ORGANIC ELECTROLUMINESCENT DEVICE EMITTING GREEN LIGHT

Abstract

The present invention relates to organic electroluminescent devices including a light-emitting layers B including a TADF material, a small full width at half maximum (FWHM) emitter S.sup.B emitting green light with an FWHM of less than or equal to 0.25 eV, and a host material H.sup.B, and an optional excitation energy transfer component EET-2. Furthermore, the present invention relates to a method for generating green light by means of an organic electroluminescent device according to the present invention.

Claims

1.-15. (canceled)

16. An organic electroluminescent device comprising: a light-emitting layer; and an exciton management layer adjacent to the light-emitting layer and comprising a triplet-triplet-annihilation (TTA) material, wherein the light-emitting layer comprises: a thermally activated delayed fluorescence (TADF) material, a small full width at half maximum (FWHM) emitter, the small FWHM emitter being to emit light with an emission maximum between 510 nm and 550 nm and with a full width at half maximum (FWHM) of less than or equal to 0.25 eV, and a host material.

17. The organic electroluminescent device according to claim 16, wherein the exciton management layer has a thickness of less than 15 nm.

18. The organic electroluminescent device according to claim 16, wherein the exciton management layer has a thickness of less than 10 nm.

19. The organic electroluminescent device according claim 16, wherein the TTA material is represented by Formula 4: ##STR00474## wherein each Ar is independently selected from the group consisting of: C.sub.6-C.sub.60-aryl, which is optionally substituted with one or more substituents selected from the group consisting of C.sub.6-C.sub.60-aryl, C.sub.3-C.sub.57-heteroaryl, halogen, and C.sub.1-C.sub.40-(hetero)alkyl; and C.sub.3-C.sub.57-heteroaryl, which is optionally substituted with one or more substituents selected from the group consisting of C.sub.6-C.sub.60-aryl, C.sub.3-C.sub.57-heteroaryl, halogen, and C.sub.1-C.sub.40-(hetero)alkyl, and wherein each A.sub.1 is independently selected from the group consisting of consisting of hydrogen; deuterium; C.sub.6-C.sub.60-aryl, which is optionally substituted with one or more substituents selected from the group consisting of C.sub.6-C.sub.60-aryl, C.sub.3-C.sub.57-heteroaryl, halogen, and C.sub.1-C.sub.40-(hetero)alkyl; C.sub.3-C.sub.57-heteroaryl, which is optionally substituted with one or more substituent selected from the group consisting of C.sub.6-C.sub.60-aryl, C.sub.3-C.sub.57-heteroaryl, halogen, and C.sub.1-C.sub.40-(hetero)alkyl; and C.sub.1-C.sub.40-(hetero)alkyl, which is optionally substituted with one or more substituents selected from the group consisting of C.sub.6-C.sub.60-aryl, C.sub.3-C.sub.57-heteroaryl, halogen, and C.sub.1-C.sub.40-(hetero)alkyl.

20. The organic electroluminescent device according to claim 16, wherein the exciton management layer comprises at least one additional emitter.

21. The organic electroluminescent device according to claim 20, wherein the at least one additional emitter in the exciton management layer is a small full width at half maximum (FWHM) emitters to emit light with a full width at half maximum (FWHM) of less than or equal to 0.25 eV and with an emission maximum between 510 nm and 550 nm.

22. The organic electroluminescent device according to claim 16, wherein the light-emitting layer further comprises: an excitation energy transfer material selected from the group consisting of a TADF material and a phosphorescence material.

23. The organic electroluminescent device according to claim 22, wherein the excitation energy transfer material is a phosphorescence material.

24. The organic electroluminescent device claim 16, wherein the small FWHM emitter comprises boron, and/or a polycyclic aromatic or heteroaromatic core structure comprising at least two aromatic rings that are fused together.

25. The organic electroluminescent device according to claim 24, wherein the polycyclic aromatic or heteroaromatic core structure comprises an anthracene derivative, a pyrene derivative, or an aza-derivative.

26. The organic electroluminescent device according to claim 16, wherein the TADF material comprises: (i) a lowermost excited singlet state energy level E(S1.sup.E) and a lowermost excited triplet state energy level E(T1.sup.E); (ii) a E.sub.ST value, which corresponds to an energy difference between the lowermost excited singlet state energy E(S1.sup.E) and the lowermost excited triplet state energy E(T1.sup.E), of less than 0.4 eV; and (iii) a photoluminescence quantum yield (PLQY) of more than 30%.

