Special Section on Solid-State Lighting: Photonics and Technologies

Exciton management for high brightness in organic light-emitting diodes

[+] Author Affiliations
Grayson Ingram, Carmen Nguyen, Zheng-Hong Lu

University of Toronto, Department of Materials Science and Engineering, 184 College Street, Ontario, Toronto M5S 3E4, Canada

J. Photon. Energy. 5(1), 050998 (Aug 13, 2015). doi:10.1117/1.JPE.5.050998
History: Received January 31, 2015; Accepted July 8, 2015
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Abstract.  Organic light-emitting diodes (OLEDs) have already been proven in display applications and show promise as the next-generation solid-state lighting technology for general illumination. A major barrier to the adoption of OLEDs for solid-state lighting is the efficiency roll-off at high brightness, which occurs at much lower current densities for OLEDs than their inorganic counterparts, in large part due to the quenching of excitons. We discuss strategies to mitigate this efficiency roll-off through management of excitons in both white and monochrome devices, including the use of phosphorescent as well as thermally activated delayed fluorescent emitters. Successful high-efficiency devices are used as case studies for how to effectively manage excitons.

Figures in this Article

Organic light-emitting diodes (OLEDs) have long been in development as the next-generation solid-state lighting source for general illumination. Over just the last few years, there has been great progress in the field in achieving high efficiencies in a range of colors that are very close to the theoretical limit for OLEDs.16 OLEDs are quickly catching up to LED lighting technology, which still has difficulties producing efficient devices in the green-yellow color range,7 where the human eye has the highest sensitivity, as demonstrated in Fig. 1(a). The efficiencies of the best-reported OLEDs of a range of wavelengths are compared with those of state-of-the-art commercial LEDs in Fig. 1. It is clear that OLEDs do not suffer from the same dip in efficiency at the eye’s peak sensitivity as occurs with LEDs. It is important to note that the OLEDs reported in Fig. 1(b) do not use optical outcoupling enhancements, and thus, the maximum achievable efficiencies would be higher. Additionally, OLEDs can be made on plastic substrates, thus enabling the fabrication of lightweight, thin, and flexible lighting panels.

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Fig. 1
F1 :

Current efficiencies of light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) at various visible wavelengths. (a) The LED efficiencies are power conversion efficiencies of commercial LEDs at 350mA/cm2.8 (b) OLED data are external quantum efficiencies of state-of-the-art phosphorescent and thermally activated delayed fluorescence (TADF) OLEDs.36,9,10 Note that these OLEDs do not use optical outcoupling enhancing techniques and the maximum achievable efficiencies would be higher. The gray dotted curve is the photopic luminosity function (the sensitivity of the human eye to different wavelengths under well-lit conditions).

One major challenge for OLEDs is a significant efficiency roll-off, a phenomenon in which there is a reduction in efficiency as the device is driven to higher brightness or current density. Traditional inorganic LEDs also experience similar behavior, often referred to as efficiency droop. Both efficiency roll-off in OLEDs and efficiency droop in LEDs describe the same effect; however, the causes of this effect differ in the two systems. A comparison of the efficiency as a function of current for organic and inorganic LEDs is presented in Fig. 2. It is clear that OLEDs suffer from a more drastic decline in efficiency at much lower currents and, hence, at lower brightness levels. This means that although OLEDs can achieve high efficiencies close to or exceeding LEDs, they often exhibit inferior performance at high brightness levels required for general illumination. In the case of OLEDs, the device area can be scaled up, an option not open to LEDs that act as point sources.

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Fig. 2
F2 :

Normalized efficiency as a function of current for an organic and inorganic LED. It should be noted that LEDs are point sources, while OLEDs can be scaled to large areas. The LED data were taken from Ref. 11.

While a detailed review of the mechanisms responsible for efficiency roll-off in OLEDs has been recently provided,12 the aim of this review is to provide the broader solid-state lighting community with an overview of the current work in OLEDs for general illumination. We begin by presenting the physics and working principles behind OLEDs, with a particular emphasis on exciton behavior. Next, we look at how excitons are manipulated and harvested in OLEDs by different types of organic emitters and device architectures. Finally, we discuss current techniques used to achieve suitable white OLEDs for lighting.

Due to the low dielectric constant in organic semiconductors, positive and negative charge carriers form strongly bound electron-hole pairs, i.e., excitons, where the two charge carriers are coulombically bound in a hydrogenic state. This is in sharp contrast to typical inorganic materials in which excitonic effects are usually negligible at room temperature. Depending on the relative orientation of the spins of the unpaired electrons in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the exciton may either be a triplet (parallel spins) or a singlet (antiparallel spins). The names singlet and triplet reflect the degeneracy of the two states; there are three degenerate triplet states and one singlet state. The properties of singlet and triplet excitons differ greatly as will be discussed below.

In an OLED, excitons are formed when electrons and holes injected from opposite electrodes meet in the device. These uncorrelated injected charge carriers form each exciton state with equal probability leading to a 31 ratio of triplet to singlet excitons.

There are two primary pathways to exciton formation in an OLED: (1) exciton formation on the host material(s) and (2) exciton formation on the dopants. In either case, excitons tend to form where there are high concentrations of electrons and holes in a specific location. Exciton formation on the host will usually occur at the interface between the electron and hole transport layers, where charges tend to accumulate owing to energetic and mobility barriers to charge transport.13,14 In some cases, in a host:dopant system the dopants will act as charge traps and charges will accumulate on the dopants.15 If there is a significant current of oppositely charged carriers in the same region, excitons will form directly on the dopants.

After formation, excitons migrate through the organic layers by hopping from one molecule to another. Although there have been reports of coherent exciton motion in crystalline organic semiconductors, the hopping model is more widely accepted for amorphous organics. The hopping motion of excitons in a neat organic layer can be described by a random walk, and it is often approximated by the diffusion equation. An exciton hopping from one molecule (donor) to another (acceptor) is referred to as energy transfer. A typical OLED will include layers and/or mixtures of different organic materials, and the motion of an exciton can include energy transfers between both identical and dissimilar molecules. There are two mechanisms through which excitons can hop from a donor to an acceptor: (1) Förster resonant energy transfer (FRET)16 and (2) Dexter energy transfer.17 These two processes are illustrated in Fig. 3.

FRET describes the energy transfer between weakly coupled dipoles. In this case, an exciton excited on a donor molecule recombines to the ground state and transfers its energy to excite an acceptor. FRET is often used to describe the hopping of an exciton between identical molecules, but can also be used to describe the energy transfer between different species, as is the case in host-dopant energy transfer in OLEDs. The range of interaction varies depending on the strength of the interaction from short range (nearest neighbors) to 10nm. This mechanism is most common when the donor and acceptor excitons are both singlets; however, it can occur as long as both the decay of the donor and the excitation of the acceptor are allowed transitions.18

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Fig. 3
F3 :

Ilustration of Förster and Dexter energy transfers.19

The rate of energy transfer through a Förster mechanism, kF, is given by Display Formula

kF=1τ(R0R)6,(1)
where R is the separation between donor and acceptor, τ is the natural lifetime of the exciton, and R0 is a characteristic length scale called the Förster radius, which is given by Display Formula
R06=9ln(10)NA128π2κ2ΦDn4J,(2)
where NA is Avogadro’s number, κ is an orientation factor, ΦD is the donor fluorescent quantum yield, n is the index of refraction of the medium containing the donor and acceptor, and J is the overlap integral of the normalized donor emission spectrum and acceptor absorption spectrum.

