Advances in Colloidal Quantum Dot Laser Diodes

Abstract

Colloidal quantum dots possess distinctive optoelectronic properties, rendering them a promising material for gain applications. Additionally, colloidal quantum dot lasers can emit light over a broad range of wavelengths, spanning from the near-infrared to the visible spectrum, which makes them suitable for various applications. The potential impact of colloidal quantum dot lasers on various industries and technologies cannot be overstated. Their continued development and optimization represent an exciting area of research that could revolutionize numerous fields. The review examines the challenges related to achieving lasing with colloidal quantum dots, discusses potential approaches to overcome these challenges, and surveys the latest advances made toward achieving this objective.

1. Introduction

Since its first demonstrations in the early 1960s [1], the laser has become a crucial tool for research, but available materials can not cover all visible light emissions. Materials that produce optical gain and sustain laser emission with tunable wavelengths would spark research interest [2, 3, 4]. Colloidal quantum dots (CQDs) are promising laser materials due to their photophysical properties, solution processability, and production capability. They offer advantages over epitaxial and dry processing methods, simplifying device preparation and reducing production costs. CQDs offer advantages of both inorganic and organic materials, allowing large-area and low-cost solution processing in device fabrication, making them a typical example of a solution-processable laser device [5]. The unique technological advantages of CQD lasers have been widely acknowledged since they first appeared [3], including on-chip optical interconnects [6, 7] and integrated photonic circuits [8, 9], wearable devices [10, 11] and advanced medical imaging and diagnostics [12], clandestine markers [13], and many others. Extensive research efforts have led to significant progress in achieving CQD lasing with both optical and electrical pumping. Advances include continuous-wave optical pumping [14], optical gain in electrically-driven QLEDs [15], on-chip integrated CQD lasers [8], and dual-function devices combining optically pumped lasing and electrically-driven LED [16].

However, despite these advances, CQD lasers are still in the stage of laboratory demonstrations. In most reported studies of CQD lasing, shape control is one of the primary strategies used to suppress Auger recombination and reduce the laser threshold. Many materials scientists have utilized various strategies from the perspective of designing and synthesizing colloidal quantum dots to suppress Auger recombination, enhance the emission efficiency of biexcitons and multiexcitons, and achieve lasing using colloidal quantum dots with different shapes, for example, spherical CQDs [17], hetero-CQDs of type I [18], type II [19] and quasi-type II [20], nanorods [21], dot-in-rods [22], nanoplatelets [23], hetero-NPLs [24], and cube-shaped perovskite CQDs [25]. Relevant studies have shown that increasing the volume of nanomaterials can effectively reduce the threshold [26, 27]. At present, continuously graded quantum dots (cg-QDs) are considered ideal laser gain materials because they can suppress Auger recombination by eliminating sharp discontinuities in the confinement potential [28].

CQD laser presents challenges due to the need for a high-quality feedback structure or optical cavity [29]. Although CQD lasers have been achieved using various cavity designs, research on this topic is not as systematic as the methods proposed to suppress Auger decay. The optical gain threshold of CQDs is still too high for practical applications, and there are still key issues that need to be addressed when coupling CQDs with optical resonators. This chapter focuses on the development of lasers based on different cavity designs, discussing the working mechanisms in optically pumped CQD lasers and the key factors influencing the performance of different optical resonators. It also presents progress made in developing high current density light-emitting diodes (LEDs) using various strategies and discusses the challenges and future development of electrically pumped CQD lasers.

2. CQD lasing with various optical resonators

Current optoelectronic technology relies on expensive and rigid epitaxially grown semiconductors, while CQDs offer tunable optical bandgap from UV to mid-IR, cost-effective processing, substrate independence, and potential long-term stability. This creates an opportunity for CQDs to revolutionize the field of laser-based optoelectronics [29]. Therefore, CQDs are compatible with nearly all types of cavity architectures [30].

The resonator cavity is essential for optical feedback in light amplification. Resonator design determines resonant modes and output beam characteristics, making it crucial for high-quality, low-threshold CQD laser development. Resonators come in various forms and shapes, ranging from simple Fabry-Perot cavities to more complex geometries that require complex theories and precise preparation facilities. Examples of resonator geometries include planar distributed Bragg reflector (DBR) cavities, distributed feedback (DFB), whispering gallery modes (WGM) cavities of different geometries, microspheres, microdiscs, photonic crystals, etc. (as shown in Figure 1). Among them, DFB and DBR resonators are the most widely used architectures for CQD lasers. Each of these geometries has unique properties that affect their lasing performance.

