Rapid synthesis of hybrid methylammonium lead iodide perovskite quantum dots and rich MnI2 substitution favouring Pb-free warm white LED applications

We present a facile room temperature synthesis of CH3NH3Pb1−xMnxI3 perovskite quantum dots (PQDs) substituting manganese (Mn2+) at the lead (Pb2+) sites to minimize environmental pollution and make it commercially feasible. By varying the concentration of Mn2+ from 0 to 60%, the PQDs exhibit strong color tunability from red to orange color suggesting successful energy transfer due to Mn2+ inclusion. We observed a high external photoluminescence quantum yield (PLQY) of 98% for unsubstituted CH3NH3PbI3 and >50% for up to 15% Mn2+ substituted PQDs. The average lifetime of PQDs was found to shorten with increasing Mn2+ replacement. We demonstrate a white LED prototype by employing the CH3NH3Pb1−xMnxI3 PQDs with green QDs on a blue LED chip. The CRI and CCT value varying from 92 to 80 and 5100 K to 2900 K, respectively, indicate the usability of the Mn2+ substituted PQDs as efficient warm white LEDs with a promising CRI and good stability.


Introduction
The discovery of hybrid perovskite quantum dots (HPQDs) (CH 3 NH 3 PbX 3 , X ¼ Cl, Br, I) has opened a new era of research and development in new generation lighting technology as quantum dot light emitting diodes (QDLEDs) and backlight applications owing to their superior narrow emission, high photoluminescence quantum yield (PLQY), wide color range, and long diffusion length with high absorption coefficients. [1][2][3][4] Moreover, the combination of organic/inorganic characteristics and an easy solution based synthesis approach at low temperature imparts a unique property to these materials. However, the presence of toxic Pb 2+ at the B site in an ABX 3 structure has restricted its commercialization. 5,6 The use of heavy metals, including lead in an electronic device has already been restricted in the European Union, and other countries are also planning to introduce similar regulations in the near future. 7 Therefore, the development of Pb 2+ free or less Pb 2+ based HPQDs that retain the excellent features of the original PQDs is obligatory. Cation exchange or substitution of Pb with divalent cations, such as, Cu 2+ , Zn 2+ , Sn 2+ , and Mn 2+ , could be a promising approach to modulate the optical and electronic properties of HPQDs. 8,9 The forbidden 4 T 1 / 6 A 1 transition of Mn 2+ makes this cation a suitable dopant to act as an economical colour emitter as well as decreasing the toxic level of the Pbbased perovskites due to its intense orange emission which remains independent of the physical and electronic conguration of the host. 6,9 The lead-free mixed halides CsSnX 3 (X ¼ Cl, Br, I) fabricated by Jellicoe and group had stability issues under ambient conditions and had a low PLQY (<10%) due to the easy oxidation of Sn(II) to Sn(IV). 10 Liu et al. reported the partial replacement of Pb 2+ with Mn 2+ in CsPb 1Àx Mn x Cl 3 PQDs (x ¼ 0.3 # x # 0.4) with maximum PLQYs up to 54% via a hot injection route. 11 Additionally, Mn 2+ doping was also reported with CsPbBr 3 , CsPbCl 3Àx Br x and CsPbI 3 inorganic perovskite QDs by different groups. 12,13 On the other hand, no reports are found on Pb substituted HPQDs and most of the studies were found on MAPbBr 3 QDs due to their high stability and PLQY. The rst solution based HPQDs were fabricated by the Pérez-Prieto group in 2014 with a PLQY of 20%. 14 Aer that, Zhang et al. reported CH 3 NH 3 PbBr 3 HPQDs in 2015 with an absolute quantum yield of up to 70% with the modication of the ligands in a room temperature process 15 and recently the PLQY reached up to 100% by a spray synthesis route. However, obtaining high QY with CH 3 NH 3 PbI 3 is still challenging with the maximum being 56% due to the higher sensitivity of iodine to moisture and air. 16,17 Therefore, we need to work on increasing the PLQY for red QDs that can be used for red LEDs and perovskite quantum dot solar cells because for photovoltaic devices a MAPbI 3 based material was found to be the best absorber.
