Probing the exciton distribution in organic light-emitting diodes using long-range energy transfer

Abstract

We present a technique to probe the exciton distribution in organic light-emitting diodes. Our method involves strategically inserting sensing layers at different locations in proximity to the emissive zone. In contrast to typical sensing layers that act as a probe of the local exciton density, the sensing layers reported here use long-range energy transfer to probe a large area around the sensing layer. We highlight the importance of material selection, such that exciton distribution could be probed without influencing the electrical and optical properties of the organic light-emitting diodes. We expect that this work will provide an invaluable tool for studying excitons in organic light-emitting diodes. PACS Nos.: 78.60.Fi, 71.35.Aa, 42.70.Jk.Résumé : Nous présentons une technique pour sonder le nombre d'excitons dans des diodes électroluminescentes organiques. Notre méthode demande d'introduire stratégiquement des couches senseurs à différentes localisations près de la zone d'émission. Contrairement aux couches senseurs typiques qui jouent le rôle de sonde de la densité locale d'excitons, les couches senseurs dont il est question ici utilisent un transfert d'énergie à longue portée pour sonder une grande surface autour de la couche senseur. Nous soulignons l'importance de la sélection du matériau, afin que la distribution d'excitons puisse être sondée sans influencer les propriétés électriques et optiques des diodes électroluminescentes organiques. Nous sommes confiants que notre travail fournira un outil très utile pour l'étude des excitons dans les diodes électroluminescences organiques. [Traduit par la Rédaction]

Understanding the Structure and Energy Transfer Process of Undoped Ultrathin Emitting Nanolayers Within Interface Exciplexes

“…Three consecutive steps of the energy transfer process of UEMLs is included: 1) pumping electrons in the ground state to electrons in the CT state (exciton generation; T 0 ∼100 fs), 2) Förster and Dexter energy transfer to UEMLs (energy transfer; τ 1 ∼100 ps), and 3) luminescence of UEMLs (relaxation luminescence; τ 2 ∼100 ns), with a distinguishing time scale is described with the energy structure of different organic interfaces in Figure 2 . ( Ingram et al, 2014 ; Menke and Holmes, 2014 ; Gould et al, 1994 ). The energy level diagrams of organic heterojunction interfaces are also exhibited in Figure 2 .…”

“…Long-range coupling of electron-hole pairs in spatially separated electron-donating and electron-accepting molecules as long as 10 nm spacer layers is reported, which is similar to type B exhibited in Figure 1B . ( Ingram et al, 2014 ; Ingram et al, 2016 ; Nakanotani et al, 2016 ). However, why the interface of the exciplex produces these positive results to the UEML and the origin of the undoped UEML within interface exciplexes is still unexplored.…”

Device design for optimal exciton harvesting

“…We placed a layer of C 60 molecules, close to the emissive zone and measured the fraction of excitons captured by this layer [16] . Any excitons captured by the C60 rapidly decay non-radiatively, thus simulating other non-radiative quenching centers that can occur in regular devices.…”

Design principles for highly efficient organic light-emitting diodes

“…This is because the transition to ground state from excited triplet states is not formally allowed since spin is not conserved in the transition. 22 Any excitons captured by C 60 rapidly decay nonradiatively, thus simulating other nonradiative quenching centers that can occur in regular devices. Since the radiative triplet to ground-state transition can occur and does so in many cases at a rate much greater than the nonradiative rate, long-range energy transfer via a Förster mechanism becomes possible.…”

“…22 configurations are depicted in Fig. We have shown that contrary to conventional wisdom in the field, many layers are not necessary to achieve high-efficiency devices, but rather reduce the device efficiency through exciton quenching and loss of charge balance due to charge carriers built up at interfaces.…”