Organic Light Emitting Diodes Advanced Materials for Light Emitting Devices

Schematic representation of the three generations of OLEDs developed to date.

Since the first demonstration of the operation of two-layer organic light emitting diodes (OLEDs), considerable improvements have been made, in particular in the material design. Initial developments were based on fluorescent OLEDs that use the emission of singlet excited states. Much attention has also been paid to the two metal-semiconductor interfaces of OLED devices  to improve the injection of holes and electrons, as well as to the optimization of the nature of dye-containing emitter layer (EML) blends. However, the first generation of OLEDs is limited by low internal quantum efficiencies (IQE) of maximum ca.25%, since upon electrical excitation of the device, spin statistics only allow for the formation of 25% of singlet excited states while 75% of the excited states are triplets.

These limitations have been successfully overcome with the second generation of OLEDs that uses phosphorescent materials for the blend of the EML. Phosphorescence is essentially obtained by using heavy metal-based emitters (such as ruthenium, iridium, platinum…) in which the metal induces a strong spin-orbit coupling (SOC) draining quantitatively all singlet excited states to the triplet excited states that are able to emit light. With this technology, an IQE of 100% can be reached. However, due to the shortage and high cost of such metals, this strategy is not viable for large scale developments.

Later on, molecules exhibiting the now well-known delayed fluorescence were introduced into devices. In this process, the triplet excited states inherently formed electrically are converted into singlet excited states through reverse intersystem crossing (RISC), yielding an IQE of 100%. For this process (commonly called Thermally Activated Delayed Fluorescence, or TADF), it has been demonstrated that a close proximity of the singlet and triplet excited states (small DES-T, the energy gap between the lowest singlet and triplet excited states) and a large SOC results in an efficient RISC. In the TADF molecules, a small DES-T is obtained by twisting the donor group from the acceptor group resulting in a small electron exchange energy (KHL) which is directly related to the energy gap between the lowest singlet and triplet excited states (DES-T = 2 KHL). In order to optimize the SOC, the singlet and triplet excited states involved in the RISC have to be defined with two different wave-functions. Following this approach, a large number of dyes (spanning from the blue to the red) have been designed and show efficient OLED’s operating properties.

Only recently, multi-resonance (MR) and vibrations induced SOC-TADF dyes have been developed and are subject to an extended amount of research. While in MR TADF, the mechanism is strongly related to the small DES-T, vibrations induced SOC-TADF are more related to the large value of the spin-orbit coupling and therefore to the control of the nature of the singlet and triplet excited states.

Within StelOrg, different research teams are interested in the design, synthesis and the study of  photophysical properties of the EML layer following different strategies, as described thereafter.

STELORG's activities in the field of Organic Light Emitting Diodes

Development of emitting TADF dyes

A. Design and Synthesis of vibration induced SOC-TADF

As explained above, the TADF mechanism turns to be not much dependent on the DES-T. While the theoretical demonstration of the influence of vibration induced SOC-TADF has been published in 2015, we have shown experimentally using curcuminoid borondifluoride-type of emitters, that NIR emitters could give external quantum efficiency as high as 10% in an OLED device. Following this work, the aim is now to have an in-depth understanding of the vibration induced SOC-TADF mechanism. To do so, synthesis of new curcuminoid borondifluorides is performed to study the influence of the DES-T on the RISC efficiency. The nature of the excited state is also playing a role on the presence/efficiency of the TADF phenomenon. We therefore aim at tuning the donor properties of such dye. The objective of this project is to understand how SOC-TADF emitters with large DES-T can show efficient RISC.

Such work will help to rationalize the design of new TADF emitters leading to 25% EQE in the NIR region. 

B. Design of TADF liquid dye

The emission properties of dyes are naturally strongly dependent on intermolecular interactions. It is possible to control these interactions through a convenient functionalization of the dyes by flexible chains, which can even lead to the formation of liquid emitters at room temperature. For some years, the literature has reported a number of liquid emitters, all based on classical fluorescent molecules. The liquid state at room temperature is currently obtained through their functionalization by branched alkyl chains.

Within StelOrg, we are developing liquid emitters. The first goal of our work is the stabilization of the liquid state by functionalizing the molecules using short siloxane chains rather than alkyl chains. We have established the remarkable efficiency of siloxane chains to confer large conjugated systems with a liquid state. For instance, we have reported a series of liquid oligofluorene showing high fluorescence in their room temperature liquid state, which further led to the first demonstration of a monolithic liquid exhibiting distributed feedback laser. The second goal of our work is the synthesis of liquid emitters showing TADF properties. These materials pave the way to the development of liquid (opto)electronic devices.

Development of new hosts for the TADF dyes in blend

In OLEDs, the nature of the host is as crucial as that of the emitter. As a rough guideline, the host has to possess a lower HOMO and a higher LUMO in comparison to those of the TADF guest. Also, the singlet excited state has to be well matched to achieve efficient Förster Resonance Energy Transfer (FRET), while the triplet excited state must be higher in energy than that of the TADF molecule one to avoid quenching of the TADF triplet and therefore disappearance of the RISC. Furthermore, the host has to be fully disorganized and amorphous in nature to allow a good dispersion and avoid emission quenching issues.

Polymeric host

PFO (Polydioctylfluorene) and F8BT (Poly(9,9-dioctylfluorene-alt-benzothiadiazole) are semiconductor hosts. While PFO is suitable for green emitters, F8BT can be used for red or near infrared (NIR) emitters. However, the F8BT triplet excited state lies low in energy inducing a quenching of the TADF of most NIR emitters. This is clearly detrimental to the external quantum efficiency of the system, “transforming” systems that belonged to the third generation of OLEDs into compounds (when using small molecular host such as CBP (4,4’-Bis(N-carbazolyl)-1,1’-biphenyl)) from the first generation of OLEDs. Despite this detrimental aspect with F8BT, the large roll-off observed in CBP is well-suppressed because the polymer plays the role of triplet excited state scavenger.

In this regard, we are designing and synthesizing new polymeric hosts with well-tuned HOMO, LUMO, singlet and triplet excited state energies. The targeted properties of such polymers also implies the absence of crystallinity allowing a better stability in time, the possibility to align/organize them (using brushing, shearing…) and the adjustment of their compatibility to dyes with different emission colors.

Liquid host

Motivated by the development of highly stretchable and deformable OLEDs, we seek to design semiconductor hosts of ultimate flexibility: i.e. liquid hosts at room temperature. These materials are solvent-free conjugated molecular liquids. The liquid state at room temperature is achieved by functionalizing the molecules with short flexible siloxane chains. This procedure has proven to be powerful to make liquid bulky conjugated systems such as oligofluorenes and various arylamines. Despite their liquid character, these hosts were found to exhibit good charge transport properties, with mobilities that compete with the ones of the best molecular glasses. We showed that this result originated to the strong microsegregating character of the siloxane chains that lead to the formation of local scale organization. Based on these results, our objective is to synthesize new liquid host and liquid TADF emitters in order to develop efficient OLEDs based on liquid photoactive layer.