Efficient Single-Layer Organic Light-Emitting Diodes Based on C545T-Alq₃ System

Abstract

We report efficient single-layer organic light-emitting diodes (OLEDs) based on 2, 3,6,7-tetrahydro-1,1,7,7,tetramethyl-1H,5H,11H-10-(2-benzothiazolyl) quinolizino-[9,9a,1gh] coumarin (C545T) and tris(8-hydroxyquinolinolato)aluminum (Alq 3 ) system. The doping concentration, as well as nickel oxide (Ni 2 O 3 ) and molybdenum oxide (MoO 3 )-modified indium tin oxide (ITO) anodes were investigated. With increasing doping concentration from 1 to 23%, the emission color varied from green to yellow; the Commission Internationale de l'Eclairage (CIE) coordinate changed from (0.29, 0.66) to (0.45, 0.53). The green-to yellow-emitting devices showed current efficiency from 6.4 to 4.2 cd/A for ITO/Ni 2 O 3 anode and 12.8 to 5.5 cd/A for ITO/ MoO 3 anode at 100 cd/m 2 , respectively, which are better than 3.8 cd/A obtained in the traditional undoped three-layered device ITO/copper-phthalocyanine (CuPc)/ N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′-diamine (R-NPD)/Alq 3 /LiF/Al and comparable to 14.4 cd/A in the doped device ITO/CuPc/R-NPD/ C545T-Alq 3 (∼1%)/Alq 3 /LiF/Al.

Heavily Doped, Charge-Balanced Fluorescent Organic Light-Emitting Diodes from Direct Charge Trapping of Dopants in Emission Layer

“…Without doping C545T, the peak at 550 nm was assigned to the pristine Alq 3 . Similar EL spectra of the doped devices were observed with the green emission; the main peak changed from 519 nm to 530 nm by doping ratio change were coincident with the previous works for Alq3 :C545T host-dopant system 13. In OLEDs, there are two common reasons responsible for the blue-shift in device emission color with varying current/voltage.…”

“…13,24,25 Moreover, direct hole injection into the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 -11 -between the HTL and the EML were also injected into the dopants through the host molecules in addition to direct hole injection. In the case of the 6% HOD, hole leakage might cause a reduction in current density due to doping being large enough to generate deep-trap sites that limit the hole currents, as compared to the shallow traps at lower doping concentrations.…”

“…13 To correlate C545T only in EML and excimer-emission peak in detail, the two devices 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 -16 -almost no overlap between the emission and absorption, which was confirmed as excimer formation by C545T. 13,41,42 In the case of 30 nm thickness of C545T in the EML, 530 nm and 573 nm of the spectra were originated from C545T exciton and strong excimer-emission, respectively. EL spectra of the host-dopant (Alq 3 + C545T) devices were observed with the green emission ranging from 519 nm to 530 nm through the change of doping ratio.…”

“…10 tandem FLOLEDs. 14 From those approaches, as well as other research reports, the highest current efficiencies were obtained by enhancing Fӧrster energy transfer with low doping concentration of C545T in Alq 3 host [13][14][15][16] and/or by increasing the hole current density via electrode modification. [8][9][10][11] Generally, 4~5% doping in EML has been widely investigated in the case of phosphorescent OLEDs (PHOLEDs) through long range dexter energy transfer compared to low doping (less than 1%) of FLOEDs with short-range Fӧrster energy transfer by dipole-dipole interaction.…”

“…13 However, the current density in PHOLEDs was enhanced as the phosphorescent doping concentration was increased in EML at their previous work. 20 In that case, they clearly asserted the device mechanism for improved devices was direct charge injection into dopants.…”