Efficiency enhancement of thermal-evaporated perovskite light-emitting diodes through process optimization and additive engineering

The ratio of precursor

The precursors commonly applied for LEDs are typically deposited using CsX and PbX₂ sources to form CsPbX₃. Early solution-processing research based on spin-coating revealed that introducing a higher amount of MABr relative to PbBr₂ in the precursor solution could effectively suppress Pb-related defects caused by uncoordinated Pb atoms, thereby yielding improved perovskite films^[58]. Similarly, in dual-source TE, attempts were made to utilize a higher CsBr ratio relative to PbBr₂. When Poly(N,N’-bis-4-butylphenyl-N,N’-bisphenyl)benzidine (poly-TPD) was used as a hole-transport layer and the CsBr:PbBr₂ ratio was varied between 1:1 and 1.5:1, devices fabricated with the 1.5:1 ratio showed inferior performance due to surface inhomogeneity of the films. Scanning electron microscope (SEM) characterization confirmed that the 1:1 film exhibited pinhole-free surfaces, whereas the 1.5:1 film displayed irregular morphology^[53]. However, as the optimization was incomplete, these devices exhibited a higher turn-on voltage compared with solution-processed counterparts, which showed a threshold voltage of approximately 3 V.

Subsequent research extended the CsBr:PbBr₂ ratio up to 5:1^[59]. Despite the high CsBr content, no significant changes in crystal structure were detected. As shown in Figure 2B, the increasing ratio led to a reduction in grain size, which was favorable for forming films with enhanced carrier confinement beneficial for LED applications. Unlike earlier research, devices were fabricated entirely by TE, with an indium tin oxdie (ITO)/MoO₃/Tris(4-carbazoyl-9-ylphenyl)amine (TCTA)/CsPbBr₃/2,2’,2’’-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)/LiF/Al structure. At a 5:1 ratio, the device achieved a luminance of 1,800 cd·m^-2 and an EQE of 1.1%.

In addition to precursor ratio engineering, other studies optimized film properties based on the molar fraction of the Cs₄PbBr₆ phase within the emissive layer, as well as by tuning the film thickness^[60]. In this case, when CsPbBr₃ and Cs₄PbBr₆ phases coexisted, carriers migrated through two distinct conductive channels and the excessive thickness of the emissive layer increased resistance or introduced shunt pathways; thus, an optimal thickness of 60 nm was employed. As illustrated in Figure 2C, the maximum EQE was achieved when the Cs₄PbBr₆ content reached 45 mol%, where the moderate presence of Cs₄PbBr₆ effectively regulated the CsPbBr₃ grain size and enabled spatially confined carrier transfer. Conversely, excessive Cs₄PbBr₆ exhibited insulating behavior, leading to the discrepancy between the ratios that yielded maximum EQE and those that yielded the highest PLQY [Figure 2D]. The optimized device exhibited a luminance of 53,486 cd·m^-2 and an EQE of 2.5%. Based on various research findings, employing a higher Cs ratio relative to Pb during deposition is beneficial for mitigating Pb-induced defect formation and facilitating the controlled incorporation of the Cs₄PbBr₆ phase; nevertheless, attention is required in emissive layer fabrication to prevent excessive Cs₄PbBr₆ formation, which could impart an undesirable insulating effect.

The rate of TE

During TE, one of the critical factors influencing thin-film characteristics is the deposition rate. While organic-inorganic MHPs are commonly applied to photovoltaic devices, TE has also been explored for fabricating inorganic CsPbBr₃ films, which are thermally stable but difficult to form into uniform layers^[32].

In thermally evaporated MHP films, grain-size control emerges as a key parameter that is strongly influenced by the TE rate. Unlike perovskite solar cells, where large grains and reduced grain-boundary density are typically targeted to promote long-range carrier transport and efficient charge collection, PeLEDs often benefit from a more finely tuned microstructure. Moderately small, uniformly distributed grains can localize charge carriers and excitons, thereby enhancing the probability of radiative recombination while suppressing nonradiative pathways associated with deep trap states and extended defect networks at grain boundaries. In TE processes, the grain size and morphology can be systematically engineered by adjusting the TE rate. A high TE rate induces a large nucleation density, and the numerous nuclei within a limited area result in a small grain size. Although such small grains are advantageous for PeLEDs, MHP films composed of very small grains can suffer from incomplete coverage and the formation of pinholes. Consequently, optimizing grain size by combining fast TE with sufficiently robust post-annealing conditions represents an essential design strategy for achieving superior TE-based PeLED performance^[61,62]. Similar to the previous approach, after optimizing the precursor ratio, the effect of deposition rate was investigated by forming films at 5, 15, and 25 Å·s^-1.

Although no phase transition was observed as a function of deposition rate, the crystallographic orientation was strongly affected. These changes in crystallinity ultimately altered the grain size, and a deposition rate of 5 Å·s^-1 fabricated large grains suitable for photovoltaic applications [Figure 2E]^[33]. In contrast, thin films with smaller grains are desirable to facilitate spatially confined carrier transport for LEDs, implying that higher deposition rates are more advantageous.

The crystallinity dependence on TE rate can be attributed to the increased nucleation density at the substrate surface under faster deposition conditions. In this context, Shin et al. evaluated the deposition-rate dependence of CsPbBr₃ films prepared for vacuum-deposited LEDs^[63]. Emitting layers were fabricated at deposition rates of 0.4, 0.9, and 2.4 Å·s^-1, and the best optoelectronic properties were obtained at 0.9 Å·s^-1 [Figure 2F-H]. Although a smaller grain size was expected at 2.4 Å·s^-1, rapid deposition of PbBr₂ resulted in the formation of microscale grains caused by residual gases. The grain-size inhomogeneity hindered spatial control of carrier transport and degraded device performance. By employing the optimized deposition rate, the improved device successfully achieved a three-fold enhancement in luminance, reaching 1,607 cd·m^-2.

In summary, while relatively high deposition rates are generally required to fabricate emitting layers suitable for LEDs, excessively fast evaporation of PbX₂ can induce grain-size nonuniformity, necessitating careful optimization of the rate. Further studies are also required to determine whether performance differences arise under single-source or sequential TE when varying the deposition rate.

The temperature of substrate

Substrate temperature during TE has been shown to exert influence over film nucleation and growth, thereby dictating the structural and optoelectronic properties of the resulting emitting layers. Elevated substrate temperatures generally promote enhanced surface diffusion of adatoms, leading to improved crystallinity, enlarged grain sizes, and reduced defect densities, which in turn contribute to higher charge carrier mobility and more balanced injection. However, excessive heating can trigger undesirable phase segregation, lattice strain, or interfacial reactions, particularly in MHPs where volatile species are prone to re-evaporation^[64,65]. Recent research demonstrates that optimized substrate temperatures affect the MHP film to smoother morphologies, reduced trap-assisted recombination, and enhanced luminance in CsPbBr₃- and mixed-halide based PeLEDs, whereas insufficient thermal activation often results in rough, polycrystalline films with inferior current efficiency. Furthermore, optimization of substrate temperature has been correlated with modifications in preferred crystallographic orientation, which critically affects light outcoupling and performance of emission. The temperature of the substrate is not merely a processing variable but an active parameter for engineering microstructure and exciton dynamics in vacuum-deposited LEDs.

