Quasi-2D Lead Halide Perovskites for Micro-and Nanolasers

WANG Jun，ZHOU Feng-xian，LI Qian，HU Zhi-ping，DU Juan*，LIU Zheng-zheng，ZHANG Ze-yu，LENG Yu-xin*

（1.School of Physics and Optoelectronic Engineering，Hangzhou Institute for Advanced Study，UCAS，Hangzhou 310024，China；2.State Key Laboratory of High Field Laser Physics and CAS Center for Excellence in Ultra-intense Laser Science，Shanghai Institute of Optics and Fine Mechanics，Chinese Academy of Sciences，Shanghai 201800，China）

Abstract：Perovskite materials and devices have obtained great progress due to their significant optoelectronic properties.Especially，quasi-two-dimensional（2D）halide perovskites have shown promising potential in optoelec⁃tronic devices due to their large exciton binding energy，strong exciton-photon coupling and enhanced stability.More⁃over，the naturally quantum-well structure of 2D perovskite allows exciton energy transfer from the small-n phase to large-n phase，promoting exciton utilization and population inversion to achieve lasing as optical gain media.Here⁃in，we will introduce the crystal structure and advanced optical properties of quasi-2D perovskite at first，and then summarize several strategies about regulating crystal orientation of quasi-2D perovskite.Finally，we will review the development of quasi-2D perovskite based micro-and nanolaser，and present a summary and prospect for quasi-2D perovskite materials and laser devices in the future.

Key words：quasi-2D perovskite；crystal orientation；gain medium；laser

1 Introduction

Perovskites have attracted considerable atten⁃tion due to their high photoluminescence quantum yield（PLQY）,large light absorption coefficient,su⁃perior carrier mobility and long carrier diffusion length,which have been studied widely in the opto⁃electronic application such as solar cell,light-emit⁃ting diode（LED）,laser and photodetector[1-12].Even though the most widely studied perovskite structure at present is the three-dimensional（3D）perovskite[13],the quasi-two-dimensional（quasi-2D）perovskites have emerged as rising star candidates in recent years due to their enlarged exciton binding energy,strengthened exciton-photon coupling and enhanced stability as compared with the 3D counterparts.These improved properties of quasi-2D perovskites can be assigned to the introduction of organic cat⁃ions,which can not only improve the stability of perovskite but also accelerate the exciton radiation recombination process[14-19].In the naturally layered quantum-well structure of quasi-2D perovskite,the exciton energy will transfer from small-nlayer to large-nlayer,promoting radiation recombination[20].Currently,quasi-2D perovskite based LED has ac⁃quired rapid progress,especially the external quan⁃tum efficiency（EQE）of green LED has exceeded 22%[21].

Due to naturally quantum-well structure,large exciton binding energy and strong exciton-photon coupling,quasi-2D perovskite has shown potential promising as optical gain media for nanolaser[22].Moreover,quasi-2D perovskite films have more fac⁃ile fabrication process and adaption to substrates than 3D perovskite structure,which are much favor⁃able for the integration with various cavity[23-25].In re⁃cent years,micro-and nanolaser devices based on quasi-2D perovskite have been realized with various cavities,including single-crystal,whispering gallery mode（WGM）,Fabry⁃Perot（F-P）cavities,distributed feedback（DFB）,distributed Bragg reflector（DBR）and photonic crystal structures[26-28].Especially,deep subwavelength quasi-2D perovskite single-mode nanolaser with simple structure has been obtained successfully[29].In addition,room temperature stabi⁃lized continuous-wave（CW）lasing also has been re⁃alized in quasi-2D perovskite[25,30-35].All these excit⁃ing micro-and nanolasers based on quasi-2D perovskites demonstrate their great potential for the development of electrically pumped lasers.

In this review,we mainly review and discuss the structure,optical properties,the crystallization orien⁃tation and the micro/nanolaser application of quasi-2D perovskite.Firstly,we introduce the structural classification of quasi-2D perovskite according to dif⁃ferent crystalline phases and organic cation.Based on unique quantum-well structure,quasi-2D perovskite shows advanced optical properties and better stabili⁃ty.Then the strategies of regulating crystal orienta⁃tion,which include additive,polymer and process en⁃gineering methods,are summarized.We mainly re⁃view recent progress of quasi-2D perovskite as gain media for nanolasers,including single-crystals,perovskite films and nanolaser arrays.Finally,we present a summary and outlook for quasi-2D perovskite materials and nanolaser in the future.

2 Structure and Properties

2.1 Structure

The standard 3D perovskites have anABX3structure,where theA-site represents monovalent cation like Cs,MA or FA,B-site ions are generally metal elements like Pb,Sn andX-site ions represent halide elements such as Cl,Br,and I[36].In this gen⁃eral structure,sixX-site ions coordinated to aB-sites ion form a[BX6]octahedra,andA-site ion is sur⁃rounded by these corner-sharing inorganic octahe⁃dra[37].The properties of the perovskite can be ad⁃justed easily by replacingA,B,andXsites ions.When Goldschmidt tolerance factor（t）is between 0.8 and 1.0,a standard 3D perovskite structure can be formed stably[38].

