Ex) Article Title, Author, Keywords
Current Optics
and Photonics
Ex) Article Title, Author, Keywords
Current Optics and Photonics 2020; 4(3): 161-173
Published online June 25, 2020 https://doi.org/10.3807/COPP.2020.4.3.161
Copyright © Optical Society of Korea.
Seunghyun Rhee1, Kyunghwan Kim1, Jeongkyun Roh2,*, and Jeonghun Kwak1,**
Corresponding author: jkroh@pusan.ac.kr
Colloidal quantum dots (QDs) have gained tremendous attention as a key material for highly advanced display technologies. The performance of QD light-emitting diodes (QLEDs) has improved significantly over the past two decades, owing to notable progress in both material development and device engineering. The brightness of QLEDs has improved by more than three orders of magnitude from that of early-stage devices, and has attained a value in the range of traditional inorganic LEDs. The emergence of high-luminance (HL) QLEDs has induced fresh demands to incorporate the unique features of QDs into a wide range of display applications, beyond indoor and mobile displays. Therefore it is necessary to assess the present status and prospects of HL-QLEDs, to expand the application domain of QD-based light sources. As part of this study, we review recent advances in HL-QLEDs. In particular, based on reports of brightness exceeding 105 cd/m2, we have summarized the major approaches toward achieving high brightness in QLEDs, in terms of material development and device engineering. Furthermore, we briefly introduce the recent progress achieved toward QD laser diodes, being the next step in the development of HL-QLEDs. This review provides general guidelines for achieving HL-QLEDs, and reveals the high potential of QDs as a universal material solution that can enable realization of a wide range of display applications.
Keywords: Quantum dots, High luminance, QLED, QD laser diode
Colloidal quantum dots (QDs), which are semiconductor nanoparticles, have attracted considerable attention because of their advantageous features, such as tunable emission spectra (from ultraviolet to infrared), remarkable photo-luminescence (PL), and narrow full-width at half-maximum emission bandwidth [1-8]. QDs have been regarded as a key material that can enable next-generation displays, given their advantages of high PL quantum yields (QYs) and cost-effective, solution-based processability [9-15]. Recently, a QD-based color-enhancement film was commercialized successfully in the display market, establishing the superiority of QDs in display applications. Intensive studies are now being conducted to realize the next technology frontier: the replacement of present-day emissive display technologies with QD light-emitting diodes (QLEDs).
The first QD-based electroluminescent device was demonstrated by Colvin
Figure 1(a) shows a broad range of display applications, with respect to required brightness. According to their purposes and operating conditions, displays require different levels of maximum brightness, the so-called
In principle, QDs are expected to be a universal material solution that can cover the entire range of applications with regard to brightness, shown in Fig. 1(a). In the early stages of development, QLEDs were able to cover the low-brightness application regime; at present, they are capable of expanding their applicability to the higher-brightness regime. QD-based light sources covering the brightness regime >105 cd/m2 are termed
Here, as an aid to boost this growth, we review the recent progress and remaining challenges of HL-QLEDs. We briefly introduce the fundamental operating principles of QLEDs, and then highlight the key factors for achieving HL-QLEDs in terms of two important aspects: material development and device engineering. We summarize the current status of HL-QLEDs with a few important milestones. This is followed by a description of remaining challenges. A review of the key factors impelling HL-QLED technology will guide us to the recent development toward a QD laser diode, which is a longstanding goal in this research field. We cover the most recent research advances in the development of QD laser diodes, and briefly discuss the future prospects. This review introduces the great potential of QDs as a universal material solution to enable a wide range of display applications, and to aid in expanding the application domain of QD-based light sources.
Intensive studies to improve the performance of QLEDs have resulted in the establishment of a standard device structure based on
TABLE 1. Summary of QLEDs with peak brightness of over 100,000 cd/m2
In this chapter, we review key advancements in HL-QLEDs with respect to two aspects: material development and device engineering. First, we overview material-based approaches for achieving HL-QLEDs, which mainly focus on improving the luminescence efficiency of QDs. Then, we cover device-related approaches for HL-QLEDs, which includes engineering of charge-injection properties to improve charge balance, management of heat dissipation, improvement of light extraction, and use of a tandem structure.
