Air Stable HyLEDs Using Efficient Electron Injection and Emitting

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Air stable HyLEDs using efficient electron injection and emitting materials Jayaraman Jayabharathi, Annadurai Prabhakaran, Venugopal Thanikachalam, Palanivel Jeeva, and Munusamy Sundharesan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01151 • Publication Date (Web): 01 Jul 2016 Downloaded from http://pubs.acs.org on July 3, 2016

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Industrial & Engineering Chemistry Research

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Air stable HyLEDs using efficient electron injection and emitting materials 5

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Jayaraman Jayabharathi*, Annadurai Prabhakaran, Venugopal Thanikachalam, Palanivel Jeeva, Munusamy Sundharesan 8

7

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Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India 10

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Abstract 1 13

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Efficient hybrid organic inorganic light emitting diodes with an electron injection layer of 3.8 nm 15

14

sized zinc oxide nanomaterials and there different emissive layers of phenanthrimidazole 17

16

derivatives 18

such

as

2-phenyl-1-(3,5-dimethylphenyl)

or

2-(p-trifluoromethylphenyl)-1-

20

19

(naphthalen-1-yl) or 2-(p-methylphenyl)-1-naphthalen-1-yl -1H-phenanthro [9,10-d] imidazoles 2

21

have been fabricated. The electroluminescent performances of the fabricated devices increased 23 24

when compared to the reference devices. The ZnO nano layer with 26 nm thickness and 2-(p25 27

26

trifluoromethylphenyl)-1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazole 29

28

as

emissive

material enhances current efficiency (ηc : 19.5 cd/A), power efficiency (ηp : 9.9 lm W-1), external 31

30

quantum efficiency (ηex :13.8 %) and luminescence (L : 17345 cd/m2 ). 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50

Keywords: HyLEDs, ZnO nanomaterials, electron injection layer; phenanthrimidazoles, current 52

51

efficiency. 53 54 56

5 *

58

57

Corresponding author. Tel.: +91 9443940735 E-mail address: [email protected]

59 60 ACS Paragon Plus Environment


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1. Introduction 5

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Organic light emitting diodes (OLEDs) are of interest because of its wide utility in 6 7

optoelectronics1-4. Low stability and high costs, however, are the challenging task for 10

9

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commercialization of OLEDs. Therefore, efforts have been taken to overcome these issues by the 12

1

development of hybrid organic-inorganic LEDs (HyLEDs)5-10. The ability of injection of electron 13 15

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in HyLEDs with indium-tin-oxide as cathode is not effective due to its high work function and 17

16

use of low work function cathode materials11-15 lowers the stability due to oxidation16,17. 18 19

Therefore, due to charge injection ability and transparency metal oxides are used as charge 20 2

21

injection layer in HyLEDs which avoid the air sensitivity problems18, 19. Quantum confinement 24

23

effects of nano metal oxides increased the band gap results better performance of HyLEDs20-22. It 25 26

is known that nano metal oxides (p-type: MoO3, WO3 and V2O523-29; n-type: ZnO, TiO2 and 27 29

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MgO30-33) are employed as electron injection layers (EIL) in hybrid LEDs. Though the doped 31

30

emitters in OLEDs improved the performance of devices, dopants concentration affects the 32 3

stability of devices34-39. In addition to that fabrication of doped devices is more expensive and 36

35

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phase separation upon heating induced ineffective energy transfer from adjacent layers to host 37 38

and to guest which lowers the efficiencies. Therefore, currently it is planned to use non-doped 39 41

40

emitters for the fabrication of HyLEDs. By considering the larger band gap and transparency, 43

42

nanocrystalline ZnO is used as injection materials in HyLEDs40-42. Although organic polymers43, 4 45

tris(8-hydroxyquinolinato)aluminium(III) (Alq3)44 and iridium complexes45 are used as emitting 46 48

47

layers in HyLEDs, this is the first report employing phenanthrimidazole derivatives namely 250

49

phenyl-1-(3,5-dimethylphenyl) 51

(DMPPI),

2-(p-trifluoromethylphenyl)-1-(naphthalen-1-yl)

52

(TMPPI) and 2-(p-methylphenyl)-1-naphthalen-1-yl -1H-phenanthro [9,10-d] imidazoles45 53 5

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(NMPPI) are employed as non-doped emitters with pristine ZnO with various thickness as 56 57 58 59 60 ACS Paragon Plus Environment

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electron injection layer. The fabricated devices showed higher efficiencies compared to the 5

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reference devices. 6 8

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2. Experimental Section 10

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Sigma-aldrich supplied 9, 10-phenanthrenequinone, substituted benzaldehydes, and 1 12

substituted amines for the synthesis of DMPPI, TMPPI and NMPPI. XRD of DMPPI were 13 15

14

recorded with Agilent Xcalibur Ruby Gemini diffractometer and radiation enhanced Mo X-ray is 17

16

used as radiation source with 10.5081 pixels mm-1 monochromator as detector. Absorption 18 19

electronic spectral measurements have been made with Perkin Elmer Lambda 35 20 2

21

spectrophotometer and diffuse reflectance spectra (DRS) were recorded employing Perkin Elmer 24

23

Lambda 35 spectrophotometer with RSA-PE-20 integrating sphere. Emission spectral 25 27

26

measurements were made with PerkinElmer LS55 spectrometer. Proton and

13

C NMR spectra

29

28

were obtained using Bruker 400 MHz NMR spectrometer and mass spectra of 31

30

phenanthrimidazoles were made with Agilent LCMS VL SD [EI mode]. CHI 630A potentiostat 32 34

3

electrochemical analyzer were used for potential measurement and Horiba Fluorocube-01-NL 36