Figure 1.

Resonator structures used for CQD lasers. (a) DBR cavities. (b) 1D DFB gratings. (c) WGM cavities. (d) Photonic crystals.

Lasing phenomena have been observed in various optical structures for CQDs, including cylindrical [31, 32, 33], spherical [34, 35], and microring [36] WGMs, as well as DFB [14, 16, 17, 18, 37, 38, 39, 40, 41, 42], F-P [43, 44], and VCSELs [45, 46]. Table 1 below summarizes the performance parameters of CQD lasers in various optical structures since Malko et al. pioneered coating CQD solid films onto the inside wall of microcapillary tubes and claimed the observation of lasing in whispering-gallery modes in 2002. Besides the resonator geometry, other factors can also affect the performance of CQD lasers, such as the quality and uniformity of the gain medium, and the optical coupling efficiency between the gain medium and the resonator.

Materials	Optical structure	Threshold	Wavelength	Pump source	Publication date	Ref.
CdSe/ZnS	WGM (Cylindrical)	1.25 mJ/cm^2	~614 nm	400 nm; 100 fs	2002	[31]
CdSe/ZnS	WGM (Cylindrical)	3 mJ/0.75 mJ	547/626 nm	532 nm	2002	[32]
CdSe/ZnS	DFB	1 uJ	583-625 nm	400 nm; 100 fs	2002	[37]
CdSe/CdZnS	WGM (spherical)	0.74 mJ/cm^2	669 nm	400 nm; 100 fs	2005	[34]
CdSe/ZnS	Fabry-Perot (Self-assembled)	3 mJ/cm^2	614 nm	390 nm; 150 fs	2010	[43]
InP/ZnS	DFB	2 mJ/cm^2	616 nm	400 nm; 80 fs	2011	[17]
CdSe/ZnS	Flexible DFB	4 mJ/cm^2	610–640 nm	355 nm; 5 ns	2011	[38]
CdSe/Zno.5; Cdo.5S CQD	VCSEL (Combined)	60 uJ/cm^2	620 nm	400 nm; 100 fs	2012	[45]
CQD	Flexible DFB	~500 uJ/cm^2	607 nm	355 nm; 5 ns	2014	[39]
CdSe/ZnCdS	2D DFB	120/280/330 uJ/cm^2	610/575/455 nm	355 nm/532 nm; 0.4 ns	2014	[40]
CdSe/CdS	VCSEL (Combined)	2.49 mJ/cm^2	597–603 nm	800 nm; 120 fs	2014	[46]
CdZnS/ZnS	WGM (Cylindrical)	25.2 mJ/cm^2	597–603 nm	385 nm; 5 ns	2015	[33]
CdSe/CdS	Fabry-Perot (Self-assembled)	10 uJ/cm^2	632 nm	400 nm; 70 fs	2015	[44]
CdSe/CdS/ZnS	2D-DFB	88 kW/cm^2	~645 nm	442 nm; 1.8 us	2015	[41]
CdSe-CdS	2D-DFB	6.4–8.4 kW/cm^2	~639 nm	CW 442 nm	2017	[14]
CdSe/CdS	WGM (spherical)	10 uJ/cm^2	~638 nm	400 nm; 100–200 fs	2018	[35]
CdSe/CdS/ZnS	WGM(microring)	22 uJ/cm^2	610 nm	405 nm; 340 fs	2018	[36]
CdSe/CdxZn1-; xSe cg-QDs	2D-DFB	9 uJ/cm^2	~628 nm	400 nm; 130 fs	2019	[18]
cg; CdSe/CdxZn1-xS; e/ZnSeo.5So.5/Z; nS	2D-DFB	5.5 uJ/cm^2	~630.9 nm	400 nm; 130 fs	2020	[16]
CdSe/CdxZn1-; xSe cg-QDs	2D-DFB	82 uJ/cm^2	~615 nm	343 nm; 190 fs	2023	[42]

Table 1.

Representative CQD lasers with various optical structures.

