Suppression of roll-off characteristics of electroluminescence at high current densities in organic light emitting diodes by introducing reduced carrier injection barriers

Abstract

We experimentally investigated suppression of the roll-off characteristics of the electroluminescence efficiency at high current densities in organic light emitting diodes (OLEDs). To increase exciton density, we propose a nonheterostructure OLED that consists of a single emitting layer of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) and layers locally with doped donors/acceptors on the cathode and anode sides. The OLED exhibited suppression of the roll-off characteristics at high current densities over 100 A/cm2 with balanced bipolar injection and transport, resulting in the high exciton density of 1024 cm−3 s−1. Furthermore, amplified spontaneous emission with a relatively low threshold of Eth=24 μJ/cm2 was obtained by optically pumping the single-layer device. However, to realize electrical excitation it was necessary to reduce the lasing threshold by two orders of magnitude or to inject a 100 times higher current density.

Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nm

“…26 Here, we discuss exciton quenching in the devices based on SHA and SPA models. 7,14 Figure 4(c) shows fitting of the roll-off characteristics of the reference device under DC and pulse operation, demonstrating good agreement with the SHA model for DC operation and SPA model for pulse operation. (See the supplementary material, Fig.…”

“…7,20,21,23,24 Cross-sectional view of the device with small active area with a line length of 2 mm and width of X ¼ 50 nm-100 lm on ITO-coated glass substrate. This is a simplified drawing of our device structure, and the real cross-sectional view of a TEM image is given in Fig.…”

“…12,13 However, very serious efficiency roll-off was observed at such high current density. 13 Roll-off characteristics originate from four main mechanisms: singlet-singlet annihilation (SSA), 5,14 singlet-triplet annihilation (STA), 15,16 singlet-polaron annihilation (SPA), 5,7,17,18 and singlet-heat annihilation (SHA). 14 These annihilation processes can be described as…”

“…This is also a fundamental issue for realizing organic semiconductor laser diodes (OSLDs), which requires extremely high exciton density to achieve light amplification. [5][6][7] To produce an OSLD, there are four principle requirements: (i) injection and transport of high current density of over a few kA cm À2 , (ii) suppression of various exciton annihilation processes, (iii) minimization of waveguide loss such as light absorption by metal electrodes, and (iv) minimization of absorption loss by triplet excitons and polarons. Exciton-polaron annihilation is the most critical issue to overcome because all polarons have a broad absorption spectrum.…”

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] To avoid these exciton annihilation processes at high current density, particularly exciton formation at organic/organic heterointerfaces that tend to allow accumulation of charge carriers, it has been recognized that expansion of the carrier recombination width is a useful method. 20,21 In this study, we demonstrate suppression of roll-off by introducing the idea of spatial separation of the "charge carrier flow, recombination, and exciton formation area" and the "exciton decay area" in fluorescence based OLEDs.…”