35

time correlated single photon counting (TCSPC) spectrometer was used for decay measurements. 38

37

ESCA-3 Mark II spectrometer (VG scientific Ltd., England) using Al Kα (1486.6 eV) radiation as 39 41

40

the source was used for X-ray photoelectron spectra (XPS) and elemental analysis of the catalyst 43

42

was confirmed by energy dispersive X-ray spectra (EDS) which was recorded with FEI Quanta 4 45

FEG. TEM images of the catalyst was obtained from TEM with 200 kV electron beam, and 46 48

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philips TEM with CCD camera was used for recording SAED pattern. Powder XRD of the 50

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nanocrystalline ZnO was recorded with Eqinox 1000 diffractometer with Cu Kα rays at 1.5406 Å 51 53

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and Atomic force microscope (AFM) was employed to probe the fabricated film thickness as well 5

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as the roughness. 56 57 58 59 60 ACS Paragon Plus Environment


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2.1.Synthesis 5

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2.1.1. 1-(3,5-dimethylphenyl)-2-phenyl-1H-phenanthro[9,10-d]imidazole(DMPPI) 6 8

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A 10

9

mixture

of

benzaldehyde/4-trifliuoromethylbenzaldehyde/4-methylbenzaldehyde

(1mmol), 3,5-dimethylaniline/1-naphthylamine (1 mmol), 9, 10-phenanthrenequinone (1mmol), 1 12

ammonium acetate (1mmol) and rutile TiO2 (1 mol %) was grained and refluxed for 30 minutes 13 15

14

at 80° C. The final solution was cooled and poured into water. Purification of the crude sample 17

16

was made by column chromatography (benzene: ethyl acetate, 9:1). Elemental composition for 19

18

C29H22N2 (cal/found) : C, 87.41/87.38; H, 5.56/5.50; N, 7.03/7.67. 1H NMR :  2.5 (s, 6H), 7.1 (t, 20 2

21

2H), 7.2-7.3 (m, 5H), 7.6 (t, 1H), 7.6-7.7 (m, 4H), 7.8 (t, 1H), 8.8 (d, 2H), 8.81 (d, 1H). 24

23

13

C

NMR :  20.8, 114.8, 115.0, 120.46, 122.03, 122.38, 122.91, 123.67, 124.79, 125.35, 125.86, 25 27

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126.23, 126.47, 126.94, 127.57, 127.60, 128.40, 130.86, 130.94, 131.39, 136.39, 137.53, 139.75, 29

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149.29, 161.09. MS: m/z. 398.0 [M+]. Calcd. 398.18. 30 32

31

2.1.2. 2-(p-trifluoromethylphenyl)-1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazole(TMPPI) 34

3

C32H19F3N2 (cal/found): C, 78.68/78.66; H, 3.92/3.91; F, 11.67/11.65; N, 5.73/5.70. 1H 36

35

NMR :  7.7 (s, 3H), 7.7-7.8 (m, 3H), 8.0 (d, 3H), 8.5-8.6 (m, 6H), 8.9 (q, 3H). 37

13

C NMR : 

39

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122.02, 122.86, 123.95, 125.56, 125.63, 125.92, 125.95, 126.62, 127.24, 127.83, 128.82, 129.13, 41

40

133.87, 147.38. MS: m/z. 488.0 [M+]. Calcd. 488.15. 42 4

43

2.1.3. 1-(naphthalen-1-yl)-2-(p-methylphenyl)-1H-phenanthro[9,10-d]imidazole(NMPPI) 46

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C32H22N2 (cal/found) : C, 88.45/88.42; H, 5.10/5.08; N, 6.45/6.41. 47

1

H NMR :  2.5 (s,

49

48

3H), 7.2 (d, 3H), 7.6 (t, 3H), 7.7 (t, 3H), 8.3 (d, 3H), 8.6 (s, 3H), 8.8 (d, 3H).

13

C NMR : 

51

50

21.03, 114.33, 121.82, 122.90, 123.91, 125.08, 127.04, 127.41, 127.71, 128.28, 149.29. MS: m/z. 52 53

434.0 [M+]. Calcd. 434. 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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2.2. Synthesis of nanocrystalline ZnO 5

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Aqueous ammonia (1:1) and zinc nitrate (0.1 g) solution were mixed slowly for 30 min 6 8

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and the white gel was kept to age for 24h. The white solid was separated and washed with water 10

9

(5 times) and ethyl alcohol (5 times). The solid was dried (100° C; 12 h) and calcinated (500° C 12

1

; 2 h rate of heating; 10° C min-1). 13 15

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2.3. HyLEDs fabrication 17

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The fabricated HyLEDs configurations: (a) ITO/ ZnO (EIL) /DMPPI (device I-IV) or 18 19

TMPPI (device V) or NMPPI (device VI) (30 nm)(EL)/molybdenum trioxide (MoO3) (10 nm) 20 2

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(HIL) /Al (100 nm); (b) ITO/DMPPI (30 nm)/MoO3 (10 nm) (HIL) /Al (100 nm) (device VII) 24

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and (c) ITO/ZnO (26 nm) (EIL) /Alq3 (EL) (100 nm)/ MoO3 (HIL) (10 nm)/Al (100 nm) (device 25 27

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VIII). The layers deposition was made using thermal evaporation method and thickness was 29

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measured with quartz crystal digital thickness monitor. Keithley 2400 was employed for current 31

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density versus voltage measurements in ambient atmosphere. 32 34

3

3. Results and Discussion 36

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Among the three green emitters DMPPI, TMPPI and NMPPI, DMPPI crystal was 37 38

obtained from a mixture of n-hexane and dichloromethane and the ORTEP is displayed in Figure 39 41

