TADF Exciplex OLED Technology
A thermally activated delayed fluorescence (TADF) exciplex is an excited-state species that can exhibit thermally activated delayed fluorescence or transfer its energy to a lower-energy emitter. TADF exciplex is formed between electron-donating and electron-accepting molecules by intermolecular charge transfer. Exciplexes can be applied as emitters and hosts in OLEDs. The wavelength of the emission of TADF exciplexes is not dependent on the band-gap value of a single compound, but the HOMO-LUMO offset between donor and acceptor molecules. As a result there is the possibility to simply produce a wide range of emitters.
Like TADF materials, exciplexes exhibit a prompt and a delayed component in their emission. However, exciplex-based systems can more easily achieve small energy difference between the singlet (S1) and triplet (T1) states (ΔEST), to facilitate reversed intersystem crossing (RISC). This is due to the electrons and holes being positioned on two different molecules. As a result, exciplex-based OLEDs have great potential to maximize the TADF mechanism and achieve theoretical unit internal quantum efficiency (IQE).
TADF Exciplex OLEDs
TADF has emerged as a revolutionary mechanism in the field of organic light-emitting diodes, enabling high-efficiency devices by harnessing both singlet and triplet excitons for light emission. Among the various TADF systems, exciplexes of intermolecular excited-state complexes formed between an electron donor (D) and an electron acceptor (A) have drawn significant attention due to their straightforward design, facile tunability, and potential for efficient electroluminescence.
A typical approach to achieve exciplex luminescence is to mix unipolar electron-donating and electron-accepting molecules or to combine a D−A type bipolar material with A or D molecules. An exciplex system can be not only regarded as the emitting layer to fabricate efficient fluorescent OLEDs, but also serve as a co-host to enhance the performance of emitters.
The common electron donors widely utilized in exciplex systems are mainly commercial materials, such as:
Intramolecular vs Intermolecular TADF
Compared with conventional intramolecular TADF, the intermolecular TADF exciplex has its unique advantages:
- Many exciplex candidates can be easily generated by simply combining suitable commercially available electron and hole transport materials. This helps avoid the tedious synthesis and purification processes needed for the molecular design of D-A molecules needed in intramolecular charge transfer.
- The bipolar nature created by the pair of electron donor and acceptor of the emission layer enhances the charge mobility, since traditional host materials have a problem of poor charge balance.
- Bipolar exciplex hosts composed of electron and hole transport materials can solve this problem of poor charge balance presented by traditional single host and improve the quantum efficiency of the device.
Types of TADF Exciplexes
By device structure and configuration, there are mainly two types of exciplexes, namely bulk TADF exciplex and interface TADF exciplex. The bulk exciplex is mixed with donors and acceptors to form a complex while the interface TADF exciplex is formed at the interface of the donor/acceptor molecules.
Bulk TADF Exciplex
The bulk exciplex, also referred to as a blend emission, is achieved by simply mixing the electron donor and the electron acceptor in a certain ratio for the preparation of an emitting layer. Exciplexes are formed when selective electron rich and deficient compounds with appropriate frontier molecular orbitals (FMO) come close enough for orbital participation, one of them being in the excited state. Bulk exciplexes are useful in OLEDs as emitters or hosts doped with guests, particularly dyes that require efficient and balanced charge transportation:
Exciplex Host
In bulk exciplex hosts, the donor serves as the hole transport material, while the acceptor functions as the electron transport material. Balanced charge transportation can be achieved by selecting donor and acceptor materials with high charge mobilities. Bulk exciplexes are typically prepared with high doping concentrations of donor and acceptor molecules, creating a mixed structure that facilitates exciplex formation. However, the mixing of donors and acceptors significantly increases the energy transfer barriers between molecules, making it challenging for electrons and holes to combine in the recombination region. Additionally, high doping concentrations can lead to exciton quenching, resulting in low external quantum efficiency (EQE) and high turn-on voltage in devices.
Example Bulk Exciplex Systems
| System | Exciplex - Donor:Acceptor | Emission Color | Maximum power efficiency / lm W-1 | Current Efficiency / cd A-1 | External Quantum Efficiency / % |
|---|---|---|---|---|---|
| Bulk TADF exciplex | DEX : PO-T2T | Green | 44.6 | 36.0 | 11.2 |
| PhOLED - bulk TADF exciplex host with Ir(MDQ)2(acac) as the phosphorescent emitter | DEX : PO-T2T | Green | 46.1 | 36.0 | 24.5 |
The outstanding device performance of the DEX:PO-T2T exciplex shows balanced charge transport properties, a broad recombination zone, and a suitable triplet energy level (ET), all indicating that the exciplex could be serving as an ideal host too. Therefore, in combination with the phosphorescent emitter Ir(MDQ)2(acac) external quantum efficiency more than doubled.