27. The organic electroluminescent device according to claim 26, wherein: the small FWHM emitter comprises a lowermost excited singlet state energy level E(S1.sup.S) and a lowermost excited triplet state energy level E(T1.sup.S); and the host material comprises a lowermost excited singlet state energy level E(S1.sup.H) and a lowermost excited triplet state energy level E(T1.sup.H), and wherein:
E(T1.sup.H)>E(T1.sup.E)(13)
E(T1.sup.E)>E(S1.sup.S)(30).

28. The organic electroluminescent device according to claim 16, wherein the exciton management layer is between the light-emitting layer and an anode of the organic electroluminescent device.

29. A method for manufacturing the organic electroluminescent device according to claim 16, the method comprising: (i) depositing the light-emitting layer via vacuum-deposition, and (ii) depositing the exciton management layer via vacuum-deposition.

30. The method according to claim 29, wherein: act (ii) is performed subsequent to act (i), or act (i) is performed subsequent to act (ii).

31. A method for generating light, the method comprising: applying an electrical current to the organic electroluminescent device according to claim 16 to generate light.

32. The method according to claim 31, wherein the light comprises an emission maximum of a main emission peak being within the wavelength from 510 nm to 550 nm.

Description

EXAMPLES

[1824] Cyclic Voltammetry

[1825] Cyclic voltammograms of solutions having concentration of 10.sup.3 mol/l of the organic molecules in dichloromethane or a suitable solvent and a suitable supporting electrolyte (e.g., 0.1 mol/I of tetrabutylammonium hexafluorophosphate) are measured. The measurements are conducted at room temperature (i.e., (approximately) 20 C.) and under nitrogen atmosphere with a three-electrode assembly (working and counter electrodes: Pt wire, reference electrode: Pt wire) and calibrated using FeCp.sub.2/FeCp.sub.2.sup.+ as internal standard. HOMO and LUMO data was corrected using ferrocene as internal standard against SCE.

[1826] Density Functional Theory Calculation

[1827] Molecular structures are optimized employing the BP86 functional and the resolution of identity approach (RI). Excitation energies are calculated using the (BP86) optimized structures employing Time-Dependent DFT (TD-DFT) methods. Def2-SVP basis sets and a m4-grid for numerical integration were used. The Turbomole program package was used for all calculations. Orbital and excited state energies are calculated with the B3LYP functional. However, herein, orbital and excited state energies are preferably determined experimentally as stated above. All orbital and excited state energies reported herein (see experimental results) have been determined experimentally.

[1828] Photophysical Measurements

[1829] Sample Pretreatment: Vacuum-Evaporation

[1830] As stated before, photophysical measurements of individual compounds (for example organic molecules or transition metal complexes) that may be included in a light-emitting layer B of the organic electroluminescent device according to the present invention (for example host materials H.sup.B, TADF materials E.sup.B, phosphorescence materials P.sup.B or small FWHM emitters S.sup.B) were typically performed using either spin-coated neat films (in case of host materials H.sup.B) or spin-coated films of the respective material in poly(methyl methacrylate) (PMMA) (e.g., for TADF materials E.sup.B, phosphorescent materials P.sup.B, and small FWHM emitters S.sup.B). These films were spin coated films and, unless stated differently for specific measurements, the concentration of the materials in the PMMA-films was 10% by weight for TADF materials E.sup.B and for phosphorescent materials P.sup.B or 1-5%, preferably 2% by weight for small FWHM emitters S.sup.B. Alternatively, and as stated previously, some photophysical measurements may also be performed from solutions of the respective molecules, for example in dichloromethane or toluene, wherein the concentration of the solution is typically chosen so that the maximum absorbance preferably is in a range of 0.1 to 0.5.

[1831] Apparatus: Spin150, Sps Euro.

[1832] The sample concentration was 1.0 mg/ml, typically dissolved in Toluene/DCM as suitable solvent.

[1833] Program: 7-30 sec. at 2000 U/min. After coating, the films were dried at 70 C. for 1 min.

[1834] For the purpose of further studying compositions of certain materials as present in the EML of organic electroluminescent devices (according to the present invention or comparative), the samples for photophysical measurements were produced from the same materials used for device fabrication by vacuum deposition of 50 nm of the respective light-emitting layer B on quartz substrates. Photophysical characterization of the samples are conducted under nitrogen atmosphere.

