Development of Solution-Processed Perovskite Semiconductors Lasers

Abstract

Lead halide perovskite is a new photovoltaic material with excellent material characteristics, such as high optical absorption coefficient, long carrier transmission length, long carrier lifetime and low defect state density. At present, the steady-state photoelectric conversion efficiency of all-perovskite laminated cells is as high as 28.0%, which has surpassed the highest efficiency of monocrystalline silicon cells (26.7%). In addition to its excellent photovoltaic properties, perovskite is also a type of direct bandgap semiconductor with low cost, solubilization, high fluorescence quantum efficiency and tunable radiation wavelength, which brings hope for the realization of electrically pumped low-cost semiconductor lasers. In recent years, a variety of perovskite lasers have emerged, ranging from the type of resonator, the wavelength and pulse width of the pump source, and the preparation process. However, the current research on perovskite lasers is only about the type of resonator, the type of perovskite and the pump wavelength, but the performance of the laser itself and the practical application of perovskite lasers are still in the initial stages. In this review, we summarize the recent developments and progress of solution-processed perovskite semiconductors lasers. We discuss the merit of solution-processed perovskite semiconductors as lasing gain materials and summarized the characteristics of a variety of perovskite lasers. In addition, in view of the issues of poor stability and high current density required to achieve electrically pumped lasers in perovskite lasers, the development trend of perovskite lasers in the future is prospected.

1. Introduction

Since semiconductor lasers came out in the 1960s [1,2], after nearly 60 years of development, their materials, processes and properties have made rapid changes, and their application scope and market scale have been expanding. Semiconductor lasers have gradually become an irreplaceable part of people’s daily life and work and have also ranked among the world’s high-end scientific and technological research. The three elements of laser generation are gain medium, pump source and optical resonant cavity. Among them, the gain medium provides the energy level structure for forming the laser, which is the internal cause of the laser generation; the pump source provides the excitation energy required to form the laser emission, which is the external cause of the laser generation; the optical resonant cavity provides feedback amplification for the laser to make stimulated emission. The intensity, directionality and monochromaticity of the lasers have been further improved.

Throughout the development of semiconductor lasers, it can be found that the development of optical gain materials has played a very important role. Lots of different materials have been developed and tested as the laser gain material. Laser gain medium materials can be divided into the following categories: solid gain media, gas gain media, liquid gain media, inorganic semiconductors, etc. [3]. Compared with other types of lasers, semiconductor lasers have attracted much attention because of their advantages of small size, fast response, low power consumption and high-efficiency [3,4]. Historically, this has been enabled using semiconductor lasers made from crystalline inorganic semiconductors such as II–VI or III–V compounds-materials heavily used in modern electronics and optoelectronics. However, crystalline inorganic semiconductors materials also have some inherent disadvantages. For example, the spectral coverage of the laser is limited due to the limited variety of inorganic semiconductors and the difficulty of the lattice doping [5]. In addition, the emission of inorganic semiconductor materials is derived from band edge radiation, the emission peak is usually narrow, and the wavelength adjustment ability is poor. In addition, most crystalline inorganic semiconductors require a complex and high-cost high-temperature fabrication process, which restricts the further development of crystalline inorganic semiconductors lasers [6].

Low-temperature solution-processed semiconductors are an emerging class of optoelectronic materials that can be processed in ink form through the wet chemistry [7]. They are technologically attractive due to their unique merits, such as facile solution processibility, lightweight, low cost and high mechanical flexibility. In addition to the above benefits offered by solution-processed semiconductors, they have the merit that their optoelectronic properties can be tailored. There are several ways to control the optical band gaps and energy levels of semiconductor materials. Solution-processed semiconductors cover organic materials, metal-halide perovskites (MHPs), and inorganic nanocrystals and quantum dots; each class of materials takes on (to lesser or greater extents) an optoelectronic tunability [7]. Solution-processed semiconductors materials as optical gain media have many incomparable advantages over traditional crystalline inorganic semiconductor materials: (i) large absorption and radiation cross-sections are conducive to high optical gain [8], (ii) abundant excited state process is conducive to the construction of a four-level system to achieve population inversion, thereby reducing the laser threshold [9] and it is also convenient to realize dynamic control of laser wavelength [10], (iii) there are many kinds of solution-processed semiconductors, which can achieve the light emission of the full spectrum from ultraviolet to near-infrared [11], (iv) solution method is easy to process and suitable for the preparation of large-area devices [5,12]. Therefore, solution-processed semiconductors gain medium is very promising to become an ideal choice for the next generation of semiconductor lasers.

In the decades following the creation of the world’s first lasers in the 1960s, solution-processed semiconductors materials including organic semiconductors (polymers), perovskites, inorganic nanocrystals and quantum dots have accounted for a large proportion of the development in this field. Progress in the controlling and understanding of these materials sciences have led to the most advanced performance of selected applications, and so far as to commercial deployments. Dominated by various weak interactions between molecules, organic molecules can self-assemble or be processed into a variety of regular micro-nano structures under mild conditions [13]. These regular micro-nano structures can be used as high-quality optical microcavity to provide structural support for the realization of low threshold laser [5,8,14]. Recent advances indicate a remarkable potential of colloidal quantum dots as an optical gain medium capable of operating under both optical and electrical pumping [15]. Latest studies include the demonstration of optically pumped continuous-wave colloidal quantum dot laser [16], the realization of optical gain by electrically pumped quantum dots [17], and the demonstration of dual-functional devices working as an optically pumped laser and an electrically excited light-emitting diodes (LEDs) [18,19].

