Journal of Luminescence 140 (2013) 7–13

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Journal of Luminescence

0022-23

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Effect of doping different dyes in Alq3 on electroluminescenceand morphology of layers using single furnace method

Mohammad Janghouri, Ezeddin Mohajerani n, Amir Khabazi, Zahra Abedi, Hosein Razavi

Laser and Plasma Research Institute, Shahid Beheshti University, G.C., Tehran 1983963113, Iran

a r t i c l e i n f o

Article history:

Received 21 October 2012

Received in revised form

26 January 2013

Accepted 6 February 2013Available online 7 March 2013

Keywords:

Low cost red OLEDs

Single furnace

AFM

Uniform mixing

13/$ - see front matter & 2013 Elsevier B.V. A

x.doi.org/10.1016/j.jlumin.2013.02.018

esponding author. Tel.: þ98 21 29904003.

ail address: e-mohajerani@sbu.ac.ir (E. Mohaj

a b s t r a c t

A method for obtaining red emission from organic-light emitting diodes has been developed by dissolving

red and green dyes in a common solvent and thermally evaporating the mixture in a single furnace.

Devices with fundamental structure of ITO/PEDOT: PSS (55 nm)/PVK (90 nm)/Alq3: porphyrin (50 nm)/Al

(180 nm) were fabricated. The emission properties and chromaticity coordinates of the devices depend

on the energy transfer between the emission of host and the absorption of the dyes. TPP and TPPNO2

doped in Alq3 showed more pure red emission compared to 3,4-TPP, and PdTPP doped in Alq3 based

devices. AFM measurement showed that the morphology of the layers depends on the type of dyes and

uniform mixing of porphyrin compounds and Alq3 at constant deposition rate. It is shown that this new

method is a promising candidate for fabrication of low cost red OLEDs at more homogeneous layer.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Organic light emitting diodes (OLEDs) and polymer light emittingdiodes (PLEDs) are of considerable interest owing to their potentialfor low cost, efficient, flexible, and large area emitting devices [1–5].Two main deposition methods including spin-coating and thermalevaporation are common in device fabrication. Performance of OLEDdevice and its optical and electrical characteristics includingefficiency, lifetime and quantum yield, greatly depends on thedeposition method [6]. Typically, OLED fabricated with thermalevaporation method exhibits much higher efficiency but spin-coating also draws great attention for easier fabrication [7–11]. OLEDfabricated with high quantum efficiency requires the application ofphosphorescence materials and efficient light out-coupling methods[12–15]. While OLED fabricated with specific emission spectrumrequires the doping of distinct dyes. It has been shown that bydoping the organic active layer with a small amount of dye, one cantune the emission layer for specific color [16–18]. Doping a hostmatrix with highly fluorescent dyes has proven to be an efficientstrategy to achieve narrow emission spectra [19–23]. In this dopedsystem, the energy of the host (donor) materials is transferred to theguest materials (acceptor) through the efficient Forster resonantenergy transfer or charge transfer processes [24–26]. Doping withevaporation takes place at least by two evaporation sources of hostand guest materials, separately [27–29]. Especially, the dopant withthe HOMO/LUMO energy levels between HOMO/LUMO energy levelsof host has to be evaporated at a precise rate, as small fluctuations ofits low concentration can significantly change the device color [30].

ll rights reserved.

erani).

Doping dye into host is possible by both mixing host and guest intheir common solution or simultaneous evaporation of the materials.In order to control the doping concentration ratio, these materials willbe deposited on substrate by controlling the related evaporationparameters using the constant evaporation rate for each of the hostand guest materials.

Up to now, for fabrication of OLED from two different materi-als, spin coating method and evaporation of two dyes at twoseparate furnaces were used. In the previous work, we reportedthe deposition of the mixture of Alq3 with one of the derivativesof TPP with additional functional group [31]. In this paper, weused a single source of evaporation to mix dyes at differentconcentrations. We have utilized a mixture of porphyrin andAlq3 to study the EL performance of the fabricated devices bysingle source evaporation method. A layer of only porphyrinmolecules is not efficient in charge transport and needs a hostsuch as Alq3 and an efficient Forster energy transfer to showefficient emission. Here, Alq3 and porphyrin compounds are ashost and guest, respectively. Therefore, porphyrin molecules areused as dopant in Alq3 with capability of hole or electrontransport to increase the injection of holes and electrons intothe layer, which leads to an increase in probability of excitonformation. Alq3 which is employed here as host material, has highelectron transferring ability in light emitting layer.