In Dexter energy transfer, the donor transfers an unpaired electron from its LUMO to the empty LUMO of the acceptor, while an electron from the HOMO of the acceptor is simultaneously transferred to the half-filled HOMO of the donor. In Dexter energy transfer, the donor and acceptor excitons are either both singlet or both triplet. The rate of Dexter energy transfer is mediated by wave function overlap, so it is only relevant for nearest neighbors. The rate of Dexter energy transfer is Display Formula

kD(2Rχ)J¯,(3)
where χ is the effective orbital radius, and J is the overlap integral of the normalized donor emission spectrum and the normalized acceptor absorption spectrum.

Once an exciton has been formed, it can undergo several different emissive or nonemissive decay pathways. The time-scale at which a decay process occurs will dictate the dominant decay pathway. A decay process that occurs more quickly will be able to better compete with alternate processes and consequently will occur more readily.

Emissive Pathways

The most common emissive pathways are fluorescence and phosphorescence. Fluorescence20 occurs through the recombination of a singlet exciton, while phosphorescence occurs through the recombination of triplet excitons. Radiative recombination of triplet excitons is spin forbidden; however, in phosphorescent materials, spin–orbit–exciton–photon interaction, typically due to the presence of a heavy metal atom, facilitates the spin flip necessary for a triplet state exciton to decay into a singlet ground state.20 Strong spin-orbital coupling also enables intersystem crossing, which is the conversion of singlet excitons into the higher spin multiplicity triplet energy state. Generally, all singlet excitons formed on phosphorescent materials are transferred to the triplet state, where they eventually decay resulting in the emission of a photon. Since the transition from a triplet state into the ground state is kinetically unfavorable, phosphorescence occurs over much longer timescales than fluorescence, typically in the microsecond domain. This allows more time for triplet excitons to undergo undesirable nonemissive pathways. However, the ability to harvest both singlet and triplet excitons allows phosphorescent materials to achieve much higher efficiencies than fluorescent emitters. As mentioned previously, in electroluminescence, singlet and triplet states are created in a 1 to 3 ratio. As only singlet excitons can decay emissively on fluorescence materials, the efficiency of fluorophores is limited to 25%. Phosphorescent OLEDs, however, can theoretically reach a quantum efficiency of 100%. Figure 4 shows a schematic diagram of fluorescent versus phosphorescent emission.

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Fig. 4
F4 :

Schematic of emissive exciton pathways. RISC and RRISC are the rates of intersystem crossing and reverse intersystem crossing, respectively. Fluorescence, phosphorescence, and TADF are defined in the text.

Another type of emission occurs from excited-state charge transfer complexes, also known as exciplexes. Exciplexes are excitons in which the electron and hole are localized on separate molecules: an electron donor molecule and an electron acceptor molecule. They can generally occur at the interface between two layers or in layers that have a mixture of two or more materials. Exciplexes can be useful as they allow the potential tailoring of emission wavelengths by carefully selecting two materials for the exciton to reside on. However, exciplexes generally exhibit low quantum yields due to a relatively large donor–acceptor separation, resulting in a relatively low wave function overlap integral between the initial and final energy states.21

Over the last few years, a new type of emitter molecule has been developed with efficiencies well above what was once thought to be the theoretical limit for fluorescent OLEDs. These emitters, called thermally activated delayed fluorescence (TADF) molecules, were first used in OLEDs by the Adachi group in 2009.22 TADF molecules are designed to contain an electron-donating (D) group and an electron-accepting (A) group, which are separated from one another by steric hinderance.23 This causes the HOMO and LUMO to be mostly localized on the electron-donating and accepting groups, respectively, thus minimizing the overlap between the HOMO and LUMO. This results in a very small energy level difference between the singlet and triplet states, ΔEST, in TADF molecules. TADF molecules can have ΔEST energies as low as 0.01 eV, while traditional fluorescence molecules have ΔEST on the range of 0.5 to 1.0 eV. TADF molecules still emit from the singlet state, but they are capable of harvesting triplet excitons as well. The small ΔEST energy allows the reverse intersystem crossing (RISC) from a triplet to a singlet energy state to occur readily at room temperature. Additionally, more recently developed TADF molecules can undergo emissive relaxation at rates >106s1, thus making them competitive against other nonradiative relaxation pathways. Many reported TADF molecules exhibit high photoluminescence efficiencies >80%.

Nonemissive Pathways/Quenching Processes

Nonemissive pathways cause excitons to release energy in the form of heat, rather than light. As excitons are capable of diffusion by undergoing successive Dexter and/or Förster energy transfer, they can migrate through an OLED and interact with other species. Two quenching processes that severely affect the efficiency of OLEDs, particularly at high current densities, are triplet-triplet annihilation (TTA) and exciton-polaron annihilation (EPA). Murawski et al. provide a more extensive exploration of mechanisms causing efficiency roll-off in OLEDs in Ref. 24.

In TTA, two triplet excitons interact resulting in a ground state molecule and a singlet or triplet exciton. This can benefit fluorescent OLEDs as some triplets are converted to emissive singlets; however, this negatively affects phosphorescent OLEDs as the system goes from having two excited states that are both capable of emitting light to only one excited state. This quenching process becomes particularly troublesome at high current densities when a high density of triplet states is being created. This causes TTA to become more prevalent, resulting in efficiency roll-off.

In EPA, the exciton is quenched by a free or trapped charge carrier. Both triplet and singlet excitons can undergo this mechanism. However, triplet polaron annihilation (TPA) tends to occur more frequently due to the long lifetime associated with triplet excitons. Since triplets require a long time to undergo phosphorescence, it is more likely for triplet excitons to undergo other pathways, such as TPA and TTA.

External Quantum Efficiency

A key parameter for OLEDs is the external quantum efficiency (EQE), which is the number of photons emitted per pair of charge carriers injected into the device. EQE is given by Display Formula

ηex=γχφeffηout=ηintηout,(4)
where γ is the charge balance factor, χ is the fraction of excitons that is captured by the emitter molecules, Φeff is the effective radiative efficiency of the emitter, and ηout is optical outcoupling efficiency. The outcoupling efficiency indicates the fraction of light emitted by the emissive layer (EML) that exits the front of the device. Reflections at interfaces can trap light in the device and reduce the EQE. The internal quantum efficiency (ηint) does not include this outcoupling factor.

In the previous sections, we discussed the effects of different emitter types on efficiency. The type of emitter used affects the effective radiative efficiency factor, Φeff, which is a combination of the photoluminescence efficiency and the spin factor. The spin factor refers to the fraction of excitons that can decay radiatively. In traditional fluorescent emitters, for example, only singlets can emit radiatively, thereby restricting the spin factor to 0.25. Contrarily, phosphorescent and TADF emitters can potentially harvest both singlet and triplet excitons and, consequently, have a spin factor of 1. The photoluminescence efficiency describes the fraction of excitons that decay emissively once formed on the emitter molecule.

