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Perspective

Toward High-Performances of Halide Light-Emitting Diodes: The Importance of Ligands Engineering

by
Le Ma
and
Feijiu Wang
*
Henan Key Laboratory of Photovoltaic Materials, Henan University, 1 Jinming Road, Kaifeng 475004, China
*
Author to whom correspondence should be addressed.
Inorganics 2023, 11(6), 230; https://doi.org/10.3390/inorganics11060230
Submission received: 12 April 2023 / Revised: 4 May 2023 / Accepted: 8 May 2023 / Published: 26 May 2023
(This article belongs to the Special Issue Recent Advances in Perovskite Optoelectronics: From Materials to Devices)
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Abstract

:
Halide perovskite light-emitting diodes (PeLEDs) have attracted great attention because of their superior optical properties, such as extremely high photoluminescence (quantum yield up to nearly 100%) of active layers with tunable wavelengths over the entire visible spectral range. With a suitable modification of halide perovskites, carrier transport materials, and their interfaces, external quantum efficiencies exceeding 10%, 25%, and 20% have been achieved for blue-colored (465 nm), green-colored (512 nm), and red-colored (640 nm) LEDs, respectively. Many strategies for pursuing high performances of devices have been successfully demonstrated, among which ligand engineering has always played an important role in the active layer. Herein, we present a perspective to illustrate the effects and roles of the ligands in cesium lead bromide light-emitting diodes. This perspective is mainly classified into three parts: (1) ligands for CsPbBr3 LEDs could improve radiative recombination of perovskites and contribute to better efficiency of LEDs; (2) ligands could confine CsPbBr3 growth for blue emission of LEDs; (3) stabilities of materials and devices become better with ligand engineering. Finally, the summary and perspective on PeLEDs are highlighted and possible solutions are provided.

