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Organic optocoupler consisting of an optimized blue organic light emitting diode and an organic photoconductor
Superlattices and Microstructures 85 (2015) 880–885
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Organic optocoupler consisting of an optimized blue organic light emitting diode and an organic photoconductor A. El Amrani a,⇑, B. Lucas b, R. Antony b a b
LPSMS, FST Errachidia, B.P. 509, Boutalamine, Errachidia, Université My Ismail, Morocco XLIM UMR 7252 – Université de Limoges/CNRS, 123 avenue Albert Thomas, 87060 Limoges Cedex, France
a r t i c l e
i n f o
Article history: Received 20 June 2015 Received in revised form 1 July 2015 Accepted 4 July 2015 Available online 6 July 2015 Keywords: OLED Photoconductor Sensitivity Current transfer ratio Pentacene Organic optocoupler Response time Stability
a b s t r a c t We present an optocoupler device based on a blue organic light-emitting diode (OLED) as input unit, and a pentacene photoconductor as output unit. The optocoupler was realized on a transparent glass substrate. The luminance was found larger than 103 cd/m2 with a blue peak emission at 450 nm for the optimized ZnO (120 nm)/ITO (150 nm)/a-NPB (40 nm)/BCP (15 nm)/Alq3 (20 nm)/Al structure. The Ids-Illum/Ids-Dark current ratio, the sensitivity and the current density transfer ratio of the optocoupler are of about 7, 101 A/W, and 101, respectively. The rise as well as full times were found faster for high bias voltages. The equilibrium regime with less persistent current was reached more quickly, as evidenced by the fast current response for higher bias voltage, indicating a more favorable recombination processes of the charge carriers. The organic optocoupler with a blue OLED reveals promising results; thus, it can be investigated as a good candidates for practical uses in organic optoelectronic circuits with high bias voltages. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The performance of organic optoelectronic devices such as the organic electroluminescent diodes (OLEDs), the organic thin films transistors (OTFTs), the organic photodetectors (OPhs), and more particulary the organic optocouplers (OOPs) are related to the molecular engineering, which can currently produce organic materials with energy gap that can be modulated very precisely in the range of 1.5 eV to 3 eV, similar to inorganic semiconductors [1]. The development of new organic photodetectors in different configurations were investigated by several research groups [2–5]. The organic optocoupler is a functional device that can transfer an input electrical signal via an optical signal to an output electrical signal. Thus, the first organic optocouplers were carried out by Yu et al. [6], the sensitivity of the device is about 101 A/W and the current transfer ratio (CTR) of about 2 103. Several other research works [7–9] have been reported on the optocoupler performance and their potential applications. Tang et al. [8] have noticed that the optocoupler with organic phototransistors may present a current transfer ratio higher than the organic photodiodes. Moreover, these last have high switching times (rise and fall times) and lower sensitivity as showed by Yao et al. [9]. Indeed, the rise time as well as the fall time of about 0.5 ls were noticed for an optocoupler based on a polyfluorene based polymer electroluminescent diode (PLED) as input unit and a photodiode using a mixture P3HT: PCBM (1:1) as photodetector. In optocoupler devices, the photoconductivity process can take place if the photon energy is higher than the energy gap of the active material, and the absorption coefficient of the material
⇑ Corresponding author. E-mail address: [email protected] (A. El Amrani). http://dx.doi.org/10.1016/j.spmi.2015.07.013 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
A. El Amrani et al. / Superlattices and Microstructures 85 (2015) 880–885
881
must be small enough for efficient charge carrier generation in the bulk of the semiconductor [10], and consequently the current increases. The maximum of generated photocurrent in the case of pentacene was observed [11] in the wavelength range varying from 350 nm to 550 nm where the absorption coefficient is weak. Thus, the higher CTR was obtained for shorter wavelength [12] because for shorter wavelength, the photons can provide more kinetic energy to overcome the coulomb interaction between exciton pairs [13]. This indicates that the blue OLED with shorter wavelength may be a good candidate for the organic optocoupler applications. Compared to inorganic optocoupler, the organic optocoupler is less expensive; it has a lower temperature process and a flexibility of realization [13]. Recent works have been reported on organic–inorganic hybrid optocouplers in order to combine the advantages of both active materials [14]. Thus, the hybrid device may be desirable for ambipolar performance optoelectronic circuits [15]. High organic performance with high blue OLED sensitivity was recently investigated [16,17]. The sensitivity of pentacene is relatively high, because the photons with a wavelength in blue can effectively contribute to the carriers photogeneration in the large volume of the semiconductor. In the present work, we report the performance of an organic optocoupler based on a blue OLED as input unit and a pentacene based photoconductor as output unit. The effect of the organic layer thicknesses of the OLED was investigated in order to obtain a optimum luminance associated with lower lighting voltage. The static as well as the dynamic behavior of the device were also studied. 