47.2: Luminescence Quenching in Blue Fluorescent OLEDs

Abstract

The radical ion of the familiar hole‐transport material NPB (i.e., NPB⋅+) absorbs strongly in the blue region of the spectrum. Therefore, it can quench blue luminescence by Förster energy transfer. A high concentration of NPB⋅+ at the interface between an NPB hole‐transport layer (HTL) and a blue light‐emitting layer (LEL) severely limits the luminescence efficiency of the emitting blue dopant. The efficiency can be improved dramatically by modifying either (a) the anode contact with a thin layer of CuPc, thereby increasing the field strength in the HTL, or (b) the cathode contact with a thin layer of Bphen, thereby reducing the field strength in the LEL. Both strategies reduce the difference in electric field strength across the interface, reduce the interfacial concentration of NPB⋅+, and suppress the quenching. Experimental evidence is provided by a spectrum of electrochemically generated NPB⋅+, simultaneous photo‐ and electroluminescence measurements on model OLEDs, and electrical characterization of the devices.

39.2: Efficient Single‐Layer Small Molecule Blue OLEDs Based on a Multifunctional Bipolar Transport Material

“…As compared to the absorption of neutral MADN, MADN + radical cation shows stronger absorption at 328 nm, 436 nm, 604 nm, 664 nm, and weaker absorption at 361 nm, 377 nm, and 397 nm, respectively. Importantly, there is no significant enhancement of absorption in the range of 450 ~ 550 nm, indicating that MADN + radical cation would not be a strong quencher of green and sky-blue fluorescence as compared to NPB + radical cations [4]. To demonstrate the efficacy of utilizing MADN as HTL, two green OLED devices I and II with structures of ITO/CF x /HTL (60 nm)/Alq 3 : 1% C545T (37.5 nm)/Alq 3 (37.5 nm)/LiF (1 nm)/Al (150 nm) have been fabricated for comparison.…”

“…One of the reasons is that the excess holes would accumulate at HTL/ETL interface and generate NPB + and Alq 3 + radical cations. It has been suggested that exciton quenching at the NPB/Alq 3 interface due to the accumulation of NPB + radical cations at the interface impacts significantly on the current efficiency [3,4]. Moreover, it has also been reported that Alq 3 cationic species would easily be produced when hole carriers exist excessively, resulting in deterioration of device lifetime [5][6][7].…”

Förster-type triplet-polaron quenching in disordered organic semiconductors

“…Nevertheless, it would be important to establish whether this mechanism more generally yields an appropriate description of the efficiency roll-off. This is urged by experimental evidence that in green-and blueemitting OLEDs TPQ is actually a more long-range process due to Förster resonant energy transfer (FRET) resulting from a triplet-polaron dipole-dipole interaction [9]. The distance (R)-dependent TPQ rate is then given by…”

“…However, accurately measuring the extinction spectra of positively or negatively charged molecules is more difficult. Recently, Young et al developed for that purpose an electrochemical method, applicable to dissolved ions [9]. Positively charged 4,4'-bis[1-naphthyl(phenyl)amino]-1,1'-biphenyl (α-NPD) was found to show strong optical absorption with a peak around a wavelength of 480 nm, explaining the occurrence of strong roll-off for blue phosphorescent OLEDs, utilizing α-NPD as a hole-transporting layer in direct contact with the emissive layer.…”

Current efficiency in organic light-emitting diodes with a hole-injection layer

“…It has been reported that charge quenching can play a significant role in determining the current efficiency of EL devices under various device configurations and drive conditions. 14,18,19 It has been suggested that exciton quenching at the NPB/Alq interface due to the accumulation of NPB + radical cations 20,21 at the interface largely determines the current efficiency. In the case of the bilayer NPB/Alq device, the electric field in the NPB layer is negligible under forward bias, and the NPB/Alq …”