Generating Light from Upper Excited Triplet States: A Contribution to the Indirect Singlet Yield of a Polymer OLED, Helping to Exceed the 25% Singlet Exciton Limit.

Vygintas Jankus,Murat Aydemir,Andrew P Monkman,Fernando B Dias

doi:10.1002/advs.201500221

Vygintas Jankus, Murat Aydemir + Show 2 more

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https://doi.org/10.1002/advs.201500221

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Abstract

The mechanisms by which light is generated in an organic light emitting diode have slowly been elucidated over the last ten years. The role of triplet annihilation has demonstrated how the “spin statistical limit” can be surpassed, but it cannot account for all light produced in the most efficient devices. Here, a further mechanism is demonstrated by which upper excited triplet states can also contribute to indirect singlet production and delayed fluorescence. Since in a device the population of these TN states is large, this indirect radiative decay channel can contribute a sizeable fraction of the total emission measured from a device. The role of intra‐ and interchain charge transfer states is critical in underpinning this mechanism.

Highlights

From the outset of research into organic light emitting diodes (OLEDs) it has been assumed that the process of charge recombination which generates excitons is controlled by random spin statistics with singlet and triplet excitons formed in the ratio 1:3
We obtain a lower limit of φDF = 0.33 for total delayed fluorescence, which is clearly much greater than the “classical” triplet annihilation (TTA) singlet triplet fusion (TF) yield of 0.055 and above the expected 0.2 yield given that in the case of PSBF, only the quintuplet TTA channel is energetically unattainable
These results indicate that a further process must contribute to the efficiency of delayed fluorescence, which we identify as induced thermally activated delayed fluorescence (TADF) via decay of TN states to the 3CT state in competition with decay to the 3(π, π*) state

Summary

Introduction

From the outset of research into organic light emitting diodes (OLEDs) it has been assumed that the process of charge recombination which generates excitons is controlled by random spin statistics with singlet and triplet excitons formed in the ratio 1:3. The initial recombination process still obeys spin statistics whilst the total singlet production yield can exceed 25%. TTA is key to our understanding of triplet exciton dynamics in organic materials and essential for a proper understanding of the use of organic materials in many applications.

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