Samrat
Das Adhikari
a,
Andrés F.
Gualdrón Reyes
b,
Subir
Paul
a,
Jeevan
Torres
a,
Beatriu
Escuder
a,
Iván
Mora-Seró
a and
Sofia
Masi
*a
aInstitute of Advanced Materials (INAM), Universitat Jaume I (UJI), Avenida de Vicent Sos Baynat, s/n, Castelló, 12071, Spain
bFacultad de Ciencias, Instituto de Ciencias Químicas, Isla Teja Universidad Austral de Chile Valdivia, 5090000, Chile
First published on 24th July 2023
Perovskite nanocrystals (PeNCs) synthesized by colloidal solution methods are an outstanding case of study due to their remarkable optical features, different from their bulk counterpart, such as a tuneable band gap and narrower photoluminescence emission, altered by the size and shape. However, the stability of these systems needs to be improved to consolidate their application in optoelectronic devices. Improved PeNC quality is associated with a less defective structure, as it affects negatively the photoluminescence quantum yield (PLQY), due to the essential, but at the same time labile interaction between the colloidal capping ligands and the perovskite core. In this sense, it would be extremely effective to obtain an alternative method to stabilize the PeNC phases and passivate the surface, in order to improve both stability and optical properties. This objective can be reached exploiting the structural benefits of the interaction between the perovskite and other organic or inorganic materials with a compatible structure and optical properties and limiting the optical drawbacks. This perspective contemplates different combinations of core/shell PeNCs and the critical steps during the synthesis, including drawbacks and challenges based on their optical properties. Additionally, it provides insights for future light emitting diode (LED) applications and advanced characterization. Finally, the existing challenges and opportunities for core/shell PeNCs are discussed.
Core/shell nanostructures for chalcogenide quantum dots have been studied over decades for the development of lighting device technologies.15 In the case of perovskite-based core/shell nanocrystals,16–18 the research is on the edge-on-the-seat for the development of perovskite LED (PeLED) performances. A detailed description of the types of core/shell structures is given in a later section, establishing how the core/shell structure is beneficial for favouring PeLED performance. In some of the recent reports, the following points are addressed, which can deal with the challenges of PeLED implementation:17
(i) optical enhancement: recovery of surface defects, with protective shelling over perovskite nanocrystals, leads to reduction of non-radiative recombination and hence increasing radiative carrier recombination (i.e., free excitons), increasing the photoluminescence quantum yield (PLQY).19 Higher PLQY offers more efficient conversion of electrical energy into light energy, which results in brighter and more efficient LEDs.20–22 Besides, the core–shell type-II heterostructure formation improves PL lifetime, stability, and LED performance.19,23–25 It is noteworthy that some of the reports on 3D/0D core/shell perovskites observed shortening of lifetime in comparison to primitive (core-only), where the 0D materials have a wide bandgap and hence the core/shell materials show type-I heterostructure formation, ensuring a faster exciton decay.21,26
(ii) Tuneable emission: the optical emission can be easily tuned by A-site variable perovskite shelling over the perovskite core due to the alloy formation with smaller/larger A-site cations, which eventually reduces the average particle size and in turn offers blue-shifting.27 This tuneability can be achieved by controlling the composition of the core and shell materials. This property allows the development of LED devices with different colours.
(iii) Stability improvement: halide perovskites experience temperature, light and moisture exposed degradation, which have been prevented by a coating with inorganic/organic materials on the core perovskite. One of the main issues in the fabrication of efficient optoelectronic technologies based on perovskite nanocrystals (PeNCs) such as LEDs is the relatively low stability of the phosphor material.28 Due to their ionic nature, the facile loss of cationic/anionic species from the perovskite structure stimulates elemental vacancy related defect states.29 In consequence, a high density of defect sites is formed, decreasing the optical performance of the active layer integrated into the device.2,30,31 In this sense, several shelling strategies are employed to restrain the influence of moisture, high temperatures and harsh polar environments on the PeNC structural integrity, without hindering the carrier transport into the LED devices, discussed vividly in a later section.
