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Research Article | Open Access

Volume 2021 |Article ID 9829374 | https://doi.org/10.34133/2021/9829374

Wei Shen, Jianbin Zhang, Ruimin Dong, Yanfeng Chen, Liu Yang, Shuo Chen, Zhan Su, Yujun Dai, Kun Cao, Lihui Liu, Shufen Chen, Wei Huang, "Stable and Efficient Red Perovskite Light-Emitting Diodes Based on Ca2+-Doped CsPbI3 Nanocrystals", Research, vol. 2021, Article ID 9829374, 11 pages, 2021. https://doi.org/10.34133/2021/9829374

Stable and Efficient Red Perovskite Light-Emitting Diodes Based on Ca2+-Doped CsPbI3 Nanocrystals

Received17 Aug 2021
Accepted15 Nov 2021
Published06 Dec 2021

Abstract

α-CsPbI3 nanocrystals (NCs) with poor stability prevent their wide applications in optoelectronic fields. Ca2+ (1.00 Å) as a new B-site doping ion can successfully boost CsPbI3 NC performance with both improved phase stability and optoelectronic properties. With a Ca2+/Pb2+ ratio of 0.40%, both phase and photoluminescence (PL) stability could be greatly enhanced. Facilitated by increased tolerance factor, the cubic phase of its solid film could be maintained after 58 days in ambient condition or 4 h accelerated aging process at 120°C. The PL stability of its solution could be preserved to 83% after 147 days in ambient condition. Even using UV light to accelerate aging, the T50 of PL could boost 1.8-folds as compared to CsPbI3 NCs. Because Ca2+ doping can dramatically decrease defect densities of films and reduce hole injection barriers, the red light-emitting diodes (LEDs) exhibited about triple enhancement for maximum the external quantum efficiency (EQE) up to 7.8% and 2.2 times enhancement for half-lifetime of LED up to 85 min. We believe it is promising to further explore high-quality CsPbI3 NC LEDs via a Ca2+-doping strategy.

1. Introduction

All-inorganic perovskite (CsPbX3, , Br, I) nanocrystals (NCs) have the potential to promote the development of the luminescence and display industry due to their high photoluminescence quantum yields (PLQYs), high color purity, and solution processability [111]. Currently, green, red, and near-infrared perovskite light-emitting diodes (LEDs) have met the need of commercial demands from the efficiency factor () [2, 12], whereas CsPbX3 NC-based LEDs are still underdeveloped. It should be noted that the high-performance optoelectronic properties of CsPbX3 NCs are dominated by their crystal phase, especially for CsPbI3 [1317]. Generally, cubic (α) CsPbI3 (direct band gap, ) exhibits good optoelectronic performance. However, the tolerance factor (τ) of α-CsPbI3 is too small to stabilize its cubic phase at room temperature. The metastable state of α-CsPbI3 can be easily transformed into the orthorhombic (δ) phase () with poor optoelectronic performance [3, 1821]. Therefore, improving phase stability of α-CsPbI3 is the foundation for achieving high-performance CsPbI3 NC LEDs [22].

According to the Goldschmidt tolerance factor (τ) function for perovskite (ABX3), doping ions with suitable size can realize precise tuning of τ to stabilize cubic phase [23, 24]. Based on this principle, A-, B-, or X-site doping can maintain α-CsPbI3. It should be noted that common methods to synthesize CsPbI3 NCs need high temperature (>170°C) [6]. As a result, organic A-site ions (methylamine or formamidine) will be easily decomposed at high temperature [24, 25]. Currently, B- or X-site doping is the hotspot for stabilization of α-CsPbI3. Due to the ionic nature of CsPbI3, X-site ions are at the corner of the PbI64− octahedron, which can easily migrate. With Br- doping as an example, though CsPbBrxI3-x NCs exhibit a cubic phase, the ion migration of Br- and I- easily leads to phase separation and degradation of optoelectronic performance [26], whereas B-site ions are at the center of the PbI64− octahedron, which can hardly migrate. Therefore, B-site doping is the efficient way to stabilize α-CsPbI3 without any side effects, such as thermal decomposition, or phase separation [2730]. On the basis of the Goldschmidt tolerance factor function, doping with small-sized B-site ions can efficiently stabilize α-CsPbI3 (Pb2+ radius 1.19 Å). For example, Mn2+ (0.67 Å) doping by direct synthesis or posttreatment methods can improve the stability of α-CsPbI3 NCs from a few days to nearly a month under ambient conditions [31, 32]. In addition, B-site doping can not only enhance the stability of CsPbI3 NCs but also boost their optoelectronic performance. Several B-site doping ions have been studied to promote the development of CsPbI3 NC LEDs, such as Mn2+ [33], Zn2+ [34, 35], Zr2+ [36], Y3+ [35], Cu2+ [37], Ni2+ [38], and Sr2+ [3941]. It should be emphasized that some doping ions, such as Zn2+, Cu2+, and Sr2+, can reduce charge injection barriers and enhance carrier transport properties. To date, the state of the art for CsPbI3 NC LEDs is facilitated by alkaline earth metal ion doping such as Sr2+ doping [3941], which has the potential to satisfy the commercial demands [23]. It should be noted that the size of Sr2+ (1.18 Å) is almost the same as Pb2+ (1.19 Å). In other words, both stability and optoelectronic performance have more room to improve by doping other small size alkaline earth metal ions, such as Ca2+ (1.00 Å).

