Influence of spin relaxation induced by molecular vibration on thermally activated delayed fluorescence

doi:10.1016/j.orgel.2017.12.036

Organic Electronics

Volume 54, March 2018, Pages 161-166

https://doi.org/10.1016/j.orgel.2017.12.036 Get rights and content

Highlights

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The model of the influence of spin relaxation caused by molecular vibration on TADF is proposed.
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The transition rate from triplet state to singlet state has two different temperature dependence.
•
The experimental phenomena could be well explained when considering spin relaxation induced by molecular vibration.
•
This process helps us to understand TADF more comprehensively.

Abstract

Thermally activated delayed fluorescence (TADF), an effective mechanism to break the 25% statistic limit of organic light-emitting diodes (OLEDs) internal quantum efficiency, has become an active topic recently. The key to germinate TADF is the achievement of efficient reverse intersystem crossing from triplet spin state to singlet state by thermal activation, which is obviously a temperature dependence process. The direct way of thermal activation is the absorption of phonon energy, in which the transition rate from triplet state to singlet state has the Boltzmann distribution function dependence of the temperature. Nevertheless, the molecular vibration could engender spin relaxation of excitons, giving rise to different temperature dependence. This could be regarded as an indirect way of thermal activation. Here, we investigate the effect of spin relaxation caused by molecular vibration on TADF and analyze the change principles of the efficiency of TADF processes versus temperature. It is found that the experimental dependence could be well explained when the spin relaxation induced by molecular vibration is considered. Therefore, the consideration of this process helps us to understand TADF more comprehensively.

Graphical abstract

Introduction

Since organic light-emitting diodes (OLEDs) were constructed by Tang and VanSlyke [1] in the 1980s, these devices have aroused widespread interest and attention. Due to the unique properties of organic materials, OLEDs have great advantages and huge development space in the application of novel electronic devices [2], [3], [4], [5]. According to the spin statistics of electron-hole pairs (excitons), the injected electrons and holes couple to form 25% singlet excitons and 75% triplet excitons in the organic light-emitting layer. The spin-forbidden of triplet excitons leads to the highest internal quantum efficiency (IQE) of only 25%. To improve OLED efficiency, it is essential to efficiently utilize the left triplet excitons for luminescence. One successful method is to use phosphorescent materials, such as Iridium (Ir) and Platinum (Pt) complexes [6], [7]. In this way, one can obtain light emission from both triplet excited state and singlet excited state by increasing spin-orbit coupling so that the IQE of the devices achieves approximately 100%. However, phosphorescent materials generally use heavy metals so they are expensive and sometimes not environmental friendly. Moreover, the materials present instability problem in practical device application, particularly for the blue light [8]. Another promising attempt to break the 25% limit is to use thermally activated delayed fluorescence (TADF) [9], [10], [11], [12], [13], [14], [15] materials, which has been given stable fluorescence and high-efficiency.

In a TADF process, excitons in the low-energy triplet excited state (T₁) can be up-converted into the high-energy singlet excited state (S₁) by thermal activation and then these singlet excitons transfer to the ground state (S₀), which is accompanied by delayed fluorescence emission [16], [17], [18]. A direct way of thermal activation is the absorption of phonons, complementing the energy gap between the singlet and the triplet state ( $Δ E_{S T}$ ). Its temperature dependence is in the form of Boltzmann distribution and this process is regarded as the reverse intersystem crossing (RISC). On the other hand, temperature induces the vibration of molecules, resulting in the fluctuations of the exchange interaction of the excitons, which gives rise to spin relaxation [19]. This process engenders different temperature dependence, which can be considered as an indirect way of thermal activation. However, all present understandings on TADF are from the direct way of thermal activation, which has the Boltzmann distribution function of temperature dependence [20], [21], [22], [23], while the indirect way of temperature due to the molecular vibration is neglected.

In this work, we suggest a model and investigate the effect of spin relaxation caused by molecular vibration on TADF. The temperature dependence of the efficiency with spin relaxation is calculated and the result is compared with present one without spin relaxation.

Section snippets

Model

The principle of TADF is schematically shown in Fig. 1. There are two different mechanisms in the process of TADF, which are the prompt fluorescence (PF) and the delayed fluorescence (DF). In the prompt fluorescence mechanism, excitons in the S₁ directly transmit to the S₀ by radiation ( $k_{r}^{S}$ ). Whereas in the delayed fluorescence mechanism, excitons in the T₁ have to be up-converted into the S₁ and then decay into the S₀, which emits delayed fluorescence. There are two channels for the transition

The value of parameters

To give a quantitative result, we take parameters $k_{r}^{S}$ , $k_{n r}^{S}$ , $k_{n r}^{T}$ , $k_{I S C}$ and $k_{R I S C}$ referring reference [25]. Then using least-squares method fitting we obtain $k_{n r 0}^{S} = 8.35 \times 10^{2} s^{- 1}$ , $k_{n r 1}^{S} = 9.84 \times 10^{7} s^{- 1}$ , $Δ E_{Q 1} = 0.16 eV$ . According to the experimental data about $k_{n r}^{T}$ at various temperatures, $k_{n r 0}^{T}$ , $k_{n r 1}^{T}$ and $Δ E_{Q 2}$ are calculated as: $k_{n r 0}^{T} = 8.01 \times 10^{2} s^{- 1}$ , $k_{n r 1}^{T} = 1.78 \times 10^{7} s^{- 1}$ , $Δ E_{Q 2} = 0.17 eV$ . Using the same method, $k_{R I S C 1}$ and $Δ E_{S T}$ can be obtained as: $k_{R I S C 1} = 8.85 \times 10^{4} s^{- 1}$ , $Δ E_{S T} = 0.0085 eV$ . In the experiment, the authors

Conclusion

In conclusion, we have investigated the effect of spin relaxation induced by molecular vibration on TADF. The relationship between the temperature with the efficiency of TADF was investigated. Our results show that the efficiency of PF, DF, RISC and Internal quantum efficiency fit better with those of experimental results when considering the spin relaxation process caused by molecular vibration, which indicates that the process should be considered in TADF. Due to the existence of this

Acknowledgements

This work is supported by the National Natural Science Foundation of the People's Republic of China (Grant No. 11574180).

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View full text

Influence of spin relaxation induced by molecular vibration on thermally activated delayed fluorescence

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Model

The value of parameters

Conclusion

Acknowledgements

J. Lumin.

Org. Electron.

Organic electroluminescent diodes

Appl. Phys. Lett.

Management of singlet and triplet excitons for efficient white organic light-emitting devices

Nature

White organic light-emitting diodes with fluorescent tube efficiency

Nature

Low-power flexible organic light-emitting diode display device

Adv. Mater.

Intrinsically stretchable polymer light-emitting devices using carbon nanotube-polymer composite electrodes

Adv. Mater.

Nearly 100% internal phosphorescence efficiency in an organic light-emitting device

J. Appl. Phys.

Highly efficient phosphorescent emission from organic electroluminescent devices

Nature

Management of singlet and triplet excitons for efficient white organic light-emitting devices

Nature

Imidazole-based excited-state intramolecular proton-transfer(ESIPT) materials: observation of thermally activated delayed fluorescence(TDF)

J. Phys. Chem.

Thermally activated delayed fluorescence from Sn4+-Porphyrin complexes and their application to organic light-emitting diodes - a novel mechanism for electroluminescence

Adv. Mater.

Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes

Appl. Phys. Lett.