Hybrid organic-inorganic halide perovskites have recently emerged as a potential new class of optoelectronic materials. The high brightness and tunable bandgap of perovskites have made it an attractive candidate for a new series of optical gain medium for low-cost semiconductor lasers [20]. So far, a variety of high-performance micro-/nanolasers have been demonstrated including 2D Ruddlesden-Popper perovskites [21], perovskite single crystals [22], and thin films [23]. Perovskite lasers have been exhibited in some architectures: resonators based on a Fabry-Perot (FP) cavity formed with parallel edge facets [22]; ring resonators in microspheres or nanoplatelets [24,25,26,27]; and random lasing in scattering films [20,28]. All the above configurations have achieved the multimode lasing over the full amplified spontaneous emission (ASE) bandwidth. However, the high-cost and high-energy-consuming synthesis approaches such as chemical vapor deposition and molecular beam epitaxy may undermine their practical applications. By comparison, the facile solution-processable lasers can not only reduce costs but also extend laser-related applications to flexible generations. In brief, despite the remarkable progress made in solution-processed perovskite semiconductors lasers, challenges and opportunities remain both basic science and a device engineering perspective. This review first introduces the structure and characteristics of perovskite materials and then states the development of perovskite laser based on different gain mediums. Finally, it summarizes the development of solution-processed perovskite semiconductors lasers, expounds its existing problems, and gives its own views on the trends of perovskite lasers in the future.

2. Perovskite Semiconductor

2.1. Crystal Structure

Any material with the same crystal structure as CaTiO₃ is collectively referred to as perovskite structure, which widely exists in nature. The general chemical formula of perovskite material is ABX₃, where a and B are two different cations, and X is the anion combined with them. Figure 1 shows a typical perovskite structure. The B cation is located in the center of the octahedron composed of X ions and embedded in the tetragonal body with the A-site ion as the apex. In common perovskite materials, A ion can be either organic or inorganic, such as CH₃NH³⁺, CH (NH₂) ³⁺, Cs⁺ and Rb⁺. B is a transition metal ion, such as Fe²⁺, Mn²⁺, Sn²⁺, and Pb²⁺. X is an oxygen or halogen ion.

2.2. Luminescence Properties

Perovskite is a direct bandgap semiconductor, which can control spectral tuning by substitution or mixing of halide components and cations. The luminous wavelength ranges from 390 nm to 790 nm and can be extended to 820 nm by mixing methyl and formamidine. MASnX₃ perovskite semiconductor has tunable emission wavelengths in excess of 900 nm but is more sensitive to air and illumination. CsPbX₃ quantum dots have also been extensively studied in recent years [29]. These perovskite nanoparticles provide a spectral range spanning 410–700 nm through halide substitution and quantum tuning, as shown in the Figure 2a,b. Their narrow photoluminescence (PL) spectrum and continuous spectral tunability enable a solid color distribution on CIE chromaticity maps that exceeds the national Television Systems Council (NTSC) standard. Some studies have shown that CsPbBr₃ perovskite quantum dots exhibit less blinking than other quantum dot systems, and excitons are insensitive to the size of quantum dots. Perovskite semiconductor has excellent optical absorption, with the absorption coefficient exceeding 10⁴ cm⁻¹ near the band edge, which can efficiently convert light into electric current and correspondingly be used as a gain material in lasers. In laser applications, the low Stokes shift reduces heat loss during the down-conversion process. Band gaps with minimal charge-trapping defects improve the efficiency of interband radiative recombination, which is critical for light-emitting devices.

Tunable emission wavelength is a beneficial characteristic of perovskite materials. The substitution of perovskite cation or halogen ion can change the bandwidth of perovskite material, and then realize the tuning of emission wavelength from visible to infrared. Since the Pb-X bond of perovskite crystals is related to the energy band structure, the band gap decreases sequentially from chlorine to bromine to iodine replacement, so the tunable emission wavelength of perovskite materials can be achieved by the replacement of halide ions [30]. In addition, the continuous tuning of the emission wavelength can also be achieved by mixing halogen elements to regulate perovskite semiconductor (see Figure 2c–e) [31,32].

2.3. Gain Properties

Laser is a process in which a gain material that can provide feedback in a cavity is excited to form a population inversion to generate optical radiation. The process of semiconductor stimulated radiation to achieve optical gain: a photon incident on the semiconductor material undergoes an electronic transition and generates a stimulated radiation photon identical to itself at the same time. The photoexcited states near the energy band of perovskite semiconductors affect charge transport and light emission. There are free carriers and excitons at the band edge. The exciton binding energy reflects the Coulomb interaction of photoexcited electron-hole pairs. The strength of the action determines the balance of the two excited particles. Unlike the exciton binding energies of conventional organic semiconductors (hundreds of millielectron volts) and inorganic semiconductors (several millielectron volts), the exciton binding energies of perovskite materials lie in between. From different experimental methods and results, there is a wide distribution range (from several millielectron volts to hundreds of millielectron volts) of its exciton binding energies by changing the stacking, structure and cation of perovskite materials [33]. The mechanism that causes the stimulated emission of perovskite semiconductors remains to be resolved. When the exciton binding energy of the semiconductor is smaller than the thermal fluctuation energy, it will easily dissociate into free carriers; otherwise, the free carriers will form excitons. For the light emission model, the exciton binding energy of perovskite is generally larger, which can obtain high quantum yields at relatively low carrier $A=\pi r^2$ relatively large exciton binding energies are important for stable lasing at room temperature.

