Yung Jin Yoon and Jin Young Kim

Perovtronics Research Center, Department of Energy Engineering, UlsanNational Institute of Scienceand Technology (UNIST),Ulsan 44919,South Korea

Abstract: The halide perovskite blue light emitting diodes (PeLEDs) attracted many researchers because of its fascinating optoelectrical properties. This review introduces the recent progress of blue PeLEDs which focuses on emissive layers and interlayers. The emissive layer covers three types of perovskite structures: perovskite nanocrystals (PeNCs), 2-dimensional (2D) and quasi-2D perovskites, and bulk (3D) perovskites. We will discuss about the remaining challenges of blue PeLEDs, such as limited performances, device instability issues, which should be solved for blue PeLEDs to realize next generation displays.

Key words: halide perovskite; light emittingdiodes; blue emission

1.Introduction

Thehalide perovskiteisapromising material withABX3formation and perovskite structure. A monovalent cation is used in the A-site; methyl ammonium (MA), formamidinium(FA), Rb,and Cs. FortheB-site,divalentmetal,typically Pb,is used.For X site, halides areused; I, Br,andCl. These halideperovskites attracted many researchers with fascinating properties, such as high charge carrier mobility[1], high photoluminescence quantum yield(PLQY)[2],high absorptioncoefficient[3],easily tunable optoelectronicbandgap[4],narrowemission spectra[5], wide color gamut[6], defect tolerance[7]. Moreover,with light weight, cheap material price, and solution processability, the halide perovskites are considered as economical materialsfor various optoelectronic devices suchassolar cells[8,9], photodetectors[10], and light emitting diodes(LEDs)[11].

The perovskite LEDs (PeLEDs) have been expectedtobe the leading candidatefor the next generation displaysdueto narrower emission spectrum than other materials, and the PeLEDs have been rapidly developed in past few years with intensive research. The external quantum efficiency (EQE) of the PeLEDs have beenenhancedfrom below1%toover 20%for green and red colors[12−14], approaching the theoretical limits[15]. However, the blue PeLEDs are showing relatively slow development.

This review introduces therecentprogressof the blue PeLEDs first, which focuses on the development of emissive layers. The emissive layer is categorized into three groups, perovskite nanocrystals (PeNCs), 2-dimensional(2D)and quasi-2Dperovskites,and bulk (3D) perovskites. Subsequently, the developments of interlayers and interfacial engineering methods will be discussed secondly. Next, we discuss the remaining huddles in blue PeLEDs such asperformancelimitations,device instability issues, and presents perspective of the blue PeLEDs.

2.Developments in emissive layer

2.1.PeLEDs with perovskite nanocrystals (PeNCs)

2.1.1. Synthesis methods of blue emissive PeNCs

The halide perovskite nanocrystals (PeNCs) are nanometersized perovskite crystals, wrapped with organic ligands. By the organic ligands, the PeNCs are dispersed in organic solvents. The presence of organic ligands increases the exciton binding energy of the PeNCs, because electrically insulating ligands confine the chargetransport behavior.Increased excitonbindingenergyenhanceradiative recombination rates, and this is the mainreasonof most of PeNCs shows higher PLQY results than other types of perovskites. Unlike two other typesofperovskites,the 2D perovskites and bulk (3D)perovskites, the PeNCs must be synthesizedbefore fabrication of emissive layers. Thefirstsynthesisof PeNCs wasreported by Schmidtet al.[5]with MAPbBr3perovskite withgreencolor emission, then, various researchonsynthesis, post-treatments, anddevice applicationsof PeNCs were published. We willbriefly introduce thesynthesis methods ofbluecolor emissive PeNCs.

Hot injection method

The general synthesis method of the PeNCs is based on the hot-injection method, which is well known and widely using method in the quantum dot fields[16]. The hot-injection method has the advantage of yielding nanoparticles with uniform size, and controlling the size and shape of the nanoparticles easily. Furthermore, methods of controlling properties of the PeNCs were quickly developed by applying the knowhow of previous hot-injection methods for quantum dots.

Fig.1. (Color online) Synthesisand engineering methods of PeNCs to improvetheperformance of the bluePeLEDs. Schematic diagramsof synthesis methodof theblueemissive PeNCs through (a)hot injectionmethod,(b)LARPmethod[20], and (c)halide exchangemethod[23]. Copyright©2015, American Chemical Society. Schematic diagrams of strategy to improvetheblue PeLEDs through (d)ligandexchange method[28], (e) halide defect passivation method[35],and(f)bipolarshellstrategy on PeNCs[18]. Copyright © 2016, JohnWileyand Sons, Copyright ©2020,American ChemicalSociety. Copyright © 2020, Springer Nature.