The structure of quasi-2D perovskite can be ob⁃tained by cutting along the<100>,<110>,and<111>plane of 3D perovskite and adding alkylammonium cation,leading to three kinds of quasi-2D perovskite with different orientations（Fig.1）[36].The general formula of<100>-oriented quasi-2D perovskites（Fig.1（a））isA'2An-1BnX3n+1,whereA’is monova⁃lent organic spacer cation,A,B,Xare the corre⁃sponding ions in 3D perovskite andnis the number of inorganic layers.<100>-oriented quasi-2D perovs⁃kite is favorable for continuous growth of inorganic plate,which is the most commonly studied quasi-2D perovskite[39].The general formulaA'2AnBnX3n+2（n>1）belongs to<110>-oriented quasi-2D perovskite（Fig.1（b））,which is found to be ideal candidate for solid-state lighting material because of its broadband white-light emission at room temperature,mainly being caused by self-trapped excitons and lattice efects[40].But it is difficult to modulate the thick⁃ness of inorganic layers accurately for stable<110>crystalline structure[41].<111>-oriented quasi-2D perovskite（Fig.1（c））has the general formulaA'2An-1⁃BnX3n+3,in which theB-site cation is usually Bi or Sb[42].This class of perovskite is one of ideal substi⁃tutes for lead-free perovskites,but with poor phase stability and excessive electron hole effective mass,resulting in lack of use for solar cell[43-44].Considering that<100>-oriented class is the mostly studied quasi-2D perovskites,our review mainly focuses on this class of quasi-2D perovskites.

Fig.1 Schematic of different oriented families of quasi-2D perovskites：（a）<100>-oriented，（b）<110>-oriented，（c）<111>-ori⁃ented［45］.

By the alternation of organic spacer cations,<100>-oriented quasi-2D perovskite structure can be further divided into Ruddlesden-Popper（RP）,Di⁃on-Jacobson（DJ）,and alternating cations in the in⁃terlayer space（ACI）structure（Fig.2）[45].The RP composition has a general formulaA'2An-1BnX3n+1,whereA’is generally monovalent organic cation[46].The inorganic layer in quasi-2D RP perovskite is separated by alkyl chains or aryl ammonium cations which are connected by relatively weak van der Waals force（Fig.2（a））[47].Paritmongkolet al.pre⁃pared quasi-2D RP perovskite（BA）2MA3Pb4I13and demonstrated a（1/2,1/2）displacement between ad⁃jacent inorganic layers[48].Because only part of the ad⁃jacent organic spacer cations overlap,quasi-2D RP perovskite has a long interlayer distance,resulting in no interaction between inorganic layers[46].The gener⁃al formula of quasi-2D DJ perovskite isA’An-1BnX3n+1,whereA’is bivalent organic spacer cation with two amino groups.Compared to RP perovskite,the inor⁃ganic layers are connected by strong hydrogen bond in⁃teraction between the organic and inorganic layers,leading to a relatively smaller interlayer space（Fig.2（b））[49].In 2018,Maoet al.firstly fabricated homolo⁃gous quasi-2D DJ perovskite⁃（3AMP）（MA）3Pb4I13,demonstrating that the inorganic layers are perfectly stacked together without displacement in the crystal plane[50].The quasi-2D ACI-phase perovskite with for⁃mulaA’AnBnX3n+1was proposed in recent years.The monovalent spacer cationsA’andA-site ions are ar⁃ranged alternately in the organic layer,resulting in a（1/2,0）displacement between inorganic layers（Fig.2（c））[51].In 2017,Soeet al.prepared quasi-2D ACI perovskites（C（NH2）3）-（CH3NH3）nPbn⁃I3n+1,which demonstrated smaller band gap compared to RP and DJ structure due to its lower interlayers distance[52].Up to now,guanidinium（GA）is the only reportedA’cation in quasi-2D ACI structure[51-52].

Fig.2 Structure diagram of quasi-2D RP-，DJ-，and ACI-phase perovskite structures：（a）（PA）2（MA）3Pb4I13，（b）（PDA）⁃（MA）3Pb4I13，（c）（GA）（MA）3Pb3I10［49，52］.

2.2 Optical Properties

Quasi-2D perovskite structure has many advan⁃tages compared to traditional 3D perovskites due to their alternately arranged organic/inorganic layers structure.They can limit charge carriers to quasi-2D range and form a multiple quantum well natural⁃ly[53].On the other hand,the huge dielectric differ⁃ence between organic and inorganic layers could in⁃fluence the electrostatic force in the electron-hole pairs,which could significantly enhance the elec⁃tron-hole binding energy（Fig.3（a））[54].The various possibilities based on multiple-quantum-well struc⁃tures make quasi-2D perovskite an interesting mate⁃rial system for room temperature photoelectric and fundamental physics applications[55].