To the best of our knowledge, Pb 2+ substitution with Mn 2+ in CH 3 NH 3 PbI 3 PQDs by a room temperature synthesis method has not been reported to date. Therefore, in the present work, we present a novel approach to obtain CH 3 NH 3 PbI 3 HPQDs with a PLQY of up to 98% and CH 3 NH 3 Pb 1Àx Mn x I 3 HPQDs via Mn 2+ substitution to reduce the toxicity of PQDs. We further demonstrate a white LED prototype by employing the as-prepared best CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs as color conversion materials, with green QDs on a blue LED chip to prove the probable commercialization of the present materials in the future for general lighting applications such as QDLEDs and backlight systems.
Synthesis of methylamine iodide (MAI). In a 250 ml round bottom ask, 20 ml methylamine, 8 ml methanol and 30 ml hydroiodic acid were mixed at 0 C in an ice bath setup with continuous stirring for 2 h. The obtained solution was placed in a vacuum oven at 60 C for 24 h to remove all the solvents. The nal product was washed two times with diethyl ether to get the MAI salt and stored at dry place.
Synthesis of CH 3 NH 3 PbI 3 and CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs. Colloidal CH 3 NH 3 PbI 3 and CH 3 NH 3 Pb 1Àx Mn x I 3 QDs were synthesized by following the LARP technique, which is described in the following section. In the typical synthesis of CH 3 NH 3 PbI 3 perovskite QDs, a mixture of 0.1 mmol (0.0159 gm) CH 3 NH 3 I and 0.1 mmol (0.0461 gm) PbI 2 was dissolved in 1 ml DMF at 60 C forming a 0.1 mmol solution. Then, 200 ml of oleic acid and 200 ml of the oleylamine were added to the perovskite solution. 40 ml of precursor perovskite solution was then injected into 6 ml of toluene with vigorous stirring at 70 C. Along with mixing the perovskite precursor in toluene, bright green emitting nanoparticles were formed within seconds. Aer centrifugation at 7000 rpm for 10 minutes to discard the larger particles, a red transparent colloidal solution was obtained. A schematic illustration of the synthesis procedure is shown in Fig. 1.
Similarly, for synthesis of CH 3 NH 3 Pb 1Àx Mn x I 3 QDs, a mixture of 0.01 mmol (0.0159 gm) of CH 3 NH 3 I and (0.1 À x) mmol of PbI 2 and (0.1x) [here x ¼ 0.05 to 0.60] mmol of MnI 2 was dissolved in 1 ml of DMF. The rest of the process was the same as the CH 3 -NH 3 PbI 3 QD synthesis. Further details of precursors and conditions are provided in Table S1. † The reason behind the selection of solvent is explained in the ESI. † In the case of iodine-based HPQDs, the selection of solvent is very important because such types of perovskite are very sensitive to the atmosphere and they are also very reactive due to the presence of I À ions. Therefore, a weak (non-polar) solvent is selected which can react very fast with the perovskite material. The reactivity of chloroform is higher than that of toluene due to the high dielectric constant of chloroform (3 ¼ 4.81). Hence only chloroform or a mixture of chloroform and toluene is used for CH 3 NH 3 PbI 3 HPQDs to promote better and fast nucleation. On the other hand, in the case of toluene the reaction kinetics is comparably slow and may lead to the destruction of the QDs during the centrifugation because of the higher sensitivity and instability of iodine-based perovskites.

Characterization
The XRD of the perovskite QDs was measured using a Bruker D8 powder XRD with Cu Ka radiation over the range of 10 < 2q < 60 with a step size of 0.02 and operating at 40 kV to 40 mA. TEM images and HRTEM patterns were recorded using a JEOL highresolution transmission electron microscope (HR-TEM) equipped with a LaB6 lament and CCD camera. Samples of different PQD samples for TEM analysis were prepared by casting 10 ml of colloidal solution onto a standard copper grid. The size distribution and particle size of PQDs were obtained from the TEM images and the d value was calculated from the HRTEM patterns with ImageJ soware. Optical UV-vis absorption spectra were measured using a Hitachi U-2900. Photoluminescence (PL) spectra of PQDs were recorded using an FS-5 Fluorescence Spectrophotometer at 420 nm excitation wavelength in the wavelength range of 500 to 820 nm. The photoluminescence decay time curves were measured using a time correlated single photon counting (TCSPC) system on an FS-5 Fluorescence Spectrophotometer PL system equipped with a 150 W xenon lamp and a 360 nm laser source respectively. The absolute quantum yield (QY) of each PQD sample was determined using a Horiba Jobin Yvon Fluorolog according to the given equation: For luminescent materials, PLQY characterization is very important for a deep understanding of molecular and light absorbing/emitting properties. Mostly, PLQY is measured using an integrating sphere. From this technique, PLQY can be determined directly. In this tool, a sphere is coated with all reective surfaces to capture all the light going in and out of the sphere. The PLQY measurement helps to nd the uorescence emission (E c ) and the scattering (L c ) of the sample and also the emission and scattering of a blank i.e. E a and L a respectively. So with the help of an integral sphere setup, spectral measurements of PLQY can be measured using the given formula;

Results and discussion
A series of Mn substituted CH 3 NH 3 PbI 3 based HPQDs were synthesized by a ligand assisted room temperature approach.