Herein, we explored Ca2+ as a new B-site doping ion to boost CsPbI3 NC performance with both improved phase stability and optoelectronic properties. Doping Ca2+ to partly replace Pb2+ must increase τ. Therefore, Ca2+ doping can dramatically improve the phase stability of CsPbI3 NCs, which suppresses the decreasing PLQY resulting from phase transition [33, 34, 40, 42]. Systematical studies on stability were done by tuning Ca2+/Pb2+ ratios (0%, 0.35%, 0.40%, and 1.20%). For the case of 0.40% Ca2+/Pb2+ ratios, Ca2+-doped CsPbI3 NCs showed enhanced phase stability to long-term storage and heat. Its solid film could exhibit the cubic phase after 58-day storage in ambient condition or 4 h accelerated aging process at 120°C. Additionally, its PL intensity in solution decayed less than 17% after 147-day storage in ambient condition. Even using UV light to accelerate aging, the half-lifetime of PL could boost 1.8-folds as compared to that of CsPbI3 NCs. Furthermore, the Ca2+-doped CsPbI3 NCs were employed as the emission layer to fabricate LED. Benefitting from the Ca2+ doping, the defect densities were decreased to 21%, its valence band maximum went up to -5.55 eV to reduce hole injection barriers, and its lower Fermi level enhanced hole transport efficiency. As a result, Ca2+-doped CsPbI3 NC-based LED exhibited about triple enhancement for maximum EQE up to 7.8%, and 2.2 times enhancement for half-lifetime of LED up to 85 min. We believe it is promising to further explore high-quality CsPbI3 NC LED via Ca2+-doping strategy.