In addition to optimizing the deposition rate, several studies have reported improved performance in LEDs through careful control of substrate temperature. When the substrate temperature was varied from 30 to 80 °C, films deposited at 30 and 60 °C exhibited uniform morphologies, whereas deposition at 80 °C resulted in the formation of clusters of grain vacancies that negatively affected carrier transport^[66]. A comparative analysis between 30 and 60 °C revealed that deposition at 60 °C produced more uniform and highly crystalline films. Furthermore, when the deposition rate was tuned from 0.5 to 1.5 Å·s^-1, the device fabricated at 1.0 Å·s^-1 showed fewer defects. Based on these optimized parameters, devices successfully achieved a luminance of 17,550 cd·m^-2 and demonstrated mechanical compatibility with flexible substrates, as shown in [Figure 3A and B].

Figure 3. Characterizations of PeLED depending on the temperature of substrate and post-annealing. (A) Current efficiency and EQE of PeLEDs fabricated by TE with varied substrate temperature and rate; (B) The image of deposited MHP films with large substrate and flexible substrate^[66]. Copyright 2020 American Chemical Society; (C) The scheme of illustrating the effect depending on the temperature of substrate; (D) Scheme of the process for fabricating the PeLED with high temperature annealing; (E) PL spectra of CsPbBr₃ post-annealing at 240 °C depending on the measured temperature; (F) Graph for relative EL intensity according to temperature from 40 to 110 °C^[72]. Copyright 2023 Elsevier; (G) PL spectra of CsPbBr₃ films with post-annealing from 200 to 350 °C and pristine film; (H) PL spectra of CsPbBr₃ films with post-annealing at 300 °C and pristine film^[73]. Copyright 2023 Royal Society of Chemistry; (I) The image of fabricating CsEuBr₃ with multi-source TE, and (J) PL spectra depending on the post-annealing from RT to 200 °C^[74]. Copyright 2021 John Wiley & Sons. PeLED: Perovskite light-emitting diode; EQE: external quantum efficiency; TE: thermal evaporation; MHP: metal halide perovskite; PL: photoluminescence; EL: electroluminescence; RT: room temperature.

In another study, substrates were thermally treated from room temperature up to 120 °C during deposition to fabricate red, green, and blue devices^[67]. Compared to room-temperature deposition, elevated substrate temperatures reduced defects and promoted the formation of grains with uniform size, leading to enhanced device performance. However, excessive heating to 120 °C increased surface roughness and degraded device efficiency. The optimal condition was identified at 100 °C, under which all three emission colors were successfully realized and characterized. Similar approaches have also been applied to lead-free MHP where CsSnBr₃ films were deposited while maintaining substrate temperature at room temperature, 50, 75, and 85 °C^[68]. Compared with deposition at room temperature, elevating the substrate temperature facilitated faster diffusion of CsBr and SnBr₂ on the substrate surface, thereby promoting the formation of the CsSnBr₃ phase. As the substrate temperature increased, both the optical and electrical properties of the films improved, while the trap density decreased, indicating enhanced crystallinity.

Conversely, lowering the substrate temperature below room temperature has also been explored for blue LED fabrication. When PbBr₂, CsBr, and CsCl were co-deposited at a slow rate, substrates held at room temperature and -10 °C were compared^[69]. The lower substrate temperature facilitated greater nucleation density, enabling rapid formation of uniform films and allowing radiative recombination to occur at lower carrier densities. As a result, blue LEDs fabricated at -10 °C achieved a luminance of 1,640 cd·m^-2, an EQE of 7.2%, and stable operation for approximately 7 min.

This research demonstrates that elevated substrate temperature promotes uniform grain formation and helps mitigate boundary defects, but excessively high temperature can induce film non-uniformity and increase defect density. In contrast, low substrate temperature enables dense nucleation and uniform small-grain films, though they require specialized deposition systems with cooling capability. The temperature-dependent changes in film characteristics are summarized in Figure 3C.

The optimization for annealing

Post-annealing has been widely recognized as a crucial strategy for improving the structural and optoelectronic quality of TE-based perovskite films and their corresponding LEDs. Post-annealing on MHPs facilitates enhanced atomic diffusion, which not only promotes improved crystallinity and enlarged grain sizes but also reduces defect densities that act as nonradiative recombination centers. Research has demonstrated that appropriately chosen annealing temperature and time lead to smoother morphologies, reduced trap states, and superior optical stability, with films retaining a high fraction of their luminescence intensity after extended storage^[70]. At the device level, optimized annealing conditions resulted in lower operating voltages, higher luminous efficiencies, and longer operational stability in PeLEDs. Nevertheless, excessive annealing temperature or prolonged heating can induce phase segregation, or interfacial degradation, emphasizing the requirement for precise optimization of the process parameters. Collectively, Research highlighted post-annealing as an indispensable tool for structural engineering, enabling both improved performance and long-term stability of TE-based PeLEDs.

After the TE, post-annealing at various temperatures has been employed to optimize thin-film properties. Huang et al. maintained a dual-source TE using CsBr and PbBr₂ at a deposition rate of 0.4 Å/s, followed by annealing at 60, 90, and 120 °C. They observed that grain size increased with annealing temperature up to 90 °C, whereas at 120 °C the average grain size decreased, and surface roughness increased, indicating that 90 °C, was the optimal annealing temperature^[71]. Based on these conditions, they fabricated LEDs with an ITO/Poly(9-vinylcarbazole) (PVK)/CsPbBr₃/TPBi/LiF/Al architecture, achieving a luminance of 39,700 cd·m^-2 and an EQE of 1.6%.

At higher annealing temperatures, organic charge transport layers may undergo structural deformation; thus, devices are often fabricated without a transport layer or by employing inorganic transport layers. As shown in Figure 3D, an LED with an Al/Si/CsPbBr₃/NiOx/Au structure was fabricated on a thermally robust silicon substrate using dual-source TE of CsBr and PbBr₂, followed by annealing at 240 °C^[72]. Compared with solution-processed films, vacuum deposition offers the advantage of enabling high-quality thin-film growth even on inorganic substrates. The electrical performance of the device was examined by applying bias under temperatures ranging from 40 to 110 °C. As shown in Figure 3E, the electrical properties improved up to 80 °C, but declined above this threshold, attributed to enhanced non-radiative recombination at elevated temperature. Figure 3F further demonstrated that at 80 °C, the device exhibited approximately twice the initial electroluminescence (EL) intensity when driven at higher current densities.