Graphical Abstract

1. Introduction

Organic-inorganic hybrid perovskite (OIHP) LEDs have shown high performance because of their high photoluminescence quantum yield (PLQYs), high carrier mobility, and unique defect-tolerant nature [1,2,3,4,5,6,7,8,9]. However, phase instability of OIHP under moisture, heat, and even oxygen greatly limits their practical application because of the volatility of the organic components in perovskites (such as methylammonium (MA), formamidinium (FA), or mixed components) and the weak chemical bonding energies between halide anions (Cl−, Br−, or I−) and metal cations (Pb2+) [10,11,12,13]. Despite many strategies for improving stability, such as surface modification with stable materials [14,15], advanced encapsulation techniques [16,17,18,19], and compositional engineering [20,21,22], the intrinsic instability of hybrid perovskite materials is still a pending issue. CsPbX3 (X = Cl−, Br−, or I−), a type of all-inorganic perovskite, presents better stability than OIHP and possesses effective luminescence emission [17,23]. CsPbBr3 has been extensively investigated in light-emitting-diode field because it could possess both green and blue emission by controlling the quantum confinement [24,25,26,27,28].
CsPbBr3 with a chemical formula of ABX3 has an octahedron structure, in which Pb and Br atoms form a corner-sharing [PbBr6]4− 3-dimensional (3D) framework and Cs atoms occupy the octahedral voids (Figure 1a) [29,30]. The structural stability and distortion of CsPbBr3 could be predicated by the Goldschmidt tolerance factor (τ), which is defined as τ = ( R C s + R P b ) / 2 ( R C s + R B r ) , where R is the ionic radius. Generally, the cubic phase structure with the τ between 0.9 to 1 is considered the most stable structure, and CsPbBr3 shows 0.92 of the value [31,32]. Additionally, CsPbBr3 has three different structural phases, which are called the cubic (α-, Pm-3m), tetragonal (β-, P4/mbm), and orthorhombic (γ-, Pnma) phase, respectively [33,34]. It is an γ-phase at room temperature, and its phase transitions happen when the temperature increases to 88 (β-phase) and 130 °C (α-phase) [35,36]. Luckily, the three phases of CsPbBr3 have similar properties, which support a wide temperature operation range for the perovskite. CsPbBr3 has been fundamentally investigated for its physics properties, such as absorption coefficient, carrier diffusion length, and carrier mobility [37,38,39]. For detailed information, many theoretical studies on the CsPbBr3 were conducted, from which the band gap, absorption coefficient, carrier effective masses, and exciton binding energy could be extracted using the density functional theory (DFT) methods. A single crystal of the CsPbBr3 exhibiting an absorbance coefficient of 105 cm−1, a carrier diffusion length of 10 µm, and a carrier mobility of >100 cm2 V−1 S−1 was reported [40,41], indicating its superiorities and potential for the optoelectronic devices such as LEDs. Note that electron and hole mobility in CsPbBr3 are similar, implying that better carrier transport balance could be realized in the applications. Additionally, solution-processable techniques for the nanocrystal, nanoplatelet, quasi-two-dimensional (quasi-2D) layer, and 3D polycrystal of perovskite film formation have been applied in high brightness and EQE LEDs. With extensive and great developments, green CsPbBr3 LEDs present very high performance with external quantum efficiencies (EQEs) exceeding 20% and outstanding brightness, which are comparable to organic and CdSe-based LEDs (Figure 1b) [42,43,44,45,46]. Rapid increases in LED performance have always involved strategies for defect passivation and carrier confinement in the active layer and improving injection/transport in devices [47,48,49]. The ligands, such as didodecyldimethylammonium bromide (DDAB), tetraoctylammonium bromide (TOAB), and n-butylammonium bromide (BABr), play vital roles to realize the above-mentioned strategies in the LEDs by controlling the on-substrate nanocrystal fabrication, forming quasi-2D perovskite, and modifying the perovskite surface [50,51,52].
Here, we discuss strategies for achieving high performance of CsPbBr3 LEDs through the ligand engineer’s effective modification of stability, radiative recombination, and morphology of perovskites. We also overview challenges and possible solutions to realize highly efficient green and blue PeLEDs, including the external quantum efficiency and operational stability of devices.

2. Toward High Radiative Recombination of CsPbBr3

Although the theoretical simulations demonstrated great defect tolerance of MHPs [58,59], the detrimental effect of defects on the perovskite LEDs was extensively reported. Several types of point defects containing vacancies, metallic Pb0, and antisite substitutions were evidenced to be associated with nonradiative trap states (Figure 2a) [60,61]. Therefore, the reduction of trap states in perovskite to minimize trap-assisted nonradiative recombination is of great importance for improving the luminescence of the emitting layer. Ligands, including Lewis bases, ammonium, and halide salts, have been considered one of the most effective strategies for defect elimination because of their extra coordination or ionic bonding to the annihilation of trap states (Figure 2b) [62]. Lewis bases, such as amine and phosphine oxide, present significant passivation effects on perovskites because of their high binding affinity [45,62,63]. In addition, hydroxyl [64,65] and carboxyl [56] were reported to effectively control the CsPbBr3 crystal size, surface coverage, and defect density, resulting in improved performance and stability of LEDs. Moreover, amine-based agents, such as 4-(2-aminoethyl)benzoic acid (ABA), DDAB, diamine-based molecules (Figure 2c), and even PMMA could effectively remove the metallic Pb site because of halide vacancy passivation-assisted suppression of metallic Pb, stripping of lead atoms, and weak hydrogen bonding with organic cations [62,66,67]. Besides the defect-induced nonradiative recombination, Auger recombination is also a nonradiative process, which is dominant at high excitation density [62]. This is one of the critical reasons for the efficiency roll-off with bias voltage increase in LEDs, especially in devices based on quantum-confined perovskite emitting layer because of the Auger recombination occurring at relatively low charge-carrier density (~1015 cm−3, Figure 2d) [68]. For bulk perovskites, the Auger recombination–dominated process usually occurs at high excitation densities of >1017 cm−3 (Figure 2e), which indicates that high-power optoelectronic devices could probably be realized by bulk perovskites. Although quantum-confined perovskites could easily result in Auger recombination, the perovskite nanocrystals (PNCs) present very similar Auger recombination rates with bulk [39], indicating that PNCs are highly possible for high brightness and EQE LEDs. A further understanding of the Auger recombination mechanism is still needed in PNCs. It is worth noting that the ligands for passivation of the trap states, improvement of the crystallization, and even control of crystal growth usually employ short chain length with high binding affinity to perovskites, which not only easily passivates the trap state but also greatly reduces negative effects on the electrical properties for devices.

