Light-emitting diodes (LEDs) have been recognized as efficient solid-state lighting sources and are widely used in the photoelectric field due to advantages such as high efficiency, a long lifetime, and a low power requirement. To improve the color-rendering index (Ra > 80) of white LEDs, the green and red contents in the spectra should be enlarged. In particular, the green light source is considered to have an important role in white LEDs [1]. Unfortunately, it is difficult to achieve high-efficiency green LEDs because of the well-known “green gap” problem [2–8]. In general, compared to the blue GaN-based LEDs counterparts, green GaN-based LEDs show much lower external quantum efficiency (EQE) and a stronger efficiency drop effect. To achieve efficient green LEDs, various efforts have been explored. For example, Saito et al. reported enhanced LEDs using the active layer consisting of the AlGaN interlayer and InGaN quantum well [9]. In addition, a series of studies about rare-earth-doped green phosphor materials also have been proposed [10–13]. However, phosphor-based green LEDs still impede the practical application due to their poor monochromaticity and wide bandwidth [14,15]. Therefore, it is still necessary to develop high-efficiency green emission materials with good monochromaticity and a narrow bandwidth.

Recently, all-inorganic perovskite CsPbX₃ (X=Cl, Br, I) quantum dots (QDs) have emerged as a new class of optical materials that have great potential for applications in the photoelectric field [16–22], because of their high optical gain, narrow emission width, and high photoluminescence quantum yield (i.e., PLQY above 85%) [23–27]. Moreover, recent reports using novel strategies on CsPbX3 QDs have enriched the applications. Ooi et al. [28] demonstrated the potential application of CsPbBr3 QDs for visible light communication. Their group also explored a high-speed UV color-converting photodetector based on CsPbBr3 [29]. Zeng’s group demonstrated that CsPbX3 QDs exhibit promising materials for high-definition QD displays and lighting devices [30]. Among the family of CsPbX3 QDs, CsPbBr3 QDs have been regarded as particularly promising for LEDs [31,32]. To date, much effort has been devoted to achieving green light emission using GaN-based LEDs that convert their blue or near-ultraviolet (UV) emission into green light via an interaction with inorganic CsPbBr3 perovskite QDs [22,33]. However, the reported CsPbBr3 QDs capped with long alkyl ligands such as oleic acid (OA) and oleylamine (OAM) synthesized by a hot-injection method exhibit unsatisfactory stabilities, which hinder their practical application in LEDs [34,35]. To improve the performance of LEDs based on CsPbBr3 QDs, some useful strategies have been proposed. The EQE of green CsPbBr3 perovskite QDs-based LEDs obviously could be enhanced through surface ligand engineering [36] and surface treatments [37,38]. Among those strategies, ligand modification of CsPbBr3 QDs also has been demonstrated to be an effective method to improve luminescent performance [39]. Replacing a long ligand (for example, an OA ligand) with a shorter ligand without degrading or destabilizing the perovskite structure is the key challenge for a CsPbBr3 QDs application. As presented in our previous report, we prepared high-quality CsPbBr3 QDs using a 2-hexyldecanoic acid (DA) with two short branched chains to replace an OA ligand with long chains in the synthesis process. During the process, the CsPbBr3 QDs with a DA ligand (CsPbBr3-DA QDs) exhibited more excellent stability and optical properties than the regular CsPbBr3 QDs with an OA ligand (CsPbBr3-OA QDs) [40]. However, to the best of our knowledge, there has been no report about CsPbBr3-DA QDs used as phosphors for green light emission devices. In this study, we aim to investigate the potential of modified CsPbBr3 QDs in light emission devices.

We synthesized high-performance CsPbBr3 QDs by using a shorter 2-hexyldecanoic acid (DA) ligand to replace an OA ligand for CsPbBr3 QDs. The ligand-modified CsPbBr3 QDs show a high PLQY of 96%, which were used as green phosphors for high-performance light emission devices. As a result, ultrapure and highly efficient green light-emitting devices based on CsPbBr3-DA QDs exhibit a luminous efficiency of 43.6 lm/W with a CIE (0.2086, 0.7635) under a 15.3 mA driving current. Furthermore, the green light-emitting devices based on CsPbBr3-DA QDs also have a more stable green emission than the devices based on CsPbBr3-OA QDs under various driving currents. Thus, we believe that ligand modification is an effective way to improve the performance of light emission devices.