Using a similar dual-source TE with CsBr and PbBr₂, researchers have also fabricated composite CsPbBr₃-Cs₄PbBr₆ thin films by tuning precursor ratios^[73]. The optimal film was obtained at a Cs/Pb ratio of 1.34, showing superior optoelectronic performance. Upon annealing from 200 to 350 °C [Figure 3G], variations in photoluminescence (PL) intensity and peak position were observed, resulting in the effective defect passivation and grain growth. The red shift in PL emission was attributed to enlarged grain size. LEDs with an Ag/n-type silicon (n-Si)/MHP/NiO/ITO configuration exhibited a luminance of 957 cd·m^-2, and at 300 °C, both enhanced EL intensity and red shift occurred [Figure 3H]. Notably, the devices retained more than 90% of their initial performance after 6 h of continuous operation, underscoring the effectiveness of defect suppression.

The dual-source TE has also been extended to lead-free perovskite blue emitters^[74]. CsEuBr₃ films were fabricated via dual-source TE of CsBr and EuBr₃ [Figure 3I]. Unlike conventional lead-free perovskites, which typically exhibit broad full-width half maximum (FWHM) due to limited defect tolerance, Eu²⁺ ions, comparable to Pb²⁺, contribute to both the valence band maximum (VBM) and conduction band minimum (CBM), thereby facilitating charge transport and yielding a narrow FWHM of ~30 nm. A comparison of samples without annealing and with annealing at 100 and 200 °C [Figure 3J] revealed behavior distinct from that of CsPbBr₃, while CsPbBr₃ exhibits a red shift upon annealing due to grain growth, CsEuBr₃ shows a blue shift, which is attributed to enhanced crystallinity and bulk CsEuBr₃ formation at 100 °C. A shoulder peak near 515 nm emerged at 100 °C due to defect states from insufficient thermal energy; however, the peak disappeared after annealing at 200 °C, leaving a dominant deep-blue emission at 448 nm.

Single-source TE using bulk CsPbBr₃ powder, rather than separate CsBr and PbBr₂ precursors, has also been reported^[34]. Post-deposition annealing at elevated temperatures from room temperature to 400 °C yielded uniform thin films. Owing to the high annealing temperature, no charge transport layer was employed, and devices with a simple ITO/CsPbBr₃/C configuration were fabricated. The optimized 350 °C annealing produced devices with a low turn-on voltage of 2.92 V and a luminance of 14,079 cd·m^-2. While post-annealing is essential to promote grain growth otherwise insufficient during TE, excessively high annealing temperature may degrade device performance due to the low glass transition temperature of organic layers. Therefore, careful optimization of annealing conditions and device architecture is crucial for achieving high-performance TE-based PeLEDs^[75].

The modification of thickness and structure

The thickness of the emitting layer and the optimization of device structure have emerged as particularly decisive factors for fabricating superior TE-based PeLEDs^[76]. Research on CsPbBr₃ nanocrystal films demonstrated that careful control of the emitting layer thickness strongly influences film morphology, PL intensity, and carrier dynamics^[77]; for example, films around 40 nm exhibited enhanced luminescence and longer carrier lifetimes, translating into higher EQE compared with thinner or thicker counterparts. Equally important, the conscious engineering of device structures through the selection and modification of charge transport layers, adjustment of energy-level alignments, and incorporation of interfacial passivation strategies has been shown to significantly improve charge injection balance, suppress non-radiative recombination, and enhance operational stability^[78-81]. The modulation of these factors indicates that a synergistic approach combining precise thickness control with rational structural design is crucial for advancing the efficiency and reliability of TE-based PeLEDs, and provides guiding principles for the development of next-generation high-performance devices.

The key factor influencing the electrical properties of LEDs is the thickness of the emitting layer^[82]. In solution-processed perovskite devices, performance was found to vary significantly with film thickness, typically in the range of 30-100 nm. Optical simulations confirmed that changes in emissive-layer thickness modify the distribution of outcoupled modes, thereby affecting the EQE^[83]. Similarly, in LED with thermally evaporated CsCu₂I₃ metal halide luminescent materials, the device performance was also strongly dependent on the emitting-layer thickness^[84]. CsCu₂I₃ is a perovskite whose structure differs from the conventional ABX₃ MHP framework, and it can be regarded as the result of a change in the most stable structure caused by the smaller ionic radius of Cu compared with Pb in the prototypical MHP CsPbBr₃, as well as differences in charge balance. CsCu₂I₃ thin films were fabricated via dual-source TE of CsI and CuI. This perovskite exhibits excellent stability, a high PLQY, and the additional advantage of being non-toxic due to the absence of lead. When the emitting-layer thickness varied from 40 to 160 nm, the fabricated LEDs showed comparable turn-on voltages; however, substantial differences were observed in EL spectra linewidths and luminance. For example, devices with an optimal thickness of 60 nm exhibited more than four times higher luminance compared with those with 160 nm thickness and a reduction in FWHM of approximately 60 nm. In all devices, the EL peak was red-shifted relative to the PL peak and showed spectral broadening, which was attributed to iodine ion migration and subsequent cluster formation. Compared with solution processing, systematic studies on the influence of emitting-layer thickness in TE-based PeLEDs remain limited. Therefore, further investigations into the optical and electrical properties of such thickness- dependent effects are needed to establish reliable design guidelines for high-performance TE-based devices.

The properties of TE-based MHP layers are also strongly affected by the underlying substrate layers, prompting extensive studies on the use of diverse organic and inorganic interlayers beyond those commonly employed in solution-processed devices. Compared with solution processing, the absence of solvents allows the use of a wider variety of charge-transport layers; however, layers located above or below the emitting layer can induce charge imbalance or unwanted nonradiative recombination caused by interfacial defects. To control such defects and improve device performance, strategies such as treating the evaporation-based emitting layer with a phenethylammonium bromide (PEABr) solution have been proposed^[85]. For improving the interface between the hole transport layer (HTL) and emitting layer, Li et al. fabricated devices by dual-source evaporation of CsBr and PbBr₂, optimizing the precursor ratio to form CsPbBr₃-Cs₄PbBr₆ composites^[86]. Continuous thermal annealing using graphite blocks, a 50 °C hotplate, and a 200 °C hotplate revealed that treatments between 72-131 °C were most effective. Based on this process, the hole injection layers poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), NiOx, and LiF were compared. PL decay measurements [Figure 4A] indicated comparable lifetimes for LiF and NiOx, suggesting low defect densities at the perovskite-interlayer interface. In contrast, PEDOT:PSS exhibited markedly shorter lifetimes, pointing to significant defect formation. Interestingly, despite its poorer interfacial quality, PEDOT:PSS yielded higher electrical performance than LiF [Figure 4B and C], which was attributed to its energy band alignment effectively blocking electron leakage. NiOx, however, demonstrated superior interfacial and electrical properties compared to both materials, suppressing electron backflow and enabling excellent hole injection, ultimately delivering 9,442 cd·m^-2 luminance and 3.26% EQE.

