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  1    Introduction

  Organic light-emitting diodes (OLEDs) have been a research focus for almost 30 years owing to their great applications in flat-panel displays, solid-state lighting and wearable electronics, since Tang's work in the late 1980s.^[1~3] Among different types of materials for high performance OLED devices, the good luminogens acting as emissive layers is generally considered as the key component, either in enhancing the performance of an OLED device or being helpful to simplify the device configuration and lower the construction costs.^[4~8]

  So far, a variety of luminogenic materials have been reported with good performance, however, most of them usually suffered from aggregation-caused quenching (ACQ): their good emission in solution are often greatly weakened or quenched at high concentration, owing to the formation of excimers or exciplexes in aggregated states.^[9] Thanks to the pioneer work of Ben Zhong Tang' group, the design of AIEgens with the characteristic of aggregation induced emission (AIE) has been proved to be an effective approach to tackle this ACQ problem.^[10] Nowadays, AIE has become a hot topic in the research field of advanced functional materials.^[11]

  On the other hand, in full-color displays, the highly efficient and stable blue emitters are badly needed, since the blue emitter can not only effectively reduce the power consumption of the devices but also be utilized to generate emission of other colors by energy transfer to a suitable emissive dopant.^[12] However, unlike their red and green counterparts with satisfactory performance, efficient and stable luminogens with standard blue or deep blue were still relatively rare, because of the intrinsic large bandgap and the encountered problems of stability.^{[13, 14]} Thus, it is urgent to develop high efficient blue and deep blue luminescent materials. Among the typical AIE luminogens, tetraphenylethylene (TPE) and many of its derivatives like triphenylethylene (triPE) enjoy the advantages of facile synthesis and outstanding AIE effects, but none of them is an efficient emitter in deep-blue region. Once some aromatic groups were introduced to enhance the LED performance of the designed AIEgens, their emissions are always red-shifted, departing from the required blue or deep-blue region. According to our previous works and the reported results of other groups, the unpleasant red-shift could be controlled in a large degree through changing the linkage mode, constructing more twisted configuration and decreasing the conjugated unit.^[15]

  Also, in the fabrication of the OLED device, there are two mainly approaches to obtain a good thin film of the organic emitting layer: vacuum deposition and solution process. In comparison with vacuum deposition, the solution process is generally accepted as a more feasible means to realize low-cost, large-area OLED displays or lighting products. But there are very few reports about spin coating of small molecules. Actually, in our previous works, we always used vacuum deposition in the fabrication of the OLED device. Thus, is it possible to develop some AIEgens applicable for the fabrication in solution process by spin coating?

  Prompted by the above points, in this communication, we utilized the special sp³ hybridization of tetraphenylmethane to break and control the intramolecular conjugation, and used silicon atom to exchange carbon atom in tetraphenylmethane as tetraphenylsilane. Around the core of tetraphenylmethane or tetraphenylsilane, TPE and triPE were utilized as the rotors. Accordingly, six AIEgens, with their structures presented in Chart 1, were successfully obtained. C-4triPE, one of the six molecules was once synthesized by Wang et al. They coordinated the triPE units around a tetrahedral junction site, with the aim to minimize the tendency of conjugated organic fragments to crystallize.^[16] And really, these AIEgens synthesized by our group could be fabricated in OLED devices by the solution process conveniently, with a maximum luminance (L_max) of 1730 cd·m^-2, a maximum current efficiency (η_{C, max}) of 2.21 cd·A^-1, a maximum power efficiency (η_{p, max}) of 0.77 lm·W^-1, a maximum external quantum efficiency (η_{ext, max}) of 1.01%, respectively. Here, we would like to present their synthesis, characterization, theoretical calculation, and photophysical properties in detail.

  

  Figure Chart 1. Chemical structure of C-4pTPE, C-4mTPE, C-4triPE, Si-4pTPE, Si-4mTPE and Si-4triPE

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  2    Results and discussion

  2.1    Synthesis

  The six target AIEgens were successfully obtained through Suzuki cross-coupling reactions (the detailed procedures and characterization data are given in the supporting information) and they were all fully characterized by ¹H-NMR and ¹³C-NMR, mass spectrometry and elemental analysis.

