Determining the Origin of Half-bandgap-voltage Electroluminescence in Bifunctional Rubrene/C60 Devices

Abstract

Lowering the driving voltage of organic light-emitting diodes (OLEDs) is an important approach to reduce their energy consumption. We have fabricated a series of bifunctional devices (OLEDs and photovoltaics) using rubrene and fullerene (C60) as the active layer, in which the electroluminescence threshold voltage(~1.1 V) was half the value of the bandgap of rubrene. Magneto-electroluminescence (MEL) response of planner heterojunction diodes exhibited a small increase in response to a low magnetic field strength (<20 mT); however, a very large decay was observed at a high magnetic field strength (>20 mT). When a hole-transport layer with a low mobility was included in these devices, the MEL response reversed in shape, and simultaneously, the EL threshold voltage became larger than the bandgap voltage. When bulk heterojunction device was examined, the amplitude of MEL curves presented an anomalous voltage-dependence. Following an analysis of the MEL responses of these devices, we proposed that the EL of half-bandgap-voltage device originated from bimolecular triplet-triplet annihilation in the rubrene film, rather than from singlet excitons that formed via an interface auger recombination. This work provides critical insight into the mechanisms of OLED emission and will help advance the applications of bifunctional devices.

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

“…It is worth noting that triplet-triplet annihilation (TTA) process has also been proposed to explain the ultralow-voltage electroluminescence from Rubrene∕C 60 device. [46][47][48][49] However, ultralow-voltage electroluminescence also have been observed in the triplet emitters-based devices. 20 As shown in Fig.…”

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

“…Motivated by a high hole mobility1234 and a long exciton diffusion length56 observed in rubrene (C 42 H 28 ) single crystal, an intense research effort has been devoted to the preparation and characterization of various rubrene molecular solid films suitable for applications in organic electronic and optoelectronic devices78910111213141516. Rubrene molecule consists of a tetracene backbone with four phenyl side rings, and each side ring lies in a plane nearly perpendicular to the tetracene backbone.…”

“…The rubrene molecular solids are formed by weak intermolecular interactions, and hence different growth methods can lead to different polymorphs24. The rubrene crystalline grains growth by vapor condensation exhibit a face-centered orthorhombic structure89111524. The high mobility observed in rubrene single crystal12 is attributed to the slipped-cofacial configuration along the long molecular-axis between two adjacent planar rubrene molecules, and this cofacial π-stack interactions significantly enhances the intermolecular charge transfer integral3.…”

“…Thus, an intense research effort has been devoted to find suitable methods of depositing rubrene polycrystalline films with similar characters1015161718. Recently, by using a vacuum deposition method, we reported a series of rubrene polycrystalline films fabricated under a fixed source temperature ( T sou ) of 300 °C and with substrate temperature ( T sub ) varied from 103 to 221 °C12.…”

Analysis of trap-assisted triplet exciton quenching by magneto- conductance in rubrene-based organic light-emitting diodes

“…文献 [1,8,24,25]中已有的研究表明, 从发光的角 度看, rubrene分子既可以作为客体分子掺杂在主体分 子里 [8,24] , 也可以作为纯薄膜用于双功能的发光与电 池器件 [1,25] . 以主客体掺杂体系作为发光层的器件, 通常可以通过主客体间的能量和电荷转移过程来提 高器件的发光效率甚至延长器件的工作寿命, 此类 器件的磁电致发光(magneto-electroluminescence, MEL) 和MC线型通常比较单一.…”