Formation of Recombination Zone in Blue Phosphorescent Organic Light-Emitting Diodes with Different Electron Transport Layers and Its Effects on Device Performance

Abstract

The effects of the electron mobilities and energy levels of different electron transport layer (ETL) materials on the performances were systemically investigated in blue phosphorescent organic light-emitting diodes. The spatial control of recombination zone (RZ) which was accompanied with triplet exciton quenching affected the balance between holes and electrons in the emission layer, resulting in the variations of the device performances. An optical micro-cavity effect in the electroluminescence (EL) spectrum around 500 nm was noticed by employing tris(8-hydroxyquinolinolato)aluminum (Alq 3 ) ETL. This was attributed to the broadening of the emission zone through the emission layer over the ETL, exhibiting the greenish color coordinates. The current efficiency of the device with 3-phenyl-4(10-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) ETL was much higher than that of the same structured device with any other ETL due to better charge balance as well as the suppression of triplet exciton quenching by the narrow RZ with low electron mobility and proper band alignment.

Recombination Zone Control without Sensing Layer and the Exciton Confinement in Green Phosphorescent OLEDs by Excluding Interface Energy Transfer

“…Figure clearly illustrates the possible RZ movement in accordance with the various EML thicknesses and QW mediated EML. Despite the improved charge balance and RZ confinement in EML layer, the observed quantum efficiencies are lower than other reported works. , The reason for having slightly lower quantum efficiency might due to the addition of NPB as HTL because its triplet energy level is lower than that of dopant Ir­(ppy) 3 which can contribute some TTA or TPA process . Another reason might be the thickness and mobility of the ETL (i.e., TPBi) and the difference of hole and electron injection barriers at respective electrodes.…”

“…This shoulder peak around 560 nm is attributed to the triplet emission from Ir­(ppy) 3. The variation in intensity of the triplet emission can be ascribed to the weak microcavity effect, , arising from the differing optical length of excitons present in the OLED architecture. − The variation in optical propagation of photons within the (partially or fully) reflective electrode could contribute a certain difference in the lifetime and intensity of the overall emission, that phenomenon termed as microcavity effect. , Especially, in the 10 nm thick EML device, the 560 nm shoulder peak is similar, but the 443 nm peak from TCTA increased with the increase of applied potential (Figure a). Remarkably, the EL intensities of all devices showed a similar red-shift (except for the 10 nm device) with respect to the applied potential, as shown in Figure (the black circle depicts the RZ of the low operation voltage, and red circle corresponds to the high operation voltage RZ) and Figure .…”

“…24,26 The reason for having slightly lower quantum efficiency might due to the addition of NPB as HTL because its triplet energy level is lower than that of dopant Ir(ppy) 3 which can contribute some TTA or TPA process. 14 Another reason might be the thickness and mobility of the ETL 17 (i.e., TPBi) and the difference of hole and electron injection barriers at respective electrodes.…”

“…The value of η ext depends on internal quantum efficiency of the molecule, charge balance, and out coupling efficiency of the architecture. − Especially, the η ext of OLED drops at higher luminance called “efficiency roll-off” occurs due to lack of exciton confinement in the emissive layer (EML) , and triplet-induced exciton quenching. − Triplet-based losses occur through a triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA) mechanism which causes decrement in the internal quantum efficiency of EML. However, the inability to confine the exciton density in the EML can induce a significant drop in device efficiency ,− despite having an efficient EML. Hence, it is essential to confine the exciton density or recombination zone (RZ) in the EML for enhanced efficiency in the OLED architecture.…”

Effects of Doping Concentration and Emission Layer Thickness on Recombination Zone and Exciton Density Control in Blue Phosphorescent Organic Light-Emitting Diodes

“…Numerous host-guest molecules have been developed for red and green phosphorescent OLEDs (Ph-OLEDs), and 100% internal quantum efficiency has been achieved by incorporating the green dopant fac-tris(2-phenylpyridinato)iridium(III) (Ir(ppy) 3 ) into the 4,4'-bis(9carbazolyl)-1,1'-biphenyl (CBP) host. 3 It is essential to choose a proper host having a triplet energy greater than that of the guest molecule, otherwise, triplet-quenching mechanisms [4][5][6][7][8][9][10] dominate in the emissive layer, which deteriorates exciton generation. Iridium-(III)-bis-[(4,6-difluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) is a frequently employed blue dopant with a high triplet energy of 2.65 eV that requires a host such as N,N'-dicarbazolyl-3,5-benzene (mCP) with an even higher triplet energy.…”