  2.2    Thermal properties

  Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to investigate the thermal properties of these six AIEgens. Figure S3 shows that they were all thermally stable and the thermal decomposition temperatures (T_d, corresponding to 5% weight loss) were all higher than 380 ℃. C-4mTPE and Si-4pTPE have relatively high glass transition temperatures (T_g) as 142℃ and 137 ℃, respectively (Table 1). Unfortunately, the DSC curves of the other four molecules showed no obvious T_gs (Figure S4). The high T_d values should be attributed to their large molecular weights. The high T_d and T_g indicate that these six molecules have excellent thermal and morphological stability, which can contribute to the fabrication of the homogeneous and stable amorphous emissive layer in OLED devices.

  Table1. The thermal, electrochemical and photophysical data of the luminogens

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  	Tda/℃	Tg/℃	Egb/eV	EHOMOc/ELUMOd (eV)	λabse/nm	PL λmax(film)/nm	ФF(solution)/%	PL λmax(aggr)f/nm	ФF(aggr)/%	α(AIE)g
  C-4pTPE	480	-	3.80	-5.57/-1.77	326	492	0.10	491	63	634
  C-4mTPE	494	142	3.95	-5.49/-1.54	314	465	0.10	469	26	257
  C-4triPE	496	-	3.72	-5.66/-1.94	333	448	0.30	451	46	135
  Si-4pTPE	471	137	3.79	-5.59/-1.80	327	487	0.080	490	36	449
  Si-4mTPE	500	-	3.94	-5.66/-1.72	315	464	0.20	469	17	85.9
  Si-4triPE	385	-	3.75	-5.70/-1.95	331	449	0.10	449	36	363
  a5% weight loss temperature measured by TGA under N2.b Band gap estimated from optical absorption band edge of the solution. c Calculated from the onset oxidation potentials of the compounds. d Estimated using empirical equations ELUMO=EHOMO+Eg. e Observed from absorption spectra in dilute THF solution. f Determined in V(THF):V(H2O)=1:99 solution. gαAIE=ФF(aggregation)/ФF(solution).

  Table1. The thermal, electrochemical and photophysical data of the luminogens

  2.3    AIE properties

  All of the six molecules are soluble in common organic solvents, such as THF, chloroform and toluene, but insoluble in water. Figure S6 in the Supporting Information shows the UV-vis absorption spectra of C-4pTPE, C-4mTPE, C-4triPE, Si-4pTPE, Si-4mTPE and Si-4triPE in THF. The UV-vis spectra of the molecules using tetraphenylmethane as core are correspondingly similar to the molecules with the tetraphenylsilane core, possibly due to the similar structure and conjugation lengths of tetraphenylmethane and tetraphenylsilane. The UV-vis spectra of C-4triPE and Si-4triPE are similar, with the maximum absorption wavelength (λ_max) at 333 nm and 331 nm, respectively. The spectra of C-4pTPE and Si-4pTPE are almost the same, with (λ_max) at 326 nm and 327 nm, respectively, a little blue-shifted in comparison with those of C-4triPE and Si-4triPE. That might be attributed to the shorter conjugation lengths of C-4triPE and Si-4triPE rather than C-4pTPE and Si-4pTPE, due to the absence of one phenyl from TPE to triPE. As for C-4mTPE and Si-4mTPE, they also have similar spectra with blue-shifted λ_max at 314 and 315 nm, respectively, suggesting that they possess lower conjugation. Due to the meta-position linkage mode, the conformation of C-4mTPE and Si-4mTPE should be more twisted between TPE and the tetraphenylmethane and tetraphenylsilane core, leading to their shorter effective conjugation lengths, as coupled with the bad conjugation effect also derived from the meta linkage mode. This confirmed the powerful control of the intramolecular conjugation by simply changing the linkage mode.

