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Abstract
Lead halide perovskites (LHPs) have made impressive progress in solid-state optoelectronics by virtue of their excellent electronic and optical features. In the past few years, the light-emitting diode (LED) adopting LHP emitters have reached a comparable level of external quantum efficiency (EQE) with organic and colloidal quantum dot LED counterparts. Apart from solution-processing, all-inorganic CsPbX3 LEDs can also be fabricated using thermal evaporation in a single run without breaking vacuum. In principle, all-evaporated LHP-LEDs in a vacuum demonstrate good uniformity and reliability in a large-area, especially full color applications where color pixelation is necessary, although their performance is still lower than that of the devices using hybrid film depositions. Herein, the understanding of vacuum-evaporated LHPs and their resulting LEDs, including the materials, film deposition and device issues, are reviewed. Additionally, guidelines toward high-performance devices and their prospects in the future are included.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As new generation of semiconductor materials, the lead halide perovskite (LHP) family has proven their worth in optoelectronics, especially in light-emitting diodes (LEDs), by virtue of the achievements reached in the past few years [1–20]. Compared to their counterparts, namely, organic LEDs (OLEDs) and semiconductor colloidal quantum dot LEDs (QD-LEDs), LHP-LEDs demonstrate a competitive or even a higher level of performance, including their high brightness, high external quantum efficiency (EQE) and ultra-wide color gamut [1–13,18–22]. Generally, the LHP emissive layer is deposited onto a substrate by solution-processing, in principle, which is supposed to be an advantageous for low manufacturing costs [1–13,18–20,23]. As a new generation of fantastic display technologies, the impressive progress of LHP-LEDs achieved in the laboratory means that industry-scale manufacturing in the next few years is critical because of the intensive homogenization competition from OLEDs, QD-LEDs and other emerging display technologies. However, solution-processing also plays an insurmountable role in hindering the further improvement of LHP-LEDs whose structural design and fabrication were severely restricted by the orthogonal selection of solvents [24–27]. Almost all the so-called solution-processed LHP-LEDs, especially high-performance LHP-LEDs, still require a high vacuum chamber for the deposition of subsequent functional layers [1–13,19,20,23]. Thus far, most device research actually concentrates on the emissive materials and their film deposition. An industry-scale LED display always requires that all pixels have a super-high uniformity over a large area, which is still a challenge for laboratory-scale solution-processing as a film deposition technology.
Fortunately, apart from solution processing, some LHPs can also be deposited using thermal evaporation in a vacuum chamber, which means good compatibility with the well-established all-evaporated LED industry [28–34]. The vacuum-evaporated LHP emissive films have an innate advantage in their uniformity of a large-area film, which is essential for LED displays. Among all LHPs, all-inorganic CsPbX3, especially CsPbBr3, demonstrated the best thermal evaporation in vacuum and is thereafter the main covering material in this review [19,28–32,34]. All-inorganic CsPbBr3 demonstrates some special features that differ from other LHP family members, for instance, the high thermal stability of all-inorganic CsPbX3 make it a candidate for high driving-current, high brightness LEDs and even laser diodes and other current-driven applications in the future [35–37]. In the past few years, vacuum-evaporated CsPbBr3 emissive films have achieved impressive progress in LEDs; however, these films are still dominated by solution-processed counterparts [28–34,38,39]. Statistically, the performance of LEDs containing a vacuum-evaporated CsPbBr3 emissive layer still falls far behind that of devices with a solution-processed CsPbBr3 emissive layer. (Table 1)
In this review, some up-to-date progress achieved in the LEDs using a vacuum-evaporated LHP emissive layer, especially CsPbBr3, is provided. Insights into the film deposition, device structure design and fabrication, and guidelines for high-performance vacuum-evaporated LHP-LEDs are presented. This review can possibly trigger subsequent studies or new possibilities in LHP-LEDs and other related subfields.