The DFB structure has proven to be an effective and versatile optical resonator for CQD lasers, with numerous research advances made in recent years, as shown in Figure 2. In CQD DFB lasers, the grating structure is typically created by periodic modulation of the refractive index or the gain/loss of the waveguide. This modulation can be achieved through various techniques, such as electron beam, holographic, and nanoimprint lithography. Among various optical structures, DFB is one of the earliest and most popular choices. In a survey of 20 representative articles on CQD lasers, DFB was used as an optical resonator in 10 of them. Adachi et al. and Fan et al. achieved quasi-continuous [41] and continuous wave [14] CQD laser emission using the DFB structure, which further increased its popularity. Due to its compatibility with a QD-LED-like device [16], DFB has become one of the feasible resonator structures for realizing electrically driven devices. The incorporation of DFB into a QD-LED-like device enables the device to operate in both electrical and optical pumping modes. This has led to the development of electrically pumped CQD DFB lasers with low-threshold current densities and high efficiencies. Recent research progress indicates that this hybrid structure can achieve high current injection [42].

Figure 2.

Representative CQD laser with DFB as optical resonator. (a) Earlier demonstrated DFB CQD laser. (b) DFB laser based on InP/ZnS CQD. (c) Flexible DFB CQD laser. (d) Surface-emitting DFB CQD laser. (e) Wavelength-tunable DFB CQD laser. (f) Quasi-continuous-wave DFB CQD laser. (g) Continuous-wave DFB CQD laser. (h) LED-coupled DFB dual-function devices. (i) DFB-based, high-current density QLED. Reprinted with permission from refs. [16, 17, 18, 37, 38, 39, 40, 41, 42].

From the statistical results in Table 1, it can be seen that WGM-based optical resonators are the second most commonly used optical structure for CQD lasers. Figure 3 shows a comparison between the various geometries based on WGM-based CQD lasers. WGM-based CQD lasers have several advantages, such as low threshold, single-mode operation, high efficiency, and wavelength tunability. The resonant modes in WGM-based cavities are confined by total internal reflection at the curved surface of the cavity, which allows for high-Q modes and strong light-matter interaction. WGM-based CQD lasers have been demonstrated in various geometries, including cylindrical, spherical, and microring resonators. These resonators can be fabricated using various techniques, such as lithography, etching, and self-assembly. WGM-based CQD lasers are a promising candidate for various applications, such as bio-sensing, spectroscopy, and on-chip optical communication systems. However, there are also challenges to be addressed, such as the optimization of the cavity geometry for specific applications, the stability of the resonant modes, and the development of efficient injection schemes for electrically pumped WGM-based CQD lasers.

Figure 3.

Representative CQD laser with WGMs as optical resonator. (a) CQD laser with cylindrical microcavities. (b) Lasing from semiconductor quantum rods in a cylindrical microcavity. (c) Quasi-continuous CQD laser with fused silica hollow fibers as the resonators. (d) CQD laser with self-assembled spherical resonators. (e) CQD laser from a microsphere resonator composite. Reprinted with permission from. (f) Active color-controlled CQD ring laser. Reprinted with permission from refs. [31, 32, 33, 34, 35, 36].

Finally, it is important to also focus on another type of optical resonator structure commonly used in other types of lasers, the Fabry-Perot cavity, also known as the vertical-cavity surface-emitting laser (VCSEL) cavity (Figure 4). Compared to WGM-based CQD lasers, VCSELs have a simpler cavity geometry and can be fabricated more easily and with higher yield. They also have excellent beam quality and can emit light perpendicular to the surface. However, VCSELs typically have a higher threshold and lower efficiency. They also suffer from wavelength tuning limitations due to the fixed cavity geometry. It is worth noting that some recent studies have also explored the use of VCSEL-like geometries for CQD lasers, such as the use of sub-wavelength metallic grating structures as the top mirror to enhance the light-matter interaction and reduce the lasing threshold. These structures have shown promising results in terms of high efficiency and wavelength tunability. In summary, while WGM-based CQD lasers and VCSELs have their own advantages and disadvantages, both have demonstrated significant progress in recent years and are expected to continue to play important roles in various fields of photonics and optoelectronics.

Figure 4.

Representative CQD laser with F-P or VCSEL as optical resonator. (a) Self-assembled F-P CQD resonator laser. (b) Single-mode CQD laser lasing from self-assembled F-P resonator. (c) Optically pumped CQD-VCSELs in red and green. (d) Colloidal nanoplatelets lasing with combining two DBRs together. Reprinted with permission from refs. [33, 43, 44, 45].

