2.1 Preparation of In Situ Generated Blue-Emitting NCs

We synthesized a mixed solution of CsPbBr3 and Cs4PbBr6 NCs by modifying a previously reported synthetic method (details provided in the Experimental Section).[41] In a typical synthesis, cesium carbonate (Cs2CO3), lead oxide (PbO), oleic acid (OA), and 1-octadecene (ODE) were added to a three-necked round-bottom flask, and metal oleate complexes were formed by heating at 120 °C. Oleylammonium bromide (OLAM-Br) was prepared separately by reacting oleylamine and hydrobromic acid (HBr), and then this was injected at low temperature under Ar into the metal oleate complexes. In this step, the reaction temperature and the Cs to Pb precursor ratio, as important factors for obtaining high-quality NCs with blue emission in the range of 460–480 nm, were carefully controlled. The CsPbBr3 to Cs4PbBr6 NC formation ratio was modulated by varying the Cs to Pb precursor feed ratio, as described in a previous report.[42] When the amount of Pb precursor exceeded the amount of Cs precursor, the formation of CsPbBr3 NCs in the orthorhombic phase was favored (Figure S1, Supporting Information). When the Cs and Pb precursor ratio was fixed at 1:1 and the reaction temperature was reduced, the growth of NCs was suppressed, and small CsPbBr3 NCs and Cs4PbBr6 NCs were simultaneously co-synthesized. The emission wavelength of the QDs was 470 nm, and a high PLQY of above 90% was achieved. This phenomenon occurred because the depletion of the Pb precursor was slower than that of the Cs precursor.[42, 43] At low temperature, the difference between the reactivities of the metal precursors was maximized, hence formation of Cs4PbBr6 NCs was promoted and the size of the CsPbBr3 NCs was reduced (Figure 1).

Schematic representation of temperature-dependent growth trend of perovskite nanoparticles. The schematic depicts the active metal precursor ion concentrations and products (CsPbBr3 and Cs4PbBr6 nanocrystals) as functions of increasing reaction temperature. The colored x-axis that gradually changes from blue to green indicates the CsPbBr3 NCs emission wavelength.

We investigated the dependence of the photophysical properties of the as-synthesized NCs on the reaction temperature in the range of 60–160 °C. These NCs exhibited two peaks in the UV–vis absorption spectra: the first excitonic peak, between 450 and 500 nm, and a sharp peak centered at 313 nm (Figure 2a). The absorption peak at 313 nm originated from optical transitions between localized states of the isolated [PbBr6]4− octahedron of Cs4PbBr6 NCs.[44-46] X-ray powder diffraction (XRD) patterns of the NCs allowed us to confirm that the first exciton peak is related to the bandgap of the luminescent CsPbBr3 NCs. As shown in Figure 2b, the sample of synthesized NCs included both orthorhombic CsPbBr3 NCs and rhombohedral Cs4PbBr6 NCs, and the Cs4PbBr6-to-CsPbBr3 ratio increased as the reaction temperature decreased. This result is consistent with the increase in the intensity of the absorption peaks at 313 nm in the UV–vis spectra. As the reaction temperature decreased from 160 to 60 °C, the first exciton peak was blueshifted, and the emission peak center shifted from the green (504 nm) to the blue (470 nm) wavelength region (Figure 2c). Transmission electron microscopy (TEM) analysis also indicated that CsPbBr3 and Cs4PbBr6 NCs coexisted in the samples, and lowering the temperature reduced the overall size of the NCs (Figure 2d–g and Figure S2 (Supporting Information)). In addition, when the particle sizes (diameters) were measured by focusing on the luminescent CsPbBr3 NCs, the average size and relative standard deviations decreased from 9.3 to 3.5 nm and from 38.1% to 9.0%, respectively, as the reaction temperature decreased from 160 to 60 °C, because of the slow crystal growth rate at low temperature (Figure 2h). Because the size of the CsPbBr3 NCs synthesized at a temperature below 160 °C was smaller than the exciton Bohr radius (≈7 nm),[47] the blueshift in the emission peaks resulted from a quantum confinement effect, and the reduction in FWHM was due to the size distribution and defect density decrease. It should be noted that as the reaction temperature decreases, the PLQY increases from 57.7% to 90.1%, contrary to previously reported general trends of increasing crystallinity and PLQY with temperature (Figure 2i).[41]

Characterization of perovskite NCs depending on the reaction temperature. a) UV–vis absorption spectra, b) XRD patterns, and c) PL spectra of samples synthesized at 60, 80, 120, and 160 °C, respectively. The insets are photographs of perovskite NCs in nonpolar solvents acquired under 365 nm UV light illumination. TEM images of the samples synthesized at d) 60 °C, e) 80 °C, f) 120 °C, and g) 160 °C; the inset of (e) shows a high-resolution TEM image the of CsPbBr3 NCs prepared at 80 °C. h) Histograms illustrating the distributions of CsPbBr3 NC sizes for the samples prepared at different reaction temperatures. i) Variations in emission wavelength, PLQY, and FWHM of the as-synthesized products with reaction temperature.