2. Fundamental properties of LHPs for LEDs
In general, LHPs are typical ionic compounds that can be expressed as (R)2(A)n−1BnX3n+1, where site A is a monovalent cation, e.g., formamidinium (FA+), methylammonium (MA+), Cs+ and their combination; site B is Pb2+; and site X is Cl-, Br-, I- and their combination [42–46]. In the LHP crystal structure, the [PbX6]4- octahedron formed by halogen anions is the core fragment, and the pivot site is always Pb2+. When n=1, a monolayer corner-sharing [PbX6]4- octahedral forms a 2D perovskite sheet that is sandwiched by two organic ligand layers (R) (Fig. 1(a)) [43,44,47]. As the n value increases, the formula can be simplified to APbX3, which corresponds to classical 3D LHPs (Fig. 1(a)) [43,45,47]. LHPs deposited in a vacuum chamber by thermal evaporation are generally 3D crystalline structures based on elemental corner-sharing [PbX6]4- octahedra because there is no symmetry breaking factor. For example, the molecular spacers used in solution-proceeded 2D LHPs can block the assembly of elemental corner-sharing [PbX6]4- octahedra out of plane [48].
Fig. 1. (a) Schematic of 3D and quasi-2D lead halide perovskites. Reproduced with permission [47] Copyright © 2015 American Chemical Society. (b) Schematic energy diagram of lead halide perovskites and their corresponding total density of states (TDOS) and the partial density of states (PDOS). Reproduced with permission [49] Copyright © 2017 American Chemical Society. (c) CIE chromaticity coordinates comparison among CsPbX3 NCs (dark points), the consumer LCD TVs (dashed white line) and the color standards of TVs (NTSC) (solid white line). Reproduced with permission [50] Copyright © 2017 American Association for the Advancement of Science. (d) The dependence of PL linewidth of FAPbI3 on temperature. Reproduced with permission [51] Copyright © 2016 The Author(s).
The energy diagram of emitters is a very important factor for the resulting LEDs because the related bandgap and states will influence the optical properties, and the energy level position of the conductive band (CV) and valence band (VB) will determine the charge carrier injection and selection of matched functional layers. The energy diagrams of LHPs are mainly determined by the orbital hybridization of lead and halogen (Fig. 1(b)) [11,49,52–54]. Regarding CB, the Pb-6p orbitals play a primary role compared to the slight contribution of the np orbitals of halogens (Cl-3p, Br-4p, I-5p) (Fig. 1(b)) [49,55]. In contrast, in the VB, which is hybridized by the X-np orbitals and the Pb-6s orbitals, the contribution of halogens is higher; thus, the energy level of VB and the resulting band gap can be widely tuned by adopting different halogen species (Fig. 1(b)) [49,54,56]. In addition to the single halogen species CsPbCl3, CsPbBr3 and CsPbI3 with a fixed band gap, alloyed halide species can also take site X to form CsPbClxBryIz (x + y+z=3) [55,57]. By changing the ratio of halogen species, the band gap can be tuned over 1 eV; correspondingly, the light emission shifts continuously from the near-infrared to the near-ultraviolet [11,58–60]. Along with their narrow linewidth, the LHP family demonstrates an ultrawide color gamut over the NSTC standards, which can reappear as almost all natural color in their resulting ‘true’ color displays (Fig. 1(c)) [50,61,62]. The strong polar X-Pb bond in LHPs leads to a Fröhlich coupling between charge carriers and longitudinal optical phonons, which is supposed to be the predominant role in determining the linewidth broadening at room temperature [51,63,64]. In comparison, other factors play minor roles in linewidth broadening. This is the reason for bulky LHPs and their nanocrystalline counterparts to demonstrate comparable linewidths and shapes of emissive spectra (Fig. 1(d)) [51–53,63,65,66]. In a vacuum-evaporated LHP film, the emissive spectra are almost the same.
The most attractive feature of LHPs, unlike typical semiconductors, is their high defect tolerance in light generation. Mostly, the bandgap of normal semiconductors is the energy difference from the top of the bonding VB to the bottom of the antibonding CB. In contrast, both the VB and CB of LHPs are antibonding (Fig. 1(b)) [67,68]. Antibonding means a raised VB maximum, even lying above the level of the X-np and Pb-6s orbitals. Defect states therefore lie within the VB [68,69]. Thus, these tolerated defects do not lead to any significant influence of the interband electron transition and resulting light generation [49,52,53,65,70,71]. However, there are still some extrinsic deep defect states caused by vacancies, halide interstitials and others [72,73]. In the LHPs deposited using vacuum thermal evaporation, there are more defects because of the excess precursors, by-products and grain boundaries during the deposition compared to solution-proceeded LHP NCs with the passivation of ligands [32,74].