A colloidal QD consists of three components: core, shell, and ligands. It is essential to select appropriate materials for each component to achieve highly luminescent QDs. Numerous attempts have been made to improve the luminescence efficiency of QDs, and the following have been identified as the key factors for achieving high luminescence efficiency: (ⅰ) selection of an appropriate core/shell structure, and (ⅱ) use of suitable ligands for stable chemical bonding with high conductivity. With regard to the core/shell structure, construction of the shell using a material with a larger band gap than that of the core enhances the luminescence efficiency of QDs noticeably. Therefore, it has become a typical structure for QDs and is called the type-I configuration. Various core/shell material combinations, such as CdSe/ZnS, CdSe/CdS, InP/ZnS, CdSe/ZnSe, and PbS/CdS, have been investigated [44-46].
Type-I core/shell QDs confine electrons and holes in the core efficiently, and consequently improve QLED performance. However, the large difference between the lattice parameters of core and shell in type-I QDs results in inhomogeneous growth of the shell, which produces internal defects in the QDs [45]. These defects are the main obstacles to achieving high luminescence efficiency (QY), because they function as nonradiative recombination centers. The QY is defined by the ratio of radiative recombination rate to the overall recombination rate: QY =
To reduce the lattice-mismatch-induced traps inside QDs, an intermediate shell is incorporated between core and outer shell. For example, Talapin
Another highly effective approach that can reduce defects caused by lattice mismatch in QDs is the use of a composition-gradient core/shell structure [4, 6, 35, 51-54]. The gradual change in composition from the core to the shell results in a significantly smoothed lattice-parameter discrepancy, which yields a large reduction of interfacial defects and nonradiative recombination. For example, Dey
In addition to the core/shell structure, the selection of an appropriate surface ligand is important for achieving high-QY QDs [55, 56]. Ligands play a significant role in both the processability and conductivity of QDs. For example, a long-hydrocarbon-chain ligand (
Inorganic halogen ions are employed as efficient short ligands to replace long-hydrocarbon ligands, to improve the carrier transport among QDs, and between QDs and adjacent layers. The halide anions I-, Br-, and Cl- have been used to stabilize colloidal materials, control interdot distance, and enhance the charge transport of QDs [57, 58]. Kang
Owing to its strong binding property, thiol ligands are also widely used as an effective ligand for high-performance QLEDs. The thiol-type ligand, which has relatively short chains for efficient carrier injection and strong anchoring properties on the QD surface, is an electron-donor group that can reduce the VBs of QDs and hole-injection barriers. Shen
Apart from the high PL QY and conductivity of QD materials, the adoption of appropriate device structure significantly affects the achievement of HL-QLEDs. The device structure, ETL, and HTL together determine the carrier-injection properties, carrier confinement in a QD’s emissive layer (EML), and the operational stability of QLEDs. Balancing of the injection rates of electrons and holes into the QD EML, and maintenance of operational stability, are the key factors in achieving HL-QLEDs [25, 33, 60]. A disparity between electron and hole injection rates induces nonradiative Auger recombination, resulting in a reduction of the luminance efficiency of a QLED. In addition, the overflow of majority carriers (electrons) to a counter CTL (HTL) accelerates the degradation of organic HTL materials, which attenuates the operational stability of devices [25, 60]. Therefore, various approaches to improving the charge-injection balance in QLEDs have been adopted, in terms of device engineering.
Charge imbalance in QLEDs arises from the different transport and injection capabilities of electrons and holes. For example, ZnO nanoparticles (NPs), the most widely used material for the ETL in a QLED, exhibit a CB energy level highly similar to that of QDs, resulting in straightforward (and occasionally spontaneous) injection of electrons. In contrast, most of the organic hole-transport materials used for the HTL, such as poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine) (TFB), poly(N,N′-bis (4-butylphenyl)-N,N′-bis(phenyl)-benzidine (poly-TPD), poly(9-vinlycarbazole) (PVK), 4,4′-bis(carbazole-9-yl)biphenyl (CBP), and Tris(4-carbazoyl-9-ylphenyl)amine (TCTA), exhibit a large difference in VB energy level from that of QDs. This disrupts efficient injection of holes. Furthermore, the large difference between the electron mobility of ZnO NPs and hole mobility of organic HTL materials worsens the charge imbalance [61, 62]. Various strategies have been recommended to alleviate this disparity, such as enhancing hole injection by modifying the HTL [21, 23-25] and reducing electron injection by ion doping [4, 63, 64] or by inserting an electron-blocking layer (EBL) [19, 65-67]. However, suppressing electron injection results in increased operational stress in QLEDs, so alternative approaches to enhancing hole injection by modulating HTLs in the QLED architecture have been proposed instead.
