Fabrication and Characterization of Large-Pixel Size OLEDs Used For Opto-Biomedical Analysis

Nguyen Nang Dinh*, Nguyen Duc Cuong, Tran Thi Thao, Do Ngoc Chung, Le Thi Hien

University of Engineering and Technology, Vietnam National University, Hanoi (UET-VNU)

Hanoi, Vietnam

*) Corresponding author, Email: dinhnn@vnu.edu.vn

Abstract— With the aim to use a “flat film-like shape” excitation source in the opto-biomedical analysis system (OBMAS), a deep-blue organic light emitting diodes (OLED) with 240 mm2

in size and an emission wavelength of 455 nm were fabricated by vacuum evaporation of low-molecular-weight polymers. In the devices N, N-Bis(naphthalen-1-yl)-. N,N-bis(phenyl) benzidine (NPB) was used for hole transport layer (HTL), 2-methyl-9,10-bis(naphthalene-2-yl)anthracene (MADN) – for emitting layer (EML) and tris(8-hydroxyquinolinato)aluminum(III) (Alq3) – for electron transport layer (ETL). The output power, the luminous efficiency, the peak wavelength and the full width at half-maximum (FWHM) of the deep-blue OLED were 1.5 mW, 1.0 cd/A, 455 nm and 100 nm, respectively, at a forward current of 30 mA. Under excitation of this excitation source, the photoluminance of the CdSe-QDs attached with listeria monocytogenes bacteria exhibit clear orange colour light. The image of the luminance was detected by a 1.3 Mpx sensitive webcam. This can be used in the biomedical analysis by using an optic microscope acting like an opto-biomedical microanalysis system (OBMAS). With use of the OBMAS one can qualitatively detect the presence of bacteria attached to the QDs through specific antibodies.

Keywords: OLED; electroluminescence, excitation source; opto-biomedical analysis.

I. INTRODUCTION During the last decade, organic light emitting diodes

(OLEDs) have been increasingly investigated due to their potential applications in many scopes, such as optoelectronics, screen for TV and cellular phones, etc [1-4]. The main advantage of OLED compared to inorganic LED is a possibly large area of the emission, that is desired for replacing LED in urban lighting. However, in order to replace the light emitting diodes (LEDs) based on inorganic semiconducting materials, it is necessary to improve both the efficiency and time of service of the OLEDs. The efficiency of the optoelectronic devices like OLEDs, is controlled by three factors: (i) equalization of injection rates of positive (hole) and negative (electron) charge carriers (ii) recombination of the charge carriers to form singlet excitons

Tran Quang TrungUniversity of Natural Science, Vietnam National

University, Ho Chi Minh City Ho Chi Minh City, Vietnam

Vu Xuan Nghia, Pham Duc Minh Biomedical & Pharmaceutical Applied Research Center,

Vietnam Military Medical University Hanoi, Vietnam

in the emitting layer (EML) and (iii) radiative decay of excitons. Recently, novel approaches to deal with these problems have been reported [5, 6] such as the addition of a hole transport layer (HTL) between the transparent anode and the emitting layer (EML) [5] and/or of an electron transport layer (ETL) sandwiched between the EML and cathode [6].

However, the efficiency of the devices as well as the long-lasting service is sometimes limited because of the large thickness of the laminar devices. So that the thickness of each layer should be controlled as thin as possible. But this results in a big difficulty in fabrication of the OLEDs with a large area of emission, the problem is how to avoid the pores and defects formed during the preparation process, for the thin organic films in particular.