The remaining two factors are largely dependent on the device design. The charge balance factor, γ, is the fraction of injected charges that form an exciton. An unbalanced number of electrons and holes can lead to free charge carries that interact destructively with excitons through EPA. Increasing the fraction of excitons captured by the emitter molecule, χ, will reduce the number of excitons that undergo quenching mechanisms on nonemitter materials. The following section will outline successful device architectures that achieved high EQE by promoting charge balance and/or the transfer of excitons to the emitter.

Most modern OLEDs employ a host:dopant EML architecture with some notable exceptions.25 In this structure, the function of charge transport is carried out primarily by the host, while emission is carried out by the dopant. The host:dopant system was adopted early in the development of OLEDs26 in order to compensate for the fact that most molecules have a higher photoluminescent quantum efficiency at low concentration, while pure films have better charge transport ability. A challenge for OLED design using a host:dopant system is to ensure that as many excitons as possible are transferred from the host to the dopant.

Choosing appropriate device architecture is crucial to the fabrication of high-efficiency OLEDs. Currently, there is no reliable way available to determine if a particular material combination will be effective without testing real devices. By examining the similarities among high-efficiency devices, however, we can identify a few useful design guidelines.

Often OLEDs employ a stack of multiple layers, such as electron and hole transport layers (ETL and HTL), electron and hole injection layers, electron and hole blocking layers, and exciton blocking layers. Charges and excitons, however, tend to accumulate at the interfaces between these organic layers due to energetic offsets (and mobility offsets for charge carriers). Near these interfaces, if there are high concentrations of charges and excitons, exciton quenching will occur, leading to reduced device efficiency. It is desirable for an OLED to be as simple as possible without sacrificing efficiency, and it is possible to remove many of these organic layers while simultaneously improving efficiency.

The device structure shown in Fig. 5(a) uses the same material as the hole transport layer and host, and has demonstrated an EQE of 29.2% with a high work function chlorinated indium tin oxide electrode.27 The efficiency of the simplified device and a comparable device using a poly(3,4-ethylenedioxythiophene) injection layer and α-N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) hole transport layer are shown in Fig. 5. The low efficiency and fast roll-off of the second device is due to poor charge balance, quenching of excitons by charge accumulation at the α-NPD/4,4′-Bis(N-carbazolyl)-1,1′-biphenyl interface, and triplet exciton transfer to the dark NPD triplet state.

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Fig. 5
F5 :

(a) Device architecture of simplified OLED using chlorinated indium tin oxide anode and (b) current efficiency of the simplified device and a comparable device using a poly(3,4-ethylenedioxythiophene) injection layer and α-NPD transport layer.27

Exciplex Forming Cohost Structure

A strategy that recently has been very successful in producing high-efficiency OLEDs is using an exciplex forming cohost.46,9

Typically, a cohost system consists of a mixture of one hole transporting material and one electron transporting material. This provides an effective medium with bipolar charge transport, which broadens the exciton formation region and, thus, limits exciton quenching. The same electron and hole transporting materials in the cohost can also be used as the ETL and HTL removing the energetic barrier to charge transport into the EML. The electron and hole transporting materials should be chosen such that the HOMO of the hole transporting material matches closely with the HOMO of the dopant and the LUMO of the electron transporting material lines up with the LUMO of the dopant. This reduces the charge build-up on the dopants due to trapping and allows excitons to form primarily on the host.

A type of cohost in which the two host materials form an exciplex has led to particularly efficient devices. The exciplex is usually excited directly rather than exciting wide-bandgap host materials. This allows very low turn-on voltages and high power efficiencies. Often the exciplex energy is close to the dopant energy, which leads to low downconversion losses compared to typical host:dopant systems. The exciplex to dopant energy transfer has also shown to be very efficient.28

Highly efficient OLEDs based on exciplex forming cohosts have been fabricated using phosphorescent emitters and have achieved EQE of 29.5, 32.3, and 35.6% for blue, green, and red phosphors, respectively,1,4,5 and 29.6% for a green TADF emitter. The EQEs as a function of luminance for these high-performance devices are shown in Fig. 6. In addition to the advantages of cohosts discussed above, it has been suggested that the horizontal orientation of the emitters plays an important role in the high performance of the devices.

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Fig. 6
F6 :

(a) Example of cohost structure5 and (b) record efficiencies for red, green, and blue cohost device. The plots are generated based on data from Refs. 4, 5, and 9.

Some studies5,29 have suggested that EQEs exceeding 30% observed in these exciplex cohost systems can be, in part, attributed to preferential dipole orientation of the emitter molecule. Analysis of Ir(ppy)2(acac) doped into TCTA:B3PYMPM cohost system has shown that the emitters exhibit anisotropic orientation with 77% of dipoles orienting horizontally. If the sample were completely isotropic, then the ratio of horizontal to vertical oriented dipoles would be 0.67:0.33.30 Energy transfer from excited dye molecules to nearby metal surfaces has been shown to occur with high probability for molecules aligned vertically or perpendicular to the metal surface.31 This coupling to surface plasmons reduces the outcoupling efficiency of the device. For this reason, emitters that have horizontally oriented dipole moments can achieve much higher EQE without special outcoupling layers. The maximum EQE for a completely isotropic phosphorescent dye is estimated to be 25%.5

It has also been shown that, at low temperatures, efficient emission can occur directly from the exciplex formed on the two host materials. This eliminates the need for a dopant material, which significantly simplifies the device fabrication. More significantly, since an exciplex is formed on an electron-donating and an electron-accepting molecule, there is a small overlap between HOMO and LUMO levels. Consequently, exciplexes can potentially possess small ΔEST values and undergo efficient reverse intersystem crossing. Similar to TADF molecules, these exciplex cohost systems can harvest both singlet and triplet states. The Kim group has shown that a fluorescent cohost system of TCTA and B3PYMPM can achieve an EQE of 10% at 195 K.32 This is much higher than the limit expected from traditional singlet exciplex emission and indicates that triplet exciplexes are also being harvested in this system. Efficient delayed fluorescence from the cohost is observed at temperatures as low as 35 K, with photoluminescence efficiency increasing to 100% from 36% at room temperature. The lack of thermal activation needed for RISC in this cohost system suggest that there must be a small energy difference between the singlet and triplet states.

Exciton Harvesting Dopants

In some cases, a particular host:dopant combination may not result in a high-efficiency OLED. This is often due to inefficient triplet exciton harvesting by the dopant. More excitons can often be harvested by increasing the dopant concentration; however, this will decrease the photoluminescent efficiency of the dopants, which often leads to lower efficiency rather than higher. One way to improve the efficiency without using a new materials combination is the exciton harvesting strategy.

The concept behind the exciton harvesting strategy is to introduce an exciton harvesting dopant (or donor), which can efficiently harvest host triplet excitons and then efficiently transfer that energy to the emissive dopant (or acceptor). This requires a harvesting dopant, which has a triplet exciton energy between that of the host and the dopant.

There are two schemes to implement the exciton harvesting strategy: intrazone exciton harvesting and interzone exciton harvesting. In the intrazone harvesting scheme, the exciton harvesting and emissive dopant are doped into the same region of the device, while in the interzone harvesting scheme, the two dopants are located in adjacent layers of the device.