Acommon synthesisprocess of blue emissivePeNCs is injecting Cs precursors into the PbBr2, PbCl2, and organic ligand mixed precursors at high injection temperature[6](Fig. 1(a)). Synthesized PeNCs are purified by centrifugation and finally dispersed in nonpolar organic solvents before use.Successful synthesisofbluePeNCswas achieved byProtesescuet al. with CsPb(Br/Cl)3perovskites with high PLQY(~90%)[6]. Interestingly, in the hot-injection method, various parts in the synthesis process affectthepropertyoftheblue PeNCs[17]. The shape of the PeNCs could be controlled by selectionoforganic ligands and theconcentration of ligandsaffectsthe PeNCs size. The injection temperaturealso determines the average size of the PeNCs. Furthermore, the emissionspectra could be adjustedbyreducing the sizeofthe nanocrystals, by the quantum confinement effect[6,18,19].

Ligand-assisted reprecipitation (LARP) method

TheLARP method is simplerthan the hot-injection method for obtaining the PeNCs. When perovskite precursors and organic ligand mixed solution is added into nonpolar solvent,nanometer-sizeperovskite crystalsareimmediately synthesized[20](Fig. 1(b)). Like hot-injection method, blue PeNCs wereobtained usingmixed halideprecursors,andwith further assistance of quantum confinement effect[21]. Since the LARP method does not require heating instruments and inert gas supply, the LARP method has great advantages in synthesisprocess steps andcould easily modify the scale ofthe synthesis.

Halide exchange post-treatment method

Afterthe PeNCssynthesis, the halide ratio in the PeNCs could be tuned more to obtain blue emissive PeNCs[22−24].The halide exchange post-treatment methodshavebeenconsideredas an efficientway to obtain blue emissivePeNCs without use of insoluble PbCl2precursors (Fig. 1(c)). Usual halideexchange processismixingextra halide precursorswith the PeNCs. Then the halides from the supplied precursors are spontaneously exchanged with the halides at the surface of the PeNCs.Because thehalidesin theperovskitecrystals are mobile, the exchanged halides on the PeNCs surface are spontaneously infiltrated into the inside of the PeNCs, forming well blended(Br andCl)halide PeNCs. Furthermore, the halide exchange method provides enough of halides to the PeNCs,thesurface halide defects of nanocrystals are naturally passivated, driven to improved PLQY.

2.1.2.PeLEDs with PeNCs

The first blue PeLEDs based on the PeNCs were produced by Songet al. with CsPb(Br/Cl)3PeNCs[25]. The PeNCs weresynthesized throughthehot-injection method, and the PeLEDs showed EQE of 0.07% and maximum luminance of 742 cd/m2. After that, various methods have been proposed to improvetheperformance ofthebluePeLEDswithPeNCs.Thecommonstrategy for improving the bluePeLEDswas the post-treatment method to improve the characteristics of the PeNCs[26,27]. Panet al. introduced ligand exchange technique toachievebettercharge injection[28](Fig.1(d)). They used didodecyldimethylammonium chloride(DDAC)ligands to control the ligand density of PeNCs as well as to supply the Cl anions to the PeNCs. They achieved performance of sky-bluePeLEDs, EQE of1.9% and maximum luminance of 35 cd/m2at490 nm emission peak.Likewise, Shinet al.used mixture ligands with DDAC and didodecyldimethylammonium bromide (DDAB) to obtain pure blue PeLEDs, showing EQE of 0.44%[29]at 470nm emissionpeak. Ochsenbeinet al. reportedthat zwitterionicsulfobetaineexchangingligands could form more stable bond with PeNCs and provide better long-term stability, and enhance the PLQY[30]. They also fabricatedbluePeLEDs with performanceofover1%of EQE at 461 nmemissionpeak.

The basic purpose of these ligand exchanging techniques is removing original ligands such as oleic acids or oleylamines. These common ligands are easyto handle and providegood colloidal stability with long alkyl chain,however, become a barrier to charge carrier injection. Therefore, exchanging ligands should have shorter alkyl chain to improve thechargecarrierinjection.In asimilarvein, theligand density in PeNCs could also be a considerable factor in the performance of blue PeLEDs[31]. High ligand density certainly confines electronic band structure of PeNCs and results high PLQY, but too highligand density lowers the performance of blue PeLEDs from inhibited charge carrier injection.On the other hand, too low ligand density lowers colloidal stability of PeNCs and leads severe aggregation that disturbs solution process. Moreover, too low ligand densityinPeNCs leads creation of defects on the surface of PeNCs and lowers the radiative recombination. Therefore, to achieve high performance blue PeLEDs, the ligand density should be controlled to appropriatelevel, which depends onwhatexchanging ligandsare used. Furthermore, the ligand density of PeNCs is unpredictably decreased in purification conditions, so, ligand exchanging techniquesrequiredelicateexperimentalsteps to obtain high quality PeNCs withoptimized liganddensity condition.