2. Experimental details For the organic optocoupler elaboration with the same transparent substrate (12 mm 12 mm), the device was realized with the structure as presented in Fig. 1. The OLED was integrated on one side of the substrate, and a pentacene (50 nm thick) was elaborated with thermal evaporation in vacuum (around 106 mbar) on the other side. The OLED (area 3 mm 8 mm) was made up of different organic layers to obtain a main emission peak in the blue based on the zinc oxide ZnO/indium tin oxide ITO/a-NPB (4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl)/BCP (bathocupuroine)/Alq3 tris(8-hydroxyquinoline)/Al aluminum structure. All the ITO/ZnO electrodes, for the OLED or the two ITO electrodes (separated by 100 lm) used to apply a voltage bias, were realized using the IBS (ion beam sputtering) technology [18]. The a-NPB (H–W Sands society) can emit the light at 470 nm, it can be used as hole transport layer as well as emitting layer [19,20]. The photodetector active area is of about 10 mm 4 mm, the current area of the photodetector is of about 50 nm 10 mm. The current intensity, luminance as well as emission spectra measurements were carried out with an automated technique associated to various instruments (a PL330P Thurlby Thandar Instruments (TTI) type, a Keithley 617 and a Keithley 175). The optoelectronic devices were tested at room temperature and in open air. The static and dynamic characteristics of the optocoupler were carried out using a computer-controlled Keithley 4200 source measure unit. 3. Results and discussion In previous works, we have shown that the photocurrent was dependent on the wavelength, the bias voltage and the illumination side of the device [21]. We noticed that with transparent electrodes (i.e. ITO), the top contact configuration yields better performance compared to the bottom contact one [21]. Thus, an antibatic behavior in the 350–600 nm wavelength range is noticed; a higher number of photogenerated charge carriers is evidenced at the emission of about 450 nm. Indeed, for the pentacene photoconductivity mechanism, the photon energy must be higher than the pentacene energy gap, and the absorption coefficient of the semiconductor must be sufficiently small, or the active layer sufficiently thin,
Vds
VOLED
Fig. 1. Schematic of the organic optocoupler structure.
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so that light can efficiently contribute to charge carrier photogeneration in the volume of the active layer [22]. The effect of the organic film thicknesses of the OLED was studied in order to obtain a maximum luminance with lower lighting voltage. The blue light emission was less studied compared to the green and the red emissions; the main reasons are poor yield of blue OLEDs like their instability [23]. The OLED with luminous powers in the order of the lW/cm2 [24] should limit the degradation phenomena, this can be induced in the organic semiconductors based optoelectronic devices under continuous supplying. 3.1. a-NPB layer thicknesses effect The Alq3 thickness was taken of about 20 nm [25]; the thickness is sufficient to play a role of an electron transport layer [26,27]. Initially, the bathocupuroine BCP of 20 nm layer thickness was deposited on the purified a-NPB films; it is used as hole blocking layer at highest occupied molecular orbital (HOMO) a-NPB/BCP interface. This interface presents high potential barrier, which leads the hole confinement in the a-NPB layer [25]. The potential barriers remain low at the lowest unoccupied molecular orbital (LUMO) a-NPB/BCP interface, the electron can easily transit from electron transport layer (ETL) toward hole transport layers HBL, then to a-NPB emitting layer. Thus, the electron can recombine inside a-NPB with holes blocked at the a-NPB/BCP interface, and consequently lead to the blue emission peak. The effect of the a-NPB layer thicknesses (between 40 and 60 nm) on the OLED performance was systematically studied. As shown in Fig. 2a, the maximum luminance of about 700 cd/m2 at VOLED of about 22 V, thus the lighting voltages of the OLEDs remain of the same order for the different a-NPB thicknesses. However, for thick a-NPB films, a shift toward high voltages has been noticed as shown in Fig. 2b. This behavior can be explained by the low conductivity of the a-NPB, which lead to the resistance increases between the OLED electrodes. Thus, for the rest of the study, the thickness of a-NPB will be taken to 40 nm. 3.2. BCP layer thicknesses effect As mentioned above, the BCP layer induces a high potential barrier of about 0.7 eV [25] at a-NPB/BCP interface that can prevent the hole injection in the BCP layer small molecules, and consequently in the Alq3, which facilitates the electron injection from the Alq3 to a-NPB layer. The BCP thickness layer has been investigated for three values varying between 10 and 20 nm. As shown in Fig. 3a, with the BCP layer thickness increases, the luminance decrease was noticed. Thus, for thicker BCP films, a shift in threshold voltage toward high voltages value was observed (Fig. 3b). For BCP thickness of
(a)
(b)
Fig. 2. (a) Luminance versus voltage for ZnO/ITO/a-NPB (X nm)/BCP (20 nm)/Alq3 (20 nm)/Al structure. (b) Current density versus voltage for ZnO/ITO/aNPB (X nm)/BCP (20 nm)/Alq3 (20 nm)/Al structure.