Optical gain is used to describe the process in which the intensity of the light incident on the gain medium increases exponentially with distance. Optical loss refers to photon scattering, non-radiative recombination and edge scattering when light is transmitted in a semiconductor medium. To achieve laser output, the gain must be greater than loss, that is, there is a positive net gain. In order to further describe the laser gain characteristics of perovskite, the optical net gain model is introduced. Due to the change in the pump spot length, the emission intensity of the sample also changes, and the net gain model formula is established according to the gain loss:

$$I=A\frac{\exp \left(g{L}_g-1\right)}{g}$$

(1)

where $I$ is the output light intensity, $A$ is a constant, $g$ is the gain coefficient, $L_g$ is the pump fringe length and the gain coefficient can be obtained by data fitting. Sutherland et al. obtained the net gain coefficient of perovskite MAPbI₃ films on silicon spheres as 103~147 cm⁻¹ and the gain bandwidth as 36~64 meV by the method of variable stripe length [26]. Through the continuous efforts of the above teams, the highest net gain coefficient of perovskite so far measured on the MAPbI₃ film obtained by atomic layer deposition is 2770~4030 cm⁻¹ [34], which is much higher than that of colloidal quantum dots and conjugated polymer films, and close to that of traditional GaAs semiconductors. The low defect density of perovskite can reduce the non-radiative recombination rate [23], thus reducing the excitation threshold and has the advantages of large absorption coefficient and high fluorescence quantum yield, which makes perovskite semiconductor as an optical gain material bring beneficial development potential for high-performance semiconductor lasers.

Table 1. Physical properties of perovskite semiconductors.

Materials	Hole/Electron Mobility (cm2/Vs)	Intrinsic Carrier Concentration (cm−3)	Carrier Lifetime (ns)	Intrinsic Resistivity (Ωm)	Thermal Conductivity (W/mK)	Dielectric Constant
MAPbBr3	20–60/20–60	5 × 109–5 × 1010 [36]	41 [36]	~108	0.1–1.4	~5.7@530 nm
MAPbI3	136/197 [37]	1016–1018 [38]	22 [36]	~1010 [36] ~105 [39] ~109 [40]	1–3 [38]	~4.7–9@vislble [41]
CsPbX3	100–240/80–290 [42]	1015–1017 [42]	1.3 (X = Br) [43]	2.1 × 1010 X = Br [43]	0.3 X = I 0.5 X = Cl	4.1–4.5 [44] X = Cl 3.2–5 [44] X = Br 5–12.7 [44] X = I

summarized the physical properties of perovskite semiconductors in the review article [35].

2.4. Carrier Dynamics

Emerging perovskite semiconductors have similar properties to traditional inorganic direct bandgap semiconductors, so many of the theories of traditional semiconductors are also applicable to perovskite semiconductors. The photophysical processes that determine the photoelectric properties of semiconductors mainly include carrier excitation, relaxation, recombination and transport. These processes usually occur in a very short time, ranging from tens of femtoseconds to a few nanoseconds. After the carriers relax to the bottom of the conduction band or the top of the valence band, they are in a non-equilibrium state, which needs to be restored by radiative and non-radiative recombination processes. These two recombination processes release energy in the form of photons and thermal energy, respectively. The whole mechanism of carrier recombination can be divided into three types: single-molecule recombination, bimolecular recombination and Auger recombination, whose dynamics follow the following differential equation [45]:

$$\frac{dn}{dt}=-k_{1}n-k_2{n}^2-k_3{n}^3$$

(2)

where $n$ is the carrier density and $k_1$, $k_2$ and $k_3$ are the unimolecular, bimolecular and Auger recombination rate constants, respectively.

Monomolecular recombination refers to a recombination process involving only one particle. In semiconductors, excitons consisting of a bound electron-hole pair all constitute a particle. Therefore, both cases of exciton recombination (radiative recombination) and trap state recombination (a single electron or hole trapped by a trap state) belong to single-molecule recombination. Conventional semiconductors prepared by solution methods have the disadvantages of high electronic disorder and a large number of bulk defects and surface traps [23,46,47], while the prepared perovskites have only limited density of trap states, making it easier to realize single-molecule recombination based on the radiative recombination process. The density of trap states strongly depends on the preparation conditions and surface treatment of the sample. At low pump intensities, the variation of trap state density can lead to different recombination lifetimes of single molecule recombination assisted by trap state. For perovskite lasers, it is of great significance to improve the crystallinity and purity of the samples to enhance the radiative recombination process of single molecules. Bimolecular recombination is the recombination of two particles, which is a recombination process involving free electrons and holes. This process is intrinsic photon-radiation recombination, and its dependence on material processing is much lower than that of single molecule recombination assisted by trapping states. For the perovskite laser, enhancing the bimolecular recombination process can also improve the luminescence efficiency.