2. Experimental

2.1. Materials

Apart from the porphyrin compounds which are synthesized inthe Department of Chemistry and prepared and purified according

M. Janghouri et al. / Journal of Luminescence 140 (2013) 7–138

to the literature procedures [32–33], all other materials includingPEDOT:PSS (poly(3,4-ethylenedi-oxythiophene):poly(styrenesulfo-nate)), PVK (polyvinylcarbazole) and Alq3 tris(8-hydroxyquinolina-to)aluminum(Alq3) are purchased from Sigma Aldrich and usedwithout any further purification. Fig. 1 depicts the structures of thematerials. Structure of meso-tetraphenylporphyrin, tetra (4-Nitro-phenyl)porphyrin, meso-tetrakis(3,4 dimethoxyphenyl) porphyrin,and tetraphenylporphyrin palladium are referred with labels TPP,TPPNO2, 3,4-TPP, PdTPP in this paper, respectively.

2.2. Fabrication of OLED

The first step of fabrication process was cleaning of ITOsubstrates by detergent, acetone, dichloromethane, ethanol,methanol and deionized water in ultrasonic bath. PEDOT: PSS ashole injection layer was spin coated on clean ITO substrate atthickness of 55 nm and baked in oven for 1 h at 120 1C. Followingthis step, PVK was also spin coated over the sample at thickness of90 nm as a hole transport layer and was baked in oven for 1 h at120 1C to soften the sharp peaks of PVK layer and to achievemore even surface. The main part in fabricating our OLED waspreparation of light emitting layer (LEL). In order to preparethis layer, porphyrin compounds and Alq3 were dissolved in

Fig. 1. Graphical structure of (a) PEDOT:PSS, (b) PVK, (c) Alq3 and (d, e) porphyrin co

(PdTPP).

Table 1The device structures.

Device Structure

Device 1 ITO/ PEDOT

Device 2 ITO/ PEDOT

Device 3 ITO/ PEDOT

Device 4 ITO/ PEDOT

dichloromethane. The solution was then left in ultrasonic bathfor 15 min to make a homogeneous solution. This solution waspoured on identical quartz furnaces and exposed to heat at 50 1Cfor 20 min until the solvent was evaporated. Finally, dried mixtureof porphyrin compounds:Alq3 with evaporation rates 0.2–0.3 nm/swas coated in evaporation chamber to make layers of 50 nmthicknesses. The aluminum cathode was deposited on the top ofthe structure through a shadow mask. The device structures andthe steps of this method are shown in Table 1 and Fig. 2. Thicknessmeasurements were performed by DekTak 8000; EL and PL offabricated OLEDs were performed by USB2000 and HR4000 OceanOptics. The current–voltage–luminance characteristics and atomicforce microscopy (AFM) measurements were checked by Keithleysource meter 2400 model, optical meter Mastech-MS6612 andeasy scan 2.

3. Results and discussions

3.1. Photoluminescence characteristics

Fig. 3a shows the photoluminescence (PL) emission of fourporphyrin compounds at long wavelengths which show a goodred chromaticity. The PL emission of four porphyrin compounds

mpounds. d)X=H,Y=H(TPP);X=NO2Y=H(TPPNO2);X=OCH3,Y=OCH3(3,4-TPP) anb e)

: PSS(55 nm)/ PVK (90 nm)/Alq3:5,10,15 wt%( 3,4-TPP ) (50 nm)/Al(180 nm)

: PSS (55 nm)/PVK (90 nm)/Alq3:5 wt%(TPP) (50 nm)/Al(180 nm)

: PSS (55 nm)/PVK (90 nm)/Alq3:5 wt%(PdTPP) (50 nm)/Al(180 nm)

: PSS(55 nm)/PVK (90 nm)/Alq3:5 wt%(TPPNO2) (50 nm)/Al(180 nm)

Fig. 2. Various steps of fabrication of light emitting layer: (a) fabrication of film, (b) left Alq3 and right blend of Alq3:TPP, (c) the device structures and (d) left ITO/

PEDOT:PSS(55 nm)/PVK(90 nm)/Alq3(50 nm)/Al(180 nm) right: ITO/PEDOT:PSS(55 nm/PVK(90 nm)/5 wt%TPP: Alq3 (50 nm)/Al(180 nm).