In the LHPs deposited using thermal evaporation in vacuum, they are polycrystalline; thus, without any effect of spatial confinement, the LHP excitons are normally Wanier-Mott type with a low binding energy down to dozens of meV [34,55,76,77]. These excitons have a high possibility of dissociating into free charges compared to the radiative recombination [78,79]. Because of the low overlap of wavefunctions between holes and electrons and their high mobilities, radiative recombination is a slow second-order bimolecular process (Fig. 2(a)) [6,75,80,81]. The many defects formed during the thermal evaporation also make a significant contribution to the consumption of excitons by a prompt first-order trap mediated non-radiative recombination [6,75,80–82]. Because of the low exciton binding energy, in LHP-LEDs the applied external electric field provides an additional pathway of consuming excitons by dissociating them into free-charge carriers [83,84]. Therefore, an effective enhancement of the crystalline quality of deposited LHPs, along with charge carrier confinement, is required for highly efficient light emission.
Fig. 2. (a) Initial time PL intensity (IPL(t = 0)) as a function of the charge carrier density in PL. Reproduced with permission [6] Copyright © 2018 American Chemical Society. (b) Schematic energy diagram of metal halide perovskites. Reproduced with permission [56] Copyright © 2019 The Author(s). (c) Schematic diagram of CsPbBr3 deposition using vacuum thermal evaporation. (d) Schematic illustration of slow bimolecular recombination. Reproduced with permission [75] Copyright © 2017 The Author(s). (e) Schematic illustration of charge carrier confinement and subsequent recombination in the perovskite multiple quantum wells. Reproduced with permission [75] Copyright © 2017 The Author(s).
Differing from the optically-excited applications, as a current-driven structure, good electric properties are essential for LEDs [18–20]. LHPs demonstrate superior charge carrier transport features, including ambipolar character, high mobility up to thousands of cm2 V-1s-1 and long lifetime charge carries [51,85–87]. The charge carrier flows through the 3D LHPs as a large polaron with radii lager than 40-50 Å, which exceeds the lattice constant and is immune from scattering with defects, other carriers, and even phonons [88–90]. Comparable hole and electron mobilities are supposed to be important merits for LHP-LEDs, which always requires balanced hole and electron injection [6,90,91]. In addition, intrinsic ion migration also makes a nonnegligible contribution to charge transport, which can be observed as hysteresis dependence of driving voltage and current [27,60,84,92]. Although this feature can be used to heal some defects and lower energy barrier for charge carrier injection, it still inevitably leads to concerns about the device stability of LHP-LEDs [93–97]. The VB of LHPs is deep and can be adjusted over 1 eV by changing halogen species, which makes the hole injection difficult, especially the blue-emitting ones (Fig. 2(b)) [49,56]. Thus, it is challenging to find matched hole transport materials with deep HOMO level, especially semiconductive polymer films, which require an additional passivation treatment before the solution-based deposition of LHPs [6,7,17,98].
3. Vacuum evaporation of LHPs
The vacuum evaporation techniques for thin-film deposition include thermal evaporation, electron beam evaporation, sputtering, molecular beam epitaxy, atomic layer deposition and others. With the consideration of the comparable device structures and fabrication of OLEDs, QD-LEDs and LHP-LEDs and the common functional materials they shared, until now, the vacuum thermal evaporation is the most commonly used technique in the LHP-LED fabrication. In principle, most members of the LHP family can be thermally evaporated [29,48,74,99,100]. Until now, most thermally evaporated LHP films have been used as absorbers in solar cells [99–102]. Organic-inorganic hybrid LHPs are not good candidate materials for vacuum thermal evaporation because of their lower thermal stability in vacuum, which will degrade rather than evaporate with heating [103,104]. For multisource co-evaporation, the organometallic precursors, especially MABr, are difficult to handle because of their high vapor pressure, and a serious contamination will occur [105–107]. In comparison, all-inorganic LHPs and their precursors demonstrate much better thermal evaporation in vacuum, and their films can be deposited by different thermal evaporation scenarios [28,34,74,96]. The CsPbX3 can be deposited by multisource co-evaporation or the subsequent evaporation of the two precursors CsX and PbX2 [28–30,32,74]. Additionally, it can also be deposited by the single-source evaporation of presynthesized raw CsPbX3 [34,108]. For both scenarios, most deposited CsPbX3 will be subjected to posttreatment for healing defects [28–30,32,34,74]. Normally, thermal annealing and solvent vapour annealing are effective treatments for enhancing the crystal quality of a deposited LHP film [28–30,32,34,74]. Moreover, some additives, especially ion liquids, have also demonstrated great defect elimination capability [109,110].
