Optical properties of molecular organic semiconductor thin films under intense electrical excitation

A dramatic spectrally narrowed edge emission (SNEE) from small molecular organic light-emitting diodes at room temperature, with a full width at half maximum of 5–10nm, is described. The results show that this emission is due to irregular waveguide modes that leak from the indium tin oxide anode to the glass substrate at a grazing angle. Measurements of variable stripe length devices exhibit an apparent weak optical gain, but there is no observable threshold bias associated with this SNEE. Hence this apparent “optical gain” is suspected to result from misalignment of the propagating leaky waveguide mode and the collecting optics.

“…Additional strong losses arise from quenching by the copious polarons, SEs, TEs, and the metal electrodes. [2][3][4][5][6] This presents a major obstacle in reaching the lasing threshold.…”

Section: Spectrally Narrowed Edge Emission From Organic Light-emittinmentioning

confidence: 99%

Spectrally narrowed edge emission from organic light-emitting diodes

Tian

Gan

Zhou

et al. 2007

“…Exciton-polaron annihilation is the most critical issue to overcome because all polarons have a broad absorption spectrum. [8][9][10][11] In our previous study, we demonstrated high current injection of over 1000 A cm À2 without device breakdown into OLEDs with an unusual device configuration of a small active area (200 Â 200 nm) on a high thermally conductive substrate by application of a short pulse voltage of less than 1 ls. 12,13 However, very serious efficiency roll-off was observed at such high current density.…”

mentioning

confidence: 99%

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

Hayashi

Nakanotani

Inoue

et al. 2015

Using e-beam nanolithography, the current injection/transport area in organic light-emitting diodes (OLEDs) was confined into a narrow linear structure with a minimum width of 50 nm. This caused suppression of Joule heating and partial separation of polarons and excitons, so the charge density where the electroluminescent efficiency decays to the half of the initial value (J0) was significantly improved. A device with a narrow current injection width of 50 nm exhibited a J0 that was almost two orders of magnitude higher compared with that of the unpatterned OLED.

“…Within the OLED devices, however, inherent low carrier mobility of organics brings about the situation that the density of charge carriers ͑i.e., polarons͒ is much higher than that of emissive excitons, thereby causing a large absorption loss of stimulated emission by polarons. 7,8 Actually, in a thin film of tris-͑8-hydroxyquinoline͒ aluminum ͑Alq͒-doped with a DCM2 laser dye, which is a host-guest type of emitting layer enabling an efficient Förster energy transfer, it is shown that the optical gain of DCM2 cannot be obtained due to the absorption loss by large amounts of anionic Alq hosts. 7 The drastic raising of mobility enough to change such a situation could hardly be expected under the carrier-transport mechanism of organics described by hopping, so that the polaronabsorption problem is a serious obstacle to the realization of OLDs.…”

mentioning

confidence: 99%

“…7,8 Actually, in a thin film of tris-͑8-hydroxyquinoline͒ aluminum ͑Alq͒-doped with a DCM2 laser dye, which is a host-guest type of emitting layer enabling an efficient Förster energy transfer, it is shown that the optical gain of DCM2 cannot be obtained due to the absorption loss by large amounts of anionic Alq hosts. 7 The drastic raising of mobility enough to change such a situation could hardly be expected under the carrier-transport mechanism of organics described by hopping, so that the polaronabsorption problem is a serious obstacle to the realization of OLDs.One of the strategies for circumventing this problem is to design the device such that the gain spectrum of a laser dye has little overlap with the absorption spectra of charge carriers. 5 For this purpose, the use of the cascade Förster energy-transfer system that is formed by simultaneous doping of two or more guests into a host matrix 9 might be an effective way, since the gain wavelength can be widely changed by the combination of guests suitably matched in their absorption and emission spectra.…”

mentioning

confidence: 99%

Using proton-transfer laser dyes for organic laser diodes

Sakai¹,

Tsuzuki²,

Itoh³

et al. 2005

104

Photopumping measurements for the dilute thin films of the representative proton-transfer laser dye, 2-͑2-hydroxyphenyl͒benzothiazole ͑HBT͒ revealed that it exhibited better stimulated-emission performance, especially at the high doping level of 26 wt %. This suggests that HBT has an ability to form high gain media, in addition to an inherent potential for widely tuning gain wavelength. Both are advantageous for overcoming the polaron-absorption problem that is a major obstacle to making organic laser diodes. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1868885͔Since the reports that the possibility of organic laser diodes ͑OLDs͒ was referred, 1,2 a considerable number of attempts have been made to date. [3][4][5][6] Although photopumped lasing has been observed in the slab waveguides made of small molecules or conjugated polymers, there has yet been no report on electrically pumped lasing from organic lightemitting diode ͑OLED͒ structures. The primary requirement for the achievement of laser radiation is that the optical gain of emissive medium must be predominant over the sum of all the losses, such as scattering and absorption; namely, the net gain must become positive. Within the OLED devices, however, inherent low carrier mobility of organics brings about the situation that the density of charge carriers ͑i.e., polarons͒ is much higher than that of emissive excitons, thereby causing a large absorption loss of stimulated emission by polarons. 7,8 Actually, in a thin film of tris-͑8-hydroxyquinoline͒ aluminum ͑Alq͒-doped with a DCM2 laser dye, which is a host-guest type of emitting layer enabling an efficient Förster energy transfer, it is shown that the optical gain of DCM2 cannot be obtained due to the absorption loss by large amounts of anionic Alq hosts. 7 The drastic raising of mobility enough to change such a situation could hardly be expected under the carrier-transport mechanism of organics described by hopping, so that the polaronabsorption problem is a serious obstacle to the realization of OLDs.One of the strategies for circumventing this problem is to design the device such that the gain spectrum of a laser dye has little overlap with the absorption spectra of charge carriers. 5 For this purpose, the use of the cascade Förster energy-transfer system that is formed by simultaneous doping of two or more guests into a host matrix 9 might be an effective way, since the gain wavelength can be widely changed by the combination of guests suitably matched in their absorption and emission spectra. Thus, for example, a further addition of adequate laser dyes into the Alq:DCM2 system might enable one to shift the gain wavelength to the region where absorption of Alq anions is relatively weak; however, it also might require a high technique for vapor codeposition of several materials with their concentration ratio exactly controlled.On the other hand, as another gain-wavelength shifting method, we have been focusing on the use of proton-transfer ͑PT͒ laser dyes. A characteristic property of PT dyes ...