15.1.1 The Emitter Materials: Heart of the OLED

Herein, we will focus on the emitting materials (see Table 15.1 and Figure 15.1), which are responsible for transforming excitons into visible light. The excitons formed in the device by recombination of positive and negative charge carriers are divided into singlet (25%) and triplet (75%) excitons due to spin statistics. In order to fabricate efficient OLEDs, emitter molecules that are able to transform both kinds of excitons into light are preferred.

Looking at the emitter materials in more detail, three main radiative decay mechanisms are used to transform electrical energy into light: fluorescence (FLUO), phosphorescence (PHOS), and TADF. In the first OLEDs, fluorescent materials were used [5]. Around 1998, it was found that phosphorescent materials could also be applied in OLEDs, which led to a drastic increase in efficiency [6, 7]. Recently, emitters based on TADF were also shown to be suitable for very efficient OLED devices [8–12]. For a more detailed description of the technical background of TADF, we refer the reader to the other chapters in this book.

A potential TADF emitter material for industrial OLED applications has to satisfy several criteria:

1. Performance: OLEDs are ultimately manufactured as display components in devices such as TVs and smartphones; further applications are lighting units for general lighting application and for the automotive industry (taillights). For both fields, the key performance indicators, device efficiency, color point of the device, and its operational stability, are critical. While the assessment of a material's potential is possible based on photoluminescence quantum efficiency (PLQY), a high PLQY is ultimately irrelevant when the proven performance in a device is lower and cannot be raised.
2. Compatibility: Apart from providing the expected performance, a TADF material also needs to be compatible with the basic OLED layout. If the reduction or oxidation potential is far away from the other materials, it cannot be implemented in a working OLED architecture (compare with Figure 15.2). The energy levels (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy) have to match the other materials in the OLED stack in order to facilitate barrier‐free charge transport, a low operating voltage, high brightness, and good stability. Another compatibility aspect, which is discussed in Section , is processing. At this point, vacuum processing still dominates industrial OLED manufacturing, meaning that nonsublimable compounds are likely not to be suitable for implementation in a commercial product in the foreseeable future.
3. Availability. To be implemented in a commercial product, TADF materials need to be available in a sufficient amount. This is somewhat connected to the number of synthetic steps and the complexity of the molecular structure, but there are no fixed guidelines since (looking at many commercial specialty chemicals and pharmaceuticals) optimization of synthetic procedures often opens straightforward routes to seemingly incredible complex structures, while relatively simple‐looking structures need to be sorted out because they cannot be made in large quantities. Availability can also encompass considerations of intellectual property and having a so‐called freedom to operate (FTO), which goes beyond the scope of this chapter [13]. What is partly connected to availability of a material is its sustainability, an aspect we previously discussed in detail [2] and will also be covered in Section 1.3.

The drastic impact of using different emitter materials on commercial products can be seen in Figure 15.3, where selected key performance indicators are plotted for various generations of Samsung Galaxy smartphones. The initial commercialization of OLED displays was possible only after the development of phosphorescent red emitters with suitable stability and efficiency [6]. Consequently, the first generation, the S1, featured fluorescent materials for green and blue pixels, while red pixels contained phosphorescent materials. After the introduction of green phosphorescent materials, the average power consumption significantly dropped from 2.5 to well below 1 W. On the other hand, the display's resolution was doubled to roughly 400 pixels per inch. Using the more efficient phosphorescent green materials allowed the display manufacturers to significantly decrease the size of green pixels, which allowed for an increase of the resolution and an improved customer experience.

15.1.2 Processing Aspects

The large majority of the OLED products available on the market are based on small molecules deposited via physical vapor deposition. The current deposition technique of choice for industrial OLED production is vacuum thermal evaporation (VTE). The organic molecules or metals are heated in a vacuum chamber with pressures below 5 × 10⁻⁵ mbar; they evaporate and then condense on a cooled substrate. The temperature needed for sublimation is most typically up to 300 °C for organics and between 500 and 1000 °C for metals. The patterning of the layers is generally done with a shadow mask.

These techniques allow for the fabrication of multilayer OLEDs with a good control of the layer thicknesses [3]. It is also convenient to employ vacuum equipment that is already used in the semiconductor industry. Moreover, the purity of the films reflects the purity of the materials; therefore pure and stable materials lead to long‐lived OLEDs. However, it is difficult to obtain homogenous layers on very large substrates, and it is very “material consuming” since the vast majority of the material end up on the walls of the vacuum chamber rather than on the substrate itself (see Figure 15.4).