Optically pumped CQD lasers have achieved significant progress, and recent advancements have allowed for electrically driven devices, which have expanded the possibilities of designing optical resonators for CQD lasers to support high-current injection. The choice of resonator geometry and material composition depends on specific application requirements, and integration with other components is crucial. Research is focused on developing efficient cavity structures such as DFB-coupled and planar microcavities, ring resonators, and photonic crystal cavities, among others. The development of suitable optical resonators for high-current injection CQD lasers is an ongoing and challenging area of research but holds promise for various applications. Advances in resonator design, material composition, and integration with other components will continue to drive progress toward practical and efficient CQD laser devices.

3. High-current-density light-emitting diodes (LEDs)

Recent research in CQD laser technology has resulted in advancements in reducing Auger recombination, leading to lower lasing thresholds and continuous wave CQD emission. However, achieving an electrically pumped device, comparable to conventional LED devices, requires a significant reduction in the threshold, which is currently relatively high. To realize an LED-style electrically-pumped CQD laser, new structural designs or electrical-driven methods may help enhance the current injection of LEDs. Innovative approaches involving materials engineering and device fabrication strategies may also be necessary.

As the device structures of organic light-emitting diodes (OLEDs), CQD light-emitting diodes (QLEDs), and perovskite light-emitting diodes (PeLEDs) are similar, the maximum injection current density of these three types of devices under direct current injection is not significantly different. However, the difference in their luminescence efficiency is substantial. The maximum external quantum efficiency (EQE) of small-molecule OLEDs is usually around 5%, while that of QLEDs and PeLEDs exceeds 20%. This is clearly reflected in the maximum brightness or output power of the devices. Currently, the highest brightness of OLEDs is about 1,500,000 cd/m² [47], while that of QLEDs and PeLEDs is 7,646,245 cd/m² [48] and 9,800,000 cd/m² [28], respectively. While QLED and PeLED technologies are based on OLED technology, CQDs are more suitable for achieving high-performance laser devices in terms of efficiency and output power.

Table 2 summarizes representative high-current density amorphous thin-film LED devices over the past 18 years. The mobility of holes and electrons in high-efficiency LED devices typically ranges from 10⁻⁴ to 10⁻²cm²V⁻¹ s⁻¹, making it challenging to effectively enhance carrier injection by increasing the mobility of functional layer materials. However, achieving high current injection is crucial for the performance of the LED devices. To this end, it is necessary to address and reduce the negative effects of Joule heating, which can increase device temperature and impact the performance and lifespan of the device. Previous studies have shown that Joule heating can cause OLEDs to overheat by tens or even hundreds of degrees Celsius, even at moderate current densities of only a few amperes per square centimeter due to their high resistivity [58]. Consequently, reducing device resistance and improving its heat dissipation ability are effective methods for mitigating Joule heating. One potential approach is to use a substrate with good thermal conductivity or to perform low-temperature testing. Hajime Nakanotani et al. conducted a study in which OLED devices with identical structures were fabricated on substrates with different thermal conductivities, namely Si, sapphire, and glass [49]. The results showed that the maximum current density achieved in these devices were 1163, 823, and 567 A/cm², respectively, indicating that the thermal conductivity of the substrate has a significant impact on Joule heating. Additionally, the implementation of optically pumped CQD lasers at low temperatures is another example of a technique utilized to mitigate Joule heating [37]. By operating the device at a lower temperature, the amount of heat generated due to Joule heating can be reduced, thus preventing thermal damage and lowing the device’s threshold.