A. El Amrani et al. / Superlattices and Microstructures 85 (2015) 880–885
(a)
(b)
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(c)
(d)
Fig. 3. (a) Luminance versus voltage for ZnO/ITO/a-NPB (40 nm)/BCP (X nm)/Alq3 (20 nm)/Al structure. (b) Current density versus voltage for ZnO/ITO/aNPB (40 nm)/BCP (X nm)/Alq3 (20 nm)/Al structure. (c) Electroluminescent spectra for ZnO/ITO/a-NPB (40 nm)/BCP (X nm)/Alq3 (20 nm)/Al structure. (d) Luminance versus current density for ZnO/ITO/a-NPB (40 nm)/BCP (15 nm)/Alq3 (20 nm)/Al structure.
10 nm, we obtain a high luminance with a green emission ; this is due to the BCP layer that is the thinnest; the thickness does not make it possible to confine the holes in the a-NPB layer. The radiative recombinations take place in the Alq3 layer that is responsible for the green color emission, at about 553 nm, as shown in Fig. 3c for the electroluminescent spectra. While increasing the BCP thickness up to 15 nm, a blue emission color was obtained; the thickness is thus sufficient to ensure the role as hole blocking layer. The best compromise of parameters in term of supplying voltage and luminance in blue was obtained for the structure ZnO (120 nm)/ITO (150 nm)/a-NPB (40 nm)/BCP (15 nm)/Alq3 (20 nm)/Al. The luminance is larger than 1000 cd/m2 with the peak emission at 450 nm. As shown in Fig. 3d, luminance increase linearly with OLED current density. The life time of blue OLEDs remain lower than that of green OLEDs [23]; also the luminance can progressively decrease with time [28]. 3.3. Static and dynamic behaviors Fig. 4a shows the source-drain current (Ids) versus applied voltages Vds between the symmetric photoconductor electrodes subjected to the different blue OLED luminances (between 50 cd/m2 and 500 cd/m2). The photocurrent increases quasi linear versus the applied bias for the different luminances. Fig. 4b shows the source-drain current (Ids) versus blue OLED luminances (between 50 cd/m2 and 500 cd/m2) for different applied voltages Vds varying between 25 V and 100 V. The relationship between luminance Lm, current under illumination Ids-Illum as well as dark current Ids-Dark is given by the following formula [29]:
Ids-Illum ¼ ALm þ Ids-Dark
ð1Þ
where A may be expressed as:
A¼
Ids-Illum Ids-Dark Iph ¼ Lm Lm
ð2Þ
where Iph is the photocurrent. The current transfer ratio CTR of the optocoupler is of about 105. In order to denote the transfer performance, the current density transfer ratio was used (Fig. 4c). Recently, the input/output areas ratio has systematically been investigated; the CTR
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(b)
(a)
(c)
Fig. 4. (a) Current versus bias voltage in dark and for different luminances at 450 nm (from 50 cd/m2 to 500 cd/m2). (b) Current versus luminance in logarithmic scale for different bias voltages (Vds varying from 25 V to 200 V). (c) Current density transfer ratio CDTR versus OLED current density for ZnO/ ITO/a-NPB (40 nm)/BCP (15 nm)/Alq3 (20 nm)/Al structure.