Auger recombination is a many-body recombination process in which the recombination of an electron with a hole is accompanied by the transfer of energy and momentum to a third particle, either an electron or a hole. Therefore, Auger recombination is strongly dependent on carrier density. As shown in the third term of Equation (2), the Auger recombination (non-radiative recombination) effect can only be detected if the pump intensity is sufficiently large. For applications with high charge densities such as lasers, Auger recombination processes can cause large energy losses. As shown in Equation (2), the mechanism of carrier recombination depends on carrier density and time. Combined with ultrafast spectroscopy technology for global fitting, the values of $k_1$, $k_2$ and $k_3$ of any material can be obtained, but a certain recombination mechanism can dominate under different pump intensities. At low pump intensities, the photoexcited minority carrier density is much smaller than the total majority carrier density, and multiparticle recombination is suppressed, so the first term in Equation (2) dominates. Under these conditions, the dynamics of carrier recombination are almost unimolecular and exhibit near uniexponential decay. At high pump intensities, the density of photogenerated carriers is large and the free electron-hole bimolecular recombination dominates. The dynamics of carrier recombination and pump intensity decay in a power-law pattern with a long tail. With time delay, the bimolecular decay dynamics will continue until the carrier density drops to the single-molecule recombination density, at which point the single-molecule type recombination will reappear and appear as a long exponential tail on the decay curve. At higher pump intensities, Auger recombination involving multiple particles will dominate, and there are few relevant studies. For the perovskite laser, the stimulated radiation process requires a high pump intensity, and the Auger recombination effect is also significant, resulting in a large energy loss [48].

2.5. Stability

The stability of materials is an important factor for practical application in devices. Although metal halide perovskites have excellent lasing properties, they are less stable [49]. The metal halide perovskite can be degraded in water, oxygen, light and heat. In the case of MAPbI₃, water reacts with MAPbI₃ and decomposes to produce MAI and PbI₂, in which the MAI produces volatile methylamine and hydrogen iodide. Oxygen penetrates the perovskite through iodide vacancies, trapping electrons and forming a highly reactive superoxide anion. Superoxide anion decomposes MAPbI₃ into PbI₂, I and H₂O. The simultaneous presence of oxygen and light greatly accelerates the degradation of MAPbI₃. The thermal decomposition of MAPbI₃ is carried out by chemical decomposition followed by the sublimation of MAI and HI. In order to improve the stability of metal halide perovskite lasers, researchers have developed a variety of methods. Exposure of perovskites to oxygen and moisture can be avoided by device encapsulation [50,51], preventing irreversible loss of volatile species from light and heat. Perovskite lasers show better stability under the protection of polymers, boron nitride films, and DBR cavities. The stability of the perovskite laser can be improved by reducing the thermal degradation of perovskite by increasing heat dissipation, wherein the sapphire substrate with high thermal conductivity can be used to assist the heat dissipation [52,53]. A perovskite laser encapsulated with boron nitride film with high thermal conductivity can accelerate heat dissipation. This method effectively combined the above two schemes [54]. In addition, the synthesis of perovskite single crystals with low trap density can inhibit the degradation of perovskite structure induced by oxygen and light, which is also one of the effective methods to achieve laser stability [55].

3. Perovskite Semiconductor Lasers

With the continuous improvement of the performance of perovskite semiconductor materials, research reports on perovskite lasers finally appeared in 2014 [23]. Since then, studies of hybrid and all-inorganic perovskite semiconductors have shown that optically pumped lasering emission can be achieved throughout the visible spectrum at room temperature [56,57]. However, there are still many challenges in the realization of electrically pumped perovskite semiconductor lasers until now. According to the different crystal types, perovskite lasers are generally divided into the polycrystalline thin-film types and low-dimensional single crystal type.

3.1. Polycrystalline Thin-Film Perovskite Lasers

The organic-inorganic hybrid perovskite material has the advantages of high intensity, high stability and easy control because of its large exciton binding energy, long carrier diffusion length and considerable quantum yield [58]. In 2014, Guichuan Xing and co-workers reported the PL properties of polycrystalline perovskite thin films under different pump light intensities [23]. With the increase of the pump light intensity, the full width half maximum (FWHM) of the luminescence peak gradually narrowed, and when it exceeded the threshold intensity 10~14 μJ/cm², the luminescence intensity (IPL) increased rapidly, the luminescence peak became sharp, and a lasing phenomenon occurred [23]. Furthermore, when the pump light irradiated the perovskite thin film, the transition from spontaneous emission to amplified spontaneous emission was realized with the increase of pump light intensity. This indicated that organic-inorganic hybrid perovskite semiconductor materials can also be used as the gain medium to achieve lasering. In addition, Stranks et al. found that the cholesteric liquid crystal (CLC) could be used as a mirror to construct a multilayer film structure as shown in Figure 3a [59]. The lasing threshold was as low as about 7 μJ/cm² (see Figure 3b), and the FWHM of the luminescence peak also reached 1.24 nm [59]. The inset of Figure 3b shows this flexible device, which lays the foundation for the development of new semiconductor lasers.

In addition to the multi-mode lasers based on mirror microcavities reported above, many studies have also been carried out on single-mode perovskite lasers based on polycrystalline thin films in recent years. Guy L. Whitworth et al. used solution processed CH₃NH₃PbI₃ perovskite as gain semiconductor and UV nanoimprinted polymer as the gratings to fabricate distributed feedback (DFB) lasers, their schematic diagram of the device shown in Figure 4a [20]. Figure 4b shows that these perovskite lasers based on the solution process achieved a laser threshold of 4 μJ/cm² and 0.4 nm of FWHM. Cha’s group reported an optically pumped single-mode laser with a two-dimensional square lattice photonic crystal (PhC) backbone structure that obtained the laser thresholds of ~200 μJ/cm² in pulse energy density at room temperature [60]. In 2019, optically pumped lasing was achieved from the perovskites (PEA)₂Cs_n−1Pb_nBr_3n+1 microcrystal film by the spin-coating technique [21]. This study reported optical pumping distributed Bragg reflectors (DBR) lasers based on a sandwich structure consisting of perovskite/PMMA/perovskite, which was shown in Figure 4c. As increasing the pump fluence above 500 μJ/cm², lasering occurred at the peak of 532 nm with a narrow FWHM of ~0.8 nm in Figure 4d. When pumped by a nanosecond pulsed laser (355 nm, pulse width 8 ns, 1 kHz), the lasing phenomenon can only occur within the separation pattern in their experiment. This study suggested that the lasers pumped by nanosecond pulses were the key basis for realizing continuous-wave pumped optical and electrical lasers [21].

