Fangchao
Zhao
and
Dongge
Ma
*
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China. E-mail: msdgma@scut.edu.cn
First published on 20th February 2017
White organic light-emitting diodes (WOLEDs) hold great promise for the fabrication of highly efficient, large-area, and flexible lighting sources. Achieving both high luminous efficiency and long operational lifetime simultaneously to meet the requirements of solid-state lighting is a considerable challenge. Alongside balanced charge transport properties and development of operationally stable blue emissive materials, device architectures and light outcoupling techniques should also be considered as matters of concern. We provide a review of recent advances on small-molecule WOLEDs including charge balance, emissive materials, device architectures, and light out-coupling techniques to address the effective approaches for high performance WOLEDs.
Regarding OLED displays, many companies have released OLED products for TVs, cell phones, smart watches, laptops, virtual reality (VR) applications, etc. For example, LG display developed 77-inch curved ultra HD OLED TV panels in 2014, and Samsung Display demonstrated a rollable 5.7′′ display that features a full-HD resolution (386 PPI) and a curvature radius of 10 mm in 2016.9 OLED displays on wearable devices such as smart watches have been widely adopted by Apple, Samsung, LG, Microsoft and so on.10 Lenovo announced the world's first OLED laptop with a 14′′ AMOLED display in 2016, and AU Optronics's 3.8′′ 90 Hz AMOLEDs for VR application have been in production since 2016.11 The OLED display market is expanding significantly and has shown a promising future for this technology.
Since the development of the first white OLED (WOLED) by J. Kido et al. in 1995, WOLEDs have attracted great attention in both academia and industry, and a tremendous upswing in technological progress has been made to achieve the requirements for lighting applications. Except for their well-known large-area, flexible and very thin properties, WOLEDs emit eye-friendly soft light, demonstrating the closest light source to natural light. Recently, LG Chemical offered the world's largest (320 × 320 mm2), extremely thin (0.88 mm) OLED lighting panel with a power efficiency of 60 lm W−1, a colour rendering index >85, and a lifetime of 40000 hours (LT70).12 OLEDWorks announced a commercially available 127 × 127 mm2 OLED lighting panel with a power efficiency of 57 lm W−1 at 300 lm, and a lifetime of 10000 hours (LT70).13 Konica Minolta Inc. exhibited 150 × 60 mm flexible OLED lighting panels based on a roll-to-roll process, which have achieved remarkable parameters of 5 g (0.06 g cm−2) in weight.14 These aforementioned characteristics of WOLEDs demonstrate their potential in lighting applications. However, the OLED lighting market is growing much more slowly than the OLED display market, which is mainly due to the remaining technical challenges for WOLEDs in solid state lighting, among which the lower luminous performance compared with that of inorganic LEDs is the key issue to be mitigated through scientific research.
White emission from OLEDs can be achieved in both small molecule and polymer systems. Focused on small-molecule WOLEDs, we provide a brief review of recent advances in WOLED technology including charge balancing, emissive materials, device architectures, and light out-coupling techniques, to address the effective approaches to practical lighting applications of WOLEDs.
In terms of device performance, the most important parameter characterizing an OLED is its EQE, which is defined as the ratio between the actually emitted photons from the device and the amount of injected charge carriers. As shown in Fig. 2, the EQE of an OLED can be formulated as
ηEQE = γηS/TΦPLηout |
However, in most WOLEDs, the different charge injection barriers and mobilities for electrons and holes, the excited state interactions as well as interactions with charge carriers (e.g. triplet–triplet annihilation, triplet–polaron quenching), and the higher refractive index of the OLED layer than the glass substrates and air make it challenging to achieve the ideal conditions for highly efficient WOLEDs. In principle, the device efficiency can be optimized through the maximization of each parameter in the EQE equation. Against these challenges, recent advances will be discussed below.
Fig. 3 Schematic diagram of charge distribution in an OLED without considering interface affection, the number of holes and electrons in the EML are equal under the ideal conditions (γ = 1). |
An OLED containing a single organic material, namely the homojunction architecture,19 is an attractive device structure due to its simplicity for device fabrication. The most simple structure to realize a balanced homojunction OLED is using an ambipolar host material which has similar charge mobility for holes and electrons. Qiao et al. demonstrated such homojunction OLEDs comprising an ambipolar host material 2,5-bis-2-9H-carbazol-9-ylphenyl-1,3,4-oxadiazole (o-CzOXD) doped with iridium complexes. For the single layer OLEDs, balanced charge carriers in the EML are realized with the help of hole-trapping and the electron-transporting role of the iridium complexes.20 For example, in the green OLED with 9 wt% Ir(ppy)3 doped with o-CzOXD, holes are mainly trapped at the dopant sites prior to passing through the doping region, resulting in a reduced hole mobility, while electron transport is even enhanced by the additional transporting channel on Ir(ppy)3 molecules. All these procedures improve the charge balance in the emissive region in the high electrical field.
Wang et al. further improved the charge injection and transport characteristics in a homojunction OLED with o-CzOXD. Homojunction top-emitting devices are composed of a MoO3-doped host as the HTL, and a Cs2CO3-doped host as the ETL. EMLs consist of a host doped with (ppy)2Ir(acac), (fbi)2Ir(acac) and (PPQ)2Ir(acac) for green, orange and red emission, respectively (Fig. 4). Top-emitting OLEDs with even higher efficiency than multi-layer heterojunction bottom-emitting devices were obtained.21 Although few reports on such homojunction WOLEDs have been reported, they show promising and feasible routes for practical application of homojunction WOLEDs.