In quasi-2D perovskite,the influence generated by confinement effect from quantum-well structure is the variation of bandgap[45].The total bandgap energy in quasi-2D RP perovskite consists of two parts:the energy determined by 3D perovskite and the extra part that caused by quantization energy of electron and hole[59].Besides traditional method of alternatingA,BandXcations in 3D perovskites,the bandgap of quasi-2D perovskites can be also adjusted by control⁃lingnvalue.Ziboucheet al.simulated the band alignment between 3D perovskites（MAPbI3,FAPbI3,MASnI3,and FASnI3）and their corresponding quasi-2D perovskites with differentnvalues using density functional theory techniques,as shown in Fig.3（b）and（c）[57].They demonstrated the bandgap of MAPbI3gradually decreases 2.41 eV to 1.50 eV withnvalue increasing from 1 to∞.Wanget al.used the self-assembly BA+precursor cation at the water-air in⁃terface as a soft template for inch-scale quasi-2D perovskite single crystal membranes BA2MAn-1Pbn‐I3n+1（n=1,2,3,4,and∞）[58].They demonstrated that the structural,optical and electrical properties of BA2MAn-1PbnI3n+1are mainly related with quantum well thickness,which can be determined bynvalues.As shown in the UV-Vis and PL spectra（Fig.3（d）and（e））of quasi-2D perovskites BA2MAn-1PbnI3n+1with differentnvalues,the bandgap shifts from 1.5 eV to 2.3 eV withnincreasing from 1 to∞.

Fig.3（a）Calculated energy level alignment（top）and dielectric constant profile（bottom）along the stacking direction in the quasi-2D perovskite BA5MA4Pb5I16［56］.Bandgap energies of（BA）2An-1SnnI3n+1（b）and（BA）2An-1PbnI3n+1（c）perovskites with different compositions and n values［57］.Photoluminescence（d）and absorption spectra（e）of quasi-2D perovskite（BA）2MAn+1PbnI3n+1［58］.（f）Schematic of energy transfer from low n value domain to high n value domain.（g）Pump-fluence dependence of the emission of quasi-2D perovskite（NMA）2（FA）Pb2BrI6［50］.

Quasi-2D perovskite tends to spontaneously ag⁃gregate in the process of synthesis leading to a quasi-2D perovskite system with differentn-values,which is favorable for the energy transfer among mixed phas⁃es.Hence,the photoexcitation could be focused on highnvalue domain,facilitating the formation of pop⁃ulation inversion,therefore is fascinating for optical gain medium[58].In 2017,Liuet al.observed the con⁃secutive photoinduced charge carriers transfer in qua⁃si-2D perovskite（BA）2（MA）n-1PbnI3n+1films with mixture of multiple perovskite phases withn=2,3,4 and≈∞[60].They found that the mixed phases natural⁃ly aligned in the order ofnalong the direction per⁃pendicular to the substrate.Consecutive photoin⁃duced electron transfer from small-nto large-nphas⁃es and hole transfer in the opposite direction were ob⁃served,which facilitated the charge contraction,and thus lay the foundation for their novel applications in optical gain medium.The room-temperature ASE from quasi-2D perovskites was firstly observed from（NMA）2（FA）Pb2Br1I6by Liet al.in 2018,with gain coefficients as high as>300 cm-1（Fig.3（f）and（g））[61].The（NMA）2（FA）Pb2Br1I6-based perovskite thin films had differentn-values,allowing rapid and efficient energy funneling from higher energy-band⁃gap quantum-wells（lownvalue domain）to lower en⁃ergy-bandgap quantum-wells（highnvalue domain）and achieving room-temperature ASE with low threshold（（19±2）μJ·cm-2）and high gain coeffi⁃cient.Moreover,the ASE stability at ambient condi⁃tions also shows better performance than 3D perovskite thin films.Zhanget al.also observed room-temperature ASE and lasing from mixednvalue phases in quasi-2D perovskites thin film of（BA）2⁃（MA）n-1PbnBr3n+1.They demonstrated an ultrafast energy transfer from lownvalue domain to highnvalue domain,concentrating photoexcitation at the lowest bandgap quantum well（n≈∞）and establishing the population inversion for stimulated emission[62].Their results showed no ASE in smallnvalues 1～3 domains,whilen=6 perovskite thin film exhibited the best ASE performance.

For semiconductor materials,excitons play an important role in charge transport and optical proper⁃ties.Excitons of perovskite belong to the Vanier ex⁃citon model and are usually observed at low tempera⁃tures[63].When the thermal energy is greater than the binding energy,the excitons rapidly ionize[64].Due to thin and deep quantum wells of quasi-2D perovskite,the spatial confinement halves the Bohr radius and quadruples the exciton binding energy compared to 3D perovskites,therefore,excitons of quasi-2D perovskite can be observed at room temperature[58].With the decrease ofnvalue,the binding energy of excitons gradually increases,which is directly relat⁃ed to the changes of quantum well and dielectric con⁃stant.Blanconet al.estimated the exciton binding energies as a function ofn[63].The exciton binding energy of BA2MAn-1PbnI3n+1crystal increased from 220 meV to 380 meV with the decrease ofnvalue（n=1～5）.The exciton effects can accelerate radia⁃tive recombination,leading to a higher photolumines⁃cent quantum yield（PLQY>80%）[14,16].

Due to its high in-plane anisotropy and excel⁃lent optoelectronic properties,quasi-2D perovskite is considered to perform a greater optical anisotropy than 3D perovskite[65].Chenet al.fabricated 2D[CH⁃（NH2）2][C（NH2）3]PbI4crystal and reported its large optical anisotropy[66].Without specially designing the crystal morphology,a large photoresponse linear dichroic ratio up to 2 and a PL linear dichroic ratio of 4.7 were found in the quasi-2D perovskite,which are equivalent or even higher than those perovskite structure with specially designed shape.This prop⁃erty makes quasi-2D perovskite an ideal candidate for polarization devices.