3.2. Single Crystals Perovskite Thin Film Lasers

Compared with polycrystalline thin-film perovskite materials, low-dimensional single crystal perovskite nanomaterials can take advantage of the regular shape and smooth interface formed by themselves to form a good optical resonator, thus achieving efficient management of incident light through excitation resonance effect [61]. Moreover, the laser based on low dimensional single crystal perovskite nanostructure also has the advantages of high-quality factor, small volume and low threshold value [62]. In 2020, Y. Zhong et.al acquired large-scale CsPbBr₃ single-crystals films (SCFs) on sapphire substrates and achieved ASE from the CsPbBr₃ SCFs with a low threshold of 8 μJ/cm² at room temperature [63], their highest values of optical gain up to 1255 ± 160 cm⁻¹ in Figure 5a. However, these large-scale CsPbBr₃ SCFs were fabricated by the chemical vapor-phase epitaxy deposition method rather than the solution process. Tian’s group prepared a high-quality CH₃NH₃PbCl₃ single-crystalline film as gain material in solution confined between a pair of DBR, which naturally formed an optical microcavity [64]. Figure 5b showed a threshold energy density of about 211 μJ/cm² in these deep-blue DBR perovskite lasers, at which the FWHM decreased from around 20 nm to 0.38 nm.

3.3. Single Crystals Perovskite Nanowires Lasers

A variety of preparation methods for perovskite nanowires have been proposed, and the most commonly used method is the solution process. One-dimensional semiconductor nanowires are nanoscale in the radial direction, which can be used as gain material and can naturally form a Fabry-Perot cavity. The light wave can be reflected back and forth between the two ends of the nanowires to continuously gain and form a stable standing wave with the same frequency and phase, thus realizing the laser emission [62]. Zhu’s group showed room-temperature and wavelength-tunable lasing from a single-crystal perovskite nanowires [22]. Polycrystalline films of lead acetate were grown on a glass substrate and immersed in a high concentration of MAX (X = Cl, Br, I) isopropanol solution. The lead acetate layer reacted with the MAX solution to form single-crystal MAPbX₃ nanowires with approximately rectangular cross sections. With the increase of pumping energy, the luminescence behavior of nanowires gradually changes from spontaneous radiation to stimulated amplification radiation, and the line width of the luminescence peak narrowed. In this study, the mixed MAPbBr_xI_3−x and MAPbCl_xBr_3−x nanowires were obtained by adjusting the ratio of halogen materials in the precursor solution. The emission wavelength of the mixed perovskite nanowires can cover the near-infrared to visible wavelengths and the minimum laser threshold was 220 nJ/cm², and the laser quantum yield can reach 100%. To improve the stability of perovskite nanowires, Y. P. Fu’s group used FA instead of MA to successfully obtain FAPbX₃ NWs, this lasing from single-crystal lead perovskite NWs was shown in Figure 6a [65]. Under the excitation of a femtosecond laser with a wavelength of 402 nm, the lasing threshold of FAPbX₃ NWs is 6.2 μJ/cm², the emission peak is 824 nm, the quality factor is 1554, and the FWHM is 0.53 nm (see Figure 6b).

H. C. Yu et al. prepared MAPbI₃ NWs on the surface of Ag film and separated them with MgF₂ to form a surface plasmon laser [66]. The device had a laser threshold of 13.5 μJ/cm² and an FWHM of 5 nm under femtosecond laser irradiation with a wavelength of 400 nm, and it can maintain good performance at a high temperature of 43.6 °C. In 2018, Jiang’s group used a gas-liquid transfer recrystallization method for synthesizing inorganic perovskite (CsPbX₃) NWs at a room temperature [67]. A femtosecond laser (1 kHz, 35 fs, 400 nm) was applied to measure the lasing behavior of NWs. This study indicated that the CsPbX₃ NWs perovskite lasing with a single mode, a low threshold of 12.33 μJ/cm² and an ultra-narrow linewidth of 0.09 nm, which is less reported in the inorganic perovskite system. Moreover, the CsPbBr₃ perovskite NWs are also used to achieve continuous-wave (CW) operation by polariton lasing at cryogenic temperature (77 K) with an excitation threshold of 6 kW/cm² [68]. Figure 7a displays fluorescence spectra from a CsPbBr₃ NW at CW excitation power densities. The intensity of this series of small peaks continues to grow until the threshold excitation power is about 6 kW/cm². In these 2–3 modes, maintaining their modal spacing becomes dominant and increases much faster than the other modes in the spectrum. This is even more evident when we curve the plotting of fluorescence intensity fitted in the energy window including the dominant modes (2.32–2.33 eV) [68]. As shown in Figure 7b, the slope above 6 kW/cm² is 9 times that below this threshold in accordance with polarized lasing. At three typical temperatures: 77 K, 171 K, and 295 K, lasing behavior was observed by the nonlinear growth in emission intensity of a few sharp peaks. It can be observed from Figure 7c–e that when the temperature is increased from 77 K to 295 K, the mode spacing increases by about an order of magnitude. This variation of the mode spacing with temperature is independent of the thermal expansion [68].