3.1 Multisource evaporation
All-inorganic LHPs can be deposited by alternately evaporating two precursors, CsX and PbX2 (Fig. 2(c)) [28–30,32,74]. By virtue of high ion migration, the heterointerface between two adjacent CsBr and PbBr2 layers will be blurred and even disappear to produce CsPbBr3 domains [29,74,84]. However, the ratio of the two precursors is difficult to handle in this operation. The two precursors can also form other LHP species with different stoichiometric ratios, such as CsPb2Br5 and Cs4PbBr6 [30,74,111]. Therefore, the deposited film could lead to a blend of several LHP species and excess precursors rather than the high purity CsPbBr3. Clearly, these by-products and excess precursor impurities will result in many defects, which leads to a low level of luminous efficiency of the LHP film and their resulting LEDs [30,74,111]. Moreover, this nonuniform emissive film contains different domains of LHP specie domains, which will make the charge carrier transport mechanism more complicated, resulting in an even worse device performance [30,74,111].
Alternatively, the CsPbBr3 film can be deposited by simultaneous co-evaporation of two precursors CsBr and PbBr2 in vacuum [28,48,99]. These two precursors would mix uniformly at the nanoscale during film deposition, and their ratio can be controlled in real time by adjusting the evaporation rate of each precursor. Similarly, two or more different PbX2 precursors can also be simultaneously co-evaporated with CsX to deposit alloyed halide perovskite films [112]. In principle, the film deposited using this scenario should demonstrate an improved film uniformity. However, on the practical level, precise control of the ratio among different precursors by adjusting the corresponding evaporation rate is difficult over the whole duration of LHP film deposition. Thus, it is still a major challenge to obtain a highly crystalline structure of deposited LHPs. In solution-processed film deposition, precursors will spontaneously react with others with a precise stoichiometric ratio, and in this dynamic growth process, excess reactants will be removed easily by the solvent [113,114]. However, in vacuum thermal evaporation, the excess precursors deposited onto the substrate cannot be removed without any assistance, which will exist as impurities or form other LHP species such as Cs4PbX6 [30,74,111].
3.2 Single-source evaporation
In addition to the multisource co-evaporation of precursors, the emissive films can also be deposited by the single-source evaporation of presynthesized CsPbX3 in high vacuum, which can fix the stoichiometry problem [34,108]. In comparison to organic-inorganic hybrid LHPs, all-inorganic CsPbX3 exhibits great thermal stability, making it more suitable for thermal evaporation [35–37]. This good thermal stability depends on the vacuum atmosphere [36]. At a low vacuum level, CsPbBr3 will decompose instead of evaporating, and at a high vacuum level of 10−5 Pa, the perovskite structure of CsPbBr3 remains stable with increasing temperature until melting and then evaporating [115–119]. However, it has been reported that at high temperature, highly activated ion migration will lead to some CsBr-rich and PbBr2-rich domains before the evaporation [34]. Therefore, at the macroscale, the deposited film still follows the stoichiometry of CsPbBr3; however, their microscopic perovskite crystal structure is lost. Based on the identical feature of ion migration, after posttreatment, all by-products will convert back to CsPbBr3 [34,36,120]. It has been observed through in situ high-resolution TEM that the crystal structure of CSPbBr3 is destroyed at a high temperature (over 800 °C) and then recovers completely as the temperature is decreased [36].
