Highly Sensitive Determination of the Polaron-Induced Optical Absorption of Organic Charge-Transport Materials

Abstract

We examine polaron-induced absorption in organic transport materials using a highly sensitive measurement technique. A hole only device is embedded into a low-loss TE2 waveguide structure, and the current induced change of the waveguide absorption is measured. The exemplary study of 2,2('),7,7(')-tetrakis(N,N-diphenylamine)-9,9(')-spiro-bifluorene (S-TAD) reveals a very low polaron absorption cross section of sigma(p) < or = 2.6 x 10(-18) cm(2) for 560 nm < or = lambda < or = 660 nm. The accuracy of this data is unsurpassed by other techniques used for the unambiguous study of polaronic species in organic thin films.

Electrical and Optical Impulse Response of High-Speed Micro-OLEDs Under UltraShort Pulse Excitation

“…Indeed, the optical time decay is related to the onset of the polaronic absorption that reduces the singlet population and hence reduces the EL [11,16].…”

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

“…9 Thus, Joule heating is suppressed in devices with a small active area, as we confirmed previously. [9][10][11][12][13]25 Furthermore, under pulse operation with a pulse width of 5 ls, a very high J max of 2.8 kA cm À2 was achieved in the linear device with X ¼ 50 nm because of further suppression of device heating. Figure 4(b) depicts external EL quantum efficiency (g ext ) À J characteristics of the devices with X ¼ 50 nm, 100 nm, 200 nm, 500 nm, and 1 lm.…”

“…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.…”

“…[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.…”

Highly efficient, diffraction-limited laser emission from a vertical external-cavity surface-emitting organic laser

“…They may find applications in the fields of, e.g., polymer optical fiber telecommunications [2], bio/chemo-sensing [3] or spectroscopy [4]. In the perspective of achieving direct [5] or indirect [6] electrical pumping of organic semiconductors, most of recent studies have been dealing with low-loss resonator structures such as distributed feedback resonators [7,8] or microcavities [9,10]. In the meantime, less attention has been paid to the question of optimizing conversion efficiency, beam quality and power scaling capability.…”