Fig. 4 Schematic architecture of the p–i–n homojunction top-emitting OLEDs and the host and dopants used by Wang et al. Reprinted with the permission from ref. 21. Copyright 2011 Wiley-VCH. |
In commonly used multi-layer heterojunction OLEDs, the charge balance is determined by charge injection and transport in devices. The efficient and balanced charge injection and transport should meet the following requirements: (1) the materials should possess suitable ionization potentials and electron affinities for energy level matching to ensure efficient injection at the interfaces between electrode/organic and organic/organic materials, and (2) the materials should have a high carrier mobility and the ability to form a high-quality morphological film. Fig. 5 shows the commonly used hole transport materials (HTMs) and electron transport materials (ETMs) with their charge carrier mobility illustrated in Tables 1 and 2, respectively. The electron mobility (μe) of ETMs are very close to the hole mobility (μh) of HTMs, which has allayed the widely recognized concern that HTMs usually have a hole mobility that is about 3 orders of magnitude higher than the electron mobility of ETMs.18 As the electron mobility has been greatly improved by the development of novel ETMs, the charge carrier balance can be brought close to unity by using doped transport layers and exciton confined structures in small-molecule OLEDs.22 For example, Zhu et al. proposed a p–i–n white OLED with TCTA:MoO3 as the HTL and BmPyPB:Cs2CO3 as the ETL. By adjusting the doping concentration of MoO3 and Cs2CO3, well balanced charge injection and transport were realized, together with high T1 of TCTA and BmPyPB to confine triplet excitons in the EML, white emission with the forward viewing EQE of 22.0% and the CRI of 83 at 1000 cd m−2 was achieved.23
In a typical heterojunction OLED, a p-type host usually shifts the recombination zone to the ETL, while an n-type host shifts it to the HTL. A mixed host with p- and n-type materials can be employed to adjust the charge balance and broaden the charge recombination zone. Zhao et al. employed a fluorescent/phosphorescent hybrid WOLED by using mixed TCTA and Bepp2 as the host of phosphorescent EMLs and the interlayer (Fig. 6). The charge carrier distribution can be well controlled by adjusting the ratio of TCTA:Bepp2 in the red and green EML and the interlayer. Therefore, the efficiency roll-off was greatly improved due to the broadened exciton distribution zone.32
Fig. 6 Device structure of a WOLED using a mixed host to adjust charge carriers. Reprinted with permission from ref. 32. Copyright 2012 Elsevier. |
Conventional fluorescent molecules have a relatively large energy difference (singlet–triplet splitting) between the S1 and T1 states. This difference results in an exciton utilization of 25% from the radiative decay of singlet excitons, while 75% of triplet excitons are quenched by competing nonradiative processes (e.g. triplet–triplet annihilation or vibronic relaxation), making the conventional fluorescent OLEDs inefficient with an upper limit EQE of only 25%, so the conventional all-fluorescent WOLEDs are not discussed in this review.
Benefitting from the development of phosphorescent materials, the efficiencies of phosphorescent WOLEDs (Ph-WOLEDs) have been greatly improved. Su et al. reported highly efficient white OLEDs with ultrathin (0.25 nm) orange EMLs between blue phosphorescent EMLs. Efficient exothermic energy transfer from the triplet states of TCTA and DCzPPy to that of FIrpic was realized, resulting in an excellent triplet energy confinement on the blue emitter FIrpic. The yellow emission was enabled by efficient energy transfer from triplet states of the host materials and FIrpic to those of the PQ2Ir. A record power efficiency of 44 lm W−1 at 1000 cd m−2 luminance in the forward direction was achieved without using any out-coupling techniques.43
Wang et al. reported an efficient Ph-WOLED by incorporating FIrpic for blue emission and (fbi)2Ir(acac) for orange emission in a single EML (Fig. 9a). The emissive mechanisms of FIrpic and (fbi)2Ir(acac) were demonstrated to be different in two parallel channels: host–guest energy transfer for blue emission and direct exciton formation for yellow emission, respectively. Unfavorable energy losses are therefore greatly reduced. The operational principle of the WOLEDs is depicted in Fig. 9b. By precisely managing all the electrically generated excitons, the resulting WOLEDs can achieve a peak forward-viewing power efficiency of 42.5 lm W−1, corresponding to an EQE of 19.3%.44
Fig. 9 (a) Energy diagram and device structure of the all-phosphor dual-color WOLEDs. The numbers in parentheses indicate the energy gaps of the materials. (b) Proposed operational principle of the Ph-WOLEDs. Reproduced with permission from ref. 44. Copyright 2009 Wiley-VCH. |
Doping free OLEDs have attracted substantial interest due to the simplicity of the fabrication process. For ultrathin un-doped WOLEDs, the thickness and position of the ultrathin un-doped EML are very critical in the efficient utilization of the generated excitons. Zhu et al. reported a highly efficient Ph-WOLED with an ultrathin un-doped orange EML sandwiched between two blue EMLs.45 In order to achieve high efficiency, two blue EMLs with the hole-transport host TCTA and the bipolar-transport host 26DCzPPy were employed, respectively. The thickness of the ultrathin un-doped orange EML has been optimized to adjust energy transfer from the blue emitter to the yellow emitter. The optimized WOLEDs show a power efficiency of 53.4 lm W−1, and an EQE of 22.2% at 1000 cd m−2 in the forward-view direction with low efficiency roll-off. And it also exhibits ideal warm-white EL spectra with the CIE coordinates of (0.41, 0.44) at a practical brightness of 1000 cd m−2.