The synthesis procedure, the selection of solvent and the thorough description are included in the ESI and the detailed chemical compositions are also given in Table S1. † Fig. 2(a-g) show the phase identication, microscopic characterization, luminescence and time resolved spectroscopy of CH 3 NH 3 PbI 3 HPQDs. Typical TEM images of the obtained HPQDs showed the homogeneous distribution of tiny spherical HPQDs with an average particle size of $3 nm and distribution of 0.03 nm as shown in Fig. 2(a) and (b). The HRTEM image of CH 3 NH 3 PbI 3 QDs (bottom part of Fig. 2(a)) showed a good crystalline structure with an interplanar distance of 0.31Å, which corresponds to the (004) plane of CH 3 NH 3 PbI 3 . 18 Fig. 2(c) shows the XRD pattern of the synthesized CH 3 -NH 3 PbI 3 HPQDs. All diffraction peaks matched well with the reported peaks of CH 3 NH 3 PbI 3 HPQDs indicating the presence of a tetragonal structure with space group I4/mcm. 19 VESTA soware was utilized to draw the crystal structure of the obtained HPQDs fused with one Pb 2+ , one CH 3 NH 3 + , and three iodine anions in the unit cell as shown in Fig. 2(d). The structural and morphological analyses indicated the formation of a CH 3 NH 3 PbI 3 perovskite structure in QD form via the present synthesis route. The visual appearance of chloroform/toluene solution of CH 3 NH 3 PbI 3 HPQDs under a 365 nm UV-light source shows a bright red emission (Fig. 2(e)). The reason behind the selection of solvent is explained in the ESI. † Fig. 2(f) shows the steady-state absorption and photoluminescence (PL) spectra of CH 3 NH 3 PbI 3 HPQDs. Broad and strong absorption in the visible region and near-infrared region revealed the promising light absorbing quality of QDs. Additionally, the sharp absorption edge of the sample also suggested a direct bandgap nature. 20 The PL emission of the obtained PQDs was also observed at 700 nm with a high intensity and color purity. The absolute PLQY of CH 3 NH 3 PbI 3 HPQDs was estimated to be 98%. To the best of our knowledge, it is the highest value reported for CH 3 NH 3 PbI 3 HPQDs prepared under ambient conditions to date. The PL decay curve can be tted well with a single exponential function for CH 3 NH 3 PbI 3 PQDs to obtain an average lifetime of 98.29 ns (Fig. 2(g)). The above discussion indicates an efficient fabrication of red HPQDs with PLQY > 98% and efficient lifetimes. 21 The XRD pattern of CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs with different concentrations of Mn 2+ substituting Pb 2+ ions is shown in Fig. 3(a). The XRD pattern indicates the presence of a tetragonal phase for all the synthesized samples even aer Mn doping. However, a minimal shiing towards higher 2q was observed for the Mn incorporated perovskites. The higher angle shiing is clear from the variation of the enlarged (110) XRD peak position with different Mn concentrations from 0% to 60% as shown in Fig. 3(b and c). The shiing can be ascribed to the substitution of Mn 2+ ions with smaller ionic radii compared to Pb 2+ ions (see    TEM images of 5%, 15%, 30% and 45% Mn 2+ ion doped CH 3 NH 3 Pb 1Àx Mn x I 3 HPQDs are shown in Fig. 4(a-d) respectively. TEM images of all HPQDs showed spherical dots with an average particle size in the range of $1 to 3 nm. The distribution density of particles decreased with the substitution of Mn 2+ ions with Pb 2+ ions in CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs owing to the sensitivity of Mn 2+ under ambient conditions compared to Pb 2+ . 