[1835] Absorption Measurements

[1836] A Thermo Scientific Evolution 201 UV-Visible Spectrophotometer is used to determine the wavelength of the absorption maximum of the sample in the wavelength region above 270 nm. This wavelength is used as excitation wavelength for photoluminescence spectral and quantum yield measurements.

[1837] Photoluminescence Spectra

[1838] Steady-state emission spectra are recorded using a Horiba Scientific, Modell FluoroMax-4 equipped with a 150 W Xenon-Arc lamp, excitation- and emissions monochromators. The samples are placed in a cuvette and flushed with nitrogen during the measurements.

[1839] Photoluminescence Quantum Yield Measurements

[1840] For photoluminescence quantum yield (PLQY) measurements an integrating sphere, the Absolute PL Quantum Yield Measurement C9920-03G system (Hamamatsu Photonics) is used. The samples are kept under nitrogen atmosphere throughout the measurement. Quantum yields are determined using the software U6039-05 and given in %. The yield is calculated using the equation:

[00003]${}_{\mathrm{PL}}=\frac{n_{\mathrm{photon}},\mathrm{emited}}{n_{\mathrm{photon}},\mathrm{absorbed}}=\frac{\frac{}{\mathrm{hc}}\left[{\mathrm{Int}}_{\mathrm{emitted}}^{\mathrm{sample}}\left(\right)-{\mathrm{Int}}_{\mathrm{absorbed}}^{\mathrm{sample}}\left(\right)\right]d}{\frac{}{\mathrm{hc}}\left[{\mathrm{Int}}_{\mathrm{emitted}}^{\mathrm{reference}}\left(\right)-{\mathrm{Int}}_{\mathrm{absorbed}}^{\mathrm{reference}}\left(\right)\right]d}$ [1841] wherein n.sub.photon denotes the photon count and Int. is the intensity. For quality assurance, anthracene in ethanol (known concentration) is used as reference.

[1842] TCSPC (Time-Correlated Single-Photon Counting)

[1843] Unless stated otherwise in the context of certain embodiments or analyses, excited state population dynamics are determined employing Edinburgh Instruments FS5 Spectrofluoremeters, equipped with an emission monochromator, a temperature stabilized photomultiplier as detector unit and a pulsed LED (310 nm central wavelength, 910 s pulse width) as excitation source. The samples are placed in a cuvette and flushed with nitrogen during the measurements.

[1844] To determine the average decay time f of a measured transient photoluminescence signal, the data is fitted with a sum of n exponential functions:

[00004]${.Math.}_{i=1}^n{A}_i\exp \left(-\frac{t}{t_i}\right)$ [1845] wherein n is an integer between 1 and 3. By weighting the specific decay time constants r, with the corresponding amplitudes A.sub.i, the excited state lifetime f is determined:

[00005]$\stackrel{\_}{}=\frac{{.Math.}_{i=1}^n{A}_i{}_i}{{.Math.}_{i=1}^n{A}_i}$

[1846] The method may be applied for fluorescence and phosphorescence materials to determine the excited state lifetimes. For TADF materials, the full decay dynamics as described below need to be gathered.

[1847] Full Decay Dynamics

[1848] The full excited state population decay dynamics over several orders of magnitude in time and signal intensity is achieved by carrying out TCSPC measurements in 4 time windows: 200 ns, 1 s, and 20 s, and a longer measurement spanning >80 s. The measured time curves are then processed in the following way:

[1849] A background correction is applied by determining the average signal level before excitation and subtracting. [1850] The time axes are aligned by taking the initial rise of the main signal as reference. [1851] The curves are scaled onto each other using overlapping measurement time regions. [1852] The processed curves are merged to one curve.

[1853] Data analysis is done using mono-exponential or bi-exponential fitting of prompt fluorescence (PF) and delayed fluorescence (DF) decays separately. By weighting the specific decay time constants .sub.i from the fits with the corresponding amplitudes A.sub.i, the average lifetime for the prompt (i.e., the prompt fluorescence lifetime) and the delayed-fluorescence (i.e., the delayed fluorescence lifetime), respectively, may be determined as follows:

[00006]$\stackrel{\_}{}=\frac{{.Math.}_{i=1}^n{A}_i{}_i}{{.Math.}_{i=1}^n{A}_i}$ [1854] wherein n is either 1 or 2.