As an excitation light, OLED possess another advantage in comparison with LEDs in the flat film-like shape that makes OLED easy to integrate monolithically with microfluidic mixers made on flat glass substrates. Kopelman et al [7] reported a fluorescent chemical sensor platform integrating an OLED device light-source with a fluorescent probe for oxygen sensor. Qiu and co-workers [8] presented an integrated PDMS microfluidic device with a 520 nm (peak wavelength) green OLED and optical fibers. Kim et al [9] reported an advanced and compact microchip coupled with a green OLED of 530 nm (peak wavelength) and a PIN photodiode for fluorescence detection which achieving a detection limit of 10-5 M Rhodamine 6G (10 M) as reported. In a lab-on-a-chip (LOC) the cross-polarized technique is needed, as it can distinguish excitation light from emission light by polarization, even if the excitation and signal light overlap in wavelength.

In this work we present recent results on the preparation and characterization of the large-pixel size OLEDs that can be served as the flat film-like shape excitation sources in the LOC for the opto-biomedical analysis. The working principle of the opto-biomedical analysis system (OBMAS) has also been investigated.

This work was supported by the MOST of Vietnam through a Project on Fundamental Scientific Research and Applications in period of 2011 –2013. Project Code: 1/2010/HD-DTNCCBUD.

2012 International Conference on Biomedical Engineering and Biotechnology

978-0-7695-4706-0/12 $26.00 © 2012 IEEE

DOI 10.1109/iCBEB.2012.219

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II. EXPERIMENTALConducting polymers or organic semiconducting

materials used in this work are the “low-molecular-weight” polymers like tris(8-hydroxyquinolinato)aluminum(III) or N, N-Bis(naphthalen-1-yl)-. N,N-bis(phenyl) benzidine. Thin or/and ultrathin films of these polymers can be made by sublimating (evaporating) in vacuum. The initial materials used for the evaporation are the polymers purchased from Sigma-Aldrich Ltd. The chemicals, their abbreviation and function in devices are listed in Table 1. All the layers in devices were made by evaporation in high vacuum, the pressure in the vacuum chamber is of ~ 2.5 10-4 Pa.

TABLE 1. INITIAL POLYMERS USED FOR PREPARATION OF OLEDS

Name Abbreviation Function N, N-Bis(naphthalen-1-yl)-. N,N-bis(phenyl) benzidine

NPB HTL

2-methyl-9,10-bis(naphthalene-2-yl)anthracene

MADN EML

Tris (8-hydroxyquinolinato) aluminum(III)

Alq3 ETL or EML

Bis(10-hydroxybenzo [h] quinolinato)-beryllium

Bebq2 ETL

The ITO-coated glass pieces with 25 mm 25 mm in size were used as substrates, further as the transparent anode. The ITO coating was etched leaving a working area of 9 mm2, on each ITO piece there are four square cells. For the large-area OLED, the working area consists of ~ 240 mm2

for one ITO piece. The substrates were ultrasonically cleaned following three steps: 15 min in acetone, 10 min in ethanol and 5 min in distilled water. Finally, the substrates were dried by nitrogen gaseous flow and kept in a argon glove-box until further use.

The organic layers NPB as HTL, MADN as EML and Alq3 as ETL were obtained by deposition through shadow mask in the high-vacuum chamber. Further, a shallow contact (ultrathin LiF layer) with aluminum cathode (Al) followed by 100 nm thick layer served as the cathode were successively evaporated. The working area of OLED pixels defined by the overlap of anode and cathode layouts is 3×3 mm2 (or 15×16 mm2 for the larger-area OLEDs). Finally, the fabricated samples were hermetically encapsulated using glass with getter before taking out of the glove-box for measurements.

To characterize optospectroscopic properties (absorption and photoluminescence) of the MADN, a solution of 1M MADN in dichloromethane (MADN + DM) was prepared.