Figure 4. Characteristics depending on the pre-deposited layer before perovskite deposition. (A) PL decay curves with different vacuum-deposited hole transport layers; (B) current density and luminance data with varied transport layers; (C) current efficiency and EQE depending on the hole transport layer^[86]. Copyright 2019 John Wiley & Sons; (D and E) transient absorption spectra with or without an 18-crown-6 layer^[87]. Copyright 2022 Elsevier; (F) XRD patterns as a function of TOABr concentration pre-deposited before CsPbBr₃ deposition; (G) illustration of CsPbBr₃ crystal evolution with TOABr treatment^[88]. Copyright 2023 American Chemical Society; (H) UV-vis and PL spectra with TAPC or CzSi as charge transport layers, and (I) PL decay curves of CsPbBr₃ deposited on two different charge transport layers^[89]. Copyright 2023 John Wiley & Sons. PL: Photoluminescence; EQE: external quantum efficiency; XRD: X-ray diffraction; UV-vis: ultraviolet-visible spectroscopy; TOABr: tetraoctylammonium bromide; TAPC: 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane; CzSi: 9,9’-spirobifluorene-based carbazole derivative.

To mitigate the poor interfacial quality associated with PEDOT:PSS, 18-crown-6, an organic molecule widely used in solution-processed devices, was introduced at the perovskite-PEDOT:PSS interface^[87]. This treatment improved conductivity, suppressed surface defects, and inhibited the formation of insulating Cs₄PbBr₆ phases, thereby enhancing operational stability. Transient absorption measurements [Figure 4D and E] confirmed reduced band-tail states and minimized red-shift of transient bleach minima, evidencing effective defect passivation. The optimized devices achieved 15,717 cd·m^-2 luminance and 1.02% EQE.

Inorganic interfacial modification has also proven effective. For example, deposition of tetraoctylammonium bromide (TOABr) between NiLiOx and perovskite layers improved film orientation and grain boundary^[88]. TOABr, with its four long alkyl chains, promoted preferential growth along the (101) and (020) orientations without introducing secondary phases [Figure 4F]. Excessive deposition marked as TOAB-2 and TOAB-3 led to TOABr impurity peaks, establishing TOAB-1 as the optimal condition. The structural improvements, evidenced in X-ray diffraction (XRD) and morphology analyses [Figure 4G], resulted in device performance of 11,133 cd·m^-2 luminance and 1.24% EQE in ITO/NiLiOx/TOABr/CsPbBr₃/TPBi/LiF/Al structures. In addition, materials widely employed in organic LEDs have been successfully adapted to PeLEDs. As illustrated in Figure 4H, emissive layers based on optimized CsPbBr₃-Cs₄PbBr₆ composites were combined with either 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole(CzSi) or 4,4’_-cyclohexylidenebis-[N,N-bis(4-methylphenyl)benzenamine] (TAPC) as HTLs^[89]. Despite exhibiting similar ultraviolet-visible (UV-vis) absorption spectra, CzSi-based devices showed tenfold stronger PL intensity and longer PL decay lifetimes compared to TAPC-based counterparts [Figure 4I]. This difference was attributed to structural deformation of the cyclohexyl ring within TAPC resulting in Pb⁰ defects. With further interfacial engineering around CzSi and the introduction of guanidinium bromide (GABr) upper layer for passivation, TE-based PeLEDs achieved record-high performance of 21.1% EQE and 75,300 cd·m^-2 luminance.

These studies demonstrate that beyond deposition rate and substrate temperature, the choice and treatment of pre-deposited transport layers critically determine the optoelectronic properties of TE-based perovskite films. Optimal selection and engineering of both organic and inorganic interlayers, combined with appropriate substrate and post-annealing conditions, are indispensable for realizing high-performance TE-based PeLEDs.

Organic additive

Organic molecules play diverse roles in regulating film crystallization, passivating defects, and tuning interfacial energetics. Small organic cations and functional ligands such as amines, phosphonic acids, and conjugated aromatic rings have been demonstrated to stabilize low-dimensional perovskite phases during deposition, thereby mitigating non-radiative recombination pathways. For instance, incorporation of PEABr^[90-92] or carbazole-based molecules^[93-95] has been shown to suppress halide vacancies and control vertical phase distribution, leading to improved film uniformity and enhanced efficiency. PEABr has been most widely employed as an organic spacer in quasi-2D MHPs, which can be considered as 3D MHPs doped with 2D MHP domains. Quasi-2D perovskites exhibit higher exciton binding energy than conventional 3D counterparts and show an n-phase distribution that depends on the number of perovskite layers sandwiched between organic spacers[Figure 5A], where larger n-phases possess smaller bandgaps. Taking advantage of these bandgap differences, a quantum-well structure can be formed, enabling improved charge transfer from wider-bandgap to narrower-bandgap regions and thus enhanced electrical characteristics. However, when multiple n-phases coexist, the bandgap variation can broaden the FWHM and inefficient energy or charge transfer may deteriorate the device performance; therefore, it is crucial to form an emitting layer with a uniform n-phase distribution. To this end, extensive studies have been conducted on the incorporation of various organic additives containing COO^- groups to improve phase uniformity and optoelectronic properties^[44,96,97]. In addition, as shown in Figure 5A, point-defect passivation and grain-boundary improvement via halide salts, as well as the interaction between phosphine oxide groups and Pb²⁺ to enhance the perovskite layer properties, represent key roles of such organic additives in quasi-2D perovskite systems.

PEABr is widely applied in solution-processed MHPs to synthesize low-dimensional structures as mentioned above, and similar approaches have been extended to TE to enhance perovskite thin films^[98]. In one study, PbBr₂ was first deposited, followed by the co-evaporation of MABr and PEABr with varying ratios as illustrated in [Figure 5B]. While MAPbBr₃ fabricated by TE alone exhibited poor optical properties, the incorporation of a small amount of PEABr improved its optical properties. However, excessive PEABr content induced the formation of a (PEA)₂PbBr₄ (PEA = phenethylammonium), resulting in blue rather than green emission. XRD analysis [Figure 5C] confirmed that a MA/PEA ratio of 1:0.25 fabricated pure MAPbBr₃, whereas a 1:0.5 ratio led to the coexistence of MAPbBr₃ and (PEA)₂PbBr₄. Importantly, at the optimal 1:0.25 ratio, pinhole defects were effectively suppressed, while the reduced refractive index minimized the mismatch with adjacent transport layers, thereby enhancing light outcoupling. Consequently, the devices achieved a luminance of 6,200 cd·m^-2 and an EQE of 0.36% [Figure 5D], outperforming other PEABr ratios. This demonstrated the potential of controlled PEABr incorporation in modifying film quality and device efficiency.

