Controlling and Optimizing Amplified Spontaneous Emission in Perovskites

Toward Thin-Film Laser Diodes with Metal Halide Perovskites

Metal halide perovskite semiconductors hold a strong promise for enabling thin-film laser diodes. Perovskites distinguish themselves from other non-epitaxial media primarily through their ability to maintain performance at high current densities, which is a critical requirement for achieving injection lasing. Coming in a wide range of varieties, numerous perovskites delivered low-threshold optical amplified spontaneous emission and optically pumped lasing when combined with a suitable optical cavity. A progression toward electrically pumped lasing requires the development of efficient light-emitting structures with reduced optical losses and high radiative efficiency at lasing-level current densities. This involves a set of important trade-offs in terms of material choice, stack and waveguide design, as well as resonator integration. In this Perspective, the key milestones are highlighted that have been achieved in the study of passive optical waveguides and light-emitting diodes, and these learnings are translated toward more complex laser diode architectures. Finally, a novel resonator integration route is proposed that is capable of relaxing optical and electrical design constraints.

Metal-Halide Perovskite Lasers: Cavity Formation and Emission Characteristics

Hybrid metal-halide perovskites (MHPs) have shown remarkable optoelectronic properties as well as facile and cost-effective processability. With the success of MHP solar cells and light-emitting diodes, MHPs have also exhibited great potential as gain media for on-chip lasers. However, to date, stable operation of optically pumped MHP lasers and electrically driven MHP lasers-an essential requirement for MHP laser's insertion into chip-scale photonic integrated circuits-is not yet demonstrated. The main obstacles include the instability of MHPs in the atmosphere, rudimentary MHP laser cavity patterning methods, and insufficient understanding of emission mechanisms in MHP materials and cavities. This review aims to provide a detailed overview of different strategies to improve the intrinsic properties of MHPs in the atmosphere and to establish an optimal MHP cavity patterning method. In addition, this review discusses different emission mechanisms in MHP materials and cavities and how to distinguish them.

Organosilicon-Based Ligand Design for High-Performance Perovskite Nanocrystal Films for Color Conversion and X-ray Imaging

Perovskite nanocrystals (PNCs) bear a huge potential for widespread applications, such as color conversion, X-ray scintillators, and active laser media. However, the poor intrinsic stability and high susceptibility to environmental stimuli including moisture and oxygen have become bottlenecks of PNC materials for commercialization. Appropriate barrier material design can efficiently improve the stability of the PNCs. Particularly, the strategy for packaging PNCs in organosilicon matrixes can integrate the advantages of inorganic-oxide-based and polymer-based encapsulation routes. However, the inert long-carbon-chain ligands (e.g., oleic acid, oleylamine) used in the current ligand systems for silicon-based encapsulation are detrimental to the cross-linking of the organosilicon matrix, resulting in performance deficiencies in the nanocrystal films, such as low transparency and large surface roughness. Herein, we propose a dual-organosilicon ligand system consisting of (3-aminopropyl)triethoxysilane (APTES) and (3-aminopropyl)triethoxysilane with pentanedioic anhydride (APTES-PA), to replace the inert long-carbon-chain ligands for improving the performance of organosilicon-coated PNC films. As a result, strongly fluorescent PNC films prepared by a facile solution-casting method demonstrate high transparency and reduced surface roughness while maintaining high stability in various harsh environments. The optimized PNC films were eventually applied in an X-ray imaging system as scintillators, showing a high spatial resolution above 20 lp/mm. By designing this promising dual organosilicon ligand system for PNC films, our work highlights the crucial influence of the molecular structure of the capping ligands on the optical performance of the PNC film.

Highly luminescent and stable CsPbBr3 perovskite nanocrystals coated with polyethersulfone for white light-emitting diode applications

Simultaneously improving the stability and photoluminescence quantum yield (PLQY) of all inorganic perovskite nanocrystals (NCs) is crucial for their practical utilization in various optoelectronic devices. Here, CsPbBr3 NCs coated with polyethersulfone (PES) were prepared via an in-situ co-precipitation method. The sulfone groups in PES bind to undercoordinated lead ion (Pb2+) on the CsPbBr3 NCs, resulting in significant reduction of surface defects, thus enhancing the PLQY from 74.2% to 88.3%. Meanwhile, the PES-coated NCs exhibit high water resistance and excellent heat and light stability, maintaining over 85% of the initial PL intensity under thermal aging (70°C, 4 h) and continuous 365 nm ultraviolet (UV) light irradiation (24 W, 8 h) conditions. By contrast, the PL intensity of the control NCs dramatically dropped to less than 40%. Finally, a diode emitting bright white light was fabricated utilizing the PES-coated CsPbBr3 NCs, which exhibits a color gamut of ~110% NTSC standard.

