Color optimization of red organic light emitting diodes (OLEDs) through dihydroxyphenyl-substituted zinc porphyrins emitters

Article abstract of DOI:10.1016/j.jphotochem.2017.03.029

Four tetrakis (dihydroxyphenyl) Zinc(II) porphyrin (Zn-TDHPP) were successfully synthesized and used as fluorescent materials in organic light-emitting diodes (OLEDs). Devices were fabricated with an indium tin oxide(ITO)/poly (3,4-ethylenedi-oxythiophene

Full text of DOI:10.1016/j.jphotochem.2017.03.029

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Title: Color optimization<!–<query id="Q1">Please check
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Doc headfor correctness.</query>–> of red organic light
emitting diodes (OLEDs) through
dihydroxyphenyl-substituted zinc porphyrins emitters
Authors: Mohammad Janghouri, Maryam Adineh
PII:
S1010-6030(16)31163-7
DOI:
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Please cite this article as: Mohammad Janghouri, Maryam Adineh, Color
optimization of red organic light emitting diodes (OLEDs) through dihydroxyphenyl-
substituted zinc porphyrins emitters, Journal of Photochemistry and Photobiology A:
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Color optimization of red organic light emitting diodes (OLEDs) through
dihydroxyphenyl-substituted zinc porphyrins emitters
Mohammad Janghouri1*, Maryam Adineh2
Email: m.janghouri@uut.ac.ir
1Faculty of Electrical Engineering, Urmia University of Technology, Band Road, Urmia, Iran
2Department of Chemistry, Shahid Beheshti University, Evin, 1983963113 Tehran, Iran
1

Graphical abstract
2

Highlights
► Novel porphyrins derivatives has been synthesized for using as fluorescent
material in OLEDs
► EL of OLEDs are explored by the spectral overlap between emissions of
PVK:PBD and absorptions of porphyrins derivatives.
► AFM measurement showed that the morphology of the layers depends on the
type of laser irradiations and solvent bath.
Abstract:
Four tetrakis (dihydroxyphenyl) Zinc(II) porphyrin (Zn-TDHPP) were successfully synthesized
and used as fluorescent materials in organic light-emitting diodes (OLEDs). Devices were
fabricated with an indium tin oxide(ITO)/poly (3,4-ethylenedi-oxythiophene):poly
(styrenesulfonate) (PEDOT:PSS) (55 nm)/ polyvinylcarbazole (PVK):(2-(4-biphenyl)-5-(4-t-
butylphenyl)1,3,4-oxadiazole) (PBD):Zn-TDHPP derivatives (80 nm)/Al (150 nm). The resultant
devices had peak emissions ranging from 206 nm to 246 nm. It has been shown that with
modification of Zn-TDHPP molecule having specific backbone with different functional groups
R (R=OH, COOH), we could change and control its optical properties such as band gap energy,
photoluminescence (PL) and electroluminescence (EL) spectra along with its electrical properties
such as current diagram of OLED devices. In addition, it is possible to modify Commission
Internationale de l’E´clairage (CIE) coordinates of EL spectrum of fabricated OLED devices for
application in the display industry. We used dichloromethane bath and Ultra-Violet laser (405
nm) for improvement of the morphology of layers. Atomic force microscopy (AFM)
measurement showed that the morphology of the layers depends on the type of laser irradiations
and solvent bath.
3

Keywords: Zn-TDHPP; Photoluminescence; Electroluminescence; Dichloromethane bath; Ultra-
Violet laser; Morphology
1. Introduction
In the last decade, considerable interest has been focused on the investigation of optical
properties of porphyrin derivatives because of their wide-spread applications in lighting,
electroluminescent devices, waveguides and memory devices.1-4 Many novel materials with blue,
green, and red colors are required to develop full-color displays. Green emitters have already
been obtained with high efficiency. With respect to green-emitting materials, great efforts have
been focused on the development and study of performance of red-emitting materials and many
compounds with excellent fluorescence properties have been investigated.5–10 Red OLEDs
usually have relatively weak brightness relative to green and blue OLEDs. Moreover, the
photoluminescence quantum yield of many red-emitting materials in films is very weak because
of self-quenching phenomenon.11 A layer of only red emitters is not efficient in current transport
and needs a host and an efficient Forster energy transfer to show efficient emission. Doping a
host matrix with highly luminescent dyes has proven to be an efficient method to obtain narrow
emission spectra.12–22 Porphyrin derivatives are promising materials for applications in OLEDs,
due to their light-harvesting via their strong absorption, narrow spectrum, suitable for vacuum
deposition, high stability, high electroluminescent intensity, with low driving voltage and tunable
emission wavelengths.23-25 The task was to alter the photophysical properties of porphyrins by
several modifications; changing the central transition metal, replacing some of the ancillary
ligands and modifying the ligand by adding suitable functional groups.
4