Devices employing the intrazone and interzone exciton harvesting strategy are shown in Figs. 7 and 8, respectively. Using the intrazone exciton harvesting scheme, the peak EQE of a red OLED was increased from 17.3 to 24.8%.33 The interzone harvesting technique was used in a greenish-yellow OLED to increase the EQE from 15.2 to 21.5%.34

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Fig. 7
F7 :

(a) Device architecture of a red OLED employing intrazone exciton harvesting and (b) efficiency improvement of optimized red OLEDs with and without intrazone exciton harvesting. The inset shows the emission spectra of the two devices.33

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Fig. 8
F8 :

(a) Device architecture of a red OLED employing interzone exciton harvesting and (b) efficiency improvement of optimized red OLEDs with and without interzone exciton harvesting.34

The energy flow pathways for the exciton harvesting strategy are shown in Fig. 9. Host triplet excitons that are not harvested directly by the emissive dopants can be harvested by the exciton harvesting dopant, which has been selected to efficiently capture host triplet excitons. The majority of these excitons are then efficiently passed to the lower-energy emissive dopants. We can describe the EQE of an OLED employing the exciton harvesting pathway by mathematically describing the pathways shown in Fig. 9. In this way, we can write the EQE as Display Formula

ηext=γηout{χAφPL,A+χD[ηDAφPL,A+(1ηDA)φPL,D]},(5)
where χD and χA denote the fraction of excitons that is trapped by the donor and acceptor molecules, respectively, ΦPL,D and ΦPL,A are the quantum yields of the donor and acceptor, and ηDA stands for the energy transfer efficiency from donor to acceptor, i.e., from green to yellow or red phosphorescent emitters.

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Fig. 9
F9 :

Schematic diagram of energy transfer processes in a two-dopant system. The dopants may be doped in the same region or in adjacent regions of a single host. EHOST, EA, ED, and E0 represent the energy levels of the host, acceptor molecule, donor molecule, and the ground state, respectively. And χA, χD, and ηDA are as defined for Eq. (5) in the text.7

Typically, the exciton harvesting strategy will be used when χA is small and a harvesting dopant is available that has a high χD and high ηDA. If the donor has a high exciton harvesting ability but low energy transfer efficiency, the donor emission will dominate. While this does not reduce the EQE, the purpose of the exciton harvesting strategy is to increase the emission from the acceptor, so we consider the third term in Eq. (6) a loss channel.

The exciton harvesting strategy inherently involves losses from the energetic downconversion of high-energy host excitons to lower-energy dopant excitons. It is important to note, however, that these downconversion losses are characteristic of any host/dopant OLED and that the introduction of an exciton harvesting dopant in addition to an emissive dopant does not introduce any additional downconversion losses but rather provides an alternative pathway for the transfer of host triplet excitons to the emissive dopant, as illustrated in Fig. 9.

It is theoretically possible to push the internal quantum efficiency of traditional fluorescent emitters by employing a phosphorescent molecule as the assistant dopant or sensitizer.35 High ISC rates in phosphors allow most singlet excitons to be converted into triplet excitons that then undergo Förster energy transfer to the singlet state of a nearby fluorescent emitter. This allows some triplet excitons to be converted into singlet ones that can be harvested by the fluorophore. Although such systems have produced devices with EQEs near 10%, some emission originates from the phosphorescent sensitizer, so these devices are often not completely fluorescent.

Recently, TADF molecules have been used as harvesting dopants36 in the exciton harvesting strategy. When used as the assistant dopant in fluorescent devices, they serve a purpose similar to phosphorescent sensitizers. TADF molecules with low photoluminescence efficiency and high RISC rates can be used to minimize emission from the sensitizer. High EQEs>10% were observed for blue, green, yellow, and red fluorescent emitters with little to no emission observed from the TADF assistant dopants. It is possible to simplify these systems by using the TADF molecule as the host material.37 The TADF material can still fulfill the role of the sensitizer by upconverting triplet state excitons to their singlet state. This, however, removes the benefit of the assistant dopant as an exciton harvester. It has been shown that utilizing a TADF molecule with low photoluminescence efficiency and a small ΔE-ST value as a host can achieve an EQE of 11.7% for a completely fluorescent device.37

OLEDs have been successfully integrated into consumer electronics, but improvements are still needed before they can be competitive for general lighting. Design of OLEDs for lighting is more difficult than for display applications due to the need for high brightness and broad spectral coverage. The requirement of high brightness is a challenge due to the efficiency roll-off of OLEDs as discussed in Sec. 3.2. Due to the narrow photoluminescent spectra of most organic emitters, multiple emitters are needed to produce high-quality white light. Typically, red, green, and blue, or red, yellow, green, and blue emitters are needed to produce high-quality white OLEDs.

Two main design choices need to be made when designing a white OLED: (1) choosing the combination of emitters used (fluorescent, phosphorescent, TADF) and (2) choosing whether to incorporate the emitters into a single layer or multiple EML architecture.

To achieve high-efficiency devices, both singlet and triplet excitons must be harvested in the device, which rules out using all fluorescent emitters. Using only phosphorescent emitters can lead to high-efficiency devices; for example, the white cascade OLED using exciton harvesting in two of the layers shown in Fig. 10 has a peak EQE of 24.5%.38 The drawback of the all-phosphorescent emitter approach is that blue phosphorescent devices tend to have poor stability due to their large energy gaps. Often a blue fluorescent emitter with better stability will be used in combination with fluorescence-phosphorescence lower-energy phosphorescent emitters instead (FP OLED).

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Fig. 10
F10 :

(a) Device configurations and (b) energy level diagrams for WOLEDs W1 to W4. The dopants employed are FIrpic for blue (B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow (Y), and Ir(MDQ)2(acac) for red (R). All doping concentrations are in wt%. (c) A photo of a large area (80mm×80mm) WOLED (W3) illuminating at 5000cd/m2 with a color rendering index of 85.38

The FP OLED takes advantage of the close match between the 25% of excitons formed in singlet states and the 25% of white light in the blue region. If all of the singlet excitons are captured by the fluorescent blue dopant and the triplet excitons are appropriately distributed between the lower-energy phosphorescent emitters, then a device with good color balance can be achieved. In practice, this is difficult due to several loss pathways present in this type of device. One loss pathway is the capture and nonradiative decay of triplet excitons by the fluorescent dopant. This channel can be limited by choosing a fluorescent dopant with a triplet energy that is higher than that of the phosphorescent emitters and host. This is not formally considered a source of losses, but the transfer of singlet excitons from the fluorescent dopant to the phosphorescent dopants is still detrimental to the device since it will lead to insufficient blue emission and produce low-quality white light.

OLEDs using single-layer architectures have demonstrated EQEs up to 25%.39,4042 In these devices, exciton management is difficult due to the close proximity of the emitters, which allows efficient Förster and Dexter energy transfer between emitters. Addressing these issues, cascade FP OLEDs with EQE as high as 19.0% have been fabricated while still maintaining a high color quality.43 In this case, emitters can be separated into adjacent layers in the device. Often a spacer layer several nanometers thick will be placed between the fluorescent emitters and lower-energy phosphors to prevent either Förster or Dexter energy transfer of singlet excitons out of the blue emitting region. In both the single layer and cascade architectures, the blue fluorescent emitter may also act as the host material.