Unlike the straightforward electron injection/transport using a ZnO NP ETL, hole injection/transport using organic HTLs encounters several obstacles, owing to the large difference in VB level versus QDs and relatively low hole mobilities. Therefore, an inverted QLED structure, which enables the use of deep-VB organic material for the HTL, was proposed to enhance hole injection and device performance. Kwak
Although the alleviation of charge-injection imbalance can enhance the operational stability of a device by suppressing excessive electron overflow from the QD EML to the HTL, Joule heating (which occurs because of high current density) is a major factor hindering stable operation and high luminance of devices. Previous studies [68-71] have reported that organic films are susceptible to thermal decomposition. Irreversible thermal degradation of organic HTLs can occur during QLED operation; the organic HTL as well as the QD emissive layer are vulnerable to Joule heating. Zhao
Therefore, it is important to use QDs that exhibit high thermal stability, and to reduce the heat generated during the operation of HL-QLEDs. Sun
Apart from the single-EML-based charge-to-photon conversion process, other methods adopting novel structures for QLEDs have been studied. Although the inherent PL QY of QDs and charge-carrier injection of QLEDs have been developed extensively, only a limited fraction of the total generated photons can be extracted to the exterior of a device [73]. The limited extraction efficiency ηext originates from Snell’s law, which states that only a portion of photons can pass through a material with a different refractive index. Therefore, reducing outcoupling loss and enhancing ηext are intuitive methods for improving the brightness of QLEDs [74]. One of the optical structures for increasing ηext is the top-emitting structure, which emits light through its top electrode. This can prevent the large outcoupling loss caused by the substrate [75, 76]. In addition, a remarkable advantage is that the light reflected from the two electrodes can be used to control the cavity effect to improve the color purity and normal-direction brightness of the QLED. Liu
Outcoupling loss also occurs due to the large difference in the refractive indices of the glass substrate (
Meanwhile, increased luminance can be obtained from a double- or triple-EML QLED structure: a tandem structure. A tandem structure adopts two or more electroluminescent (EL) units in a single device, to increase the efficiency and brightness of the QLED. As the multiple EL units are operated at equal current density, the number of photons generated increases proportionally to that the number of of EL units. Zhang
The significant improvement in QD technologies, in terms of materials and devices, has generated a fresh opportunity to achieve a longstanding goal in this field: the development of a QD-based laser diode. This is the missing piece among QD-based light sources, which would enlarge the application area of projection-type displays by enabling distinguishing features of QDs such as conveniently tunable emission color, flexibility, low weight, and inexpensive processability.
The development of QD laser diodes has been hindered by the intrinsic nature of QDs, wherein light amplification occurs only under multiexciton conditions because of multifold degeneracy of the band-edge states, and excitons decay rapidly by nonradiative Auger recombination [77, 78]. This results in an ultrashort lifetime of optical gain, and complicates lasing in QDs. However, because of the notable progress in material engineering, state-of-the-art QDs exhibit substantially suppressed Auger recombination, which has opened up potential approaches to achieving QD laser diodes [4, 79]. In this chapter, we briefly review the current status of and future prospects for QD laser diodes, with the most recent research advancements.
A fundamental problem plaguing the development of QD laser diodes is the multiexciton nature of their optical gain [77, 78]. When multiexciton species exist in QDs, rapid nonradiative Auger recombination dominates over radiative recombination, thereby deactivating optical gain [80]. This impedes the realization of lasing in QDs, which requires successive light amplification by spontaneous emission. Furthermore, achievement of lasing in QDs becomes even more challenging when slow electrical excitation is used as a pumping source. Therefore, the development of QDs with suppressed Auger recombination is a prerequisite for the development of QD laser diodes.
Because of the tremendous progress in material engineering (reviewed in Chapter 2), state-of-the art QDs exhibit substantially suppressed Auger decay. As a result, notable progress toward QD laser diodes has been achieved recently. One of the most important milestones was attained by Lim
The demonstration of optical gain under electrical excitation resulted in the next step toward QD laser diodes: integration of an optical cavity that can provide an optical-feedback path in an LED structure. Various types of optical cavities, such as Fabry-Perot, whispering-gallery-mode, and distributed-feedback (DFB), can be considered. Roh
Two significant obstacles to achieving QD laser diodes (population inversion under electrical pumping, and integration of optical cavity into the LED structure) have been addressed. The remaining challenge is, in principle, to combine those two efforts: achieving population inversion in a QLED integrated with an optical cavity, under electrical excitation. To increase current densities to a level sufficient for achieving population inversion, short-pulsed operation can be employed to prevent massive heat generation inside the device, or a high-thermal-conductivity substrate can be used for heat management. This should be accompanied by efforts to reduce the lasing threshold, to accommodate this development. The recent demonstration of sub-single-exciton lasing in charged QDs is a good example of a feasible method for reducing the lasing threshold [82].