Scheme 1 A schematic representation of type-I and type-II heterojunctions in typical core/shell nanostructures. |
Conceptually, type-I semiconductors are favourable for LED and lasing applications, due to the exciton confinement, whereas type-II semiconductors are extensively used for catalysis and energy cascade applications, due to the charge separation. However, in a recent report by Ye et al., efficient LED device performance has been achieved from type-II perovskite/semiconductor core/shell nanostructures.25 Shelling of the nanostructured crystals can be classified as single core/shell type and multiple core/shell type. In a single core/shell, there is a single shelling material, which can be an inorganic semiconductor,19,23,24 a multidimensional perovskite core/shell alloy,21 or organic polymer coated PeNCs.33–35 In the case of multiple core/shell nanoparticles, they are formed when many smaller sized nanocrystals are coated with a single shelling material. This type of shelling has been studied extensively in the case of cubic-shaped perovskite nanocrystals, where several arrangements of nanocubes can be easily encapsulated by organic or inorganic protective materials.36
Considering all the possible perovskite nanocrystal-based core/shell materials, a classification is presented in Scheme 2, and their particulars are elaborated in the following sections.
Fig. 1 (a) A schematic of MAPbBr3 (M-PE) perovskite core/poly(norepinephrine) (PNE) shell nanocrystals, and (b) the corresponding TEM images of wide and enlarged views showing a 10 nm thick shell of PNE micelles over perovskite. Reproduced with permission from ref. 39, Copyright © 2020, American Chemical Society. (c) Schematic of the BPA-induced core–shell formation mechanism. (d) TEM image of a (FA0.7MA0.1 GA0.2)0.87Cs0.13PbBr3 core/BPA shell nanocrystal. Reproduced with permission from ref. 40 Spring Nature. |
Lee and coworkers recently came up with a breakthrough finding regarding the shelling of perovskite nanocrystals with organic molecules for PeLED applications.40 They found out that post-treatment of a 3D perovskite film of (FA0.7MA0.1 GA0.2)0.87Cs0.13PbBr3 with benzylphosphonic acid (BPA) results in BPA capped core–shell nanocrystals, which show a very strong PLQE of 85%. This report indicates that discrete single core/shell nanostructures are formed as BPA molecules penetrate the large grain of perovskite thin films. A schematic of such a mechanism is presented in Fig. 1c. The TEM image of BPA-shelling over perovskite nanocrystals is presented in Fig. 1d. Based on this observation, it has been concluded that the shelling of BPA ligands results in the binding of non-bonded Pb atoms, thereby reducing trap density, minimizing carrier extraction, and increasing carrier confinement, ultimately leading to enhanced luminous efficiency. These beneficial gains accompanying the transformation into a core–shell nanocluster are manifested by the unprecedented LED performance exhibited by the system. LEDs fabricated with the core–shell nanocluster system exhibited a maximum current efficiency of 151 cd A−1 and a maximum EQE of 28.9% with a maximum brightness of 473990 cd m−2, which is comparable with the highest brightness of state-of-the-art inorganic quantum dot LEDs, and the record performance for PeLEDs.41
Proper organic capping also allows an increase of performance for photocatalytic applications. As an example, the stability improvement has been demonstrated in alcoholic media through the introduction of organic coordinated ligands.42 However, the instability in water is a significant concern that needs to be addressed to utilize perovskite nanocrystals in various applications. Very recently, Rao and co-workers have reported a short-chain multidentate bolaamphiphilic ligand (NKE-3), able to passivate the perovskite surface with an ionic terminal bearing amino functionalities and interact with water molecules with a terminal glutamic acid, leading to a water suspended colloidal solution of PeNCs.43 This system allowed an efficient long-range dipole-based FRET from perovskites to Rhodamine B isothiocyanate (RITC) in water, with FRET efficiencies ranging from 96% to 98%. Additional research is required to enhance the operational durability of PeLEDs that are coated with an organic material under moist conditions to make them suitable for practical applications.
The use of core–shell nanoparticles with organic/polymer shelling for optoelectronic devices has to provide enough electrical integration as the coating is non-conductive in most cases. Thus, the major synthetic challenge would be to reap the benefits of organic shelling, such as enhanced stability, without compromising the optoelectronic properties of perovskites. One way to achieve this goal is obviously tuning the thickness of the organic shell to an optimum value. In addition, the characteristics of each ligand also will influence the selection of organic molecules for shelling. Molecules that can directly interact with the metal could be better candidates as shell materials compared to those which only form a non-interacting shell as evidenced by the findings of Lee and co-workers.40
The epitaxial growth of the shell material, which faces strain related to lattice mismatch, is evidently the primary challenge in designing any core/shell material. In this context, the lattice constant of CsPbBr3 is 5.85 Å and CdS is 5.83 Å,24,52 which closely match the values of lattice constants. This fact enables the formation of a less strained interface in the CsPbBr3/CdS nanostructure.19,53–55 The synthesis and optical properties of a CsPbBr3/CdS core/shell heterostructure were investigated by Kipkorir et al.24 The study revealed that CdS capping effectively suppresses defect states at the surface of core CsPbBr3 nanocrystals. This core/shell nanocrystals exhibit a nearly seven-fold increase in time-resolved PL lifetime compared to pristine CsPbBr3 nanocrystals, see Fig. 2a, which is an additional advantage of the core/shell structure in addition to surface passivation. The heterostructure formation facilitates the formation of quasi type-II band alignment, see Fig. 2b, where the nearly isoenergetic conduction band of core CsPbBr3 and shell CdS offers delocalization of electrons across two semiconductors, prompting charge separation.