3. Blue Emission of LEDs

Component engineering of Cl and Br in the CsPbBrxCl3−x and quantum confinement engineering of CsPbBr3 to achieve a suitable bandgap for blue emitting are both effective strategies [69,70,71]. However, a mix of halides always results in phase separation and lattice distortion, which presents a negative effect on LEDs. In addition, Cl vacancies in mixed halide perovskites easily create relatively deeper defect levels in the bandgap, which could capture the carriers and increase the nonradiative recombination [72]. Moreover, the defects, which act as hoping sites to induce the halide ion migration and result in the perovskite being more vulnerable under operational conditions, have also been reported [73]. Compared with Cl vacancies in CsPbBrxCl3-x, the CsPbBr3 exhibits strong “defect-tolerant” nature, which could reduce the related nonradiative recombination [74]. Therefore, the fabrication of quantum-confined CsPbBr3, such as quasi-2D layers, nanoplatelets (NPs), and quantum dots (QDs), has attracted much attention for blue-emission LEDs. For achieving quantum-confined perovskites, ligands-assisted growth is an effective strategy and could fine-tune the size and even shape, resulting in a suitable bandgap for LED applications [75,76]. For example, the use of a cheap and facile solution-processable method to tune the emission of MAPbBr3 from ~2.31 to 2.83 eV was reported by changing the organic ligand and its solute concentrations [77]. Quasi-2D CsPbBr3-based perovskites with the formula of A2(CsPbBr3)n-1PbBr4, where A is an organic ammonium ligand and n is the phase order, have presented a high potential for the blue LED application because of high quantum confinement resulting in enlarging the bandgap from 2.4 to ~3.07 eV (determined by the n value) [78]. Using ligand engineering for the perovskite, the n phase could be controlled for blue emission. For example, mixed ligands of phenylethylammonium (PEA+) and propylamine ion (PA+) in perovskite solution could form PEAxPA2−x(CsPbBr3)n−1PbBr4, which could inhibit the n = 2 phase generation and increase the n = 3 phase formation because of their thermodynamic stability differences (Figure 3a) [79,80]. The authors also introduced PEABr to passivate defects that are generated during perovskite crystallization. The PEABr-passivated PEAxPA2−x(CsPbBr3)n−1PbBr4 exhibits an EQE of 7.51% and a brightness of 1765 cd m−2 and a low turn-on voltage of 3.07 V. Bifunctional ligands of 4-(2-aminoethyl)benzoic acid (ABA) [81] and γ-aminobutyric acid (GABA) [82] not only have a similar role for the quasi-2D phase engineering and passivating but also perform longer operational stability because of the strong interaction between perovskite phase [71]. CsPbBr3 NPs with great quantum confinement effect have been synthesized with uniformly and precisely controllable thickness [83,84,85]. The thickness of CsPbBr3 could be precisely controlled at a monolayer level for the blue LED design and fabrication [86]. However, the nucleation process of CsPbBr3 could induce a large number of surface defects, such as Br vacancies, resulting in low EQE performance of the devices. To remove the surface defects of CsPbBr3, HBr was employed in the perovskite precursor solution to increase Br- and eliminate Br vacancy [84]. Similarly, ligands, such as DDAB, 2,2-(ethylenedioxy) bis(ethylammonium) sulfate (EDBESO4), and MABr [67,87,88], have been also utilized for the surface passivation in the CsPbBr3 NPs-based LEDs for improving their performance. Moreover, ligands for the interfacial engineering and crystal growth controlling were also demonstrated (Figure 3b) [72,89,90], yet the EQE of the pure blue emission has not exceeded 2%. Compared with NPs, QDs-based CsPbBr3 presents great progress and higher performance in the blue-emitting LEDs. C. Bi et al. employed CsPbBr3 quantum dots with 4 nm size for blue PeLEDs fabrication, which exhibit great performance with an EQE of 4.7% with pure-blue emission at 470 nm, a brightness of 3850 cd m−2, and a half-lifetime of 12 h, respectively. The results are attributed to the HBr etching imperfect octahedrons with vacancy defects and removing excess carboxylate ligands from the QDs surface. Auger recombination in the low-dimensional semiconductor easily occurs because of strongly bound excitons. A larger dielectric constant of inorganic ligands, such as ZnBr2 and ZnCl2, was induced in the quantum dot system to reduce the dielectric confinement and suppress Auger recombination [91,92]. ZnBr2 with Br− could also passivate uncoordinated sites and exchange with the initial organic ligands on the QDs surface to improve the charge mobility between adjacent modified QDs [Figure 3c]. The resulted CsPbBr3 QDs-based LED shows a pure-blue emission at 469 nm, low roll-off EQE, high luminance of 12,060 cd m−2, and a high EQE of 10.3% (Figure 3d,e) [91]. Other effective strategies, including in-phase transformation from cubic to rhombic dodecahedron CsPbBr3 QDs, on-substrate fabrication of QDs, and bipolar-shell resurfacing of QDs by ligands for blue LEDs application, have also exhibited great performance [28,48,93], strongly suggesting that CsPbBr3 QDs provides more unique nature for the blue LED developments. Most of the important reports on the use of CsPbBr3 NPL, quasi-2D, and QDs-emitting layers for blue LEDs have been summarized in Table 1.