The large waste of material during vacuum deposition prompted academic and industrial researchers to develop solution processing methodologies to allow for OLEDs fabricated by printing and coating techniques. The advantages of this concept would be evident: They could be performed at atmospheric pressure, are better suited to large‐area devices, and are much less material consuming. With inkjet printing, for example, one can deposit a droplet exactly where it is needed instead of depositing material everywhere and using a shadow mask. Both coating and printing are suitable for large‐scale fabrication and incorporation into roll‐to‐roll fabrication lines [14]. Several challenges need to be addressed prior to reaching market readiness for this approach:

1. Blending of the functional layers during or after deposition [15, 16], a phenomenon also called interlayer diffusion, may occur when a solvent used for the deposition of further layers is able to dissolve already deposited layers. This issue can be prevented with orthogonal solvent strategies, i.e. using a solvent that is not able to dissolve previously cast layers. This strategy requires tuning of the material properties to achieve sufficient solubility in the desired solvent while preventing solvation in other solvents. Often, the hole‐transport layer is processed by this approach.
2. Morphology and process parameter impact on device properties, which is more challenging for solution processing [17, 18]. Morphological defects like crystalline grains in functional layers may act as charge traps [19], while aggregation causes emission quenching, which decreases the efficiency and may also affect the lifetime of the devices [11]. Such morphology defects can be avoided by using materials with a low crystallization tendency, which corresponds to a low lattice energy and a good solubility, or by immobilizing the relevant molecules, e.g. by attaching them to a polymeric backbone [20–27].
3. Potential necessity of crosslinking. Both morphological problems and troubles related to blending of layers during processing may be addressed with crosslinking [16, 28–35] of each layer after deposition, which prevents the dissolving of the layer during or after processing and stabilizes it against diffusion during device operation. Known approaches cover photochemical [36, 37], chemical [31, 35], thermal [32], and electrochemical techniques [33, 34]. Among various thermal reactions, simple click‐chemistry‐based processes are of particular interest [11, 21, 38–40]. A covalently anchored emitter prevents interlayer diffusion and promises to enhance the device lifetime drastically [30, 31].

From an industrial perspective, it is unclear if and when solution processing methodologies will be mature enough to be considered for the industrial manufacturing of OLEDs in general. Comparing solution processing with vacuum processing, in most cases both efficiency and device lifetime are lower, while the turn‐on voltage is higher for the former technique, even when identical materials are used.

15.1.3 Sustainability Aspects

Looking at potential supply bottlenecks as a result of the incorporation of rare metals, we concluded in an earlier study [2] that the growth of the market share of OLED technology for flat‐screen displays will not be hindered due to the low amount of material incorporated per device and the comparably low sales numbers. However, we pointed out that future industrial mass market applications such as lighting panels and especially the hypothetical use as smart labels could not be possible without finding alternatives for elements such as iridium, which is being used in PHOLED displays now (see Figure 15.3). The replacement of phosphorescent materials with TADF materials could solve this problem.

However, sustainability goes beyond avoiding supply bottlenecks: The key idea of the concept of “elemental sustainability” is to ensure that all extant elements in the periodic table are kept available for future generations [41]. Apart from iridium, the rare element indium, which is being used in indium tin oxide (ITO), could also be critical here.

15.1.4 Realization of Efficient and Stable Blue OLEDs

15.1.4.1 The Blue Gap

Apart from sustainability concerns, there is also a technical issue that favors TADF over PHOS materials: the blue gap, which denotes the current trade‐off between the efficiency and the stability of blue emitters. Figure 15.3 indicates that as of today, blue pixels of commercial products contain FLUO materials, even though they are less efficient. The reason for this is that – even after almost 20 years of industrial and academic research on the field of PHOS emitters – science failed to produce a blue PHOS material that combines efficiency, stability, and a proper color point. If a blue emitter were to show high efficiency and long lifetime, as TADF is promising to do, this would lead to great opportunities to create better products with even lower power consumption and better resolution, something that is of great commercial interest at present.