The thickness of the films was automatically controlled by using a quart crystal microbalance (QCM) during evaporation. For comparison, the different devices were made, as follows (the number in brackets shows the thickness of the films in nanometer):

ITO/NPB(50)/Alq3(40)/LiF(0.5)/Al ITO/NPB(50)/Alq3(40)/Bebq2(20)/LiF(0.5)/Al ITO/NPB(50)/Bebq2(40)/Alq3(20)/LiF(0.5)/Al ITO/NPB(50)/MADN(50)/Alq3(20)/LiF(0.5)/Al

The ultraviolet-visible absorption spectra were carried-out on a JASCO UV-VIS-NIR V570. Photoluminescence (PL) and Electroluminescence (EL) spectra were measured, respectively on a FL3-2 spectrophotometer and a “Darsa Pro-5000 system” with use of “Keithley-2400” dc-source. The luminance of the OLEDs was estimated by using a source measure unit of “Keithley 236 SMU” as the dc-source and “Minolta CS-100A” as the luminance meter. The luminous efficiency was determined as the ratio of the luminance density (cd/m2) and corresponding current density (A/m2).

For detecting the presence of any bacteria, the DNA of the last was attached to CdSe-QDs in a liquid solution. A normal optic microscope was used with adding two cross-polarizers and an OLED served as the flat excitation source, instead of the illuminating lamp. The PL images of samples were taken by a high resolution webcam.

III. RESULTS AND DISCUSSION Comparing with Alq3 (C27H18AlN3O3), MADN (C35H24)

has a less complicated molecular structure (Fig. 1). However, MADN is a prospective polymer that can be used for the EML in OLEDs, because MADN is stable at temperatures higher 330 oC and the sublimation efficiency is larger 99%. Moreover, the bandgap of MADN is larger than that of Alq3 [10].

a. b.Figure 1. The molecular structure of Alq3 (a) and MADN (b)

Figure 2 shows the absorption and photoluminescence spectrum of the 1M MADN in dichloromethane. From this figure one can see that MADN exhibits a peak at ~ 379 nm in UV-VIS, in agreement with reported data in [10].

Figure 2. Absorption (a) and photoluminescence spectra (b) of the MADN + DM solution. Excitation wavelength is = 325 nm.

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The absorption spectra of the MADN + DM solution is broad, ranging from 350 to 450 nm. Under excitation of He-Cd laser beam at = 325 nm, the obtained PL spectrum exhibits a peak at ~ 453 nm. One can expect that the MADN used for the EML in OLEDs can emit a deep-blue colour, when applying an appropriate potential onto the transparent ITO anode. Indeed, from the EL spectra of three different OLEDs in Fig. 3 one can see that for the MADN device, the EL peak is observed at ~ 455 nm, this is quite consistent with the wavelength of the PL peak.

Figure 3. EL spectra of different laminar OLEDs:

“1” - ITO/NPB/Alq3/LiF/Al

“2” - ITO/NPB/Alq3/Bedq2/LiF/Al

“3” - ITO/NPB/MADN/LiF/Al

“4” - ITO/NPB/MADN/Alq3LiF/Al

In Fig. 3 there are presented also EL curves of two other devices, the first is the OLED with using Alq3 served simultaneously as EML and ETL, while the second one is the OLED with using Alq3 and Bebq2 served as EML and ETL, respectively. Although the addition of Bebq2 as the ETL in OLED does not change the feature of the EL spectrum, the luminance efficiency could be enhanced. The Alq3 diode emits green colour at the EL peak at ~ 527 nm, the full width of half maximum (FWHM) of the Alq3-OLED is about 100 nm. The FWHM of the MADN-OLED is almost the same value. The FWHM of the OLEDs is much larger than that of inorganic LEDs, for instance the FWHM of InGaN-LED is of 30 nm [11]. However, the large FWHM is just an advantage of the OLED in using for the flat film-like shape excitation source.

The luminous efficiency vs. luminance of the devices is plotted in Fig. 4. From this figure one can see that at the same value of the luminance, the MADN-OLED with ETL (Alq3) exhibits much larger luminance efficiency. The effect of both the HTL and ETL on the enhancement of electroluminescence efficiency or (luminance efficiency) was well demonstrated, associated with the equalization process of injection rates of holes and electrons. Indeed, The electroluminescence efficiency can be determined by using an expression obtained by Tsutsui and Saito in [12]:

r f (1)

where is a double charge injection factor which is dependent on the processes of carrier injection and is maximal ( = 1) if a balanced charge injection into the emission layer of the device is achieved, i.e. the number of injected negative charges (electrons) equals the number of injected positive charges (holes); r quantifies the efficiency of the formation of a singlet exciton from a positive and a negative polaron, and f is the photoluminescence quantum efficiency.