Besides high efficiency, a long operational lifetime is also essential for WOLED technology to achieve practical application. After more than a decade of research, significant breakthroughs in the lifetime of green and red Ph-OLEDs have been achieved, with the reported half lifetime (T50) from 1000 cd m−2 higher than 106 h.46,47 However, the operational stability of all Ph-WOLEDs remains a significant challenge due to the significantly shorter lifetime of blue phosphorescent emitters. In contrast to their red and green counterparts, these blue Ph-OLEDs retain lifetimes on the order of a few tens to hundreds of hours.48–51 The degradation mechanism of these shorter lifetime blue Ph-OLEDs has been attributed in part to energetically driven formation of traps formed due to bimolecular triplet–polaron quenching (TPQ).52 When a triplet exciton (∼2.8 eV) on the phosphor encounters a polaron (∼3.3 eV) on a nearby host molecule, the exciton will be quenched, and the polaron is excited to a higher energy level (>6 eV).53 This high-energy polaron in blue emitting materials is sufficiently high to break up and degrade the emissive molecules through bond cleavage. As an example, the commonly used sky-blue emitter, FIrpic, is subjected to broken Ir–O bonds after excitation, followed by the loss of CO2 and the formation of FIrpic+.54 The long lifetime (microsecond scale)55 of triplet states in phosphorescent OLEDs increases the likelihood of these destructive events, especially in contrast to the likelihood of these events occurring in fluorescent emitters with significantly shorter exciton lifetimes (nanoseconds).56 The Forrest group reported a novel blue phosphorescent device with a graded dopant concentration profile in a broadened emissive layer to suppress TPQ, resulting in a lifetime of 616 ± 10 hours (time to degradation to 80% of the initial 1000 cd m−2 initial luminance), representing the most efficient sky-blue phosphorescent FIrpic emitter.7 However, this reported lifetime still lags far behind the practical values, due to the intrinsic chemical degradation of blue phosphorscent emitters.48,57
Inspired by the phosphor sensitized fluorescent monochrome OLEDs,59,60 hybrid WOLEDs have been reported for more than ten years.61–63 However, a substantial breakthrough in device efficiency was not achieved until 2006, when Sun et al. proposed a smart architecture by placing an ambipolar spacer layer CBP between fluorescent and phosphorescent EMLs, as shown in Fig. 10a. The device exhibited a maximum total EQE of 18.7 ± 0.5% and a maximum total power efficiency of 37.6 ± 0.6 lm W−1.64 Two exciton formation regions were formed at the interfaces between carrier transport layers (HTL for hole and ETL for electron) and blue fluorescent EMLs. As the design concept shown in Fig. 10b, the singlet excitons with shorter diffusion length are harvested by the fluorescent emitter 4,40-bis(9-ethyl-3-carbazovinylene)-1,10-biphenyl (BCzVBi), while the triplet excitons can migrate to red and green phosphorescent EMLs for radiative decay due to the long diffusion length of triplet excitons (460 ± 30 Å). Although the singlet and triplet excitons are effectively separated, CBP molecules with a lower triplet energy than that of the blue emitter serve as triplet traps which quench a considerable amount of triplet excitons, resulting in a forward EQE of only 10% at 500 cd m−2.
Fig. 10 (a) Device structure for a hybrid WOLED based on the concept of separating fluorescent and phosphorescent EMLs by a spacer layer. (b) Proposed energy transfer mechanism in this device. Reproduced with permission from Ref. 64. Copyright 2006 Nature Publishing Group. |
Moreover, the use of the interlayer in hybrid WOLEDs has several disadvantages that limit the device efficiency. Firstly, the voltage drop across the interlayer is not negligible, leading to lower power efficiency. Moreover, the interlayer brings additional interfaces which could induce energy losses by increasing the possibility of charge imbalance and exciplex formation.58 Finally, the additional fabrication step for an interlayer is also unfavorable for commercial applications. To mitigate the aforementioned issue, Schwartz and co-workers employed a blue fluorophore with high triplet energy between red and green phosphorescent EMLs to harvest triplet excitons.65 The device structure and possible exciton transfer mechanisms are shown in Fig. 11a. The key feature in this WOLED is that the blue fluorophore 4P-NPD with a high triplet state (2.3 eV) can effectively avoid trapping triplets by transferring them to the adjacent phosphorescent EMLs, while the singlet excitons are directly used for blue fluorescence within this layer. Based on this triplet harvesting concept, highly efficient hybrid WOLEDs with reduced efficiency roll-off were achieved. By the introduction of a p-doped HTL and an n-doped ETL, the device can achieve a power efficiency of 22.0 lm W−1, corresponding to 10.4% EQE at 1000 cd m−2. The triplet harvesting concept inspired subsequent work on hybrid WOLEDs; some fluorophores with high triplet energy were synthesized to be multi-functional as blue emitters and phosphorescent hosts, and hybrid WOLEDs with simpler structures were developed.66–68
Fig. 11 (a) Device structure of hybrid WOLEDs and possible exciton transfer mechanisms proposed by Schwartz et al. (b) EL spectra of the hybrid WOLEDs at different current densities. Reproduced with permission from ref. 65. Copyright 2007 Wiley-VCH. |
Owing to the relatively lower triplet energy of 4P-NPD (2.3 eV) compared to that of Ir(ppy)3 (2.5 eV), a possible Dexter energy transfer from the triplet state of Ir(ppy)3 to the lower lying non-radiative triplet state of 4P-NPD may occur, which results in energy loss and thus the obtained device efficiency is far behind the expected value for the hybrid WOLEDs in ref. 66. Moreover, the shifting of the recombination zone with increased driving voltage leads to unstable white spectra at different brightnesses (Fig. 11b), which is also a common issue for hybrid WOLEDs.