Another main strategy to increase the luminescence efficiency of PeNCs is the halide defect passivation. The halide defectsare a major factor that creates trap-states in electronic band of the PeNCs and lowers radiative recombination in the PeNCs. Therefore, removing the halide defects is an efficient waytoimprovethePeLEDsand lotsof methods were proposed[17]. Sincethehalide defects could bepassivated by just supply enough of halides to PeNCs, in most cases, the halides were provided with proper exchanging ligand pairs to improve chargecarrier injection as mentioned[28,32,33].However, thereareseveral interesting methods to passivate the halide defects without using halides for blue PeLEDs. By using tetrabutylammonium p-toluenesulfonate (TBSA) and sodium dodecylbenzenesulfonate (SDSA), Yeet al. successfully exchanged halidesin thePeNCs andpassivated halide vacancies[34]. They achieved blue PeLEDs with performance of 2.6%of EQE for 496 nm emission peak. To passivate the Cl vacancies,Zheng and coworkersused n-dodecylammoniumthiocynate (DAT)[35]. The thiocyanate group in DAT acted as pseudo halides, passivated the halide vacancies, and removed electronic trapstates (Fig.1(e)). The blue PeLEDs basedonthisPeNCs achieved EQE of 6.3% at 471 nm emission peak.

Recently, Donget al. reported surface engineering methodonCsPbBr3quantum dotsfor blue PeLEDs[18](Fig.1(f)).Theyused 4 nmsized PeNCs witha strong quantum confinement effect to achieve blue emission spectra. The surfaces of PeNCs were treated with isopropylammonium bromide (IPABr)to passivate theBrvacancies on the surface and to form anionic inner shell. After then, a NABr solution was added to exchange the IPA+on the surface with Na+and to form outershell. ThePeNCs with bipolar shell structure exhibited near-unity PLQY result. The PeLEDs with this PeNCs achieved EQE of 12.3% and maximum luminance of ~ 250 cd/m2at 479 nmemissionpeak, whichis the highestperformance of the blue PeLEDs withPeNCs. This bipolar shellstrategy suggested that stable blue PeNCs inks could be achieved without use of any organic ligands. The ionic bipolar shell successfully passivated halidedefectsand greatly reducedcharge carrier injection barrier, so resulted with the highest performance of blue PeLEDs with PeNCs for now. Even more, as the PeNCsinthismethodwerebased onasingle halideperovskite, the blue PeLEDs from this method could be expected to have enhanced spectral stability due to the lack of halidesegregation.

2.2.PeLEDs with 2-dimensional (2D) and quasi-2D perovskites

2Dperovskites and quasi-2Dperovskiteshave formation of R2An–1BnX3n+1, where R is an aryl or alkyl group, andnvalue represents the number of layers of perovskite crystals.Consideringthatn= 1 is2D perovskite andn=∞ is3Dperovskite, the perovskites with thenvalue bigger than 1 called as quasi-2D perovskites (Fig. 2(a)). These 2D and quasi-2D perovskites are formed with the assistanceof organic spacing molecules,andimportantly, the numberofperovskite layers are tuned by selection of the organic spacing molecules and by controlling the condition of those molecules. For ideal case(n=1),the 2D perovskitecouldbe expressedas(R-NH3)2BX4,also known as the Ruddlesden-Popper perovskite[36]. The big advantage of the 2D and quasi-2D perovskite for blue PeLEDs isthat blue emissive perovskite could be obtained without use of any Cl precursors by quantum confinement effect. The first PeLEDs with 2D perovskites was suggested by Lianget al.[37]using 2-phenylethylammonium(PEA)as spacingmolecule. The fabricated (PEA)2PbBr4film showed luminescence peak at 410 nm and the PeLEDs resulted EQE of 0.04%.Chenget al. used 4-phenylbutylamine (4-PBA) for the 2D perovskites[38]. They made sky-bluecolor perovskites films and PeLEDs showed EQE of 0.015%. Unfortunately, too much organic spacing molecules in the film can disrupt and degrade the charge carrier transport, andcan lead to decrease of radiative excitons in the emissive layers. Moreover, the performance of the PeLEDs using 2D perovskites is limited by the strong exciton-phonon coupling[39]. Therefore, pure 2D perovskites have limits for emissive applications.

Fig. 2. (Color online) (a) Schematic diagrams of the Ruddlesden-Popper perovskite and quasi-2D perovskites[44]. Copyright © 2019, Springer Nature. (b) Emission spectra and (c) absorption spectra of quasi-2D perovskites with control of n values by adjustments of optical spacing molecule concentration[56]. Copyright © 2018, Springer Nature. (d) Schematic diagram of energy transfer in a quasi-2D perovskite film with mixed n values[45]. Copyright © 2019, Springer Nature. Operational stability of blue PeLEDs based on quasi-2D perovskite with (e) single halide composition[44] and (f) mixed halide composition[52]. Copyright © 2019 and 2020, Springer Nature.