M. Janghouri et al. / Journal of Luminescence 140 (2013) 7–13 9

show two peaks, broadened for the first peak, with more intensityfrom 651 to 658 nm in comparison with the second peak from713 to 721 nm. The PL spectra of the mixture of Alq3:TPP solutionand film are shown in Fig. 3b. For the solution there are two peaksat green and red emission region related to Alq3 and TPPrespectively, indicating that incomplete energy transfer occursfrom Alq3 to TPP due to the great distance between the molecules.On the other hand, when the premixed material was thermallyevaporated, the evaporated molecules were compressed into asolid film. Energy transfer then occurred between the molecules,then, the emissions of TPP could be observed. Fig. 4 shows thespectral overlap between Alq3 emission and porphyrin com-pounds absorption which is favorable for energy transfer fromAlq3 as host to porphyrin compounds as guest. The absorbancecharacteristics of the solutions of porphyrin compounds display asharp and intense soret band accompanied by four weakerQ-bands at longer wavelengths in the visible range. But in themetallic porphyrin compound, the condition is different, a nar-rower soret band and two Q-bands appeared. Table 2 shows peakabsorption data for porphyrin compounds.

3.2. Electroluminescence of OLED devices

We fabricated light-emitting layers consisting of a matrixmaterial, Alq3 as green dye and porphyrin compounds as reddye. The mixture of the two dyes was used for evaporation with asingle evaporation source instead of employing two separatematerials at two evaporation sources. Fig. 5 shows the EL spectraof the fabricated OLED for different concentrations of 3,4-TPPdoped in Alq3 at different applied voltages. The peak at 538 nmrelated to Alq3 and two peaks at 661 nm and 724 nm correspondto 3,4-TPP. The appearance of Alq3 peak indicates that the

complete energy transfer did not take place from Alq3 to3,4-TPP molecules at different doping concentration. Concentra-tion variation of 3,4-TPP into Alq3 with 100:5 and 100:10 weightratios have higher intensity at 538 nm but contrarily 100:15 ratioshows more intense peak at 661 nm. Thus an efficient ratio existsfor high intensity peak at 661 nm. Electrical characteristic of threesamples at different concentrations of 3,4-TPP doped into Alq3 byone evaporation source method is shown in Fig. 6. The currentdensity of the fabricated OLED with a co-evaporated layer ofAlq3:3,4-TPP¼100:5 is more than that of two others. Since theconductivity of Alq3 is higher than that of porphyrin compounds,by increasing porphyrin content, the trap density increases andthe conductivity of the device decreases [34]. Synthesizingporphyrin with peak absorption spectrum close to Alq3’s emissionis possible by functional group replacement in phenyl rings whichleads to peak absorption spectrum variation of porphyrin asshown in Table 2. By following this step, we have used otherporphyrin compounds. In order to study the effect of mixingthe host and guest on EL performance of the device, we fabricatedlight emitting layers consisting of Alq3:TPP,Alq3:TPPNO2

,Alq3:PdTPP, with ratio 100:5 wt%. In Fig. 7a it is clear that forTPP and TPPNO2 in Alq3, the broad peak was removed which isthe result of much better energy transfer but for 3,4-TPP andPdTPP complete energy transfer did not take place. Fig. 7b showsEL spectra with 5 wt% of TPP in Alq3 at different voltages. Noemission from Alq3 was observed in the TPP doped Alq3 baseddevice. Hence, the choice of TPP as guest is obviously better thanother porphyrin-doped OLEDs. To estimate the transition energyfrom Alq3 to the porphyrin dopant, we measured the emissionspectra of Alq3 and absorption spectra of these compounds. Fig. 4shows a good overlap between the Q-bands of porphyrin com-pounds absorption spectra and Alq3 emission. This overlap

Fig. 4. The absorption spectra of porphyrin compounds and the PL spectrum

of Alq3.

Table 2Peak absorption spectra for porphyrin compounds.

labs (nm) l1 (nm) l2 (nm) l3 (nm) l4 (nm)

TPP 516 549 592 649

TPPNO2 517 551 591 647

3,4-TPP 520 558 596 651

PdTPP 522 591 648 –

Fig. 3. (a) Photoluminescence characteristic of porphyrin compounds and (b) PL

spectra of the mixture of Alq3:TPP solution and film.

Fig. 5. The EL spectra with different concentrations of 3,4-TPP in Alq3 at different

applied voltages: (a) 5 wt%, (b) 10 wt% and (c)15 wt%.

M. Janghouri et al. / Journal of Luminescence 140 (2013) 7–1310

Fig. 6. Current density–voltage characteristic of three samples with different

concentrations of 3,4-TPP doped in Alq3.

Fig. 7. The normalized EL spectra with 5 wt% of porphyrin compounds in Alq3 and

(b) EL spectra with 5 wt% of TPP in Alq3 at different voltages.

Fig. 8. The luminance–voltage relationship of OLEDs with 5 wt% concentration of

porphyrin compounds doped in Alq3.

Table 3Characteristics of the EL devices.