One method of addressing the issue of insufficient blue emission is through the use of a blue TADF molecule. TADF molecules naturally possess high triplet energies, allowing the triplet states to be easily distributed to the lower-energy phosphors and preventing the back-energy transfer of triplets from the phosphors to the blue fluorophore. Furthermore, highly efficient and stable blue TADF molecules have been developed, which ensures sufficient blue emission by harvesting triplet states for delayed fluorescence. Using the blue TADF molecule, 2CzPN, with an organic phosphor, doped into adjacent host layers, the Qiu group has reported a white OLED with an EQE of 19.6%.44

Another advantage of utilizing a blue TADF material is that, unlike traditional fluorophores, it is not susceptible to the external heavy atom effect.45 A nearby heavy metal atom, such as the ones present in phosphorescent compounds, can induce singlet-triplet intersystem crossing in traditional fluorescent emitters. This can present a substantial lost pathway in traditional fluorophores with a small radiative decay rate. The efficiency of conventional fluorophores can be expressed as follows: Display Formula

φf=krkr+kISC+knr,(6)
where kr, kISC, and knr are the radiative decay rate, the rate of intersystem crossing, and the nonradiative decay rate, respectively. With increased ISC due to the nearby heavy metal and no change to kr and knr, there is a decrease in the overall fluorescent efficiency. This effect has been confirmed through studies of films doped with a traditional fluorophore and a TADF molecule. Significant quenching was found in the traditional fluorophore films and little change in the TADF efficiency with increasing phosphor concentrations.

TADF molecules have also been used for all the emitters (RGB) in a white OLED, achieving an EQE of 17%.46 Due to the good hole and electron transporting properties of TADF molecules, they have also been used as a host material in white phosphorescent OLEDs. A high EQE of 22.8% was demonstrated.47

In summary, this review paper has presented several strategies for achieving high efficiency by managing excitons in OLEDs. Both material selection and device architecture play an important role. Certain types of emitter molecules are intrinsically limited due to spin statistics and are incapable of harvesting a large fraction of excitons formed through electrical excitation. In recent years, TADF-based OLEDs have been able to achieve record-breaking efficiencies through a combination of harvesting both singlet and triplet excitons and through the preferential horizontal orientation of dipoles.

It has been found that the minimization of energy barriers at the interfaces will generally result in more efficient devices. This prevents the build-up of charges at interfaces, which can interact with excitons through nondesirable quenching processes. Cohost systems are excellent examples of devices designed to minimize charge trapping and downconversion losses. Cohost devices have been able to achieve EQEs>30%.

Similar strategies utilized in monochrome OLEDs can be applied to produce white OLEDs. Combinations of traditional fluorescent, phosphorescent, and/or TADF molecules that emit at different wavelengths have been used to achieve efficiencies >20% with good color quality.

We wish to acknowledge funding support from Canada Foundation for Innovation, NSERC, and Connaught Global Challenge Fund. Z. H. L. is the Canada Research Chair in Organic Optoelectronics funded by the Canadian Federal Government.