2.3 Stability

Although 3D perovskite exhibits excellent pho⁃toelectric properties when used as laser gain medi⁃um,its poor stability hindered its further applica⁃tion.In general,quasi-2D perovskites exhibit signif⁃icantly enhanced stability as compared with the 3D counterpart,due to their hydrophobicity of organic groups and stronger interlay interaction[46-47,49,67-70].The organic spacer cations act as a barrier on the surface of perovskite film,preventing perovskite structure from direct contact of moisture and diffu⁃sion of water（Fig.4（a））[35].Spanopouloset al.proved quasi-2D RP perovskite performed better en⁃vironmental stability by preparing a series of quasi-2D perovskites with different organic spacer cations[butylamine（BA）,pentylamine（PA）and hexylamine（HA）][67].They demonstrated that the hydrophobici⁃ty increased with the length of the organic spacer molecules increasing（Fig.4（b））[67].Moreover,over⁃looking the little effect on the optical properties,in⁃creasing the length of organic spacer can improve the air,heat,and light stability of quasi-2D perovskite.Finally,the obtained（PA）2（MA）2PbnI7films on crystalline Si can be air stable for more than 450 d（relative humidity=（20%-80%）±5%）.

Fig.4（a）Schematic of organic spacer cations blocking water molecules in quasi-2D perovskite［35］.（b）Contact angles between a water droplet and the perovskite films：（A）MAPbI3，（B）（BA）2MA2Pb3I10，（C）（PA）2MA2Pb3I10，and（D）（HA）2MA2Pb3I10［67］.（c）Schematic of 2D/3D perovskite［71］.（d）Evolution of radiance according to time passed under a constant voltage of LEDs with pure 3D，2D/3D and quasi-2D perovskites［72］.

Moreover,the organic layers in quasi-2D perovskite can act as a barrier in the structure,which can prevent inorganic cations from migrating（Fig.4（c））[73-74].Therefore,quasi-2D perovskites can be used to modify the surface of 3D bulk perovskite and enhance device performance.Hanet al.designed surface-quasi-2D（（PEA）2PbI4）/bulk-3D（FA0.98Cs0.02PbI3）hetero-phased perovskite nanograins for long-term-stable LEDs[72].They found that quasi-2D surface functionalized perovskite dem⁃onstrated not only significantly reduced trap density and ion migration,but also rapid radiative recombi⁃nation due to its spatially and potentially confined charge carriers.As a result,the hetero-phased perovskite LEDs showed substantial improvement in operational lifetime compared to conventional pure 3D or quasi-2D counterparts（Fig.4（d））[72].On the other hand,quasi-2D perovskites have a better ther⁃modynamic stability compared to traditional 3D perovskites due to the alternately arrangement of or⁃ganic/inorganic layers and stronger interaction among organic spacer cations andBX6octahedra[75].Compared to RP ones,quasi-2D DJ-phase perovskites usually have better stability,since the strong hydrogen bond in DJ perovskite is more stable than the van der Waals interaction in RP perovskite[49].Moreover,the closer interlayer dis⁃tance in DJ perovskite is favorable for the interaction of adjacent inorganic layers,which performs en⁃hanced optical properties and thermodynamics sta⁃bility.Zhaoet al.prepared quasi-2D DJ perovskite（FPP）PbI4thin films,which exhibits phase stability with the temperature up to 230℃.In contrast,the RP perovskite（PMA）2PbI4almost degraded com⁃pletely at 157℃[76].Moreover,the photostability is also enhanced in quasi-2D perovskites,since the hy⁃drophobic organic spacer effectively passivates the perovskite crystal lattice against moisture and oxy⁃gen,which reduces the formation of reactive oxygen species under light[77].

3 Crystallization Regulation

Quasi-2D perovskites show excellent photoelec⁃tric property due to their special crystal structure and have been widely used in the field of optoelec⁃tronic devices.The crystal phase structures affect their photoelectric properties,since the crystalliza⁃tion orientation of quasi-2D perovskite films could influence the charge transfer effectively[78].The accu⁃rate regulation of crystal orientation can effectively promote photo-generated carrier transport and in⁃duce radiative recombination.Therefore,controlling the crystal orientation of quasi-2D perovskite films is crucial for manufacturing efficient devices[78-79].As for semiconductor lasers,the gain property of gain media is critical for the performance,which has great correlation with the crystallization quality of perovskite crystals and films[80].The accumulation of heat and defects of gain media mainly hinder the per⁃formance of optical-pump laser.Therefore,several strategies can be used to enhance structural stability and suppress the formation of photoinduced defects of quasi-2D perovskite,such as compositional engi⁃neering or external additives.