Chemicals and reagents: Cs2CO3 (99.9%) and PbBr2 (99.9%) were purchased from Xi’an Polymer Light Technology Corp. OAm (>90%, Adamas), OA (>90%, Adamas), DA (>98%, TCI), octadecene (ODE, >90%, Adamas) were used without further purification.

Synthesis of CsPbBr3-OA QDs and CsPbBr3-DA QDs: ODE (4 mL), Cs2CO3 (81.5 mg), and 0.5 mL OA were loaded into the 100 mL three-neck flask 1 with stirring; PbBr2 (0.188 mmol) and ODE (5 mL) were loaded into another 100 mL three-neck flask 2 with stirring. The two flasks were degassed at 120°C for 1 h under vacuum and nitrogen flow, then OA (0.5 mL) and OAm (0.5 mL) were quickly injected into flask 2, and the temperature rose to 150°C in 2 min in a nitrogen environment. A 0.4 mL of Cs-oleate solution was quickly injected. After 5 s, the crude reaction solution was cooled by the ice water bath. The synthesis of CsPbBr3-DA QDs was similar to that of CsPbBr3-OA QDs, except that the OA ligands were replaced by DA ligands.

Fabrication of CsPbBr3 QDs films: The QDs films were fabricated by spin-coating 400 μL of 10 mg/mL solution of CsPbBr3 QDs (in n-octane) onto glass substrates (1.5cm×1.5cm) in air. The speed of spin-coating was 2500 r/min (15 s). Thus, the ∼300nm thick CsPbBr3-OA QDs films could be obtained. The CsPbBr3-DA QDs films with the same thickness were also fabricated by this method.

Green LEDs fabrication: First, the 10 mg CsPbBr3 QDs were dissolved in 1 mL n-octane to obtain a homogenous solution. Then, the mixture was put in a vacuum chamber to get rid of the bubbles inside. Finally, the 6μL CsPbBr3 QDs solution was directly dropping-coated into the LED groove using a pipettor. Then, the LED was baked at 50°C to volatilize n-octane and to form the QDs layer. The thickness of the layer created by 6 μL QDs solution was estimated to be about 40 nm, and also the film thickness has a linear relationship with the dropped solution.

Materials and devices characterization: The X-ray diffraction (XRD) analysis was performed on XRD-6100 (Shimadzu, Japan). The purified samples were redispersed in 0.5 mL n-octane and then dropped on 0.8 cm by 0.8 cm glass substrates followed by solvent evaporation. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were performed using an electron microscope (Libra 200 FE, Carl Zeiss AG, Oberkochen, Germany). The absorption spectra were measured by a UV-vis spectrophotometer (UV-2100, Shimadzu, Kyoto, Japan). Samples with the same thickness were prepared by dropping the QDs solution (300 μL of 10 mg/mL solution of CsPbBr3 QDs) onto glass substrates. The photoluminescence (PL) measurements were conducted by a fluorescence spectrophotometer (Cary Eclipse, Agilent Technologies, Inc., Santa Clara, CA, USA) equipped with a Xe lamp. PL decay curves were detected using an EPL-405 nanosecond laser (Edinburgh Instruments Ltd., Livingston, Scotland, UK). The Fourier transform infrared (FTIR) analysis (KBr pellet method) was recorded using a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). XPS spectra were recorded on a Thermo Fisher ESCALAB Xi+ spectrometer. The atomic force microscopy (AFM) images were analyzed by an atomic force microscope (MFP-3D-BIO, Asylum Research, Goleta, CA, USA). The characteristics of the fabricated LED devices were measured using one spectrograph of PR670 (Photo Research, New York, USA) with an analyzer system.

