Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters

Abstract

Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔEST ) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with ΔEST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.

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

“…The 1 CT– 3 CT gap in PSBF can be estimated from the previous results of King et al, who observed a temperature activation energy for DF in PSBF films of ≈14 meV 36. This was attributed to the gap between the 1 CT state and the 1 (π, π*) S 1 state, however, this is too small for the gap measured spectroscopically, and in light of our recent findings on TADF, we can now understand this data as being a measure of the exchange energy in the CT manifold 29. The small exchange energy is comparable with other CT systems where there is near orthogonally between HOMO and LUMO level and is the case for the PSBF where we have previously shown that the charge transfer occurs across the orthogonal spiro unit, mediate by spiro conjugation and is in line with the results of Mehes et al on spiro charge transfer molecules 28.…”

“…Thus, the T N decay channel via the 3 CT state can be an intrinsic mechanism for harvesting triplet states into singlets 29. Also, many CT small molecules which give mixed TADF and TTA, when the lowest lying 3 LE state lies below the CT manifold still give good OLED performance.…”

Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

“…Material synthesis has been reported previously [8,16]. Solutions (10 −5 -10 −4 M) of all materials were degassed using five freeze-thaw cycles.…”

“…The TADF mechanism uses the thermal energy to assist reverse intersystem crossing and promote the up-conversion of lower triplet states into more energetic but emissive singlet states [8]. The efficiency of the TADF mechanism is controlled by two main parameters, the energy splitting between the singlet and triplet states ( E ST ) and the efficiency of non-radiative pathways available for the excited singlet and triplet states to decay [9,10].…”

“…In this case, the yield of singlet emissive states is highly dependent on the relative energy order of the excited singlet and triplet energy levels in the molecule, and the maximum total singlet yield is limited to 62.5%. OLEDs with TTA contribution as large as 40% have been demonstrated but the relative merit of this approach is still unsatisfactory [6,7].The TADF mechanism uses the thermal energy to assist reverse intersystem crossing and promote the up-conversion of lower triplet states into more energetic but emissive singlet states [8]. The efficiency of the TADF mechanism is controlled by two main parameters, the energy splitting between the singlet and triplet states ( E ST ) and the efficiency of non-radiative pathways available for the excited singlet and triplet states to decay [9,10].…”