2.2 Stability of In Situ Generated Perovskite NCs

In order to analyze the stability of the in situ generated CsPbBr3–Cs4PbBr6 NCs (denoted by ISNCs), we synthesized small CsPbBr3 QDs (denoted by C-QD113) as a control group using a conventional method based on hot injection of Cs-oleate at 90 °C with control of the OA and oleylamine (OLAM) ligands (details provided in the Figure S3 in the Supporting Information and the Experimental Section).[48] Conventional CsPbBr3 QDs (denoted by C-QD113) were obtained by hot injection of Cs-oleate at 90 °C with control of the OA and OLAM ligands. In general, small CsPbBr3 QDs have high surface energies and a large number of defects, and hence they can be ripened easily, resulting in a photoluminescence (PL) redshift. Upon examining the colloidal stability of ISNCs that had been stored under an environment with relative humidity of 65% and room temperature of 25 °C, little change in emission peak and FWHM was observed over 120 days, and 90% of the initial PLQY was retained (Figure 3a). By contrast, the emission wavelength of the C-QD113 without Cs4PbBr6 rapidly redshifted, moving from 458 to 475 nm within 30 days, even at room temperature (Figure 3d). To compare in detail the thermal stability of the prepared QDs, they were dispersed in toluene and incubated for a period of time at 120 °C. The C-QD113 were easily ripened and fused together at high temperatures, and the emission gradually shifted to longer wavelengths, with a green emission at 500 nm observed after 60 min of incubation (Figure 3e). The XRD data showed that a nonluminescent CsPb2Br5 tetragonal phase was formed simultaneously (Figure 3f). In the case of the ISNCs, the intensity of the emission centered at ≈510 nm increased slightly with time during the annealing process, but there was no shift in the position of the emission peak maximum and the blue emission was maintained with a slight decrease in PL intensity (Figure 3b). Furthermore, other crystal structures such as CsPb2Br5 were not generated, and the diffraction patterns indicated that the crystallinity of the ISNCs was improved (Figure 3c). These results clearly demonstrated that the Cs4PbBr6 effectively suppressed heat-induced aggregation and decomposition of small CsPbBr3 QDs. These significant changes could be attributed to a reduction in surface energy via surface passivation. When comparing the water contact angles of ISNC and C-QD113 films, the contact angle of the ISNC film was 107.2°, i.e., greater than that of the C-QD113 film, which was 65.4° (Figure S4, Supporting Information). The increased contact angle indicates a reduction in surface energy due to effective passivation of unsaturated atoms by hydrophobic ligands.[32, 34]

Stability of in situ generated perovskite NCs. Variations in emission wavelength, PLQY, and FWHM of a) in situ generated CsPbBr3–Cs4PbBr6 NCs (ISNCs) and d) conventional CsPbBr3 QDs (C-QD113) with incubation time under ambient conditions (relative humidity: 65%, temperature: 25 °C). Emission stability of b) ISNCs and e) C-QD113 in toluene at 120 °C; the insets show photographs of the respective NCs acquired under UV light illumination. XRD patterns of c) ISNCs and f) C-QD113 after annealing in toluene at 120 °C over various periods of time.

2.3 Effects of Cs4PbBr6 NCs on CsPbBr3 QDs Quality

We hypothesized that the cause of the high PLQY and good thermal stability of the ISNCs was related to the presence of the Cs4PbBr6 NCs generated with the CsPbBr3 QDs. To investigate the role of the Cs4PbBr6 NCs, we conducted a systematic study to monitor the differences in the optical properties, defect levels, and morphology of CsPbBr3 QDs with different amounts of added Cs4PbBr6 NCs. For this purpose, nonluminescent pure Cs4PbBr6 NCs (denoted by NC416) with diameters of 13.6 nm were prepared separately via a previously reported method (Figure S5, Supporting Information).[49] CsPbBr3 QDs were separated from a crude solution and then mixed with Cs4PbBr6 NCs in different weight ratios from 0 to 5 in a nonpolar solvent. The separation of the CsPbBr3 QDs from the crude solution involved extraction via a size selection process using methyl acetate as an antisolvent; a clear emission spectrum peaking at 477 nm was observed for the separated CsPbBr3 QDs. Because Cs4PbBr6 NCs and relatively large CsPbBr3 QDs were removed during the separation process, the absorption peak at 313 nm disappeared, and the PL peak was slightly blueshifted. The XRD and TEM results for the CsPbBr3 QD sample verified that the rhombohedral Cs4PbBr6 phase was not present and that the sample consisted entirely of CsPbBr3 QDs of 4.3 nm in size (Figure S6, Supporting Information).