4. Stabilities of Materials and Devices

Compared with the other organic–inorganic halide perovskites, CsPbBr3 presents good moisture, light, and thermal stability under a wide temperature range. CsPbBr3 exhibiting better light stabilities than organic cation-based perovskites has been reported, and a comparison of the photostability of several typical halide perovskites is presented [95,96], implying that CsPbBr3 could be better for the stabilized-light-involved devices. Akbulatov et al. performed comprehensive research on the photostability of different halide perovskites. The CsPbBr3 film presents a higher degree of photostability without any prominent degradation, while the MAPbBr3 exhibits serious degeneration under continuous illumination for 900 h [97]. Moreover, the CsPbBr3 absorption bands increased even with long-term illumination, which also indicates the great photostability of the perovskite films. A comparison of the photostability of MAPbI3, MAPbBr3, and CsPbBr3 was also conducted, which demonstrates that CsPbBr3 displays superior stability compared with its counterparts. The photostability in the CsPbBr3 QDs was also addressed [98], which indicates that the ligands-modified samples present much-improved photostability because of the elimination of the surface dangling bonds. Note that ligands on the QD surface could be eliminated because of the weak bound energy to QDs and dissolvability in a solvent, which could have a great impact on its stability. This phenomenon implies that the ligands are of great importance for the low dimensional perovskites [31]. In addition, the thermal and humidity stability of CsPbBr3 have also been experimentally addressed, which shows negligible degradation below 350 °C and no evident change under the humidity of ~30% and temperature of ~25 °C [36,99]. The above-mentioned findings imply that Cs in the CsPbBr3 could result in more suitable Goldschmidt tolerance and low formation energy of perovskite systems. Despite good stability of CsPbBr3, its applications in the LEDs still show poor operational lifetime (T50 < 10 min, T50 is the time required for luminance to reach half of the initial value) [82,100]. The improved strategies, such as bipolar-shell QD fabrication, quasi-2D perovskites film formation, and interfacial engineering, have been reported [48,80,81]; however, the results are still far from our expectations (thousands of hours, or even over ten thousand hours). Very recently, the T50 lifetime presented a record value of over 30,000 h for green PLEDs at an initial luminance of 100 cd m−2, which employed benzylphosphonic acid (BPA) additive for modifying three-dimensional polycrystalline perovskite films [101]. Another impressive report on the long operational stability is utilizing in situ solution-grown perovskite single crystals (SCs) for the LEDs, which enables great device performance with a high luminance of 86,000 cd m−2, a peak external quantum efficiency of 11.2%, and a stability value of 12,500 h at an initial luminance of 100 cd m−2, respectively [102]. These results imply that the bulk film with the passivated surface could greatly improve the stability of devices.