  In order to investigate the AIE characteristics of these six luminogens, their fluorescent behaviors were studied. We chose THF as good solvent and water as poor solvent due to their miscibility. They are all nearly nonemissive when readily dissolved in pure THF, but with the water fraction increasing, the PL intensity gradually increased. The detail data including PL change and fluorescent images of the fluorophores in THF and THF/water mixtures are shown in the Figure S5 in the supporting information. Taking C-4mTPE as an example. Figure 1 clearly shows the PL change, fluorescent image and quantum yield in THF and THF/water mixtures. The PL curves of C-4mTPE is nearly a flat line coinciding to the abscissa when dissolved in pure THF, demonstrating the faint emission property in the solution state. And the PL intensity of C-4mTPE remains low in aqueous mixtures with less than 50% water fraction but starts to increase swiftly afterwards. The emission intensity reaches its maximum at 95% water fraction. From pure THF solution to the THF/water mixture with 95% water content, the PL intensity rises by 105-fold, which can also be verified by visual observations. When illuminated under 365 nm UV lamp, its THF solution emitted no observable light, but intense emission was clearly observed from the THF/water mixture with 95% water content (Figure 1A). Similar phenomena are also observed for other molecules. Clearly, the emission is induced by the aggregate formation, confirming that they are AIE-active. In dilute solution, the rotation of multiple phenyl rings has consumed the energy of the excitons through the nonradiative relaxation channel and thus quenched the light emission of the dye molecules. In aggregate state, the intramolecular rotation is restricted, thus allowing the luminogens to emit intensely.

  

  Figure 1. (A) PL spectra of C-4triPE in different THF/H₂O ratio (f_W=water fraction). Inset: photos of C-4triPE in THF/water mixtures (f_W=0% and 90%) under UV lamp illumination. Concentration=10 μmol/L, excitation wavelength: 310 nm. (B) Plots of fluorescence quantum yields of C-4pTPE, C-4mTPE, C-4triPE, Si-4pTPE, Si-4mTPE and Si-4triPE, determined in THF/H₂O solutions by using 9, 10-diphenylanthracene (Ф=90% in cyclohexane) as standard versus water fractions

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  The solution-state fluorescence quantum yields (Ф_{F, S}) of C-4triPE, C-4pTPE, C-4mTPE, Si-4triPE, Si-4pTPE and Si-4mTPE estimated by using 0.1 mol/L 9, 10-diphenylan-thrancene (Ф_F=90% in cyclohexane) as reference are merely 0.30%, 0.10%, 0.10%, 0.10%, 0.080% and 0.20%, respectively, indicating that they are all weak emitters in solution (Figure 1B). From pure solution in THF to the aggregated state in a 99% aqueous mixture, the fluorescence quantum yields (Ф_F) of C-4triPE, C-4pTPE, C-4mTPE, Si-4triPE, Si-4pTPE and Si-4mTPE increased up to 46%, 63%, 26%, 36%, 36% and 17%, respectively, once again proving they are all AIE-active. The Ф_F of different solutions and aggregated state were calculated according to the previous literature.^[17]

  We further investigated the PL behaviors of the six AIEgens in the solid state. As shown in Figure S7, the emissions of their thin films are observed at 449~492 nm in the blue region, similar to the PL as nanoparticles in THF/water mixtures (Table 1). However, the solid state emissions of C-4pTPE and Si-4pTPE are significantly red-shifted compared to those of C-4triPE, Si-4triPE, C-4mTPE and Si-4mTPE, with a PL peak at 492 and 487 nm, indicating that C-4pTPE and Si-4pTPE are more conjugated than C-4triPE, Si-4triPE and C-4mTPE, Si-4mTPE, as the result of the decreased conjugation and the more twisted structure.

