Charge Transporting Semi-Conducting Material and Electronic Device Comprising It

Abstract

The present invention relates to a charge transporting semi-conducting material. The charge transporting semi-conducting material may include optionally at least one electrical dopant, and a branched or cross-linked charge transporting polymer that includes 1,2,3-triazole cross-linking units of at least one of the general formulae Ia and/or Ib herein.

The charge transporting polymer can include ethylene building units substituted with at least one pending side group including a conjugated system of delocalised electrons. Also provided herein are processes for obtaining the charge transporting semi-conducting material.

Claims

1. Charge transporting semi-conducting material comprising: a) optionally at least one electrical dopant, and b) a branched or cross-linked charge transporting polymer comprising 1,2,3-triazole cross-linking units of at least one of the general formulae Ia and/or Ib, ##STR00017## wherein aa) Pol.sup.1-Pol.sup.4 are independently selected chains of the charge-transporting polymer, bb) X.sup.1, X.sup.2, X.sup.3, and X.sup.4 are independently selected spacer units or, independently, represent direct bonding of Pol.sup.1-Pol.sup.4 to the 1,2,3-triazole ring, cc) each of R and R′ is independently selected from H, halogen or a carbon-containing group, wherein the charge transporting polymer comprises ethylene building units substituted with at least one pending side group comprising a conjugated system of delocalised electrons, the charge transporting semi-conducting material being obtainable by a process comprising: i) providing a solution containing aaa) a first precursor charge transporting polymer comprising at least one covalently attached azide group and optionally at least one acetylenic group; and/or a second precursor charge transporting polymer comprising at least one covalently attached acetylenic group and optionally at least one azide group; and optionally at least one crosslinking agent comprising at least two functional groups selected from azide and/or acetylenic group, bbb) optionally at least one electrical dopant, ccc) at least one solvent, ii) depositing the solution on a substrate, iii) removing the solvent, and v) reacting the azide and acetylenic groups to effect crosslinking, wherein at least one of the first and second precursor charge transporting polymer comprises ethylene building units substituted with at least one pending side group comprising a conjugated system of delocalised electrons.

2. Charge transporting semi-conducting material according to claim 1, wherein the average number of azide and/or acetylenic groups per molecule in each the first precursor charge transporting polymer, the second precursor charge transporting polymer and the crosslinking agent is greater than 2.

3. Charge transporting semi-conducting material according to claim 1, wherein, in the pending side group, the conjugated system of delocalized electrons is comprised in a carbocyclic or heterocyclic structural moiety.

4. Charge transporting semi-conducting material according to claim 3, wherein the conjugated system of delocalized electrons comprised in the carbocyclic or heterocyclic structural moiety is aromatic.

5. Charge transporting semi-conducting material according to claim 4, wherein the pending side group comprises at least two rings which are independently selected from aromatic and heteroaromatic rings.

6. Charge transporting semi-conducting material according to claim 1, wherein the pending side group comprises at least one trivalent nitrogen atom.

7. Charge transporting semi-conducting material according to claim 6, wherein the trivalent nitrogen atom is substituted with three carbocyclic or heterocyclic rings which are independently selected from aromatic and heteroaromatic rings.

8. Charge transporting semi-conducting material according to claim 6, wherein the pending side group is selected from ##STR00018##

9. Charge transporting semi-conducting material according to claim 1, wherein the electrical dopant is a p-dopant.

10. Charge transporting semi-conducting material according to claim 1, wherein the electrical dopant is selected from [3]-radialene compounds, wherein each bridgehead carbon atom is substituted by a nitrile group, C.sub.6-C.sub.14 perfluorinated aryl or C.sub.2-C.sub.14 perfluorinated heteroaryl, wherein up to three fluorine atoms in the perfluorinated substituents may optionally be replaced by groups independently selected from nitrile or trifluoromethyl.

11. First precursor charge transporting polymer of claim 1 comprising at least one covalently attached azide group and optionally at least one acetylenic group and having the pending side groups as defined in any of claims 3 to 8.

12. Second precursor charge transporting polymer of claim 1 comprising at least one covalently attached acetylenic group and optionally at least one azide group and having the pending side groups as defined in any of claims 3 to 8.