Figure 5. Improvement of perovskite films and devices by organic additives. (A) The structure of quasi-2D MHPs with organic spacer (left) and the effect of diverse organic additive (right); (B) Schematic illustration of PEABr incorporation into MAPbBr₃; (C) XRD patterns with varying MABr:PEABr ratios from 1:0 to 1:0.5; (D) luminance-voltage characteristics of LEDs fabricated with different MABr:PEABr ratios^[98]. Copyright 2017 American Chemical Society; (E) energy level alignment of perovskite layers with different BABr contents for quasi-2D structures; (F) electroluminescence image of a 0.5B/BC-based LED under 5 V bias^[100]. Copyright 2020 John Wiley & Sons; (G) mechanism of formation energies for CsPbBr₃ and Cs₄PbBr₆ with or without MABr during CsPbBr₃ fabrication; (H) GIWAXS pattern of pristine CsPbBr₃, and (I) GIWAXS pattern of CsPbBr₃ with MABr incorporation^[101]. Copyright 2025 John Wiley & Sons. MHPs: Metal halide perovskites; PEABr: phenethylammonium bromide; MAPbBr₃: methylammonium lead bromide; XRD: X-ray diffraction; LED: light-emitting diode; BABr: butylammonium bromide; GIWAXS: grazing-incidence wide-angle X-ray scattering; MABr: methylammonium bromide.

Beyond MAPbBr₃, the formation of quasi-2D perovskites via PEABr incorporation has also been investigated, enabling multi-quantum-well (MQW) structures that enhance both optical and electrical properties^[99]. PEABr was introduced during CsPbBr₃ deposition, and subsequent thermal annealing promoted grain growth, producing uniform, defect-suppressed films. Efficient exciton transfer from low- to high-dimensional domains was confirmed, leading to improved device performance. Optimized devices, incorporating modified charge transport layers, achieved an EQE of 6.5% and luminance of 7,400 cd·m^-2.

In contrast to the aromatic PEABr, alkyl-chain ammonium additives have been utilized to improve low-dimensional MHPs and grain boundaries. Sequential TE of CsBr, PbBr₂, and n-butylammonium bromide (BABr) produced films with quasi-2D/3D coexisting, resembling a core-shell morphology^[100]. By tuning the BABr/(BABr + CsBr) ratio between 0 and 0.67 and applying post-annealing at 160 °C for 5 min, the most uniform nanoflake-like morphology was obtained at a ratio of 0.45. As shown in Figure 5E, X-ray photoelectron spectroscopy (XPS) and Tauc plot analysis confirmed favorable band alignment, and the fabricated LEDs [ITO/Copper(II) phthalocyanine (CuPc)/MHP/TPBi/LiF/Ag] exhibited 8,400 cd·m^-2 luminance and 5.3% EQE and the devices displayed bright EL at 5 V [Figure 5F]. These results indicate that the high annealing temperature imposes limitations on the choice of hole transport materials, such as CuPc. This highlights the trade-off between film crystallinity and process compatibility in TE systems.

Another strategy involved the incorporation of MABr during CsPbBr₃ growth to simultaneously achieve defect passivation and crystallinity enhancement^[101]. The low formation energy of MAPbBr₃ enabled it to act as nucleation sites, upon which CsPbBr₃ crystals grew. Meanwhile, excess CsBr and PbBr₂ facilitated the formation of Cs₄PbBr₆ around CsPbBr₃ domains, further reducing defects and improving film quality. As shown in Figure 5G, crystallization followed an energy hierarchy from MAPbBr₃ to CsPbBr₃ to Cs₄PbBr₆. PL spectra confirmed enhanced intensity and narrower FWHM upon MABr incorporation, while grazing-incidence wide-angle X-ray scattering revealed characteristic ring patterns at q = 0.97 and 2.08 Å^-1, indicating randomly oriented Cs₄PbBr₆ domains [Figure 5H and I]. Based on this, LEDs using mixed Cs₄PbBr₆/MA_xCs_1-xPbBr₃ films achieved a record-high luminance of 246,211 cd·m^-2 and an EQE of 5.97%.

Beyond ammonium-based spacers, bis[2-[(oxo)diphenylphosphino]phenyl] ether (DPEPO) was co-deposited with PbBr₂ and CsBr to delay crystallization, thereby forming uniform films^[102]. Phosphine oxide groups interact with Pb²⁺ ions through P=O coordination, stabilizing local bonding and mitigating trap formation^[103,104]. With DPEPO treatment, Pb-ion coordination was strengthened, grain boundaries were improved, and trap density was reduced, enabling uniform film growth. This strategy led to devices exhibiting outstanding performance, with luminance of 35,083 cd·m^-2 and an EQE of 16.91%. These results underline the promise of functional phosphine oxide additives in defect suppression and film uniformity control.

Research has also been extended to blue and red devices, which typically show inferior performance compared to their green counterparts^[105]. For instance, post-deposition treatment with metformin hydrochloride improved film uniformity and defect passivation in multi-source TE-based CsBr/CsCl/PbBr₂ films^[106]. This approach not only enhanced electrical properties but also effectively mitigated anion exchange, a key issue compromising color purity in blue LEDs, thus ensuring spectral stability under high bias. Similarly, post-deposition treatment with Phenethylammonium chloride (PEACl) followed by GABr yielded synergistic improvements^[107]. While PEACl incorporation alone induced a blue shift in PL spectra due to Cl^- incorporation [Figure 6A], the subsequent addition of GABr led to a slight red shift from Br^- incorporation [Figure 6B]. The cations, consisting of PEA⁺ and Guanidinium (GA⁺), did not alter the perovskite crystal structure but contributed defect passivation by donating electrons from N–H bonds to undercoordinated Pb²⁺. As a result, the devices exhibited a twofold increase in EQE, achieving 6.09% [Figure 6C].

Figure 6. Data related to performance improvement by organic additives. (A) PL spectra changes upon incorporation of PEACl into CsPbBr₃; (B) PL spectra of CsPbBr₃ treated with PEACl only and with both PEACl and GABr; (C) EQE vs. current density of pristine CsPbBr₃, PEACl-treated, and GABr-treated films^[107]. Copyright 2025 Springer; (D) schematic illustration of perovskite modification in CsPbBrI₂ upon addition of 2-F-TPPO; (E) optical image of a large-area perovskite film deposited by vacuum evaporation, and (F) PL peak position of large-area CsPbBrI₂:2-F-TPPO films^[108]. Copyright 2025 Springer. PL: Photoluminescence; PEACl: phenethylammonium chloride; GABr: guanidinium bromide; EQE: external quantum efficiency; TPPO: triphenylphosphine oxide.

Finally, fluorinated organic additives have been introduced to red devices. For example, 2-F-triphenylphosphine oxide (2-F-TPPO) combined the defect-passivation capabilities of P=O moieties with the stabilizing effect of fluorine^[108]. Fluorine incorporation, known to enhance electrical stability and suppress anion exchange^[24,109], facilitated smaller grain sizes and uniform film morphology. As shown in Figure 6D, 2-F-TPPO treatment suppressed uncoordinated Pb²⁺ and improved optoelectronic properties compared to conventional TPPO. As shown in Figure 6E and F, large-area films (9 cm × 10 cm) were successfully fabricated using this method, maintaining uniform PL peak positions across the surface. This scalability showed the practical potential of vacuum deposition with organic additives for display technologies.