Enhancement of Optical Gain in Colloidal CdSe/CdS/ZnS Quantum Dots through Nanosecond Optical Pumping

Optical gain and lasing in colloidal nanocrystals are often hindered by sub-nanosecond rapid Auger non-radiative recombination, especially under continuous wave optical or electrical excitation. This study demonstrates amplified spontaneous emission (ASE) from CdSe/CdS/ZnS quantum dot (QD) solids through prolonged pulsed optical pumping over 10 ns. The incorporation of CdS and ZnS double shells on CdSe QDs effectively decelerates the Auger process in multiexcitonic states by extending the electron wave function and enhancing dielectric screening. Furthermore, we engineer smooth, densely packed QD solid films that efficiently guide the optical mode, achieving substantial net gain values under nanosecond pumping. The proposed approach helps observe ASE with gain thresholds of 0.84 and 1.5 mJ/cm2 under optical pumping pulse widths of 6 and 15 ns, respectively. This advancement can promote continuous pumping in colloidal QD gain systems, opening new avenues for optoelectronic applications.

Toward Nonepitaxial Laser Diodes

Thin-film organic, colloidal quantum dot, and metal halide perovskite semiconductors are all being pursued in the quest for a wavelength-tunable diode laser technology that does not require epitaxial growth on a traditional semiconductor substrate. Despite promising demonstrations of efficient light-emitting diodes and low-threshold optically pumped lasing in each case, there are still fundamental and practical barriers that must be overcome to reliably achieve injection lasing. This review outlines the historical development and recent advances of each material system on the path to a diode laser. Common challenges in resonator design, electrical injection, and heat dissipation are highlighted, as well as the different optical gain physics that make each system unique. The evidence to date suggests that continued progress for organic and colloidal quantum dot laser diodes will likely hinge on the development of new materials or indirect pumping schemes, while improvements in device architecture and film processing are most critical for perovskite lasers. In all cases, systematic progress will require methods that can quantify how close new devices get with respect to their electrical lasing thresholds. We conclude by discussing the current status of nonepitaxial laser diodes in the historical context of their epitaxial counterparts, which suggests that there is reason to be optimistic for the future.

Competition of Carrier Kinetics Contributes to Amplified Spontaneous Emission in Quasi-2D/3D (PBA)2MAn-1PbnBr3n+1 Thin Films under Strip Light Mode

Quasi-2D halide perovskites have potential in lasing due to their amplified spontaneous emission (ASE) properties. The ASE of (PBA)₂MA_n-1Pb_nBr_3n+1 thin films has been confirmed by photoluminescence (PL) testing using stripe light excitation (SLE). The ASE threshold decreases with decreasing environmental temperature (T_E) or increasing number of inorganic layers (n). Using the transient absorption technique, the Auger recombination and the cooling process of the high-activity carrier are accelerated with the decrease of n or T_E. A new ASE mechanism is proposed where high-activity carriers directly emit photons under photon perturbation from adjacent sites, leading to the accumulation and amplification of emitted photons only in the SLE region for ASE to occur. In addition, the reduction of n promotes light scattering between nano-thin layers, which supports a rapid increase in the ASE signal after the ASE threshold is crossed.

From optical pumping to electrical pumping: the threshold overestimation in metal halide perovskites

The threshold carrier density, conventionally evaluated from optical pumping, is a key reference parameter towards electrically pumped lasers with the widely acknowledged assumption that optically excited charge carriers relax to the band edge through an ultrafast process. However, the characteristically slow carrier cooling in perovskites challenges this assumption. Here, we investigate the optical pumping of state-of-the-art bromide- and iodine-based perovskites. We find that the threshold decreases by one order of magnitude with decreasing excitation energy from 3.10 eV to 2.48 eV for methylammonium lead bromide perovskite (MAPbBr₃), indicating that the low-energy photon excitation facilitates faster cooling and hence enables efficient carrier accumulation for population inversion. Our results are then interpreted due to the coupling of phonon scattering in connection with the band structure of perovskites. This effect is further verified in the two-photon pumping process, where the carriers relax to the band edge with a smaller difference in phonon momentum that speeds up the carrier cooling process. Furthermore, by extrapolating the optical pumping threshold to the band edge excitation as an analog of the electrical carrier injection to the perovskite, we obtain a critical threshold carrier density of ∼1.9 × 10¹⁷ cm^-3, which is one order of magnitude lower than that estimated from the conventional approach. Our work thus highlights the feasibility of metal halide perovskites for electrically pumped lasers.