This paper has reviewed recent studies aimed at overcoming the obstacles to achieving HL-QLEDs. The status of the development of QD laser diodes was also examined briefly. Various efforts to achieve HL-QLEDs, in terms of material development and device engineering, have been summarized. With regard to material development, the construction of an appropriate QD core/shell structure that can minimize lattice-mismatch-induced traps has been the major focus of research. The use of alloyed or gradient shell structures was observed to be the most efficient method for developing highly luminescent QDs. In addition, the selection of appropriate surface ligands also plays a significant role in realizing high luminance in QDs. Short ligands are preferred, so as not to distort charge transport and injection. Strong chemical bonding with QDs is also important for producing high-quality film with few surface traps. In terms of device engineering, designing a device architecture that can suppress nonradiative Auger recombination by improving charge balance has been the most important research direction for achieving HL-QLEDs. In addition, the moderation of Joule heating, improvement in light-extraction efficiency, and use of tandem structures have also exhibited their effectiveness in improving the luminance of QLEDs.
Notwithstanding this notable progress, several issues remain to be addressed. The first of these is the degraded luminescence efficiency of QDs in films. Although QDs in solution exhibited near-unity PL QY, the PL QY of a QD film still displays considerable suppression, owing to the inherent defects formed during fabrication, and nonradiative Auger recombination by dot interaction. Another issue is the development (or identification) of a better HTL material with a higher hole mobility, closer VB to that of QDs, and most importantly high thermal stability. Present HTL technologies rely substantially on organic materials, but diverse types of materials should be employed and investigated. By solving those remaining challenges, the domain of application for QD-based light sources will be widened, offering fresh opportunities for a wide range of display applications.
Current Optics and Photonics 2020; 4(3): 161-173
Published online June 25, 2020 https://doi.org/10.3807/COPP.2020.4.3.161
Copyright © Optical Society of Korea.
Seunghyun Rhee1, Kyunghwan Kim1, Jeongkyun Roh2,*, and Jeonghun Kwak1,**
1
Correspondence to:jkroh@pusan.ac.kr
Colloidal quantum dots (QDs) have gained tremendous attention as a key material for highly advanced display technologies. The performance of QD light-emitting diodes (QLEDs) has improved significantly over the past two decades, owing to notable progress in both material development and device engineering. The brightness of QLEDs has improved by more than three orders of magnitude from that of early-stage devices, and has attained a value in the range of traditional inorganic LEDs. The emergence of high-luminance (HL) QLEDs has induced fresh demands to incorporate the unique features of QDs into a wide range of display applications, beyond indoor and mobile displays. Therefore it is necessary to assess the present status and prospects of HL-QLEDs, to expand the application domain of QD-based light sources. As part of this study, we review recent advances in HL-QLEDs. In particular, based on reports of brightness exceeding 105 cd/m2, we have summarized the major approaches toward achieving high brightness in QLEDs, in terms of material development and device engineering. Furthermore, we briefly introduce the recent progress achieved toward QD laser diodes, being the next step in the development of HL-QLEDs. This review provides general guidelines for achieving HL-QLEDs, and reveals the high potential of QDs as a universal material solution that can enable realization of a wide range of display applications.
Keywords: Quantum dots, High luminance, QLED, QD laser diode
Colloidal quantum dots (QDs), which are semiconductor nanoparticles, have attracted considerable attention because of their advantageous features, such as tunable emission spectra (from ultraviolet to infrared), remarkable photo-luminescence (PL), and narrow full-width at half-maximum emission bandwidth [1-8]. QDs have been regarded as a key material that can enable next-generation displays, given their advantages of high PL quantum yields (QYs) and cost-effective, solution-based processability [9-15]. Recently, a QD-based color-enhancement film was commercialized successfully in the display market, establishing the superiority of QDs in display applications. Intensive studies are now being conducted to realize the next technology frontier: the replacement of present-day emissive display technologies with QD light-emitting diodes (QLEDs).