  2.4    Theoretical calculations

  To further understand the structure-property relationship at the molecular level, Density Functional Theory (DFT) calculations (B3LYP/6-31g*) of the six AIEgens were carried out to obtain their optimized structures and orbital distributions. As demonstrated in Figure S8, for C-4pTPE, the electron clouds of HOMO and LUMO energy levels are almost the same and mainly located on the two TPE branches. For Si-4pTPE, the HOMO is mainly located on the TPE moieties, while the LUMO is dispersed on the whole molecule. And for C-4mTPE and Si-4mTPE the HOMO and LUMO are both located on one TPE moiety of four sides. So the energy levels of C-4pTPE, C-4mTPE, Si-4pTPE and Si-4mTPE only partly overlapped, which might lead to intramolecular charge transfer (ICT) effects. And the molecular structures of these four molecules were highly twisted, which would weaken the ICT effect. For C-4triPE and Si-4triPE, the HOMO and LUMO are nearly distributed in average of the whole molecules, indicating that there was no intramolecular charge transfer (ICT). For those molecules, there were no regular distribution of the HOMO and LUMO electron clouds, leading to weak ICT effect. Thus, the nonpolar properties of these AIEgens partially ensured their blue emissions in their OLED devices.

  2.5    Electrochemical properties

  The electrochemical properties of C-4triPE, C-4pTPE, C-4mTPE, Si-4triPE, Si-4pTPE and Si-4mTPE were investigated by cyclic voltammetry (CV) (Figure S9). The HOMO energy levels were calculated to be 5.66, 5.57, 5.49, 5.70, 5.59 and 5.60 eV, respectively, according to the following equation: HOMO=-(4.8-E_(Fc/Fc+)+E_ox) eV, suggesting that C-4mTPE might have the best hole-injection ability among the six AIEgens. Their LUMOs were obtained from the optical band gap energies as 1.94, 1.77, 1.54, 1.95, 1.80 and 1.72 eV for C-4triPE, C-4pTPE, C-4mTPE, Si-4triPE, Si-4pTPE and Si-4mTPE, respectively. Thus, C-4triPE and Si-4triPE had a better electronic injection capacity, which might contribute to their good performance in OLED devices. Furthermore, the higher band gap energies of C-4mTPE and Si-4mTPE (3.95 and 3.94 eV) have clearly demonstrated their shorter effective conjugation lengths, which is closely related to their twisted conformations (Table 1). Therefore, it could be rationalized that the turn-on voltages of C-4mTPE and Si-4mTPE would be decreased in comparison with those of other four molecules.

  2.6    Electroluminescence

  As a result of their good thermal stabilities and efficient light emission in the solid state, we fabricated non-doped OLED devices using these AIEgens as emitting layers with a configuration of indium tin oxide (ITO)/PEDOT: PSS (30 nm)/EML (30 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (100 nm) by spin-coating, in which, the ITO was utilized as anode, PEDOT: PSS (ploy (3, 4-enthylenedioxy-thiophene): poly (styrenesulfonate)) worked as the hole-injection layer, TmPyPB acted as the electron-transporting and hole-blocking layer, and Liq worked as the electron-transporting material, respectively. However, when using C-4pTPE as the emitting layer to fabricate OLED devices, its bad solubility makes it very difficult to be spin-coated. Thus, we did not get the EL data of C-4pTPE. The performance and EL data of the other five molecules are depicted and summarized in Figure 2 and S11 and Table 2. Devices A~E emit bright sky blue EL in the range of 458~502 nm, which are close to the PL spectra of their amorphous thin films. This indicates that both the EL and PL spectra are originated from the same radiative decay of the singlet excitions. The OLED device E based on Si-4pTPE turned on at a voltage of 8.5 V and exhibited the best performance with a maximum luminance (L_max) of 156.5 cd·m^-2, a maximum current efficiency (η_{C, max}) of 1.41 cd·A^-1, a maximum power efficiency (η_{p, max}) of 0.47 lm·W^-1 and a maximum external quantum efficiency (η_{ext, max}) of 0.62%, respectively. For device E and D based on Si-4pTPE and Si-4mTPE, the structure is more twisted for Si-4mTPE due to the meta-linkage, thus the emission of device D was blue-shifted by about 18 nm. And for devices E and C based on Si-4pTPE and Si-4triPE, the configuration length of Si-4triPE was a little shorter than that of Si-4pTPE due to the lack of one phenyl ring. Thus the emission of device C was blue-shifted by about 16 nm. However, the performance of device C and D were not so good as device E. And the emissions of the AIEgens were all red-shifted in their nondoped OLED devices in comparison with those of their thin films.

