Elsevier

Thin Solid Films

Volume 660, 30 August 2018, Pages 166-170
Thin Solid Films

Temperature dependence of the driving properties for a green thermally activated delayed fluorescence device with a mixed host

https://doi.org/10.1016/j.tsf.2018.05.053Get rights and content

Highlights

  • Current efficiency do not significantly change despite exciton transfer activated by heat.

  • Rise time depends on mobility and ratio of reverse intersystem crossing as temperature.

  • The decay is mainly affected by the ratio of reverse intersystem crossing.

Abstract

The temperature dependence of organic light-emitting diodes (OLEDs) with green thermally activated delayed fluorescence (TADF) was investigated in this study. The temperature-dependent driving properties of an OLED device based on TADF were analyzed and its transient electroluminescence characteristics at various temperatures were investigated. TADF materials feature an additional mechanism called reverse intersystem crossing (RISC) that is forbidden in conventional fluorescence. The ratio of RISC is known to be proportional to temperature. We found that the luminance of the device with a TADF material was dependent on the temperature when under fixed voltage but not when under a fixed current density. In addition, the spectrum, luminance, and power efficiency were slightly shifted as the temperature was varied from −20 °C to 50 °C. Moreover, with increasing temperature and the use of a mixed host, the rising- and decay-time properties were improved. Lastly, the dependence of device performance on the host ratio was analyzed. The results revealed that temperature dependence of rising time was due to influence of increase in both mobility and ratio of RISC, while that of decay time was mainly attributed to ratio of RISC.

Introduction

With advancements in organic light-emitting diode (OLED) materials, next-generation dopant materials have also been developed. The first-generation dopants were fluorescent materials with a conventional luminescence mechanism using only singlet-state exciton [1, 2]. Because fluorescence materials are limited to singlet-state excitons, their internal quantum efficiency (IQE) is limited to a maximum of 25%. The study of OLED materials led to the development of new materials that utilize triplet-state excitons. These materials feature an additional luminescence mechanism in which spin–orbit coupling (SOC) facilitates the conversion from a singlet state into triplet states via intersystem crossing. These second-generation dopants are referred to as phosphorescent materials [3, 4]. IQEs that approach 100% can be achieved with devices based on phosphorescent materials; however, expensive heavy metals such as platinum or iridium are required to induce SOC.

The fluorescence mechanism for third-generation dopant materials is known as thermally activated delayed fluorescence (TADF). These materials are used to enhance device properties such as external quantum efficiency (EQE) without using expensive metals. TADF provides an additional mechanism for the conversion from triplet-state to singlet-state excitons via reverse intersystem crossing (RISC). The RISC mechanism is enabled via the design of molecules with a narrow energy difference between their singlet and triplet states, denoted by ΔEst. Adachi et al. reported that the EQE of a device with TADF can reach that of a device with phosphorescence; this finding spurred the interest of many researchers who have since studied OLED materials to enhance the properties of OLEDs without using heavy metals. Thus far, the highest reported EQEs of OLEDs with a TADF material are greater than 30% [[5], [6], [7], [8], [9]].

Time-resolved electroluminescence (EL) measurements represent a powerful tool for investigating the behavior of charge carriers in emissive layer (EML). For instance, the charge-carrier mobility, exciton lifetime, and the recombination process can be investigated [[10], [11], [12], [13], [14], [15], [16]]. The driving properties of devices with a TADF material are reported herein. The temperature dependence of the performance of the TADF-based devices was analyzed by measuring their brightness, EL spectrum, and transient EL characteristics as varying temperature. Furthermore, the mixed-host efficacy of the devices was analyzed, wherein the effect of the host ratio on the temperature dependence of their performance was studied.

Section snippets

Experimental detail

Fig. 1 schematizes the energy-band diagram of the OLEDs. Organic layers were deposited using a thermal evaporator. 1,3-Bis(N-carbazolyl)benzene (mCP) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) were used as p- and n-type host materials, respectively, and a green TADF material, 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), was used as the dopant. We varied the mCP:TPBi host ratio from 2:1 to 1:1 to 1:2 to compare among the devices with different host

Results and discussion

We first analyzed the temperature dependence of the devices with 4CzIPN; these devices are based on a TADF system. The temperature dependence of the TADF-based material is represented as kRISC ~ exp(–ΔEst/kT) [17] where kRISC is the ratio of RISC, k is the Boltzmann constant, T is temperature, and ΔEst is the energy difference between the singlet and triplet states. It shows that kRISC is proportional to temperature. With increasing temperature, the brightness of the device might increase in

Conclusion

We investigated the temperature dependence of OLEDs based on TADF as a dopant material. Results indicated that the current efficiency did not change significantly upon changes in the temperature; the power efficiency should also be considered when designing TADF-based OLEDs because it can lead to an increase in voltage. It might come from poor charge moving characteristics at low temperature. Moreover, we found that the FWHM in the EL spectrum of the device with the TADF material slightly

Acknowledgements

This research was supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under the Industrial Technology Innovation Program (No. 10048317).

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