Study of EL and Transport Properties of Zn Metal Complex Based Materials for OLED Applications

Abstract

Organic semiconductors have attracted lot of attention in academic and technological community due to its potential applications in organic light emitting diodes (OLEDs), organic photovoltaic cells etc. OLED is being considered as one of the most promising technology for flat-panel displays and in general lighting. The most important part of OLED devices is the electroluminescence layer, for which polymer and small molecular weight metal chelats can be used. The small molecule based metal chelats 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 an emissive layer because of their wide spectral response in the visible region. Two zinc complexes bis(8-hydroxy quinolinate) zinc (Znq2) and bis(2-methyl 8- hydroxy quinolinate) zinc Zn(mq)2 have been synthesized and characterized by different characterization techniques (FTIR, TGA, UV visible and Photoluminescence Spectroscopy). Both the photoluminescence and electroluminescence properties are extensively studied. The Photoluminescence properties of thin films of Znq2 shows maximum absorption at 380 nm, extinction coefficient of 7.788 × 103 moles-1LCm-1, photoluminescence peak at 542 nm and a quantum yield of 41 % and that of Zn(mq)2 shows maximum absorption at 385 nm, an extinction coefficient of 1.869 × 103 moles-1LCm-1 photoluminescence peak at 530 nm and a quantum yield of 21 %. Organic light emitting diode have been fabricated with the structure ITO/α-NPD(40 nm)/Znq2(35 nm)/BCP(6 nm)/Alq3(30 nm)/LiF(1 nm)/Al(100 nm) which shows a broad electroluminescence peak at 558 nm . The I-V characteristics of the device shows turn on voltage about 10.5 V and a maximum brightness 623 cd/m2 at 18 V. Similarly an organic light emitting diode of Zn(mq)2 as an emissive material with the structure ITO/α-NPD(40 nm)/Zn(mq)2(35 nm)/BCP(6 nm)/Alq3(30 nm)/LiF(1 nm)/Al(100 nm) have been fabricated. An electroluminescence peak at 539 nm has been observed. The I-V characteristics of the device Zn(mq)2 as an emissive material ) shows turn on voltage about 4.5 V and a maximum brightness 752 cd/m2 at 18 V. Hole only devices of both the Zinc metal complexes [Znq2 and Zn(mq)2] have been fabricated. The injection limited current behavior has been observed for both the complexes. I-V characteristic of both the devices shows a temperature independent behaviour. Flower Nordiem model fits for both and the injection barrier height have been calculated for Znq2 and Zn(mq)2 device respectively.