Device	Maximum current density (A/cm^2)	Maximum luminance (cd/cm^2)	Wavelength	Active area	Electrical driven	Publication date	Ref.
Green OLED	12, 000 (Si); 514 (Glass)	1,500,000	~540 nm	25 um (circle) shadow mask	DC	2005	[47]
Green OLED	1163 (Si); 823 (Sapphire); 567 (Glass)		~540 nm	50 um (circle) shadow mask	DC	2005	[48]
Red OLED	800		~610 nm	100 um × 100 um shadow mask	DC	2011	[49]
Red QLED	~1	42,000	640 nm	2 mm× 2 mm; shadow mask	DC	2014	[50]
Blue OLED	2800		~460 nm	2 um× 0.05 um; E-beam lithography	Pulsed 5 us	2015	[51]
Blue OLED	275		~460 nm	112.5 um (circle) photolithography	Pulsed 5 us	2016	[52]
Green OLED	400/1000		~511 nm	0.07 mm^2; 50 um (circle) shadow mask	Pulsed 0.25 us	2017	[53]
Blue OLED	615	~1,000,000	~460 nm		Pulsed	2018	[54]
Green PeLED	620			100 um (circle) shadow mask	Pulsed 2 us	2018	[55]
Red QLED	18 (CW); 1040 (Pulesd)		~614 nm	50 um× 300 um current-focusing	CW; Pulsed 1 us	2018	[15]
QLEDs	1 ~ 2	356,000 (R),; 614,000 (G), 62,600 (B)	620/545/480 nm	2 mm× 2 mm; shadow mask	DC	2019	[56]
Green QLEDs	3.88	1,680,000	540 nm		DC	2019	[57]
Green PeLED	28.9 (CW)	7,646,245	519 nm	0.05 um (circle) photolithography	DC; Pulsed 2 us	2020	[58]
Blue QLEDs	~1	88,900	457 nm		DC	2020	[59]
Green PeLED	10, 000	1200 kW sr^−1 m^−2	~760 nm	0.01 mm^2; shadow mask	Pulsed 30 ns	2021	[60]
Red QLED	3.4 (CW); 1170 (Pulesd)	9,800,000	~614 nm	2.25 mm^2; 50 um× 300 um current-focusing	DC; Pulsed 2 us	2022	[28]
Red QLED	557		~614 nm	50 um× 290 um current-focusing	Pulsed 1 us	2023	[42]

Table 2.

Representative high-current-density light-emitting diodes.

In addition to these methods, pulsed driving can be used to control the impact of Joule heating. In amorphous thin-film LEDs, using short-pulsed driving can easily increase the injected current density to over 1000 A/cm² [15, 28, 48, 52, 54, 60]. Additionally, the width of the electrical driving pulses gradually becomes shorter, transitioning from microseconds [53, 56] to nanoseconds [54, 60]. Using short pulses with a low duty ratio for electrical injection can reduce the effects of Joule heating while maintaining the desired current injection level.

The injected current density in amorphous thin-film LED devices is influenced by the driving method and is closely related to the active area, as shown in Figure 5, which illustrates the maximum injected current density as a function of the emission area’s size. When the active area is 1–10 mm², the maximum injected current typically ranges from 1 to 4 A/cm² [28, 51, 57]. However, when the active area is reduced to 0.01–0.1 mm², the maximum injected current density can reach up to 600 A/cm² [42, 56]. The range of 0.001–0.01 mm² for the active area is currently the most commonly used for high-current injection LEDs, with the lowest current density exceeding 1000 A/cm² [15, 28], and the highest exceeding 10,000 A/cm² even under DC driving [47]. However, reducing the active area below 0.001mm² will not result in further increases in the maximum injected current [48, 52]. As the active area decreases in amorphous thin-film LEDs, the increase in injected current density can cause thermal effects that affect the device’s electrical and optical performance. Balancing the active area and injected current density is crucial for achieving better performance in LED design.

Figure 5.

Enhance the injection current density vs varying the active area and using pulsed-driven mode. The injection current density plotted as a function active area for OLEDs (red circles with pulsed driven [52, 53, 54], red solid circles with DC driven [47, 49, 50]), QLEDs (red pentagram with pulsed driven [15, 28, 42], red solid pentagram with DC driven [15, 28, 57]), and PeLEDs (green hexagon with pulsed driven [48, 56, 60], green solid hexagon with DC driven [48]).

The previous section discussed the effects of the driving method and the active area on the injection current. However, there are also challenges in achieving small-area LEDs. Currently, there are three main methods for obtaining effective areas ranging from tens to thousands of square micrometer, as shown in Figure 6. These methods are electron beam lithography [52], photolithography [48], and current-focusing method achieved by depositing wide bandgap LiF in the functional layer [42]. Each method has its own advantages and disadvantages. The first two can be combined with advanced processing technology to obtain high-quality patterns, but the processing inevitably produces negative effects on the substrate, especially the transparent electrode, which is difficult to eliminate. The last method is particularly suitable for amorphous thin film LEDs, as it can obtain a controllable emitting area without significantly affecting the device performance and fabrication process [15, 28, 42]. However, attention should be paid to the insulation properties of the thinner LiF under high electric fields and the possible manifestation of current diffusion in this structure. Under the premise of not affecting the device’s performance, it may be possible to achieve true current-focusing by introducing similar insulation layers in both the hole and electron transport layers.