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1a. Dihedral angles between phenyl ring and phenanthrimidazole ring is ca. 4.7° and 3, 543

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dimethylphenyl ring and phenanthrimidazole ring is ca. 82°. At this large angle 3,5-dimethyl aryl 4 45

ring is twisted with respect to imidazole plane. The decomposition temperatures (Td) and glass46 48

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transition temperatures (Tg) measured by TGA and DSC are as follows: DMPPI; 261° and 82° C, 50

49

TMPPI; 265° and 91° C, NMPPI: 268° and 85° C, respectively (Figure 1b & c). The obtained 51 53

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high glass-transition temperature shows larger thermal stability of the green emitting materials. 5

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The absorption/emission of phenanthrimidazoles such as DMPPI, TMPPI and NMPPI in CH2Cl2, 56 57 58 59 60 ACS Paragon Plus Environment


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are 342/428 nm, 335/420 nm and 339/431 nm, respectively and the electronic spectral maxima 5

4

of the film is red-shifted when compared to solution which may due to π–π stacking and non6 8

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coplanar geometry of phenanthrimidazoles which suppress the aggregation of the molecules 10

9

(Figure 1d). Using the fluorescence yield () of DMPPI (0.45), TMPPI (0.51) and NMPPI 12

1

(0.48) and decay lifetime (τ) of DMPPI (1.45 ns), TMPPI (1.49 ns) and NMPPI (1.36 ns), the 13 15

14

radiative rate constants for DMPPI (3.1 × 108 s-1), TMPPI (3.4 ×108 s-1) and NMPPI (3.5 × 108 s17

16

1

18

) and non-radiative rate constants for DMPPI (3.8 × 108 s-1), TMPPI (3.3 × 108 s-1) and NMPPI

19

(2.8 × 108 s-1) are calculated (Figure 1e). The HOMO [EHOMO = 4.8 + Eoxi] energies are 2

21

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calculated for DMPPI (-5.6 eV), TMPPI (-5.5 eV) and NMPPI (-5.4 eV) from their 23 24

corresponding onset potentials (Figure 1f). The LUMO energies of DMPPI (-2.5 eV), TMPPI (25 27

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2.3 eV) and NMPPI (-2.1 eV) has been deduced from ELUMO = HOMO-1239/λonset relationship 29

28

(Table 1). The HOMO is populate on phenanthro[9,10-d]-imidazole plane and LUMO 30

is

31

centered on the aryl ring attached to C-2 carbon with a small fraction on the imidazole unit of all 32 34

3

the synthesized phenanthrimidazoles (Figure S1). 36

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The powder XRD of ZnO nanomaterials and JCPDS ZnO (36-1451) are displayed in 37 39

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Figure 2a. The average crystal size of pristine ZnO is 3.8 nm. The emissions observed at 352 41

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(3.52 eV) and 496 nm (2.5 eV) are band-edge and deep-level emissions (DLE) of pristine ZnO46 43

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(Figure 2b). With respect to the band gap of bulk47 ZnO the blue shifted absorption onset at 330 4 46

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nm (3.76 eV) is observed for 3.8 nm sized ZnO. The size calculated from XRD and TEM is in 48

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agreement with each other (Figure 3a) and SAED pattern shows that the synthesized ZnO 49 50

nanocrystal is of zincite structure (Figure 3b). EDX shows the presence of zinc and oxygen and 51 53

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absence of other elements revealing the purity of the material (Figure 3c). The oxidation state of 5

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the constituent elements in the synthesized material was deduced using XPS (Figure 4). The 56 57 58 59 60 ACS Paragon Plus Environment

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observed two symmetrical peaks α (528.1) and β (533.8) shows that O1s is asymmetric and 5

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revealing two types of oxygen present in ZnO. Zn2p located at 1024.0 and 1052.1 eV shows the 6 8

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+2 oxidation of zinc in pristine ZnO. As the band gap(Eg) synthesized ZnO nanomaterials is 10

9

larger than that of bulk ZnO the former could be employed as electron injection materials in 1 12

HyLEDs fabrication and the Eg broadening in the synthesized ZnO may lead to enhance the 13 15

14

electroluminescent efficiencies. 17

16

For an efficient HyLEDs the solid film of nano ZnO with 0.5 %, 1.0 %, 1.5 % and 2.0 % 18 19

concentrations should uniformly cover the ITO layer and the surface morphology was analysed 20 2

21

through AFM (Figure 5a). The AFM provides the thickness (15, 18, 24 and 26 nm) and root 24

23

mean square (RMS) roughness (3.82, 3.51, 2.83, and 2.58 nm, bare ITO- 4.49 nm) of the nano 25 27

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ZnO layer prepared from 0.5, 1.0, 1.5 and 2.0 % solutions. The observed absorption edge of 3.30 29

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eV (Figure 5b) of the film does not correspond to the Eg of ZnO: it is the Eg between the ZnO 31

30

CB and DMPPI LUMO. The amorphous nature of ZnO film remains unchanged upto 100° C and 32 34

3

this is seen from the recorded XRD. The above results indicate that the suitability of the 36

35

synthesized nano ZnO as EIL in HyLEDs. The average lifetime [1.30 ns (bare-4.49 nm); 1.95 ns 37 38

(15 nm); 2.25 ns (18 nm); 2.40 ns (24 nm) and 2.65 ns (26 nm)] are obtained from decay curves 39 41

40

which are shown in Figure 5c. The obtained results indicate that increased lifetime with 26 nm 43

42

thickness of ZnO layer hinders the quenching of exciton effectively. The likely mechanisms of 4 45

quenching are (i) surface quenching and (ii) energy transfer (nonradiative) quenching. The 46 48

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former can takesplace when the binding energy of the exciton is lower than band offset46 50