2. Result and Discussion

Ca2+-doped CsPbI3 NCs were synthesized by using different feed ratios of Ca(Ac)2/PbI2 (0%, 15%, 25%, and 35%) via hot injection method (Experimental Section). As shown in Table S1, the real Ca2+/Pb2+ ratios for Ca2+-doped CsPbI3 NCs were determined by ICP-MS. According to ICP-MS results, the real Ca2+/Pb2+ ratios were 0%, 0.35%, 0.40%, and 1.20% for the condition of 0%, 15%, 25%, and 35% feed ratios of Ca(Ac)2/PbI2. Such results implied that Ca2+ was successfully doped into CsPbI3 NCs. The Pb2+ is located in the center of the octahedron (PbI64-). The exchange and migration of the B-site ion is more difficult as compared to that of A-site and X-site ions. Therefore, B-site ions do not easily incorporate into the CsPbI3 lattice, and the actual doping ratio of the B-site ion is much lower than the feed ratio [18, 34, 38, 40, 43]. The size of Ca2+ (1.00 Å) is smaller than Pb2+ (1.19 Å), and Ca2+ doping must induce the lattice contraction, which can be confirmed by XRD. Figure 1(a) shows XRD patterns for Ca2+-doped CsPbI3 NC films with different real Ca2+/Pb2+ ratios. The patterns for all films match well with α-CsPbI3 (PDF#97-018-1288), and the diffraction peaks at 14.09° and 28.58° correspond to the (100) and (200) planes, respectively [34]. It should be emphasized that the splitting peaks at around 14° may be related to γ-CsPbI3 [44, 45], while the characterization patterns for γ-CsPbI3 from 25-30° cannot be observed. It is possible that a slight lattice distortion occurred due to the influence of water and oxygen during the test [20, 46]. With increasing Ca2+/Pb2+ ratio, no additional diffraction peaks were observed, which indicated that doping Ca2+ did not significantly change the CsPbI3 crystal phase. Their fine XRD patterns are shown in Figure 1(b). There is a slight shift toward a higher diffraction angle for the α-CsPbI3 (200) plane (28.62° for 0%, 28.77° for 0.35%, 28.78° for 0.40%, and 28.82° for 1.20%), which verifies the lattice contraction resulted from the partial replacement of Pb2+ (1.19 Å). An enhanced peak at 20° is attributed to the (110) crystal plane for CsPbI3. Based on the HRTEM images, the cubic NCs become truncated cubic NCs (Figure S1a-d). Therefore, (110) crystal planes are more exposed and enhance the peak at 20° [47, 48]. Furthermore, the morphology and size characterization for Ca2+-doped CsPbI3 NCs were studied by TEM. As shown in Figures 1(c)–1(f), with increasing Ca2+/Pb2+ ratio, Ca2+-doped CsPbI3 NCs maintain a cubic shape with uniform size distribution. The average size decreased from , , , and 9.89 ± 1.07 nm (Figure S2), which is mainly because Ca2+ doping influences lattice contraction or nucleation and growth processes. HRTEM images (Figures 1(g)–1(j)) reveal that Ca2+-doped CsPbI3 NCs are highly crystalline with displayed lattice fringes. The lattice distances of all samples are 0.31 nm corresponding to the (200) plane of α-CsPbI3 [49]. This is due to the limited TEM accuracy, which makes it difficult to distinguish the difference of 0.001 nm. According to the XRD test results and Scherer’s formula, the lattice distances of (200) are 0.311 nm (0%), 0.310 nm (0.8%), 0.310 nm (3.1%), and 0.309 nm (5.0%). Therefore, only XRD results can confirm that Ca2+ is successfully doped into CsPbI3 NCs.

Furthermore, the elemental mapping images were measured by EDS (Figure S3a-d). We chose the elemental mapping area of Ca2+-doped CsPbI3 NCs in a high-angle annular dark field scanning transmission electron microscopy image (HAADF-STEM) (Figure 2(a)). Elemental mapping images of Cs, Pb, Ca, and I (Figures 2(b)–2(e)) can be observed clearly. We merged these elemental mapping images to obtain the overlapped image (Figure 2(f)), which shows that the positions of Cs, Pb, Ca, and I are uniformly distributed in NCs. Therefore, these results can directly demonstrate that Ca2+ is doped in CsPbI3 NCs.

The optical properties of Ca2+-doped CsPbI3 NCs are shown in Figure 3. The UV-vis absorbance and PL spectra of Ca2+-doped CsPbI3 NCs are compared in Figure 3(a) and Figure S4a-b. With the ratio of Ca/Pb increasing, both of their UV-vis absorption and PL peaks appear as blue shifts due to their lattice contraction, size decrease, and Ca2+ orbitals to influence the electronic structure of NCs [34, 40, 50, 51] (Figure S4a-b and Figure S2). Figure 3(b) summarizes the UV-vis absorption peaks, PL peaks, and PLQY evolution with different ratios of Ca/Pb. The UV-vis absorption peaks exhibit a blue shift from 666 nm to 665, 664, and 661 nm with the ratio of Ca/Pb increasing from 0% to 0.35%, 0.40%, and 1.20%, respectively. Simultaneously, their PL peaks also exhibit a blue shift from 682 nm to 676 nm with increase of the ratio of Ca/Pb. Furthermore, benefitting from Ca2+ doping, their PLQYs can be improved. With increasing Ca/Pb ratios from 0% to 0.40%, their PLQYs increased from 89% (±1.5%) to 93% (±1.3%), whereas further increasing the Ca/Pb ratio to 1.20% led PLQY lower to 91% (±1.4%). This is due to that the smaller NCs () with larger surface-to-volume ratio must have more surface defects. In addition, Ca2+ cannot be as emission centers. Therefore, excess Ca2+ doping may decrease PLQY. Figure 2(c) shows the decay curves of Ca2+-doped CsPbI3 NCs. All of the decay curves can be fitted by a double-exponential (Table S2). The average PL lifetimes of Ca2+-doped CsPbI3 NCs are 71.76 ns (0%), 77.47 ns (0.35%), 119.47 ns (0.40%), and 92.62 ns (1.20%). Such results demonstrate that the improved τ facilitated by Ca2+ doping can reduce the lattice distortion and phase transition to maintain PLQYs at a relatively high level.