After mixing the separated CsPbBr3 QDs (denoted by S-QD113) and NC416 for 1 h at room temperature in a weight ratio of 1:0 to 1:5, changes in morphology and optical properties were observed in a nonpolar solvent (Figure 4). As the relative NC416 amount was increased, the absorption intensity at 313 nm increased, while the first exciton peaks of S-QD113 gradually decreased in intensity and were blueshifted (Figure 4a). The emission peaks were also shifted to shorter wavelength, and the PL intensity was concomitantly improved (Figure 4b and Figure S7 (Supporting Information)). In addition to the optical properties, changes in particle size and morphology were observed. In the 1:2 ratio mixed solution, the size of the S-QD113 decreased from 4.32 to 3.87 nm, while the NC416 size increased from 13.38 to 14.51 nm (Figure 4c and Figure S8 (Supporting Information)). The morphology of the NC416 transformed from hexagonal to truncated diamond and assembled into zigzag shapes (Figure S9, Supporting Information). The etching of S-QD113 and the variation in the NC416 shape are a result of the high surface energy of the small CsPbBr3 QDs and the structural lability of perovskite as a function of its ligand environment. The CsPbBr3 phase was successfully converted into the Cs4PbBr6 phase and vice versa using excess OLAM and OA in a previous study.[50] In our system, NC416 had a relative excess of the OLAM ligand, so small S-QD113 were easily etched and the NC416 size was increased (Figure S10, Supporting Information).

Characterization of the interaction between CsPbBr3 QDs and Cs4PbBr6 NCs. a) UV–vis absorption and b) PL spectra for various ratios of separately prepared CsPbBr3 QDs (S-QD113) and Cs4PbBr6 NCs (NC416). c) NC size histograms before and after mixing S-QD113 and NC416 at a weight ratio of 1:2. d) PL decay curves of S-QD113 and mixed S-QD113 and NC416. e) Photographs acquired under UV light illumination of the as-prepared sample (left) and the sample after storage for 12 h under ambient conditions (right). f) PL spectra of film in part A and B of the sample slide after 0 and 12 h. Part A of the film was coated with S-QD113 only and part B was sequentially coated with NC416 and then S-QD113. g) Schematics of S-QD113 surfaces produced by mixing S-QD113 and NC416 in weight ratios from 1:0 to 1:2. Imperfect octahedrons on the S-QD113 surface were peeled off upon addition of NC416.

Time resolved photoluminescence (TRPL) measurements were performed to determine the effects of the PLQY enhancement along with NC etching on the optical spectroscopic characteristics (Figure 4d). Decay curves were used to analyze the excited state radiative relaxation dynamics. The S-QD113 decay curve was fitted to a biexponential function with a 3.89 ns average lifetime. When S-QD113 was mixed with NC416, the decay curve of the resultant mixture fitted a monoexponential function well, and the average lifetime was gradually increased to 5.25 ns. This increment in the average lifetime could be a result of various effects, such as energy transfer between S-QD113 and NC416, a S-QD113 size effect, and a reduction in trap-state density; hence, we decided to examine each of these possible causes in turn. First, Cs4PbBr6 NCs have a wider bandgap than CsPbBr3 NCs, so energy transfer is possible when the distance between the two materials is sufficiently close. Xuan et al. reported that the lifetime and PLQY were increased in a perovskite composite, in a study in which CsPbBr3 NCs were embedded in Cs4PbBr6 NCs.[38] In addition, Chen et al. synthesized CsPbBr3-embedded Cs4PbBr6 and analyzed the effect of metal halide interlayer in determining their photoluminescence excitation (PLE) properties.[51] To investigate the energy transfer, we acquired PLE and PL spectra of S-QD113 and ISNCs for various the excitation wavelengths (Figure S11, Supporting Information). The S-QD113 emission intensity gradually decreased as the excitation wavelength increased, in line with the trend observed for the PLE spectrum. The PLE spectrum of the ISNCs included a sharp drop centered at around 313 nm corresponding to the Cs4PbBr6 NC absorption peak. These results demonstrate that the origin of the PL of the ISNCs can be attributed to band edge emission of the CsPbBr3 QDs rather than energy transfer from the Cs4PbBr6 NCs. In the latter case, the ISNC emission would have improved significantly before the Cs4PbBr6 NC absorption region. Therefore, we excluded the energy transfer effect in ISNCs. Second, for perovskites, lifetimes tend to decrease as bandgaps widen.[52-55] Since S-QD113 is in the strong quantum confinement regime, the lifetime is expected to decrease with the decrease in particle size. However, in our study, when NC416 was mixed with S-QD113, the size of the S-QD113 particles decreased slightly because of surface etching. This result, indicating a longer lifetime for smaller NCs, was contrary to our expectation. Therefore, we conclude that surface passivation effects are likely to be the main reasons for the longer lifetime, as reported previously.[56, 57] The Cs4PbBr6 NCs might promote the elimination of defects and radiative recombination in the CsPbBr3 QDs.