13. A cross-linked charge transporting polymer of claim 1 as defined in claim 1.

14. A process for preparing a charge transporting semi-conducting material according to claim 1, comprising the steps of: i) providing a solution containing a) a first precursor charge-transporting polymer comprising at least one covalently attached azide group and optionally at least one acetylenic group; and/or a second precursor charge-transporting polymer comprising at least one covalently attached acetylenic group and optionally one azide group; and optionally at least one crosslinking agent comprising at least two functional groups selected from azide and/or acetylenic group, b) optionally at least one electrical dopant, c) at least one solvent, ii) depositing the solution on a substrate, iii) removing the solvent, and iv) reacting the azide and acetylenic groups to effect crosslinking, wherein at least one of the first and second precursor charge transporting polymer comprises ethylene building units substituted with at least one pending side group comprising a conjugated system of delocalized electrons.

15. Electronic device comprising a semi-conducting layer comprising the charge transporting semi-conducting material according to claim 1.

16. Solution as defined in the step i) of claim 14, the solution containing a) a first precursor charge-transporting polymer comprising at least one covalently attached azide group and optionally at least one acetylenic group; and/or a second precursor charge-transporting polymer comprising at least one covalently attached acetylenic group and optionally one azide group; and optionally at least one crosslinking agent comprising at least two functional groups selected from azide and/or acetylenic group, b) at least one electrical dopant, c) at least one solvent, wherein at least one of the first and second precursor charge transporting polymer comprises ethylene building units substituted with at least one pending side group comprising a conjugated system of delocalized electrons and the solvent comprises at least 1 wt % of a nitrile compound.

17. Charge transporting semi-conducting material according to claim 1, wherein the step of reacting the azide and acetylenic groups comprises heating.

18. Charge transporting semi-conducting material according to claim 1, wherein the average number of azide and/or acetylenic groups per molecule in each the first precursor charge transporting polymer, the second precursor charge transporting polymer and the crosslinking agent is greater than 2.05.

19. Process for preparing charge transporting semi-conducting material according to claim 14, wherein the step of reacting the azide and acetylenic groups comprises heating.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0113] FIG. 1: Current density of the blue OLED in dependence on the voltage;

[0114] FIG. 2: Graph of luminance of the blue OLED in dependence on time;

EXAMPLES

[0115] Exemplary semiconducting material for comparison with semiconducting materials comprising main-chain charge transport polymer of the previous application WO 2014/037512 was prepared from precursor polymers PPF1 and PPF2

##STR00011##

General Methods.

[0116] Gel permeation chromatography (GPC) measurements of polymer molecular weights were carried out on Agilent 1100 Series (Agilent, USA) normal-temperature size exclusion chromatograph, equipped with a refractive index detector and one column PL Gel MIXED-B (Polymer Laboratories, U.K.); the eluent was tetrahydrofuran (THF), and the flow rate was 1 mL/min. Number-average molecular weights (M.sub.n) and polydispersity indexes (PDI) of the obtained polymers were determined based on calibration with polystyrene standards obtained from Polymer Standards Service (PSS, Germany).

Starting Materials for Polymer Preparation

1-(azidomethyl)-4-vinylbenzene (1)

[0117] ##STR00012##

[0118] 5.00 g 1-(chloromethyl)-4-vinylbenzene (32.8 mmol, 1.00 eq), 6.40 g (98.7 mmol, 3.00 eq) natrium azide and 0.52 g (1.40 mmol, 0.04 eq) dicyclohexyl-18-crown-6 were suspended in absolute N,N-dimethylformamide (DMF) and stirred for 24 h at room temperature (RT). The solvent was then removed on rotary evaporator, the mixture suspended in 100 mL diethyl ether and extracted three times with 50 mL brine. After drying over anhydrous sodium sulfate, the ether solution was evaporated with silica and the obtained material was filled in a chromatographic column and eluted with n-heptane/ethyl acetate gradient. Vacuum evaporation of the combined eluate gave the desired product in form of clear colourless oil.

[0119] Yield: 4.80 g (30.2 mmol, 92%)

[0120] IR (ATR, cm.sup.−1) 2928; 2875; 2090; 1629; 1512; 1444; 1406; 1343; 1249; 1204; 1116; 1017; 989; 909; 847; 821; 766; 720; 669; 558

[0121] .sup.1H NMR (500 MHz; chloroform-d) [ppm] δ 7.41 (dd, J=8.2 and 1.9 Hz, 2H); 7.26 (dd, J=8.2 and 1.6 Hz, 2H); 6.71 (ddd; J=17.7, 10.9 and 2.1 Hz, 1H); 5.76 (ddd, J=17.6, 2.5 and 0.9 Hz, 1H); 5.27 (ddd, J=10.9, 2.4 and 0.9 Hz, 1H); 4.30 (s, 2H).