In this paper, we synthesized tetrakis (dihydroxyphenyl) zinc(II) porphyrin (Zn-TDHPP) and
compared the results with tetrakis(p-carboxyphenyl) zinc(II) porphyrin (ZnTCPP) which has
been previously studied.26- 27 The effects of the evaporation rate of dyes and temperature on
morphology of films studied in many reports,28–30 but to the best of our knowledge there is no
report on the investigation of morphology of films when a laser irradiation and dichloromethane
bath simultaneously was applied. Here, PVK:PBD matrix and Zn-TDHPP derivatives are used as
host and guest, respectively. Therefore, Zn-TDHPPs are acted as dopant in PVK:PBD matrix. It
is interesting to know how the structures of the Zn-TDHPPs influence the OLEDs and to
estimate the EL performance of the Zn-TDHPPs-doped OLED devices.
2. Experimental
2.1. Synthesis of Dyes
2.1.1. Tetrakis dimethoxyphenyl porphyrins
Propionic acid (150 mL) was brought to a boil, and then, the appropriate
dimethoxybenzaldehyde (20 mmoL) was added, followed by pyrrole (20 mmoL) being added
dropwise. After refluxing for 1.5 h, the solution was cooled to room temperature and then
crystallized over 3 days. The product was filtrated and washed with hot water and methanol. In
the case of tetrakis(3,4-dimethoxyphenyl)porphyrin and tetrakis(2,5-dimethoxyphenyl) porphyrin
no precipitate was achieved, so the propionic acid was removed from crude. The solid black
residue was purified using column chromatography with dichloromethane and methanol (50:1)
as eluents. Tetrakis(3,4-dimethoxyphenyl)porphyrin: yield: 350 mg (4%), 1H NMR (CDCl3, 300
MHz), δ, ppm: 8.9 (s, 8H) , 7.8 (s, 4H), 7.7 (d, 4H) , 7.2 (d, 4H), 4.1 (s, 12H); 3.9 (s, 12H), -2.70
(s, 2H). 13C NMR (CDCl3, 100 MHz), δ, ppm: 148.8, 147.0, 134.7, 131.1, 130.0, 127.3, 126.7,
5

119.8, 118.2, 110.2, 109.4, 108.7, 56.0, 55.9; UV–Vis, (CHCl3, λmax): 421 (sorret); 518.; 556;
592; 649.
Tetrakis(2,3-dimethoxyphenyl)porphyrin: yield: 680 mg (8%), 1H NMR (300 MHz, CDCl3,
25℃): δ, ppm, 8.7(s, 8H), 7.2(m, 4H), 7.1(d, 4H), 7.0(m, 4H), 3.9 (s, 12H), 1.8 (s, 12H) -2.8(s,
2H). Tetrakis(2,4-dimethoxyphenyl)porphyrin: Yield: 682 mg (8%), 1H NMR (CDCl3, 300 MHz,
25℃ ), δ, ppm, 8.7 (s, 8H), 7.7 (s, 4H), 7.0 (s, 8H), 4.1 (s, 12H), 1.8 (s, 12H) , -2.7 (s, 2H).
Tetrakis(2,5-dimethoxyphenyl)porphyrin: yield: 500 mg (6%) ,1H NMR (CDCl3, 300 MHz,
25℃), δ, ppm, 9 (s, 8H), 7.4 (d, 4H), 7.1 (s, 8H)-2.8(s, 2H), 3.9 (s, 12H), 1.9 (s, 12H) .
2.1.2. Tetrakis(dihydroxyphenyl)porphyrins (TDHPP)
The appropriate tetrakis(dimethoxyphenyl)porphyrin (0.5 mmoL) was dissolved in the minimum
amount of dry dichloromethane. The solution was cooled in a methanol-liquid nitrogen bath (to -
98 °C). Boron tribromide (6 mmoL) was slowly added to this solution. The mixture was stirred
for 1 h at -98 °C, and then allowed to come to room temperature, and was then stirred overnight
at room temperature. Water (20 mL) was added to quench the ice-cold reaction mixture.
Neutralization with triethylamine produced a brown-purple precipitate, which was collected by
filtration and washed with water and dichloromethane. Characterization for 3,4-TDHPP: yield:
75%, 1H NMR (300 MHz, DMSOd6, 25ᵒC): δ, ppm 9.4 (s, 8H), 8.9 (s, 8H), 7.7 (s, 4H), 7.4 (s,
4H), 7.1 (s, 4H), -2.9 (s, 2H). 13C NMR (75 MHz, DMSOd6, 25 °C): δ, ppm 145.0, 144.1, 132.8,
131.6, 126.7, 122.9, 120.4, 115.9, 114.7. IR (KBr): ν, cm-1 3386, 1653, 1597, 1508, 1433, 1346,
1260, 1201, 1108, 928, 871, 801, 711. UV-vis (EtOH, λmax, nm ): 428 (soret), 520 , 559 , 592 ,
653.
6