Fig. 2 (a) Time-resolved PL lifetime of the CsPbBr3/CdS core/shell nanostructure showing enhanced lifetime comparison to the pristine. (b) Schematic of the presentation of carrier recombination of the carrier transfer process of pristine CsPbBr3 and CsPbBr3/CdS core/shell nanostructures. Reproduced from ref. 24 under the terms of the CC-BY license. (c) PL lifetime of CsPbBr3/ZnS showing 15 times enhancement in comparison to the control (CsPbBr3-oleylammonium bromide capped). (d) Schematic of the CsPbBr3/ZnS core/shell band structure showing typical charge separation in the semiconductor heterojunction. (e) HR-TEM image of the typical heterojunction of the CsPbBr3–ZnS core/shell heterojunction. Reproduced from ref. 23 with permission, Copyright © 2020, American Chemical Society. |
Another exciting performance of ZnS nanolayer coating over CsPbBr3 nanocubes has been reported by Ravi et al.,23 which also established nearly 15 times extended PL lifetime of the CsPbBr3/ZnS core/shell nanostructure in comparison to core-only CsPbBr3 nanocrystals, see Fig. 2c. As explained in the previous paragraph, heterostructure formation due to the coating of a metal chalcogenide layer over the perovskite electronically facilitates the PL lifetime. In this example, the valence band maximum is in a similar position to that of CsPbBr3, and the conduction band minimum is significantly higher than that of CsPbBr3, and then they form a quasi-type-II heterostructure, where the hole is delocalised with the core and shell and the electron is localised in the core material, see Fig. 2d. From this observation, the significant PL lifetime enhancement is explained. The high-resolution TEM image of the epitaxial heterojunction of CsPbBr3 and ZnS is presented in Fig. 2e. In this work, the authors have also studied the water and photostability of the nanostructure.
From these recently developed reports on perovskite/chalcogenide core/shell nanostructures, PL lifetime enhancement is the common effect that has been established. In this context, a longer lifetime can result in a more stable and consistent emission of light over time, which is important for practical applications of LEDs, where uniform and long-lasting lighting is needed, which could be a potential aspect of CsPbBr3/MS (M = Zn, Cd) core/shell nanocrystals for future implementation as a PeLED.
Lead chalcogenide is found to be an efficient chalcogenide-based shelling semiconductor because of common metal ions (Pb2+, herein). In a report by Zhang et al., PbS shelling CsPbI3 nanocrystals are constructed for the application of efficient and stable PeLEDs. The PbS-shelling not only passivated the CsPbI3, but also induced a switch in its semiconducting behaviour from n-type to ambipolar. As a result, the PeLED's electroluminescence occurred in a p–i–n structure.46 Furthermore, the effectiveness of PbS-shelling is demonstrated by its tolerance to polar solvents in a case study involving CsPbBr3/PbS core/shell nanostructures.45
The perovskite/chalcogenide semiconductors have been synthesized successfully; however, there are some limitations in the synthesis of a wide range of metal chalcogenide shelling following the colloidal synthesis strategy. The challenge in this synthesis lies in the following factors: (i) separate growth of the chalcogenide nanocrystals, (ii) degradation of perovskite nanocrystals in the presence of the shelling precursor materials, and (iii) reaction temperature that leads to uncontrolled growth of shelling materials, and at the same time perovskite nanocrystal ripening/agglomeration. To overcome these synthesis obstacles, Ravi et al. synthesized CsPbBr3/ZnS core/shell nanocrystals using Zn(DDTC)2 as a single molecular precursor to grow a ZnS shell epitaxially over the CsPbBr3 core, where the shelling procedure do not interfere to degrade the core perovskite. The single molecular precursor slowly releases Zn and S at lower temperature, and hence the chances of separate growth of ZnS quantum dots are ruled out. CsPbBr3/ZnS nanocrystals are synthesized by the low temperature ZnS shell growth and the slow release of the precursor, which eventually prevents the separate nucleation of the ZnS impurity phase.23