49

whereas the energy transfer (non radiative) quenching is dominant when there is overlapping of 51 52

absorption of electron injection material and emission of emissive materials48-51 (Figure 5d). 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment


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The electroluminescent performances of the fabricated devices using nano ZnO as EIL 5

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with various thickness (15 nm, 18 nm, 24 nm and 26 nm) and DMPPI, TMPPI and NMPPI as 6 8

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non-doped emitters have been carried out (Figure 6). The efficiencies of the devices fabricated 10

9

by the combination of ZnO as EIL and DMPPI as EL (devices (I-IV)) increased when compared 1 12

to the reference devices VII (0 % ZnO) & VIII (0 % DMPPI). Among the devices (I-IV), device 13 15

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IV with 26 nm thickness of ZnO nanolayer and DMPPI as non-doped emitter exhibit maximum 17

16

luminescence (L) of 17182 cd/m2 with external quantum efficiency (ηex) of 13.2 %, current 19

18

efficiency (ηc) of 19.2 cd/A and power efficiency (ηp) of 9.6 lmW-1 at 18.1 V but the reference 20 2

21

devices VII (0 % ZnO) and VIII [Alq3 (EL)] shows the luminance (L) of 1451 cd/m2 at 23.0 V 24

23

and 1814 cd/m2 at 22.8 V, respectively. These results show increase of efficiency with thickness 25 27

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of ZnO nano layer and this may be due to the fact that as the thickness increased the surface 29

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coverage becomes uniform which will increase the electron injection52. As more electrons are 31

30

injected, the e- - h+ balance is improved and so efficiencies are higher than that of other devices. 32 34

3

Among the non-doped emissive layers such as DMPPI, TMPPI and NMPPI, combination 36

35

of 26 nm thickness of ZnO as EIL and TMPPI as emissive layer exhibit maximum luminance (L) 38

37

of 17345 cd/m2 with external quantum efficiency (ηex) of 13.8 %, current (ηc) and power (ηp) 39 41

40

43

42

efficiencies of 19.5 cd/A and 9.9 lm W-1, respectively. This high efficiency may be due to the presence of electron withdrawing effect of trifluoromethyl group at p-position of the aldehydic 4 45

ring in the emissive layer which lowers the HOMO energy (-5.4 eV) and thereby lowering the 46 48

47

energy barrier (Figure 7a) so that the leakage of hole is avoided which result in higher 50

49

efficiencies. The fabricated HyLEDs (Figure 7b) shows green emission around 475 nm. 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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4. Conclusion 5

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In conclusion, the efficient HyLEDs have been fabricated using the ITO/ZnO 6 8

7

nanomaterials as a transparent cathode. The device with the 26 nm thickness of ZnO nano layer 10

9

with TMPPI as emissive layer shows higher efficiencies ηc (19.5 cd/A), ηp (9.9 lm W-1) and ηex 1 12

(13.8 %) compared to other devices due to improved energy level alignment. As larger number 13 15

14

of electrons are injected into the emissive layer from 26 nm thickness of ZnO, e- - h+ balance 17

16

becomes to be improved and hence the higher efficiencies. 18 19

5. Acknowledgment 20 2

21

The authors thank the Council of Scientific and Industrial Research (01/(2707)/13EMR-II), 24

23

Department of Science and Technology (EMR/2014/000094), Defence Research and 25 26

Development Organization (Naval Research Board) (213/MAT/10-11) and University Grant 27 29

28

Commission [36-21/2008] for financial support. 30 31 32 3

Supporting Information: 34 36

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Device with 26 nm thickness of ZnO as EIL and 1-(3,5-dimethylphenyl)-2-phenyl-1H38

37

phenanthro[9,10-d]imidazole as non-doped emitter exhibit maximum luminescence (L) of 17182 39 40

cd/m2 with external quantum efficiency (ηex) of 13.2 %, current efficiency (ηc) of 19.2 cd/A and 43

42

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power efficiency (ηp) of 9.6 lmW-1 at 18.1 V. 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment


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References 5

4

(1) Burroughes, J.H.; Bradley, D.D.C.; Brown, A.R.; Marks, R.N.; Mackay, K.; Friend, R.H.; 6 8

7

Burn, P.L.; Holmes, A.B. Light-emitting diodes based on conjugated polymers. Nature, 10

9

1990, 347, 539. 1 12

(2) Forrest S.R. The path to ubiquitous and low cost organic electronic appliances on plastic. 13 15

14

Nature, 2004, 428, 911. 17

16

(3) Blom, P.W.M.; Berntsen, A.J.M.; Liedenbaum, C.; Schoo, H.F.M.; Croonen, Y.; Van de 18 19

Weijer, P.V.D. Efficiency and stability of polymer light-emitting diodes. J. Mater. Sci. 20 2

21

Mater. Electron. 2000, 11, 105. 24

23

(4) Bernius, M.T.; Inbasekaran, M.; O’Brien J.; Wu, W.S. Progress with light-emitting 25 27

26

polymers. Adv. Mater. 2000, 12, 1737. 28 30

29

(5) Morii, K.; Ishida, M.; Takashima, T.; Shimoda, T.; Wang, Q.; Nazeeruddin M.K.; Graetzel, 32

31

M. Encapsulation-free hybrid organic-inorganic light emitting diodes. Appl. Phys. Lett., 2006, 34

3

89,183510. 35 37

36

(6) Bolink, H.J.; Coronado, E.; Repetto D.; Sessolo, M. Air stable hybrid organic-inorganic light 39

38

emitting diodes using ZnO as the cathode. Appl. Phys. Lett., 2007, 91, 223501. 40 41