  

  Figure 2. Changes in (A) current density and luminance with the applied voltage, (B) current efficiency with the current density. Inset: the simple configuration of the device, (C) Power efficiency with the current density, (D) External quantum efficiency with the current density. Device configuration: ITO/PEDOT:PSS (30 nm)/(Device E) Si-4pTPE (30 nm), (Device F) mCP:Si-4pTPE (90:10, 30 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (100 nm)

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  Table2. EL performances of devices A~F. Device configuration: ITO/PEDOT:PSS (30 nm)/(A) C-4triPE (30 nm) (B) C-4mTPE (30 nm) (C) Si-4triPE (30 nm) (D) Si-4mTPE (100 nm) (E) Si-4pTPE (30 nm) (F) mCP : Si-4pTPE (90:10, 30 nm) /TmPyPB (50 nm)/ Liq (1 nm)/Al (100 nm)

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  Device	λEL/nm	Von/V	Lmax/(cd·m-2)	ηP, max/(lm·W-1)	ηC, max/(cd·A-1)	ηext, max/%	CIE (x, y)
  A	458	5.5	445.1	0.32	0.56	0.32	0.21, 0.26
  B	502	7.5	97.78	0.95	0.36	0.16	0.27, 0.37
  C	480	5.5	122.4	0.08	0.19	0.09	0.24, 0.30
  D	478	14.5	33.11	0.08	0.40	0.23	0.20, 0.25
  E	496	8.5	156.5	0.47	1.41	0.62	0.26, 0.37
  F	478	8	1730	0.77	2.21	1.01	0.21, 0.29
  bbreviations: Von=turn-on voltage at 1 cd·m-2, Lmax=maximum luminance, ηP, max, ηC, max and ηext, max=maximum power, current and external efficiencies, respectively. CIE=Commission International de l'Eclairage coordinates.

  Table2. EL performances of devices A~F. Device configuration: ITO/PEDOT:PSS (30 nm)/(A) C-4triPE (30 nm) (B) C-4mTPE (30 nm) (C) Si-4triPE (30 nm) (D) Si-4mTPE (100 nm) (E) Si-4pTPE (30 nm) (F) mCP : Si-4pTPE (90:10, 30 nm) /TmPyPB (50 nm)/ Liq (1 nm)/Al (100 nm)

  Considering the good performance of device E based on Si-4pTPE, we further fabricated the doped OLED device F through spin-coating with a configuration of ITO/PEDOT: PSS (30 nm)/mCP:Si-4pTPE (90:10, 30 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (100 nm), in order to further improve the device performance. Figure 2 shows the current characteristics, current efficiency, power efficiency and external quantum efficiency versus current density curves of the OLED device F, with the detail data listed in Table 2. The emission of device F is in the blue region (λ_max=478 nm), blue-shifted 18 nm in comparison with that of the nondoped device E. The maximum luminance (L_max), maximum current efficiency (η_{C, max}), maximum power efficiency (η_{p, max}) and maximum external quantum efficiency (η_{ext, max}) of device F all have considerable increases at 1730 cd·m^-2, 2.21 cd·A^-1, 0.77 lm·W^-1 and 1.01%, respectively, much better than those of device E.

  3    Conclusion

  In summary, with aim to construct spin-coating AIEgens for blue OLED devices, six AIEgens of C-4triPE, C-4pTPE, C-4mTPE, Si-4triPE, Si-4pTPE and Si-4mTPE were synthesized, by utilizing tetraphenylmethane and tetraphenylsilane as the core. Through the adjustment of the linkage mode and the number of phenyl rotors to control the intramolecular conjugation, we can tune the emission light with λ_max from 448 to 492 nm. When fabricated into OLED devices through spin-coating as the emitters, Si-4pTPE in device E exhibited the best performance among the nondoped devices. Further utilized in doped OLED device F, even better performance was achieved, with a blue emission at 478 nm and L_max, η_{C, max}, η_{p, max}, η_{ext, max}at 1730 cd·m^-2, 2.21 cd·A^-1, 0.77 lm·W^-1, 1.01%, respectively, while the CIE chromaticity coordinates of (0.20, 0.25).