(i) 3D/3D core/shell heterostructures consist of a 3D perovskite core and a 3D perovskite shell, where the two perovskites have different compositions or crystal structures. In the case of 3D/3D core/shell heterostructures, shelling of 3D perovskites over 3D perovskite nanocubes has been reported to provide better photophysical performances. In this way, Yang and coworkers50 have developed the colloidal synthesis of core/shell PeNCs, through the epitaxial growth of CsPbBr3 on FAPbBr3 PeNCs, see the schematic presented in Fig. 3c. The core-only FAPbBr3 nanocrystals exhibit an average particle size of 8.2 ± 1.2 nm. Upon shelling with CsPbBr3 at an FA:Cs molar ratio of 1, the average particle size increases to 11.2 ± 1.1 nm, see Fig. 3d. The resultant FAPbBr3@CsPbBr3 nanocrystal composite solutions exhibit a PLQY of up to 93% and are stable under ambient conditions for 70 days and resistant to continuous UV illumination for 50 h. The combination of both kinds of nanoparticles induces the emergence of alloyed FAxCs1−xPbBr3 at the core/shell interface, favoring the surface defect passivation and thereby the improvement of radiative carrier recombination. This modification can be observed through the electroluminescence (EL) spectra of the materials, achieving a displacement of the EL peak position of the PeNCs by varying the thickness of the CsPbBr3 shell. In this way, green-LEDs were fabricated with a maximum EQE of ∼8.1%, luminance (L) of ∼1758 cd m−2, initial operation voltage (VON) = 2.6 V and a maximum operational lifetime (T50) of ∼ 47 min, at L = 100 cd m−2.
Fig. 3 3D/3D core/shell: (a) schematic presentation of the epitaxial growth of CsPbBr3 on FAPbBr3 PeNCs, and their TEM images and particle size histogram of (b) core-only CsPbBr3 and (c) core/shell CsPbBr3/FAPbBr3 PeNCs. Reproduced with permission from ref. 50, under the terms and conditions from creative commons CC-BY license. 3D/2D core/shell: (d) schematic of the 2D/3D core/shell atomic model and (e) corresponding TEM image of a MAPbBr3/(S/RPEA2PbBr4) core/shell nanocrystal. Reproduced from ref. 25 with permission, Copyright © 2020, American Chemical Society. 3D/0D core/shell: (f) schematic of the atomic model of the FAPbBr3 core and the sequential growth of Cs4PbBr6 through the creation of a mixed alloy transition layer of FAxCs1−xPbBr3, and their TEM images and particle size histogram of (g) core-only FAPbBr3 and (h) core/shell FAPbBr3/Cs4PbBr6 PeNCs. Reproduced from ref. 21 with permission, Copyright © 2020, American Chemical Society. |
(ii) 3D/2D core/shell heterostructures consist of a three-dimensional (3D) perovskite core and a two-dimensional (2D) perovskite shell. In the case of 3D/2D, 2D perovskites are composed of hybrid organic-inorganic materials and their heterojunctions are formed through van der Waals epitaxy. In the case of van der Waals heterostructures,56 the capping ligands (act as a spacer cation) attached to the halides can have possible interaction with a next inorganic layer(s) surrounding the 3D perovskite nanocubes. Eventually, this type of interaction leads to the development of a 3D perovskite structure featuring a layer of 2D perovskite.57,58 In principle, the category of such a kind of shelling is a single core/shell type, where the individual nanocubes are shelled by 2D perovskite layers. As an example, a single layer of guanidinium lead halide over 3D perovskites acts as the 2D perovskite shell over 3D perovskite nanocrystals, which has been studied in a few recent reports. This configuration passivates the surface trap states and improves the carrier transportation for the solar cell performance. The benefits of guanidium cations are attributed to their strong hydrogen bonding ability with the surface halides, which in turn facilitates the surface defect recovery.58–61 3D/2D core/shell perovskites are regarded as multidimensional perovskites, where Chen et al. revealed that the multidimensional type-I heterostructure formation between 3D MAPbBr3 and 2D (octylammonium)2PbBr4 leads to multiphoton absorption, which are useful for multiphoton imaging applications.62 In a recent report by Ye et al.,25 spin-polarised luminescence behaviour has been studied, where a chiral organic ammonium bromide molecule is introduced to provide a 2D perovskite layer coating, which formed a 3D/2D core/shell with a type-II heterostructure. In this study, the chiral shell acts as a spin-filter to select the spin to inject into the achiral core, which eventually imparts the spin polarised properties to the achiral MAPbBr3 core. This effect is finally applied to the room temperature spin-LED application to obtain spin polarised electroluminescence. Fig. 3a provides the schematic of such an achiral MAPbBr3 core/chiral 2D organic ammonium bromide shell (upper panel), and the TEM image (lower panel) of MAPbBr3/(S/R)-(+/−)-α-methylbenzenemethanammonium lead bromide (S/RPEA2PbBr4) core/shell. A thin layer of S/RPEA2PbBr4 2D perovskite surrounding the achiral MAPbBr3 core is clearly observed from the TEM image (Fig. 3b), where the thin layer controls the spin-selective injection into the core MAPbBr3.