The emissive layers with quasi-2D perovskites have less organicspacingmoleculesinthe film, resultedwithimproved optoelectronic properties byreduced exciton-phonon coupling. Additionally, the quasi-2D perovskites form an efficient energy level structure. When a quasi-2D perovskites are formed,the film usually has a mixed phase state with severalnvalues, not a uniform phase with a singlenvalue. This mixed phasestate naturally constructs energy funnel structure that providesbetter chargetransport, higherradiative recombination chances, and improved device performance[40](Fig. 2(d)).Several successful results for PeLEDs using quasi-2D perovskites have been reported, supported by nice stability and pinhole-free morphology[41−45]. The most important part determining the performance of the blue PeLEDs with quasi-2D perovskite is organic spacing molecules, which determine the number of perovskite crystal layers and charge carrier transport behavior, so various organic molecules have been tested for the quasi-2D perovskites and PeLEDs.

Kumaret al. proposed a blue PeLEDswithquasi-2D perovskite that haveformation of OLA2MAn–1PbnBr3n+1,wheren=3–5 and OLA= oleylammonium[46]. Theygot device performance ofEQEof0.024% and maximum luminance of~1 cd/m2at456nm emissionpeak. By using n-butylammonium (BA),theCongreveet al. achieveda bluePeLEDs EQE of 0.0054%[47]. With introduction of 2-phenoxyethylamine(PEOA) group as spacing molecules, device performance was improved to EQE of 1.1% and maximum luminance of 19.25 cd/m2at 480 nm emission peak[48]. Wanget al. reported quasi-2D perovskites with ethylammonium bromide(EABr)[49]. They achieved deep blue PeLEDs with EQE of 2.6%and maximum luminance of 100 cd/m2at 473 nm emission peak. Vashishthaet al. achieved successful blue PeLEDs with quasi-2Dperovskites with BA,withperformanceof EQE of 6.2%and maximum luminance3340cd/m2at 487emission peak[50]. Byusing mixed organicmoleculeswithguanidine(GA) andPEA,Zhanget al. reportedblue PeLEDs withEQE of 8.2%at 492 nmemission peak[51]. Chuet al. boostedperformance of blue PeLEDs with quasi-2D perovskites over 10%[52].They used PEA molecules to form quasi-2D phases and used EA molecule to control the crystal structure and achieved EQE of 12.1%, which is the highest performance result with quasi-2D perovskites for blue PeLEDs for now.

The blue PeLEDs with quasi-2D perovskite with single halide composition, Br, have intensively been studied due to easy of fabrication, free from the halide segregation ensuring stable color spectra (Fig. 2(e)), and excellent performances.However, mostofquasi-2Dperovskitesexhibited skybluecolor, ataround490nmemission peak. Oneof the main reasons for the sky-bluecolor result is that itishard to obtain puresingle phase quasi-2D perovskites with lownvalue through solutionprocess. Theemissionspectra of quasi-2D perovskites at mixed phase state is determined by the largestnvalue in the film. Therefore, if the film could not have pure phases with lownvalue, which is hard to achieve, the emissive layer could not exhibit the pure blue color emission.Additionally, as the emission peak of the quasi-2D perovskites is not continuous withnvalues, it is also hard to obtain desired specific emission peak at around 470 nm. Therefore, obtaining pure blue color PeLEDs around 470 nm with quasi-2Dperovskiteswithasinglehalide compositionis difficult.Moreover, the emissionspectra from single halide quasi-2D perovskite films haveshownbroaderormultiple emission peaks that reduces great advantages of the PeLEDs, by uncontrolledradiativerecombination from the mixedphases in the films.Severalattemptswereconductedto obtain pure blue emission spectra with quasi-2D perovskites, with participation of Cl elements[53−56], and device performances were improved up to EQE of 3.1% by Zhanget al. at emission peak at 474 nm[57]. However, as participation of Cl elements means there could be the halide segregation problem, improving the spectral stability of the quasi-2D based pure blue PeLEDs with mixed halide composition is a major remaining problem(Fig. 2(f)).

2.3.PeLEDs with bulk (3D) perovskites

Low-dimensional perovskites have alternative way to obtainblue emissionwith assistance ofthequantum confinement effect, and the properties can be enhanced by organic molecules.However, it is inevitable to use Cl elementto achieve the blue emission with bulk perovskites, which is the biggest problem toobtain highly efficient blue PeLEDs. The inorganic Cl precursors are rarely solved in most of solvents,which leads poor film morphology resulting limited performances (Fig. 3(a)). Fortunately, Cl precursors with organic counter cations have better solubility and demonstrated the potential of bulk perovskites for bluePeLEDs.

