOS) technique. Non-doped Alq3 sample shows a discrete trap level located at ~1.39 eV below the lowest unoccupied molecular orbital band in addition to near-band-edge transitions at 2.2 - 3.6 eV. The pronounced 1.39 eV level is attributable to an intrinsic nature of Alq3 and can be active as an efficient generationrecombination (GR) center that may impact the photophysical properties. On the other hand, Qddoped Alq3 sample exhibits a new deep level at ~2.40 eV with increasing the double carrier injection rate, corresponding to the highest occupied molecular orbital band of the Qd. Simultaneously, this GR center is subject to charge up positively due to the presence of holes injected into the Qd doping level for Qd-doped Alq3. INTRODUCTION Since the first report of efficient electroluminescence (EL) from an organic heterojunction device using tris(8-hydroxyquinoline) aluminum (Alq3) as the emissive material, organic light-emitting diodes (OLEDs) utilizing fluorescent molecules have attracted considerable interest for flat panel display applications [1]. Up to date, Alq3 has been widely used as the emitting layer and/or the electron-transporting layer for OLEDs based on small molecules. However, there is still a lack of understanding concerning the nature of carrier transport in this material. In general, organic materials show a marked tendency to have electronic traps due to their weak molecular bonds and structural disorder. These traps introduce energy levels inside the band gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) bands of the organic layers. So far, the optical and electrical properties of Alq3 have been studied in detail by using various physical and electrical techniques such as time-resolved photoluminescence (PL), time of flight (TOF), current-voltage (I-V), and impedance spectroscopy, but the fundamental electronic properties have still not been well understood. In particular, in-depth data concerning band gap states in Alq3 have yet to be clarified experimentally. Deep-level optical spectroscopy (DLOS) is well known as a powerful tool for the characterization of electronic deep levels in the band gap of semiconductors [2,3]. This technique measures changes in the depletion region capacitance under optical excitation and can provide detailed mapping of the deep levels that would be undetectable by thermal emission techniques
such as deep-level transient spectroscopy (DLTS) and thermally stimulated currents (TSC). In th
Data Loading...