22 It is well known that agglomeration in QDs will be less owing to the high surface energy. As we explained in the structural section, according to Vegard's law, the lattice constants and cell volume are directly proportional to the ionic radii of cations present in the compound. Hence, with the substitution of Mn 2+ ions instead of Pb 2+ ions the lattice constant is observed to decrease resulting in cell volume contraction. Such reduction in cell volume due to Mn 2+ substitution in CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs further results in the reduction of particle size. Fig. 5(a) shows the steady-state absorption and PL of CH 3 -NH 3 Pb 1Àx Mn x I 3 HPQDs with the variation of Mn 2+ from 0 mol% to 45 mol%. Fig. 6(a-c) show the variation of absorption and PL spectra of CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs with continuous variation of Mn 2+ ion concentration from 0 mol% to 60 mol%. The absorption spectra showed a blue shi aer the replacement of Pb 2+ with Mn 2+ . The emission peaks of CH 3 NH 3 Pb 1Àx Mn x I 3 HPQDs also shied from 700 to 600 nm indicating successful replacement of Pb 2+ in CH 3 NH 3 PbI 3 HPQDs. 23a The changes in the color of HPQDs suggest the successful energy transfer from PQDs to Mn 2+ dopants in the CH 3 NH 3 Pb 1Àx Mn x I 3 perovskite samples as shown in Fig. 4(e). Fig. 4(b) displays the 3D scan of the variation of PL emission intensity and wavelength with the increase in Mn 2+ concentration. PL emission intensity was found to decrease along with a blue shi in color emission as clearly visible in the photographs of the resulting CH 3 NH 3 -Pb 1Àx Mn x I 3 HPQD colloidal solutions (Fig. 5(d) and Fig. 6(a-c)). Furthermore, to better enable the comparison of color variation, the inuence of Pb 2+ to Mn 2+ cation exchange on the color coordinates of CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs is shown in the ESI † and Fig. 6(d and e).
The PL emission spectra reveal an interesting broadening for Mn content along with an obvious blue shi due to the combined effect of quantum connement and Mn incorporation. It is well known that agglomeration in QDs will be less owing to the high surface energy. Moreover, as the ionic radius of Mn 2+ ions is lower than that of Pb 2+ ions, the lattice volume starts to shrink with the incorporation of Mn 2+ ions into the CH 3 NH 3 PbI 3 host. And hence the particle size starts to decrease with the incorporation of Mn 2+ ions in place of Pb 2+ ions as is clear from the TEM image and corresponding statistical distribution. As the particle size starts to decrease from approximately 4 nm to 1.5 nm, the quantum connement plays a role for the blue shiing of PL emission. Fig. 6(a) shows the steadystate absorption and PL of CH 3 NH 3 Pb 1Àx Mn x I 3 HPQDs with the variation of Mn 2+ from 0 mol% to 60 mol%. The absorption spectra showed a blue shi aer the replacement of Mn 2+ in Pb 2+ sites. However, careful examinations of PL emission spectra reveal an interesting broadening with the increase in Mn concentration as shown in Fig. S3. † The emission spectra of CH 3 NH 3 Pb 1Àx Mn x I 3 with x ¼ 0, 0.05, 0.15, 0.30 and 0.60 are shown in Fig. S3(i-v) † respectively. The emission spectra of CH 3 NH 3 PbI 3 shows a broad band around 700 nm which is blue shied compared to its bulk counterpart (Fig. S2 †). With the increase in Mn 2+ concentration, the peak position blue shied due to the quantum connement effect as discussed earlier.
Hassan et al. have also observed similar results to ours. 23b Fig. 4 TEM characterization of CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs with different Mn 2+ concentration and their corresponding statistical analysis graph of particle size distribution (a) 5% with average particle size 3.5 nm, (b) 15% with average particle size 3 nm, (c) 30% with average particle size 2 nm, and (d) 45% with average particle size 1.5 nm.