5. Perspectives and Summary

We have concluded that the ligand engineering for the achievement of great optical properties of CsPbBr3 improved performances of CsPbBr3-based blue LEDs and their stability. From the surface passivation and interfacial modification to the stabilization and quantum confinement of the perovskites, the ligands have demonstrated the great importance in the LED applications. Great achievements have been attained in CsPbBr3-based LEDs, especially, their high EQE of green-emitting devices. Still, many crucial issues—such as the low efficiency of pure blue LEDs (center of emitting wavelength about 465 nm) and devices’ operation stabilities—exist and need to be further addressed. The selected suitable ligands, which could eliminate the trap states in the perovskites with negligible effect on the carrier injection and radiative recombination, could further improve the EQE of the LEDs. The use of short organic or inorganic ligands, such as trimethylammonium, BPA, and ZnBr2, could meet the requirements. For the stability of devices, most promising strategies should be the use of polycrystal bulk film or even single-crystal film with ligands-passivated surface for the device’s fabrication [101,102], which could not only produce high performance but also reduce the efficiency roll-off because PLQY of single-crystal perovskites could realize nearly 100% at a wide range of excitation density (related to a broad range of bias voltage in LEDs) [62]. A universal strategy has been reported on preparing ultrathin CsPbBr3 and other perovskites (Figure 4) [103], in which CsPbBr3 with a thickness of 9.5 nm is greatly suitable for LED applications. With the technology and development of the perovskites, ligands, and combining the ligands with high-quality perovskites, high performance of perovskite-based LEDs with great EQE, brightness, and stability could be realized in the near future.