3.1 Solvent Engineering

Quasi-2D perovskite films are mainly fabricated through simple solution process,in which the polari⁃ty,viscosity of solvent and the coordination will affect the crystallization of quasi-2D perovskite.Especial⁃ly,the crystallization rate of the perovskite phase can be controlled by using specific solvent.Dimethyl sulfoxide（DMSO）and N,N-dimethylformamide（DMF）are commonly used as solvent for the crystalli⁃zation of perovskite[81-82].When DMF is used as sol⁃vent for perovskite precursor only,DMF can be evapo⁃rated rapidly at the gas-liquid interface,which can cause random-orientated perovskite grains in the in⁃ner layer of perovskite films（Case 1 in Fig.5（a））[83].In comparation,when the mixture of DMF and DMSO are used as solvent,the nucleation process will be slowed down due to the formation of intermediate from DMSO and perovskite precursor and then the crystal grows perpendicular to the substrate（Case 2 in Fig.5（a））.In 2018,Hamillet al.investigated the role of different solvent（DMSO,DMPU,DMAC,NMP,GBL,EC,PC,et al.）in the precursors contain⁃ing Pb2+.They introduce Gutmann’s donor number（DN）to represent the coordination ability of solvent with Pb2+[84].LowDNvalue solvent has little effect on Pb2+,which is conducive to the combination of Pb2+and I-.By contrast,those solvents with highDNcould coordinate more strongly with Pb2+,in turn inhibiting I-coordination and stalling perovskite crystalliza⁃tion.Varying the concentration of highDNadditives in precursor solutions could tunes the strength of lead solvent interactions effectively,allowing finer control over the crystallization and morphology of perovskite active layers（Fig.5（b））.

Fig.5（a）Illustration of crystallization process without DMSO addition（Case 1）and with DMSO addition（Case 2）to DMF sol⁃vent［83］.（b）DN of different solvent［84］.（c）Illustration of defect passivation by BMIM+ions［87］.（d）Schematic of crystal structure change and defect passivation by crown and MPEG-MAA［88］.

In addition,ionic liquid can be also used as sol⁃vent to regulate the crystallization of quasi-2D perovskite[85].The physical properties of ionic liquid,such as high conductivity and low viscosity can be adjusted by varying the combination of cations and anions.In 2016,Yanget al.firstly reported the ap⁃plication of ionic liquid（1-benzyl-3-meth-ylimidazo⁃lium chloride）in perovskite solar cells based on quasi-2D perovskite（HC（NH2）2PbI3）0.85（CH3NH3-PbBr3）0.15.They demonstrated that ionic liquid can effectively promote the reduction of the electron trapstate density at perovskite surface.Finally they fabricated quasi-2D perovskite-based solar cell with improved power conversion efficiency（PCE）of 16.09%[86].In 2021,Penget al.treated Pb-based perovskite thin films with ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate（BMIMBF4）,which can effectively passivate defect due to the spontaneous distribution of ions on film top surface and the crystal surface of the polycrystalline perovskite（Fig.5（c））[87].Then,they obtained highly efficient LED with EQE of 22.9%.

3.2 Additives Engineering

Additive can be usually used to improve the quality of perovskite film and the performance of de⁃vice through the control of crystal orientation and surface modification[89].In 2020,Liet al.adopted CH3NH3Cl（MACl）as an additive to obtain a（PTA）2⁃（MA）3Pb4I13film with highly oriented crystal struc⁃ture and uniform surface morphology[90],which enable superior charge transfer capability and low-density defect states.The highly oriented crystal structure can be attributed to the interaction between Cl-of MACl additive and Pb2+.Finally,the optimized de⁃vice showed PCE of 11.53% and enhanced stability.Then Huanget al.also used MACl as additive to mod⁃ifying quasi-2D RP perovskite（PEA）2（MA）4Pb5I16films through facile one-step process[91].They demon⁃strated that the MACl additive can form an intermedi⁃ate phase to facilitate the vertical alignment of quasi-2D RP perovskite film on the substrate.Finally,they improved the PCE of solar cell from 0.66% to 9.7%by introducing MACl as additive.

Besides,ammonium chloride（NH4Cl）can also be used as effect additive for quasi-2D perovskite precursor solution to regulate the crystal orienta⁃tion[82].In 2018,Fuet al.adopted NH4Cl and NH4SCN as additive to obtain quasi-2D perovskite（PEA）2（MA）4Pb5I16film with better crystallinity and preferential orientation[92].Both additives can improve the crystallinity of perovskite film,but com⁃pared with NH4Cl,NH4SCN is more effective.How⁃ever,tiny pinholes appeared in the film with only NH4SCN as additive,compact and uniform films are obtained with only NH4Cl or both additives.More⁃over,the addition of NH4Cl increases the concentra⁃tion of Cl-at the perovskite interface in solar cell,which can passivate the electron trap and improves the external quantum effect of electroluminescence.Then they fabricated（PEA）2（MA）4Pb5I16-based so⁃lar cell with PCE of 14.1%.In 2020,Yuanet al.used NH4Cl additive to inhibit PbI2-DMF-contained solvate phases（PDS）precipitation for the first time,thus achieved the regulation of perovskite crystal nu⁃cleus density in solution[93].NH4Cl additive can change the growth behavior of quasi-2D perovskite from top to bottom into the main growth mechanism,and increase the grain size by inhibiting nucleation.Vertically oriented grains obtained at room tempera⁃ture solution can overcome the problem of poor con⁃ductivity of quasi-2D perovskite layers and signifi⁃cantly improve the energy conversion efficiency of solar cells[93].There have been many polymer addi⁃tives can be used to regulate the crystallization pro⁃cess of quasi-2D perovskite films,which generally contain hydroxyl,carboxyl,carbonyl and other hydro⁃philic groups,coordinating with Pb2+in perovskite.In 2021,Liuet al.used 18-crown-6 and poly（ethyl⁃ene glycol）methyl ether acrylate（MPEG-MAA）as additives to obtain quasi-2D perovskite PEABr∶CsPbBr3film[88].They found that the C—O—C bond in the additives can reduced the self-polymerization of organic ligands,enabling higher-quality nanocrys⁃tals（Fig.5（d））.Then,they fabricated a highly effi⁃cient green light LED with a maximum EQE of 28.1%.In 2022,Caoet al.used（biodegradabili⁃ty）poly（butylene adipate-coterephthalate）polymer（PBAT）as additive to fabricate CsMAFA-based perovskite film with high quality[78].They proved that carbonyl groups and benzene rings of PBAT polymer can passivate the undercoordinated Pb2+and defects of perovskite,which can regulate the crystal⁃lization of perovskite films.They obtained a highly efficient perovskite solar cell with PCE of 22.07%.