However, the PL emission peak can be deconvoluted into two distinguished peaks centred at 634 and 600 nm. According to the results of Hassan et al., in the present research, 634 nm emissions can be assigned to the characteristic emission of the CH 3 NH 3 PbI 3 perovskite host. Zou et al. 23c have reported the dominant broad emission band peaking at $600 nm of the perovskite host, which can be readily ascribed to the 4 T 1 / 6 A 1 transition of Mn 2+ doped in CH 3 NH 3 Pb 1Àx Mn x I 3 . When the Mn 2+ concentration is more than 30%, the contribution from Mn 2+ is dominant over the host perovskite and hence at 60% Mn 2+ doping only a 600 nm peak is visible. Furthermore, CH 3 -NH 3 Pb 1Àx Mn x I 3 (x ¼ 0) and CH 3 NH 3 Pb 1Àx Mn x I 3 (x ¼ 0.3) PQDs exhibit almost the same PL excitation spectra when monitoring their excitonic or Mn 2+ -related emissions. The similar excitation features suggested a clear energy transfer from the CH 3 NH 3 PbI 3 host Mn 2+ . 23 Time-resolved PL spectroscopy was used to correlate the variation of the PL emission efficiency of the CH 3 NH 3 Pb 1Àx -Mn x I 3 (x ¼ 0 to 0.60) as shown in Fig. 5(c) and S4(a-c). † For CH 3 NH 3 PbI 3 and CH 3 NH 3 Pb 0.95 Mn 0.05 I 3 PQDs, single exponential PL decay was observed reecting the homogeneous distribution of the elements. The result was similar to PL decay in Mn 2+ : ZnSe and Mn 2+ :CdS/ZnS nano-crystals as reported earlier. 24,25 However, for more than 5% Mn 2+ doping, the PL decay curve could be tted well with a biexponential function.
For more than 5% Mn 2+ doping, the PL decay curve could be tted well with a bi-exponential function as follows where, A, A 1 , A 2 and, t represent constants and time, respectively, while s 1 and s 2 represent short-lived and long-lived decay lifetimes that originated from the trap-assisted recombination and carrier radiative recombination, separately. The average lifetime of Mn 2+ doped PQDs can be measured as per the formula The average lifetime of 0 to 60% Mn 2+ doped CH 3 NH 3 PbI 3 PQDs gradually decreases as the emission shis to a shorter wavelength because of the increased content of Mn 2+ (Mn 2+ has a lower atomic no. than Pb 2+ ). CH 3 NH 3 PbI 3 and CH 3 NH 3 -Pb 0.95 Mn 0.05 I 3 PQDs have a longer lifetime that can be attributed to the lower non-radiative energy transfer, trap-defects and surface states of the QDs. The decrease in s av with increase in Mn 2+ concentration in CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs is shown in Fig. S4. † The maximum color purity (details are given in the ESI †) was estimated to be 97% for CH 3 NH 3 PbI 3 and it continually decreased with the elevation of the doping concentration of Mn 2+ (Fig. 5(f)). The effect on PLQY of Mn 2+ concentration in CH 3 NH 3 PbI 3 PQDs is shown in Fig. 5(f). The total external PLQY of CH 3 NH 3 PbI 3 and CH 3 NH 3 Pb 0.95 Mn 0.05 I 3 was observed to be 98% and 86%, respectively, and then decreased with further increase in Mn 2+ concentration. Promising PLQY has been obtained at even low concentrations of Mn 2+ (up to 20%). It is the highest concentration when compared to analogous results for Mn 2+ doped nanocrystals. [26][27][28][29] The absolute PLQY, average lifetime and color purity of PQDs with different concentrations of MnI 2 are summarized in Table 1 and a plot between the PLQY and color purity is also shown in Fig. S5. † It is evident that with the Mn 2+ ion substitution in CsMn 1Àx Pb x I 3 PQDs, the PLQY is observed to drastically reduce from 98% to 8%. The reason for such a decrease could be attributed to the change in the energy band structure due to Mn 2+ ion substitution. In the case of manganese ion substitution, the electrons are distributed between t g 2 and e g bands according to crystal eld stabilization theory. However, for Mn 2+ ions the unpaired electrons in the d orbital remain distributed in the low energy t g 2 band leaving behind empty e g bands. In the case of excitation, the electrons in the t g 2 band may get excited to the e g band, which is essentially a non-radiative transformation owing to its low energy difference, and this mechanism is shown in Fig. S6. † In such a scenario, the excitation of electrons leads to a radiation quenching effect. Thus low photo luminescence quantum yields (PLQYs) and reduced lifetimes are observed in the case of Mn 2+ ion substitution.
