Phosphorescent materials can find wide application in flat-panel-display and solid state lighting technologies. The main focuses of this project is to design and synthesis of small molecules for phosphorescent organic light-emitting diodes (PhOLEDs). In chapter one discussed about the general introduction related OLED technology. The necessity of attention is to understand how the chemical structures of molecules relate to their electronic and optical properties, morphological stability and using this concept to design and synthesize novel organic compounds and test their suitability as an emitting layer in PhOLED devices.
In the second and third chapters discuss about facile synthesis and characterization of iridium(III) complexes containing an N-ethylcarbazole?thiazole and N-ethylcarbazole?benzthiazole main ligands using a tandem reaction for solution processed phosphorescent organic light-emitting diodes. A new series of highly efficient phosphorescent Ir(III) complexes were synthesized and their photophysical, electrochemical, and electroluminescent (EL) properties were investigated. The Ir(III) complexes, including (Et-Cz?Tz)2Ir(pic), (Et-Cz?Tz)2Ir(pic-N-O), (Et-Cz?Tz)2Ir(EO2?pic), (Et-Cz?Tz)2Ir(EO2?pic-N-O), (Et-Cz-BTz)2Ir(pic), (Et-Cz-BTz)2Ir(pic-N-O), (Et-Cz-BTz)2Ir(EO2-pic), and (Et-Cz-BTz)2Ir(EO2-pic-N-O) are comprised of linked N-ethylcarbazole (Et-Cz) and thiazole (Tz), benzothiazole (BTz) units as the main ligands and picolinic acid (pic) and picolinic acid N-oxide (pic-N-O) as ancillary ligands. In addition, some of the Ir(III) complexes contain an ethylene oxide solubilizing group attached to the ancillary ligands via a tandem reaction. High performance, solution processable PhOLEDs, fabricated using (Et-Cz?Tz)2Ir(EO2?pic), were observed to have a maximum external quantum efficiency of 6.08% and a luminance efficiency of 10.98 cd A-1. This is the first report on the use of EO2?pic and EO2?pic-NO ancillary ligands for the synthesis of solution processable Ir(III) complexes via a tandem reaction. The performances of the PhOLEDs based on these Ir(III) complexes correlate well with the theoretical properties predicted by using density functional theory calculations. Among the eight Ir(III) complexes, the (Et-Cz-BTz)2Ir(EO2-pic) showed the best luminous efficiency of 60 cd/A and external quantum efficiency of 19% with CIE coordinates (0.467, 0.524), which is one of the best efficiencies for solution-processed yellow PhOLEDs using heteroleptic Ir(III) complexes as an emitting layer.
The fourth topic is related to the easy route to red emitting homoleptic Ir(III) complex for highly efficient solution-processed PhOLED. In this study, we chose a smart combination of thiophene and phenylquinoline as a single cyclometalated ligand for homoleptic Ir (III) complexes, tris(4-phenyl-2-(thiophen-2-yl)quinoline)iridium [(Th-PQ)3Ir]. The introduction of electron-donating thiophene into the ligand frame in (Th-PQ)3Ir improves the quantum yield, and induced red shift in the photoluminance (PL) spectrum due to the decrease in the ligand’s triplet energy with increasing π-conjugation. On the other hand, the strong electron-accepting character of the quinoline group can effectively reduce the 3MLCT exited energy of the cyclometalated Ir(III) complex. Using (Th-PQ)3Ir dopant, we achieved a PhOLED that emits in the deep red (CIE coordinates (0.64, 0.34)), very high luminous current efficiency of ~26 cd/A and an external quantum efficiency (EQE) of ~21 % which is the highest recorded current efficiency and EQE of a solution?processed red-emitting OLEDs to date.
Chapter five has discussed about electron transporting unit linked multifunctional Ir(III) complex is a promising strategy to improve the performance of solution-processed PhOLEDs. The oxadiazole based electron transporting (ET) unit was glued to the heteroleptic Ir(III) complex (TPQIr-ET) and used as a dopant for PhOLEDs. It shows superior device performance than the dopant (TPQIr) without ET unit due to the balanced charge carrier injection by ET unit, which has never been observed before in PhOLEDs. TPQIr-ET shows the EQE of 20.59%, which is 25% higher than that of the TPQIr. This work provides the ?rst successful example of the use of a dopant with the ET group in PhOLEDs to realize e?cient device performance.
The last section is referred to the highly efficient bluish green PhOLEDs based on heteroleptic Ir(III) complexes with phenylpyridine main skeleton. A new series of heteroleptic iridium(III) complexes, bis(2-phenylpyridinato-N,C2’) iridium (2-(2′,4′-difluorophenyl)-4-methylpyridine), (ppy)2Ir(dfpmpy) and bis(2-(2′,4′-difluorophenyl)-4-methylpyridinato-N,C2’)iridium(2-phenylpyridine) (dfpmpy)2Ir(ppy), have been synthesized by using phenylpyridine as a main skeleton for bluish green PhOLEDs. The Ir(III) complexes showed high thermal stability and high photoluminescent (PL) quantum yields of 95 ± 4% simultaneously. As a result, the PhOLEDs with the heteroleptic Ir(III) complexes showed excellent performances approaching 100% internal quantum efficiency with a very high external quantum efficiency (EQE) of ~27%, a low turn-on voltage of 2.4 V, high power efficiency of ~85 lm/W, and very low efficiency roll-off up to 20,000 cd/m2.