Fig. 5. Current responses of pentacene under constant luminance (of about 200 cd/m2 at 450 nm) and for different bias voltages (Vds varying from 25 V to 200 V).
was enhanced to 1.87% with a relatively smaller OLED area [17]. Indeed, we noticed that, for IOLED (10 mm 4 mm) and Ids-Illum (50 nm 10 mm), the maximum current density transfer ratio (CDTR) reached about 2.5 101. The Ids-Illum/Ids-Dark current ratio of about 7 was obtained at Vds = 100 V for 200 cd/m2 (Fig. 5). Thus, Fig. 5 shows the response times of photocurrent for different voltages (of 25 V to 200 V); the equilibrium regime is reached more quickly as evidenced by the fast current response for high applied voltages. The rise process as well as the decay process of the current are consistent with an exponential decay. When the light is switched off, the relaxation of the current is accompanied with a less persistent photocurrent, which indicates an important recombination processes of the charge carriers. This behavior is also associated to the minority carrier lifetime, which leads to the come back to the initial state. 4. Conclusion We report the optoelectronic coupling of a blue OLED with a pentacene based photoconductor. The important parameters in term of supplying voltage and luminance in blue were obtained for ZnO (120 nm)/ITO (150 nm)/a-NPB (40 nm)/BCP
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(15 nm)/Alq3 (20 nm)/Al optimized structure. The luminance is larger than 103 cd/m2 with a peak emission at 450 nm. The Ids-Illum/Ids-Dark current ratio, the sensitivity and the current density transfer ratio of the optocoupler are of about 7, 101 A/W and 101, respectively. Although the speed of the organic optocoupler is low, the rise time as well as fall time remain higher than a second. However, the blue OLED coupling with pentacene photosensor reveals promising results for advanced optoelectronic applications with blue emitter unit. References [1] N. Karl, Organic Semiconductors, in: O. Madelung, M. Schulz, H. Weiss (Eds.), Landolt-Boernstein (New Series), Group III, vol. 17 Semicondcuctors, Subvolume 17i, page 106, Springer, Berlin, 1985. [2] J.Y. Park, H.M. Le, G.T. Kim, H. Park, Y.W. Park, I.N. Kang, D.H. Hwang, H.K. Shim, The electroluminescent and photodiode device made of a polymer blend, Synth. Met. 79 (1996) 177. [3] K.S. Narayan, T.B. Singh, Nanocrystalline titanium dioxide-dispersed semiconducting polymer photodetectors, Appl. Phys. Lett. 74 (1999) 3456–3458. [4] J.M. Lupton, R. Koeppe, J.G. Müller, J. Feldmann, U. Scherf, U. Lemmer, Organic microcavity photodiodes, Adv. Mater. 15 (2003) 1471–1474. [5] S.R. Forrest, Active optoelectronics using thin-film organic semiconductors, IEEE J. Select. Topics Quantum Electron. 6 (2000) 1072–1083. [6] G. Yu, K. Pakbaz, A.J. Heeger, Photonic devices made with semiconducting conjugated polymers: new developments, Synth. Met. 71 (1995) 2241. [7] G. Dong, Y. Hu, C. Jiang, L. Wang, Y. Qiu, Organic photocouplers consisting of organic light-emitting diodes and organic photoresistors, Appl. Phys. Lett. 88 (2006) 051110. [8] Q. Tang, L. Li, Y. Song, Y. Liu, H. Li, W. Xu, Y. Liu, W. Hu, D. Zhu, Photoswitches and phototransistors from organic single-crystalline submicro/nanometer ribbons, Adv. Mater. 19 (2007) 2624–2628. [9] Y. Yao, H.Y. Chen, J. Huang, Y. Yang, Low voltage and fast speed all-polymeric optocouplers, Appl. Phys. Lett. 90 (2007) 053509. [10] K. Kim, Y.K. Yoon, M.-O. Mun, S.P. Park, S.S. Kim, S. Im, J.H. Kim, Optical properties of solid pentacene, J. Supercond. 15 (2002) 595–598. [11] Y.-Y. Noh, K. Yase, D.