Temperature-Dependent Amplified Spontaneous Emission in CsPbBr3 Thin Films Deposited by Single-Step RF-Magnetron Sputtering

Due to their high optical efficiency, low-cost fabrication and wide variety in composition and bandgap, halide perovskites are recognized nowadays as real contenders for the development of the next generation of optoelectronic devices, which, among others, often require high quality over large areas which is readily attainable by vacuum deposition. Here, we report the amplified spontaneous emission (ASE) properties of two CsPbBr₃ films obtained by single-step RF-magnetron sputtering from a target containing precursors with variable compositions. Both the samples show ASE over a broad range of temperatures from 10 K up to 270 K. The ASE threshold results strongly temperature dependent, with the best performance occurring at about 50 K (down to 100 µJ/cm²), whereas at higher temperatures, there is evidence of thermally induced optical quenching. The observed temperature dependence is consistent with exciton detrapping up to about 50 K. At higher temperatures, progressive free exciton dissociation favors higher carrier mobility and increases trapping at defect states with consequent emission reduction and increased thresholds. The reported results open the way for effective large-area, high quality, organic solution-free deposited perovskite thin films for optoelectronic applications, with a remarkable capability to finely tune their physical properties.

Vacuum-Vapor-Deposited 0D/3D All-Inorganic Perovskite Composite Films toward Low-Threshold Amplified Spontaneous Emission and Lasing

Vacuum vapor deposition (VVD) is a promising way to advancing the commercialization of perovskite light sources owing to its convenience for wafer-scale mass production and compatibility with silicon photonics manufacturing infrastructure. However, the light emission performance of VVD-grown perovskites still lags far behind that of the conventional solution-processed counterparts due to their inferior luminescence properties. Here, a 0D/3D cesium-lead-bromide perovskite composite film is prepared on Si/SiO₂ substrates through composition modulation with the VVD method, which exhibits an ultralow amplified spontaneous emission (ASE) threshold down to 14.3 µJ cm^-2 in the optimal films, which is on par with that of the solution-processed counterparts. Meanwhile, they also display intriguing operational stability with negligible emission intensity decay under continuous excitation above ASE threshold for 4 h in the air. The outstanding ASE performance mainly originates from the reduced trap density and weakened electron-phonon coupling in the 3D CsPbBr₃ phase enabled by the incorporation of the 0D Cs₄ PbBr₆ phase. Finally, by integrating the composite film with the distributed feedback (DFB) cavity, DFB lasing is achieved with a low threshold of 18.2 µJ cm^-2 under nanosecond-pulsed laser pumping, which highlights the potential of VVD-processed perovskites for developing high-performance lasers.

Optical Properties of Perovskite-Organic Multiple Quantum Wells

A comprehensive study of the optical properties of CsPbBr₃ perovskite multiple quantum wells (MQW) with organic barrier layers is presented. Quantum confinement is observed by a blue-shift in absorption and emission spectra with decreasing well width and agrees well with simulations of the confinement energies. A large increase of emission intensity with thinner layers is observed, with a photoluminescence quantum yield up to 32 times higher than that of bulk layers. Amplified spontaneous emission (ASE) measurements show very low thresholds down to 7.3 µJ cm^-2 for a perovskite thickness of 8.7 nm, significantly lower than previously observed for CsPbBr₃ thin-films. With their increased photoluminescence efficiency and low ASE thresholds, MQW structures with CsPbBr₃ are excellent candidates for high-efficiency perovskite-based LEDs and lasers.

Amplified Spontaneous Emission from Thermally Evaporated High-Quality Thin Films of CsPb(Br1-xYx)3 (Y = I, Cl) Perovskites

As a wavelength-tunable lasing material, perovskites are now generating a lot of scientific attention. Conventional solution-processed CsPbX₃ perovskite films sometimes suffer unavoidable pinhole defects and poor surface morphology, severely limiting their performance on amplified spontaneous emission (ASE) and lasing application. Herein, a thermal evaporation approach is explored in our work to achieve a uniform and high-coverage CsPb(Br_1-xY_x)₃ (Y = I, Cl) perovskites polycrystalline thin film. The ASE of these films was studied using a picosecond laser system. The ASE profile increases rapidly over the narrow peak in relation to the laser pump intensity, confirming the development of stimulated emission. ASE began when the energy density threshold was reached and ranged between 25 and 170 μJ/cm² per pulse for perovskite materials when replacing I with Br and then Cl. This work emphasizes the notable optical properties of high-quality perovskite thin films, leading to possible accessible uses in optoelectronic applications.