[1855] The ratio of delayed and prompt fluorescence (n-value) is calculated by the integration of respective photoluminescence decay s in time.

[00007]$\frac{I_{\mathrm{DF}}\left(t\right)\mathrm{dt}}{I_{\mathrm{PF}}\left(t\right)\mathrm{dt}}=n$

[1856] Transient Photoluminescence Measurements with Spectral Resolution

[1857] In transient photoluminescence (PL) measurements with spectral resolution, PL spectra at defined delay times after pulsed optical excitation are recorded.

[1858] An exemplary device for measuring transient PL spectra includes: [1859] a pulsed laser (eMOPA, CryLas) with a central wavelength of 355 nm and a pulse width of 1 ns to excite the sample. [1860] a sample chamber configured to house a sample that can be either evacuated or flushed with nitrogen. [1861] a spectrograph (SpectraPro HRS) to disperse light emitted from the sample. [1862] a CCD camera (Princeton Instruments PI-MAX4) for wavelength resolved detection of the dispersed emitted light, with integrated timing generator for synchronization with the pulsed laser. [1863] a personal computer configured to analyze the signal from the CCD camera imported thereinto.

[1864] In the course of the measurement, the sample is placed in the sample chamber and irradiated with the pulsed laser. Emitted light from the sample is taken in a 90 degree direction with respect to the irradiation direction of the laser pulses. It is dispersed by the spectrograph and directed onto the detector (the CCD camera in the exemplary device), thus obtaining a wavelength resolved emission spectrum. The time delay between laser irradiation and detection, and the duration (i.e., the gate time) of detection are controlled by the timing generator.

[1865] It should be noted, that transient photoluminescence may be measured by a device different from the one described in the exemplary device.

[1866] Production and Characterization of Organic Electroluminescence Devices

[1867] Via vacuum-deposition methods OLED devices including organic molecules according to the invention can be produced. If a layer contains more than one compound, the weight-percentage of one or more compounds is given in %. The total weight-percentage values amount to 100%, thus if a value is not given, the fraction of this compound equals to the difference between the given values and 100%.

[1868] The not fully optimized OLEDs are characterized using standard methods and measuring electroluminescence spectra, the external quantum efficiency (in %) in dependency on the intensity, calculated using the light detected by the photodiode, and the current. The FWHM of the devices is determined from the electroluminescence spectra as stated previously for photoluminescence spectra (fluorescence or phosphorescence). The reported FWHM refers to the main emission peak (i.e., the peak with the highest emission intensity). The OLED device lifetime is extracted from the change of the luminance during operation at constant current density. The LT50 value corresponds to the time, where the measured luminance decreased to 50% of the initial luminance, analogously LT80 corresponds to the time point, at which the measured luminance decreased to 80% of the initial luminance, LT97 to the time point, at which the measured luminance decreased to 97% of the initial luminance etc.

[1869] Accelerated lifetime measurements are performed (e.g., applying increased current densities). Exemplarily LT80 values at 500 cd/m.sup.2 are determined using the following equation:

[00008]$\mathrm{LT}80\left(500\frac{{\mathrm{cd}}^2}{m^2}\right)=\mathrm{LT}80\left(L_0\right){\left(\frac{L_0}{500\frac{{\mathrm{cd}}^2}{m^2}}\right)}^{1.6}$ [1870] wherein L.sub.0 denotes the initial luminance at the applied current density. The values correspond to the average of several pixels (typically two to eight).

[1871] Experimental Results

TABLE-US-00001 TABLE 1TTA Stack materials [00417] HBM1 (Hole-blocking material) [00418] EBM1 (electron-blocking material) [00419] HTL1 (Hole-transport material) [00420] ETL1 (electron-transport material) [00421] ETL2 (electron-transport material) [00422] ETL3 (electron-transport material) [00423] NDP-9 (hole-injection material) TTA materials [00424] TTA1 Properties of the TTA materials. Example E.sup.HOMO E.sup.LUMO E(S1) E(T1) compound [eV] [eV] [eV] [eV] TTA TTA1.sup.1 5.89 2.63 3.16 .sup.1measured in neat film.