Overall, these studies collectively demonstrate that the incorporation of organic additives during TE not only improves defect passivation and crystallinity but also enables fine-tuning of optical and electronic properties across green, blue, and red PeLEDs. Optimization of precursor ratios and post-annealing conditions is crucial, as device performance consistently surpasses that of conventional AX/BX₂-based perovskite devices. Nevertheless, while multi-source and sequential TEs with organic additives have been widely explored, single-source TE with organic incorporation remains largely uncharted. Future research in this direction could bridge the gap toward scalable, industrially viable processes for high-performance perovskite optoelectronics.

Inorganic additive

Inorganic species have likewise proven indispensable for optimizing film quality and device performance. Alkali metals (e.g., Li⁺, Na⁺, Rb⁺) and alkaline earth elements have been incorporated to regulate dynamics of crystallization, stabilize the perovskite lattice, and suppress ion migration under bias. These dopants can effectively reduce defect densities at grain boundaries while promoting enhanced thermal and operational stability. Halide additives, including Cl^- and Br^- co-evaporation, serve to passivate anion vacancies and refine crystal orientation, which is particularly relevant for narrow FWHM and achieving high color purity. Transition-metal oxides and halides have also been explored as passivating agents at perovskite-electrode interfaces, where they alleviate non-radiative recombination and mitigate interfacial degradation under electrical stress. Collectively, inorganic additive strategies have enabled significant improvements in performance.

In dual-source TE of MHP thin films, one of the primary strategies for improving crystallinity is the incorporation of AX and BX₂ as an additive. For instance, when fabricating FAPbI₃ films from formamidinium iodide (FAI) and PbI₂ via TE, the resulting material is predominantly the yellow δ-phase. However, when CsI is introduced in a multi-source TE, the stable black α-phase of FAPbI₃ can be synthesized as illustrated in [Figure 7A]^[110]. The incorporation of Cs⁺ ions occupies the vacant A-sites, suppressing the formation of large δ-phase grains and enabling uniform films with smaller grain sizes in the α-phase. The differences with and without CsI incorporation are clearly observed in the SEM images in [Figure 7B and C]. Consequently, LEDs based on uniform FACsPbI₃ films achieved a high EQE of 10.25%.

Figure 7. Variation of perovskite film properties with inorganic additives. (A) Illustration of phase transformation in FAPbI₃ upon CsI addition; (B) SEM image of pristine FAPbI₃ film; (C) SEM image after CsI incorporation^[110]. Copyright 2025 Springer; (D) Cross-sectional HAADF-STEM image of alternating six-layer deposition of NaF and CsPbBr₃ by vacuum evaporation (E) XRD patterns of CsPbBr₃ depending on the presence of NaF interlayers^[118]. Copyright 2025 American Chemical Society; (F) schematic illustration of sequential deposition of RbBr/PbBr₂ followed by CsBr/PbBr₂ and the corresponding film formation; (G) PL spectra as a function of RbBr/CsBr ratio, and (H) optical images under UV illumination^[119]. Copyright 2025 Elsevier. FAPbI₃: Formamidinium lead iodide; FA: formamidinium; SEM: scanning electron microscopy; HAADF-STEM: high-angle annular dark-field scanning transmission electron microscopy; XRD: X-ray diffraction; PL: photoluminescence; UV: ultraviolet.

A similar strategy has been applied in the fabrication of blue PeLEDs using CsPb(Cl/Br)₃. In this case, dual-source TE employing CsCl and PbBr₂ was compared with multi-source TE using CsCl, CsBr, and PbBr₂^[111]. The study revealed that employing two CsX sources led to the formation of Cs₄Pb(Cl/Br)₆, and that optimizing the ratio of CsPb(Cl/Br)₃ to Cs₄Pb(Cl/Br)₆ yielded improved device performance.

Analogous to alkali-metal doping widely employed in perovskite quantum dots^[39,112-114], recent studies have introduced inorganic alkali halides into TE to enhance device efficiency. The effect of the alkali metal strongly depends on its ionic radius: ions of suitable size can occupy the A-site, while ions that are too small or too large tend to segregate at the grain boundaries, thereby exerting different influences^[115]. For example, incorporation of LiBr during deposition introduces Li⁺ and Br^- ions, which help suppress bromide vacancies and lower the activation energy for ion diffusion, facilitating mass transport within the perovskite lattice^[116]. Similarly, the introduction of NaBr together with CsBr and PbBr₂ effectively suppresses the formation of Cs₄PbBr₆, a phase that hinders charge injection^[117]. By characterizing XRD patterns of MHPs, films deposited with CsBr:PbBr₂:NaBr ratios of 9-x:4:x exhibit a reduction in Cs₄PbBr₆ peaks for x ≥ 1, while maintaining the CsPbBr₃ phase. At x = 1, the surface defect density of the films was also minimized.

TE further enables the construction of stacked structures that are difficult to realize in solution processing. For instance, alternating 1 nm NaF and 10 nm CsPbBr₃ layers produced nanoscale grains with suppressed defect formation^[118]. Owing to the small lattice mismatch between NaF and CsPbBr₃, NaF serves as an effective defect passivator at the perovskite interface. Cross-sectional high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images [Figure 7D] confirm the presence of six alternating layers. When CsPbBr₃ films with thicknesses of 20 and 40 nm were alternately deposited with NaF, the XRD patterns [Figure 7E] revealed that the intensities of the (110) and (220) peaks decreased, while the (112) peak intensity increased. This indicates that the crystallographic orientation becomes more enhanced in the MHP regions closer to the NaF layer. Additional GABr treatment at the interface with the charge transport layer further reduced interfacial defects, leading to devices with an EQE of 7.21% and a luminance as high as 35,289 cd·m^-2.

In another approach, Rb⁺ incorporation was investigated as an alternative to Cs⁺ for blue LED emission without the addition of chloride^[119]. Dual-source TE of RbBr and PbBr₂, as well as CsBr and PbBr₂, was carried out [Figure 7F], and the resulting optical and electrical properties were studied as a function of Rb:Cs ratio. Because the ionic radius of Rb⁺ is smaller than that of Cs⁺, increasing Rb content induced a blue shift in the emission and enhanced the migration activation energy, thereby suppressing ion migration. The PL peak shift with increasing Rb fraction is shown in Figure 7G, while the EL images under applied bias for devices with different Rb ratios are displayed in Figure 7H.

Overall, the incorporation of inorganic additives during TE has served primarily to control defects, suppress undesired phase formation, and enhance crystallinity, leading to improved efficiency and stability, analogous to the role of organic additives. Nevertheless, unlike the well-established A-site or halide additives, systematic B-site doping via TE has not yet been explored. Furthermore, there have been no reports of simultaneous incorporation of multiple inorganic additives. These directions represent promising opportunities for further improving the performance of PeLEDs.