1-(prop-2-in-1-yloxy)-4-vinylbenzene (2)

[0122] ##STR00013##

[0123] A solution of 11.0 g propargyl bromide (92.6 mmol, 1.50 equiv), 7.41 g 4-vinylphenol (freshly freed of the stabilizer, 61.7 mmol, 1.00 eq) and 10.4 g (185 mmol; 3.00 eq) KOH in 50 mL water and 120 mL acetone was stirred for 48 h at RT. The reaction mixture was then diluted with 100 mL ethyl acetate, extracted five times with 50 mL brine, dried over Na.sub.2SO.sub.4, filtered, the filtrate evaporated with silica and the resulting material column-eluted with n-heptane/ethyl acetate gradient. Vacuum evaporation of the combined eluate gave the desired product in form of clear yellowish oil.

[0124] Yield: 7.81 g (49.4 mmol, 80%)

[0125] IR (ATR, cm.sup.−1) 3275; 2923; 2132; 1816; 1628; 1603; 1574; 1508; 1452; 1410; 1372; 1321; 1302; 1226; 1178; 1119; 1017; 971; 906; 834; 740; 704; 674; 570

[0126] .sup.1H NMR (500 MHz; chloroform-d) [ppm] δ 7.42-7.29 (m, 2H); 6.99-6.86 (m, 2H); 6.65 (dd, J=17.6 and 10.9 Hz, 1H); 5.61 (dd, J=17.6 and 0.9 Hz, 1H); 5.13 (dd, J=10.9 and 0.9 Hz, 1H); 4.66 (d, J=2.4 Hz, 2H); 2.50 (t, J=2.4 Hz, 1H)

3,6-dibutoxy-9H-carbazole (3)

[0127] ##STR00014##

[0128] At RT, 57 mL absolute 1-butanol (154 mmol, 10.0 eq) were dropwise added under stirring to a suspension 11.1 g NaH (460 mmol, 10.0 eq) in dry DMF. After gas release ceased, the mixture was stirred for an additional hour and the resulting sodium butanolate solution was then, under inert atmosphere, added to a suspension 35.0 g CuI (18.0 mmol, 4 eq) in 50 mL dry DMF. The reaction mixture has been stirred at 120° C. for 1 h, filtered through diatomaceous earth and evaporated with silica. Column elution with n-hexane/ethyl acetate gradient afforded, after eluate evaporation, the desired product as a white solid.

[0129] Yield: 12.3 g (39.5 mmol, 86%)

[0130] .sup.1H NMR (500 MHz; THF-d.sub.8) [ppm] δ 9.81 (s, 1H); 7.51 (d, J=2.4 Hz, 2H); 7.23 (dd, J=8.8 and 0.5 Hz, 2H); 6.94 (dd, J=8.7 and 2.4 Hz, 2H); 4.04 (t, J=6.5 Hz, 4H); 1.88-1.75 (m, 4H); 1.65-1.47 (m, 4H); 1.01 (t, J=7.4 Hz, 6H)

[0131] .sup.13C NMR (126 MHz; chloroform-d) [ppm] δ 153.2; 135.4; 123.9; 115.9; 111.5; 104.1; 68.9; 31.74; 19.5; 14.1

4-(bis(4-(3,6-dibutoxy-9H-carbazol-9-yl)phenyl)amino)benzaldehyde (4)

[0132] ##STR00015##

[0133] 9.00 g (3) (29.0 mmol, 2.4 eq), 6.30 g 4-(bis(4-iodophenyl)amino)benzaldehyde (29.0 mmol, 1.00 eq), 1.60 g copper bronze (25.0 mmol, 2.10 eq), 0.63 g [18]crown-6 (2.40 mmol, 0.20 eq) and 5.00 g K.sub.2CO.sub.3 (36.0 mmol, 3.00 eq) were stirred in 30 mL dry o-dichlorobenzene at 200° C. for 24 h. Then, the reaction mixture was diluted with 400 mL toluene, filtered through diatomaceous earth, the filtrate was three times washed with 100 mL brine, dried over sodium sulfate and vacuum evaporated to dryness. Recrystallization from toluene afforded the desired product as a yellowish solid.