Sun et al. suppressed the aforementioned intrinsically mutual quenching between the fluorescent emitter and the phosphorescent emitter by introducing mixed hosts with bipolar-transport properties to construct the blue fluorescent EML, and Ir(ppy)2(acac) with lower triplet energy (2.4 eV) as the green emitter (Fig. 12a).69 The optimized device exhibits impressive EL performances with a turn-on voltage of 3.1 V. The maximum forward-view EQE, current efficiency, and power efficiency of the device are 19.0%, 45.2 cd A−1, and 41.7 lm W−1, respectively. And at the practical brightness of 1000 cd m−2, they still retain 17.0%, 40.5 cd A−1, and 34.3 lm W−1, exhibiting less pronounced efficiency roll-off. The EL spectra of the hybrid WOLED turned out to be stable within the investigated brightness ranges (Fig. 12b). The above advantages are attributed to the broad exciton recombination zone within the EML owing to the ambipolar transporting properties of the mixture of TCTA and TmPyPb, as well as the efficient utilization of excitons in the blue emissive zone at a low blue dopant concentration.
Fig. 12 (a) Energy level diagram and energy transfer scheme in hybrid WOLEDs proposed by Sun et al. (b) EL performance of the hybrid WOLED; inset: normalized EL spectra of the hybrid WOLED at different current densities. Reproduced with permission from ref. 69. Copyright 2014 Wiley-VCH. |
In 2014, Duan et al. reported the first hybrid WOLEDs with 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN) as the blue TADF emitter to suppress the intrinsically mutual quenching between the fluorescent emitter and the phosphorescent emitter(s). By wisely taking advantage of the energy levels of the materials and the trapping ability of the dopant, the devices exhibit stable warm emission with a maximum forward viewing EQE of 22.6% and a power efficiency (PE) of 47.1 lm W−1.74 As the external heavy-atom effect of the phosphors may quench the conventional fluorescence due to the increased singlet–triplet ISC rate constant and the radiative or non-radiative decay rate of triplet excitons in the fluorophores, Duan et al. found that the TADF fluorophore is much less sensitive to the external heavy-atom effect perturber than the conventional fluorophores due to the almost unchanged ISC rate and the enhanced rate of reverse ISC. By doping a yellow phosphor and a blue fluorophore with TADF into a single EML, hybrid WOLEDs with a maximum forward-viewing EQE of 19.6% and a maximum forward-viewing power efficiency of 50.2 lm W−1 are achieved.75
Recently, Wu et al. demonstrated high-performance TADF-based hybrid WOLEDs by employing a TADF molecule DMAC-DPS as the blue emitter combined with red and green phosphorescent emitters.76Fig. 14a shows the device structure and the operational mechanism of the resulting hybrid R–G–B WOLEDs. Due to the high triplet energy of DMAC-DPS, cascaded energy transfer from the blue emitter to the green and then the red emitter contributes to efficient utilization of excitons. The EL performance of the resulting WOLEDs is also dependent on the doping concentration of Ir(ppy)2(acac); therefore, the regulation of doping concentration facilitates the precise manipulation of charges and excitons, leading to efficient hybrid WOLEDs with a forward-viewing maximum EQE, current efficiency, and power efficiency of 23.0%, 51.0 cd A−1, and 51.7 lm W−1, respectively, which are even comparable to those of the state-of-the-art all phosphorescent WOLEDs. Moreover, the EL spectra exhibit remarkable colour stability with a high CRI of 89, CIE coordinates of (0.438, 0.438) and a CCT of 3231 K at 1000 cd m−2 (Fig. 14b).