1
2,3-TDHPP: yield: 80%, H NMR (300 MHz, DMSO-d6, 25℃): δ, ppm 9.7 (s, 8H), 8.7(s, 8H),
7.2(m, 4H), 7.1(d, 4H), 7.0(m, 4H), -2.8(s, 2H). 13C NMR (75 MHz, DMSO-d6, 25℃): δ, ppm,
146.5, 145.5, 129.1, 128.6, 127.8, 126.8, 118.1, 116.8, 116.1. IR (KBr, cm-1): 3423, 2923, 2853,
1615, 1586, 1466, 1346, 1204, 1145, 927, 844, 804, 768, 730. UV-Vis (EtOH, λmax, nm): 420
(soret), 516, 550, 591, 647.
2,4-TDHPP: yield: 78%, 1H NMR (300 MHz, DMSO-d6, 25℃): δ, ppm 9.1 (s, 8H), 8.8(s, 8H),
7.7(s, 4H), 7.0(s, 8H), -2.7(s, 2H). 13C NMR (75 MHz, DMSO-d6, 25℃): δ, ppm, 146.5, 145.5,
129.1, 128.6, 127.8, 126.8, 118.1, 116.8, 116.1
2.1.3. Zn-tetrakis (dihydroxyphenyl)porphyrins
A mixture of TDHPP and 1.2 eq Zn(OAc)2.4H2O in methanol were stirred at room temperature.
The metalation process was followed by UV-Vis spectroscopy. After the completion of the
reaction, the solvent was evaporated with a rotary evaporator at low temperature. The crude
product was washed several times with water to remove the excess zinc salt and then dried at low
pressure using a vacuum pump.
Zn-2,3-TDHPP: yield: 95%, 1H NMR (300 MHz, DMSO-d6, 25℃): δ, ppm 9.7 (s, 8H), 8.8 (s,
8H), 7.4 (m, 4H), 7.2 (d, 4H), 7.0 (m, 4H). 13C NMR (75 MHz, DMSO-d6, 25℃): δ, ppm, 146.6,
145.5, 129.2, 128.8, 128.0, 126.8, 118.3, 116.9, 115.9. IR (KBr, cm-1): 3426, 2926, 2851, 1620,
1586, 1466, 1346, 1210, 1145, 927, 844, 809, 768, 735,547, 469. UV-Vis (EtOH ,λmax, nm ): 425
5
  4.5310
(soret,
), 556 , 592. Elemental Analysis Calc. for C44H28N4O8Zn: C, 65.56; H, 3.50;
N, 6.95; Found C, 65.58; H, 3.45; N, 6.98%.
Zn-3,4-TDHPP: yield: 95%, 1H NMR (300 MHz, DMSO-d6, 25℃): δ, ppm 9.4 (s, 8H), 8.8 (s,
8H), 7.7 (s, 4H), 7.4 (s, 4H), 7.1 (s, 4H), 13C NMR (75 MHz, DMSO-d6, 25℃): δ, ppm 145.0,
144.1, 132.8, 131.6, 126.7, 122.9, 120.4, 115.9, 114.7. UV-Vis (EtOH ,λmax, nm ): 428 (soret,
7