[0134] Yield: 9.00 g (10.1 mmol, 84%) .sup.1H NMR (500 MHz; THF-d.sub.8) [ppm] δ 9.86 (s, 1H); 7.85-7.77 (m, 2H); 7.66-7.60 (m, 8H); 7.57-7.49 (m, 4H); 7.38 (d, J=8.9 Hz, 4H); 7.32-7.26 (m, 2H); 7.00 (dd, J=8.9 and 2.4 Hz, 4H); 4.09 (t, J=6.5 Hz, 8H); 1.85-1.79 (m, 8H); 1.64-1.49 (m, 8H); 1.02 (t, J=7.4 Hz, 12H)

[0135] .sup.13C NMR (126 MHz; CDCl.sub.3) [ppm] δ 190.6; 153.7; 153.1; 144.5; 136.2; 135.2; 131.7; 130.1; 127.9; 127.3; 123.9; 120.5; 115.9; 110.7; 104.1; 68.9; 31.7; 19.5; 14.1

4-(3,6-dibutoxy-9H-carbazol-9-yl)-N-(4-(3,6-dibutoxy-9H-carbazol-9-yl)phenyl)-N-(4-vinylphenyl)aniline (5)

[0136] ##STR00016##

[0137] A suspension 9.60 g methyl triphenyl phosphonium bromide (26.9 mmol, 3 eq) and 2.9 g potassium tert-butoxide (26.1 mmol, 2.90 eq) in dry 1,4-dioxane has been stirred for 2 h at 0° C., then, 8.00 g 4-(bis(4-(3,6-dibutoxy-9H-carbazol-9-yl)phenyl)amino)benzaldehyde (8.97 mmol, 1.00 eq) were added in form of a solution in 200 mL dry toluene, while maintaining the temperature 0° C., the mixture has been stirred at this temperature for further 30 min, washed three times with 300 mL brine, dried over sodium sulfate, filtered and vacuum evaporated with silica. After column elution with n-heptane:toluene (1:2 v/v) and eluate evaporation, the desired product was obtained as a white solid.

[0138] Yield: 7.23 g (8.12 mmol, 91%)

[0139] .sup.1H NMR (500 MHz; THF-d.sub.8) [ppm] δ 7.64 (d, J=2.5 Hz, 4H); 7.56-7.46 (m, 10H); 7.46-7.41 (m, 4H); 7.36 (d; J=8.9 Hz, 4H); 7.31-7.27 (m, 4H); 7.02 (dd, J=8.9 and 2.4 Hz, 4H); 6.75 (dd, J=17.6 and 10.9 Hz, 1H); 5.75 (d, J=17.6 Hz, 1H); 5.19 (d, J=11.0 Hz, 1H); 4.10 (t, J=6.5 Hz, 8H); 1.87-1.82 (m, 8H); 1.62-1.54 (m, 8H); 1.04 (t, J=7.4 Hz, 12H)

[0140] .sup.13C NMR (126 MHz, chloroform-d) [ppm] δ 153.4; 146.9; 146.0; 136.3; 133.0; 132.9; 129.0; 128.2; 127.6; 127.4; 124.9; 124.5; 123.5; 115.7; 110.6; 103.9; 68.8; 31.6; 19.4; 13.9

Typical Co-Polymerization Procedures.

Poly(4-(3,6-dibutoxy-9H-carbazol-9-yl)-N-(4-(3,6-dibutoxy-9H-carbazol-9-yl)phenyl)-N(4-vinylphenyl)aniline).SUB.50.-ran-(1-(azidomethyl)-4-vinylbenzene).SUB.50.] (PPF1)

[0141] 0.50 eq 4-(3,6-dibutoxy-9H-carbazol-9-yl)-N-(4-(3,6-dibutoxy-9H-carbazol-9-yl)phenyl)-N(4-vinylphenyl)aniline (5) and 0.50 eq 1-(azidomethyl)-4-vinylbenzene (1) were stirred with 0.02 eq azo-bis(isobutyronitrile) (AIBN) in toluene at overall mass concentration 0.1 g/mL for 72 h at 50° C., cooled to RT and the resulting polymer was precipitated from n-heptane:ethyl acetate 4:1 v/v mixture. The precipitate was collected using a PTFE filter (20 μm porosity), dried dissolved in toluene to a solution having mass concentration 0.05 g/mL and reprecipitated. After drying, the desired product was obtained as a white solid in 61% yield.