Figure 6.

Representative pattern methods for the small-area LED. (a) E-beam lithography [52]. (b) Photolithography [48]. (c) Current-focusing [42]. Reprinted with permission from refs. [42, 48, 52].

4. Toward electrical pumping CQD lasing

Electrically pumped CQD lasers have unique optical properties that make them promising for optoelectronic applications, but developing an optical resonator that is compatible with CQD electrical injection is crucial. To achieve this, careful consideration is required for material selection, resonator structure design, CQD material characteristics, compatibility with high-efficiency QLED structures, and fabrication process compatibility. Future development of electrically pumped CQD lasers will require continued improvements in resonator design, such as developing new microcavity structures and design theories to eliminate light scattering, and optimizing the integration processes for resonant cavity and QLED device fabrication to minimize optical losses. With continued research and development in these areas, electrically pumped CQD lasers have the potential to become highly efficient and tunable light sources for various optoelectronic applications.

Yue Wang et al. have pointed out that the scattering problem caused by the nonuniformity of film thickness in CQD results in difficulties or obtaining high-quality factor optical resonant cavities [30]. Klimov et al. have specifically discussed this issue in several review articles [26, 27, 61]. As discussed in the first part of this article, DFB is currently the most commonly used optical resonant cavity for CQD. In recent years, Klimov et al. have made significant progress in the use of DFB structures in continuous-wave pumped CQD lasers [14], LED, and CQD laser dual-function devices [16], and thus, the DFB structure is considered one of the optional structures for electrically pumped devices. Another optical structure, the planar microcavity or VCSEL, is considered unsuitable for CQD due to high optical. In fact, even in the field of optically pumped CQD lasers, or even QLED, devices based on planar microcavities are rare. Currently, the optically pumped VCSEL CQD laser uses a combination structure [45, 46], where two DBRs are combined physically. There have been no reports of monolithic VCSEL CQD devices. The principle of using planar microcavities to adjust the emission characteristics of CQD is simple in theory, but in practice, there are few reports, and most of them are based on metal microcavities [62, 63, 64, 65, 66], which do not significantly improve device performance or regulate emission and laser properties. Wang et al.’s report is one of the few research results that demonstrate the advantages of planar microcavities [67]. If we can combine the above discussion and analyze why the use of planar optical microcavities cannot effectively regulate the emission and laser properties of CQD, it may be possible to develop a new resonant cavity structure to assist in achieving electrically driven CQD lasers.

The use of planar microcavities or VCSELs to regulate the emission and laser properties of CQD lasers has been considered unsuitable due to high optical losses and complex preparation processes. However, a recent study by Wang et al. demonstrates successful use of planar microcavities to regulate CQD laser properties, which may be due to targeted improvements and optimized preparation processes. Overcoming optical losses and preparing high-quality factor planar microcavities remains a challenge. Nonetheless, planar microcavities offer advantages such as simplicity, control, scalability, and wider wavelength tuning range compared to distributed feedback structures. Combining the advantages of planar microcavities and distributed feedback structures may lead to better solutions for improving CQD laser performance. Further research is needed to explore the effective regulation of CQD laser properties using planar microcavities or other optical structures and to develop new resonant cavity structures for electrically driven CQD lasers.

In this case, researchers need to explore new methods to address these issues. Gao et al.’s research has shown that interface engineering can effectively reduce optical losses caused by interface states [68], thereby improving the quality factor and laser characteristics of microcavities (Figure 7).

Figure 7.

Schematic representation of the laser device and the surface properties of N2F4 films. (a) Illustrates the ideal schematic of the laser device. (b) Shows a cross-sectional SEM image of the device. (c) Displays the actual schematic of the interface between the perovskite gain layer and top DBR. (d) Presents an atomic force microscopy image of the TOL-N2F4 and EA-N2F4 films, with a horizontal scale bar of 10 μm. (e) Shows the corresponding height profile of the TOL-N2F4 and EA-N2F4 films at the 100-micron scale. Reprinted with permission from ref. [68].

