As one of the most important inventions in the 20th century, the laser has been widely termed “the fastest knife,” “the most accurate ruler,” and “the brightest light.” A laser represents light amplification by the stimulated emission of radiation, which can explain the lasing generation process. Organic lasers, as important components of lasers, have received extensive attention in the field of optoelectronics for various applications owing to their advantages, such as facile fabrication, low cost, and ease of integration. Recently, organic lasers have been used extensively, and the design and synthesis of a series of promising organic-laser gain media have intensified. The development of optically pumped organic lasers has progressed significantly; however, the aim of electrically pumped organic lasers remains a worldwide research problem. Owing to the three key components of organic lasers, the fabrication of optical resonators is crucial to realize efficient and stable electrically pumped organic lasers.

Here, we summarize the recent novel developments of electrically pumped organic lasers, from material selection to device optimization. Three types of optical resonators are typically used in electrically pumped organic lasers: distributed feedback structures (DFBs), distributed Bragg reflectors (DBRs), and whispering gallery mode (WGM). To realize electrically pumped organic lasers, the following criteria must be satisfied: sufficient current injection and exciton densities to induce population inversion, clear threshold behavior of current-density-dependent spectral narrowing and output-power enhancement, and observation of both spatial and temporal coherence. Figure 1 shows the organic four-level system under electrical excitation and the lasing behavior of the output power and FWHM (full width at halt maxima) as a function of current density. In Section 2.1, we summarize the recent developments of electrically pumped organic lasers based on DFB resonators. DFB resonators are considered to be among the most effective structures for realizing electrically pumped organic lasers. They are essentially periodic Bragg-grating structures that can provide effective optical feedback for lasing oscillations via Bragg scattering. Figure 2 illustrates an electrically pumped organic laser with a DFB structure, in which an electroluminescent device incorporates a mixed-order distributed feedback SiO₂ grating into an organic light-emitting diode (OLED) structure and emits blue lasing. Figure 3 shows an electrically driven organic laser with an integrated device structure that efficiently couples an OLED with exceptionally high internal-light generation and a polymer-distributed feedback laser. Under electrical driving, a threshold in the light output versus the drive current with a narrow emission spectrum and the formation of a beam above the threshold are observed. Section 2.2 summarizes the recent novel developments of electrically pumped organic lasers based on DBR resonators. DBR resonators are periodic structures comprising alternating layers with different refractive indices, where the optical thickness of each layer corresponds to a quarter of the reflected wavelength. Owing to their vertical structure, DBR resonators can be integrated well with OLEDs. Additionally, they can effectively confine light to the cavity by reflecting it, thereby realizing laser oscillation. Figure 4 shows a DBR microcavity organic laser that observes electrically pumped quasi-continuous-wave lasing at an extremely low current density. Polariton lasing originates from the leakage of coherent photons from macroscopic exciton-polariton (EP) condensates via stimulated scattering, which is also known as the Bose?Einstein condensation (BEC) of EPs. Because population inversion is no longer required, polariton lasers are considered an alternative that may facilitate the development of practical electrically pumped organic lasers with much lower thresholds. As shown in Figure 5, strong coupling between excitons and cavity photons, referred to as room-temperature polariton lasing, is successfully observed in planar DBR cavities containing two new fluorene-based oligomers, BSFCz and BSTFCz. Within the WGM resonators, light is trapped owing to the total internal reflection at the interface, thereby realizing lasing oscillation. Section 2.3 summarizes the recent novel developments of electrically pumped organic lasers based on WGM resonators. WGM resonators typically possess high quality factors; however, their small size renders their fabrication and integration difficult. Figure 6 shows the realization of large microdisk arrays based on organic single crystals with the observation of WGM lasing from these microresonators.

This study focuses on electrically pumped organic lasers. Based on different optical resonators, we summarize the recent novel developments of electrically pumped organic lasers from material selection to device optimization and then discuss the future development trends of electrically pumped organic lasers. Recent efforts toward electrically pumped organic lasers are important in the roadmap of organic lasers, which not only help us clarify the role of material synthesis, resonator design, device optimization, and photophysics in electrically pumped lasing but also provide insights into the fundamental knowledge, technologies, and strategies for solving the worldwide research problem of electrically pumped organic lasers.