Fig. 3. (Color online)(a) Surface images of bulk (3D)perovskite with varying Clcontentsin thefilmand(b) corresponding PL spectraof bulk perovskites[60].Copyright ©2015,AmericanChemicalSociety. EL spectraoperatedunder different appliedvoltagesof the bluePeLEDsbasedon the(c)single Asite perovskite (CsPbX3)[65] and(d)triple Asite perovskite ((Cs/MA/FA)PbX3)[61].Copyright© 2021, American Chemical Society. Copyright © 2017, JohnWiley andSons.

The first blue PeLEDs with bulk perovskite was reported byKumawatet al.with MAPb(Br1–xClx)3(x=0–1), with clear blue color emission[58]. They showed the emission spectra could be easily tuned withcontrol of the halide precursor ratio. They also successfully fabricated blue PeLEDs with EQE of~1 × 10–4% andmaximumluminance of~2cd/m2at 482nm emission peak. This limited performance could be attribute to poor morphology with many pinholes in the film[59]. Several attempts of modifying precursor materials were conducted to improve the solution processability andqualityof the perovskite films. For example, lead acetate was introduced in precursor solution to achieve better solution processability and pinhole-free film[60].

Producing emissive layers through mixed A-site cations could bean effective way to obtain high-quality films. Bymixing the A-site cations, the formation energy of the perovskite could be increased,the defectdensity inthe perovskite crystals could be reduced, and the device performance could be enhanced.Moreover, withreduced defect density,the halide segregation could be reduced to have better spectral stability(Figs. 3(c) and 3(d)). Kimet al. usedthree typesofcations(Cs, MA, FA) for bulk perovskites and successfully fabricated blue PeLEDswithEQEof 1.7% at475 nmemissionpeak[61].Yuanet al. developed this mixed cation concept. They used five typesofcations to fabricate thebulkperovskiteand achieved EQE of 2.01% at 484 nm emission peak[62]. The mixed cation perovskites were further developed with vaporassisted crystallization technique. Karlssonet al. used three cations (Rb, Cs, FA) for perovskite films and the PeLEDs resultedEQE of11.0% and maximum luminance of2180 cd/m2,which is the highest performance of blue PeLEDs with bulk perovskites for now[63].

Though mixing cation is effective strategy to improve the performance ofthe blue PeLEDs,the bulk perovskites with organic cations have some problems that moisture absorbing properties of organic cation combined Cl precursors,and resulted with high defect density in the pure blue color PeLEDs. However,the inorganic bulkperovskitesfor blue PeLEDs are much more difficult to fabricate due to even lower solubilityofinorganicCl sources. To solvethe problem of insoluble Cl precursors, alternative ways to get wide optical bandgap were tried. Wanget al. reported that introducing the RbcationsintotheCs basedperovskite causes lattice distortion and widens the optical bandgap[64]. They successfully fabricated inorganic blue PeLEDs with RbCl precursorswith EQE of 0.062% for blue PeLEDs. Yoonet al. introduced anion exchange technique forinorganic bulkbluePeLEDs[65]. They fabricatedbulk CsPbBr3filmand then exchangedanionswith simple post-treatments and blue PeLEDsshowed EQE of 0.32% with excellent stability. With assistance of additives,Chenget al. reported simple fabrication method to obtain high-quality inorganic perovskite films[66]. By adding polyoxyethylene sorbitan monolaurate (Tween) and tetraphenylphosphoniumbromide (TPPB) into the precursor solution, the solutionprocessability wasimproved withcontrolledsolution wettability,the filmmorphology was improved, and the halide segregation was effectively prevented. The device showed EQE of 4.13% and maximum luminance of 9352 cd/m2at 482 nm peak.

The bulk perovskites have disadvantages such as difficultiesin film morphology, exciton quenching between chargetransport layersandemissive layerdue to lackof electronic bandconfiningorganicmolecules,and halidesegregations. However, the bulk perovskites have advantages on cheap precursors, simpler fabrication methods, stable to most of non-polar solvents which enables fabricating organic layers on top of emissive layers by solution processing methods. In addition, thebulk perovskitesarefree fromthe charge blockingorganic molecules,the bulkperovskitecould have better maximum luminancewith higher currentinjection when the material quality and stability are guaranteed.

3.Developments in charge transport/ injection layers

3.1.Hole transport/ injection layers (HTL/HIL)

With excellent wetting property to form all three types of perovskite emissivelayers, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate) (PEDOT:PSS) is widely used for blue PeLEDs, and the p–i–n structure that using PEDOT:PSS as a substrateis common structure fortheblue PeLEDs.However,due to the deep valence band maximum of blue emissive perovskites andshallow HOMO (highest occupiedmolecular orbital) of the PEDOT:PSS, there is a large energy barrier to inject holes from PEDOT:PSS layer to the emissive perovskite layers.Moreover,the acidic propertyofthe PEDOT:PSSlayer could be potential origin of degradation of devices. Therefore, to match the energylevels withemissivelayer, toimprovethe charge injection, to match the charge injection balance, and to enhance the device stability, various genuine HTL/HIL materials andrelatedengineering techniques to improve the properties of HTL/HIL were suggested for all three types of blue emissive perovskitematerials(Fig. 4(a)).