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• 

  Figure Chart 1  Chemical structure of C-4pTPE, C-4mTPE, C-4triPE, Si-4pTPE, Si-4mTPE and Si-4triPE

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  Figure 1  (A) PL spectra of C-4triPE in different THF/H₂O ratio (f_W=water fraction). Inset: photos of C-4triPE in THF/water mixtures (f_W=0% and 90%) under UV lamp illumination. Concentration=10 μmol/L, excitation wavelength: 310 nm. (B) Plots of fluorescence quantum yields of C-4pTPE, C-4mTPE, C-4triPE, Si-4pTPE, Si-4mTPE and Si-4triPE, determined in THF/H₂O solutions by using 9, 10-diphenylanthracene (Ф=90% in cyclohexane) as standard versus water fractions

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  Figure 2  Changes in (A) current density and luminance with the applied voltage, (B) current efficiency with the current density. Inset: the simple configuration of the device, (C) Power efficiency with the current density, (D) External quantum efficiency with the current density. Device configuration: ITO/PEDOT:PSS (30 nm)/(Device E) Si-4pTPE (30 nm), (Device F) mCP:Si-4pTPE (90:10, 30 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (100 nm)

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  Table 1.  The thermal, electrochemical and photophysical data of the luminogens

  	Tda/℃	Tg/℃	Egb/eV	EHOMOc/ELUMOd (eV)	λabse/nm	PL λmax(film)/nm	ФF(solution)/%	PL λmax(aggr)f/nm	ФF(aggr)/%	α(AIE)g
  C-4pTPE	480	-	3.80	-5.57/-1.77	326	492	0.10	491	63	634
  C-4mTPE	494	142	3.95	-5.49/-1.54	314	465	0.10	469	26	257
  C-4triPE	496	-	3.72	-5.66/-1.94	333	448	0.30	451	46	135
  Si-4pTPE	471	137	3.79	-5.59/-1.80	327	487	0.080	490	36	449
  Si-4mTPE	500	-	3.94	-5.66/-1.72	315	464	0.20	469	17	85.9
  Si-4triPE	385	-	3.75	-5.70/-1.95	331	449	0.10	449	36	363
  a5% weight loss temperature measured by TGA under N2.b Band gap estimated from optical absorption band edge of the solution. c Calculated from the onset oxidation potentials of the compounds. d Estimated using empirical equations ELUMO=EHOMO+Eg. e Observed from absorption spectra in dilute THF solution. f Determined in V(THF):V(H2O)=1:99 solution. gαAIE=ФF(aggregation)/ФF(solution).

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  Table 2.  EL performances of devices A~F. Device configuration: ITO/PEDOT:PSS (30 nm)/(A) C-4triPE (30 nm) (B) C-4mTPE (30 nm) (C) Si-4triPE (30 nm) (D) Si-4mTPE (100 nm) (E) Si-4pTPE (30 nm) (F) mCP : Si-4pTPE (90:10, 30 nm) /TmPyPB (50 nm)/ Liq (1 nm)/Al (100 nm)

  Device	λEL/nm	Von/V	Lmax/(cd·m-2)	ηP, max/(lm·W-1)	ηC, max/(cd·A-1)	ηext, max/%	CIE (x, y)
  A	458	5.5	445.1	0.32	0.56	0.32	0.21, 0.26
  B	502	7.5	97.78	0.95	0.36	0.16	0.27, 0.37
  C	480	5.5	122.4	0.08	0.19	0.09	0.24, 0.30
  D	478	14.5	33.11	0.08	0.40	0.23	0.20, 0.25
  E	496	8.5	156.5	0.47	1.41	0.62	0.26, 0.37
  F	478	8	1730	0.77	2.21	1.01	0.21, 0.29
  bbreviations: Von=turn-on voltage at 1 cd·m-2, Lmax=maximum luminance, ηP, max, ηC, max and ηext, max=maximum power, current and external efficiencies, respectively. CIE=Commission International de l'Eclairage coordinates.

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