3.4. Single Crystals Perovskite Microplates/Nanoplatelets Lasers

Two-dimensional (2D) single-crystals perovskite microplates/nanoplatelets have great application potential in whispering gallery mode (WGM) micro-nano lasers. Perovskite microplates/nanoplatelets are two-dimensional micro-nano materials. Usually, cubic or tetraconal halogen perovskites have highly symmetrical isotropic crystal structures and tend to grow into cube-shaped perovskites without any ligands or surfactants. In this optical resonant cavity, light wave can form continuous total internal reflection and form a stable propagation mode under certain conditions, which is usually called a WGM. Therefore, this type of resonant cavity is a good choice for making perovskite micro-nano lasers. In 2015, Tyagi et al. prepared MAPbBr₃ nanoplatelets with a thickness as low as 1.2 nm for the first time by solution method with the aid of surface inhibitors [69]. Figure 8a indicated the purified product consists of a colloidal solution of 2D nanoplatelets of submicron level dimensions. The absorption spectrum of the 2D nanoplatelets was dominated by a single sharp exciton absorption feature at 431 nm in Figure 8b, which occurred by 503 meV compared to the bulk exciton absorption at 525 nm blueshift. Liao et al. synthesized MAPbBr₃ microdisk with transverse sizes of 1–10 μm by the self-growth method in the solution [27]. Under the excitation of femtosecond laser at the wavelength of 400 nm, the laser threshold was (3.6 ± 0.5) μJ/cm², the FWHM was 1.1 nm, and the quality factor was 430 as shown in Figure 8c,d. By adjusting the symmetry of the microplate’s shape, the output can be converted from a multimode laser to a single-mode laser. Liu’s group studied the output mode of the perovskite laser [70], and these CH₃NH₃PbBr₃ perovskite microplates were synthesized by a simple one-step in the solution process.

With the continuous in-depth study of perovskite semiconductor materials, researchers have found that perovskite materials also exhibit nonlinear optical properties. Wei Zhang et al. simultaneously prepared MAPbBr₃ 1D microwires (MWs) and 2D square microplates (MPs) using a liquid phase synthesis method. Excited under a pulsed laser with a wavelength of 900 nm, both MAPbBr₃ MWs and MPs showed a two-photon absorption [71]. A maximum Q factor of about 920 was obtained by varying the MPs at the edge length. Figure 9a indicated that the FWHM at 552.3 nm narrowed rapidly from 22 to 0.6 nm with the increasing of the two-photon-pumped (TPP) fluence around the onset power, achieving a high quality (Q ≈ 920) and a low-threshold (E_th ≈ 62 µJ/cm²) lasing action. In 2016, BinYang and their team used femtosecond laser excitation of MAPbBr₃ microdisks to study two-photon pump-amplified spontaneous emission and obtained their tunable emission spectrum from 500 nm to 570 nm [72]. They demonstrated that the photoluminescence properties of the microdisks were dominated by the reabsorption effect under two-photon excitation. In addition, it was found that the interband emission from the near-surface region and the photocarrier diffusion from the near-surface region to the inner region were important for single-photon excitation. The aforementioned two dynamic processes were illustrated in Figure 9b,c. In 2017, the linear and nonlinear light emission characteristics of MAPbBr₃ microplates with different sizes were investigated, and their lasing performance was characterized by two-photon excitation at 800 nm (150 fs, 1 kHz) [73]. Figure 9d clearly showed that the lasing threshold decreased linearly as the lateral dimension of the microplate decreased from 90 μm to 20 μm. Yisheng Gao et al. synthesized a high-quality MAPbBr₃ perovskite microstructure by solution precipitation [74]. The insets of Figure 9e display the high-resolution SEM images of the microplate and microrod. Under the intense laser pumping at 1240 nm, 100 fs, and 1 kHz, an obvious optical limit effect could be observed. Interband photoluminescence was observed at 540 nm. By increasing the pump density, three-photon excitation lasing in MAPbBr₃ perovskite microplate was achieved for the first time at room temperature. The measured three-photon absorption coefficient γ was 2.26 × 10⁻⁵cm³/GW², which was obtained by fitting the data in Figure 9e. Through further observation of the three-photon excited whispering gallery mode laser, it was found that the hybrid lead halide perovskite also had a very large fifth-order nonlinearity, which was of great significance for practical applications such as optical switches.

3.5. Perovskite Quantum Dots Lasers

Perovskite quantum dots (QDs) refer to perovskite materials that are less than 100 nm in three dimensions. They have garnered recent attention due to their unique versatility as laser gain materials, such as low cost, easy synthesis process, tunable emission wavelength and high photoluminescence quantum yield. It is one of the materials with potential development and is expected to replace the traditional II–VI, III–V and IV–VI colloidal QDs. When perovskite QDs do not have an external cavity, multiple scattering between QDs produces amplification, causing the lasering modes to fluctuate randomly. In 2015, Yakunin et al. demonstrated a low-threshold ASE from caesium lead halide perovskites CsPbX₃ (X = Cl, Br or I, or mixed Cl/Br and Br/I systems) nanocrystals with an optical gain coefficient of ∼450 cm⁻¹ and threshold of ∼5.3 μJ/cm² [24] (see Figure 10d). As shown in Figure 10a–c, the ASE from CsPbX₃ NCs was tuned from 440 to 700 nm. At last, they achieved random lasing from CsPbX₃ NCs films without the optical cavity and WGM lasing employing a silica sphere as the resonant cavity. In addition to realizing perovskite QDs lasing-based silica spheres and micro capillaries, well-designed DBR can also serve as an optical resonant cavity [75,76,77]. Sun and co-workers first realized a vertical-cavity surface-emitting laser (VCSEL) based on perovskite QDs, displaying a low threshold of ~9 μJ/cm² and favorable stability. Their device architecture was a sandwiched structure of DBR/CsPbBr₃ QDs/DBR in Figure 10e. This low lasing threshold can result from the large absorption cross-section of the perovskite QDs, high PLQY, low Auger loss, and the good match between the gain profile and the stop band of the DBRs [75]. In 2017, Huang et al. fabricated a perovskite VECSL with an ultralow threshold of ~0.39 μJ/cm² [77]. These VCSELs consisted of a CsPbBr₃ QD thin film and two highly reflective DBRs. Spectacularly, the realization of all-inorganic CsPbBr₃ QDs contributed to high device stability and enabled stable device operation under both femtosecond and quasi-CW nanosecond pulse pumping at ambient conditions [77].