As shown in Figure 8, Lin and coauthors have proposed a novel design strategy for microcavities light and laser devices. The non-quarter wave microcavity structural design is a promising approach for enhancing the performance of optoelectronic devices, such as OLEDs [69] and Pe LEDs [70]. The use of non-quarter-wave DBRs in MOLEDs leads to a higher EQE and narrower emission spectrum compared to quarter-wave DBRs. Additionally, the use of non-quarter wave DBRs in metal-dielectric microcavities can provide further enhancements in device performance. This novel microcavity design presents new research opportunities for electrically driven lasers based on planar microcavity structures. The proposed design enables flexible tuning of the cavity resonance modes by adjusting the thickness of the optical spacer layer while maintaining high-performance QLED devices. This approach facilitates efficient coupling between the microcavity modes and the CQD emitting layer, enhancing the laser’s overall performance. By employing this new microcavity design, researchers can achieve effective control over the emission characteristics of CQD lasers while maintaining high device performance.

Figure 8.

Structures of OLED and PeLED with non-quarter wave microcavity structural design. (a) Structure and cross-sectional SEM image of MOLEDs. (b) EL spectra of MOLEDs with different non-quarter wave DBRs. (c) Schematic of a metal-dielectric microcavity with different DBRs, including quarter wave DBR and non-quarter wave DBR, forming a Fabry-Perot resonator that surrounds a cavity medium of refractive index n and thickness L. (d) Cross-sectional SEM image of the perovskite microcavity device. (e) the refractive index of each layer and standing wave electric field distribution in the designed MPeLED. Reprinted with permission from refs. [69, 70].

The application of non-quarter-wave microcavity designs in OLED and PeLED devices is a promising approach for optimizing the performance of CQD technology. The design approach solves the challenges associated with balancing the optical and electrical performance of CQD devices, and enables the optimization of both aspects in tandem. By employing non-quarter-wave DBRs, the actual reflection interface of the microcavity can be relocated inside the DBR, effectively mitigating the optical interface issues that typically arise when reducing the nanometer functional layer and contacting the optical resonant cavity. A novel optical resonant cavity structure has been designed for CQD lasers based on this approach, which improves the coupling efficiency between the microcavity mode and the CQD emitting layer, while also resolving optical interface issues, as illustrated in Figure 9. The proposed planar microcavity structure based on non-quarter-wave DBR offers a promising avenue for the development of high-efficiency optoelectronic devices based on colloidal quantum dots. Its unique design enables flexible tuning of the cavity resonance modes and efficient coupling with the CQD emitting layer, which are critical for achieving high-performance CQD devices. The proposed approach provides a simple and effective solution for realizing efficient coupling between the CQD emitting layer and the microcavity modes, which is critical for achieving high-performance CQD lasers.

Figure 9.

A proposed planar microcavity structure based on non-quarter-wave DBR for high-efficiency electrical injection based on CQD.

5. Summary and outlook

After years of extensive research, electrically driven CQD lasers are on the verge of becoming a reality. However, researchers have found that the challenges faced by CQD lasers are similar to those encountered by organic semiconductor lasers [71, 72]. Therefore, there may be potential synergies between these two technologies, and it is essential to explore how to leverage these synergies to the fullest [27, 61]. The latest research progress in CQD has leveraged the design approach of inorganic optoelectronic devices to achieve high current injection through current focusing [73, 74], enabling CQD optical-pumping lasers and LED functional devices [16]. This approach is also inspired by the DFB electrical-driven organic devices [75]. Although many teams have made significant efforts in this regard, the differences between CQD materials and devices have made it challenging to implement promising ideas. Based on the basic principles of optics and materials science [76], and considering the characteristics of colloidal quantum dots, the characteristics of QLED are studied. Developing a microcavity for efficient electrical injection of CQDs, overcoming optical-electrical balance issues [69].

In summary, the realization of electrically driven CQD lasers still lacks several critical pieces. To address this issue, the novel optical microcavity design theory mentioned in this article, as well as the optical resonant cavity constructed based on the characteristics of CQD materials and devices, successfully separates the optical and electrical interfaces of the electrically driven device. This separation may become a crucial puzzle piece in achieving electrically driven CQD lasers. The potential synergies between CQD and organic semiconductor lasers should be explored further, and leveraging the latest research progress in the field of inorganic optoelectronic devices and organic lasers [77, 78] may hold the key to realizing electrically driven CQD lasers. The novel optical microcavity design theory and the constructed optical resonant cavity may provide an essential breakthrough in this regard. Recent advancements in CQD lasers indicate that unlocking the potential synergies between different technologies will play a critical role in achieving breakthrough progress [79]. Perhaps, the final implementation of electrically driven CQD lasers may depend on the synergistic effects of numerous research fields.