3.3 Process Engineering

In the crystallization process of quasi-2D perovskite films,temperature,humidity and rotation speed can affect the quality.In 2016,Tsaiet al.re⁃ported a hot-coating process for the fabrication of high-quality（BA）2（MA）n-1PbnI3n+1perovskite film[94].Compared with the spin coated film at room tempera⁃ture,the morphology of the quasi-2D perovskite film prepared by hot-casting is superior and the film qual⁃ity is improved.The PCE of obtained the solar cells reaches 12.52% by using hot-casting method,and the stability of the device is greatly improved.The unpackaged devices can maintain an efficiency of 65% or more in standard（AM1.5G）illumination for more than 2 250.In 2019,Yanget al.found that CsPbIBr2film has better growth on In2S3films com⁃pared with TiO2films,which can be attribute to the reduction of Gibbs free energy on the In2S3film sur⁃face,promoting the formation of perovskite film with smooth,pinhole-free and high crystallinity[95].And they obtained a high efficiency solar cell with a PCE of 5.59% with reduced hysteresis.In 2021,Luoet al.realized the directional growth of FA0.75MA0.25⁃PbI3perovskite filmviaquasi-2D-seed（（BDA）PbI4）induction[96].In this seed induction method,the 2D seed solution is spin-coated on the substrate,which can act as templates for the growth of 3D perovskite with the desired facet orientation.Then the precur⁃sor containing 3D perovskite is spin-coated on the first film.The 2D seeds can be transformed into the grain boundary after completing the orientationinduced growth.They found that this process can control crystallization and facilitate the epitaxial growth of 3D perovskite,resulting in a film with the desired growth direction.The high-performance perovskite based solar cell was fabricated by using seed-induced method with PCE of 23.95% and high fill factor of 0.847.

4 Perovskite Lasers

Organic-inorganic hybrid quasi-2D perovskites have recently attracted great attention in optical and optoelectronic applications due to their inherent nat⁃ural quantum-well structure[25].Thin films of quasi-2D perovskites typically contain a mixture of do⁃mains with different layers.Within such inhomoge⁃neous domain distribution,it is assumed that the en⁃ergy rapidly funnels from the high-bandgap domains to the low-bandgap domains and finally emits radia⁃tively,resulting in rapid charge-carrier localization and increased carrier concentration in acceptors[97].This process improves the rate of radiative recombi⁃nation and facilitates the buildup of population inver⁃sion.Therefore,quasi-2D perovskites are promising gain media for laser applications[98].

4.1 Perovskite Single⁃crystal Lasers

Fig.6（a）The optical image of（BA）2PbI4（n=1）single crystal bottom surface［68］.（b）Tunable lasing spectra from quasi-2D perovskite（BA）2PbI4 single crystals［68］.（c）Schematic diagram of random lasing in a quasi-2D perovskite microrod single crystal［99］.（d）Random lasing spectra and microscopy images with pump fluence of 12 μJ/cm2 with increased time［99］.（e）Schematic diagram of the homologous（BA）（MA）n-1PbnI3n+1 lasers with various thickness of the inorganic layer［100］.（f）Evo⁃lution of spontaneous emission to lasing spectra under various excitation fluence for（BA）（MA）n-1PbnI3n+1［100］.（g）Lasing spectra of the RP microflakes with n=3，4，5 at 78 K［100］.（h）Temperature dependence of lasing thresholds for RP micro⁃flakes with n=3，4，5［100］.