-Y. Kim, Highly sensitive thin-film organic phototransistors: effect of wavelength of light source on device performance, J. Appl. Phys. 98 (2005) 074505-1–074505-7. [12] Q. Sun, D. Li, G. Dong, X. Jin, L. Duan, L. Wang, Y. Qiu, Improved organic optocouplers based on a deep blue fluorescent OLED and an optimized bilayer heterojunction photosensor, Sensors Actuat. B: Chem. 188 (2013) 879–885. [13] Q. Sun, G. Dong, L. Wang, Y. Qiu, Organic optocouplers, Sci. China Chem. 54 (2011) 1017–1026. [14] D. Li, G. Dong, W. Li, L. Wang, High performance organic–inorganic perovskite-optocoupler based on low-voltage and fast response perovskite compound photodetector, Scientific Reports/5: 7902/http://dx.doi.org/10.1038/srep07902. [15] T.D. Anthopoulos, Electro-optical circuits based on light-sensing ambipolar organic field-effect transistors, App. Phys. Lett. 91 (2007) 113513-1– 113513-3. [16] D. Li, G. Dong, L. Duan, D. Zhang, L. Wang, Y. Qiu, High-performance organic optocouplers based on an organic photodiode with high blue light sensitivity, IEEE Elect. Dev. Lett. 34 (2013) 1295–1297. [17] D. Li, W. Li, L. Duan, G. Zhang, Highly Integrable organic optocouplers on a patterned double-side indium tin oxide substrate with high isolation voltage, IEEE Elect. Dev. Lett. 36 (2015) 171–173. [18] B. Lucas, W. Rammal, A. Moliton, ITO films realized at room-temperature by ion sputtering for high-performance flexible organic light-emitting diodes, Eur. Phys. J. Appl. Phys. 34 (2006) 179–187. [19] Y. Kijima, N. Asai, S.I. Tamura, A blue organic light emitting diode, Jpn. J. Appl. Phys. 38 (9A) (1999) 5274–5277. [20] T. Tsuji, S. Naka, H. Okada, H. Onnagawa, Nondoped-type white organic electroluminescent devices utilizing complementary color and exciton diffusion, Appl. Phys. Lett. 81 (2002) 3329. [21] A. El Amrani, B. Lucas, F. Hijazi, A. Skaiky, T. Trigaud, M. Aldissi, Transparent pentacene-based photoconductor: high photoconductivity effect, Eur. Phys. J. Appl. Phys. 51 (2010) 33207–33211. [22] A. El Amrani, B. Lucas, F. Hijazi, A. Moliton, Visible light effect on the performance of photocouplers/phototransistors based on pentacene, Mater. Sci. Eng. B 147 (2008) 303–306. [23] J. Park, S.Y. Jung, J.Y. Lee, Y.G. Baek, High efficiency in blue organic light-emitting diodes using an anthracene-based emitting material, Thin Solid Films 516 (2008) 2917–2921. [24] T. Yu, W. Su, W. Li, R. Hua, B. Chu, B. Li, Ultraviolet electroluminescence from organic light-emitting diode with cerium(III)–crown ether complex, Sol.Stat. Electr. 51 (2007) 894–899. [25] J.-H. Lee, H.-S. Woo, T.-W. Kim, J.-W. Park, Blue organic light-emitting diodes with carbazole-based, Opt. Mater. 21 (2002) 225–229. [26] A. Pais, A. Banerjee, D. Klotzkin, I. Papautsky, High-sensitivity, disposable lab-on-a-chip with thin-film organic electronics for fluorescence detection, R. Soc. Chem. 8 (2008). 794–800/795. [27] C. Schmitz, M. Thelakkat, H.-W. Schmidt, A combinatorial study of the dependence of organic LED characteristics on layer thickness, Adv. Mater. 11 (1999) 822–826. [28] M. Chakaroun, R. Antony, A.P.A. Fischer, B. Ratier, A. Moliton, M.W. Lee, A. Boudrioua, Enhanced electron injection and stability in organic lightemitting devices using an ion beam assisted cathode, Sol. Stat. Sci. 15 (2013) 84–90. [29] S. Mansouri, L. El Mir, A.A. Al-Ghamdi, O.A. Al-Hartomy, S.A.F. Al Said, F. Yakuphanoglu, Characterization and modeling of TIPS-pentacene-poly(3hexyl) thiophene blend organic thin film transistor, Synth. Met. 185–186 (2013) 153–158.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Organic optocoupler consisting of an optimized blue organic light emitting diode and an organic photoconductor A. El Amrani a,⇑, B. Lucas b, R. Antony b a b
LPSMS, FST Errachidia, B.P. 509, Boutalamine, Errachidia, Université My Ismail, Morocco XLIM UMR 7252 – Université de Limoges/CNRS, 123 avenue Albert Thomas, 87060 Limoges Cedex, France
a r t i c l e
i n f o
Article history: Received 20 June 2015 Received in revised form 1 July 2015 Accepted 4 July 2015 Available online 6 July 2015 Keywords: OLED Photoconductor Sensitivity Current transfer ratio Pentacene Organic optocoupler Response time Stability