The first QD-based electroluminescent device was demonstrated by Colvin
Figure 1(a) shows a broad range of display applications, with respect to required brightness. According to their purposes and operating conditions, displays require different levels of maximum brightness, the so-called
In principle, QDs are expected to be a universal material solution that can cover the entire range of applications with regard to brightness, shown in Fig. 1(a). In the early stages of development, QLEDs were able to cover the low-brightness application regime; at present, they are capable of expanding their applicability to the higher-brightness regime. QD-based light sources covering the brightness regime >105 cd/m2 are termed
Here, as an aid to boost this growth, we review the recent progress and remaining challenges of HL-QLEDs. We briefly introduce the fundamental operating principles of QLEDs, and then highlight the key factors for achieving HL-QLEDs in terms of two important aspects: material development and device engineering. We summarize the current status of HL-QLEDs with a few important milestones. This is followed by a description of remaining challenges. A review of the key factors impelling HL-QLED technology will guide us to the recent development toward a QD laser diode, which is a longstanding goal in this research field. We cover the most recent research advances in the development of QD laser diodes, and briefly discuss the future prospects. This review introduces the great potential of QDs as a universal material solution to enable a wide range of display applications, and to aid in expanding the application domain of QD-based light sources.
Intensive studies to improve the performance of QLEDs have resulted in the establishment of a standard device structure based on
TABLE 1.. Summary of QLEDs with peak brightness of over 100,000 cd/m2.
In this chapter, we review key advancements in HL-QLEDs with respect to two aspects: material development and device engineering. First, we overview material-based approaches for achieving HL-QLEDs, which mainly focus on improving the luminescence efficiency of QDs. Then, we cover device-related approaches for HL-QLEDs, which includes engineering of charge-injection properties to improve charge balance, management of heat dissipation, improvement of light extraction, and use of a tandem structure.
A colloidal QD consists of three components: core, shell, and ligands. It is essential to select appropriate materials for each component to achieve highly luminescent QDs. Numerous attempts have been made to improve the luminescence efficiency of QDs, and the following have been identified as the key factors for achieving high luminescence efficiency: (ⅰ) selection of an appropriate core/shell structure, and (ⅱ) use of suitable ligands for stable chemical bonding with high conductivity. With regard to the core/shell structure, construction of the shell using a material with a larger band gap than that of the core enhances the luminescence efficiency of QDs noticeably. Therefore, it has become a typical structure for QDs and is called the type-I configuration. Various core/shell material combinations, such as CdSe/ZnS, CdSe/CdS, InP/ZnS, CdSe/ZnSe, and PbS/CdS, have been investigated [44-46].
Type-I core/shell QDs confine electrons and holes in the core efficiently, and consequently improve QLED performance. However, the large difference between the lattice parameters of core and shell in type-I QDs results in inhomogeneous growth of the shell, which produces internal defects in the QDs [45]. These defects are the main obstacles to achieving high luminescence efficiency (QY), because they function as nonradiative recombination centers. The QY is defined by the ratio of radiative recombination rate to the overall recombination rate: QY =
To reduce the lattice-mismatch-induced traps inside QDs, an intermediate shell is incorporated between core and outer shell. For example, Talapin
Another highly effective approach that can reduce defects caused by lattice mismatch in QDs is the use of a composition-gradient core/shell structure [4, 6, 35, 51-54]. The gradual change in composition from the core to the shell results in a significantly smoothed lattice-parameter discrepancy, which yields a large reduction of interfacial defects and nonradiative recombination. For example, Dey
In addition to the core/shell structure, the selection of an appropriate surface ligand is important for achieving high-QY QDs [55, 56]. Ligands play a significant role in both the processability and conductivity of QDs. For example, a long-hydrocarbon-chain ligand (
Inorganic halogen ions are employed as efficient short ligands to replace long-hydrocarbon ligands, to improve the carrier transport among QDs, and between QDs and adjacent layers. The halide anions I-, Br-, and Cl- have been used to stabilize colloidal materials, control interdot distance, and enhance the charge transport of QDs [57, 58]. Kang
Owing to its strong binding property, thiol ligands are also widely used as an effective ligand for high-performance QLEDs. The thiol-type ligand, which has relatively short chains for efficient carrier injection and strong anchoring properties on the QD surface, is an electron-donor group that can reduce the VBs of QDs and hole-injection barriers. Shen
Apart from the high PL QY and conductivity of QD materials, the adoption of appropriate device structure significantly affects the achievement of HL-QLEDs. The device structure, ETL, and HTL together determine the carrier-injection properties, carrier confinement in a QD’s emissive layer (EML), and the operational stability of QLEDs. Balancing of the injection rates of electrons and holes into the QD EML, and maintenance of operational stability, are the key factors in achieving HL-QLEDs [25, 33, 60]. A disparity between electron and hole injection rates induces nonradiative Auger recombination, resulting in a reduction of the luminance efficiency of a QLED. In addition, the overflow of majority carriers (electrons) to a counter CTL (HTL) accelerates the degradation of organic HTL materials, which attenuates the operational stability of devices [25, 60]. Therefore, various approaches to improving the charge-injection balance in QLEDs have been adopted, in terms of device engineering.