(7) Bolink, H.J.; Coronado, E.; Orozco J.; Sessolo, M. Efficient polymer light-emitting diode 42 4

43

using air-stable metal oxides as electrodes. Adv. Mater., 2009, 21, 79. 46

45

(8) Bolink, H.J.; Coronado, E.; Repetto, D.; Sessolo, M.; Barea, E.; Bisquert, J.; Garcia 47 48

Belmonte, G.; Prochazka J; Kavan, L. Inverted solution processable OLEDs using a metal 49 51

50

oxide as an electron injection contact. Adv.Funct. Mater., 2008, 18, 145. 53

52

(9) Kabra, D.; Song, M.H.; Wenger, B.; Friend R.H.; Snaith, H.J. High efficiency composite 54 56

5

metal oxide-polymer electroluminescent devices: A morphological and material based 57 58 59 60 ACS Paragon Plus Environment

Page 11 of 27


11 1 3

2

investigation. Adv. Mater., 2008, 20, 3447. 5

4

(10) Morii, K.; Kawase T.; Satoshi, I. High efficiency and stability in air of the encapsulation6 8

7

free hybrid organic-inorganic light-emitting diode. Appl. Phys. Lett., 2008, 92, 213304. 10

9

(11) Chu, T.Y.; Chen, S.Y.; Chen, J.F.; Chen, C.H. Ultrathin electron injection layer on indium1 12

tin oxide bottom cathode for highly efficient inverted organic light emitting diodes. Jpn. J. 13 15

14

Appl. Phys., Part 1. 2006, 45, 4948. 17

16

(12) Zhou, X.; Pfeiffer, M.; Huang, J.S.; Nimoth, J.B.; Qin, D.S.; Werner, A.; Drechsel, J.; 18 19

Maenning, B.; Leo, K. Low-voltage inverted transparent vacuum deposited organic light20 2

21

emitting diodes using electrical doping. Appl. Phys. Lett. 2002, 81, 922. 24

23

(13) Chu, T.Y.; Chen, J.F.; Chen, S.Y.; Chen, C.J.; Chen, C.H. Highly efficient and stable 25 27

26

inverted bottom-emission organic light emitting devices. Appl. Phys.Lett. 2006, 89, 29

28

053503. 31

30

(14) Chen, S.Y.; Chu, T.Y.; Chen, J.F.; Su, C.Y.; Chen, C.H. Stable inverted bottom-emission 32 34

3

organic electroluminescent devices with molecular doping and morphology important. 36

35

Appl. Phys.Lett. 2006, 89, 053518. 37 38

(15) Xiong, T.; Wang, F.; Qiao, X.; Ma, D. Cesium hydroxide doped tris-(8-hydroxyquinoline) 39 41

40

aluminium as an effective electron injection layer in inverted bottom-emission organic light 43

42

emitting diodes. Appl. Phys. Lett. 2008, 92, 263305. 45

4

(16) Andrade, B.W.D’; Forrest, S.R.; Chwang, A.B. Operational stability of electro 46 48

47

phosphorescent devices containing p and n doped transport layers. Appl. Phys. Lett. 2003, 50

49

83, 3858. 51 53

52

(17) Parthasarathy, G.; Shen, C.; Kahn, A.; Forrest, S.R. Lithium doping of semiconducting 5

54

organic charge transport materials. J. Appl. Phys. 2001, 89, 4986. 56 57 58 59 60 ACS Paragon Plus Environment


Page 12 of 27

12 1 3

2

(18) Morii, K.; Ishida, M.; Takashima, T.; Shimoda, T.; Wang, Q.; Nazeeruddin, M.K.; Gratzel, 5

4

M. Encapsulation-free hybrid organic-inorganic light-emitting diodes. Appl. Phys. Lett. 6 8

7

2006, 89, 183510. 10

9

(19) (a) Haque, S.A.; Koops, S.; Tokmoldin, N.; Durrant, J.R.; Huang, J.S.; Bradley D.D.C. 1 12

Palomares, E. A. Multilayered polymer light-emitting diode using a nanocrystalline metal13 15

14

oxide film as a charge-injection electrode. Adv. Mater., 2007, 19, 683; (b) Shariffudin, S.S.; 17

16

Abidin, N.Z.; Yahya, N.Z.; Aziz, A.A.; Herman, S.H.; Rusop M. Hybrid organic-inorganic 18 19

light emitting diode using ZnO nanorods as electron transport layer. RSM IEEE Micro and 20 2

21

Nanoelectronics, 2013, 340. 24

23

(20) Monticone, S.; Tufeu, R.; Kanaev, A.V.; Scolan E.; Sanchez, C. Quantum size effect in 25 27

26

TiO2 nanoparticles: does it exist. Appl. Surf. Sci., 2000, 565, 162. 29

28

(21) Cozzoli, P.D.; Curri, M.L.; Agostiano, A.; Leo G.; Lomascolo, M. ZnO nanocrystals by a 31

30

non-hydrolytic route: synthesis and characterization. J. Phys. Chem. B, 2003, 107, 4756. 32 34

3

(22) (a) Bolink, H.J.; Brine, H.; Coronado E.; Sessolo, M. Hybrid organic-inorganic light 36

35

emitting diodes: effect of the metal oxide. J. Mater. Chem., 2010, 20, 4047; (b) Hyunkoo, 37 38

L.; Kang C.M.; Park, M.; Kwak J.; and Lee, C. Improved Efficiency of Inverted Organic 39 41

40

Light-Emitting Diodes Using Tin Dioxide Nanoparticles as an Electron Injection Layer, 43

42

ACS Appl. Mater. Interfaces, 2013, 5, 1977. 4 45

(23) Chan I.M.; Hong, F.C. Improved performance of the single-layer and double-layer organic 46 48