The detailed synthesis of CsPbBr3-DA QDs can be found in the previous report [40]. The pristine CsPbBr3-OA QDs were synthesized via the general hot-injection method using lead bromide and cesium oleate as precursors [16]. The synthesis scheme of the modified CsPbBr3 QDs by using DA as a ligand to replace the regular OA ligand is schematically shown in Fig. 1(a). To confirm the successful synthesis of CsPbBr3 QDs with a DA ligand, HRTEM, XRD, and FTIR spectra were recorded. Figure 1(b) shows the XRD patterns of the CsPbBr3-OA QDs and CsPbBr3-DA QDs. Both samples exhibit distinctive XRD peaks at 2θ=15.2°, 21.5°, and 30.7°, which can be indexed to the (100), (110), and (200) planes of the CsPbBr3 cubic phase [41–44], respectively. In addition, both XRD results of the CsPbBr3 QDs are well indexed to the standard cubic CsPbBr3 phase, without impure phases. It also can be seen that the crystallinities of the CsPbBr3-DA QDs are notably higher than that of the CsPbBr3-OA QDs. To investigate the surface ligand, the corresponding FTIR spectra were measured, as shown in Fig. 1(c). The peaks of CH3 reveal the presence of the ligand on the surface of both CsPbBr3 QDs. The peaks of 718cm−1 are the peaks of -(CH2)- which are the long-chain saturated hydrocarbons, further confirming the presence of the ligands. In particular, the peaks of 1463cm−1 (-COOH-) for DA ligand and OA ligand imply that the CsPbBr3 QDs are well capped with the OA/DA ligand after a cleaning process. HRTEM images reveal a cubic shape of the CsPbBr3-OA and CsPbBr3-DA QDs, with the average sizes of 11.66 nm and 12.44 nm, respectively [Fig. 1(d)]. In addition, the same lattice spacing of 0.58 nm for both CsPbBr3 QDs is clearly observed from the HRTEM images [Fig. 1(d)], which is consistent with the (200) crystal plane of the cubic CsPbBr3 structure. The HRTEM image also demonstrates that the CsPbBr3 QDs have high crystallinity. Interestingly, the CsPbBr3-DA QDs are found to exhibit a high PLQY of 96%, while the PLQY of CsPbBr3-OA QDs is only 84% [left inset of Fig. 1(d)].

To better investigate the emitting properties of the CsPbBr3 QDs, we further conducted the absorption and PL spectra of CsPbBr3-OA and CsPbBr3-DA QDs films. Figure 2(a) shows the PL peak of both QDs centered at around 516 nm, while their corresponding absorption peaks [Fig. 2(b)] appear at around 509 nm, indicating a slight Stokes shift [45]. It is important to note the FWHM values obtained from the PL spectra are 20 nm and 17 nm for CsPbBr3-OA and CsPbBr3-DA QDs, respectively, indicating high-purity green emission for both QDs. Moreover, the PL intensity of CsPbBr3-DA QDs film is obviously enhanced under the same measurement condition. The time-resolved PL decay spectroscopy was used to study the lifetime of both QDs films. Biexponential behaviors are observed for both QDs films, indicating both QDs with the intact crystal structure and efficient radiative recombination. The inset table of Fig. 2(c) presents a summary of the comparison of the results for the two QDs consisting of a fast-decay component (τr), a slow-decay component (τnr), and the calculated average lifetime (τavg). Generally, the fast decay (τr) process originates from trap-assisted recombination, while the slow decay (τnr) process presents the free-charge carrier radiative recombination. CsPbBr3-DA QDs film shows an average lifetime (τavg) of 34.6 ns, which is much longer than those of CsPbBr3-OA QDs films (18.9 ns). The longer PL lifetime of CsPbBr3-DA QDs films indicates lower trap state densities, which may be ascribed to a reduction of the nonradiative pathways. To further demonstrate the formation of two CsPbBr3 QDs, the X-ray photoelectron spectroscopy (XPS) comparative analyses of CsPbBr3-OA and CsPbBr3-DA QDs were performed. The result reveals that both CsPbBr3-OA QDs and CsPbBr3-DA QDs are comprised of Cs, Pb, and Br elements, shown in Fig. 2(d). Both CsPbBr3-OA and CsPbBr3-DA QDs comprise all peaks labeled, which are consistent with the previous report [46]. The high-resolution XPS spectra corresponding to the core levels of Cs 3d (723.5 and 737.5 eV), Pb 4f (137.4 and 142.2 eV), and Br 3d (68 eV) are recorded [see Figs. 2(e), 2(f), and 2(g)]. The XPS results show no differences for both CsPbBr3 QDs, implying that their formation is well identified and both of them have been successfully synthesized.