Figure 15.5 indicates the impact of the blue gap in the current blue fluorescent emitters. All OLED displays currently require relatively large blue pixel areas to reach enough brightness in the display. In smartphones with red, green, and blue pixels, the blue pixel makes up to 52% of the total area. Having TADF or PHOS pixels with a much higher efficiency would enable the display manufacturers to make smaller blue pixels to yield the same amount of light, which would pave the way again to increase the display resolution. Additionally, customers would benefit from a longer battery life of their mobile device, which is closely connected to the power consumption of the display.

OLED TVs have a more complex stack architecture, essentially making white light from red‐, green‐, and blue‐emitting layers and then using color filters to separate the colors again for the different pixels. Having a more efficient blue, which again makes up about 50% of the display area, would effectively reduce the power consumption of the TV by switching to a less complex stack design, which could potentially reduce manufacturing costs.

15.2.1 Current Status of Blue TADF Technology: Academia

The general approach for emitter design is to combine electron‐rich (donor) and electron‐poor (acceptor) moieties in a molecule, so that the HOMO and LUMO orbitals are separated, thus decreasing the singlet–triplet gap (ΔE_ST). Typical examples for donor moieties are heterocyclic structures such as carbazole or dimethylacridine (DMAC) derivatives, while acceptor moieties vary from benzonitriles to triazines or sulfones. In this subsection, we collected a list with blue TADF materials published between 2012 and the beginning of 2017. Table 15.2 contains all relevant references together with information concerning the responsible principal investigator (PI), the used donor and acceptor types, the best OLED performance, and references to the molecular structure of key emitters, which are collected in Figures 15.6–15.10 (for a discussion of the different molecular structures, kindly refer to one of the other chapters in this book).

From the collected data, two main conclusions can be derived. First, academic research proves that it should be possible to achieve very high performance with blue TADF technology. Even at very low CIE_y coordinates between 0.1 and 0.2, efficiencies in the order of 20% EQE were realized.

Second, there is still a lot of room for improvement. In general, only few academic researchers in the TADF field deal with the degradation, and stability parameters are hardly ever reported. One noteworthy exception is a study by the group of Lian Duan, which reports promising performance values, displayed in Figure 15.11 [77]. These results are in themselves excellent for the device lifetime, compared with what academic researchers published, and among the best in the field looking at color point and efficiency. However, much like most references displayed in Table 15.2, they reveal that the overall stability is still too low for industrial application and that the efficiency roll‐off is strong, reducing the excellent initial efficiency by more than a factor of two at a practical luminance of 500 cd m⁻².

Looking at the molecular building blocks, it is apparent that most researchers focus on a narrow set of donor unit that contains triarylamine cores, which are often fused to five‐membered carbazole‐type donors or bridged to six‐membered derivatives. The used bridges range from sp³‐hybridized carbon (→ acridanes) to oxygen and sulfur (→ phenoxazine, phenothiazine) to silicon (→ azasiline). The acceptor units on the other hand cover more chemical space, ranging from aromatic systems substituted with electron‐withdrawing groups to fused nitrogen‐containing heterocycles. Often, other heteroatoms such as sulfur, phosphorous, and boron are found here.

15.2.2 Current Status of Blue TADF Technology: Industry

During the last 2 years, we have witnessed breathtaking improvement of TADF materials, which now can surpass conventional PHOS materials in terms of color point and efficiency, with similar stability. A main driving force behind this progress was the development of suitable screening algorithms for molecular design, which make use of density functional theory (DFT) calculations. These computational tools drastically increased the efficiency of material development by allowing to only synthesize highly efficient TADF materials, thus leading to very short material development cycles with a very steep learning curve per cycle.

Results based on this development approach were recently published by the team at CYNORA [90]. We reported several blue materials, among them a blue emitter reaching 14% EQE at 500 cd m⁻² and a lifetime LT₈₀ of 420 h at 500 cd m⁻² starting luminance [90]. This material featured an emission maximum below 480 nm.

In here, we briefly discuss the properties of two other recently developed materials, TADF 1 and TADF 2, which are shown in Figures 15.12 and 15.13, as well as Table 15.3.

The electrooptic JVL characteristics of this material in a simple device architecture, using the literature‐known host mCBP [92] (short for 4,4′‐bis(3‐methylcarbazol‐9‐yl)‐2,2′‐biphenyl), show a voltage of about 4.2 V at 500 cd m⁻². The so‐called blue index, which can be calculated by dividing the current efficiency in cd A⁻¹ and the CIE_y color coordinate, approaches 80 at 500 cd m⁻² in a bottom‐emitting device.