TABLE-US-00002 TABLE 1H Host materials H.sup.B [00425] mCBP = H.sup.B-1 [00426] PYD2 = H.sup.B-2 [00427] H.sup.B-3 [00428] H.sup.B-4 [00429] H.sup.B-5 [00430] H.sup.B-6 [00431] H.sup.B-7 [00432] H.sup.B-8 [00433] H.sup.B-9 [00434] H.sup.B-10 [00435] H.sup.B-11 [00436] H.sup.B-12 [00437] H.sup.B-13 [00438] H.sup.B-14 [00439] H.sup.B-15 [00440] H.sup.B-16 Properties of the host materials. Example E.sup.HOMO E.sup.LUMO E(S1) E(T1) compound [eV] [eV] [eV] [eV] H.sup.B HBM1 2.91 2.94 EBM1 5.54 2.46 3.08 2.36 mCBP 6.02 2.42 3.6 2.82 PYD2 6.08 2.55 3.53 2.81 H.sup.B-3 5.66 2.35 3.31 2.71 H.sup.B-4 5.85 2.43 3.42 2.84 H.sup.B-5 5.91 2.89 2.79 H.sup.B-6 5.94 2.93 3.01 2.78 H.sup.B-7 3.27 2.71 H.sup.B-8 2.94 2.70 H.sup.B-9 5.97 3.10 2.88 2.77 H.sup.B-10 3.15 2.75 H.sup.B-11 6.04 3.10 2.94 2.86 H.sup.B-12 6.23 3.02 3.21 2.76 H.sup.B-13 6.23 3.12 3.21 2.76 H.sup.B-14 5.99 2.48 3.51 2.97 H.sup.B-15 5.64 2.36 3.28 2.70 H.sup.B-16 5.68 2.55 3.13 2.81

TABLE-US-00003 TABLE 1E TADF materials E.sup.B [00441] E.sup.B-1 [00442] E.sup.B-2 [00443] E.sup.B-3 [00444] E.sup.B-4 [00445] E.sup.B-5 [00446] E.sup.B-6 [00447] E.sup.B-7 [00448] E.sup.B-8 [00449] E.sup.B-9 [00450] E.sup.B-10 [00451] E.sup.B-11 [00452] E.sup.B-12 [00453] E.sup.B-13 [00454] E.sup.B-14 [00455] E.sup.B-15 [00456] E.sup.B-16 [00457] E.sup.B-17 [00458] E.sup.B-18 [00459] E.sup.B-19 [00460] E.sup.B-20 [00461] E.sup.B-21 Properties of the TADF materials E.sup.B. Example E.sup.HOMO E.sup.LUMO E(S1) E(T1) max.sup.PMMA FWHM PLQY compound [eV] [eV] [eV] [eV] [nm] [eV] [%] E.sup.B E.sup.B-1 5.97 3.28 2.69 2.63 518 0.43 61 E.sup.B-2 5.97 3.31 2.66 2.72 526 0.43 43 E.sup.B-3 5.92 3.25 2.67 2.65 517 0.40 73 E.sup.B-4 6.00 3.37 2.63 2.65 525 0.40 54 E.sup.B-5 5.95 3.27 2.68 2.64 508 0.41 72 E.sup.B-6 5.94 3.24 2.70 2.64 509 0.41 74 E.sup.B-7 5.94 3.24 2.70 2.66 509 0.41 71 E.sup.B-8 5.93 3.33 2.60 2.59 525 0.39 71 E.sup.B-9 5.89 3.15 2.74 2.64 498 0.40 81 E.sup.B-10 5.99 3.34 2.65 2.65 520 0.42 54 E.sup.B-11 5.79 3.15 2.77 2.81 514 0.50 63 E.sup.B-12 6.07 3.19 2.88 2.80 477 0.42 83 E.sup.B-13 6.15 3.13 3.02 2.79 454 0.44 72 E.sup.B-14 6.03 3.01 3.02 2.97 459 0.45 72 E.sup.B-15 5.79 3.02 2.77 2.82 511 0.49 64 E.sup.B-16 5.71 3.07 2.64 2.59 517 0.38 68 E.sup.B-17 5.79 3.02 2.77 2.77 523 0.51 49 E.sup.B-18 5.80 3.04 2.76 522 0.52 52 E.sup.B-19 5.80 3.14 2.67 540 0.50 38 E.sup.B-20 5.71 3.06 2.65 2.60 510 0.37 69 E.sup.B-21 5.79 2.96 2.84 2.88 502 0.51 66