Sample:

(100:5wt%)lPL

(nm)

lEL

(nm)

CIE16 V

(X,Y)

Max:Luminescence

(cd/m2)

EL color

TPP 652 656 (0.71,0.27) 71 Deep red

TPPNO2 653 654 (0.62,0.31) 140 Red

3,4-TPP 658 538,661 (0.38,0.51) 312 Yellow

PdTPP 651 507,651 (0.36,0.48) 188 Yellow

Commission international del’Eclairage (CIE) coordinates of the emitted light

(1931).

Fig. 9. Variation in the CIE 1931 chromaticity coordinate for devices with 5 wt%

concentration of porphyrin compound doped Alq3.

M. Janghouri et al. / Journal of Luminescence 140 (2013) 7–13 11

guarantees the Forster energy transfer from Alq3 as host, toporphyrin compounds as guest. For the emission of Alq3 at515 nm, as shown in Table 2, the QY10-band absorption of TPP,TPPNO2 displays a blue shift, and the QY10-band absorption of

3,4-TPP, PdTPP with Alq3 emission displays a red shift. It isobvious that the spectral overlap between the QY10-band absorp-tion of TPP, TPPNO2 and Alq3 emission, should be better than thatbetween 3,4-TPP, PdTPP and Alq3 emission. Here we observe veryhighly efficient nonradiative Forster energy transfer from Alq3 toTPP and TPPNO2 relative to 3,4-TPP and PdTPP. In addition, tohave efficient Forster energy transfer, apart from having overlap

M. Janghouri et al. / Journal of Luminescence 140 (2013) 7–1312

between absorption spectrum of the guest and EL spectrum of thehost, uniform mixing of the two species is also required. This isbecause the range of Forster energy transfer is only few nan-ometers [35,36]. The luminance–voltage properties of the OLEDswith 5 wt% concentrations of porphyrin compounds are shown inFig. 8. The maximum brightness of emissions is 71–312 cd/m2 forthe devices at different concentration of porphyrin dopant. It canbe seen that the organic LED with a co-evaporated layer of

Fig. 10. AFM image for concentration of 100:5 wt% (a) 3,4-TPP, (b) TPP,

3,4-TPP:Alq3 has the highest luminance and the organic LED witha co-evaporated layer of TPP:Alq3 has the lowest luminance atspecific voltage. Furthermore, we believe that the high luminanceof the 5 wt% 3.4-TPP doped in Alq3 could be due to more portionsof green emission form Alq3 in the EL spectrum, compared tothose of other OLEDs. Generally, green emission is more efficientthan red and blue emission. Table 3 shows characteristics of theEL devices. Fig. 9 shows the CIE coordinates for the devices. The

(c) TPPNO2 and (d) PdTPP doped in Alq3 at evaporation rate 0.2 A/s.

M. Janghouri et al. / Journal of Luminescence 140 (2013) 7–13 13

result of CIE(X, Y) coordinate and EL spectrum of TPP indicatedthat TPP is more red shifted relative to other compounds. Fig. 10shows AFM image of the premixed of two dyes for scanning area10.1�10.1 mm2 at evaporation rate 0.2 A/s. The average root-mean-square (RMS) surface morphology of films with depositionrate 0.2 A/s for {TPP, 3,4-TPP, TPPNO2, PdTPP} in Alq3 are about12.86 nm, 15.87 nm, 17.40 nm, 27.77 nm respectively. Weconcluded that the morphology of films depends on the type ofdyes and uniform mixing of porphyrin compounds and Alq3 atconstant deposition rate. The evaporation of the mixture of Alq3

and porphyrin compounds in a single furnace is very differentfrom evaporation rate for separate green and red dyes. This canlead to achieve emissive layer with very low red concentrationsand an optimized emission.

4. Conclusion

We have investigated the emission properties of four porphyrincompounds in tris (8-hydroxyquinolinato) aluminum (Alq3) filmsorganic light emitting diodes (OLEDs) with a single evaporationsource method. TPP, TPPNO2, exhibit red emission with high colorpurity. We observed that 3,4-TPP changes the driving voltage andalso the shift of I–V diagram to high voltage by increasing theporphyrin concentration. The maximum luminescence wasobserved for the 3,4-TPP at doping concentration of 5 wt% in Alq3.AFM measurements showed that morphology of the films dependson type of dyes and uniform mixing of porphyrin compounds andAlq3 at constant deposition rate. Finally it is shown that theadvantages of this method can be: low cost, homogeneity, usingonly single furnace and more effective energy transfer.

Acknowledgments

Authors would like to thank ‘‘Iran National Science Founda-tion: INSF’’ for their financial support.

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