Synthesis of solid-state MHPs

A significant step toward scalable TE of MHP emitters lies in developing optimized precursors for single-source TEs. Conventional dual-source TE suffers from composition drift and limited stoichiometric control, leading to phase inhomogeneity and unstable emission^[120]. Recent advances in powder synthesis, including ball-milling^[121], solution methods^[122], and other physical methods^[123-126] [Figure 8A], have enabled uniform vapor transport and reproducible thin-film formation. Modifying precursor crystallinity, and compositional purity minimizes halide loss during evaporation, improving film stoichiometry, defect passivation, and spectral stability. These strategies not only facilitate reproducibility but also open the way toward finely tuned bandgap engineering and device-to-device uniformity in large-area displays.

Figure 8. Solid-state synthesis of perovskites and powder preparation by ball milling. (A) Schematic illustration of various solid-state synthesis methods for perovskites; (B) Schematic mechanism of the formation of Cs₄PbBr₆ and CsPbBr₃; (C) Comparison of PLQY values of three synthesized powders with those of perovskite nanocrystals prepared by the same method^[130]. Copyright 2022 Nature; (D) PL spectra of Cs₄PbX₆ synthesized by ball milling with different halides; (E) Schematic illustration of the single-source deposition process; (F) Optical image of Cs₄PbX₆ films fabricated by single-source TE^[131]. Copyright 2023 American Chemistry Society.

The most widely adopted method for synthesizing solid-state MHP powders is ball milling. Two common variants are wet ball milling, in which solution-phase precursors are mixed, and dry ball milling, which employs only solid precursors. Long-chain ligands such as oleic acid (OA) and oleylamine (OAm), conventionally used for quantum dot synthesis, were introduced during the milling process to prepare powders emitting in the sky-blue and green spectral ranges^[127-129]. These ligands facilitate nanocrystal formation by widening the interparticle distance. More recently, high-energy dry ball milling has emerged as a robust strategy to produce powders with uniform particle size distributions^[130].

Comparable to previous studies employing mixed CsPbBr₃ and Cs₄PbBr₆ phases in both solution and TE, solid-state MHP synthesis also exhibited phase evolution and changes in optical properties as a function of milling time. Milling durations ranging from 20 min to 48 h revealed that the most prominent CsPbBr₃ phase fraction was achieved after 180 min of processing, corresponding to the highest optical quality. To elucidate this transformation, the reaction pathway was analyzed [Figure 8B]. Three representative cases were proposed: (1) CsPbBr₃ serving as the primary phase with excess CsBr attaching at the surface to yield Cs₄PbBr₆; (2) CsBr and PbBr₂ combining in specific ratios to generate both Cs₄PbBr₆ and CsPbBr₃; and (3) Cs₄PbBr₆ binding with PbBr₂ to coexist with CsPbBr₃. Across all scenarios, the optimal composition for photophysical performance corresponded to a Cs₄PbBr₆:CsPbBr₃ ratio of 93:7 wt%, with Case 1 producing the most superior optical characteristics. As shown in Figure 8C, the optical properties were consistent with those reported for solution-processed perovskite nanocrystals, further validating the milling-based approach.

Dry ball milling has also been extended to the synthesis of Cs₄PbBr₆ powders (X = Cl, Br, I), enabling emission tunability from blue to red^[131]. As illustrated in Figure 8D, varying the halide composition shifted the PL peak from 410 to 690 nm, with FWHM values spanning 17-60 nm closely resembling those of solution-synthesized perovskites. Moreover, these powders were successfully employed as single-source TE precursors: after placement in tungsten boats, they yielded high-quality thin films via TE [Figure 8E]. Interestingly, for iodine-containing powders, both α- and δ-CsPbI₃ phases emerged, attributed to thermally induced phase transitions during deposition. This highlights the critical importance of minimizing deposition time to suppress thermal degradation and decomposition, particularly when employing ball-milled powders. The resulting thin films, when exposed to UV illumination, exhibited distinct emission images [Figure 8F]. This demonstrates that dry-milled powders are not only compositionally tunable but also directly compatible with TE workflows, underscoring their potential as scalable sources for high-quality thin films.

Solution-mediated synthesis routes for perovskite powders have also been reported. Here, long-chain ligands are typically removed by centrifugation or vacuum oven drying, enabling the recovery of solid-state nanocrystals in large quantities^[132-134]. For microscale and larger powders, slow crystallization at low temperature from precursor solutions has been widely employed to yield single crystals with fewer defects^[135-141]. For instance, CsPbBr₃/Cs₄PbBr₆ composites were obtained by dissolving cesium acetate (CsAc) and PbBr₂ in Dimethyl sulfoxide (DMSO), followed by HBr-assisted precipitation and DMSO washing^[142]. The yield and PLQY strongly depended on the CsAc/PbBr₂ ratio and washing solvent volume, reaching up to ~70% yield and 97% PLQY. As shown in Figure 9A, powders synthesized with a CsAc/PbBr₂ ratio of 4 displayed bright luminescence under UV excitation. Corresponding XRD patterns indicated that ratios above 5 produced unidentified phases with CsBr peaks, whereas ratios below 4 predominantly yielded Cs₄PbBr₆ [Figure 9B]. Consistent with other sections, coexisting CsPbBr₃ and Cs₄PbBr₆ phases enhanced emission efficiency by facilitating carrier transfer and surface defect passivation, while also improving stability under ambient exposure [Figure 9C].

Figure 9. Data on solid-state perovskite synthesis using solution processing. (A) Optical and UV-illuminated images of powders synthesized with a 4:1 ratio of CsAc:PbBr₂; (B) XRD patterns as a function of CsAc:PbBr₂ ratio and the amount of DMSO used for washing; (C) PLQY stability test of solid-state CsPbBr₃ and Cs₄PbBr₆ powders compared with CsPbBr₃ nanocrystals upon air exposure^[142]. Copyright 2018 American Chemistry Society; (D) Optical images of powders under UV and ambient light with CsBr:PbBr₂ ratios varied from 1:1 to 6.5:1; (E) PLQY data as a function of CsBr:PbBr₂ ratio; (F) Comparison of the stability of powders synthesized at high temperature with SiO₂-coated materials and quantum dots^[143]. Copyright 2021 John Wiley & Sons. PLQY: Photoluminescence quantum yield; PL: photoluminescence; TE: thermal evaporation.

An alternative approach involved mixing CsBr and PbBr₂ with OA and OAm in Dimethylformamide (DMF)/DMSO solvents, followed by HBr addition, to form perovskite powders with nanometer-scale dimensions^[143]. Unlike the CsAc-based method, no unidentified phases were detected even at CsBr/PbBr₂ ratios as high as 6:1. As shown in Figure 9D, powders exhibited strong luminescence under UV excitation, and at a ratio of 6, the Cs₄PbBr₆:CsPbBr₃ mixture achieved optimal defect passivation and the highest PLQY [Figure 9E]. Thermal stability, a prerequisite for TE, was further validated by comparing bare CsPbBr₃ powders with SiO₂-coated counterparts and perovskite quantum dots. As illustrated in Figure 9F, the powders exhibited excellent stability up to 100 °C. Nevertheless, given that vacuum deposition processes often require substantially higher thermal loads, the design of powders with enhanced high-temperature stability remains a critical challenge for future research.