To investigate in detail the optical properties of the NCs, the temperature-dependent PL of the S-QD113 and ISNC samples was measured (Figure S12, Supporting Information). The PL intensity of the S-QD113 sample gradually decreased as the temperature increased, in agreement with previous reports.[58-60] In metal halide perovskites, because of the low exciton binding energy, excitons are dissociated into free charge carriers by thermal energy, promoting nonradiative decay.[60-62] However, the ISNCs displayed a constant PL intensity, for the entire temperature range from 20 to 300 K. As the exciton binding energies of NCs are highly dependent on the size of the NCs,[59, 62] we expect that the exciton binding energy of the S-QD113 (≈4.32 nm) and ISNC (≈4.33 nm) samples would be similar. Therefore, the constant ISNC PL intensity as a function of temperature suggests that nonradiative decay paths are substantially reduced by the presence of Cs4PbBr6.

In addition, we induced an interaction between the S-QD113 and NC416 samples on the substrate to identify the effects of defect passivation in the solid state. First, half of the substrate was covered with Kapton tape (3M #5413) and then NC416 was spin coated on the exposed part. Subsequently, the experiment proceeded in order with the removal of ligands with methyl acetate, peeling off the tape, and coating S-QD113 over the entire substrate. As a result, half of the substrate was covered with only S-QD113, and the other coated with a double layer, with S-QD113 and NC416 in contact at the layer interface (Figure S13, Supporting Information). For this as-prepared film, initially there was no difference in emission between the two regions (A and B); however, the PL intensity was greatly improved in the double layer (layer B) after 12 h (Figure 4e,f). Although the interaction was relatively slow in the solid state, the S-QD113 defects were also passivated similar to the solution. We summarized the above-described mechanistic findings in a schematic illustration (Figure 4g). Surface reconstruction takes place at the interfaces between the CsPbBr3 QDs and Cs4PbBr6 NCs, and imperfect octahedrons on the CsPbBr3 QD surfaces are reduced during the etching process. In this process, the CsPbBr3 QD crystal size decreases, resulting in a blueshift of the emission wavelength, and the optical properties and colloidal stability are increased by the addition of Cs4PbBr6 NCs.

2.4 Fabrication and Characterization of Blue Perovskite LEDs

Encouraged by the high PLQY and stability of the ISNCs, we constructed LEDs with glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(p-styrene sulfonate) (PEDOT:PSS)/poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4ʺ-(N-(4-sec-butylphenyl)diphenylamine)] (TFB)+ poly(9-vinylcarbazole) (PVK) (30 nm)/ISNCs (20 nm)/2,2ʺ,2ʺ-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) (70 nm)/LiF (1 nm)/Al (100 nm) structures. The thickness of each layer was confirmed via the acquisition of cross-sectional scanning electron microscopy (SEM) images (Figure 5a). Energy band diagrams for the materials employed in the LEDs are shown in Figure 5b. Due to the low Cs4PbBr6 HOMO energy level (7.2 eV), a triple hole injection layer including a polymer with a deep work function was used for efficient hole injection. The device performance characteristics of the ISNCs LEDs are shown in Figure 5c–f and Table S1 (Supporting Information). The LEDs fabricated with ISNCs exhibited a maximum luminance of 23 cd m−2 and EQE of 4.65% at wavelength of 480 nm. Histogram of maximum EQEs of ISNCs LEDs are shown in Figure S14 (Supporting Information).

Blue perovskite LED device structure and performance characteristics. a) Cross-sectional SEM image of ISNCs LED. b) Energy band diagram of the materials employed in LEDs. c) Current density–voltage (J–V) curve and luminance–voltage (L–V) curve, d) luminous efficiency–current density (LE–J) curve, and e) EQE–current density (EQE–J) curve of ISNCs LEDs. f) EL spectra of ISNCs LEDs at various voltages.

One of the most important problems for blue-emitting metal halide perovskites is the spectral instability that results from halide segregation. LED emission spectra measured using various voltage bias values are shown in Figure 5e. Because the ISNCs exhibited blue emission at 480 nm without halide mixing, the ISNC LEDs exhibited stable electroluminescence (EL) emission spectra over the entire range of device operating voltages.