The vacuum-evaporated emissive films are polycrystalline and contain many CsPbBr3 and other LHP species grains, whose boundaries and impurities would mediate a prompt nonradiative recombination of excitons as trap states [34,65,70,82]. Due to the high defect tolerance, these LHP films still demonstrate a high level of light emission [28–34,52]. However, even if the deposited CsPbBr3 film is a perfect single crystal without any defects, it is still difficult to obtain efficient light emission as high as that of the nanocrystalline emitter counterparts used in LEDs. Because of the negligible wavefunction overlap of electrons and holes, there is a small chance that opposite charge carriers will meet each other, which means a low-efficiency radiative recombination for LEDs (Fig. 2(d)) [6,75,80,81]. Thus, the LHP-LEDs are dominated by LHP nanocrystals, quantum dots, quantum wells, nanoplatelets and other nanostructures that confine electrons and holes in a small domain with dimensions of nanometers to facilitate their radiative recombination (Fig. 2(e)) [1–17]. Without effective charge carrier confinement, sufficient exciton binding energy and effective passivation of surface defects, it is difficult to further improve the light emission to a comparable level of solution-processed LHP NCs [1–17,28–33,75,80,81].
3.3 Role of the substrate
In most crystal growth, the substrate plays an important, even a critical-level role. The LHP vapour deposits onto an organic semiconductor substrate to form a solid thin film, which is also a crystallization process; thus the role played by organic semiconductor substrate during the crystal growth is considerable. Until now, there are still a few works focusing onto this topic [99,121]. The surface potential of organic substrate was proved to play an important role in the crystallization during the solution-processed film deposition [122,123]. A solution-processed LHP film deposited on top of a hydrophilic PEDOT:PSS substrate demonstrated a smaller grain compared to hydrophobic substrates [122,123]. In the vacuum-evaporated LHPs, the role of organic semiconductor substrate played in film deposition is still a complicated issue. Different organic semiconductor substrates show significant variety in LHP film composition and morphology [99,121]. It is confirmed that without the orthorhombic limitation of solvent, the substrate selection and the device structure design of the whole LHP-LED are more flexible. The complicated influences on organic semiconductor substrate caused by the subsequent solution-processing onto it are also eliminated completely.
3.4 In situ treatment and post treatment of LHP films
In single-source vacuum thermal evaporation, the macroscale-ordered structure of CsPbBr3 will be destroyed, and the vapor should be a nanoscale cluster of CsPbBr3 (Fig. 3(a)). Because of the enhanced ion migration at high temperature, some CsBr-rich and PbBr2-rich domains form in the raw CsPbBr3 [34]. The deposited LHP grains are much smaller than the solution-processed grains, which means more boundaries [126]. Thus, a treatment is required to improve the crystal quality of the deposited LHPs. Thermal annealing is a simple and effective post-treatment for enhancing the crystalline quality of the deposited CsPbBr3 film [48,100,127]. However, if the annealing temperature is too high, the deposited CsPbBr3 films will lose their light emission [34]. It is noted that there is a significant discrepancy about whether thermal annealing can enhance the light emission of a vacuum-evaporated CsPbBr3 film, which means that there is still an unknown mechanism about the crystallization of LHP during thermal evaporation that needs to be uncovered [34,48]. In situ temperature management of substrate was proved to make a significant influence on the size of the deposited LHP crystal [121]. As decreasing temperature from room temperature to -2 °C, the deposited LHP grain size increases from 100 nm to micrometer level [121]. In addition, solvent vapor annealing, ion liquid additives and precursors like PEABr were also used to heal the defects of deposited LHP [74,100]. Until now, most treatment works still concentrated onto the LHP absorber in solar cells.
Fig. 3. (a) In situ TEM image of CsPbBr3 nanocrystal in high vacuum (1.2×10−5 Pa). Consecutive clip images at 690 K from 0 to 10 s. Scale bars: 10 nm. Reproduced with permission [36] Copyright © 2019 Author(s). (b) Schematic device architecture of the all-vacuum-evaporated perovskite LEDs. Reproduced with permission [33] Copyright © 2020 Author(s). (c) Schematic of doping effect at the interface caused by ion migration under applied electric field. Reproduced with permission [124] Copyright © 2018 American Chemical Society. (d) TEM image of CsPbBr3 nanocrystals embedded Cs4PbBr6 matrix. Reproduced with permission [125] Copyright © 2018 The Authors.