In 2021, Edward H. Sargent and co-workers reported a self-assembly passivation method that relied on sodium—an assembly director that enhanced the attractive forces between nearby CsPbBr₃ QDs and induced the formation of high-quality cubic superlattices [78]. Figure 10f,g showed the effect of atomic-size ligands on the self-assembly interactions of CsPbBr₃ QDs. These self-assembly perovskite quantum-dot superlattices structures were utilized as the resonant cavity and the gain medium, realizing nanosecond-sustained lasing with a threshold of 25 μJ/cm² in Figure 10h. In 2022, Zhang’s group developed a new approach to realize multicolor lasering in the special structure of the perovskite QDs superlattice [79]. The alloy superlattice samples based on perovskite QDs were approximately 10 times more stable than perovskite single-crystal alloy NWs with poor band gap stability, exhibiting significant PL spectral changes within 3 days [80]. Furthermore, the carrier transport dynamics demonstrated the energy transport process in the alloy superlattice, which elucidated the core difficulty of achieving a multicolor perovskite lasers [79].

3.6. Others

In addition to the conventional perovskite lasers mentioned above, several other types of perovskite lasers based on the solution process have been reported in recent years. Wang et al. prepared the CH₃NH₃PbBr₃ perovskite microrod using the solution-processed one-step precipitation method [81]. This perovskite microrod formed a whispering gallery mode microcavity, which was different from the Fabry-Perot cavity. This structure was excited by a femtosecond laser at a wavelength of 400 nm, with a lasering threshold of 2.37 μJ/cm², an FWHM as low as 0.1 nm, and a quality factor as high as 5000. Surface-plasmon (SP) is an excited state with a large enhancement of the electromagnetic field localized at the metal–dielectric interface [82]. SP can provide a powerful platform to tailor the spontaneous emission, thus lasing the low-dimensional perovskite structures in a nanoscale regime [83]. SPs arise from a metal layer or conducting layer, and transfer along the semiconductor-metal interface. In 2017, Wang and co-workers demonstrated that the laser threshold of CsPbBr₃ perovskite microrod with Al nanoparticles (NPs) layer was drastically decreased by more than 20%, and the output intensity was significantly increased by more than an order of magnitude due to plasmonic resonances [84]. Wu et al. proposed a new approach to improve the ASE performance of MAPbI₃ perovskite film via utilization of Au nanorods-doped PMMA [85]. These MAPbI₃ films were prepared by the modified two-step process. Finally, the ASE threshold of the MAPbI₃ perovskite films was obviously decreased from 26.5 to 16.9 μJ/cm², which mainly resulted from the surface passivation of the PMMA layer. In the same year, the reduction of the lasing threshold of CsPbBr₃ perovskite nanocubes was also realized via the surface plasmonic effect of Au NPs by Leng’s group [86].

Table 2. Performances comparison of solution-processed perovskite semiconductors lasers.

Materials	Structure	Laser Mode	Wavelength	Pump Laser	Threshold	FWHM	Year
MAPbX3	Polycrystalline thin film	ASE	390–790 nm	600 nm, 150 fs	44 kW/cm2	N.A.	2014 [23]
MAPbI3	Polycrystalline thin film	ASE	780 nm	530 nm, 4 ns	76 µJ/cm2	1.24	2015 [59]
MAPbI3	Polycrystalline thin film	DFB	784 nm	515 nm, 200 fs	4 µJ/cm2	0.4	2016 [20]
MAPbI3	Polycrystalline thin film	PhC	780 nm	532 nm, 400 ps	200 µJ/cm2	N.A.	2016 [60]
(PEA)2Csn−1PbnBr3n+1	Polycrystalline thin film	VCSEL	532 nm	355 nm, 8 ns	500 µJ/cm2	0.8	2019 [21]
MAPbCl3	Single crystals thin film	VCSEL	414–435 nm	355 nm, 8 ns	211 µJ/cm2	0.38	2020 [64]
MAPbX3	Single crystals NWs	FP	500–780 nm	402 nm, 150 fs	220 nJ/cm2	0.22	2015 [22]
(FAxMA1−x)Pb(Br3−yIy)	Single crystals NWs	FP	490–824 nm	402 nm, 150 fs	2.6 µJ/cm2	0.24	2016 [65]
MAPbI3	Single crystals NWs	FP	776–784 nm	400 nm, 120 fs	13.5 µJ/cm2	5	2016 [66]
CsPbX3	Single crystals NWs	FP	420–650 nm	405 nm, CW	12.3 µJ/cm2	0.09	2018 [67]
MAPbClxBr3−x	Microdisk	WGM	525–557 nm	400 nm, 150 fs	3.6 µJ/cm2	1.1	2015 [27]
MAPbBr3	Microplates	FP&WGM	552.3 nm	900 nm, 150 fs	62 µJ/cm2	0.6	2016 [71]
MAPbBr3	Microdisks	ASE	500–570 nm	1064 nm, 10 ns	2.2 mJ/cm2	N.A.	2016 [72]
CsPbX3	Quantum dots	ASE	440–700 nm	400 nm, 100 fs	6 µJ/cm2	N.A.	2015 [24]
CsPbX3	Quantum dots	VCSEL	440–700 nm	400 nm, 100 fs	9 µJ/cm2	0.6	2017 [75]
CsPbBr3	Quantum dots	VCSEL	522 nm	400 nm, 50 fs 355 nm, 5 nm	0.39 µJ/cm2 98 µJ/cm2	0.9	2017 [77]
CsPbBr3	Quantum dots	ASE	536 nm	355 nm, 2 ns	25 µJ/cm2	0.4	2021 [78]
CsPbBr3	Quantum dots	ASE	480–508 nm	400 nm, 40 fs	30 µJ/cm2	0.13	2022 [79]
MAPbBr3	NCs	SP	554 nm	800 nm, 100 fs	10 µJ/cm2	3	2021 [87]
CsPbBr3	NCs	SP	532 nm	400 nm, 250 fs	46.8 µJ/cm2	20.9	2022 [88]