Fig. 14 (a) Proposed energy diagram of hybrid WOLEDs with a blue TADF molecule. (b) EL performance of hybrid WOLEDs with a blue TADF molecule; inset: normalized EL spectra of the hybrid WOLED at different luminance. Reproduced with permission from ref. 76. Copyright 2016 Wiley-VCH. |
In addition to the utilization of TADF materials, the exciplex with the TADF effect has recently drawn substantial attention. Chen et al. demonstrated an efficient blue exciplex system formed by a new hole transport material (4-dimesitylboryl)phenyltriphenylamine (TPAPB) and a commonly used electron transport material TPBi. The resulting exciplex exhibited a high photoluminescence quantum yield of 44.1%. Using only these two organic materials, a high performance blue-emitting OLED was fabricated with a spectral peak at 468 nm and a maximum EQE of 7.0 ± 0.4%. Furthermore, a yellow phosphorescent dopant Ir(2-phq)3 was doped in the above exciplex system with a low concentration of 0.5 wt%, and a WOLED with a power efficiency of 25.1 ± 0.1 lm W−1 and an EQE of 14.0 ± 0.2 at 100 cd m−2 was obtained.77
Liu et al. reported an efficient TADF exciplex system formed in a mixed film comprising a hole-transporting molecule 4,40-bis(9-carbazolyl)-2,20-dimethylbiphenyl (CDBP) and an electron-transporting molecule ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (POT2T).78 Both CDBP and PO-T2T have PL emission peaks smaller than 400 nm, whereas the CDBP:PO-T2T mixed film shows a broad and significantly red-shifted PL spectrum with a peak at 476 nm, which is attributed to exciplex emission. By doping green phosphor Ir(ppy)2(acac)) and red phosphor Ir(MDQ)2(acac) into the blue TADF exciplex, they fabricated a hybrid WOLED with a maximum EQE and power efficiency of 25.5% and 84.1 lm W−1, respectively. Although the device achieved high maximum efficiencies, they decreased to 14.8%, and 24.2 lm W−1 at 1000 cd m−2, exhibiting severe efficiency roll-off, and the EL spectra showed an obvious color change with increased luminance with a CRI of 76.
The overall EL performances of hybrid WOLEDs with blue TADF emitters surpass those with conventional fluorescent emitters, and even are comparable to those of all phosphorescent WOLEDs. Even so, they still suffer from some big challenges. In the TADF mechanism, triplet excitons must be converted into luminescent singlet excitons for fluorescence via a reverse intersystem crossing (RISC) mechanism. This additional process before fluorescence emission delays the luminescence process, resulting in an increased fluorescence lifetime of up to several microseconds.72 Due to the inherently long delayed lifetime of TADF molecules, the probability of the exciton quenching processes such as singlet–triplet annihilation (STA), triplet–triplet annihilation (TTA), and exciton–polaron quenching progressively increases with increasing luminance, leading to severe efficiency roll-off at high brightness. This roll-off in efficiency directly correlates with degradation in lifetime as evidenced by the previously mentioned, state of the art blue TADF emitter, DMAC-DPS, with the lifetime of approximately one hour.79 Moreover, to date, deep blue TADF molecules are very rare. Though continual innovation will slowly improve the performance and lifetime of TADF materials, the aforementioned issues currently limit the use of blue TADF materials from practical applications in WOLEDs.
Recent conceptual advances based on TADF sensitization such as the TADF-assisted dopant and the sensitizing TADF host have been proposed to boost triplet harvesting. Adachi et al. employed TADF molecules as assistant dopants with a wide-energy-gap host and conventional fluorescent emitters in fluorescent OLEDs in which TADF dopants are separated from fluorescent dopants by host molecules to prevent Dexter energy transfer between them that will cause energy losses. As a result, maximum EQEs of 13.4%, 15.8%, 18.0% and 17.5% were achieved for blue, green, yellow and red OLEDs.80 This concept was extended to WOLEDs by employing a blue TADF emitter bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMACDPS) as the exciton donor and green and red fluorescent emitters dispersing in the host mCP as exciton acceptors. The exciton donor and acceptors are separated into two different layers with a 2 nm spacer layer inserted between them, causing carrier recombination mainly on the blue TADF molecules. The generated singlet excitons either directly or by RISC can be resonantly transferred to the red and green fluorescent acceptors through the spacer layer, which eliminates direct carrier recombination on fluorescent dopants, leading to a WOLED with an EQE of 12% and a CIE of (0.25, 0.31).81
According to Fermi's Golden rule,82 small singlet–triplet splitting (ΔEST) for realizing efficient reverse ISC usually conflicts with the fast radiative decay rate for high PLQY in TADF emitters. Duan et al. directly used TADF materials as sensitizing hosts to excite fluorescent dopants, which breaks the trade-off between small ΔEST and high radiative decay rates, leading to a high-efficiency orange-fluorescent OLED with a EQE of 12.2%.83
Ma et al. demonstrated high-efficiency fluorescent WOLEDs with low efficiency roll-off employing DMAC-DPS as the host for a conventional orange fluorescent emitter. Concentration of the dopant was optimized to prevent energy loss from the Dexter energy transfer from the triplet energy of the TADF host to that of the dopant.84 The resulting WOLEDs using DMAC-DPS as the host exhibit excellent EL performances with the maximum forward viewing power efficiency and EQE of 48.0 lm W−1 and 14.0%, respectively. At a practical brightness of 1000 cd m−2, they remain as high as 29.3 lm W−1, and 13.0%, indicating a much improved efficiency roll-off compared with the TADF emitters-based WOLEDs. Fluorescent WOLEDs with DMAC-DPS as the TADF assistant dopant were also studied in this work, showing inferior performance to the TADF host devices, which is attributed to the broad exciton distribution in the TADF host with its ambipolar transport properties. The exciton recombination zone in the WOLEDs using DMAC-DPS as the host with its ambipolar-transporting properties. Based on the sensitizing TADF host concept, Ma et al. also demonstrated efficient R–G–B fluorescent WOLEDs by emissive layer design to manipulate charges and excitons in the emissive regions, leading to very high performance of over 18% EQE and stable EL spectra with a high CRI of 82 for the reported fluorescent WOLEDs.85
Since all fluorescent WOLEDs are attracting more and more interest due to their long operational lifetime, high color purity, and potential to be manufactured at low cost in next-generation display and lighting applications. It seems that TADF-sensitized fluorescent WOLEDs with high efficiency, good reliability, and low efficiency roll-off would offer a new avenue for the future development of high-performance WOLEDs. However, serving as the fluorescent sensitizer as well as blue emitters, operationally stable blue TADF materials still face challenges.