4. LEDs exploiting vacuum-evaporated LHP emitters
In a sense, solution-processed LHP-LEDs can be considered as a derivative of LHP photovoltaics and QD-LEDs, which share almost the same device structure, even identical functional layers (Fig. 3(b)) [1–17,128]. Similarly, vacuum-evaporated LHP-LEDs also adopt the device structure frame of solution-processed ones, including the ITO anode modification layer of PEDOT:PSS and a semiconductor polymer hole transport layer [28–34]. A highly conductive PEDOT:PSS film is an excellent ITO anode modification layer used in a variety of thin-film optoelectronic applications [74]. Generally, a semiconductor polymer hole transport layer, such as poly-TPD and PVK, is believed to work as an energy steppingstone to facilitate the injection of holes into the LHP emissive layer [28]. Because of the metallic feature, PEDOT:PSS is supposed to be an exciton quencher of the adhesive emissive layer and the semiconductor polymer hole transport layer also worked as a buffer layer for eliminating the quenching of LHPs [129,130]. However, without any semiconductor polymer hole transport layer between the PEDOT:PSS and LHP layers, some LHP-LEDs still demonstrate high brightness, low driving voltages and high EQEs [30,34,74]. Because of the high ion migration of LHPs, a p-type doping layer will be formed at the PEDOP:PSS/LHP interface facilitated by external bias in LEDs [84,124,131,132]. This p-type doping layer will effectively decrease the hole injection barrier from PEDOT:PSS into LHPs, leading to a low driving voltage, although their VB is much deeper than the Fermi level of PEDOP:PSS (Fig. 3(c)) [84,124,131,132]. On the contrary, in some LHP-LEDs using a vacuum-evaporated CsPbBr3 emissive layer deposited onto a PEDOT:PSS film, the driving voltage is much higher, and the origin is probably the low conductivity of by-product Cs4PbBr6 [30,74,111].
To achieve highly efficient LHP-LEDs, balanced charge carrier injection is essential. To balance superior hole injection, efficient electron injection is required. For the cathode side, a small molecular electron transport layer, an electron injection or interfacial buffer layer and a thin-film metal cathode were used [28–34]. Because the CB of LHPs is deep, most organic electron transport materials can match the energy level alignment for barrier-free electron injection well [11,49,52–54,56]. However, the electron mobility of most organic electron transport materials is orders of magnitude lower than that of LHPs [6]. Using n-type doping, the conductivity of the electron transport layer can be enhanced significantly [133,134]. However, small-size n-type dopants such as lithium, cesium and compound Cs2CO3 would diffuse into the emissive layer [135,136]. Whether these hot atoms and ions heal or damage the crystal structure of the deposited LHPs is still unknown. To replace Cs and Cs2CO3, some organic n-type dopants like Lewis acids can be used for LHP-LEDs. Moreover, the evaporation temperatures of this kind of inorganic n-type dopant, LiF interfacial buffer layer and metal electrode film are much higher than those of other materials, which would lead to thermal annealing of the deposited LHPs.
Alternatively, some inverted structures were adopted in solution-processed LHP-LEDs, which can fix the unmatched electron injection. There are a few electron transport polymers that can be used in inverted structure LHP-LEDs, the most used electron transport layer is a solution-processed ZnO nanocrystal film on an ITO cathode substrate [137–140]. To modify the energy level alignment and eliminate potential quenching caused by ZnO, an ultrathin PEIE film is used before LHP film deposition [139,140]. Subsequently, an organic hole transport layer CBP and hole injection layer MoO3 are evaporated [140,141]. To prevent the high temperature from evaporating MoO3, an organic p-type dopant F4-TCNQ or HAT-CN can be used for efficient hole injection, which can be evaporated at a much lower temperature [136]. Thus far, there are still no vacuum-evaporated LHP-LEDs using an inverted structure.
For most LHP-LEDs using a vacuum-evaporated emissive layer, solution-processed deposition of other functional layers was still used, which abandoned some advantages of the vacuum-evaporated LHPs in LEDs [28–34]. The energy level of different LHPs can be tuned to over 1 eV by using different halogen species [56]. To date, only a few semiconductor polymer candidates can be used in solution-processed LEDs; however, the orthorhombic issue is a further limitation [1–17]. Thus, on the practical level, it is difficult to determine the optimized device structure for all, even only one, LHP species emitter in LEDs. All-evaporated LHP-LEDs in a single run without breaking vacuum may be an option for further improvement of the device performance because of their good reliability and reproducibility. Without the limitation of solution processing in device fabrication, the device structure design, the selection of matched functional layers and reliability control of all-evaporated LHP-LEDs in vacuum are easy to handle. However, until now, only a few all-evaporated devices in vacuum have been reported, and their performance is still far behind that of the devices using a solution-processed LHP emissive layer [29,119].