Quasi-2D perovskite single crystals have advan⁃tages of rectangular boundary,low defect density and pure phase,which can contribute to the realiza⁃tion of quasi-2D perovskite laser.In 2018,Raghavanet al.prepared high-quality RP quasi-2D perovskite（BA）2（MA）n-1PbnI3n+1（n=1,2,3）single crystals with millimeter size through slow evaporation at con⁃stant temperature（SECT）solution-growth pro⁃cess[68].The single crystal with pyramidal structure has good crystallinity,phase purity and spectral uni⁃formity（Fig.6（a））.Then they obtained tunable la⁃ser with low threshold at room temperature.The la⁃ser thresholds are 2.85 μJ/cm2（n=1）,3.02 μJ/cm2（n=2）and 3.21 μJ/cm2（n=3）（Fig.6（b））respec⁃tively.Then in 2020,this group adopted SECT method to fabricate quasi-2D perovskite FA-（N-MP⁃DA）PbBr4（N-MPDA=N-1-methylpropane-1,3-diam⁃monium）single crystal microrods（Fig.6（c））[99].They observed high-quality,low threshold（～0.5 μJ/cm2）random lasing in microrods at room tempera⁃ture,which can retain after 2 h under continuous las⁃ing irradiation（Fig.6（d））.In 2019,Lianget al.in⁃vestigated the mechanism of lasing and loss in quasi-2D perovskite（BA）（MA）n-1PbnI3n+1thin microflakes and discovered the effect of Auger recombination and exciton phonon coupling on lasing[100].They proved that multicolor lasing can be achieved from the large-nRPPs（n≥3）in the spectral range of 620-680 nm（Fig.6（e）-（g））.With decreasingn,the lasing threshold increases significantly and the char⁃acteristic temperature decreases as 49,25,20 K forn=5,4,3,respectively（Fig.6（h））.Most recently,Zhanget al.fabricated high-quality quasi-2D perovskite single crystals with mechanically exfoliat⁃ed flakes with high crystallinity.Then they embed⁃ded PEA2PbI4in a vertical microcavity to realized single-mode continuous-wave laser with low thresh⁃old 5.7 W/cm2at room temperature[101].

4.2 Quasi⁃2D Perovskite Film Lasers

Quasi-2D perovskite films are not only simple in preparation,but also have high quality,good surface morphology and low defect density,which have great potential as gain medium in micro-and nanolaser.In 2018,Edwardet al.prepared（C12H25NH3）2PbI4perovskite film,which can be used as optical medi⁃um in an optical microcavity[102].Then,they observed biexciton lasing in a vertical cavity（comprising a distributed Bragg reflector and a metal mirror）above 125 K,with a threshold peak excitation densi⁃ty of 5.6×1018cm-3.In 2019,Matthewet al.inves⁃tigated the feasibility of quasi-2D perovskite for elec⁃trically pumped lasers.They obtained optically driv⁃en DFB lasers from low-dimensional NMA-based perovskite with a FWHM of less than 1 nm.They stated that the current density probably needs to reach 4-10 kA/cm2to achieve the emission of elec⁃trically pumped laser[23].In 2022,Zhong's group ob⁃tained single-mode vertical-cavity lasing from CsPb⁃Cl1.5Br1.5-based quasi-2D perovskites between two DBR mirrors with low threshold of 9.2 μJ/cm2（Fig.7（a））[24].They found the perovskite film using 2,2-diphenylethylammonium bromide（DPEABr）as li⁃gand has a large-n-phase narrow distribution,which enable enough exciton energy transfer and inhabit Auger recombination（Fig.7（b））.

In 2021,Liuet al.proposed an ultra-simple“sandwich”cavity structure based on the quasi-2D perovskite thin film to enhance the interaction be⁃tween light and matter at the nanoscale（Fig.7（c））[29].The room temperature single-mode picosecond laser has been successfully achieved with low threshold（～10 μJ/cm2）,narrow line width（～0.3 nm）and high degree of polarization（～81%）in the deep subwave⁃length scale 40 nm perovskite film and 10 nm glue layer（Fig.7（d））.The laser device overcame the dif⁃fraction limit without the help of metal plasma,which was the smallest size of all-medium semicon⁃ductor laser.This realization of deep subwavelength scale quasi-2D perovskite laser could provide a strong driving force for electrically pumped perovskite laser devices with high gain and effective electron-hole re⁃combination to balance the trade-off between carrier injection and transmission distance in subwave⁃length scale[29].In 2022,Liet al.prepared CsPbBr3（BABr）x/CsPbBr3perovskite planar heterojunction structure as gain medium of laser[103].Importantly,this structure can facilitate the transfer of excited states from the broadband quasi-2D perovskite to narrow-band 3D perovskite.Then they obtained ver⁃tical cavity surface-emitting laser based on planar heterojunction structure with low threshold of（46±15）μJ/cm2.

Fig.7（a）Schematic of the CsPbCl1.5Br1.5-based laser［24］.（b）Schematic of the effect of narrow domain distribution on ASE［24］.（c）Schematic of the quasi-2D perovskite vertical cavity under two-photon excitation［29］.（d）Single mode lasing spectra of qua⁃si-2D perovskite laser［29］.（e）Chemical structures of quasi-2D perovskites P2F8 and N2F8［107］.（f）Schematic of the DFB cavity pattern design（left）and top-view SEM image（right）［107］.（g）Lasing spectra of quasi-2D perovskites with different grating periods［107］.（h）PL images of NW arrays and spatially resolved lasing spectra of single NW［25］.（i）PL images of（BA）2（MA）n-1PbnBr3n+1 microrings with varied diameter［62］.（j）PL spectra collected from（BA）2（MA）n-1PbnBr3n+1 microrings with varied diameters［62］.