[0142] .sup.1H NMR (500 MHz, chloroform-d) [ppm] δ 7.85-6.15 (aromatic); 4.29-3.40 (alkoxy, benzylic); 2.48-0.55 (aliphatic+backbone); monomer ratio .sup.1H-NMR: (m=50, n=50); T.sub.g=160° C.; M.sub.w=25.0 kg mol.sup.−1; M.sub.r, =12.2 kg mol.sup.−1

Poly(4-(3,6-dibutoxy-9H-carbazol-9-yl)-N-(4-(3,6-dibutoxy-9H-carbazol-9-yl)phenyl)-N(4-vinylphenyl)aniline).sub.57-ran-(1-(prop-2-in-1-yloxy)-4-vinylbenzene).sub.43] (PPF2)

[0143] 0.50 eq 4-(3,6-dibutoxy-9H-carbazol-9-yl)-N-(4-(3,6-dibutoxy-9H-carbazol-9-yl)phenyl)-N(4-vinylphenyl)aniline (5) and 0.50 eq 1-(propargyloxy)-4-vinylbenzene (2) were stirred with 0.02 eq azo-bis(isobutyronitrile) (AIBN) in toluene at overall mass concentration 0.1 g/mL for 72 h at 50° C., cooled to RT and the resulting polymer was precipitated from n-heptane:ethyl acetate 4:1 v/v mixture. The precipitate was collected using a PTFE filter (20 μm porosity), dried, dissolved in toluene to a solution having mass concentration 0.05 g/mL and reprecipitated. After drying, the desired product was obtained as a white solid in 66% yield.

[0144] .sup.1H NMR (500 MHz, chloroform-d) [ppm] δ 7.85-6.15 (aromatic); 4.29-3.40 (alkoxy; benzylic); 2.48-0.55 (aliphatic+backbone); monomer ratio .sup.1H-NMR: (m=57, n=43);

[0145] T.sub.g=160° C.; M.sub.w=14.1 kg mol.sup.−1; M.sub.n, =9.28 kg mol.sup.−1.

Conductivity and Stability of a Doped Crosslinked Layer

[0146] An anisole solution containing 1.74 wt % of the first precursor charge-transporting polymer PPF1, 0.75 wt % p-dopant PR-1 and 2.02 wt % the second precursor charge-transporting polymer PPF2 was prepared and spin-coated on ITO substrate for 30 s at 1000 rpm. The mass ratio PPF1:PPF2 8.6:10 corresponds to molar ratio between azide and acetylene crosslinking units 1:1. After drying and baking on hot plate in nitrogen atmosphere for 30 min at 180° C., conductivity and UV absorbance of the formed thin film at the wavelengths 340 nm (characteristic for charge transport structural moieties of the polymer) and 540 nm (characteristic absorption band of the active state of the p-dopant) were measured.

[0147] The formed film was spin-rinsed with toluene after 10 s soaking-time before spinning After 30 min drying at 80° C., the conductivity and UV absorbance were measured again.

[0148] The experiment was repeated with mass ratio PPF1:PPF2 2.9:10 which corresponds to molar ratio between azide and acetylene crosslinking units 1:3, and with precursor polymers PP3 and PP4 of the previous application WO2014/037512, in the same molar ratios 1:1 and 1:3 between the azide-substituted and the acetylene-substituted crosslinking units in both precursor polymers as in the above example.

[0149] Whereas the observed conductivities in the range 10.sup.−5-10.sup.4 S.Math.cm.sup.−1 confirmed the applicability of the tested material for charge transport, from the relative changes in polymer and dopant absorbance, the stability of the layer in terms of the polymer wash-off and dopant wash-out can be estimated from the effect of the rinsing on the crosslinked layer.