Host-dopant system

The host-dopant system, well established in organic LEDs, offers new opportunities for PeLEDs. By embedding highly emissive perovskite nanocrystals or reduced-dimensional domains as dopants within wide-bandgap host matrices, concentration quenching can be suppressed, Auger recombination reduced, and exciton confinement enhanced. The host environment simultaneously controls charge transport pathways, facilitating balanced carrier injection while protecting the emissive domains from environmental degradation. Importantly, rational host selection - whether organic semiconductors with tailored frontier orbitals or insulating scaffolds with controlled polarity - enables tunable recombination dynamics and suppression of non-radiative decay, thereby bridging the performance gap between PeLEDs and organic LED structures. This cross-disciplinary perspective highlights the potential of host-dopant systems to accelerate the commercialization of PeLEDs.

In host-dopant systems, the host material is typically chosen to possess a wider bandgap than the dopant^[144]. When a narrower bandgap dopant material is incorporated at concentrations of up to ~10%, spatially confined excitons and charge carriers, corresponding to the Cs₄PbBr₆-CsPbBr₃ structure, promote radiative recombination, enhancing the PLQY. For example, the lead-free perovskite CsCu₂I₃ was synthesized via sintering and subsequently employed as a dopant in various organic host materials. As shown in Figure 10A, three different wide-bandgap organic hosts with distinct band alignments were investigated for their ability to improve device performance^[145]. Among these, tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) was found to be the most effective host because the CBM and VBM of CsCu₂I₃ align favorably within the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TmPyPB. This alignment enabled efficient exciton and carrier management within the perovskite, resulting in nearly a 20-fold increase in PLQY. Furthermore, as illustrated in Figure 10B, EL spectra revealed a single sharp emission peak without secondary features, confirming the success of this host-dopant strategy. The optimized devices achieved a high EQE of 7.4% along with operational stability exceeding 20 h.

Figure 10. Characteristics of host-dopant systems and MQWs. (A) Energy alignment of three organic hosts with CsCu₂I₃; (B) PL spectra of perovskite-doped and undoped CsCu₂I₃ using three organic host materials compared with bulk CsCu₂I₃^[145]. Copyright 2021 American Chemistry Society; (C) PL spectra of MAPbCl₃ with and without Coumarin 153 doping^[146]. Copyright 2018 John Wiley & Sons; (D) Schematic illustration of MQW structure formation using single-crystal CsPbBr₃ and three organic materials; (E) Band gap variation as a function of TPBi thickness in single QW structures formed with TPBi and CsPbBr₃; (F) Corresponding changes in PL peak position^[148]. Copyright 2021 John Wiley & Sons. PL: Photoluminescence; MAPbCl₃: methylammonium lead chloride; MA: methylammonium; MQW: multiple quantum well; QW: quantum well; TPBi: 2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole).

Conversely, research has also explored the use of perovskites as hosts with organic dopants^[146]. To this end, MAPbCl₃ films were fabricated by dual-source vacuum deposition of MACl and PbCl₂, yielding a wide-bandgap perovskite host. When doped with coumarin, the molecular frontier orbitals of coumarin resided within the HOMO-LUMO gap of MAPbCl₃, facilitating efficient energy transfer. At 1% coumarin doping, the highest PL intensity was observed. As shown in Figure 10C, pristine MAPbCl₃ exhibited a PL peak at ~400 nm, whereas coumarin doping induced a red-shifted, broadened emission peak in the green region. This spectral shift suggests absorption by the perovskite followed by energy transfer to the coumarin dopant. Compared with conventional organic hosts, MAPbCl₃-based devices demonstrated enhanced EQE, reduced turn-on voltage, and overall improved electrical performance.

One important set of challenges in host-dopant systems remains to be fully resolved. Although embedding MHPs in inorganic or organic host matrices can effectively suppress nonradiative recombination, excessive spatial isolation of the emissive domains can reduce percolation pathways and hinder charge transport, leading to increased driving voltages and reduced current efficiency at higher luminance. In addition, efficiency roll-off often arises from poor carrier balance at high brightness, where limited charge mobility in the host or insufficient charge injection into the dopant phase can cause exciton quenching or charge accumulation. From a processing standpoint, co-evaporation in TE-based fabrication presents practical difficulties in precisely controlling the individual deposition rates of host and dopant, especially for multi-component stacks and very low dopant concentrations, which can result in compositional drift and device-to-device variability. Addressing these related issues of transport, carrier balance, and process control will be crucial for translating host-dopant strategies into robust, scalable PeLEDs^[147].

Multi-quantum well structure

Another powerful strategy involves MQW structures, wherein alternating layers of wide- and narrow-bandgap perovskites confine carriers and excitons within emissive wells. This engineered energy landscape reduces carrier diffusion losses, enhances radiative recombination rates, and enables efficient color purity tuning, such as a host-dopant system. MQWs further mitigate ion migration by providing structural barriers, thus improving operational stability. Advanced deposition protocols have demonstrated controllable well thickness and composition, unlocking improved EQE and operational lifetimes.

The formation of MQW structures requires the sequential TE of wide-bandgap organic barrier materials and perovskite layers. Since the optoelectronic properties strongly depend on the thickness of each layer, precise thickness control is critical. For this reason, single-source TE has been primarily employed as the deposition method. As shown in Figure 10D, single-crystalline CsPbBr₃ was synthesized, ground into an evaporation source, and subsequently deposited together with organic barrier materials such as TPBi, tris-(8-hydroxyquinoline)aluminum (Alq₃), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) to construct multilayer MQW structures^[148]. In the case of single-quantum well (QW) structures, the PL peak position was shifted depending on the thickness of the CsPbBr₃ layer, while the extent of blue-shift varied with the type of organic barrier employed. The largest blue shift was observed in CsPbBr₃/TPBi single-QWs [Figure 10E]. This behavior can be explained by the different band alignments: TPBi and Alq₃ exhibit type-I band alignment with CsPbBr₃, enabling both electrons and holes to transfer into the perovskite layer, whereas BCP exhibits type-II band alignment, driving electrons into the BCP layer. Due to its larger barrier height compared to Alq₃ and BCP, TPBi induced nearly a twofold stronger blue shift, which is evident in the PL spectra [Figure 10F].

Optimization of such MQW structures has revealed that ultrathin CsPbBr₃ layers combined with TPBi barriers enable the fabrication of blue-emitting perovskite materials without the need for chloride incorporation. Conversely, MQW structures with relatively thick CsPbBr₃ layers exhibited a red shift in their PL spectra^[149]. Compared with bulk CsPbBr₃, all MQW configurations demonstrated superior optical properties owing to efficient exciton confinement and energy transfer. However, despite these promising results, the direct integration of MQW perovskite structures into operational LEDs has not yet been realized, underscoring the need for further research on device-level implementation.