Promise to electrically pumped colloidal quantum dot lasers

The laser has been recognized as one of the most successful and significant technological achievements of the 20th century. Undoubtedly, its applications are versatile, ranging from mundane tasks to cutting-edge scientific research, e.g., displays, lighting, optical communications, remote sensing, and medical treatments. Thus, it is believed that laser technology will continue to shape the world and change our way of life. Among all laser technologies, solution-processed colloidal quantum dot (CQD) laser diodes have attracted intensive attention due to their low cost, flexibility, simple processing, and ease of integration.1Kagan C.R. Lifshitz E. Sargent E.H. et al.Building devices from colloidal quantum dots.Science. 2016; 353: aac5523Crossref PubMed Scopus (827) Google Scholar Although optically pumped CQD lasers have been demonstrated for more than two decades, developing their electrically driven counterparts is challenging.2Park Y.S. Roh J. Diroll B.T. et al.Colloidal quantum dot lasers.Nat. Rev. Mater. 2021; 6: 382-401Crossref Scopus (134) Google Scholar One of the major obstacles in CQD lasers is the intrinsic optoelectronic property, such as high density of surface/trap states that can facilitate non-radiative recombination and deteriorate carrier injection efficiency. Additionally, the scattering effect of CQDs after film formation significantly influences the lasing threshold. Moreover, fundamental optimizations are required to achieve suitable optical resonators for high-gain feedback to amplify the photons. The traditional resonator of Fabry-Perot (F-P) cavity may not be effective for CQD lasers due to the high optical loss and the low quality factor (Q-factor). Consequently, alternative resonators with various designs, such as distributed feedback Bragg (DFB) reflectors and whispering gallery mode (WGM) cavities, have been extensively explored. Another challenge of CQD lasers is the compatibility between the fabrication processes of electrically driven devices and optical resonant cavities. Generally, efficient QD light-emitting diodes (QLEDs) employ an organic-inorganic hybrid structure to couple with an optical resonator, leading to defects and poor interface quality. The main reason lies in the difficulty to achieve a well-ordered optical and current arrangement when the charge-transporting layer and the optical resonator have to be integrated with the organic-inorganic hybrid structure, which directly contacts the electrode. Although significant progress has been made in developing CQD lasers in recent years, electrically driven CQD lasers have never been realized. In order to improve the carrier injection efficiency and minimize Auger recombination, a range of strategies have been implemented, such as compositional and shape control, surface passivation, ligand exchange, and interface engineering. Consequently, the low-threshold CQD lasers under optical pumping and highly efficient QLEDs have been demonstrated, respectively. These advancements play a pivotal role in establishing a solid foundation for the ultimate realization of electrically driven CQD lasers. Ongoing research is striving to unlock the full potential of CQD lasers, focusing on addressing their intrinsic material properties and developing new resonators. Recently, Klimov's group achieved remarkable progress in addressing the challenges of electrically driven CQD lasers.3Ahn N. Livache C. Pinchetti V. et al.Electrically driven amplified spontaneous emission from colloidal quantum dots.Nature. 2023; 617: 79-85Crossref PubMed Scopus (6) Google Scholar To suppress Auger recombination, a novel structure of continuously graded QDs (cg-QDs; shown in Figure 1A) was utilized, which shared some similarity with the conventional CdSe/Cd1−xZnxSe cg-QDs but with a reduced thickness of the graded layer. This approach led to a longer biexciton Auger recombination lifetime of 1.9 ns and an elevated biexciton quantum yield of 38%, resulting in large optical gain coefficients and low excitation thresholds for both the band-edge and excited-state transitions. Typically, it is feasible to observe lasing from the gain materials constructed with DFB structure as the resonator (Figure 1B). In contrast, a Bragg reflection waveguide (BRW) mode has been utilized to produce amplified spontaneous emission (ASE) under electrical operation. For instance, an inverted architecture with an optimized charge-transport layer was employed (Figure 1C). Furthermore, this approach created a tiny "current-focusing" structure by introducing a shaped insulating space (ca. 30 μm in width) between the hole injection and the transport layers (Figure 1D), leading to a higher current density with minimal impact on the electrical properties.3Ahn N. Livache C. Pinchetti V. et al.Electrically driven amplified spontaneous emission from colloidal quantum dots.Nature. 2023; 617: 79-85Crossref PubMed Scopus (6) Google Scholar The unique current-focusing configuration dramatically accounted for the electrically driven lasers. By optimizing the indium tin oxide (ITO) cathode, a high current density close to 2,000 A/cm2 was achieved, allowing for laser operation. One of the key innovations of this study lies in the integration of the multilayer CQD devices with a DBR-Ag cavity. These findings demonstrate the importance of leveraging the advanced experiences of the other laser devices, such as organic lasers, perovskite lasers, and even new inorganic vertical-cavity surface-emitting lasers (VCSELs), which show similar challenges. Recent research on lasing from CQDs has taken the advantages of the design approaches commonly used in inorganic optoelectronic devices, facilitating efficient charge carrier injection through current focusing.3Ahn N. Livache C. Pinchetti V. et al.Electrically driven amplified spontaneous emission from colloidal quantum dots.Nature. 2023; 617: 79-85Crossref PubMed Scopus (6) Google Scholar,5Lim J. Park Y.S. Klimov V.I. Optical gain in colloidal quantum dots achieved with direct-current electrical pumping.Nat. Mater. 2018; 17: 42-49Crossref PubMed Google Scholar This innovative approach has enabled the development of optically pumped CQD lasers and dual-functional LEDs.4Roh J. Park Y.S. Lim J. et al.Optically pumped colloidal-quantum-dot lasing in LED-like devices with an integrated optical cavity.Nat. Commun. 2020; 11: 271Crossref PubMed Scopus (81) Google Scholar Although extensive research has been conducted over the years, the realization of continuously and electrically driven CQD lasers remains a challenge. It is essential to explore potential synergies among different technologies. The state-of-the-art CQD lasers indicate that the realization of the full potential of these devices would be beneficial from the synergistic effects between the different technologies.3Ahn N. Livache C. Pinchetti V. et al.Electrically driven amplified spontaneous emission from colloidal quantum dots.Nature. 2023; 617: 79-85Crossref PubMed Scopus (6) Google Scholar In order to address the challenges of materials and the device design faced by CQD lasers, two key aspects have to be considered, i.e., reducing Auger recombination and constructing high-Q optical resonators compatible with the efficient charge injection. This research was supported by the National Natural Science Foundation of China (nos. 62175189 and 61975256). G.X. acknowledges funding support from the joint China-Sweden Mobility program (no. 52211530052). J.H. acknowledges financial support from the Perovskite Thin-Film Innovation Technology Centre at OSCAR (no. YZCXPT2022104). The authors declare no competing interests.