(iii) 3D/0D core/shell heterostructures consist of a 3D perovskite core and zero-dimensional (0D) perovskite nanoparticles on the surface. 3D/0D core/shell perovskites refer to the epitaxial shelling of a 0D perovskite onto a 3D perovskite. Although FAPbBr3 (with a bandgap of 2.18 eV) has been found to be a superior ultrapure green emitter compared to CsPbBr3 (with a bandgap of 2.25 eV), the hybrid organic-inorganic FAPbBr3 is much less stable than the all-inorganic CsPbBr3. However, by shelling the FAPbBr3 core with 0D Cs4PbBr6 perovskite, a recent study by Zeng et al.21 has demonstrated high stability with improved emission. The presented heterojunction is of the type-I category, wherein the core material has a bandgap of approximately 2.18 eV and the shell material has a bandgap of around 4.0 eV. Fig. 3f depicts the atomic model of the FAPbBr3 core and the sequential growth of Cs4PbBr6 through the creation of a mixed alloy transition layer of FAxCs1−xPbBr3. The shelling process involves the epitaxial growth of Cs4PbBr6 over FAPbBr3 nanocubes, with Cs+ ions partially penetrating the FAPbBr3 lattice and reducing the lattice mismatch. The shelling is carried out seamlessly through the FA1−xCsxPbBr3 transition layer. The TEM images of FAPbBr3 core-only and FAPbBr3/Cs4PbBr6 core/shell nanocrystals are provided in Fig. 3g and h. Xu et al.51 reported multiple core/shell perovskite nanocrystals embedded in another perovskite matrix useful for LED applications. They have prepared a green-emitting LED based on a CsPbBr3/Cs4PbBr6 nanostructure using a solution growth thin film deposition technique.
Despite the success in the synthesis of core/shell perovskite nanocrystals, limitations can be encountered during the synthesis, where the first and foremost is the mixed phase formation instead of the core/shell. Despite the benefits of shelling upon the van der Waals interaction for 2D perovskites, their morphology is typically large sheets and it is hard to control the length of the sheets, which might be challenging for shelling over a 3D cubic shaped perovskite material, since it is very hard to grow a crystal over the cubic-shaped nanocrystals. The nanocubes have six planes and twelve edges, which makes it quite challenging to shell them with a different material, due to the limited formation potential caused by the restricted degree of freedom. In this context, Xu et al. designed the morphology of the MAPbBr3 core material as spherical in MAPbBr3/S/RPEA2PbBr4 core/shell perovskite. It is anticipated that the shelling of the 2D material has less formation potential. In some reports, the challenges are overcome to have shelling over nanocubes, and among those reports, the synthesis of perovskite/perovskite Cs1−xFAxPbBryI3−y core/shell nanocrystals is carried out following a modified hot-injection method, where FABr in isopropyl alcohol and the protective alkylammonium iodide ligand in isopropyl alcohol was injected sequentially into colloidally dissolved lead iodide solution in the presence of oleic acid and oleylamine at 140 °C and 150 °C, respectively. In the following step, Cs-oleate was injected at 200 °C, and the reaction was quenched by rapid cooling in an ice bath. In this example, isopropyl alcohol helps to dissolve organic cations and acts as an antisolvent to crystallize hybrid core/shell perovskite nanocrystals.57
In this regard, the synthesis challenges of core/shell perovskite/perovskite nanocrystals irrespective of the dimensions of the perovskites have been solved partially; however, some fundamental concepts are still in the embryo stage to elaborate. Table 1 depicts the synthesis challenges and their possible outcome:
Limitations | Possible outcome |
---|---|
•The concept of epitaxy formation between iso- or multi-dimensional perovskites still needs to be disclosed properly, which is necessary to design wide varieties of perovskite core/shelling | • In the case of the 3D/2D system, the van der Waals epitaxy forms between the organic spacer of 2D perovskite with metal halide octahedra; therefore, the design of the organic molecule (spacer cation) with a suitable functional group for better interaction |
• In the case of the 3D/3D and 3D/0D systems, it is important to understand the lattice constant of both the materials | |
•Two-step preparation method sometimes leads to the further abrupt growth of core materials alongside the growth of shelling materials | • Improved purification of the core materials is very important, since the unreacted precursors can react further to promote the control/abrupt growth |
• Reaction parameters such as temperature and reaction time need to be considered for the growth of shelling materials | |
•Uncontrolled growth of the shelling material | • Dropwise addition of the shelling material as a function of time instead of one-time addition might help to prevent the uncontrolled growth of shelling materials |