TABLE-US-00004 TABLE 1P Phosphorescence materials P.sup.B [00462] Ir(ppy).sub.3 [00463] P.sup.B-2 [00464] P.sup.B-3 [00465] P.sup.B-4 Properties of the materials. Example HOMO LUMO.sub.CV E.sup.LUMO E(S1) E(T1) max.sup.PMMA FWHM compound [eV] [eV] [eV] [eV] [eV] [nm] [eV] P.sup.B Ir(ppy).sub.3 5.36 2.56 509 0.38 P.sup.B-2 5.33 2.32 2.57.sup.b 522 0.34 P.sup.B-3 5.80 2.67 2.88.sup.c 482 0.40 P.sup.B-4 5.24 [1872] wherein LUMO.sub.CV is the energy of the lowest unoccupied molecular orbital, which is determined by Cyclic voltammetry. .sup.aThe emission spectrum was recorded from a solution of Ir(ppy).sub.3 in chloroform. .sup.bThe emission spectrum was recorded from a 0.001 mg/mL solution of P.sup.B-2 in dichloromethane. .sup.cThe emission spectrum was recorded from a 0.001 mg/mL solution of P.sup.B-3 in toluene.

TABLE-US-00005 TABLE 1S Small FWHM emitters S.sup.B [00466] S.sup.B-1 [00467] S.sup.B-2 [00468] S.sup.B-3 [00469] S.sup.B-4 [00470] S.sup.B-5 [00471] S.sup.B-6 [00472] S.sup.B-7 [00473] S.sup.B-8 Properties of the Small FWHM emitters S.sup.B. Example E.sup.HOMO E.sup.LUMO E(S1) E(T1) max.sup.PMMA FWHM compound [eV] [eV] [eV] [eV] [nm] [eV] S.sup.B S.sup.B-1 5.54 3.10 2.44 2.12 538 0.21 S.sup.B-2 5.53 3.04 2.49 2.26 525 0.18 S.sup.B-3 5.55 3.05 2.50 2.22 520 0.18 S.sup.B-4 5.48 3.05 2.43 2.25 537 0.17 S.sup.B-5 5.47 3.01 2.46 2.58 527 0.15 S.sup.B-6 5.56 3.03 2.53 2.19 518 0.22 S.sup.B-7 5.48 2.97 2.53 2.23 521 0.25 S.sup.B-8* 5.86 3.40 2.46 517 0.10

TABLE-US-00006 TABLE 2 Setup 1 of exemplary organic electroluminescent devices (OLEDs). Layer Thickness Material 10 100 nm Al 9 2 nm Liq 8 10 nm ETL2: Liq = 50:50 7 10 nm ETL3 6 = B 60 nm H.sup.B: E.sup.B: EET-2: S.sup.B 5 = EXL X, see Device TTA1: S.sup.B = 99:1 results 1 4 10 nm X EBM1 3 60 nm NPB 2 9 nm NPB: NDP-9 = 95:5 1 50 nm ITO substrate glass

[1873] Results I: Variation of the thickness of the exciton management layer EXL adjacent to the light-emitting layer B (emission layer, 6)

[1874] Composition of the light-emitting layer B of devices D1-D3 (the percentages refer to weight

TABLE-US-00007 Layer D1-D3 Emission H.sup.B (66.3%): layer (5) E.sup.B (30%): EET-2 (3%): S.sup.B (0.7%)

[1875] Setup 1 from Table 2 was used, wherein H.sup.B-15 was used as host material H.sup.B, E.sup.B-16 was used as TADF material E.sup.B, P.sup.B-2 was used as excitation energy transfer component EET-2 (here exemplarily a phosphorescence material P.sup.B), and S.sup.B-1 was used as the small FWHM emitter S.sup.B.

Device Results I

[1876]

TABLE-US-00008 relative Thickness Voltage EQE at lifetime of layer at 10 1000 LT95 at EXL X FWHM .sub.max mA/cm.sup.2 cd/m.sup.2 15 Device [nm] [eV] [nm] CIEx CIEy [Volt] [%] mA/m.sup.2 D1 0 0.18 533 0.33 0.64 4.1 27.4 1.00 D2 2 0.18 533 0.33 0.64 4.2 26.9 1.12 D3 4 0.18 533 0.33 0.64 4.5 26.9 1.31