Numerous substrate modification techniques have been implemented to enhance light extraction from the substrate mode. The most commonly used method is to attach a light scattering film to the back-surfaces or using microlense to prohibit the total internal reflection at the glass/air interface by increasing the critical angle.89,90 Möller et al. employed microlens arrays with 10 mm diameter poly-dimethyl-siloxane lenses attached to the glass substrates of a green OLED, which widens the escape cone for total internally reflected light incident at the air–substrate boundary. Thus the extraction of light at angles higher than the critical angle of glass is enhanced, resulting in a factor of 1.5 enhancement over unlensed substrates.91 Chen et al. proposed a very simple and cost-effective method to roughen the substrate surface by simply sand-blasting the edges and back-side surface of the glass substrates, 20% enhancement of forward efficiency has been achieved due to the light scattering effect.90 Lee et al. reported another simple method using O2 plasma treatment on the back side of a PET substrate to form a sub-micrometer-sized pattern (Fig. 16), which contributes to a 70% enhancement of luminance by improving light extraction from the substrate to air.92
Fig. 16 Spontaneously formed nanopatterns on polymer films for flexible OLEDs. Reproduced with permission from ref. 92. Copyright 2015 Wiley-VCH. |
Fig. 17 Schematic light extraction of the waveguide mode in OLEDs with a high index substrate compared to that with a low index substrate. |
However, it should be noted that it is rather costly to obtain glass substrates with a high refractive index, which is partly due to instability in the glass forming process and lower chemical durability with lower contents of SiO2. A high index polymer film is a good substitute for high index glasses.98 Kim et al. employed an industrial-grade version of polyethylene naphthalate (PEN) as the high index substrate (n ∼ 1.7) for OLEDs, another high index polymer polyamide-imide (PAI) was coated on PEN to provide a smooth surface for preparing indium zinc oxide (IZO) and a relatively good index matching between PEN and IZO (Fig. 18). As a result, waveguided modes were effectively extracted to the substrate. Combined with built-in scattering properties of PEN which enables the extraction of those substrate-confined modes to the air without any substrate structuring or additional microlens array films, a green OLED with an EQE of 29.6% (98.7 lm W−1) was achieved with an enhancement factor of 1.32.99
Fig. 18 Device structure of OLEDs with the high-index substrate reported by Kim et al. Reproduced with permission from ref. 99. Copyright 2015 Wiley-VCH. |
Fig. 19 Scheme of intensity distribution of the waveguide mode in the light-emitting devices with a light scattering layer. Reproduced with permission from ref. 100. Copyright The Royal Society of Chemistry 2015. |
Li et al. demonstrated a light scattering layer with nanoparticles of barium strontium titanate dispersed in a polymer, single-walled carbon nanotubes and silver nanowires stacked and embedded in the surface of the scattering layer to form the transparent electrode (Fig. 20). Based on the flexible substrates with a scattering layer, a 2.5 fold enhancement of the current efficiency at 10000 cd m−2 was reported for green and white polymer OLEDs.101
Fig. 20 Schematic illustration of light scattering by nanoparticles in the SWNT/Ag NW-nanocomposite substrate. Reprinted with permission from ref. 101. Copyright Nature Publishing Group 2014. |
Kumar et al. fabricated a nanostructured ITO (NSITO) between a conducting anode and a glass substrate using a glancing angle deposition method by radio frequency (RF) sputter. The refractive index of the scattering layer varies from 1.2 to 1.9 depending on the deposition angle. OLEDs with these films inserted between glass and conducting ITO were found to have enhanced out-coupling efficiency in comparison to the reference OLED by about 80%.102
It is not easy to control the random scattering center of the internal scattering layers discussed above, which makes them not exactly reproducible. Nanoimprint lithography for the nanostructure is another choice for developing light extracted substrates. Using an ultraviolet (UV) curable polymer resin and UV nanoimprint lithography, Jeon et al. fabricated two-dimensional nanostructural pillars with a UV curable resin and SiNx on the glass substrate directly (Fig. 21) and 50% enhancement of the EL intensity was achieved compared with the conventional device.103 Moreover, OLEDs with the pillars in the substrates exhibit interesting emission patterns depending on the symmetry and dimensions of the pillars.
Fig. 21 (a) Vertical structure and materials of OLEDs with a 2D nanostructure in the substrate. (b) SEM image of the imprinted polymer pillars. Reproduced with permission from ref. 103. Copyright American Institute of Physics 2016. |
In 2008, the Forrest group fabricated a low-refractive-index grid (n = 1.45) using standard photolithographic methods between ITO and the organic layers, which allowed redirecting light traveling and improving extraction of waveguided light into the glass and air modes. Combined with microlens arrays on the substrate surface, an out-coupling enhancement of ∼2.3 times over that using a conventional flat glass substrate was achieved.104 Recently they demonstrated a non-diffractive dielectric grid layer between ITO and the glass substrate using photolithographic methods (Fig. 22), which couples out all waveguide mode power into the substrate with a minimal impact on the wavelength, the viewing angle and molecular dipole alignment.105 Importantly, because the grid lies below the anode, its design and fabrication are completely independent of the PHOLED structure and performance. This characteristic of the sub-anode grid allows the complete freedom in materials and device structure needed to achieve a fully optimized emitting device.