More complex exciplex systems have been designed in order to provide a multistep energy transfer channel to reduce efficiency roll-off. Quaternary bulk exciplex complex system can be fabricated with the widely used host material mCP, TADF bule emitters DMAC-DPS and TPXZPO as electron-donating materials, and PO-T2Tas the electron-accepting material. This quaternary bulk exciplex complex system contains three different exciplexes of mCP:PO-T2T exciplex host, DMAC-DPS:PO-T2T and TPXZPO:PO-T2T exciplex dopants. Within the exciplex complex, effective exciton confinement and multiple RISC channels are achieved by the doping of high triplet TADF electron-donating materials with PO-T2T to reduce energy loss and maximise the utilization of excitons. In comparison to the binary and ternary exciplex devices, the quaternary exciplex device exhibited high external quantum efficiency of 16.68% with high PLQY and RISC rate constant of 5.96 × 106 s−1. The internal multistep exciplex to exciplex energy transfer channels also ensure a slight efficient roll-off. The external quantum efficiency remained at 16.60% and 15.74% at a luminance of 1000 cd/m2 and 5000 cd/m2, respectively.
Interface TADF Exciplex
Interface exciplex, on the other hand, is formed by separating the donor layer and the acceptor layer to generate exciplex at the interface. Interface exciplex formed at the interface of the donor and acceptor layers can reduce the exciton quenching and enhance the possibility of radiative transition. Interface exciplex can not only form between the adjacent donor and acceptor, but also between the donor and acceptor inserted by a spacer layer. This donor/spacer/acceptor configuration enlarges the charge transport exciton distribution region and gives convenience to adjust the charge transport exciton radius, which can reduce ΔEST with improved rate of RISC.
Like the bulk exciplex, interface exciplex can be employed as efficient TADF emitters or TADF hosts for dopants with enhanced charge transport properties. Interface exciplex-type OLEDs exhibit many advantages, such as:
- Simplified device architectures
- Lower driving voltages
For interface exciplex, the concentration quenching effect of triplet excitons is almost inevitable especially at high current density due to the narrow exciton distribution area at the interface. Also, the RISC rate is always lower than the ISC rate in the interface exciplex system, which leads to the non-radiative transition processes of triplet excitons. However, an efficient energy transfer process can effectively guard against the waste of interface exciplex triplet excitons.
To serve effectively as an exciplex host the following features are required:
- The T1 of the exciplex formed at the interface should be higher than that of the dopants to prevent reverse energy transfer process.
- The emission spectrum of exciplex should overlap well with the absorption spectrum of the dopant to achieve efficient energy transfer.
Examples Interface TADF Exciplex
| Hole Transport Layer | Spacer / Dopants | Electron Transport Layer | Emission Color | Maximum power efficiency / lm W-1 | Current Efficiency / cd A-1 | External Quantum Efficiency / % |
|---|---|---|---|---|---|---|
| TCTA | 3P-T2T | Yellow | 23.6 | 22.5 | 7.7 | |
| TAPC | 26DCzPPy | PO-T2T | Orange | 76 | 24.0 |
It is a well-established fact that in OLEDs, the recombination process of electrons and holes needs to overcome the potential barrier between carrier transport layers and light-emitting layer. This problem can be overcome if the interface exciplex is used as host so that the electron transport and hole transport layers are in direct contact at the interface:
| Interface Exciplex Host | Dopants | CIE coordinate | CRI | Current Efficiency / cd A-1 | External Quantum Efficiency / % |
|---|---|---|---|---|---|
| mCBP:PO-T2T | SpiroAC-TRZ, Ir(MDQ)2(acac) | (0.35,0.39) | 80 | 52.8 | 22.9 |
Photopysical Properties of TADF Exciplexes
Highly efficient TADF exciplexes exhibit several key photophysical properties with high PLQY, broad emission spectrum, small ∆EST that enables high rate of RISC for low efficiency roll-offs and great device stability with tuneable emission that covers almost the entire visible spectrum from blue to red. TADF exciplexes also show short lifetimes (in the microsecond range) to minimize triplet-triplet annihilation and exciton quenching, enhancing operational stability.