The morphology of the CsPbBr3 QDs films plays a very important role in achieving a bright green LED. To further probe the film qualities of the CsPbBr3-OA QDs and CsPbBr3-DA QDs films, we used AFM to characterize the morphology of CsPbBr3-OA and CsPbBr3-DA QDs films. Figure 3(a) shows that the CsPbBr3-OA QDs film has a poor morphology with a high root mean square (RMS) surface average roughness of RMS≈5.61nm. However, after the DA ligand modification, CsPbBr3-DA QDs perovskite films with better morphology are obtained and show a significant reduction in the RMS roughness; namely 3.26 nm for the CsPbBr3-DA QDs films [see Fig. 3(b)].

To demonstrate the green light emission conversion of QDs, the blue LEDs with an emission wavelength of 450 nm were used as an excitation source. The green LED devices were fabricated using CsPbBr3-OA QDs and CsPbBr3-DA QDs films as the active layer. We have designed a series of CsPbBr3 QDs layers to explore the suitable quantity for the green LEDs and obtain nearly pure green emission. The emission spectrum of both CsPbBr3 QDs as-coated LEDs has been recorded under room temperature. Figures 4(a) and 4(b) present the evolution of QDs coated on LEDs under the ambient conditions. Both of them undergo a change from blue to green light, indicating the CsPbBr3 QDs layers have a favorable color conversion ability that converts blue light emission into a green one. Finally, the green light emission of the CsPbBr3-OA QDs coated LEDs has a weak brightness, while the green LEDs with CsPbBr3-DA QDs are much brighter with less QDs solution. Both CsPbBr3-OA QDs and CsPbBr3-DA QDs based LEDs have been explored under a constant driving current of 31.1 mA during the coating process. The spectrum variation in the processing of LEDs with different amounts of QDs is shown in Fig. 4(c). The two emission peaks of 450 nm and ∼530nm are contributed by the blue InGaN chips and CsPbBr3-OA QDs, respectively. The quantity of the QDs solution addition is a key parameter for pure green LEDs. With an increase of the quantity of the QDs solution, the main blue emission peak of 450 nm reduces gradually, accompanied by an increase in the green emission intensity. Finally, the blue emission peak of 450 nm almost disappears and the green emission peak of 530 nm becomes the main peak, implying that the QDs enable absorption of the emission of the blue InGaN chip completely and output the green emission intensively. It is obvious that the intensity of green emission increases to a maximum value with the CsPbBr3-OA QDs up to 18 μL and then decreases with the addition of the QDs. Similarly, the addition of the CsPbBr3-DA QDs onto blue LEDs causes a similar emission behavior. Compared to CsPbBr3-OA QDs, a much smaller amount of the CsPbBr3-DA QDs solution is required for pure green emission. In other words, the thickness of CsPbBr3-DA QDs films is smaller than that of CsPbBr3-OA QDs films due to the different amounts of the QDs solution required under the same experimental conditions [including the concentration (10 mg/mL), room temperature and air]. We ascribe this to the higher PLQY of CsPbBr3-DA QDs solution.