Achieving Up-Conversion Amplified Spontaneous Emission through Spin Alignment between Coherent Light-Emitting Excitons in Perovskite Microstructures

Metal hybrid perovskites have presented interesting infrared-to-visible up-conversion light-emitting lasing properties through multi-photon absorption. Here, when the optical pumping switches between circular and linear polarization, up-conversion amplified spontaneous emission (ASE) intensity exhibits large and small amplitudes, respectively, leading to a positive up-conversion ΔASE in the CsPbBr3 perovskite microrods. This observed phenomenon demonstrates that the coherent interaction between coherent light-emitting excitons is indeed established at the up-conversion ASE regime in the CsPbBr3 perovskite microrods. In addition, the positive up-conversion ΔASE indicates the orbital magnetic dipoles between coherent light-emitting excitons are conserved during up-conversion ASE action. Essentially, the up-conversion ΔASE results provide evidence that shows up-conversion ASE can be realized by the orbit−orbit polarization interaction between light-emitting excitons. Moreover, up-conversion ASE proportionally increased as the pumping fluence increased, which shows that orbit–orbit polarization interaction can be gradually enhanced between coherent light-emitting excitons by increasing pumping density in the CsPbBr3 perovskite microrods. Substantially, our studies provide a fundamental understanding of the spin alignment between coherent light-emitting excitons towards developing spin-dependent nonlinear lasing actions in metal halide perovskites.

Metal oxide nanoparticle-modified ITO electrode for high-performance solution-processed perovskite photodetectors

Low dark current density plays a key role in determining the overall performance of perovskite photodetectors (PPDs). To achieve this goal, a hole transport layer (HTL) on the ITO side and a hole blocking layer (HBL) on the metal electrode side are commonly introduced in PPDs. Unlike traditional approaches, we realized a high-performance solution-processed broadband PPD using metal oxide (MO) nanoparticles (NPs) as the HBL on the ITO electrode and PC61BM as another HBL on the metal electrode side to reduce the device dark current. The PPDs based on TiO2 and SnO2 NP-modified layers show similar device performances at -0.5 V: a greater than 105 on/off ratio; over 100 dB linear dynamic range (LDR) under different visible light illumination; around 0.2 A W-1 responsivity (R); greater than 1012 jones detectivity (D*); and ∼20 μs rise time of the device. The MO NP interfacial layer can significantly suppress charge injection in the dark, while the accumulated photogenerated charges at the interface between the MO layer and the perovskite layer introduce band bending, leading to dramatically increased current under illumination. Therefore, the dark current density of the devices is significantly reduced and the optical gain is drastically enhanced. However, after UV illumination, the dark current of the TiO2 device dramatically increases while the dark current of the SnO2 device can stay the same as before since the UV illumination-induced conductivity and barrier height changes in the TiO2 layer cannot recover after removing the UV irradiation. These results indicate that the TiO2 NP layer is suitable for making a vis-NIR photodetector, while the SnO2 NP layer is a good candidate for UV-vis-NIR photodetectors. The facile solution-processed high-performance perovskite photodetector using MO NP-modified ITO is highly compatible with low cost, flexible, and large-area electronics.

One- and Two-Photon Excited Photoluminescence and Suppression of Thermal Quenching of CsSnBr3 Microsquare and Micropyramid

Thermal photoluminescence (PL) quenching is fundamentally important for perovskite optoelectronic applications. Herein, we investigated PL characteristics of CsSnBr₃ microsquares and micropyramids synthesized by chemical vapor deposition (CVD) and their PL quenching behavior at high temperature. These microstructures have favorable PL performances in ambient atmosphere. Under two-photon excitation, we observed whispering gallery modes (WGMs) in microsquares and amplified spontaneous emission (ASE) in micropyramids. Reversible PL losses due to thermal effect were observed for both samples. Monotonic blue shifts in PL emission upon temperature increase suggest a band gap widening associated with an emphanisis effect. Temperature-dependent spectral line width analysis reveals that a line width broadening is attributed to the dominant electron-longitudinal optical phonon interaction. The estimated activation energy of thermally assisted nonradiative recombination for CsSnBr₃ microsquares and micropyramids is over 310 meV by the Arrhenius equation, which is higher than CsPbBr₃. These results prove that CsSnBr₃ exhibits better thermal stability than Pb-based perovskites.