5
  3.0910
), 559 , 602. Elemental Analysis Calc. for C44H28N4O8Zn: C, 65.56; H, 3.50; N,
6.95; Found C, 65.62; H, 3.43; N, 6.92%.
Zn-2,4-TDHPP: yield: 96%, 1H NMR (300 MHz, DMSO-d6, 25℃): δ, ppm 9.4 (s, 8H), 8.8 (s,
5
  2.2410
8H), 7.7 (s, 4H), 6.9 (s, 8H). UV-Vis (EtOH ,λmax, nm ): 425 (soret,
), 556 , 597.
Elemental Analysis Calc. for C44H28N4O8Zn: C, 65.56; H, 3.50; N, 6.95; Found C, 65.57; H,
3.48; N, 6.96%.
Zn-2,5-TDHPP: yield: 95%, 1H NMR (300 MHz, DMSOd6, 25℃): δ, ppm 9.7 (s, 8H), 9 (s, 8H),
5
  2.8410
7.4 (d, 4H), 7.1 (s, 8H). UV-Vis (EtOH ,λmax, nm ): 424 (soret,
), 556 , 591.
Elemental Analysis Calc. for C44H28N4O8Zn: C, 65.56; H, 3.50; N, 6.95; Found C, 65.60; H,
3.44; N, 6.94%.
2.2. Materials
Apart from the zinc(II) porphyrin compounds which are synthesized in the Department of
Chemistry and prepared and purified according to the literature procedures,31–33 all other
materials including PEDOT:PSS (poly(3,4-ethylenedi-oxythiophene):poly(styrenesulfo-nate))
PVK (polyvinylcarbazole) and PBD (2-(4-biphenyl)-5-(4-t-butylphenyl)1,3,4-oxadiazole), are
purchased from Sigma-Aldrich and used without any further purification. Figure 1 depicts the
structures of the zinc(II) porphyrin derivatives. Structure of Zn-2,3-TDHPP, Zn-3,4-TDHPP, Zn-
-2,4TDHPP, Zn-2,5-TDHPP and Zn-TCPP are designated as (1), (2), (3), (4) and (5) in this
work, respectively.
2.3. Physical Measurements
8

Nuclear magnetic resonance, 1H NMR and 13C NMR, spectra were recorded in dimethyl
sulfoxide-d6 (DMSO-d6) using a 300 MHz Bruker instrument. Thickness measurements were
performed by DekTak 8000. EL and PL measurements of the fabricated OLEDs were performed
by USB2000 and HR4000 Ocean Optic. The current–voltage–luminance characteristics and
AFM measurements were checked by Keithley source meter 2400 model, optical meter Mastech-
MS6612 and Easy Scan 2, respectively.
2.4. Fabrication of OLEDs
Fabrication of OLED is the sequence of deposition of different materials. Figure 2a depicts the
structure of the fabricated OLED. Glass substrates, coated with ITO (sheet resistance of 70
Ω/m^2), were used as the conducting anode. The ratio of the zinc(II) porphyrins for each type
were 6% Wt. in PVK:PBD (100:40). PEDOT:PSS as a hole injection layer was spin coated at
3000 rpm in 30 s on clean ITO substrate to a thickness of 55 nm and was baked in an oven for
one hour at 110°C. The PVK:PBD:zinc(II) porphyrins in dichloromethane with a concentration
of 1:0.4:0.06 mg/mL was spin coated at 3000 rpm in in 30 s over the sample to a thickness of 80
nm and baked at 80 C for 1 h. As shown in Fig. 2b, in step 1, the PVK:PBD:zinc(II) porphyrins
was exposed to solvent bath for 30 min to make a homogeneous wet film. In step 2, a UV laser
(20 mW) was used to heat the wet film to achieve much smoother surface. In other words, the
laser operates as a heating source to warm up the film surface, and the solvent bath is for
moistening it. Finally, a metallic cathode of Al was deposited on the emissive layer at 8 ×10-5
mbar by thermal evaporation. The active area of the prepared samples was 3× 3 mm2.
3. Results and discussion
3.1. Photoluminescence characteristics
9

We have investigated four types of porphyrin dyes, (1), (2), (3) and (4) and compared them with
previously studied dye (5). A diode laser at 405 nm was used as an excitation light source to take
the PL spectrum of all synthesized materials. Figure 3 depicts the PL spectra of five zinc(II)
porphyrin derivatives. The PL spectra of compounds (1-5) was obtained in 10-5 mol lit-1
dichloromethane solution.
It seems the emission wavelength of the Zn-TDHPP complexes can be tuned simply by the
changing of the type substituted group. The maxima of emission peaks show that a good
correlation exists between the nature of the substituent group and π-conjugated ligand in the
structure of the Zn-porphyrin complexes and emission wavelength. The difference in emission
peaks and emission intensity can be understood by considering the band gap energy for Zn-
porphyrin complexes. The compound (5) and Zn-TDHPPs emit in the orange-red emission
region of the electromagnetic spectrum, with an intense peak in the, 626-649 nm region. This
could be referred to excess functional groups of hydroxyl in Zn-TDHPP derivatives. These
functional groups affect the π-electron distribution of Zn-TDHPP ring and lead to creation and
modification of molecular energy levels. Modifications to Zn-TDHPP structure could cause
changes the band gap energy, and these amendments also could cause changes to PL spectra.
Table 1 shows photophysical property of compounds.
3.2. Electroluminescence
To consider the potential of Zn-TDHPP as electro sensitizers for OLED, their OLEDs
performances are tested. The normalized EL spectra of (5) and Zn-TDHPPs as an OLED device
in PVK:PBD host is given in Figure 4. The OLED performance is dependent on the substituent
type R in Zn-TDHPP. All devices show an intense peak in the wavelength range of 602 nm to
10