In addition,there have been several reports about perovskite-based CW lasers[104-106].In 2019,Philippet al.realized CW ASE from phase-stabilized perovskites（Cs0.1（MA0.17FA0.83）0.9Pb0.84（I0.84Br0.16）2.68）at a temperature up to 120 K[105].The threshold of obtained CW ASE is 387 W/cm2at 80 K.This phase-stabilized perovskite retains the room-temper⁃ature phase at low temperature and has the potential to support CW laser at higher temperature.In 2020,Qinet al.investigated the effect of singlet and trip⁃let states on particle number inversion in FAPbBr3-based quasi-2D perovskite containing different cat⁃ions and prepared a stable green quasi-2D laser with CW laser pump[107].They prepared PEABr-based perovskite（P2F8）films with high triplet energy and NMABr-based perovskite（N2F8）films（n=8）with low triplet energy respectively（Fig.7（e））.They found that singlet-triplet exciton annihilation is the main intrinsic mechanism causing lasing loss.Then,they fabricated a DFB laser by rotating quasi-2D perovskite film onto a substrate with 2D grating（Fig.7（f））.Finally,they successfully obtained CW laser based on P2F8 and N2F8 with threshold of 59 W/cm2and 45 W/cm2,respectively（Fig.7（g））.In 2021,Luet al.demonstrated stable pure blue emis⁃sion～471 nm under CW laser irradiation with high power density of 81 W/cm2from mixed-Br/Cl Csbased quasi-2D perovskite films by a facile vapor an⁃ion exchange（VAE）process[101].The advantage of VAE method is that Cl can be introduced into the lat⁃tice without destroying the morphology,overcoming low solubility of Cl in the precursor solution.In ad⁃dition,changing the VAE processing time can effec⁃tively control the bandgap of perovskite film,result⁃ing in tunable ASE changing from 537 nm to 475 nm.

4.3 Quasi⁃2D Perovskite Lasers Array

In 2018,Zhanget al.prepared quasi-2D RP perovskite BA2FAn-1PbnBr3n+1film withn=3,and achieved ASE at room temperature.Finally they fab⁃ricated nanowire laser array with high quality（Q）factor（～1 800）and low lasing threshold of～27-31 μJ/cm2by using polydimethylsiloxane（PDMS）tem⁃plate confined solution-growth method（Fig.7（h））[107].They indicated that high-energy small-n-value QWs（n=2,4,5）acting as energy antenna captured more energies from the excited light,then an ultrafast en⁃ergy transfer to low-energy large-n-value QWs（n>5）,leads to rapid accumulation of excitons and therefore induces population inversion for the ASE process.Later,they prepared quasi-2D RP perovskite BA2MAn-1PbnBr3n+1film and achieved ASE from the film withn=6 with low threshold of 13.6 μJ/cm2[62].Furthermore,they fabricated micror⁃ings whispering-gallery-mode（WGM）laser array based on quasi-2D perovskite with high quality（Q）factor of 2 600,as show in Fig.7（i）.Moreover,they demonstrated different numbers of resonance modes can be obtained on RP perovskite microrings with different diameter（Fig.7（j））[62].

5 Conclusion and Outlook

Layered quasi-2D halide perovskites have been studied widely as an advanced class of semiconduc⁃tor materials due to their unique optoelectronic prop⁃erties.In this review,the crystal structure,optical properties and stability of quasi-2D lead halide perovskites are mainly reviewed.Then the recent re⁃search progress of crystal growth regulating is intro⁃duced and the applications in laser devices are briefly discussed.Compared to traditional 3D perovskites,quasi-2D perovskites have many fascinating proper⁃ties,such as enhanced stability,flexibility in struc⁃ture adjustment,and ultra-fast energy transfer pro⁃cess.Therefore,quasi-2D perovskites materials have great potential in the field of optoelectronic de⁃vices like high-efficient LED due to their advantages of unique multiple quantum-well structure and high gain coefficient.The charge carriers transfer pro⁃cess in quasi-2D perovskite with differentnvalue phases facilitate the population inversion,which pro⁃duce possibilities for the realization of low threshold laser.However,multiplen-values phase also has negative effects like multipeak emission and lack of repeatability.Therefore,the accurate control of phase purity is important research direct,which might be supported by the development of new pro⁃cessing methods.In addition,some details about electron properties,such as electron-phonon cou⁃pling,symmetry effects,exciton optical physics and their effects remain unclear,which hinder the im⁃provement of quasi-2D perovskite optoelectronic de⁃vices.

It is well known that the ultimate goal for semi⁃conductor laser is electrical pump lasing,which is the biggest challenge for perovskite micro/nanolaser.There are several issues needed to be resolved tremen⁃dously,such as the poor stability,high internal resis⁃tance and poor thermal conductance of gain media.As for the device,the excessive leakage current,charge imbalance and Joule heating also can reduce the device performance,hindering the realization of electrical pump lasing.Therefore,to develop perovskite-based electrically pumped lasers,several urgent strategies should be performed including mate⁃rial engineering especially thermal management of gain media,cavity engineering and charge balance of laser device.As the perovskite materials and opto⁃electronic devices developing rapidly,we believe in the emergence of electrically pumped lasers in the near future.

Response Letter is available for this paper at:http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20220179.