In the presence of light-emitting components, the emission colors can be analyzed by a spectrum that shows the corresponding integrated intensity of the blue and green lights. As shown in Figs. 4(d) and 4(g), the percentage of CsPbBr3-OA QDs based green emission increases with the amount of QDs. The energy transfer efficiency from blue to green light is presumed to be important for achieving a green LED and estimated by the ratio of the light integration. For example, it is calculated to be 79.4% at 24 μL. The figures reveal that the coated LEDs exhibit nearly pure green light when the coated volume of the CsPbBr3-OA QDs and the CsPbBr3-DA QDs solutions reaches 60 μL and 18 μL, respectively. The smaller required amount of the CsPbBr3-DA QDs solution can be attributed to the better optical properties of the CsPbBr3-DA QDs compared to the CsPbBr3-OA QDs. Furthermore, the intensity of the green light from LEDs based on CsPbBr3-OA QDs increases slowly. We ascribe this phenomenon to the aggregation of the CsPbBr3-OA QDs during the emitting process, while the CsPbBr3-DA QDs might maintain better stability. We have confirmed that there is strong binding energy between the DA ligand and the QDs in our previous report; thus, the ligand could not lose easily during the emission process [40]. To investigate the quantification of the green emission light of the QDs coated LED, we have explored the light purity analyses on both CsPbBr3-OA QDs and CsPbBr3-DA QDs samples [Figs. 4(d) and 4(g)]. From blue to green, the color variation of the CsPbBr3-OA samples is achieved by adding the QDs solution up to 60 μL, while the CsPbBr3-DA samples change into the green region with only 18 μL. In terms of the calculation about light purity, the dominant wavelength plays an important role in the characterizing of the spectral light, and it can be speculated by the coordinates of the QDs coated LED and pure green light in the Commission Internationale Ed I’eclairage (CIE) diagram. Figures 4(e) and 4(h) show the CIE chromaticity coordinates of the spectrum, which reveals the process from blue to nearly pure green, with the increasing volume of the QDs solution. Finally, the chromaticity coordinates of the nearly pure green LEDs are reached at (0.2547, 0.7266) and (0.2086, 0.7635) for CsPbBr3-OA and CsPbBr3-DA QDs films, respectively. In addition, it is worth noting that the emission wavelength of the CsPbBr3-OA QDs red shifted from 525 to 536 nm along with the addition of the solution, while the CsPbBr3-DA QDs showed a small red shift. These red shifts can be ascribed to the QDs particles aggregation in solid-state, which is caused by the high temperature during the process.

To further reveal the properties of the pure green emission, the emission spectra of the green LEDs under different currents have been measured. The voltage-current characteristics and the FWHM of both CsPbBr3-OA QDs and CsPbBr3-DA QDs coated on blue LEDs have been investigated. As shown in Figs. 5(a) and 5(d), the intensity of the pure green light increases gradually for both green LEDs along with the driving current increase from 0.4 mA to 64.7 mA. For CsPbBr3-OA QDs coated LEDs, the FWHM varies from 17 to 20 nm and the emission peak position changes from 538 to 540 nm when the driving current increases from 0.4 to 64.7 mA. The changes could be attributed to the aggregation of the CsPbBr3-OA QDs under high temperatures induced by the increasing current [Fig. 5(b)]. This phenomenon of thermally induced aggregation of CsPbBr3 QDs has also been reported in other previous research [47–49]. In contrast, CsPbBr3-DA QDs coated LEDs exhibit robust stabilities even under a high injection current (i.e., the FWHM remains 22 nm and the emission peak keeps the same position, respectively), as shown in Fig. 5(e). The corresponding CIE coordinates of the CsPbBr3-OA QDs coated LEDs and CsPbBr3-DA coated LEDs are (0.2547, 0.7266) and (0.2086, 0.7635), respectively, exhibiting no shift under the different injection current, as shown in Figs. 5(c) and 5(f). Notably, the power efficiency (43.6 lm/W) of CsPbBr3-DA QDs coated LEDs is much higher than that of the CsPbBr3-OA QDs coated LEDs (30.9 lm/W) under a 15.3 mA driving current. These results demonstrate that DA ligand modification for CsPbBr3 QDs is a useful strategy to improve the performance of green LED devices.

In summary, we have demonstrated that CsPbBr3 QDs with DA ligand modification exhibit better properties than CsPbBr3 QDs with a traditional OA ligand. Subsequently, we have fabricated green LEDs based on both CsPbBr3-OA QDs and CsPbBr3-DA QDs under the ambient conditions. Compared to CsPbBr3-OA QDs coated LEDs, the LEDs coated by CsPbBr3-DA QDs show higher purity, higher thermal stability, and stronger green light intensity. This work promotes the investigation of LEDs based on ligand-modified perovskite QDs.