Charge imbalance in QLEDs arises from the different transport and injection capabilities of electrons and holes. For example, ZnO nanoparticles (NPs), the most widely used material for the ETL in a QLED, exhibit a CB energy level highly similar to that of QDs, resulting in straightforward (and occasionally spontaneous) injection of electrons. In contrast, most of the organic hole-transport materials used for the HTL, such as poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine) (TFB), poly(N,N′-bis (4-butylphenyl)-N,N′-bis(phenyl)-benzidine (poly-TPD), poly(9-vinlycarbazole) (PVK), 4,4′-bis(carbazole-9-yl)biphenyl (CBP), and Tris(4-carbazoyl-9-ylphenyl)amine (TCTA), exhibit a large difference in VB energy level from that of QDs. This disrupts efficient injection of holes. Furthermore, the large difference between the electron mobility of ZnO NPs and hole mobility of organic HTL materials worsens the charge imbalance [61, 62]. Various strategies have been recommended to alleviate this disparity, such as enhancing hole injection by modifying the HTL [21, 23-25] and reducing electron injection by ion doping [4, 63, 64] or by inserting an electron-blocking layer (EBL) [19, 65-67]. However, suppressing electron injection results in increased operational stress in QLEDs, so alternative approaches to enhancing hole injection by modulating HTLs in the QLED architecture have been proposed instead.
Unlike the straightforward electron injection/transport using a ZnO NP ETL, hole injection/transport using organic HTLs encounters several obstacles, owing to the large difference in VB level versus QDs and relatively low hole mobilities. Therefore, an inverted QLED structure, which enables the use of deep-VB organic material for the HTL, was proposed to enhance hole injection and device performance. Kwak
Although the alleviation of charge-injection imbalance can enhance the operational stability of a device by suppressing excessive electron overflow from the QD EML to the HTL, Joule heating (which occurs because of high current density) is a major factor hindering stable operation and high luminance of devices. Previous studies [68-71] have reported that organic films are susceptible to thermal decomposition. Irreversible thermal degradation of organic HTLs can occur during QLED operation; the organic HTL as well as the QD emissive layer are vulnerable to Joule heating. Zhao
Therefore, it is important to use QDs that exhibit high thermal stability, and to reduce the heat generated during the operation of HL-QLEDs. Sun
Apart from the single-EML-based charge-to-photon conversion process, other methods adopting novel structures for QLEDs have been studied. Although the inherent PL QY of QDs and charge-carrier injection of QLEDs have been developed extensively, only a limited fraction of the total generated photons can be extracted to the exterior of a device [73]. The limited extraction efficiency ηext originates from Snell’s law, which states that only a portion of photons can pass through a material with a different refractive index. Therefore, reducing outcoupling loss and enhancing ηext are intuitive methods for improving the brightness of QLEDs [74]. One of the optical structures for increasing ηext is the top-emitting structure, which emits light through its top electrode. This can prevent the large outcoupling loss caused by the substrate [75, 76]. In addition, a remarkable advantage is that the light reflected from the two electrodes can be used to control the cavity effect to improve the color purity and normal-direction brightness of the QLED. Liu
Outcoupling loss also occurs due to the large difference in the refractive indices of the glass substrate (
Meanwhile, increased luminance can be obtained from a double- or triple-EML QLED structure: a tandem structure. A tandem structure adopts two or more electroluminescent (EL) units in a single device, to increase the efficiency and brightness of the QLED. As the multiple EL units are operated at equal current density, the number of photons generated increases proportionally to that the number of of EL units. Zhang
The significant improvement in QD technologies, in terms of materials and devices, has generated a fresh opportunity to achieve a longstanding goal in this field: the development of a QD-based laser diode. This is the missing piece among QD-based light sources, which would enlarge the application area of projection-type displays by enabling distinguishing features of QDs such as conveniently tunable emission color, flexibility, low weight, and inexpensive processability.