The obtained CH 3 NH 3 Pb 1Àx Mn x I 3 PQDs were utilized to fabricate LED devices. In the present case, the perovskite emitters were integrated as color converting layers combined with blue LEDs and the color parameters of the obtained W-LEDs are shown in Fig. 7. From the CIE chromaticity diagram ( Fig. 7(a)) it is clear that the connection lines of the color point of blue LED with green and red PQDs always pass through the white emission and hence the combination can produce white light consistently. Therefore, white light emissions were simulated by the linear combination of the emission of green and red PQDs with blue LEDs. Color correlated temperature (CCT) and color rendering index (CRI) values of the corresponding white emission were evaluated with the help of methods reported earlier. 30,31 The evaluated data were used to plot a CCT-CRI diagram as shown in Fig. 7(b). The simulation results indicated that warm white light with CCT < 5000 K and CRI > 80 (blue shaded area) can be simply realized in the current combination. In the present simulation, a high CRI of 92 was obtained at a CCT of 5100 K and the lowest CCT value of 2900 K with a CRI of 80 was obtained. The simulation results indicated that the present combination can produce an efficient warm white light with a promising CRI. 32 Therefore, a series of WLEDs were fabricated by encapsulating a blue InGaN LED chip with various amounts of green and red PQDs to validate the simulation results. The electroluminescence (EL) spectra of the fabricated WLED are shown in Fig. 7(c). The inset of Fig. 7(c) presents the fabricated WLED with high brightness. WLEDs with chromaticity coordinates at (0.34, 0.37), a CRI of 91 and a CCT of 4000 K were obtained and are shown as star points in Fig. 7(a). Different color rendering parameters for the fabricated WLEDs are shown in Fig. 7(d). The gure indicated the promising CRI nature of the obtained WLEDs. The CCT values can be tuned with the change in red to green ratios from cool light (11 000 K) to warm light (3800 K) as shown in Fig. 7(e).
The shaded region in Fig. 7(e) conrmed the presence of warm white light emission from the present materials by controlling the amount of green and red emitting PQDs. 33 The results are consistent with the simulation results. The combination of a blue LED chip and green and red PQDs results in efficient warm white light (4000 K) with a high CRI (91) along with an improved stability since only $10% of the initial intensity is lost aer 200 h under accelerated aging conditions (85 C and 85% relative humidity) as can be observed in Fig. 7(f). From the above discussion it is clear that, the present PQDs can act as an efficient red emitting material for warm white light emitting applications.

Conclusion
In conclusion, we have offered an inexpensive and speedy room temperature synthesis method for the fabrication of CH 3 NH 3 -Pb 1Àx Mn x I 3 PQDs with spherical shape and a particle size of $1.5 to 3 nm. Absorbance and PL emission studies show that with increasing Mn 2+ substitution of Pb 2+ , in CH 3 NH 3 PbI 3 , the PQDs produce an emission colour varying from red to orange due to efficient energy transfer from QDs to Mn 2+ ions. The unsubstituted PQDs showed PLQYs of nearly 98% while with 5% Mn 2+ substitution the PLQY decreases to 86% and further declines with increasing concentration of Mn 2+ . The average lifetime of pure (CH 3 NH 3 PbI 3 ) and PQDs with very low concentration of Mn 2+ was observed to be longer than that of the PQDs with higher Mn 2+ concentration. The fabricated series of WLEDs combining a blue InGaN LED chip with various amounts of green and red PQDs produced warm white light with CCT < 5000 K and CRI > 80. The best WLED with chromaticity coordinates at (0.34, 0.37), a CRI of 91 and a CCT of 4000 K was obtained with an improved stability since only $10% of the initial intensity is lost aer 200 h under accelerated aging conditions (85 C and 85% relative humidity). These results conrm the commercial usability of the present PQDs for general lighting applications as environment friendly QD-LEDs and backlight systems.

Conflicts of interest
There are no conicts to declare.