Abstract
Spin-dependent nonlinear processes in organic materials such as singlet-fission and triplet-triplet annihilation could increase the performance for photovoltaics, detectors, and light emitting diodes. Rubrene/C60 light emitting diodes exhibit a distinct low voltage (half-bandgap) threshold for emission. Two origins for the low voltage turn-on have been proposed: (i) Auger assisted energy up-conversion, and (ii) triplet-triplet annihilation. We test these proposals by systematically altering the rubrene/C60 interface kinetics by introducing thin interlayers. Quantitative analysis of the unmodified rubrene/C60 device suggests that higher order processes can be ruled out as the origin of the sub-bandgap turn-on. Rather, band-to-band recombination is the most likely radiative recombination process. However, insertion of a bathocuproine layer yields a 3-fold increase in luminance compared to the unmodified device. This indicates that suppression of parasitic interface processes by judicious modification of the interface allows a triplet-triplet annihilation channel to be observed.
Highlights
Spin-dependent nonlinear processes in organic materials such as singlet-fission and triplettriplet annihilation could increase the performance for photovoltaics, detectors, and light emitting diodes
The active layers were evaporated onto Glass/ITO/ PEDOT:PSS:Nafion substrates, with PEDOT:PSS:Nafion acting as the hole injection layer (HIL), and capped by BCP/Al acting as electron injection layer
Bandgap will refer to the Highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) separation, while optical gap will refer to the absorption onset
Summary
Spin-dependent nonlinear processes in organic materials such as singlet-fission and triplettriplet annihilation could increase the performance for photovoltaics, detectors, and light emitting diodes. One of the most commonly studied devices uses the small-molecule rubrene as emitter and hole transporter and the fullerene C60 as the electron transporter In these devices, two distinct mechanisms have been proposed for the low-voltage EL: an Auger-assisted energy up-conversion process at the heterojunction interface[6,7], or Dexter transfer of triplet charge transfer (CT) states into triplet exciton states, followed by TTA to produce an emitting singlet[8,9,10]. Two distinct mechanisms have been proposed for the low-voltage EL: an Auger-assisted energy up-conversion process at the heterojunction interface[6,7], or Dexter transfer of triplet charge transfer (CT) states into triplet exciton states, followed by TTA to produce an emitting singlet[8,9,10] In both cases, the non-linear charge dynamics at the rubrene/C60 interface are crucial to the mechanism. When parasitic interface pathways are suppressed, evidence for a TTA process is observed
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