Study On Structure Optimization And Performance Of Phosphorescent Organic Light Emitting Diodes Based On Ir(ppy)3

Short lifetime and low efficiency is the "bottleneck" restricting the wide application of organic light emitting diodes (OLEDs). How to improve the luminous efficiency and prolong their operation lifetime is paid close attention by domestic and foreign researchers. In this paper, we chose a typical phosphorescent OLEDs (PHOLEDs) of ITO/NPB/CBP:Ir(ppy)3/TPBi/Alq3/LiF/Al, and optimized its structure. The influences of its structure optimization on its the optical and electrical properties were studied in detail.Firstly, we used CBP and TPBi as the double host materials to fabricate the double emitting layer (D-EML) OLEDs of ITO/NPB/CBP:Ir(ppy)3/TPBi: Ir(ppy)3/TPBi/Alq3/LiF/Al. We studied the influence of double emitting layer thickness on the properties of the devices. Two groups of D-EML PHOLEDs with the different thickness of CBP and TPBi were produced. We found that when the thickness of CBP and TPBi is20nm and1Onm, respectively, the device shows the best performance. Its maximum brightness is51355cd/m2. Its maximum current efficiency is31.1cd/A, which is1.52times of single emitting layer (S-EML) OLED. Its maximum power efficiency is15.3lm/W, which is 1.90times of S-EML OLED. When the initial brightness is250cd/m2, its lifetime is76h under without encapsulation, which is15times of S-EML OLED.Secondly, we carried out n-type doping at electron transport layer for the above optimized D-EML OLED, that is using TPBi:LiF instead of Alq3, and studied the effect of n-type doping on properties of device. We found that the properties of n-type doping device were improved further. Its maximum current efficiency is40.5cd/A, which is1.98times of S-EML OLED. Its maximum power efficiency is23.7lm/W, which is2.95times of S-EML OLED. When the initial brightness is250cd/m2, its lifetime is220h under without encapsulation, which is44times of S-EML OLED, respectively.Thirdly, keeping the total thickness of devices unchanged, we introduced concavo-convex structure at Alq3/LiF interface for the above optimized D-EML OLED. The performance of the device with different height and numbers of interface protrusions were tested, where the height of Alq3protrusions are5,10,15and20nm, and the number are1and2, respectively. Results show that the device with1Onm height and2protrusions shows the best performance, in which the maximum current efficiency and the maximum power efficiency is47.3cd/A and26.01m/W, increased by52%and70%comparison to the optimized D-EML OLED, respectively.Then keeping the total thickness of devices unchanged, we introduced concavo-convex structure at CBP:Ir(ppy)3/TPBi:Ir(ppy)3, TPBi:Ir(ppy)3/TPBi, TPBi/Alq3and Alq3/LiF interfaces for the above optimized D-EML OLED. The performance of the device with different height and numbers of protrusions were tested. The height of CBP:Ir(ppy)3protrusions are10and15nm, and the number are1and2, respectively. We found that the device with lOnm height and2protrusions shows the best performance, in which the maximum current efficiency and the maximum power efficiency is50.6cd/A and29.01m/W, being1.63and1.90times of the optimized D-EML OLED, respectively.Finally, keeping the total thickness of devices unchanged, we introduced concavo-convex structure at NPB/CBP:Ir(ppy)3, CBP:Ir(ppy)3/TPBi:Ir(ppy)3, TPBi:Ir(ppy)3/TPBi, TPBi/Alq3and Alq3/LiF interfaces for the above optimized D-EML OLED. The performance of the device with different height and numbers of protrusions were tested. The height of NPB protrusions are10,15and20nm, and the number are1and2, respectively. We found that the device with1Onm height and2protrusions shows the best performance, in which the maximum current efficiency and the maximum power efficiency is52.6cd/A and32.91m/W, being1.69and2.15times of the optimized D-EML OLED, respectively.