Kim  K.  et al., “Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes,” Nat. Commun.. 5, , 1 –8 (2014). 2041-1723 CrossRef
Lee  C. W., and Lee  J. Y., “Above 30% external quantum efficiency in blue phosphorescent organic light-emitting diodes using pyrido[2, 3-b]indole derivatives as host materials,” Adv. Mater.. 25, , 5450 –5454 (2013). 0935-9648 CrossRef
Hirata  S.  et al., “Highly efficient blue electroluminescence based on thermally activated delayed fluorescence,” Nat. Mater.. 14, , 330 –336 (2014). 1476-1122 CrossRef
Shin  H.  et al., “Blue phosphorescent organic light-emitting diodes using an exciplex forming co-host with the external quantum efficiency of theoretical limit,” Adv. Mater.. 26, , 4730 –4734 (2014). 0935-9648 CrossRef
Kim  K. H.  et al., “Highly efficient organic light-emitting diodes with phosphorescent emitters having high quantum yield and horizontal orientation of transition dipole moments,” Adv. Mater.. 26, , 3844 –3847 (2014). 0935-9648 CrossRef
Sun  J. W.  et al., “A fluorescent organic light-emitting diode with 30% external quantum efficiency,” Adv. Mater.. 26, (32 ), 5684 –5688 (2014). 0935-9648 CrossRef
Chang  Y.-L., and Lu  Z. H., “White organic light-emitting diodes for solid-state lighting,” J. Display Technol.. 9, (6 ), 459 –468 (2013). 1551-319X CrossRef
Lumileds, , “Luxeon Rebel and Luxeon Rebel ES color portfolio datasheet, DS68,” 2014, http://www.mouser.com/pdfdocs/PhilipsLumileds_DS68.pdf (10  July  2015).
Kim  K.  et al., “Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes,” Nat. Commun.. 5, , 1 –8 (2014). 2041-1723 CrossRef
Nakanotani  H.  et al., “High-efficiency organic light-emitting diodes with fluorescent emitters,” Nat. Commun.. 5, , 4016  (2014). 2041-1723 CrossRef
Lin  R. M.  et al., “Inserting a p-InGaN layer before the p-AlGaN electron blocking layer suppresses efficiency droop in InGaN-based light-emitting diodes,” Appl. Phys. Lett.. 101, , 081120  (2012). 0003-6951 CrossRef
Murawski  C., , Leo  K., and Gather  M. C., “Efficiency roll-off in organic light-emitting diodes,” Adv. Mater.. 25, , 6801 –6827 (2013). 0935-9648 CrossRef
Wang  Z. B.  et al., “Controlling carrier accumulation and exciton formation in organic light emitting diodes,” Appl. Phys. Lett.. 96, (4 ), 043303  (2010). 0003-6951 CrossRef
Crone  B. K.  et al., “Device model investigation of bilayer organic light emitting diodes,” J. Appl. Phys.. 87, (4 ), 1974  (2000). 0021-8979 CrossRef
Weichsel  C.  et al., “Storage of charge carriers on emitter molecules in organic light-emitting diodes,” Phys. Rev. B. 86, (7 ), 075204  (2012).CrossRef
Förster  T., “Transfer mechanisms of electronic excitation energy,” Discuss. Faraday Soc.. 27, , 7 –17 (1959). 0014-7664 CrossRef
Dexter  D. L., “A theory of sensitized luminescence in solids,” J. Chem. Phys.. 21, (5 ), 836 –850 (1953). 0021-9606 CrossRef
Braslavsky  S. E.  et al., “Pitfalls and limitations in the practical use of Förster’s theory of resonance energy transfer,” Photochem. Photobiol. Sci.. 7, (12 ), 1444 –1448 (2008). 1474-905X CrossRef
Feron  K.  et al., “Organic solar cells: understanding the role of Förster resonance energy transfer,” Int. J. Mol. Sci.. 13, (12 ), 17019 –17047 (2012). 1422-0067 CrossRef
Singh  J., , Baessler  H., and Kugler  S., “A direct approach to study radiative emission from triplet excitations in molecular semiconductors and conjugated polymers,” J. Chem. Phys.. 129, (4 ), 041103  (2008). 0021-9606 CrossRef
Kim  J. H.  et al., “Highly fluorescent and color-tunable exciplex emission from poly(N-vinylcarbazole) film containing nanostructured supramolecular acceptors,” Adv. Funct. Mater.. 24, , 2746 –2753 (2014). 1616-301X CrossRef
Endo  A.  et al., “Thermally activated delayed fluorescence from Sn(4+)-porphyrin complexes and their application to organic light emitting diodes—a novel mechanism for electroluminescence,” Adv. Mater.. 21, , 4802 –4806 (2009). 0935-9648 CrossRef
Uoyama  H.  et al., “Highly efficient organic light-emitting diodes from delayed fluorescence,” Nature. 492, (7428 ), 234 –238 (2012).CrossRef
Murawski  C., , Leo  K., and Gather  M. C., “Efficiency roll-off in organic light-emitting diodes,” Adv. Mater.. 25, , 6801 –6827 (2013). 0935-9648 CrossRef
Wang  Q.  et al., “A non-doped phosphorescent organic light-emitting device with above 31% external quantum efficiency,” Adv. Mater.. 26, , 8107 –8113 (2014).CrossRef
Tang  C. W., , VanSlyke  S. A., and Chen  C. H., “Electroluminescence of doped organic thin films,” J. Appl. Phys.. 65, (9 ), 3610  (1989). 0021-8979 CrossRef
Helander  M. G.  et al., “Chlorinated indium tin oxide electrodes with high work function for organic device compatibility,” Science. 332, (6032 ), 944 –947 (2011). 0036-8075 CrossRef
Park  Y.-S.  et al., “Exciplex-forming co-host for organic light-emitting diodes with ultimate efficiency,” Adv. Funct. Mater.. 23, (39 ), 4914 –4920 (2013). 1616-301X CrossRef
Kim  S. Y.  et al., “Organic light-emitting diodes with 30% external quantum efficiency based on a horizontally oriented emitter,” Adv. Funct. Mater.. 23, , 3896 –3900 (2013). 1616-301X CrossRef
Frischeisen  J.  et al., “Determination of molecular dipole orientation in doped fluorescent organic thin films by photoluminescence measurements,” Appl. Phys. Lett.. 96, , 073302  (2010). 0003-6951 CrossRef
Weber  W. H., and Eagen  C. F., “Energy transfer from an excited dye molecule to the surface plasmons of an adjacent metal,” Opt. Lett.. 4, (8 ), 236  (1979). 0146-9592 CrossRef
Park  Y. S., , Kim  K. H., and Kim  J. J., “Efficient triplet harvesting by fluorescent molecules through exciplexes for high efficiency organic light-emitting diodes,” Appl. Phys. Lett.. 102, (15 ), 153306  (2013). 0003-6951 CrossRef
Chang  Y.-L.  et al., “Enhancing the efficiency of simplified red phosphorescent organic light emitting diodes by exciton harvesting,” Org. Electron.. 13, (5 ), 925 –931 (2012). 1566-1199 CrossRef
Chang  Y.-L.  et al., “Highly efficient greenish-yellow phosphorescent organic light-emitting diodes based on interzone exciton transfer,” Adv. Funct. Mater.. 23, (25 ), 3204 –3211 (2013). 1616-301X CrossRef
Baldo  M. A., , Thompson  M. E., and Forrest  S. R., “High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer,” Nature. 403, , 1998 –2001 (2000).CrossRef
Nakanotani  H.  et al., “High-efficiency organic light-emitting diodes with fluorescent emitters,” Nat. Commun.. 5, , 4016  (2014). 2041-1723 CrossRef
Zhang  D.  et al., “High-efficiency fluorescent organic light-emitting devices using sensitizing hosts with a small singlet-triplet exchange energy,” Adv. Mater.. 26, , 5050 –5055 (2014). 0935-9648 CrossRef
Chang  Y.-L.  et al., “Highly efficient warm white organic light-emitting diodes by triplet exciton conversion,” Adv. Funct. Mater.. 23, (6 ), 705 –712 (2013). 1616-301X CrossRef
Liu  X.-K.  et al., “Novel blue fluorophor with high triplet energy level for high performance single-emitting-layer fluorescence and phosphorescence hybrid white organic light-emitting diodes,” Chem. Mater.. 25, (21 ), 4454 –4459 (2013). 0897-4756 CrossRef
Ye  J.  et al., “Management of singlet and triplet excitons in a single emission layer: a simple approach for a high-efficiency fluorescence/phosphorescence hybrid white organic light-emitting device,” Adv. Mater.. 24, (25 ), 3410 –3414 (2012). 0935-9648 CrossRef
Liu  X.-K.  et al., “Novel blue fluorophor with high triplet energy level for high performance single-emitting-layer fluorescence and phosphorescence hybrid white organic light-emitting diodes,” Chem. Mater.. 25, (21 ), 4454 –4459 (2013). 0897-4756 CrossRef
Ye  J.  et al., “Management of singlet and triplet excitons in a single emission layer: a simple approach for a high-efficiency fluorescence/phosphorescence hybrid white organic light-emitting device,” Adv. Mater.. 24, (25 ), 3410 –3414 (2012). 0935-9648 CrossRef
Sun  N.  et al., “High-performance hybrid white organic light-emitting devices without interlayer between fluorescent and phosphorescent emissive regions,” Adv. Mater.. 26, (10 ), 1617 –1621 (2014). 0935-9648 CrossRef
Zhang  D.  et al., “Highly efficient and color-stable hybrid warm white organic light-emitting diodes using a blue material with thermally activated delayed fluorescence,” J. Mater. Chem. C Mater. Opt. Electron. Devices. 2, , 8191 –8197 (2014).CrossRef
Zhang  D.  et al., “Highly efficient hybrid warm white organic light-emitting diodes using a blue thermally activated delayed fluorescence emitter: exploiting the external heavy-atom effect,” Light Sci. Appl.. 4, , 1 –7 (2015).CrossRef
Nishide  J.  et al., “High-efficiency white organic light-emitting diodes using thermally activated delayed fluorescence,” Appl. Phys. Lett.. 104, (23 ), 233304  (2014). 0003-6951 CrossRef
Kim  M., and Lee  J. Y., “Donor–acceptor type material as a triplet host for high efficiency white phosphorescent organic light-emitting diodes,” Synth. Met.. 199, , 105 –109 (2015). 0379-6779 CrossRef

Grayson Ingram received his BSc degree in engineering physics from the University of Alberta in 2012. He is currently a PhD student working under the supervision of Zheng-Hong Lu in the Department of Materials Science and Engineering at the University of Toronto. His research focus is on the properties of excitons in organic optoelectronic devices including both OLEDs and OPVs.

Carmen Nguyen is a PhD degree candidate in materials science and engineering at the University of Toronto. She received her bachelor’s degree in applied science in nanotechnology engineering at the University of Waterloo in 2013. Her current research focuses on understanding energy transfer processes in multi-dopant systems in order to develop white organic light emitting diodes for application in solid-state lighting.