a b s t r a c t We present an optocoupler device based on a blue organic light-emitting diode (OLED) as input unit, and a pentacene photoconductor as output unit. The optocoupler was realized on a transparent glass substrate. The luminance was found larger than 103 cd/m2 with a blue peak emission at 450 nm for the optimized ZnO (120 nm)/ITO (150 nm)/a-NPB (40 nm)/BCP (15 nm)/Alq3 (20 nm)/Al structure. The Ids-Illum/Ids-Dark current ratio, the sensitivity and the current density transfer ratio of the optocoupler are of about 7, 101 A/W, and 101, respectively. The rise as well as full times were found faster for high bias voltages. The equilibrium regime with less persistent current was reached more quickly, as evidenced by the fast current response for higher bias voltage, indicating a more favorable recombination processes of the charge carriers. The organic optocoupler with a blue OLED reveals promising results; thus, it can be investigated as a good candidates for practical uses in organic optoelectronic circuits with high bias voltages. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The performance of organic optoelectronic devices such as the organic electroluminescent diodes (OLEDs), the organic thin films transistors (OTFTs), the organic photodetectors (OPhs), and more particulary the organic optocouplers (OOPs) are related to the molecular engineering, which can currently produce organic materials with energy gap that can be modulated very precisely in the range of 1.5 eV to 3 eV, similar to inorganic semiconductors [1]. The development of new organic photodetectors in different configurations were investigated by several research groups [2–5]. The organic optocoupler is a functional device that can transfer an input electrical signal via an optical signal to an output electrical signal. Thus, the first organic optocouplers were carried out by Yu et al. [6], the sensitivity of the device is about 101 A/W and the current transfer ratio (CTR) of about 2 103. Several other research works [7–9] have been reported on the optocoupler performance and their potential applications. Tang et al. [8] have noticed that the optocoupler with organic phototransistors may present a current transfer ratio higher than the organic photodiodes. Moreover, these last have high switching times (rise and fall times) and lower sensitivity as showed by Yao et al. [9]. Indeed, the rise time as well as the fall time of about 0.5 ls were noticed for an optocoupler based on a polyfluorene based polymer electroluminescent diode (PLED) as input unit and a photodiode using a mixture P3HT: PCBM (1:1) as photodetector. In optocoupler devices, the photoconductivity process can take place if the photon energy is higher than the energy gap of the active material, and the absorption coefficient of the material
⇑ Corresponding author. E-mail address: [email protected] (A. El Amrani). http://dx.doi.org/10.1016/j.spmi.2015.07.013 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
A. El Amrani et al. / Superlattices and Microstructures 85 (2015) 880–885
881
must be small enough for efficient charge carrier generation in the bulk of the semiconductor [10], and consequently the current increases. The maximum of generated photocurrent in the case of pentacene was observed [11] in the wavelength range varying from 350 nm to 550 nm where the absorption coefficient is weak. Thus, the higher CTR was obtained for shorter wavelength [12] because for shorter wavelength, the photons can provide more kinetic energy to overcome the coulomb interaction between exciton pairs [13]. This indicates that the blue OLED with shorter wavelength may be a good candidate for the organic optocoupler applications. Compared to inorganic optocoupler, the organic optocoupler is less expensive; it has a lower temperature process and a flexibility of realization [13]. Recent works have been reported on organic–inorganic hybrid optocouplers in order to combine the advantages of both active materials [14]. Thus, the hybrid device may be desirable for ambipolar performance optoelectronic circuits [15]. High organic performance with high blue OLED sensitivity was recently investigated [16,17]. The sensitivity of pentacene is relatively high, because the photons with a wavelength in blue can effectively contribute to the carriers photogeneration in the large volume of the semiconductor. In the present work, we report the performance of an organic optocoupler based on a blue OLED as input unit and a pentacene based photoconductor as output unit. The effect of the organic layer thicknesses of the OLED was investigated in order to obtain a optimum luminance associated with lower lighting voltage. The static as well as the dynamic behavior of the device were also studied. 