• Shelling must be done at considerably lower temperature to prevent a faster growth |
Fig. 4 (a) EL spectra and (b) EQE of ultrabright green-LEDs using PeNCs:NiOx composites as active layers. Reproduced under the terms of the CC-BY license.71 Copyright 2023, The Authors. American Chemical Society. (c) Typical PL spectra of individual CsPbBr3 (sea-green curve), CsPbMnX3@SiO2 (pink curve), and nanocrystal combination (blue curve) under UV illumination. The inset of Fig. 4c shows the white-light emission produced by the CsPbBr3+ CsPbMnX3@SiO2 PeNC mixture. (d) CRI and CCT of a WLED device fabricated from the nanocrystal combination as a function of diverse driving currents during device operation. Reproduced with permission.73 Copyright 2019, John Wiley & Sons, Inc. (e) Fabricated LCD display with its corresponding (f) PL spectrum and (g) estimated CIE color coordinates for white color emission produced by the LCD prototype (black line) compared with the NTSC standard color gamut area of CIE1931 (dashed line). Reproduced under the terms of the CC-BY license.74 Copyright 2022, The Authors. John Wiley & Sons, Inc. |
Gonzalez-Pedro et al. showed the positive passivation effect of the shelling of CsPbX3 cores with amino-functionalized silanes on the PLQY.72 On the other hand, Leng and coworkers73 have reported the preparation of ultrathin core/shell structured CsPbMnX3 PeNCs@SiO2 composites (X = Cl, Br) through a reverse microemulsion method under room conditions. In this synthetic route, the addition of trioctylphosphine as a multibranched capping agent is pivotal to avoid the structural deterioration of the perovskite during the hydrolysis of silane species, also hindering the direct interaction between the nanocrystals and water molecules formed from the hydrolysis process. In this context, Cs(Pb/Mn)X3 PENCSs@SiO2 composites show an enhanced resistance to heat, with a PLQY of ∼50.5%. Furthermore, this material does not exhibit any anion-exchange with other perovskite nanoparticles such as CsPbBr3, maintaining its initial PL features. The mixture produced between the green- and stable yellow-light emissions from above Br- and Mn-PeNCs respectively can produce white-light emission without any signal of spectral shift, see Fig. 4c, combined with a blue on-chip LED device, generating white-LEDs (WLEDs) with an improved luminous efficiency of ∼68.4 lmW−1, correlated color temperature (CCT) ranging from 3857 K to 5934 K and a high color-rendering index (CRI) of ∼90%, see Fig. 4d. This fact demonstrates the viability to fabricate white LEDs (WLEDs) with high color purity and suitable performance.
Dirin and coworkers75 have developed the synthesis of colloidal CsPbX3 PeNCs covered with metal oxide gel coatings by introducing non-hydrolytic sol–gel reactions. Here, alumina-, zirconia- and titania-based gel coatings were prepared and studied to improve the stability of PeNCs in polar environments. By reacting the corresponding metal halide MX3 (M = Al, Zr, Ti; X = Cl, Br, and I) and metal alkoxide M(OR)3 (R = t-Bu), the subsequent addition of the PeNC solution and the precipitation step with acetone, coated nanoparticles with a charged surface were obtained, being dispersible in water and some alcohols. At this stage, metal oxide gel coatings have replaced organic capping ligands from the material surface. While the crude gel-coated NCs contain excess gel, which can be washed away by precipitating them in a nonpolar to less-polar solvent (e.g., diethyl ether or hexane), after several rounds of washing (up to 5), thin metal oxide gel coatings ∼1 nm are achieved. However, some differences in the stability of the coatings influence the final PL properties of the PeNCs. The presence of an Al coating on PeNCs generates nanoparticles for any halide, with a full width and half maximum (FWHM) of ∼20 nm and PLQY values up to 90%. In contrast, the Zr- and Ti-coatings are not efficient compared to their Al analogues, hindering the removal of part of Zr-coverage during the purification step (causing a decrease in PLQY to obtain values between 40 and 50%), or the complete quenching of the PL features in the presence of Ti-coverage. In this scenario, a titania layer can act as an electron scavenger, which in contrast could be beneficial for photocatalytic systems. Accordingly, some limitations emerge which hinder the synthesis of high-quality PeNC based composites such as the partial removal of the inorganic shell during the purification step, the reactivity of the shell itself or the use of precursors reactive towards the nanoparticle core, and those inorganic species that incorporate carrier traps into the perovskite, decreasing the optical performance in the final composite. Hence, the choice of a convenient metal oxide layer would be pivotal to establish which PeNC dispersions are suitable to enhance the optical performance of LED devices.
