Electrical and Optical Properties of Electroluminescent Materialsused for Organic Light Emitting Device

Abstract

Organic semiconductors have attracted lot of attention in technological community due to its potential applications in organic light emitting diodes (OLEDs), organic photovoltaic cells and field effect transistor etc. OLED is being considered as one of the most promising technology for flat-panel displays and in general lighting. The one of the important layer of OLED devices is the electroluminescence layer, for which polymer and small molecular weight metal chelates can be used. The small molecule based metal chelates are the appropriate candidates as they are processed by conventional vacuum deposition techniques. Small molecule based Zn metal complexes can be used in OLED devices as electron and hole transport layer. It can also be used as an emissive layer because of their wide spectral response in the visible region. Zinc metal complex viz. {[(2-(2-hydroxyphenyl)benzoxazole)2-methyl -8-hydoxyquinoline} zinc [Zn(HPB)mq] have been synthesized and characterized by different characterization techniques (TGA, UV visible and Photoluminescence Spectroscopy). The thermo gravimetric analysis shows the degradation of Zn(HPB)mq at 3500C. Both the photoluminescence and electroluminescence properties are extensively studied. The UV-Visible spectrum shows the absorption maxima at 380 nm. The photoluminescence spectra of the solution of Zn(HPB)mq exhibits maxima at 525 nm. Organic light emitting diode device has been fabricated with the structure ITO/ -NPD(40 nm)/Zn(HPB)mq(35 nm)/LiF(1 nm)/Al(100 nm), which shows a peak in electroluminescence at 545 nm . Moreover, electron only device of the Zinc metal complexes [Zn(HPB)mq] has been fabricated. The current –voltage characteristics of this device shows ohmic conduction at lower voltage and the space charge limited conduction is observed at higher voltage. The optical and electrical properties of Copper phthalocyanine (CuPc) have been studied.The thermo gravimetric analysis shows the degradation of CuPc at 4500C. The UV-Vis spectrum of CuPc shows the absorption maxima shows at 270 nm and the maxima in photoluminescence of CuPc has been found at 410 nm. The hole-only device by using CuPc as organic semiconductor has also been fabricated. The current- voltage characteristics of this organic material shows ohmic conduction at lower voltage , trap charge limited conduction (TCLC) at moderate voltage and trap free space charge limited conduction at higher voltage .The trap energy and Fermi energy from the I-V data has also been calculated. At lower voltage the traps are shallow traps and at higher voltage the traps are deep traps.