Janget al. improved wettability, compatibility of PEDOT:PSS, whichresultsbetter perovskite crystal growth process, with assistance of conjugate polyelectrolytes (CPEs)[67]. In-troducing the neutral HTL/HIL material with deep HOMO levelis also effective methodtoimprove thechargeinjection and chemical stability. The polymers that have deeper HOMO level, such as poly[N,N′-bis(4-butylphenyl)-N,N′-bisphenyl-benzidine] (poly-TPD)[29,68], Poly(9-vinylcarbazole)(PVK)[69,70], CBP[30], havewidely been usedfor better charge injection with PeNCs. Additionally, the energy level of these polymeric HTL/HIL could be modified with further engineering methods[71]. Gangishettyet al.reported the successfulcharge injection barrierreduction using poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl) diphenylamine)) (TFB)and Nafion perfluorinated ionomer (PFI) bilayer structure[72](Fig.4(b)). Shinet al.also reported holeinjectionbarrier could be reduced withCPEs withformation of dipole moment, improving EQE of blue PeLEDs with PeNCs to 1.34% at 470 nm emission peak[73].

However,the polymeric HTL/HILcould disturb perovskite crystal growthdue to low wettability of precursor solutions in polar solvents for the bulk perovskites. Considering the disadvantages of organic HTL/HIL, the inorganic oxide HTL/HILcouldbe promisingHTL/HIL forthebulk blue LEDs[74]. Renet al. introduced PSS Na to form a dipole moment on the NiOxlayers to modify the energy level of the Ni-Oxlayers (Fig. 4(c)). Withmodifiedenergy level, the holeinjection wasimproved, andnon-radiativerecombination was reduced. The performance of the blue PeLEDs was improved to EQE of 1.45%[75].

3.2.Electron transport/ injection layers (ETL/EIL)

As the p–i–n structure is common structure in the blue PeLEDs field, usually the ETL/EIL are fabricated upon the blue emissive perovskitelayers. Themain problemis that theperovskite layers have weak chemical stability and are easily decomposed with polar solvents, makingdifficult to fabricate ETL/EIL with solution processing methods. Therefore, most of the ETL/EIL for blue PeLEDs are formed through a thermal evaporation method, which isadifficult method tocontrol the chemical properties of the materials. In addition, most of theETL/EIL materials usedforbluePeLEDs already have shallower energy level than perovskite materials (Fig. 4(a)), so the bottle neck of charge injection is HTL/HIL usually. As a result,not manyapproaches havebeen reported to make significant changes to control the properties of the ETL/EIL materials itself for blue PeLEDs.

For the n–i–p structure, perovskite emissive layers are fabricated onto the ETL/EIL. Because most known ETL/EIL materialsare organicmaterials that disturb the useof polarsolvents due to poor wetting properties, bulk perovskites are almost impossibleto fabricate onto the organicETL/EILmaterials.However, the PeNCs or the quasi-2D perovskite emissive layers already have PEDOT:PSS material that has superior wettingproperties for the p-i-n structure. Therefore,onlya few n–i–p structure based PeLEDs with bulk perovskites were tested with oxideETL/EIL materials[60,61]. But,sincethe typical oxide ETL/EIL materials have a deeper conduction band energylevelthan that of the blueemissive bulk perovskites,the basic development strategy is modifying conduction band energylevel to have shallowerlevel to reduce the electroninjection barriers. For instance, Sadhanalaet al. successfully demonstrated Mg-doped ZnO could be used as a successful ETL/EIL for bluePeLEDs with betterdeviceelectronicband structure[60].

3.3.Recombination zone control

It is obvious that the electron and hole transport/injection behavior shouldbe carefully controlledto recombine with each other in the proposed recombination zone, the emissive perovskite layers, as well as to balance the charge carrier injection. However, as theperovskiteemissivelayers are very thin, less than few tens of nanometers, the charge carriers could be recombined not at the emissive layer but at the adjacent interlayers.Generally,the position of the recombination zone could be controlled by adjusting the thickness of interlayers[76]. Liet al. successfully tuned the recombination zone by tuning the thickness of the PEDOT:PSS layer[45]and improved device performances with blue PeLEDs.