The development of QD laser diodes has been hindered by the intrinsic nature of QDs, wherein light amplification occurs only under multiexciton conditions because of multifold degeneracy of the band-edge states, and excitons decay rapidly by nonradiative Auger recombination [77, 78]. This results in an ultrashort lifetime of optical gain, and complicates lasing in QDs. However, because of the notable progress in material engineering, state-of-the-art QDs exhibit substantially suppressed Auger recombination, which has opened up potential approaches to achieving QD laser diodes [4, 79]. In this chapter, we briefly review the current status of and future prospects for QD laser diodes, with the most recent research advancements.
A fundamental problem plaguing the development of QD laser diodes is the multiexciton nature of their optical gain [77, 78]. When multiexciton species exist in QDs, rapid nonradiative Auger recombination dominates over radiative recombination, thereby deactivating optical gain [80]. This impedes the realization of lasing in QDs, which requires successive light amplification by spontaneous emission. Furthermore, achievement of lasing in QDs becomes even more challenging when slow electrical excitation is used as a pumping source. Therefore, the development of QDs with suppressed Auger recombination is a prerequisite for the development of QD laser diodes.
Because of the tremendous progress in material engineering (reviewed in Chapter 2), state-of-the art QDs exhibit substantially suppressed Auger decay. As a result, notable progress toward QD laser diodes has been achieved recently. One of the most important milestones was attained by Lim
The demonstration of optical gain under electrical excitation resulted in the next step toward QD laser diodes: integration of an optical cavity that can provide an optical-feedback path in an LED structure. Various types of optical cavities, such as Fabry-Perot, whispering-gallery-mode, and distributed-feedback (DFB), can be considered. Roh
Two significant obstacles to achieving QD laser diodes (population inversion under electrical pumping, and integration of optical cavity into the LED structure) have been addressed. The remaining challenge is, in principle, to combine those two efforts: achieving population inversion in a QLED integrated with an optical cavity, under electrical excitation. To increase current densities to a level sufficient for achieving population inversion, short-pulsed operation can be employed to prevent massive heat generation inside the device, or a high-thermal-conductivity substrate can be used for heat management. This should be accompanied by efforts to reduce the lasing threshold, to accommodate this development. The recent demonstration of sub-single-exciton lasing in charged QDs is a good example of a feasible method for reducing the lasing threshold [82].
This paper has reviewed recent studies aimed at overcoming the obstacles to achieving HL-QLEDs. The status of the development of QD laser diodes was also examined briefly. Various efforts to achieve HL-QLEDs, in terms of material development and device engineering, have been summarized. With regard to material development, the construction of an appropriate QD core/shell structure that can minimize lattice-mismatch-induced traps has been the major focus of research. The use of alloyed or gradient shell structures was observed to be the most efficient method for developing highly luminescent QDs. In addition, the selection of appropriate surface ligands also plays a significant role in realizing high luminance in QDs. Short ligands are preferred, so as not to distort charge transport and injection. Strong chemical bonding with QDs is also important for producing high-quality film with few surface traps. In terms of device engineering, designing a device architecture that can suppress nonradiative Auger recombination by improving charge balance has been the most important research direction for achieving HL-QLEDs. In addition, the moderation of Joule heating, improvement in light-extraction efficiency, and use of tandem structures have also exhibited their effectiveness in improving the luminance of QLEDs.
Notwithstanding this notable progress, several issues remain to be addressed. The first of these is the degraded luminescence efficiency of QDs in films. Although QDs in solution exhibited near-unity PL QY, the PL QY of a QD film still displays considerable suppression, owing to the inherent defects formed during fabrication, and nonradiative Auger recombination by dot interaction. Another issue is the development (or identification) of a better HTL material with a higher hole mobility, closer VB to that of QDs, and most importantly high thermal stability. Present HTL technologies rely substantially on organic materials, but diverse types of materials should be employed and investigated. By solving those remaining challenges, the domain of application for QD-based light sources will be widened, offering fresh opportunities for a wide range of display applications.
TABLE 1. Summary of QLEDs with peak brightness of over 100,000 cd/m2