2. Experimental details For the organic optocoupler elaboration with the same transparent substrate (12 mm 12 mm), the device was realized with the structure as presented in Fig. 1. The OLED was integrated on one side of the substrate, and a pentacene (50 nm thick) was elaborated with thermal evaporation in vacuum (around 106 mbar) on the other side. The OLED (area 3 mm 8 mm) was made up of different organic layers to obtain a main emission peak in the blue based on the zinc oxide ZnO/indium tin oxide ITO/a-NPB (4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl)/BCP (bathocupuroine)/Alq3 tris(8-hydroxyquinoline)/Al aluminum structure. All the ITO/ZnO electrodes, for the OLED or the two ITO electrodes (separated by 100 lm) used to apply a voltage bias, were realized using the IBS (ion beam sputtering) technology [18]. The a-NPB (H–W Sands society) can emit the light at 470 nm, it can be used as hole transport layer as well as emitting layer [19,20]. The photodetector active area is of about 10 mm 4 mm, the current area of the photodetector is of about 50 nm 10 mm. The current intensity, luminance as well as emission spectra measurements were carried out with an automated technique associated to various instruments (a PL330P Thurlby Thandar Instruments (TTI) type, a Keithley 617 and a Keithley 175). The optoelectronic devices were tested at room temperature and in open air. The static and dynamic characteristics of the optocoupler were carried out using a computer-controlled Keithley 4200 source measure unit. 3. Results and discussion In previous works, we have shown that the photocurrent was dependent on the wavelength, the bias voltage and the illumination side of the device [21]. We noticed that with transparent electrodes (i.e. ITO), the top contact configuration yields better performance compared to the bottom contact one [21]. Thus, an antibatic behavior in the 350–600 nm wavelength range is noticed; a higher number of photogenerated charge carriers is evidenced at the emission of about 450 nm. Indeed, for the pentacene photoconductivity mechanism, the photon energy must be higher than the pentacene energy gap, and the absorption coefficient of the semiconductor must be sufficiently small, or the active layer sufficiently thin,
Vds
VOLED
Fig. 1. Schematic of the organic optocoupler structure.
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so that light can efficiently contribute to charge carrier photogeneration in the volume of the active layer [22]. The effect of the organic film thicknesses of the OLED was studied in order to obtain a maximum luminance with lower lighting voltage. The blue light emission was less studied compared to the green and the red emissions; the main reasons are poor yield of blue OLEDs like their instability [23]. The OLED with luminous powers in the order of the lW/cm2 [24] should limit the degradation phenomena, this can be induced in the organic semiconductors based optoelectronic devices under continuous supplying. 3.1. a-NPB layer thicknesses effect The Alq3 thickness was taken of about 20 nm [25]; the thickness is sufficient to play a role of an electron transport layer [26,27]. Initially, the bathocupuroine BCP of 20 nm layer thickness was deposited on the purified a-NPB films; it is used as hole blocking layer at highest occupied molecular orbital (HOMO) a-NPB/BCP interface. This interface presents high potential barrier, which leads the hole confinement in the a-NPB layer [25]. The potential barriers remain low at the lowest unoccupied molecular orbital (LUMO) a-NPB/BCP interface, the electron can easily transit from electron transport layer (ETL) toward hole transport layers HBL, then to a-NPB emitting layer. Thus, the electron can recombine inside a-NPB with holes blocked at the a-NPB/BCP interface, and consequently lead to the blue emission peak. The effect of the a-NPB layer thicknesses (between 40 and 60 nm) on the OLED performance was systematically studied. As shown in Fig. 2a, the maximum luminance of about 700 cd/m2 at VOLED of about 22 V, thus the lighting voltages