An organic semiconductor laser device usually refers to the structure and processing of organic optoelectronic devices. These are low-cost devices that consist of a pumping source, a laser medium, and an optical feedback layout to induce particles from the ground state to higher energy levels in order to achieve population inversion. They are totally divided into different types of excitations, including optical excitation, gas discharge excitation, electrical excitation, and nuclear excitation, depending on the state of the gain material. To achieve population inversion, quasi-four-level materials are chosen as the laser medium, including gas, liquid, solid, and semiconductor. Optical feedback layout is utilized to increase the effective length of the working medium and determine the output characteristics of the laser, such as monochromaticity, directionality, and coherence.

In general, organic semiconductor lasers can be simplified to a structure with organic semiconductor materials as the intermediate gain media and two mirrors as the light feedback structures. One of the mirrors is a semi-transparent mirror (95% reflectivity). If the pump sources provide energy above the threshold requirement to generate a large number of particles in the excited state of the gain medium organic material, thus achieving population inversion, and if a portion of the light emitted spontaneously by the gain medium hits the mirror vertically, it enables a back-and-forth reflection between the mirror and the semi-transparent mirror, repeatedly traversing the organic gain medium during this process. Furthermore, the light can trigger the gain material to produce stimulated emission instead of spontaneous emission consecutively, thus achieving amplification. If the total gain of the pumping source equals or exceeds the loss caused by incomplete reflection and absorption of the mirror, as well as the absorption in the organic material during the reflection process, a continuous laser emission can be maintained. After abundant diffraction and amplification in the optical resonant cavity, coherent light that is perpendicular to the axis of the resonant cavity is emitted from the semi-transparent mirror, forming the laser eventually.

In order to realize population inversion, the material must have a metastable level. Due to the fact that the rate of stimulated emission equals light absorption in thermal equilibrium, the number of particles at the higher energy level must be less than at the lower energy level. As a result, the material of a two-level system cannot achieve population inversion. Comparatively, with traditional laser materials, the organic gain material possesses a three or quasi-four-level distribution. This allows the excitation to stay in the excited state steadily, making population inversion easily achievable when excited—A necessary condition for obtaining a laser. Unlike the three-energy level system, it is easier to achieve population inversion between energy levels E₂ and E₃ since the number of particles in the sub-stable energy level E₂ is lower. This results in a decline of the excitation threshold. Thus, organic crystals, organic small molecules, and conjugated polymers with a four-level system are usually chosen as the gain medium. In organic semiconductor lasers with different materials and structures, many measures are taken to reduce the excitation threshold, which is a comprehensive result of multiple conditions, such as modal gain and absorption. To illustrate these methods more systematically, gain, loss, and Q factor are introduced to describe the conditions of continuous laser emission in a more intuitive manner.

As one part of the organic laser device, the configuration of the microcavity can influence the performance of organic lasers by adjusting its size or changing its type. Typical optical microcavities include Fabry–Pérot microcavities, distributed Bragg reflector structures (DBR), whispering gallery mode resonators (WGM), and distributed feedback structures (DFB).

The continuous emission of light in an organic laser can be understood as an increase in intensity rather than a decrease as it passes through the gain medium between the two reflective surfaces. In other words, the modal gain must be equal to or exceed the losses existing in the structure for the device to achieve excitation conditions. The Q factor is necessary to describe the ability of the optical feedback structure to retain light.

According to the excited emission theory, when the gain medium in the excited state interacts with an incident photon, it releases an additional photon with the same direction and frequency as the incident photon, leading to light amplification and modal gain. This phenomenon corresponds to the particle density in the excited state within the gain medium in a significant manner. Additionally, the particle density in the excited state is highly dependent on the pump strength, determining the excitation threshold, which is the minimum pumping strength required for modal gain to exceed modal loss.

To reduce the excitation threshold, both aspects need to be examined. Regarding modal gain, its magnitude is not only related to the pump strength but fundamentally depends on the basic properties of the gain medium itself. Exploring materials and different doping processes that can achieve population inversion at lower pumping strength is an efficient technique used to increase modal gain and minimize the threshold of laser excitation in the organic semiconductor.

The resonant cavity parameters directly affect the density of emitted photons, thereby influencing both the laser threshold and slope efficiency. By increasing the Q factor from 2000 to 4000 ^[1], the cavity loss and laser threshold decrease due to the rise in photon lifetime and excitation generation within the cavity. On the other hand, constructing a high-Q microcavity structure that continuously reduces mode loss is another effective way to decrease the excitation threshold.

Recently, remarkable improvements in the performance of Organic Semiconductor Lasers (OSLs) have been achieved in parallel with the development of Organic Light Emitting Diodes (OLEDs). State-of-the-art OLED materials and device architectures provide the foundation for OSLs and OSLDs (Organic Semiconductor Laser Diode). However, operating OSLs under the continuous-wave (CW) regime and electrical excitation with a low current threshold remains a significant challenge due to various loss mechanisms, such as excitonic annihilations, excited-state absorptions, and cavity losses that hinder their operation. In accordance with the law of conservation of energy, reducing system losses is an effective way to increase efficiency while maintaining constant input gain. By decreasing overall excitonic and photonic losses, the threshold decreases, and the slope efficiency increases.

Organic semiconductor lasers can be divided into two types: optically pumped organic lasers and electrically pumped organic lasers, based on the variation in pumping modes. These two types of laser devices exhibit significant differences in theory, structure, and methods employed to reach the excitation threshold conditions. Therefore, they should be presented separately.

The various loss mechanisms in OSLs and OSLDs, which contribute to an increase in the laser threshold and a decrease in the slope efficiency, can be categorized into excitonic loss and photonic loss mechanisms. Excitonic losses quench the excitons in an active layer and include singlet-singlet annihilation (SSA), singlet-triplet annihilation (STA), singlet-polaron annihilation (SPA), singlet exciton dissociation by an external electric field, and exciton quenching at the interface of a metallic electrode. On the other hand, photonic losses, which annihilate emitted photons, consist of cavity losses, such as the absorption of photons by singlet and triplet excitons ^[1], as well as polarons and the absorption by electrodes. The singlet and triplet absorptions (TAs) are quantified by singlet and TA cross-sections, which indicate the probability of interaction between photons and excitons in each excitonic state.

After comprehensive analysis, it is found that the laser threshold decreases significantly with STA, SSA, and SPA declining as 75% of triplet excitations of total excitations are forced to generate directly from electron-hole recombination, leading to a reduction in photon density. Among them, SPA is the most critical excitation loss mechanism in electrical excitation. In contrast, TTA can decrease the laser threshold by reducing the density of triplet excitations. Long-lived triplet excitons generally cause the absorption of emitted photons, inducing significant optical losses for light amplification. This loss mechanism is noticeable when the excited state absorption and the emission spectra overlap. These reabsorption losses are important mechanisms that affect the laser threshold and slope efficiency. Additionally, reducing the production and annihilation of triplet excitons and harvesting the triplet excitations by utilizing thermally activated delayed fluorescence (TADF) emitters or phosphorescence can achieve light amplification and efficiency enhancement, opening up a novel guideline aimed at realizing an OSLD.