Additionally, introducing a very thin insulating materials between the interlayers and the emissivelayers could support the charge carriers to recombine at the emissive perovskite layers. The thininsulating layers allowchargeinjectionbytunnelling effect and provide energy barrierto prevent the leakage of holes to ETL/EIL or electrons to HTL/HIL[42]. Yuanet al.introducedthin LiF layerforbothHTL/HIL and ETL/EIL interfaces (Fig. 4(d)), prevented charge carrier leakage, and successfully confined recombination zone with improved performance of blue PeLEDs with3D perovskites[62].Furthermore, because the emissive perovskite materials can react with ZnO ETL[77], introducing thin insulating layers could be protection[78]toimprove chemical stabilityof blue PeLEDs and could be effective method to control the recombination zone.

4.Remaining challenges

4.1.Quantum efficiency of emissive layers

The EQE of the PeLEDs is determined by internal quantumefficiency (IQE)and theoutcoupling efficiency of the device. The IQE is the product of charge carrier balance,the fraction of excitons capable of radiative decay, and the effective radiativequantum yield[15]. Toenhancethe deviceperformance, the IQE must be improved. As the fraction of excitons capable of radiative decay isrelated tomaterialcharacteristicand hard to control,andthecharge carrierbalance could be simply adjusted through optimizing the interlayers and device structure, therefore, the main challenges to improvethe device efficiency ismatter of improving theeffective radiative quantum yield, which is also related to the PLQY of emissivelayers. As widerbandgap materialstendto have more sub-bandgap trap states[79], the blue emissive perovskite materials tend to have more non-radiative trap-assisted recombination thanthegreen,red, andnear-IR emissive perovskite materials, resulting lower PLQY. Therefore, the underlying strategy for improving the blue PeLEDs isimproving the effectiveradiative quantum yieldof the device through the passivation and prevention of defects in emissive layers that creates the sub-bandgap trap states.

For PeNCs, surface defects are the main cause of trap states that lowersthe effective radiativerecombination. The common ligands, oleic acids and/or oleylamines, could beeasily detached with purification process or environmental reasons,so exchangingthese ligandstotheligandsthatmake stronger bond iseffective wayto prevent and passivate the surface defects of PeNCs. For now, various ligands, such as ligands thathave secondary amines[70],diaminegroups[28,29,80],bifunctional groups[81], aromatic groups[82], zwitterionic ligands[30], shorter alkyl chains[68], and much more results have been showed improved PLQY for blue emissive PeNCs. Another strategy to improvethe PLQYof the PeNCs isdoping metal ions, passivating non-radiative surface cationic vacancies[83]. Successful improvements of PLQY for blue emissive PeNCs were conductedwith Mn2+[84],Zn2+[85],Sn2+,Cd2+[86],Nd3+[87], metal cation doping.

For 2D and quasi-2D perovskites, various successful defect engineering techniques werereported[32,88], however, reducing exciton-phonon interaction is another key issue to improve the effective radiative quantum yield. With control of ligand configuration,Gonget al. successfullysuppressed electron-phonon coupling and improved PLQY of the blue quasi-2D perovskites to 79%[89]. Penget al.also reported suppressed phonon coupling and enhanced PLQY withcontrol of phases of quasi-2D perovskites[90]. As energy level of the bulk perovskites are not confined and the carriers in the bulk perovskite are tends to flowout to interlayers, thebulk perovskites for blue emission usually showed lower PLQY values.However, several successful improvements of PLQY with defect engineeringin grain boundaries inbulk perovskiteswere reported in other fields, and we could easily expect that those strategies can also work for blue PeLEDs[91,92].

Hopefully, attemptstoreduce the defectand trap states in the emissive layers have shown successful results with improved PLQY of emissive layers reaching near unity value,andenhancedEQE of the blue PeLEDs.However,despite the extremely high PLQY values, the device performance of the blue PeLEDs remained at around over 10% of EQE. Consideringthe other color PeLEDs resultedaround 25% of EQE which is reaching the theoretical limits, it seems there is still some room for improving the performance of the blue PeLEDs. Assumingthe emissivelayers areperfect, toimprove the device performance, the interlayers should be seriously discussed. The common trend of improving the device performancethroughinterlayer engineering wasdealingwith the HTL/HIL because the HTL/HIL have inferior carrier transport/injection behavior than the ETL/EIL due to charge injection barriers.Fortunately, manysuccessfulattempts on the HTL/HIL improved charge carrier transport/ injection properties, reaching that of commonly used ETL/EIL to balance the charge carrier injection. To achieve the theoreticallimitof EQE withblue PeLEDs, itis timeto develop ETL/EIL materials to have superior charge transport/ injection behavior than conventional materials, andfor HTL/HILtoo.Fromsimple strategies tomodify theinterfaces to havebetterelectronic bandstructure to synthesizing innovative materials should be conducted to reach extremely efficient bluePeLEDs.