In addition to boosting their PLQYs, the increase in τ for Ca2+ doping in CsPbI3 can much improve their stability. We measured the stability of their solutions and solid films. Firstly, all samples were stored in the conditions of and 40−50% humidity. Figure 4(a) shows PL intensity evolution with storage time. After a 147-day storage, PL intensity of CsPbI3 NCs decreased to 36% of the initial one, whereas PL stabilities of Ca2+-doped CsPbI3 NCs exhibited much improvement. After 147-day storage, an obvious color bleaching was observed for the CsPbI3 NC solution, while the red color of Ca2+-doped CsPbI3 NC solutions can be maintained (Figure 4(b)). PL intensities of Ca2+-doped CsPbI3 NC solutions can be preserved to 80% (0.35%), 83% (0.40%), and 76% (1.20%) of the initial intensities after 147-day storage (Figure 4(a) and Figure S5). We further studied their morphology changes by TEM. The size of CsPbI3 NCs became nonuniform and noncubic shape and tended to be aggregated, which must lead to their PL decrease (Figure 4(c) and Figure S5). In contrast, cubic shape and uniform size distribution of Ca2+-doped CsPbI3 NCs can be kept with relatively high PL performance (Figure 4(c) and Figure S5). Secondly, Ca2+-doped CsPbI3 NCs exhibited improved stability against to UV. All of the samples were placed under 365 nm UV light (8 W), and their PL intensities decayed with UV irradiation time increasing. We periodically measured their PL spectra, which were used to calculate their half-lifetimes (T50) under UV light (Figure S6 and Figure S7a). On the basis of these data, T50 were 52 min (0%), 74 min (0.35%), 92 min (0.40%), and 85 min (1.20%), separately. About 1.8 times enhancement of UV stability was observed for the 0.40% Ca/Pb ratio. Further increasing UV irradiation time to 100 min, it was hard to observe the red color for CsPbI3 NCs, while Ca2+-doped CsPbI3 NCs could still emit red PL (Figure S7b). As a result, Ca2+-doped CsPbI3 NCs exhibit enhanced UV stability as compared to CsPbI3 NCs.

According to the above results, Ca2+-doped CsPbI3 NC solutions exhibited improved stability in ambient condition and UV light, whereas the application of these NCs must form solid films. Therefore, we further studied their stability in solid films, such as in ambient condition (40−50% RH at ) and heating condition (120°C). As shown in Figure 5(a), α-CsPbI3 (cubic phase) film gradually converted to δ-CsPbI3 (orthorhombic phase) film after 28-day storage time, and its color changed from red to yellow (Figure 5(e)). However, Ca2+-doped CsPbI3 films exhibited better crystal phase stability. The crystal phase of their films (, 1.20%) could be maintained to the cubic phase after 58-day storage, which exhibited good PL performance (Figures 5(e) and 5(f)). Such results demonstrate that the phase stability of CsPbI3 can be enhanced in ambient condition facilitated by Ca2+ doping. Additionally, we verified their thermal stability in the solid state. All the films were placed on a hot plate at 120°C, and we periodically measured their XRD patterns. The CsPbI3 film exhibited the poorest phase stability, and its phase gradually converted to the orthorhombic phase after 120°C heating for 2 h (Figure S8). On the basis of previous reports, the small size effect can reduce CsPbI3 lattice distortion to partly prevent phase transition [39, 52]. As an ionic nature of CsPbI3, all ion migrations can be accelerated at high temperature. As a result, ion migration and crystal fusion easily occur in the solid state, which leads to increasing the size of CsPbI3 NCs. The enlarged size of CsPbI3 must induce phase transition [14, 53, 54]. Benefitting from small-sized Ca2+ doping, the increase in τ should improve the phase stability of CsPbI3. With increase of the Ca/Pb ratio from 0% to 1.20%, their cubic phase could be maintained at least for 4 h at 120°C (Figure S8). Therefore, the thermal stability of CsPbI3 films can be enhanced via Ca2+ doping.