Interestingly, Manna and coworkers76 have performed the synthesis of CsPbBr3 PeNCs covered with a mesoporous SiO2 layer (m-SiO2), in the presence of stoichiometric contents of molten salts based on NaNO3, KNO3 and KBr salts, under room conditions, at 320–350 °C. Under these conditions, PeNCs@m-SiO2 composites are obtained. These materials show a PLQY of ∼89 ± 10%, stable PL performance up to 3 h at 180 °C, in saline water at 90 °C up to 24 h, even resisting into an aqua regia (strong acidic environment) system for at least 30 days. The main ions such as K+ and Br− from the molten salts favor the compensation of surface defects in the nanocrystal. Accordingly, these materials were used as green-color converters into a prototype display, where the composite embedded into a polymer film was mounted into a customized 7-in. LCD system with a magenta LED backlight (composed of a commercial blue-LED and a red-emitting K2SiF6:Mn4+), see Fig. 4e. The final device produces a white-light emission, see Fig. 4f with CIE color coordinates of (0.3067, 0.3271), and a correlated color temperature of 6861 K, covering 92% of the NTSC standard color gamut area of CIE1931, see Fig. 4g. This color gamut is higher compared to that offered by a commercial phosphor LCD display, for instance, that contained in a Dell XPS 15 7590 laptop. This contribution highlights that high-quality PeNC composites can be used in real systems, focusing on future device commercialization.
This strategy is also adopted in the case of PbS QD embedded perovskite matrices. Through a ligand exchange process, native oleates anchored to PbS QDs are replaced by a perovskite ligand (for instance, FAPbI3 ligand), see Fig. 5a, and the polarity of the ligand itself makes them compatible with the polar solvent in which the perovskite is soluble. The perovskite matrix in the presence of the QDs will show different structural and optical properties.79,80 It has been demonstrated that after annealing, a monolayer of the perovskite can be formed around the PbS QDs, and this guarantees passivation of the PbS structural defects, shielding them from degradation upon exposure to ambient air for a prolonged time, see Fig. 5b and b′, and enhancing their optical and electronic properties. Pb-halide passivation leads to PbS QDs/perovskite with optical properties that are well-described by a theoretical model, based on a type I finite-barrier heterostructure between the PbS QD and the perovskite matrix. Ning et al. have successfully synthesized polyheterocrystalline solid multiple core/shell PbS/MAPbI3 NCs, characterized by an epitaxial junction. Due to the type I junction commented above, the quantum confined infrared emission coming from the PbS QD core fulfils the criteria for advanced LED device performances.81 Finally, these PbS/perovskite systems are good candidates for short-wave infrared (SWIR) photodetectors. Yin et al.82 fabricated photodiodes with an EQE enhancement of 2.5 times compared with control devices, reaching an internal quantum efficiency of over 90%. On the other hand, a large area application has been reported, in which the judicious choice of the solvent (specifically 2,6-difluoropyridine, DFP) led to the preparation of stable PbS QD colloidal solutions in the presence of the MAPbI3 (MAPI) ligand (Fig. 5c). To provide a wide analysis of the photophysical properties of PbS QDs, nanoparticles were grown with a particle size, from 2.7 to 3.7 nm diameter, corresponding to excitonic absorption peaks from 855 to 1070 nm.83 After the ligand exchange process, these absorption edges/PL peak positions were redshifted from 892 to 1127 nm, see Fig. 5d, associated with the reduction of the quantum confinement effect by the action of the perovskite ligand. This feature is pivotal to prepare IR absorption layers through blade-coating deposition and improve the performance of PbS QD-based solar cells achieving photoconversion efficiencies of ∼8.7% (Fig. 5e).83 Considering that an improved carrier mobility is favored into the IR-active layer, this feature could be pivotal to promote the carrier injection into the MAPI-capped PbS QDs and fabricate promising IR-LEDs with high optical performance.