4.2.Operational stability and color stability

Thelowformation energy is a greatadvantage ofhalide perovskites, enabling formation of highly crystalline filmswith solution processing at room temperature condition.However,the low formation energyhasanotheraspect. The low formation energy causes easy and frequent creation of ionic defects which eventually deform and damage the perovskite crystals. Theseionic defects sites notonly serve asthe non-radiative recombination centers, but also impede materialstability andoperational stabilityof PeLEDs[93,94]. Therefore,increasing formation energy ofthe perovskite canbe an efficient way to improve the operational stability of blue PeLEDs. To improve the formation energy of theperovskite,strategy of mixing the A-site cations seems an efficient method. Successfully improved film and device stability results werereported by severalgroups,and evenenhanced stability was reported with mixed A-site cations in photovoltaic fields[95]. Additionally, byusing mixed A-site materialsforthe perovskite materials, the lattice strain of the perovskite could be controlled to improve the phase stability[96].

However,state-of-artmixing A-site techniquerequires many A-site cations[62], which requires delicate control of fabricationconditions,andit seemsmostof possible candidates have been tested and hard to find other promising candidates. Even with the state-of-art mixed A-site cation strategy,the stability ofbluePeLEDsis farfrom the commercialization for now. Therefore, another additional strategy to improve the stability of blue PeLEDs that can be combined with mixedA-sitestrategy should be proposed.As mentioned above, exchanging organic ligands or spacers for PeNCs and quasi-2D perovskite could beone of promisingmethods to enhance the device stability. Recently, combining 2D perovskite with 3D perovskite strategy was proposed in perovskite photovoltaic devicesfield[97]. The combinedperovskite structure exhibited enhanced carrier lifetime and improved device performance and stabilityresult.Likewise,mixing 2D/3D perovskite strategy could be a promising method for the 3D blue PeLEDs. In addition, introducing the pseudo halide anions to perovskite materialscouldalso bea promising candidate method to improve the device stability[8,35].

Another bigremaining problemthat must be solvedis color instability. The main cause of this color instability comes from the mobile characteristic of halides in the halide perovskite crystals[98]. The mobile halides in the mixed halide perovskites induce halide segregation phenomena due to thermodynamic reason,which gathersthe sametypeof halides in the crystal, creates unnecessaryadditional emissive phase,changes overall emission spectra of the perovskite. Interestingly, the origin of halide segregation was also attributed to the ionicdefects, which is main reasonfordegrading operationalstabilityand performanceof bluePeLEDs. Tosuppress the mobile property, eliminating the created ionic defects could be efficient method, and successful stability enhancements were reportedwithvariousionic defect passivation methods[32,35,44,65]. Thereis also another efficient methodto suppress the mobile property, that increasing the formation energy and prevents ionic defects. The formation energy could beincreasedby usingmixedA-site cationstrategy[99], and this strategy resulted successfulenhancementsinboth operational stability and color stability for blue PeLEDs[44,62,100], especially for blue PeLEDs with the 3D perovskites[61,63,64,101].

However,nomatterhow well the perovskitecompositionis controlled,itis impossible topermanentlyprevent the halide segregation with mixed halide compositions, because the halide segregation strongly affected by uncontrollable entropic factors. Fortunately, we already knowtheperfect method to prevent the halide segregation, that formingthe perovskite with a single halide, Br. Though the Br only perovskites need additional delicate size control to get quantum confinementeffect toexhibit blue coloremission, but at least therearelotsof methods andpossibilities to broadenthe bandgap for Br only perovskites, while it is almost impossible to prevent the halide segregation with mixed halide perovskites.

5.Conclusions and outlook

Weintroducedrecent developments intheblue PeLEDs(Table 1) and remaining challenges, low quantum efficiency of emissive materials and device instability. With the deep understand of material characteristics, the bluePeLEDsshowed dramatic developments in past few years. To improve device performance, the strategy of increasing the formation energy through mixing A-site cations have been effective way for all three types of perovskites. Reducing Cl contents in the perovskite material and reducing the size of the perovskite crystal could be effective method to obtain efficient and color stable emissive perovskite layer for sky-blue PeLEDs. For the pure blue PeLEDs with narrow emission spectra, the participation of the Cl element seems inevitable for now, so to realize the highly efficient pure blue PeLEDs, the halide defect engineering technique is essential to handle the trap states in the mixed halide perovskites. However, the Cl element should eventually be eliminated to avoid halide segregation and to getenhanced colorstability for purebluePeLEDs infuture. Additionally, advanced interfacial engineering techniquesandinnovativematerialsforinterlayers shouldbe developed to improve charge carrier injection and to reach the theoretical performance limit.Withimproved performance and long-term device stability, the blue perovskite LEDs will bethe best choice for next generationdisplays.

Table 1. Recent advances of blue PeLEDs.

Acknowledgements

This work was supported by "the Research Project Funded by U-K Brand"(1.210037.01, 1.200041.01) of UNIST(Ulsan National Institute of Science & Technology). This work was supported by Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) fundedbyMinistryofScienceand ICT (NRF-2021M3H4A1A02049634).