645 nm. This wavelength range is important due to the effect of the modification of backbone
structure of Zn-TDHPP derivatives on this range. The electroluminescence spectral peaks shifted
from low wavelength to the high wavelength region when the constituent group was varied from
electron-withdrawing (COOH) to electron-donating (OH) type. The Zn-TDHPPs and (5) are
dissolved in a solution of PVK:PBD as the host materials and dichloromethane as a solvent, then
spin-coated on the prepared substrates. A layer containing only porphyrin materials show low
electrical conduction, therefore, (5) and Zn-TDHPPs are doped into a host matrix. It is evident
from Fig. 5, that the PL of PVK:PBD as a host material has comprehensive overlap with the
absorption spectra of (1). Apparently, in host matrix layer, PVK polymer acts as a hole transfer
material, while PBD small molecule is electron transfer material. In the emissive layer, holes in
highest occupied molecular orbital (HOMO) level have columbic interaction with the electrons
in lowest unoccupied molecular orbital (LUMO) levels, which contribute to the formation of
excitons in this layer. The annihilation of these excitons emit photons that have the same energy
as energy band gap of dopants or higher than it. The energy transfer should exist between host
material and emissive dopants for exciton transfer to HOMO and LUMO levels of emissive
dopants. An efficient energy transfer requires a good overlap between the PL spectrum of the
host material and the absorption spectrum of emissive dopants. The compound (1) bands (425-
563 nm) overlap with PVK:PBD PL (435 nm), which accounts for efficient energy transfer
according to both the Forster and Dexter/superexchange model.34 In addition, according to Figure
6, efficient emissive exciton energy transfer has been occurred, to achieve a good balance of
holes and electrons, both hole and electron-transporting functions should be incorporated into a
single bipolar host material. It should be noted that a high ratio of PBD leads to a lower stability
of the device due to the decrease in the ratio of PVK. A PVK to PBD ratio of 100:40 is suggested
11

as an effective ratio,35-36 which decreases the risk of excimer formation and increases the stability
of the device. The current density (J) versus applied voltage (V) characteristics of devices (1-5)
is shown in Fig.7. This show (5) has the highest current density of 148 mA/cm2, and the high
current density probably suggests that the concentration of (5) sites in the blend is sufficient to
provide the conducting channel for the prior charge injection and transport. The current density
of the (5) increased the fastest in the lower applied voltage 4.6 V. On the other hand, the current
density of (5), (1), (2) and (4) showed very similar characteristics on the current–voltage curve.
The order of driving voltage is (3) > (4) > (2) > (1) > (5). The driving voltage of devices (1-5)
approximately are 4.6-8.5 V, indicating that with increasing of driving voltage the electron
injection barrier from PBD into (5) and Zn-TDHPPs increased, which induced space charge and
prevented further injection of carriers from the Al into the bulk current. At high operating
voltage, due to thermal aging and impurity diffusion in a high electric field, the device lifetime
decreased. Figure 8 shows luminance-voltage characteristics of the devices. The maximum
brightness of emissions is 700–2654 cd/m2 for the devices at different dye dopants. It was
established that the luminescence increased in the order: (3), (4), (2), (1) and (5), respectively.
The turn-on voltages of devices (1-5) approximately are 5.4-8.9 V. The order of turn-on voltage
is (3)> (4)> (2)> (1)> (5). Figure 9 shows luminance decay time characteristics of the devices.
Device (1-3) are more stable than device (4-5). It is evident that with increasing of time, all
samples depict relatively similar behavior in luminance change. Figure 10 and Table 2 show the
CIE coordinates and data for the devices. The CIE coordinates for (5), (1), (2), (3), and (5) in
PVK:PBD were 0.56, 0.40; 0.61, 0.31; 0.60, 0.36, 0.58, 0.38 and 0.59, 0.35, respectively.
Notably, (5) and Zn-TDHPPs-doped PVK:PBD showed orange–red emission. For investigation
of surface roughness of Zn-TDHPPs, compound (1) was selected, and effect of laser irradiation
12