Ir(dmp)₃, short for iridium (III) tris[3‐(2,6‐dimethylphenyl)‐7‐methylimidazo[1,2‐f] phenanthridine], is currently representing one of the most stable blue iridium emitters with a rather blue color point and decent efficiency. In a recent study by Forrest and coworkers [91], the basic performance was reported in the same host used to obtain the data shown in Table 15.3. The LT₈₀ at 500 cd m⁻² as well as the blue index of Ir(dmp)₃ was estimated from the values given in the publication. Even using a simple, nonoptimized screening architecture and normal R&D‐grade purity, the stability LT₈₀ at this starting luminance is in the order of 100 h, which is in the range of the best phosphorescent materials when also considering the better color and the higher efficiency [91] (Table 15.3). Considering that phosphorescent materials are under investigation since 1997 [6], while TADF has just been studied since 2011 [8], these results demonstrate the breathtaking development curve for TADF.

Moreover, deep‐blue materials with high performance have as well been developed. Emitter TADF 2, for example, displayed performances up to 20% in OLEDs with a CIE_y value of 0.17 (Figure 15.14). The efficiency of this emitter reached similarly high performance as the best materials displayed in Table 15.2.

15.3.1 General Remarks

Forrest proposed three main approaches to combine both device lifetime and efficiency in blue OLEDs [93]: (i) reducing the excited state lifetime, (ii) reducing the exciton density, and (iii) reducing the effectiveness of bimolecular recombination processes that can promote the excitation to very high energies.

Apart from that, general rules concerning emitter design do apply, too. For example, it is crucial to use robust, stable compounds that are not prone to decomposition or morphological changes such as crystallization, which need to be provided in excellent purity [94].

To understand various aspects of the use of TADF as hosts, it is critical to understand the fundamental differences between Förster‐type and Dexter‐type energy transfer, the two key mechanisms for the energy transfer between two species (Figure 15.15).

Because of the exchange of electrons during the so‐called Dexter transfer [95], it is mainly important that the distance between two species that are involved in a transfer step is in the order of 0.5–2 nm – for both singlet and triplet excitons. This short‐range transfer is the only way for triplet excitons to migrate. During the transfer, the total spin of the system is conserved. It relies on orbital overlap. The rate for this process can be expressed with

15.1

where r is the distance between the species, L the sum of the van der Waals radii of the species, and J the spectral overlap, defined by

15.2

The Förster transfer [96] does not rely on a direct exchange of electrons, but on dipole–dipole interaction. The spin of each participating species remains, which allows only singlet excitons to be transferred with this process. The rate for this process can be expressed with

15.3

where, in addition to the factors defined above, the radiative decay rate of the donating species, k_r, and the photoluminescence quantum yield (PLQY), φ, are of relevance.

The rate is not an exponential function of r, which allows for the Förster process also to occur when r = 20 nm.

15.3.2 First Attempts of Using TADF as Hosts

The fact that energy transfer from TADF molecules to fluorescent emitters is possible was established early: In 2008, Li and coworkers demonstrated that a green‐emitting Cu(I)‐TADF emitter is strongly quenched when introducing a neighboring layer with a low triplet energy. Thus, introducing a DPVBi layer between the TADF‐containing emission layer (EML) and the ETL reduced the efficiency of the device from 16.7 to 5.7 lm W⁻¹ [97]. In 2009, it was shown that the triplet energy of the ETL can affect the efficiency of a TADF OLED due to quenching of TADF triplet excitons by low‐lying triplets of fluorescent molecules [98]. Because of the conceptual analogies between TADF‐type emitters and ambipolar host materials [99–102], it is fair to speculate that the first uses of TADF molecules for host applications were realized unbeknownst to the researchers, which was already discussed elsewhere [10].

15.3.3 Discussion of Various Concepts

Often, the term hyperfluorescence is associated with the use of TADF materials as hosts in OLEDs [65, 103]. This is problematic, since hyperfluorescence was already established several decades ago [104] in medical imaging to describe an increased FLUO during fluorescein angiography, a technique used by ophthalmologists to study the retina.

Also, there are different ways of implementing TADF materials as hosts, which are often described with this term in a confusing manner. Because of this, we will treat the various concepts in separate sections in here.

