Identifying the efficient inter-conversion between singlet and triplet charge-transfer states by magneto-electroluminescence study

Abstract

Using the magneto-electroluminescence (MEL) as a tool, we demonstrated the efficient inter-conversion between singlet and triplet charge-transfer (CT) states in exciplex-based organic light-emitting diodes (OLEDs). Results show that the MEL of exciplex-based device is larger than that of exciton-based device by a factor of 3.2. The emission of exciplex-based devices comes from the direct intermolecular electron-hole pair recombination and their spin exchange energy is much smaller, which causes the efficient inter-conversion between singlet and triplet states. This argument was supported by the consistent evolutions of the MEL and EL spectra versus applied bias and donor concentrations. Finally, the bandgap effects on the MEL as well as the external quantum efficiency of exciplex-based devices were discussed. Our findings of MEL may offer a feasible way to unravel underlying mechanisms that limit the EL efficiency in the OLEDs.

Effects of idling time between depositions of organic layers and metal electrode in organic photovoltaic device

“…Instead of non-emissive SubPc and C 60 , tris-(8-hydroxyquinoline) aluminum (Alq 3 ) was used in the study because of its excellent luminescence properties [23][24][25]. Stacks of Alq 3 (20 nm)/BPhen (7 nm, kept in 10 À6 Torr for T = 10, 15, 20, 25 and 30 min)/Mg:Ag (0, 100 nm) are prepared on glass substrates.…”

Current–voltage characteristics of organic heterostructure devices with insulating spacer layers

“…We assume that the current under forward bias involves the formation and recombination of exciplexes. For many donor/acceptor systems, including the Tetracene/C 60 structures mentioned above and the m-MTDATA/Bphen system to be discussed below, this is indeed the dominant process [14][15][16][17][18]. When a forward bias voltage is applied, electrons injected into the acceptor layer and holes injected into the donor layer meet at the interface.…”

Modulations in line shapes of magnetoconductance curves for diodes of pentacene:fullerene charge transfer complexes

“…In organic diodes http://dx.doi.org/10.1016/j.orgel.2014.08.053 1566-1199/Ó 2014 Elsevier B.V. All rights reserved. using donor/acceptor (D/A) molecules as the active layer, the application of a magnetic field modulates the ISC of an intermolecular excited state, that is charge transfer (CT) complex state [7,12,13]. Since the e-h separation distance of CT complex states is relatively long (or the Columbic interaction is relatively weak) as compared to that of intramolecular excited states (excitons), the electric field, such as the applied bias voltage on the device or built-in electric field (E built-in ), possibly dissociates the CT complex states, reducing the influence of the applied magnetic field on the CT complex states and suppresses the device's MFEs [12,14].…”

“…Several models have been proposed to explain the mechanisms of MFEs [1,2,4,7,10,11]. Since the generation of MFEs in organic semiconductor devices usually correlates with excited states, the impact of the formation of excited states on MFEs is a major research issue [6,7,[11][12][13].…”

“…Several models have been proposed to explain the mechanisms of MFEs [1,2,4,7,10,11]. Since the generation of MFEs in organic semiconductor devices usually correlates with excited states, the impact of the formation of excited states on MFEs is a major research issue [6,7,[11][12][13].Frankevich et al proposed the intersystem crossing (ISC) of excited states with relatively long electron-hole (e-h) separation distance in conjugated molecules; that is, polaron pairs (PPs), can be changed by applying a magnetic field [1]. Since singlet and triplet states have different photo-physical properties, varying the relative populations between singlet and triplet states results in the MFEs in organic semiconductor devices [2,7,11].…”