To identify the positive effect of Ca2+-doped CsPbI3 NCs for LEDs, we used Ca2+-doped CsPbI3 NC films as emitting layers to fabricate red LEDs. Figure 6(a) is the energy-level diagram of LED. The LED architecture consists of a multiple-layered structure of ITO (170 nm)/PEDOT:PSS (30 nm)/Ca2+-doped CsPbI3 NCs (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) shown in Figure S9. Herein, ITO is the anode; PEDOT:PSS is the hole-injection layer; TPBi and LiF are the electron-transport and electron-injection layer, respectively; and Al is the cathode. Figure 6(b) presents the current density as functions of voltage. The LED with Ca2+-doped CsPbI3 NCs () exhibited obviously enhanced current density, which implied the decreasing of defect density as well as the improvement for carrier transport efficiency.

Firstly, hole-only devices with a structure of ITO/PEDOT:PSS/CsPbI3 NCs or Ca2+-doped CsPbI3 NCs/MoO3/Al (Figure S10) were used to quantitatively measure the defect density of NCs films. The defect density () is calculated according to the following equation: where and are the vacuum dielectric constant and the relative dielectric constant, respectively; is the trap-filled limit voltage; is the thickness of the NC film; and is the elementary electronic charge. By assuming that [34], the defect densities of CsPbI3 and Ca2+-doped CsPbI3 NC films were and , respectively. This means that the defect density could be decreased to 21.1% facilitated by 0.40% Ca2+ doping. Such results match with those results of PLQY and TRPL. In addition, the hole migration rates were estimated by fitting the space-charge-limited-current region (SCLC) with Child’s law: where is the vacuum permittivity; is the average relative dielectric constant of CsPbI3 (); is the thickness of the perovskite film; and , μ, and are the measured current density, carrier migration rates, and applied voltage, respectively [34]. The hole mobilities of CsPbI3 and Ca2+-doped CsPbI3 NC films were and , respectively. With the Ca2+ doping, the hole mobility increased, which enhanced hole transport efficiency.

Then, the improvement for carrier transport efficiency can be confirmed by ultraviolet photoelectron spectra (UPS). Combining with their optical bandgaps (Eg) for Ca2+-doped CsPbI3 NC film (Figure S11a) and their UPS spectra of CsPbI3 NCs (Figure S11b), their conduction band minimum (CBM), valence band maximum (VBM), and Fermi level can be obtained. For CsPbI3 NCs, its CBM, VBM, and Fermi level were -3.79 eV, -5.60 eV, and -3.93 eV, respectively. Benefitting from Ca2+ doping (0.40%), its CBM, VBM, and Fermi level shifted to -3.73 eV, -5.55 eV, and -4.05 eV, respectively. The higher VBM reduces the energy barrier between PEDOT:PSS and the emission layer to boost hole transport efficiency. Additionally, the lower Fermi level reveals the transformation of CsPbI3 NCs from n-type to a more nearly ambipolar nature facilitated by Ca2+ doping, which also enhances hole transport efficiency [34, 37].

Benefitting from the enhanced current density, the Ca2+-doped CsPbI3 NC LEDs exhibited stronger brightness than CsPbI3 NC LEDs in the whole driving voltage range (Figure 6(c) and Figure S12). In the case of 0.40% Ca2+ doping, LED had a maximum luminance of 790 cd/m2 (7.2 V), which was doubled as compared to the nondoping one. In addition, the maximum and average EQEs for Ca2+-doped CsPbI3 NC (0.40%) LEDs were 7.8% and 6.3%, both of which enhanced to about 3 times as compared to the nondoping one, respectively (Figure 6(d) and Figure S13). Therefore, our work using Ca2+ as a new B-site doping ion to boost CsPbI3 NC LEDs is quite promising (Table S3). Their electroluminescent (EL) spectra exhibit stable and sharp peaks at 683 nm with a full width at half-maximum (FWHM) of 35 nm on various driving voltages (Figure 6(e) and Figure S14). A bright red emission could be observed at 5.0 V (inset in Figure 6(e)), and its Commission Internationale del’Eclairage (CIE) color coordinate was (0.72, 0.27) (Figure S15). Furthermore, the operational stability of the LED was evaluated at a constant current density of 5.0 mA/cm2. The luminance of CsPbI3 NC LEDs decreases to half at 39 min, while the half-lifetime of Ca2+-doped CsPbI3 NC (0.40%) LEDs could significantly increase to 85 min. A 2.2-fold improvement for the half-lifetime of LED confirmed that Ca2+ doping is a powerful strategy to promote both stability of CsPbI3 NCs and their LEDs. In a word, the boosting of both the efficiency and the stability of LEDs is mainly attributed to a decrease in defect density, improvement on hole injection efficiency, and reduction in phase transition.