Figure 4. The luminous efficiency vs. luminance of MADN device without ETL (bottom curve) and with the ETL (top curve).

The abrupt increase in efficiency occurred at the value of luminance around 110 cd/m2. This relates to the most effective current range corresponding polarized potentials that was applied onto the transparent anode (ITO), where the current density in the I-V characteristic raised with an abrupt value.

The deep-blue OLED with MADN as the EML was used for a flat excitation source excitation in so called “opto-biomedical microanalysis system” (OBMAS), which is shown in Fig. 5. For testing the working principle of the OBMAS, the colloidal CdSe quantum dots (CdSe-QDs) with 18 – 20 nm in size were used, this material emits luminance with a peak at ~ 655 nm [13-15]. CdSe-QDs were attached with the DNA of some bacteria, for instance, listeria monocytogenes bacteria (LMB). The deep-blue light emitted from the OLED light was polarized by the first polarizer to distinguish excitation light and signal light (Fig. 5). The last emitted from the sample cell containing colloidal CdSe-QDs attached with the LMB. The signal light or luminous emission was recorded by using a webcam with the resolution of 1.3 Mpx. The results on observing the presence of the LMB are shown in Fig. 6.

Without using polarizer 1, the image of the light emitted from the deep-blue OLED is strongly bright (Fig. 6a) and with using polarizer 1 and 2, the image became dark (Fig. 6b). This proves that the light was completely polarized by polarize 1. When the sample cell was put on the pathway of the polarized excitation light, we observed the luminance image with the orange colour (Fig. 6c). The image reflects the photoluminescence of the CdSe-QDs under excitation of the deep-blue polarized light from the OLED. This clearly shows that using OBMAS one can qualitatively detect the presence of bacteria. However, to carry-out a fast

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bio-medical analysis, further we need to intergrate also a microfluidic device to the OBMAS and to attach the bacteria to the QDs through specific antibodies.

Figure 5. Schema of an opto-biomedical analysis system based on an optic microscope combined with two cross-polarizers and a photodigital camera or a sensitive webcam.

a. b. c.

Figure 6. Image of the excitation beam without (a), with polarizer 1 (b) and image of luminous emission of CdSe-QDs attached with LMB (c).

IV. CONCLUSIONS Deep-blue OLEDs with 240 mm2 in size and an emission

wavelength of 455 nm were fabricated by vacuum evaporation of low-molecular-weight polymers, such as NPB for HTL, MADN – for EML and Alq3 – for ETL. The emission spectrum possesses a full width of half maximum (FWHM) of 100 nm. Using these OLEDs for a so called “flat film-like shape” excitation source, the photoluminescence of the CdSe-QDs attached with listeria monocytogenes bacteria exhibited clear orange colour light. The image of the luminance was detected due to taking a photograph by a sensitive webcam. This suggests a very useful application for the biomedical analysis by using an optic microscope incorporated with two cross-polarizers that can work as an opto-biomedical microanalysis system (OBMAS). With use of the OBMAS one can qualitatively detect the presence of the bacteria, once the last are attached to the QDs through specific antibodies.

ACKNOWLEDGMENTS One of us (T.T.T.) thanks Prof. N.N.Dinh for the

financial support during her M.Sc. study in UET-VNU.

REFERENCES [1] J. S. Salafsky, “Exciton dissociation, charge transport, and

recombination in ultrathin, conjugated polymer-TiO2 nanocrystal intermixed composites,” Phys. Rev. B vol. 59, pp. 10885–10894, 1999.