Fig. 5 (a) Photographs of the ligand exchange process between oleate-capped PbS QDs dispersed in octane and FAPbI3 (FAPI) precursors: (i) before the interaction with PbI2 + FAI in DMF and (ii) after the ligand exchange to produce FAPI-capped PbS QDs dispersed in DMF. (b) Typical XRD patterns of the FAPI films in the presence of different concentrations of PbS QDs at (b) 0 day and (b′) after 30 days of aging under ambient air. Reproduced from ref. 80 with permission, Copyright © 2020, American Chemical Society. (c) Schematic illustration of the stabilization of MAPbI3-capped PbS QDs in the presence of DFP. (d) Optical properties of PbS QDs before (dashed lines) and after (solid lines) ligand exchange in the presence of DFP. (e) J–V characteristics of the record PbS QD based solar cell by using MAPI-capped PbS QD active layers. The inset of (e) exhibits the respective structure of the solar cell. Reproduced under the terms of the CC-BY-NC-ND license from ref. 83, Copyright © 2021, The Authors, American Chemical Society. |
An important aspect regarding the preparation of LEDs is that by tuning the distance between adjacent PbS quantum dots, it is possible to improve the LED efficiencies84 to achieve the optimum balance between charge injection and radiative exciton recombination.84 Moreover, by varying the size of the quantum dots, the emission wavelengths between 800 and 1850 nm can be tuned.
To summarize the positive effects of using core/shell PeNCs for fabricating efficient and stable LEDs, we can mention that the introduction of organic/inorganic shelling and the formation of different types of heterostructures promote the surface passivation of the perovskites. Mainly, the density of halide vacancies and non-coordinated Pb (and thereby the content of carrier traps) is suppressed, favoring the emergence of high-quality PeNC emitting layers with improved radiative recombination and high PLQY. In this context, the higher the PLQY, more efficient electrons-to-light conversion will be reached by the PeNC-based LEDs, enhancing their performance. Moreover, core/shell PeNCs can exhibit a longer PL lifetime compared to pristine PeNCs, and the composition of the core and shell materials can be modulated, so that their incorporation as emitting layers can produce multicolor LED devices with a brighter, more stable and consistent light emission during their operation. Simultaneously, the coverage of PeNCs with organic/inorganic shells avoids the direct interaction or permeation of oxygen or polar molecules such as water or alcohols, allows PeNC processing under harsh conditions such as acids, saline water, and high temperatures, and hinders the anion-exchange between different kinds of halide perovskites. These facts extend the long-term stability of luminescent materials. Therefore, the fabrication of efficient LEDs with EQE values close to 30%, with high luminance/current parameters, and the introduction of stable PeNCs as color converters into the real prototypes of LCD systems are possible, mediating the generation of white light emission with enhanced color quality and CRI. In this scenario and considering the above benefits of the core/shell PeNCs, the PeNC-based LEDs are considered as promising energy-efficient and rapidly developing lighting technologies for future commercialization.
In perspective, the development of new characterization techniques based on Transmission Electron Microscopy (TEM) also opened wide perspectives toward the understanding of the core–shell structure. Four-dimensional scanning TEM (4D-STEM) is an emerging technology to extract not only the intensity signal but also the phase information of the specimen.88 Moreover, cation inversion at the interface in core–shell nanoparticles has been investigated recently by performing electron energy-loss spectroscopy (EELS) measurements with atomic resolution to map the ions in the shell and in the core structure.89 In addition, by X-ray absorption spectroscopy (XAS) for determining the local geometric and/or electronic structure, the chemical composition and distribution of atoms in the crystal structure would be accurately determined.
In addition, the engineering of the LED device is straightforward and adheres to a traditional device architecture, which employs an inverted structure. In such a traditional configuration, a hole transport layer (PEDOT:PSS) is deposited, followed by a blocking layer (Poly-TPD), on top of an ITO-coated glass substrate. Next, the active layer consisting of core/shell perovskite NCs is deposited. On top of the active layer, an electron transport layer (PO-T2T or TPBi) is applied, followed by a buffer layer of LiF, and finally, aluminum is deposited.
Furthermore, the core/shell system offers the possibility to change the fundamental properties of matter by changing the NC size and the shell thickness, thus making it strongly coupled to light.92 Potentially, systems consisting of emitters separately surrounded by an inorganic shell look rather promising for polariton LEDs,93 and in turn for developing high speed LEDs for communication systems. The potential of strong coupling to control light–matter interactions is far ranging with application in quantum information, polaritonic chemistry, lasing and even the development of novel coherent light sources.94–96 Recent advances in heterostructures based on core/shell systems give ground to believe that the shell capping layer could indeed be used in many practical devices by exploiting the strong coupling effect.97
This journal is © The Royal Society of Chemistry 2023 |