Zheng-Hong Lu received his BSc degree in physics in 1983 from China’s Yunnan University and his PhD degree in engineering physics in 1990 from the Ecole Polytechnique, Canada. He is a full professor and a Tier I Canada research chair in organic optoelectronics at the University of Toronto. His lab focuses on developing materials and devices for OLED flat-panel display, solid-state lighting and solar cells. He has authored and co-authored over 200 papers in leading journals such as Nature, Science, Applied Physics Letters, etc., and has filed more than 20 patent applications.

© 2015 Society of Photo-Optical Instrumentation Engineers

Citation

Grayson Ingram ; Carmen Nguyen and Zheng-Hong Lu
"Exciton management for high brightness in organic light-emitting diodes", J. Photon. Energy. 5(1), 050998 (Aug 13, 2015). ; http://dx.doi.org/10.1117/1.JPE.5.050998


Figures

Graphic Jump Location
Fig. 1
F1 :

Current efficiencies of light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) at various visible wavelengths. (a) The LED efficiencies are power conversion efficiencies of commercial LEDs at 350mA/cm2.8 (b) OLED data are external quantum efficiencies of state-of-the-art phosphorescent and thermally activated delayed fluorescence (TADF) OLEDs.36,9,10 Note that these OLEDs do not use optical outcoupling enhancing techniques and the maximum achievable efficiencies would be higher. The gray dotted curve is the photopic luminosity function (the sensitivity of the human eye to different wavelengths under well-lit conditions).

Graphic Jump Location
Fig. 2
F2 :

Normalized efficiency as a function of current for an organic and inorganic LED. It should be noted that LEDs are point sources, while OLEDs can be scaled to large areas. The LED data were taken from Ref. 11.

Graphic Jump Location
Fig. 3
F3 :

Ilustration of Förster and Dexter energy transfers.19

Graphic Jump Location
Fig. 4
F4 :

Schematic of emissive exciton pathways. RISC and RRISC are the rates of intersystem crossing and reverse intersystem crossing, respectively. Fluorescence, phosphorescence, and TADF are defined in the text.

Graphic Jump Location
Fig. 5
F5 :

(a) Device architecture of simplified OLED using chlorinated indium tin oxide anode and (b) current efficiency of the simplified device and a comparable device using a poly(3,4-ethylenedioxythiophene) injection layer and α-NPD transport layer.27

Graphic Jump Location
Fig. 6
F6 :

(a) Example of cohost structure5 and (b) record efficiencies for red, green, and blue cohost device. The plots are generated based on data from Refs. 4, 5, and 9.

Graphic Jump Location
Fig. 7
F7 :

(a) Device architecture of a red OLED employing intrazone exciton harvesting and (b) efficiency improvement of optimized red OLEDs with and without intrazone exciton harvesting. The inset shows the emission spectra of the two devices.33

Graphic Jump Location
Fig. 8
F8 :

(a) Device architecture of a red OLED employing interzone exciton harvesting and (b) efficiency improvement of optimized red OLEDs with and without interzone exciton harvesting.34

Graphic Jump Location
Fig. 9
F9 :

Schematic diagram of energy transfer processes in a two-dopant system. The dopants may be doped in the same region or in adjacent regions of a single host. EHOST, EA, ED, and E0 represent the energy levels of the host, acceptor molecule, donor molecule, and the ground state, respectively. And χA, χD, and ηDA are as defined for Eq. (5) in the text.7

Graphic Jump Location
Fig. 10
F10 :

(a) Device configurations and (b) energy level diagrams for WOLEDs W1 to W4. The dopants employed are FIrpic for blue (B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow (Y), and Ir(MDQ)2(acac) for red (R). All doping concentrations are in wt%. (c) A photo of a large area (80mm×80mm) WOLED (W3) illuminating at 5000cd/m2 with a color rendering index of 85.38