3. Conclusion

In summary, we explored stable and high-performance CsPbI3 NCs based on Ca2+ (1.00 Å) doping. With a Ca2+/Pb2+ ratio of 0.40%, the phase stability could be greatly enhanced. Ca2+-doped CsPbI3 NC solid films could maintain the cubic phase after 58-day storage in ambient condition or 4 h accelerated aging process at 120°C. Moreover, the PL stability could be also improved. The PL intensity of Ca2+-doped CsPbI3 NC solutions could be preserved to 83% after 147-day storage in ambient condition. Even using UV light to accelerate aging, the T50 of PL could be boosted 1.8-folds as compared to that of CsPbI3 NCs. Red LED based on Ca2+-doped CsPbI3 NCs exhibited about triple enhancement for maximum EQE up to 7.8% and 2.2 times enhancement for half-lifetime of LED up to 85 min. These were mainly attributed to the decreased defect densities of films and reduced hole injection barrier facilitated by Ca2+ doping. Ca2+ as a new B-site doping ion can efficiently boost both stability and performance for CsPbI3 NC LEDs, which has the potential to promote the development of CsPbI3 NC LEDs.

4. Experimental Section

4.1. Chemical Materials

Oleic acid (OA, 90%), 1-octadecene (ODE, 90%), methyl acetate (MeOAc, 99%), hydriodic acid (HI, 57%) oleylamine (OLA, 80-90%), cesium carbonate (Cs2CO3, 99.99%), calcium acetate (Ca(Ac)2, 95%), and lead iodide (PbI2, 99.99%) were purchased from Aladdin. Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonate) (PEDOT:PSS, CLEVIOS P VP AI 4083) was purchased from Heraeus Materials Technology Co. Ltd. 2,2,2-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was purchased from Nichem Fine Technology Co. Ltd. All the chemicals were directly used without further purification.

4.2. Synthesis of Cs-OA

0.39 g Cs2CO3, 18 mL ODE, and 2.0 mL OA were mixed into a 100 mL three-neck flask and dried in vacuum at 120°C for 1 h. Then, the mixture was heated to 150°C under N2 until the Cs2CO3 powders were completely dissolved to form a transparent solution. The solution was cooled down to room temperature via ice-water bath and preheated to 110°C before use.

4.3. Synthesis of OLA-HI

20 mL OLA and 2 mL HI were mixed into a 100 mL three-neck flask. Then, the solution was heated to 120°C for 2 h under vacuum to remove the water. Then, the solution was cooled down to 60°С to obtain the OLA-HI solution and preheated to 90°C before use.

4.4. Synthesis of Ca2+-Doped CsPbI3 NCs

In a typical synthesis, PbI2 (0.4 mmol), Ca(Ac)2 (0, 0.06, 0.10, and 0.14 mmol), and 10 mL ODE were mixed into a 100 mL three-neck flask. The mixture was degassed and dried in vacuum for 1 h at 120°C. Then, 1.0 mL OLA, OA, and preheated OLA-HI were injected into the reaction flask, separately. The mixed solution became clear and was kept under vacuum for 30 min at 120°C. Finally, the temperature was increased to 260°C, and 1.0 mL Cs-OA solution was injected immediately. After 1 min, the reaction flask was placed into an ice-water bath to stop the reaction.

4.5. Purification of NCs

The as-prepared NC solution was mixed with the same volume of MeOAc and was centrifuged at 15000 rpm for 5 min at 19°C to remove the supernatant. The precipitate was redispersed into toluene. Then, the NC toluene solution was mixed with the same volume of MeOAc and was centrifuged at 15000 rpm for 5 min at 19°C. After removing the supernatant, the precipitate was dispersed into toluene. Finally, the redispersed NC solution was centrifuged at 15000 rpm for 5 min at 19°C to discard the precipitate, and the supernatant was preserved for characterization and device fabrication.

