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The efficiency of phosphorescent organic light emitting diodes (OLEDs) shows a decrease with increasing luminance ("roll-off"). One of the contributions to the roll-off is triplet-triplet annihilation (TTA). TTA is the process of energy transfer from one exciton to another, after which the excited exciton decays non-radiatively to the lowest triplet state. There is ongoing debate on the mechanism of TTA [1, 2]: (i) when can TTA be described as a single step Förster-type process? (ii) when does exciton diffusion enhance the TTA rate? (iii) is exciton diffusion the result of Förster or Dexter-type interaction processes? (iv) when does the exciton confinement on the guest molecules become insufficient, so that also diffusion via the matrix can contribute to the TTA rate?
In previous work [2] it was shown that it is possible to determine when exciton diffusion starts playing a role by using a refined analysis method of time-resolved photoluminescence (PL) measurements. This was done for a single system, CBP:Ir(ppy)2(acac) [3, 4]. In this study, we will go beyond this work by studying the role of exciton confinement by systematically varying the host material, for the case of the three phosphorescent guest molecules Ir(ppy)2(acac) (green), Ir(bt)2(acac) (yellow) and Ir(MDQ)2(acac) (red). In total, thirteen systems are included. We find a systematic monotonic decrease of the TTA rate coefficient with increasing confinement energy, which we attribute to diffusion via the host, as long as the confinement energy is smaller than approximately 0.3 eV. For TPD:Ir(ppy)2(acac), e.g., (confinement energy only 0.15 eV), we find an enhancement of the TTA rate of more than a factor of two as compared to the value obtained for a system with excellent confinement, such as TCTA:Ir(ppy)2(acac) (confinement energy 0.45 eV). This enhancement is found to be thermally activated.
For systems with a strong exciton confinement, guest-guest exciton diffusion starts playing role for guest concentrations above 6 mol%. We have employed measurements of the temperature dependence of the TTA rate, which is found to be thermally activated, to study the mechanism of guest-guest exciton diffusion.
The experimental results are analyzed more in-depth using kinetic Monte Carlo simulations, within which all processes are treated in a mechanistic manner. The simulations include the effect of triplet energy disorder. For systems with strong confinement, such as TCTA:Ir(ppy)2(acac), TTA can be described well assuming Förster-type TTA processes with a Förster radius of 3.5 nm. Exciton guest-guest diffusion can be equally well be described using Förster-type processes with a Förster radius of 2.7 nm or Dexter-type processes using a wavefunction decay length of 0.35 nm and rate to the first neighbor molecules of kD,1 ~ 10^9 s-1. These results enable us to more precisely determine the optimal degree of confinement and guest concentration, required in high efficiency OLEDs.
[1] Y. Zhang, S.R. Forrest, Chemical Physics letters, 590 (2013) 106-110
[2] H. van Eersel et al., Journal of Applied Physics, 117 (2015) 1155002
[3] L. Zhang et al., Chemical Physics Letters, 652 (2016) 143-147
[4] L. Zhang et al., Chemical Physics Letters, 662 (2016) 221-227
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