47

light emitting diodes by nickel oxide coated indium tin oxide anode. Thin Solid Films, 50

49

2004, 450, 304. 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

Page 13 of 27


13 1 3

2

(24) Li, J.; Yahiro, M.; Ishida, K.; Yamada H.; Matsushige, K.; Enhanced performance of 5

4

organiclight emitting device by insertion of conducting/insulating anodic buffer layer. 6 8

7

Synth. Met., 2005, 151, 141. 10

9

(25) Im, H.C.; Choo, D.C.; Kim, T.W.; Kim, J.H.; Seo J.H.; Kim, Y.K. Highly efficient organic 1 12

light-emitting diodes fabricated utilizing nickel-oxide buffer layers between the anodes and 13 15

14

the hole transport layers. Thin Solid Films, 2007, 515, 5099. 17

16

(26) Wu, J.; Hou, J.; Cheng, Y.; Xie Z.; Wang, L.; Efficient top-emitting organic... modified 18 19

silver anode. Semicond. Sci. Technol., 2007, 22, 824. 20 2

21

(27) Zhang H.M.; Choy, W.C.H. Indium Tin Oxide Modified by Au and Vanadium Pentoxide as 24

23

an Efficient Anode for Organic Light-Emitting Devices. IEEE Trans. Electron Devices, 25 27

26

2008, 55, 2517. 29

28

(28) Meyer, J.; Winkler, T.; Hamwi, S.; Schmale, S.; Johannes, H.H.; Weimann, T.; Hinze, P.; 31

30

Kowalsky W.; Riedl, T. Transparent inverted organic light-emitting diodes with a tungsten 32 34

3

oxide buffer layer. Adv. Mater., 2008, 20, 3839. 36

35

(29) Zilberberg, K.; Trost, S.; Schmidt H.; Riedl, T. Solution processed vanadium pent oxide as 37 38

charge extraction layer for organic solar cells. Adv. Energy Mater., 2011, 1, 377. 39 41

40

(30) Bolink, H.J.; Brine, H.; Coronado E.; Sessolo, M. Phosphorescent Hybrid Organic43

42

Inorganic Light-Emitting Diodes. Adv. Mater., 2010, 22, 2198. 4 45

(31) Lee, B.R.; Choi, H.; Sun Park, J.; Lee, H.J.; Kim, S.O.; Kim J.Y.; Song, M.H. Surface 46 48

47

modification of metal oxide using ionic liquid molecules in hybrid organic–inorganic 50

49

optoelectronic devices. J. Mater. Chem., 2011, 21, 2051. 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment


Page 14 of 27

14 1 3

2

(32) Choi, H.W.; Kim, S.Y.; Kim W.K.; Lee, J.L.; Enhancement of electron injection in inverted 5

4

top-emitting organic light-emitting diodes using insulating magnesium oxide buffer-layer. 6 8

7

Appl. Phys. Lett., 2005, 87, 082102. 10

9

(33) (a) Tokmoldin, N.; Griffiths, N.; Bradley D.D.C.; Haque, S.A. A hybrid inorganic–organic 1 12

semiconductor light-emitting diode using ZrO2 as an electron-injection Layer. Adv. Mater., 13 15

14

2009, 21, 3475; (b) Jin, X.; Lei, Y.; Ruifeng, H.; Wei, Y.; Junbiao P.; Yong, C.; Color 17

16

tuning in inverted blue light-emitting diodes based on a polyfluorene derivative by adjusting 18 19

the thickness of the light-emitting layer, J. Mater. Chem. C., 2015, 3, 9819. 20 2

21

(34) Mullen K.; Scherf, U. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2006. 24

23

(35) Yersin H. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008. 25 27

26

(36) Lee, M.T.; Chen, H.H.; Liao, C.H.; Tsai C.H.; Chen, C.H. Stable styrylamine-doped blue 29

28

organic electroluminescent device based on 2-methyl-9,10-di(2-naphthyl)anthracene. Appl. 31

30

Phys. Lett., 2004, 85, 3301. 32 34

3

(37) Wang, P.F.; Hong, Z.R.; Xie, Z.Y.; Tong, S.W.; Wong, O.Y.; Lee, C.S.; Wong, N.B.; Hung 36

35

L.S.; Lee, S.T. A bis-salicylaldiminato schiff base and its zinc complex as new highly 37 38

fluorescent red dopants for high performance organic electroluminescence devices. Chem. 39 41

40

Commun., 2003, 14, 1664. 43

42

(38) Li, Y. Multifunctional platinum porphyrin dendrimers as emitters in undoped 4 45

phosphorescent based light-emitting devices. Appl. Phys. Lett., 2006, 89, 061125. 46 48

47

(39) Tong, Q.X.; Lai, S.L.; Chan, M.Y.; Tang, J.X.; Kwong, H.L.; Lee C.S.; Lee, S.T. High Tg 50

49

triphenylamine-based starburst hole–transporting material for organic light-emitting 51 53

52

devices. Chem. Mater., 2007, 19, 5851. 54 5 56 57 58 59 60 ACS Paragon Plus Environment

Page 15 of 27


15 1 3

2

(40) Hau, S.K.; Yip, H.L.; Baek, N.S.; Zou, J.; O’Malley, K.; Jen, A.K.Y. Air-stable inverted 5

4

flexible polymer solar cells using zinc oxide nanoparticles as an electron selective layer. 6 8

7

Appl. Phys. Lett. 2008, 92, 253301. 10

9

(41) Ong, B.S.; Li, C.; Li, Y.; Wu, Y.; Loutfy, R. Stable, Solution-processed, high-mobility ZnO 1 12