4.6. Fabrication of LED Devices

Indium tin oxide- (ITO-) coated glass substrates were cleaned by ultrasonic cleaning for 15 min in acetone, ethanol, and deionized water, separately, and dried with nitrogen flow. The clean ITO glasses were treated by UV-ozone for 15 min. Then, PEDOT:PSS solutions were spin-coated onto the ITO substrates at 3000 rpm for 30 s and annealed at 120°C for 15 min in air. NCs were spin-coated onto the PEDOT:PSS layer at 1500 rpm for 30 s. This process was repeated five times. Finally, 40 nm of the TPBi layer, 1 nm of the LiF layer, and 100 nm of Al electrode were deposited in sequence by a thermal evaporation system under a high vacuum (). The active area of the devices was 10 mm2 as defined by the overlapping area of the ITO and Al electrodes.

4.7. Characterizations

The ultraviolet-visible (UV-Vis) absorption spectra of NC solutions were measured by a PerkinElmer Lambda 35S instrument in transmission mode. PL spectra were collected by a RF6000 spectrofluorometer with an excitation wavelength of 500 nm. The PL lifetimes of NCs were measured by a FLS920 fluorescence spectrometer with a pulse laser at 375 nm. The chemical compositions were measured by a PerkinElmer NexION 2000 inductively coupled plasma mass spectrometry (ICP-MS). X-ray diffraction (XRD) data were collected by a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (). Photoluminescence fluorescence quantum yield (PLQY), which is defined as the ratio of emitted photons to absorbed ones, was determined by a FLS920 fluorescence spectrometer equipped with an integrating sphere. The morphology and size of NCs were confirmed by transmission electron microscope (TEM) (Hitachi, HT7700), high-resolution TEM (HRTEM) (Talos, F200X), and energy-dispersive X-ray spectroscopy (EDS). The electroluminescent (EL) spectra and luminance- (L-) current density- (J-) voltage (V) characteristics were collected by using a Keithley 2400 source and PR-655 spectra scan spectrophotometer (Photo Research). The characterization for LED devices was measured at room temperature in air.

Data Availability

All data needed to evaluate the conclusions in the paper are presented in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Conflicts of Interest

The authors declare no conflict of interest.

Authors’ Contributions

W. Shen and J. Zhang contributed equally to this work and are listed as co-first authors. S. Chen and W. Huang conceived and designed the research and revised the manuscript. W. Shen and J. Zhang performed the synthesis, data analysis, and article writing. R. Dong, Y. Chen, and S. Chen prepared and characterized the LED. L. Yang, Z. Su, and Y. Dai acquired the stability data. L. Liu and K. Cao assisted in all data analysis and interpretation. All authors participated in drafting the manuscript and approved the final version.

Acknowledgments

This work was supported by the National Major Fundamental Research Program of China (Grant No. 91833306), the National Natural Science Foundation of China (Grant Nos. 62074083, 62005131, and 61705111), the Natural Science Foundation of Jiangsu Province (Grant No. BM2012010), the Natural Science Fund for Colleges and Universities in Jiangsu Province (Grant No. 20KJA510005), the Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant No. YX030003), NUPTSF (Grant Nos. NY219158 and NY220025), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX20_0754).

Supplementary Materials

Supplementary 1. Figure S1: HRTEM images of Ca2+-doped CsPbI3 NCs. Figure S2: statistical histogram particle size. Figure S3: EDX spectrum of Ca2+-doped CsPbI3 NCs. Figure S4: normalized UV-vis absorption and normalized PL spectra. Figure S5: the PL evolution of Ca2+-doped CsPbI3 NC solutions with increase of storing time. Figure S6: the PL area evolution with increasing irradiation time. Figure S7: the PL evolution of Ca2+-doped CsPbI3 NCs under 365 nm. Figure S8: XRD patterns evolution for Ca2+-doped CsPbI3 NC thin films at 120°C. Figure S9. the cross-section SEM image of LEDs. Figure S10: J-V curves of “hole-only” devices. Figure S11: Eg and UPS of Ca2+-doped CsPbI3 NCs. Figure S12: current density and luminescence for LEDs. Figure S13: histogram of EQEs for 10 LEDs. Figure S14: (a–d) normalized EL spectra of Ca2+-doped CsPbI3 NCs. Figure S15: the corresponding CIE coordinates for the EL spectra. Table S1: ICP-MS data. Table S2: the time-resolved PL decays. Table S3: current CsPbI3 NC LED performance with similar structure. (Supplementary Materials)

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