Funding

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No.12174086).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Cubic structure of CsPbBr3 perovskite; (b) evolution of external quantum efficiencies (EQEs) of green-LEDs employing CsPbBr3-emitting layer. The EQE data was collected from references [43,44,45,47,48,50,52,53,54,55,56,57].
Figure 1. (a) Cubic structure of CsPbBr3 perovskite; (b) evolution of external quantum efficiencies (EQEs) of green-LEDs employing CsPbBr3-emitting layer. The EQE data was collected from references [43,44,45,47,48,50,52,53,54,55,56,57].
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Figure 2. (a) Point defects in perovskites; (b) categories of passivating ligands for colloidal PNCs; (c) diamine-based molecules for perovskite passivation; related PLQY-excitation density plots with (d) varying trap-assisted nonradiative recombination rate constants in quantum-confined perovskites and (e) varying nonradiative monomolecular recombination coefficients in bulk perovskites. Figures adapted with permission from Springer Nature Ltd.
Figure 2. (a) Point defects in perovskites; (b) categories of passivating ligands for colloidal PNCs; (c) diamine-based molecules for perovskite passivation; related PLQY-excitation density plots with (d) varying trap-assisted nonradiative recombination rate constants in quantum-confined perovskites and (e) varying nonradiative monomolecular recombination coefficients in bulk perovskites. Figures adapted with permission from Springer Nature Ltd.
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Figure 3. Schematic diagram of the effect of PEA2PbBr4 on the quasi-2D perovskite phases (a), C18A/C4 and NH4+/C18A/C4 for CsPbBr3 nanoplatelets (b), and ZnBr2 for CsPbBr3 QD modification (c). Current density, luminance (d), and EQE (e) of the CsPbBr3 LEDs using ZnBr2-modified QDs. Copyright from American Chemical Society.
Figure 3. Schematic diagram of the effect of PEA2PbBr4 on the quasi-2D perovskite phases (a), C18A/C4 and NH4+/C18A/C4 for CsPbBr3 nanoplatelets (b), and ZnBr2 for CsPbBr3 QD modification (c). Current density, luminance (d), and EQE (e) of the CsPbBr3 LEDs using ZnBr2-modified QDs. Copyright from American Chemical Society.
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Figure 4. AFM topography images of the fabricated ultrathin perovskites by a universal strategy. Scale bar: 2 µm (A = organic/inorganic cation, B = metal cation and X = halide ion). Copyright from Wiley-VCH.
Figure 4. AFM topography images of the fabricated ultrathin perovskites by a universal strategy. Scale bar: 2 µm (A = organic/inorganic cation, B = metal cation and X = halide ion). Copyright from Wiley-VCH.
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Table
Table 1. Summary of main blue LEDs based on the CsPbBr3 NPL, quasi-2D, and QDs emitting layer.
Table 1. Summary of main blue LEDs based on the CsPbBr3 NPL, quasi-2D, and QDs emitting layer.
YearEmitting LayerEQE
(%)		Brightness
(cd m−2)		EL Peak
(nm)		Stability
(min)		Ref.
2018HBr-treated CsPbBr3 NPL0.12462463-[94]
2019poly(triarylamine) modified CsPbBr3 NPL0.3-464-[89]
2019DDAB-treated CsPbBr3 NPL1.4241.84690.7[67]
2021PEI-modified CsPbBr3 NPL0.8631465-[72]
2022SA-modified CsPbBr3 NPL3.1881.84606.2[88]
2022NH4Br- and PEABr-modified CsPbBr3 NPL274463-[90]
2022EDBeSO4-modified CsPbBr3 NPL1.7769146220[87]
2019PA2(CsPb Br3)n−1PbBr41.455735487220 at 150 cd m−2[80]
2020PEAxPA2−x(CsPbBr3)n−1PbBr47.51176548866[79]
2020GABA-treated PEA2(CsPbBr3)n−1PbBr46.32004782.5 at 200 cd m−2[82]
2020ABA2PbBr4-modified PEAxPA2−x(CsPbBr3)n−1PbBr411.151348681.3[81]
2020Bipolar-shell-protected 4 nm CsPbBr3 QDs12.3~45047920 at 90 cd m−2[48]
2021DDDAM- and PEA-treated 4 nm CsPbBr3 QDs4.73850470720[73]
2022ZnBr2-treated 4 nm CsPbBr3 QDs10.3120604691500 at 115 cd m−2[91]
2022Hydrobromide-treated CsPbBr3 QDs6.6280.84801.83 at 80 cd m−2[28]
2022Br-MBA+-treated CsPbBr3 quantum dots17.9%~2500480120 at 100 cd m−2[93]
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Ma, L.; Wang, F. Toward High-Performances of Halide Light-Emitting Diodes: The Importance of Ligands Engineering. Inorganics 2023, 11, 230. https://doi.org/10.3390/inorganics11060230

AMA Style

Ma L, Wang F. Toward High-Performances of Halide Light-Emitting Diodes: The Importance of Ligands Engineering. Inorganics. 2023; 11(6):230. https://doi.org/10.3390/inorganics11060230

Chicago/Turabian Style

Ma, Le, and Feijiu Wang. 2023. "Toward High-Performances of Halide Light-Emitting Diodes: The Importance of Ligands Engineering" Inorganics 11, no. 6: 230. https://doi.org/10.3390/inorganics11060230

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