Fig. 22 Schematic cut-away view of an OLED with a sub-anode grid. Reprinted with permission from ref. 105. Copyright Nature Publishing Group 2015. |
Lee et al. demonstrated a spontaneously formed ripple-shaped nanostructure of ZnO for an inverted PLED (Fig. 23) through the reorganization of gel particles of zinc acetate dihydrate [Zn(CH3COO)22H2O] during the slow drying process. As a result, the out-coupling efficiency of 79% was achieved when the ripple period was ∼300 nm, which is primarily attributed to the reduction in the in-plane wave vectors of the waveguide modes caused by the Bragg grating vector.110 Except for the extraction of the waveguide mode, the surface plasmon mode should also be reduced by the periodical cathode morphology.
Fig. 23 Device architecture of inverted PLEDs with a ripple-shaped structure. Reprinted with permission from ref. 110. Copyright Nature Publishing Group 2014. |
Since the coupling to SP modes is a near-field effect, the amount of dissipated power depends on the distance to the metal. Therefore increasing the distance between the emitter and the cathode is a simple method to reduce the coupling between emitting dipoles and the SP mode. In 2010, Leo's group simulated the quantification of energy loss channels as a function of ETL thickness in a red phosphorescent OLED. As shown in Fig. 24, the SP mode is notably extracted to the waveguide mode with increasing ETL thickness, reaching a negligible level for thicknesses >200 nm.114 The thick ETL involves an n-doped transport layer to maintain electrical characteristics. Qu et al. used a 240 nm ETL with Bphen:Cs separating the EML from the Al planar electrode, which showed an operating voltage comparable to conventional devices at the same luminance. Due to the reduced SP modes, the thick ETL contributes to a 50% increase in the EQE.105
Fig. 24 Simulation of the distribution of energy loss channels in a red bottom OLED, as well as measured EQE as a function of the ETL thickness and comparison to simulation results. Reprinted with permission from ref. 114. Copyright American Institute of Physics 2010. |
As the increased ETL thickness reduces the SP modes by shifting them to waveguide modes rather than extracting them out, no significant increase of the overall efficiency is implemented only with a thick ETL. Therefore, waveguide mode extraction should be accompanied by SP mode shifting to realize high out-coupling efficiency. A more promising approach for a strongly reduced coupling to SP modes is the employment of oriented emitters that have their transition dipole moment oriented parallel to the substrate plane.115 In this case, the coupling of horizontally oriented dipoles to SP modes is significantly reduced in comparison to perpendicular dipoles. A bunch of horizontally oriented fluorescent molecules with linear-shaped molecules in an isotropic host matrix film has been reported.116–118 For phosphorescent emitters which are usually bulky metal–organic complexes with isotropic dopant orientation,119,120 a non-isotropic, predominantly parallel emitter orientation in the well-known red phosphorescent emitter Ir(MDQ)2(acac) blended in an α-NPD matrix (Fig. 25) was reported, and an out-coupling efficiency increase of ∼50% compared to the isotropic case was achieved,121 demonstrating a promising approach for light out-coupling in phosphorescent guest–host systems. Kim et al. demonstrated that the green phosphorescent emitter Ir(ppy)2(acac) has a preferred non-isotropic orientation with a horizontal to vertical dipole ratio of 0.77:0.23 in their device, resulting in an EQE of 30% for the green OLEDs.122
Fig. 25 Predominantly parallel emitter orientation in Ir(MDQ)2(acac): α-NPD. Reprinted with permission from ref. 121. Copyright 2011 Elsevier. |
Transparent metal-free OLEDs are other optimal devices used for significantly reducing power dissipation into SP modes.123 Lee et al. proposed a transparent green OLED with a 60 nm IZO as the anode and a 70 nm ITO as the cathode. Without interaction between the metal surface and the emitting dipoles, SP modes in the transparent OLED are significantly reduced. By integrating a micro-cone array on one side and a half-sphere lens on the other side, a total EQE of 62.9% was achieved for the green phosphorescent OLED.124
Instead of a typical planar metal layer with heavy SP loss, a corrugated metal layer can extract SP modes by being Bragg-scattered into visible light. The dispersion of SP modes traveling between two semi-infinite layers is described in Fig. 26. Since the wave vector of SP modes is larger than the in-plane wave vector of light for all frequencies, no coupling between SPs and light is possible in the case of planar interfaces. Therefore, in a device such as an OLED where an emitting dye molecule excites SPs from the optical near field, power that is coupled to SPs is dissipated as heat and therefore lost. A periodically corrugated microstructure allows Bragg scattering of SPP modes, thus SPP modes can be shifted partly into the light cone.