5. Summary and future prospects
Vacuum-evaporated LHPs have achieved impressive progress in solar cells; however, they are still dominated by their solution-processed counterparts in LEDs. In principle, the area of LHP-LED panel fabricated using vacuum thermal evaporation can reach a comparable level of commercial OLED displays, though most current research works still focus on small-area devices. Among all LHP family members, all-inorganic CsPbX3 is more suitable for vacuum thermal evaporation; CsPbBr3 is the most intensively studied emitter used for LEDs; and the maximum brightness and EQE can reach 15,745 cd m-2 and 8.86%, respectively [38]. For the vacuum-evaporated LHP-LEDs, the predominant consideration is the deposition of highly efficient LHP emissive layer. The CsPbBr3 film can be deposited by using the sequential evaporation or co-evaporation of two precursors of CsBr and PbBr2; however, their molar ratio is difficult to handle to obtain a stoichiometric CsPbBr3 during the whole deposition process. Alternatively, pre-synthesized CsPbBr3 can be directly evaporated as the emissive film. During evaporation, the crystal structure and stoichiometric ratio of CsPbBr3 cannot be controlled uniformly, and some CsBr-rich and PbBr2-rich domains will form because of thermally facilitated ion migration. Thus, even though raw CsPbBr3 is highly crystalline, the deposited film is polycrystalline and contains several components.
Currently, to achieve comparable or even higher performance from vacuum-evaporated LHP-LEDs with respect to the solution-processed device counterparts, the predominant challenge is a highly efficient LHP emissive film deposited using vacuum thermal evaporation. There are two main reasons that limit the light emitting efficiency of vacuum-evaporated LHP film. Firstly, the LHP crystal structure and even the ABX3 stoichiometric ratio are destroyed during the thermal evaporation, and the resulting LHP emissive film deposited onto substrates are low-quality polycrystalline, which contains high density of defects. Therefore, obtaining a highly-crystalline LHP emissive film is essential. An in situ treatment, such as substrate heating, during the LHP film deposition could be helpful to improve their crystal quality. By virtue of ion migration of LHP, a post-treatment, including thermal annealing, solvent vapour annealing and using additives, can be used for healing defects of the deposited emissive film. However, these kinds of post-treatment steps will require to break vacuum, which trades off the advantage of vacuum-evaporated LHP-LEDs. Additionally, the substrate on which LHP film is deposited is also an important factor in determining the emissive film quality. Secondly, the low-efficiency light emission is determined by their delocalized charge carriers and slow bimolecular radiative recombination even in a vacuum-evaporated LHP single crystal. Based on the same idea of the solution-processed LHP-LEDs, a feasible scenario is the nanocrystallization of evaporated LHPs. The simultaneous co-evaporation of LHPs with a spacer could be a potential pathway to obtain monocrystalline LHPs, which is similar to the CsPbBr3/Cs4PbBr6 composite structure (Fig. 3(d)) [125]. For selecting the spacer material, there are three requirements: First, the spacers should be a good evaporating material such as small molecular semiconductors. Second, the spacers should control the dimension of LHP grains deposited. And third, the spacers should passivate the LHP grain boundary defects.
In principle, solution-processed LHP-LEDs have a relative advantage in device fabrication cost because high vacuum is not essential for emissive LHP layer deposition. Until now, the performance of all-solution processed LHP-LEDs has still fallen behind their counterparts using hybrid film depositions of vacuum thermal evaporation and solution processing. However, it is worth emphasizing that one of the key features of vacuum-evaporated LHP-LEDs is their flexibility in device structure design, fabrication and optimization, especially for large-area applications. In this sense, an all-vacuum-evaporated LHP-LED is the most appropriate device which demonstrates this key feature. Although the solution-processed LHPs have made a breakthrough in large-area monochrome LEDs, the all-vacuum-evaporated LHP-LEDs will still play a predominant role in full color display requiring a highly uniform pattern of pixels. Moreover, all-evaporated LHP-LEDs in vacuum hardly output any toxic waste solution during manufacturing, which is more environmentally friendly and should be taken into consideration in terms of cost.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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