Tables

References

Kim  K.  et al., “Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes,” Nat. Commun.. 5, , 1 –8 (2014). 2041-1723 CrossRef
Lee  C. W., and Lee  J. Y., “Above 30% external quantum efficiency in blue phosphorescent organic light-emitting diodes using pyrido[2, 3-b]indole derivatives as host materials,” Adv. Mater.. 25, , 5450 –5454 (2013). 0935-9648 CrossRef
Hirata  S.  et al., “Highly efficient blue electroluminescence based on thermally activated delayed fluorescence,” Nat. Mater.. 14, , 330 –336 (2014). 1476-1122 CrossRef
Shin  H.  et al., “Blue phosphorescent organic light-emitting diodes using an exciplex forming co-host with the external quantum efficiency of theoretical limit,” Adv. Mater.. 26, , 4730 –4734 (2014). 0935-9648 CrossRef
Kim  K. H.  et al., “Highly efficient organic light-emitting diodes with phosphorescent emitters having high quantum yield and horizontal orientation of transition dipole moments,” Adv. Mater.. 26, , 3844 –3847 (2014). 0935-9648 CrossRef
Sun  J. W.  et al., “A fluorescent organic light-emitting diode with 30% external quantum efficiency,” Adv. Mater.. 26, (32 ), 5684 –5688 (2014). 0935-9648 CrossRef
Chang  Y.-L., and Lu  Z. H., “White organic light-emitting diodes for solid-state lighting,” J. Display Technol.. 9, (6 ), 459 –468 (2013). 1551-319X CrossRef
Lumileds, , “Luxeon Rebel and Luxeon Rebel ES color portfolio datasheet, DS68,” 2014, http://www.mouser.com/pdfdocs/PhilipsLumileds_DS68.pdf (10  July  2015).
Kim  K.  et al., “Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes,” Nat. Commun.. 5, , 1 –8 (2014). 2041-1723 CrossRef
Nakanotani  H.  et al., “High-efficiency organic light-emitting diodes with fluorescent emitters,” Nat. Commun.. 5, , 4016  (2014). 2041-1723 CrossRef
Lin  R. M.  et al., “Inserting a p-InGaN layer before the p-AlGaN electron blocking layer suppresses efficiency droop in InGaN-based light-emitting diodes,” Appl. Phys. Lett.. 101, , 081120  (2012). 0003-6951 CrossRef
Murawski  C., , Leo  K., and Gather  M. C., “Efficiency roll-off in organic light-emitting diodes,” Adv. Mater.. 25, , 6801 –6827 (2013). 0935-9648 CrossRef
Wang  Z. B.  et al., “Controlling carrier accumulation and exciton formation in organic light emitting diodes,” Appl. Phys. Lett.. 96, (4 ), 043303  (2010). 0003-6951 CrossRef
Crone  B. K.  et al., “Device model investigation of bilayer organic light emitting diodes,” J. Appl. Phys.. 87, (4 ), 1974  (2000). 0021-8979 CrossRef
Weichsel  C.  et al., “Storage of charge carriers on emitter molecules in organic light-emitting diodes,” Phys. Rev. B. 86, (7 ), 075204  (2012).CrossRef
Förster  T., “Transfer mechanisms of electronic excitation energy,” Discuss. Faraday Soc.. 27, , 7 –17 (1959). 0014-7664 CrossRef
Dexter  D. L., “A theory of sensitized luminescence in solids,” J. Chem. Phys.. 21, (5 ), 836 –850 (1953). 0021-9606 CrossRef
Braslavsky  S. E.  et al., “Pitfalls and limitations in the practical use of Förster’s theory of resonance energy transfer,” Photochem. Photobiol. Sci.. 7, (12 ), 1444 –1448 (2008). 1474-905X CrossRef
Feron  K.  et al., “Organic solar cells: understanding the role of Förster resonance energy transfer,” Int. J. Mol. Sci.. 13, (12 ), 17019 –17047 (2012). 1422-0067 CrossRef
Singh  J., , Baessler  H., and Kugler  S., “A direct approach to study radiative emission from triplet excitations in molecular semiconductors and conjugated polymers,” J. Chem. Phys.. 129, (4 ), 041103  (2008). 0021-9606 CrossRef
Kim  J. H.  et al., “Highly fluorescent and color-tunable exciplex emission from poly(N-vinylcarbazole) film containing nanostructured supramolecular acceptors,” Adv. Funct. Mater.. 24, , 2746 –2753 (2014). 1616-301X CrossRef
Endo  A.  et al., “Thermally activated delayed fluorescence from Sn(4+)-porphyrin complexes and their application to organic light emitting diodes—a novel mechanism for electroluminescence,” Adv. Mater.. 21, , 4802 –4806 (2009). 0935-9648 CrossRef
Uoyama  H.  et al., “Highly efficient organic light-emitting diodes from delayed fluorescence,” Nature. 492, (7428 ), 234 –238 (2012).CrossRef
Murawski  C., , Leo  K., and Gather  M. C., “Efficiency roll-off in organic light-emitting diodes,” Adv. Mater.. 25, , 6801 –6827 (2013). 0935-9648 CrossRef
Wang  Q.  et al., “A non-doped phosphorescent organic light-emitting device with above 31% external quantum efficiency,” Adv. Mater.. 26, , 8107 –8113 (2014).CrossRef
Tang  C. W., , VanSlyke  S. A., and Chen  C. H., “Electroluminescence of doped organic thin films,” J. Appl. Phys.. 65, (9 ), 3610  (1989). 0021-8979 CrossRef
Helander  M. G.  et al., “Chlorinated indium tin oxide electrodes with high work function for organic device compatibility,” Science. 332, (6032 ), 944 –947 (2011). 0036-8075 CrossRef
Park  Y.-S.  et al., “Exciplex-forming co-host for organic light-emitting diodes with ultimate efficiency,” Adv. Funct. Mater.. 23, (39 ), 4914 –4920 (2013). 1616-301X CrossRef
Kim  S. Y.  et al., “Organic light-emitting diodes with 30% external quantum efficiency based on a horizontally oriented emitter,” Adv. Funct. Mater.. 23, , 3896 –3900 (2013). 1616-301X CrossRef
Frischeisen  J.  et al., “Determination of molecular dipole orientation in doped fluorescent organic thin films by photoluminescence measurements,” Appl. Phys. Lett.. 96, , 073302  (2010). 0003-6951 CrossRef
Weber  W. H., and Eagen  C. F., “Energy transfer from an excited dye molecule to the surface plasmons of an adjacent metal,” Opt. Lett.. 4, (8 ), 236  (1979). 0146-9592 CrossRef
Park  Y. S., , Kim  K. H., and Kim  J. J., “Efficient triplet harvesting by fluorescent molecules through exciplexes for high efficiency organic light-emitting diodes,” Appl. Phys. Lett.. 102, (15 ), 153306  (2013). 0003-6951 CrossRef
Chang  Y.-L.  et al., “Enhancing the efficiency of simplified red phosphorescent organic light emitting diodes by exciton harvesting,” Org. Electron.. 13, (5 ), 925 –931 (2012). 1566-1199 CrossRef
Chang  Y.-L.  et al., “Highly efficient greenish-yellow phosphorescent organic light-emitting diodes based on interzone exciton transfer,” Adv. Funct. Mater.. 23, (25 ), 3204 –3211 (2013). 1616-301X CrossRef
Baldo  M. A., , Thompson  M. E., and Forrest  S. R., “High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer,” Nature. 403, , 1998 –2001 (2000).CrossRef
Nakanotani  H.  et al., “High-efficiency organic light-emitting diodes with fluorescent emitters,” Nat. Commun.. 5, , 4016  (2014). 2041-1723 CrossRef
Zhang  D.  et al., “High-efficiency fluorescent organic light-emitting devices using sensitizing hosts with a small singlet-triplet exchange energy,” Adv. Mater.. 26, , 5050 –5055 (2014). 0935-9648 CrossRef
Chang  Y.-L.  et al., “Highly efficient warm white organic light-emitting diodes by triplet exciton conversion,” Adv. Funct. Mater.. 23, (6 ), 705 –712 (2013). 1616-301X CrossRef
Liu  X.-K.  et al., “Novel blue fluorophor with high triplet energy level for high performance single-emitting-layer fluorescence and phosphorescence hybrid white organic light-emitting diodes,” Chem. Mater.. 25, (21 ), 4454 –4459 (2013). 0897-4756 CrossRef
Ye  J.  et al., “Management of singlet and triplet excitons in a single emission layer: a simple approach for a high-efficiency fluorescence/phosphorescence hybrid white organic light-emitting device,” Adv. Mater.. 24, (25 ), 3410 –3414 (2012). 0935-9648 CrossRef
Liu  X.-K.  et al., “Novel blue fluorophor with high triplet energy level for high performance single-emitting-layer fluorescence and phosphorescence hybrid white organic light-emitting diodes,” Chem. Mater.. 25, (21 ), 4454 –4459 (2013). 0897-4756 CrossRef
Ye  J.  et al., “Management of singlet and triplet excitons in a single emission layer: a simple approach for a high-efficiency fluorescence/phosphorescence hybrid white organic light-emitting device,” Adv. Mater.. 24, (25 ), 3410 –3414 (2012). 0935-9648 CrossRef
Sun  N.  et al., “High-performance hybrid white organic light-emitting devices without interlayer between fluorescent and phosphorescent emissive regions,” Adv. Mater.. 26, (10 ), 1617 –1621 (2014). 0935-9648 CrossRef
Zhang  D.  et al., “Highly efficient and color-stable hybrid warm white organic light-emitting diodes using a blue material with thermally activated delayed fluorescence,” J. Mater. Chem. C Mater. Opt. Electron. Devices. 2, , 8191 –8197 (2014).CrossRef
Zhang  D.  et al., “Highly efficient hybrid warm white organic light-emitting diodes using a blue thermally activated delayed fluorescence emitter: exploiting the external heavy-atom effect,” Light Sci. Appl.. 4, , 1 –7 (2015).CrossRef
Nishide  J.  et al., “High-efficiency white organic light-emitting diodes using thermally activated delayed fluorescence,” Appl. Phys. Lett.. 104, (23 ), 233304  (2014). 0003-6951 CrossRef
Kim  M., and Lee  J. Y., “Donor–acceptor type material as a triplet host for high efficiency white phosphorescent organic light-emitting diodes,” Synth. Met.. 199, , 105 –109 (2015). 0379-6779 CrossRef

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