Thin-film transistors. J. Am. Chem. Soc., 2007, 129, 2750. 13 15

14

(42) Nayak, P.K.; Yang, J.; Kim, J.; Chung, S.; Jeong, J.; Lee, C.; Hong, Y. Spin-coated Ga17

16

doped ZnO transparent conducting thin films for organic light-emitting diodes. J. Phys. 18 19

D, 2009, 42, 035102. 20 2

21

(43) (a) Vasilopoulou, M.; Palilis, L.C.; Georgiadou, D.G.; Douvas, A.M.; Argitis, P.; Kennou, 23 24

S.; Sygellou, L.; Papadimitropoulos, G.; Kostis, I.; Stathopoulos, N.A.; Davazoglou, D. 25 27

26

Reduction of Tungsten Oxide: A path towards dual functionality utilization for efficient 29

28

anode and cathode interfacial layers in organic light-emitting diodes. Adv. Funct. Mater., 30 32

31

2011, 21, 1489; (b) Sessolo, M; Bolink, H.J; Brine, H; Prima-Garcia, H; Tena-Zaera, 34

3

R. Zinc oxide nanocrystals as electron injecting building blocks for plastic light sources. J. 36

35

Mater. Chem., 2012, 22, 4916; (c) Sessolo, M.; Bolink, H.J. Hybrid organic–inorganic 37 39

38

light-emitting diodes. Adv. Mater., 2011, 23, 1829; (d) Zhong, C.; Liu, S.; Huang, F.; Wu, 41

40

H.; Cao, Y. Highly efficient electron injection from tin oxide/cross-linkable amino42 43

functionalized polyfluorene interface in inverted organic light emitting devices. Chem. 4 46

45

Mater., 2011, 23, 4870. 48

47

(44) Chang, C.H.; Huang, J.L.; Wu, S.W. Mo-doped GZO films used as anodes or cathodes for 49 50

highly efficient flexible blue, green and red phosphorescent organic light-emitting diodes. J. 51 53

52

Mater. Chem. C, 2015, 3, 12048. 54 5 56 57 58 59 60 ACS Paragon Plus Environment


Page 16 of 27

16 1 3

2

(45) (a) Hong, K.; Kim, K.; Kim, S.; Lee, I.; Cho, H.; Yoo, S.; Choi, H.W;. Yang Lee, N.; 5

4

Heung Tak, Y.; Lee, J.L. Optical properties of WO3/Ag/WO3 multilayer as transport 6 8

7

cathode in top-emitting organic light emitting diodes. J. Phys. Chem. C, 2011, 115, 3453; 10

9

(b) Jayabharathi, J.; Prabhakaran, A.; Karunakaran, C.; Thanikachalam. V.; Sundharesan 1 12

M. Structural, optical and photoconductivity characteristics of pristine FeO·Fe2O3 and 13 15

14

NTPI–FeO·Fe2O3 nanocomposite: aggregation induced emission enhancement of 17

16

fluorescent organic nanoprobe of thiophene appended phenanthrimidazole derivative, RSC 18 19

Adv., 2016, 6, 18718; (c) Thanikachalam, V.; Arunpandiyan A.; Jayabharathi, J.; 20 2

21

Karunakaran C.; Ramanathan, P. Spectroscopic Studies on Photoelectron Transfer from 224

23

(furan-2-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole to ZnO, Cu-doped ZnO and Ag25 27

26

doped ZnO, J. Fluoresc., 2014, 24, 1447; (d) Ying, Z.; Lai, S.L; Tong, Q.X.; Chan, M.Y.; 29

28

Ng, T.W.; Wen, Z.C.; Zhang, G.Q.; Lee, S.T.; Kwong, H.L.; Lee, C.S. Synthesis and 31

30

characterization 32

of

phenanthrimidazole

derivatives

for

applications

in

organic

34

3

electroluminescent devices, J. Mater. Chem., 2011, 21, 8206; (e) Jayabharathi, J.; 36

35

Karunakaran C.; Arunpandiyan A.; Vinayagamoorthy P. Styryl phenanthrimidazole37 38

fluorescence switched on by core/shell BaTiO3/ZnO and Mn-doped TiO2/ZnO nanospheres 39 41

40

and switched off by the core nanoparticles, RSC Adv., 2014, 4, 59908; (f) Karunakaran, C.; 43

42

Jayabharathi, J.; Venkatesh Perumal M.; Thanikachalam,V.; Thakur, P.K. Electronic 4 45

properties of phenanthrimidazoles as hole transport materials in organic light emitting 46 48

47

devices and in photoelectron transfer to ZnO nanoparticles. J. Phys. Org. Chem., 2013, 26, 50

49

386; (g) Jayabharathi, J.; Ramanathan, P.; Karunakaran, C.; Thanikachalam, V. Fused 51 53

52

Methoxynaphthyl Phenanthrimidazole Semiconductors as Functional Layer in High 5

54

Efficient OLEDs, J. Fluoresc., 2016, 26, 307; (h) Jayabharathi, J.; Kalaiarasi V.; 56 57 58 59 60 ACS Paragon Plus Environment

Page 17 of 27


17 1 3

2

Thanikachalam, V.; Jayamoorthy, K.; Estimation of Excited State Dipolemoments from 5

4

Solvatochromic Shifts–Effect of pH, J. Fluoresc., 2014, 24, 599; (i) Jayabharathi, J.; 6 8

7

Thanikachalam, V.; Ramanathan, P.; Arunpandiyan , A. Radiative and Nonradioactive 10

9

Electron Transfer in Donor–Acceptor Phenanthrimidazoles, J. Fluoresc., 2014, 24, 1603. 1 13

12

(46) Ozgur, U.; Alivov, Y.I.; Liu, C.; Teke, A.; Reshchikov, M.A.; Dogan, S.; Avrutin, V.; Cho 14 16