and dichloromethane bath on the morphology of film was studied. Figures 11 shows AFM
images of the (1) in PVK:PBD thin films, before and after irradiant of the samples. Before
irradiant, a lot of craters can be seen on the film surface, which increased the roughness of the
film. When the film was irradiated for 1 h, these craters on the surface of the PVK:(1):PBD
disappeared, indicating softening and contraction of the film at dichloromethane bath. Therefore,
the surface roughness of the PVK:(1):PBD layer was certainly lower than the film without
irradiation. We believe that the morphology of films relies on overlaps between the absorption of
(1) and the laser radiation. There is a small ratio of overlap between the spectrum of the laser and
the absorption spectra of the (1). Hence, it is expected energy transfer laser emission to the (1) to
occur to some extent. The effect of the solvent bath was also taken into account. When the
solvent bath was not employed, and only the laser radiation was applied, the smoothest surface
was not achieved. Also, the solvent bath was found to be an important factor for controlling the
morphology of the prepared films. The average root-mean-square (RMS) surface morphology of
(1) doped in PVK: PBD before and after laser irradiation was 9.4 nm, 5.4 nm, respectively. The
introduction of different functionality into porphyrins can be a convenient approach to study
their properties. We chose catechol-substituted porphyrins and compared them with (5). These
porphyrins are quite stable in our experimental condition. Degradation studies have not been
performed on these porphyrins; however, we obtained emission spectrum (Fig.12.) of the film
before and after the laser irradiation for the (1). The result shows that before and after laser
irradiation the PL spectra of the film PVK:(1): PBD is similar. No noticeable change observed.
So, it seems porphyrins maintain their structures.
4. Conclusion
13

In conclusion, it has been demonstrated that modification of the molecular structure of emissive
dopant could change the optical and electrical properties. The EL results show that the
luminescent of the Zn-TDHPP depends on functional groups, which shows the EL color of the
Zn-TDHPPs from orange to red emission can be tuned. The OLED performance for studied
porpphyrins were ranked: (5) > (1) > (2) > (4) > (3); the order was the same as extinction
coefficient for Zn-TDHPPs. Despite higher luminescence for (5) compare to Zn-TDHPPs this
porphyrins showed better luminescence decay time. In the EL spectra, Zn-TDHPPs show a long
red shift rather than (5) EL spectra, demonstrating that effective energy transfer is taking place in
the emissive layer. So Zn-TDHPPs can be used for fabrication of deep red OLED devices for
application in display industry. We believe that the electroluminescence performance of OLED
devices based on Zn-TDHPP depends on overlaps between the absorption of the Zn-TDHPP and
the emission of PVK:PBD. Finally, the luminescence of devices could be adjusted dramatically
by aforementioned modifications of molecular structure. AFM measurement showed that the
morphology of the layers depends on the type of laser irradiations and solvent bath.
Acknowledgements
The author would like to thank the Urmia University of Technology and Shahid Beheshti
University for supporting this work.
14

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Figure captions
Fig.1. the structures of the Zinc porphyrin derivatives.
Fig.2. a) Schematic structures of the OLEDs. b) Step 1, effect of dichloromethane bath and step
2, laser irradiation on morphology of fabricated film.
Fig.3. PL spectra of compounds (1-5) in dichloromethane solution 10-5mol lit-1 (λexc=405 nm).
Fig.4. Electroluminescence spectra of devices.
Fig. 5. PL and absorption spectra of PVK:PBD and (1), respectively.
Fig.6. Schematic showing the energy levels of device and the dynamic process of EL emission in
(5).
Fig.7. Current density of Zinc porphyrin based device.
Fig.8. The luminance – voltage relationship of OLEDs with different dyes.
Fig.9. Luminance decay time for devices (1-5).
Fig.10. Variation in the CIE 1931 chromaticity coordinates for devices.
Fig.11. AFM images of the (1) in PVK: PBD thin films, a) before, b) after dichloromethane bath
and laser irradiation
Fig.12. PL spectra of the mixture of PVK: (1): PBD film. Before and after laser irradiation.
18

Fig.1.

a
b
Fig.2.

Fig.3.

Fig.4.

Fig. 5.

Fig.6.

Fig.7.

Fig.8.

Fig.9.

Fig.10.

a
b
Fig.11.