[0150] The results further amended with estimation of relative change in dopant absorbance after long-term illumination of the deposited layer with blue light are summarized in Table 1

TABLE-US-00001 TABLE 1 Material (molar ratio of azide and acetylene Dopant crosslinking Polymer deterioration units given) Dopant wash-out % wash-off % by blue light % PPF1:PPF2 1:3 11 2.0 6.0 PPF1:PPF2 1:1 0.3 1.0 0.1 PP3:PP4 1:3 0.3 0.1 8.3

[0151] Table 1 shows different behaviour of crosslinked semiconducting materials of present invention in comparison with materials of previous application. The results demonstrate that in materials of present application, better fit between processing stability of the crosslinked semiconducting layer and its operational stability can be achieved.

Bottom Emission Blue OLED

[0152] On 90 nm thick indium tin oxide (ITO) layer fabricated on a glass substrate, 50 nm thick crosslinked hole-transporting layer from PPF1 and PPF2 in weight ratio doped with 20 wt. % PR1 based on the overall polymer weight was cast by spin-coating from 2 wt. % tolueneanisole solution. After drying and baking in an inert atmosphere at 180° C. for 40 minutes, a doped crosslinked layer having thickness 40 nm was obtained. Following layers were prepared on top of the crosslinked layer by vacuum deposition: 90 nm undoped electron blocking layer composed from N.sup.4,N.sup.4″-di(naphtalen-1-yl)-N.sup.4,N.sup.4″-diphenyl-[1,1′:4′,1″-terphenyl]-4,4″-diamine, 20 nm blue fluorescent emitting layer composed of ABH113 (obtained from Sun Fine Chem (SFC), Korea) doped with NUBD370 (also from SFC, host:emitter ratio 95:5 by weight), 30 nm electron transporting layer composed of 2-(4-(9,10-di(naphtalen-2-yl)anthracen-2-yl)phenyl)-1-phenyl-1H-benzo[d]imidazole (CAS 561064-11-7) and lithium 8-hydroxyquinolinolate (CAS 850918-68-2) in weight ratio 1:1 and 100 nm thick Al cathode. At current density 15 mA/cm.sup.2, the OLED had operational voltage 4.8 V, quantum efficiency 5.2% and current efficiency 5.5 cd/A (see also FIGS. 1 and 2). The lifetime of the OLED, expressed as LT97 (the time necessary for luminance decrease to 97% of its initial value), was 60 hours.

[0153] The results are summarized in table 2.

TABLE-US-00002 TABLE 2 V Qeff @ 15 mA/cm.sup.2 @ 15 mA/cm.sup.2 LT97 Operational material (V) (%) CiEy @ 15 mA/cm.sup.2 stability PPF1:PPF2 1:1 4.8 5.2% 0.11 60 h yes PP3:PP4 1:1 5.7 5.7% 0.09 — no PP3:PP4 1:3 5.5 6.1% 0.09 30 h yes

[0154] These results confirm that whereas all crosslinked p-doped materials exhibit sufficient conductivity and negligible polymer wash-off and dopant wash-out, they differ in their OLED device performance, especially in terms of lifetime and operational stability. It appears that whereas the crosslinked polymer of the previous application which is formed from precursor polymers having tough backbone requires a stoichiometric excess of acetylenic groups over complementary azide groups (see in Table 1, e.g. better polymer wash-off and dopant wash-out at 3:1 molar ratio), more flexible polymers of the present invention enable that the best cross-linking is achieved if the stoichiometric ratio between the complementary groups is set around 1:1, the value which is optimal for OLED operational stability.

[0155] This unexpected result enables designing crosslinked p-doped materials with very low content of residual unreacted crosslinking groups. Consequently, cross-linked p-doped materials according to present invention enable designing electronic devices with high reproducibility and favourable stability of their luminance and operational voltage at constant operational current density.

[0156] FIGS. 1 and 2 compare current-voltage characteristics and lifetime of the blue OLED prepared in the example above (curve (c), circles) and the same OLED prepared under stress conditions of the HTL baking prolonged to 120 minutes (curve (b), rhombi). The results show robustness of the inventive doped material, wherein on expense of slightly higher operational voltage, efficiency and device-lifetime can be further increased by prolonged heat treatment of the doped layer, without a substantial change in spectral characteristics of the device.

[0157] The results demonstrate that a crosslinked charge transporting layer comprising semiconducting material according to the invention can be successfully used in organic electronic devices.

[0158] The features disclosed in the foregoing description may, both separately and in any combination thereof, be material for realizing various embodiments of the invention which is generically defined in the independent claims.