15

S.J.; and Morkoc, H. A comprehensive review of ZnO materials and devices. J. Appl. 18

17

Phys., 2005, 98, 041301. (b) Jun M.; Bradley M.S.; Bulovic, V. Photoluminescence 20

19

quenching of tris-(8-hydroxyquinoline) aluminum thin films at interfaces with metal oxide 21 23

2

films of different conductivities. Phys. Rev. B, 2009, 79, 235205. 25

24

(47) Monticone, S.; Tufeu R.; Kanaev, A.V. Complex nature of the UV and visible fluorescence 26 27

of colloidal ZnO nanoparticles. J. Phys. Chem. B, 1998, 102, 2854. 28 30

29

(48) Wu, Y.; Zhou, Y.C.; Wu, H.R.; Zhan, Y.Q.; Zhou, J.; Zhang, S.T.; Zhao, J.M.; Wang, Z.J.; 32

31

Ding, X.M.; Hou, X.Y. Metal-induced photoluminescence quenching of tri-(83 34

hydroxyquinoline) aluminum. Appl. Phys. Lett., 2005, 87, 044104. 35 37

36

(49) Chance, R.R.; Prock, A.; Silbey, R. Comments on the classical theory of energy transfer. J. 39

38

Chem. Phys., 1975, 62, 2245. 40 42

41

(50) Patel, N.K.; Cina, S.; Burroughes, J.H. High-Efficiency Organic Light-Emitting Diodes. 4

43

IEEE J. Sel. Top. Quantum Electron., 2002, 8, 346. 45 46

(51) Hill, I.G.; Milliron, D.; Schwartz, J.; Kahn, A. Organic semiconductor interfaces: electronic 47 49

48

structure and transport properties. Appl. Surf. Sci., 2000, 166, 354. 51

50

(52) Baxter J.B.; Schmuttenmaer, C.A. Conductivity of ZnO Nanowires, nanoparticles, and thin 52 53

films using time resolved terahertz spectroscopy. J. Phys. Chem. B. 2006, 110, 25229. 54 5 56 57 58 59 60 ACS Paragon Plus Environment


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18 1 3

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Figure Captions 5

4

Figure 1: (a) Single crystal XRD of DMPPPI; (b) TGA of DMPPPI, TMPPI and NMPPI; (c) 6 8

7

DSC of DMPPPI, TMPPI and NMPPI (d) UV-Visible and PL spectra of DMPPPI, 10

9

TMPPI and NMPPI; (e) Life time spectra of DMPPPI, TMPPI and NMPPI and (f) 1 12

Cyclic voltammogram of DMPPPI, TMPPI and NMPPI. 13 15

14

Figure 2: (a) Powder X-Ray diffraction of pristine ZnO and (b) UV and PL of pristine ZnO 16 18

17

Figure 3: (a) HR-TEM image of pristine ZnO; (b) SAED image of pristine ZnO and (c) EDX 19 20

spectrum of pristine ZnO. 21 2 24

23

Figure 4: (a) XPS of pristine ZnO; (b) Binding energy peaks of zinc and (c) Binding energy 26

25

peaks of oxygen 27 29

28

Figure 5: (a) AFM images of films of devices I-IV along with VII; (b) UV-spectrum of film IV; 30 31

(c) Life time spectra of films I-VIII and (d) (i) Exciton dissociation at ZnO/DMPPPI 32 34

3

interface and (ii) Non radiative energy transfer from DMPPPI to ZnO. 36

35

Figure 6: (a) Current density - Voltage; (b) Luminescence - Voltage; (c) Current efficiency37 39

38

Current density; (d) Power efficiency - Current density and (e) External quantum 41

40

efficiency- Current density of devices with different ZnO nanomaterials thicknesses. 42 43

Figure 7: (a) Schematic energy level diagram of fabricated HyLEDs and (b) Normalized 4 46

45

electroluminescent spectra of HyLEDs 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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2

Table 1. Photophysical, thermal properties and device characteristics

8

7

6

5

4

Parameters Photophysical & thermal :

DMPPI

TMPPI

NMPPI

-

-

-

13

12

1

10

λab (nm)

342

395

339

λem (nm)

428

420

431

Ф

0.45

0.51

0.48

τ (ns)

1.45

1.49

1.36

HOMO (eV)

-5.6

-5.4

-5.5

LUMO (eV)

-2.5

-2.3

-2.1

kr (108 S-1 )

3.1

3.4

3.5

knr (108 S-1 )

3.8

3.3

2.8

Td (°C)

261

265

268

Tg (°C)

82

91

85

Device:

IV/I/II/III (26/15/18/24 nm)

V (26 nm)

VI (26 nm)

V1000 (V)

18.1/21.8/21.2/20.2

16.0

17.9

L (cd/m2)

17182/14215/15710/16631

17345

16989

ɳex (%)

13.2/7.9/ 8.5/ 9.2

13.8

6.5

ɳc (cd/ A-1)

19.2/15.4/16.1/17.0

19.5

15.3

ɳp (lm W-1)

9.6/6.8/7.5/8.1

9.9

7.2

9

18

17

16

15

14

20

19

25

24

23

2

21

27

26

32

31

30

29

28

39

38

37

36

35

34

3

46

45

4

43

42

41

40

48

47

VII/VIII: V1000 (V); 23.0/22.8, L (cd/m2); 1451/1814, ɳex (%); 6.0/3.01, ɳc (cd/ A-1); 12.1/4.9, ɳp (lm W-1); 5.1/3.5 50

49 51 52 53 54 5 56 57 58 59 60

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Air Stable HyLEDs Using Efficient Electron Injection and Emitting

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