Effects of photon recycling and scattering in high-performance perovskite solar cells

Efficient external radiation is essential for solar cells to achieve high power conversion efficiency (PCE). The classical limit of 1/2n² (n, refractive index) for electroluminescence quantum efficiency (ELQE) has recently been approached by perovskite solar cells (PSCs). Photon recycling (PR) and light scattering can provide an opportunity to surpass this limit. We investigate the role of PR and scattering in practical device operation using a radiative PSC with an ELQE (13.7% at 1 sun) that significantly surpasses the classical limit (7.4%). We experimentally analyze the contributions of PR and scattering to this strong radiation. A novel optical model reveals an increase of 39 mV in the voltage of our PSC. This analysis can provide design principles for future PSCs to approach the Shockley-Queisser efficiency limit.

Electrical Pumping of Perovskite Diodes: Toward Stimulated Emission

The success of metal halide perovskites in photovoltaic and light-emitting diodes (LEDs) motivates their application as a solid-state thin-film laser. Various perovskites have shown optically pumped stimulated emission of lasing and amplified spontaneous emission (ASE), yet the ultimate goal of electrically pumped stimulated emission has not been achieved. As an essential step toward this goal, here, a perovskite diode structure that simultaneously exhibits stable operation at high current density (≈1 kA cm-2 ) and optically excited ASE (with a threshold of 180 µJ cm-2 ) is reported. This diode structure achieves an electroluminescence quantum efficiency of 0.8% at 850 A cm-2 , which is estimated to be ≈3% of the charge carrier population required to reach ASE in the same device. It is shown that the formation of a large angle waveguide mode and the reduction of parasitic absorption losses are two major design principles for diodes to obtain a positive gain for stimulated emission. In addition to its prospect as a perovskite laser, a new application of electrically pumped ASE is proposed as an ideal perovskite LED architecture allowing 100% external radiation efficiency.

Why Hybrid Tin-Based Perovskites Simultaneously Improve the Structural Stability and Charge Carriers' Lifetime: Ab Initio Quantum Dynamics

Much effort has been dedicated to boost the development of lead-free perovskite solar cells. However, their performance and stability are still much less competitive to the lead-bearing counterparts. By exploiting a mixed Sn-Ge cation strategy for the development of lead-free perovskites, we perform ab initio electronic structure calculations and quantum dynamics simulations on MASn_0.5Ge_0.5I₃ and compare them to MASnI₃. The calculations demonstrate that the hybrid cation strategy can improve simultaneously the perovskite stability and the lifetime of charge carriers. The stability increases due to a larger space of possible structures within the favorable range of the structural parameters, such as the Goldschmidt tolerance and octahedron factors. By exploring the larger structure space, mixed perovskites find stable configurations with lower free energies and better fitting components that exhibit reduced fluctuations around the equilibrium geometries. Charge carriers live longer in mixed perovskites because cation mixing results in an additional and moderate disorder that separates electrons and holes, reducing their interactions while still maintaining efficient band-like charge transport. These general and fundamental principles established by the analysis of the simulation results are useful for the design of advanced materials for solar energy and construction of optoelectronic devices.

Computational Study of Dipole Radiation in Re-Absorbing Perovskite Semiconductors for Optoelectronics

Compared to organic emitters, perovskite materials generally have a small Stokes shift and correspondingly large re-absorption of dipole emission. Classical optical modelling methods ignoring re-absorption do not provide an adequate description of the observed light emission properties. Here, optical modelling methods and design rules for perovskite light-emitting diodes are presented. The transfer-matrix formalism is used to quantify the Poynting vectors generated by a dipole radiating inside a perovskite optoelectronic device. A strategy is presented to deal with non-radiative coupling to nearby emissive material that can otherwise lead to non-physical divergence in the calculation. Stability issues are also investigated regarding coherence of the light propagating in the substrate and the absence of a light absorber in the system. The benefit of the photon recycling effect is taken into account by recursive calculation of the dipole generation profile. The simulation results predict that a high external quantum efficiency of ≈40% is achievable in formamidinium lead triiodide-based perovskite light-emitting diodes, by optimization of microcavity, dipole orientation, and photon recycling effects. Contrary to conventional device structures currently reported, this work highlights the benefits of thick charge transport layers and thick perovskite with small Stokes shift.