15.3.3.2 TADF as Host for Fluorescent Materials

Lee and coworkers pointed out a valid issue of this approach: the necessity of a three‐component EML [106]. While the studies discussed in the previous section suggest that it should be possible to technically realize three‐component EMLs where the relative amount of the components differs over three orders of magnitude, transferring them into mass production will be challenging and require optimization of the processing technology. To avoid these costly and elaborate expenses would require two‐component setups of TADF hosts and fluorescent dyes.

In a pioneer work, Duan demonstrated the combination of PIC‐TRZ and DIC‐TRZ, which both emit around 500 nm, with DDAF, an orange‐emitting fluorescent dye that emits at 580 nm (Figure 15.17) [107, 108]. Using Alq₃ as a conventional fluorescent host, DDAF gave a peak efficiency in the order of 3% EQE, while the TADF‐type hosts gave a peak efficiency of 11% EQE. In both cases, doping concentrations of 1 wt% were used. In this work, it is proposed to use fluorescent molecules with sterically demanding substituents in future approaches in order to reduce charge trapping on the dopant.

The suitability of TADF materials for host applications seems to be connected to the absence of concentration quenching. While many emitting materials get less efficient at high doping concentrations [109], other materials may actually require high concentrations to emit efficiently (so‐called aggregation‐induced emission (AIE)) [110, 111].

An ideal TADF‐type host material would actually not show any photophysical changes when modifying the concentration. Apart from Cu(I)‐type TADF emitters that are usually devoid of concentration quenching even in neat films or powder [11, 39, 112–114], this issue has been investigated for a great number of metal‐free TADF emitters as well [64, 115, 116].

Interestingly, the presence of DMAC (Figure 15.18) as a donor seems to reduce concentration quenching. While it has been speculated that steric hindrance (resulting in a reduced probability for Dexter transfer processes) could be responsible for this beneficial behavior [117]; this is yet to be investigated systematically.

Just recently, phenoxazine and phenothiazine (Figure 15.19) have been identified as alternatives for DMAC donors, yielding high efficiencies well over 15% EQE and even improved roll‐off behavior in nondoped devices compared with conventional OLEDs with DPEPO as host materals [117].

15.3.3.3 TADF as Host for Phosphorescent Emitters

In addition, TADF‐type hosts were used in combination with phosphorescent emitters [108]. The driving motivation is again the triplet energy: As stated earlier, phosphorescent emitters emit via a transition between the T₁ and S₀ levels, and, again, the triplet energy of host materials needs to be larger than the triplet energy of the dopant. Especially when moving toward phosphorescent high‐energy emission, TADF emitters are particularly attractive for such applications because of the small singlet–triplet splitting, ΔE_S1–T1.

A broader review on TADF and ambipolar host materials has been previously published [10]. Some examples are shown in Figure 15.20. D2ACN was published in 2008 in a study describing its use as a host material in red PHOLEDs [118]. After the realization of the beneficial properties of TADF materials as dopants in 2011 [8], the related molecule Spiro‐CN was used in a TADF OLED [119].

Combining the orange emitter PO‐01 in 10 wt% concentration with the TADF‐type host BBIC‐TRZ (Figure 15.21) gave excellent performance values with efficiencies in the order of 20% EQE, almost negligible roll‐off up to brightness values of 2000 cd m⁻² and a high operational stability of LT₅₀ = 1900 h at 1000 cd m⁻² starting luminance under constant current conditions [108]. It is speculated that a potential reason for this surprisingly high stability is the possibility of energy transfer from the host to the dopant via both Förster and Dexter processes. Since TADF hosts are capable of transforming triplet into singlet excitons, a higher ratio of the favorable Förster‐type transfer processes can occur, reducing the triplet exciton density. Interaction between triplet excitons and polarons – so‐called triplet‐polaron quenching (TPQ) – is assumed to be a major degradation pathway in phosphorescent OLEDs [93, 120, 121].

While it seems intriguing to fuse the properties of TADF‐type emitters with the proven performance of phosphorescent materials, there are some drawbacks. For instance, part of the benefits of the metal‐free TADF systems (sustainability, freedom of broad patents, potential cost issues) is lost when combining them with heavy‐metal‐containing systems. Most importantly, since it is not yet proven if the reason for the instability of blue PHOLEDs is tied to the phosphorescent dopant, the hosts, or both, using TADF‐type hosts is no tool to solve any issues based on degradation of the phosphorescent dopants themselves.