- Narrow ∆EST: The charge transfer nature of the exciplex state inherently results in a small ∆EST, which enables efficient intersystem crossing (ISC) and reverse intersystem crossing (RISC) process. Compared to the conventional host/guest strategies, bulk exciplex presents many advantages of bipolarity, small ∆EST, and low driving voltage. Interfacial exciplex also shows several advantages of barrier-free charge injection, unimpeded charge transport, and the energy-saving direct exciton formation at the interface. Due to the fast and efficient reverse intersystem-crossing (RISC) process gained from the narrow ∆EST, exciplex host is capable of regulating singlet/triplet exciton populations as well as in the dopant emitters to further enhance device efficiency and stability.
| Exciplex - Donor:Acceptor | Dopant | CIE Coordinates | External Quantum Efficiency / % |
|---|---|---|---|
| BPhCz:SFX-PIM-TRZ | BN-TP | (0.27, 0.69) | 31% at 1000 cd m−2 |
- Broad Emission Spectrum: Exciplex emission is a rather unique form of luminescence from an excited state complex formed between two or multiple donor/acceptor materials in contact in OLEDs. Such emissions are typically of longer wavelengths, boarder spectral widths and longer decay times comparing with those of the two materials involved. Exciplex emission is typically broad due to the CT state’s diffuse electronic nature. As the fraction of CT in the exciplex increase, the photoluminescence (PL) spectra of exciplexes are normally redshifted, broad, and featureless without the vibrational progression comparing to the PL spectra of the constituent molecules. Moreover, featureless broad emission spectra of the exciplexes have promoted the introduction of the exciplex emission into WOLEDs for good color quality.
| Exciplex - Donor:Acceptor | Emission Color | CRI | Maximum Power Efficiency / lm W-1 | Current Efficiency / cd A-1 | External Quantum Efficiency / % |
|---|---|---|---|---|---|
| mCBP : PO-T2T | White | 85 | 46.0 | 36.4 | 21.1 |
- Tuneable Emission: By varying the donor-acceptor combination, host matrix, the thickness of the spacer between the interface, exciplexes can emit across the visible spectrum, from blue to red. For example, while OLEDs based on POZ-DBPHZ exhibit orange-yellow light emission, interface exciplex formed by POZ-DBPHZ and m-MTDATA showed deep-red emission of 741 nm. By employing a deep blue emitter TAPC and m-MTDATA, green and orange-red exciplex-based organic light-emitting diodes (OLEDs) can be realized. OLEDs based on the exciplexes show emission peaks located at 524 nm and 596 nm and broader FWHM reaching 102 nm and 117 nm, respectively. The wide FWHMs are beneficial for developing high-color-quality white OLEDs. Also, by adopting OCT as both a blue emitter and exciplex acceptor, three-color white OLEDs fabricated by simultaneously combining the blue intrinsic emission with the green and orange-red exciplex emissions demonstrate very good white emission with a maximum color rendering index of 97.
Conclusion
Highly efficient TADF exciplexes represent a transformative advancement in organic optoelectronics. The emission wavelength of an exciplex-based system is purely dependent on the HOMO−LUMO offset between donor and acceptor counterparts rather than on the band gaps of its individual compounds, it presents a great opportunity to develop a wide range of exciplex emitters or host systems and the electroluminescence of an exciplex-based OLED can be notably boosted by the astute selection of donor and acceptor materials. By leveraging their unique charge transfer characteristics and tuneable emission properties, researchers have unlocked new possibilities for high-performance OLEDs, sensors, and energy devices. Continued innovation in material design, device engineering, and fundamental understanding of exciplex behaviour will pave the way for broader adoption and further breakthroughs, solidifying TADF exciplexes as a cornerstone of modern photonic technologies.
TADF Materials
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What is Intramolecular Charge Transfer (ICT)?
Intramolecular charge transfer (ICT) refers to the transfer of charge within a single molecule (intra = “within” in Latin). In molecules containing one or more electron donor and acceptor groups, ICT can occur if the molecule is in an excited state.
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Exciplex and Excimer Absorption and Emission
An exciplex (or excited complex) is a complex formed from two different molecules, where one molecule is in an excited state (donor) and the other is in the ground state (acceptor). Both are typically conjugated semiconducting molecules. This short-lived complex will occur when the molecules are in close proximity to one another, and it results in a lower energy than if the two existed separately in their excited and ground states.
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- J. Gu et al. (2022); Intermolecular TADF: bulk and interface exciplexes, J. Mater. Chem. C, 10, 4521-4532; DOI: 10.1039/D1TC04950J.
- J. Guo et al. (2021); Recent progress on organic exciplex materials with different donor–acceptor contacting modes for luminescent applications, J. Mater. Chem. C, 9, 16843-16858; DOI: 10.1039/D1TC04330G.
- Shao et al. (2022); Recent Advances of Interface Exciplex in Organic Light-Emitting Diodes, Micromachines, 13(2), 298; DOI: 10.3390/mi13020298.