[2] V. M. Burlakov, K. Kawata, H. E. Assender, G. A. D. Briggs, A. Ruseckas, and I. D. W. Samuel, “Discrete hopping model of exciton transport in disordered media,” Phys. Rev. B vol. 72, pp. 075206-1 ÷ 075206-5, 2005.

[3] A. Petrella, M. Tamborra, P. D. Cozzoli, M. L. Curri, M. Striccoli, P. Cosma, G. M. Farinola, F. Babudri, F. Naso, and A. Agostiano, “TiO2 nanocrystals–MEH-PPV composite thin films as photoactive material,” Thin Solid Films, vol. 451/452, pp. 64 – 68, 2004.

[4] H. Spanggaard and F. C. Kerbs, “A brief history of the development of organic and polymeric photovoltaics,” Sol. Energy. Mat. Sol. Cells, vol. 83, pp. 125–146, 2004.

[5] D. E. Markov and P. W. M. Blom, “Migration-assisted energy transfer at conjugated polymer/metal interfaces,” Phys. Rev. B vol. 72, pp. 161401(R)–161404(R), 2005.

[6] K. Kawata, V. M. Burlakov, M. J. Carey, H. E. Assender, G. A. D. Briggs, A. Ruseckas, and I. D. W. Samuel, “Description of exciton transport in a TiO2/MEH, PPV heterojunction photovoltaic material,” Sol. Energy Mat. Sol Cells vol. 87, pp.715–724, 2005.

[7] V. Savvate’ev, Z. Chen-Esterlit, J. W. Aylott, B. Choudhury, C. H. Kim, L. Zou, J. H. Friedl, R. Shinar, J. Shinar, and R. Kopelman, “Integrated organic light-emitting device/fluorescence-based chemical sensors,” Appl. Phys. Lett., vol. 81, pp. 4652–4557, 2002.

[8] B. Yao, G. Luo, L. Wang, Y. Gao, G. Lei, K.Ren, L. Chen, Y. Wang, Y. Hu, and Y. Qiu, “A microfluidic device using a green organic light emitting diode as an integrated excitation source,” Lab on a Chip, vol. 5, pp. 1041–1047, 2005.

[9] J. H. Kim, K. S. Shin, K. K. Paek, Y. H. Kim, Y. M. Kim, Y. K. Kim, T. S. Kim, J. Y. Kang, E. G. Yang, S. S. Kim, and B. K. Ju, Proc. Micro TAS 2004 (8-th International Conference on Miniaturized System for Chemistry and Life Science, eds. T. Laurell, J. Nilsson, K. Jensen, D.J. Harrison and J.P. Kutter, The Royal Society of Chemistry, Cambridge, UK, 2004, pp. 428–432.

[10] Catalogue of “Lumtec”- Luminescence Technology Corp., OLED materials, 2011.

[11] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, T. Yamada, and T. Mukai, “Superbright Green InGaN Single-Quantum-Well-Structure Light-Emitting Diodes,” Jpn. J. Appl. Phys. Vol. 34 pp. L1332–L1335, 1995.

[12] T. Tsutsui and S. Saito, “Intrinsically conducting polymers: an emerging technology,” Edited by M. Aldissi. NATO ASI Series E: Applied Sciences, vol 246, pp.123–127, 1993.

[13] M. J. Archer and J. L. Liu, “Bacteriophage T4 Nanoparticles as Materials in Sensor Applications: Variables That Influence Their Organization and Assembly on Surfaces,” Sensors, vol. 9(8) pp. 6298 – 6311, 2009.

[14] N. K. Pettya, T. J. Evansa, P. C. Finerana, and G. P. C. Salmond, “Bio- technological exploitation of bacteriophage research,” Trends. Biotechnol., Vol. 25, pp. 7–15, 2006.

[15] M. Manchester and P. Singh, “Virus-based nanoparticles (VNPs): platform technologies for diagnostic imaging,” Adv Drug Deliv Rev, vol. 58 (2006) 1505 – 1522, 2006.

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