Rubrene/Fullerene Heterostructures with a Half‐Gap Electroluminescence Threshold and Large Photovoltage

Abstract

Tremendous interest in organic semiconductors for the development of light-weight and flexible electronic devices has been generated, owing to their ease of fabrication and suitability to large-area applications. [1][2][3][4][5] However, there are still many unexplored possibilities offered by these conjugated materials. [6,7] We demonstrate here a new structure that permits efficient integration of both light-and current-generation functions from a rubrene/fullerene heterostructure into an efficient organic dual device (ODD). [8][9][10] The device behaves like a compound semiconductor device: The electroluminescence (EL) turn-on voltage is < 1 V with the characteristic color of rubrene. The solar power conversion efficiency reaches 3 % with a 5.3 mA cm -2 short-circuit current density and almost 1 V open-circuit voltage under AM 1.5 illumination. Surprisingly, the EL turn-on voltage is about half the value of the rubrene bandgap (2.2 eV), a fact that cannot be explained using current models of charge injection into organic semiconductors. A physical interpretation is proposed in terms of the so-called Auger fountain mechanism, [11] which we could implement into our molecular heterojunction. During the EL process in heterojunction devices, holes and electrons are injected into the hole and electron conducting layers, respectively. They then recombine near the interface to raise a photon that has a color characteristic of the recombination region. Whereas, for the photovoltaic (PV) effect in organic materials, electron-hole pairs created by a broadband light source form excitons that dissociate at the donor-acceptor (D-A) junction. It then generates a net bipolar current flow across the device. Interest in organic-based solar cells (OSCs) has grown because of experience and understanding gained from organic light-emitting diode (OLED) developments. The general understanding treats light and current generation as competing phenomena. Indeed, the working principle of organic solar cells is that photoluminescence (PL) is quenched by ultrafast charge transfer from the donor to the acceptor.[4] However, we have successfully integrated both efficient light-and current-generating functions in a discrete ODD device. 5,6,11,12-Tetraphenylnaphthacene, commonly known as rubrene, is used as a hole-transporting material, while fullerene (C 60 ) is used as an electron-transporting material. Both rubrene and fullerene are widely studied semiconductors, with among the highest field-effect mobility for holes and electrons, respectively. [12,13] Moreover, rubrene is also currently used as a yellow dopant for achieving OLEDs with white-light emission suitable for display and lighting applications, and fullerene is omnipresent as an acceptor in efficient OSCs. [14,15] We have made heterojunction devices that combine 35 nm thick rubrene and 25 nm thick C 60 layers sandwiched between transparent indium tin oxide (ITO) and 60 nm thick metal electrodes. A 40 nm thick layer of poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDO...

Ultralow-voltage Auger-electron-stimulated organic light-emitting diodes

“…[17][18][19][20][21] This process is referred to as Auger recombination. Auger recombination is a common electronic process in inorganic semiconductors 40 and quantum dots, in particular when the carrier concentration is high.…”

“…This type of OLED has operating voltages well below the HOMO-LUMO energy gaps of the emitter molecules; i.e., the output photon energies are higher than the input electrical energies. [17][18][19][20][21] In this report, we provide a review on the electronic processes at organic heterojunctions and Auger-electron-stimulated ultralow-voltage OLEDs.…”

Light-emitting Organic Photovoltaic Devices Based on Rubrene/PTCDI-C13 Stack

“…Pandey et al reported a similar device using a combination of rubrene and fullerene. 3 Their device showed maximum luminance of ca. .…”

“…Recently, organic optoelectronic devices, such as, organic lightemitting diodes (OLEDs), 1 organic photovoltaic device (OPVs), 2,3 organic photodiodes (OPDs), [4][5][6][7] and organic phototransistor 8 have been actively studied. Because a certain organic material has a characteristic optical property, a new functional device is expected, for example, that has both light-emitting and photo-sensing abilities.…”

“…4,5 Bright emission over 9,000 cd/m 2 and high ratio of photo-and dark-conductivity of 10 4 under reverse bias condition were demonstrated in the pyrazole-derivative based devices. Also, polymer based multifunction diodes were realized by emitter doping dyes such as rubrene or tris(2-phenylpyridinato)iridium (III) (Ir(ppy) 3 ) into poly(3-hexylthophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend. 6,7 In this study, we have investigated an organic multi-function diode with OLED, OPV, and OPD mode.…”