summarizes the representative works on solution-processed perovskite lasers in recent years.

In 2020, Atwate and co-workers demonstrated that single-mode up-conversion plasmonic lasing from MAPbBr₃ perovskite NCs with a low threshold (10 μJ/cm²) and a calculated ultrasmall mode volume (~0.06 λ³) at 6 K [87]. The MAPbBr₃ perovskite NCs were synthesized by doping MAPbBr₃ film with chlorine as shown in Figure 11a,b. Figure 11a showed a MAPbBr₃ perovskite NCs integrated with an Al₂O₃/TiN (5 nm/80 nm) plasmonic cavity and inset of SEM image. The device was deposited on a silicon-based substrate in which TiN served as a promising resonance-tunable plasmonic platform. In 2021, Lan’s group reported polycrystalline CsPbBr₃ NPs composed of QDs on a thin Au film, exhibiting optically-controlled quantum size effect [89]. However, it is regrettable that their CsPbBr₃ NPs were synthesized by using chemical vapor deposition rather than the solution. Most recently, Lin et al. proposed an on-chip fabricated hybrid photon-plasma system consisting of a perovskite laser structure coupled to a long-range surface plasmon polariton (LRSPP) waveguide, obtaining a low threshold and propagation length in excess of 100 µm [88]. Perovskite NCs were synthesized by a solution method [90]. In this system, the CsPbBr₃ NCs were drop-casted on the patterned samples and then the NCs were forced into the pattern. When the pump energy density exceeded 46.8 μJ/cm², the emission intensity increased and the nonlinearity of FWHM decreased. These results suggested that SPs could not only improve the performance of perovskite lasers but also enable different applications in optical communications and sensor-related devices.

4. Conclusions

Perovskite semiconductor materials have the advantages of high optical gain, large absorption coefficient, long carrier life, solution processing and so on, which is an excellent gain material to achieve low threshold laser. The combination of perovskite materials and optical microcavities can further reduce the laser threshold value. At present, it is necessary to further study the stability, optical gain, threshold and other basic properties of perovskite materials. In this review, we summarized the recent developments and progress of solution-processed perovskite semiconductors lasers. We discussed the merit of solution-processed perovskite semiconductors as lasing gain materials and summarized the characteristics of a variety of perovskite lasers. Research progress in recent years has shown that the application of perovskite and microcavity structures in lasers has a positive effect on reducing the laser threshold. Moreover, the preparation method of the solution process, as well as the flexible devices, are the unique advantages of perovskite lasers compared to other conventional semiconductor lasers.

Despite the great progresses made in perovskite semiconductor materials and perovskite semiconductor lasers, there are still many issues to be solved. Firstly, from the perspective of materials, whether it is chemical vapor deposition or solution crystallization, the prepared perovskite single crystal samples have a certain degree of randomness. To realize the large-scale and commercialization of perovskite lasers, we should also explore a method to prepare homogeneous and reproducible perovskite samples. Furthermore, since most of the perovskite samples in use today contain the heavy metal Pb, which is harmful to the human body and unfriendly to the environment, it is necessary to strengthen the search for new perovskite systems with good optoelectronic properties and less lead or lead-free. Secondly, the Q value of perovskite laser emission still needs to be improved. Further improvement of film forming quality and Q value of optical microcavity are the next important research directions. Compared with traditional III-V compounds semiconductor materials, organic and inorganic hybrid perovskite materials have relatively poor stability, and the organic molecular layer in the material structure is very sensitive to non-polar solvents. There are problems of easy decomposition in air and easy dissolution in water and organic solvents. Finally, but more importantly, a theoretical explanation of the photophysics of perovskite NCs is required to better explain the quantum size effect of perovskite crystals, which can guide research directions for regulating their electronic, optical and defective nature [82]. In response to the above problems, researchers have proposed a variety of solutions, including a small amount of doping to improve the stability of the material phase, reduce the defects of perovskite crystals, design pure inorganic perovskite, and improve the packaging process to encapsulate the polymer layer on the device surface. Therefore, the future trend of the perovskite-based laser is to optimize cavity design and improve device stability to achieve perovskite lasing under continuous optical or even electric pumping at room temperature, which is still key in this field.