Bi et al. demonstrated a corrugated red OLED with integrating dual-periodic corrugation by employing a laser interference lithography technique. The light trapped in the SPP modes associated with both top and bottom electrode/organic interfaces is efficiently extracted from the OLEDs by adjusting appropriate periods of two set corrugations, and a 29% enhancement in the current efficiency has been obtained.125 Chen et al. reported a facile method to fabricate an ITO nanomesh by nanosphere lithography. As shown in Fig. 27, firstly a polystyrene (PS) self-assembled monolayer was fabricated on a glass substrate. The diameter of PS nanospheres was reduced from 500 nm to ∼270 nm after reactive ion (RIE) etching. ITO was then deposited on the PS monolayer. The ITO nanomesh was obtained by removing (lifting off) PS nanospheres and excess ITO on nanospheres. As a result, the corrugated OLED including the corrugated metal cathode was obtained based on the ITO nanomesh, which significantly reduces SP losses for radiation from the vertical dipoles. In addition, making use of very different refractive indices of ITO mesh and PEDOT:PSS, internal extraction was further enhanced. By combining this internal extraction structure and the attached extraction lens, a very high EQE of nearly 62% (264.3 lm W−1) was achieved with a green phosphorescent OLED.126
Fig. 27 Schematic diagram and EL performances of an ITO nanomesh OLED reported by Chen et al. Reproduced with permission from ref. 126. Copyright 2015 Wiley-VCH. |
Fig. 28 Schematic device structure and photograph for the flexible white OLEDs using PEAN as the anode. Reproduced with permission from ref. 127. Copyright © 2014 American Chemical Society. |
By employing the internal extraction structure comprising a high refractive-index layer and a light scattering layer to extract the waveguide modes, and using an external extraction structure to extract the substrate modes, Konica Minolta Inc. demonstrated all-phosphorescent WOLEDs with a state of the art power efficiency of 130 lm W−1 (CCT = 2857 K) at 1000 cd m−2 in 2015, even at 5000 cd m−2, the WOLED still maintains 110 lm W−1,128 indicating a highly meaningful step for WOLEDs to enter into the general lighting market.
The merits of WOLEDs such as high efficiency, low driving voltage, flexibility, and low energy consumption have made them the most promising candidate for the next-generation full-colour displays and solid-state lighting sources. Flexible WOLEDs will be the ultimate choice in the smart display and lighting industry, because of their advantages including portability and large-size and low-cost production. However, the reliability of flexible WOLEDs by the limitation of the transparent electrode on the flexible substrate and the encapsulation method still need to be further improved.
To date, the most efficient WOLEDs are grown in high-vacuum systems using thermal evaporation sources. New manufacturing techniques like roll-to-roll, inject printing and slot-die coating offer better choices for mass production of low-cost WOLEDs, and these processes may eventually make the cost of producing WOLEDs competitive, especially for polymeric WOLEDs. However, their luminous efficiency remains a big challenge compared with small-molecule WOLEDs.
Abbreviation | IUPAC name of material |
---|---|
Ir(ppy)3 | fac-Tris(2-phenylpyridine)iridium(III) |
Ir(ppy)2(acac) | Bis(2-phenylpyridinato-N,C20)iridium (acetylacetonate) |
(fbi)2Ir(acac) | Bis(2-(9,9-diethyl-fluoren-2-yl)-1-phenyl-1H-benzo[d]imidazolato)(actylacetonate)iridium(III) |
(PPQ)2Ir(acac) | Bis(2,4-diphenylquinolyl-N,C2′)iridium(acetylacetonate) |
Ir(2-phq)3 | Tris(2-phenylquinoline)iridium(III) |
PO-01 | (Acetylacetonato)bis[2-(thieno[3,2-c]pyridin-4-yl)phenyl]iridium(III) |
Ir(bt)2(acac) | Bis(2-phenyl benzothiozolato-N,C20)iridium (acetylacetonate) |
Ir(MDQ)2(acac) | Bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)iridium(III) |
FIrpic | Bis(3,5-diuoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) |
Fir6 | Bis(2,4-diuorophenylpyridinato)tetrakis(1-pyrazolyl)borate iridium(III) |
4P-NPD | N,N′-Di-(1-naphthalenyl)-N,N′-diphenyl-[1,10:40,1′′:4′′,1000-quaterphenyl]-4,4000-diamine |
CBP | 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl |
TPD | N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine |
NPB | N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine |
TCTA | 4,40,4′′-Tris(carbazol-9-yl)-triphenylamine |
TAPC | 1,10-Bis[4-(di-p-tolylamino)phenyl]cyclohexane |
DCzppy | 2,6-Bis(3-(9H-carbazol-9-yl)phenyl)pyridine |
26DCzppy | 2,6-Bis(3-(9H-carbazol-9-yl)phenyl)pyridine |
Bepp2 | Bis(2-(2-hydroxyphenyl)-pyridine)beryllium |
TPBi | 2,20,2′′-(1,3,5-Benzenetriyl)tris-[1-phenyl-1H-benzimidazole] |
TmPyPB | 1,3,5-Tri(m-pyrid-3-yl-phenyl)benzene |
Bphen | 4,7-Diphenyl-1,10-phenanthroline |
BmPyPB | 1,3-Bis(3,5-dipyrid-3-yl-phenyl)benzene |
DPEPO | Bis[2-(diphenylphosphino)phenyl]ether oxide |
DMAC-DPS | Bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone |
This journal is © the Partner Organisations 2017 |