of the OLEDs remain of the same order for the different a-NPB thicknesses. However, for thick a-NPB films, a shift toward high voltages has been noticed as shown in Fig. 2b. This behavior can be explained by the low conductivity of the a-NPB, which lead to the resistance increases between the OLED electrodes. Thus, for the rest of the study, the thickness of a-NPB will be taken to 40 nm. 3.2. BCP layer thicknesses effect As mentioned above, the BCP layer induces a high potential barrier of about 0.7 eV [25] at a-NPB/BCP interface that can prevent the hole injection in the BCP layer small molecules, and consequently in the Alq3, which facilitates the electron injection from the Alq3 to a-NPB layer. The BCP thickness layer has been investigated for three values varying between 10 and 20 nm. As shown in Fig. 3a, with the BCP layer thickness increases, the luminance decrease was noticed. Thus, for thicker BCP films, a shift in threshold voltage toward high voltages value was observed (Fig. 3b). For BCP thickness of
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(b)
Fig. 2. (a) Luminance versus voltage for ZnO/ITO/a-NPB (X nm)/BCP (20 nm)/Alq3 (20 nm)/Al structure. (b) Current density versus voltage for ZnO/ITO/aNPB (X nm)/BCP (20 nm)/Alq3 (20 nm)/Al structure.
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Fig. 3. (a) Luminance versus voltage for ZnO/ITO/a-NPB (40 nm)/BCP (X nm)/Alq3 (20 nm)/Al structure. (b) Current density versus voltage for ZnO/ITO/aNPB (40 nm)/BCP (X nm)/Alq3 (20 nm)/Al structure. (c) Electroluminescent spectra for ZnO/ITO/a-NPB (40 nm)/BCP (X nm)/Alq3 (20 nm)/Al structure. (d) Luminance versus current density for ZnO/ITO/a-NPB (40 nm)/BCP (15 nm)/Alq3 (20 nm)/Al structure.
10 nm, we obtain a high luminance with a green emission ; this is due to the BCP layer that is the thinnest; the thickness does not make it possible to confine the holes in the a-NPB layer. The radiative recombinations take place in the Alq3 layer that is responsible for the green color emission, at about 553 nm, as shown in Fig. 3c for the electroluminescent spectra. While increasing the BCP thickness up to 15 nm, a blue emission color was obtained; the thickness is thus sufficient to ensure the role as hole blocking layer. The best compromise of parameters in term of supplying voltage and luminance in blue was obtained for the structure ZnO (120 nm)/ITO (150 nm)/a-NPB (40 nm)/BCP (15 nm)/Alq3 (20 nm)/Al. The luminance is larger than 1000 cd/m2 with the peak emission at 450 nm. As shown in Fig. 3d, luminance increase linearly with OLED current density. The life time of blue OLEDs remain lower than that of green OLEDs [23]; also the luminance can progressively decrease with time [28]. 3.3. Static and dynamic behaviors Fig. 4a shows the source-drain current (Ids) versus applied voltages Vds between the symmetric photoconductor electrodes subjected to the different blue OLED luminances (between 50 cd/m2 and 500 cd/m2). The photocurrent increases quasi linear versus the applied bias for the different luminances. Fig. 4b shows the source-drain current (Ids) versus blue OLED luminances (between 50 cd/m2 and 500 cd/m2) for different applied voltages Vds varying between 25 V and 100 V. The relationship between luminance Lm, current under illumination Ids-Illum as well as dark current Ids-Dark is given by the following formula [29]:
Ids-Illum ¼ ALm þ Ids-Dark
ð1Þ
where A may be expressed as:
A¼
Ids-Illum Ids-Dark Iph ¼ Lm Lm
ð2Þ
where Iph is the photocurrent. The current transfer ratio CTR of the optocoupler is of about 105. In order to denote the transfer performance, the current density transfer ratio was used (Fig. 4c). Recently, the input/output areas ratio has systematically been investigated; the CTR
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(b)
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Fig. 4. (a) Current versus bias voltage in dark and for different luminances at 450 nm (from 50 cd/m2 to 500 cd/m2). (b) Current versus luminance in logarithmic scale for different bias voltages (Vds varying from 25 V to 200 V). (c) Current density transfer ratio CDTR versus OLED current density for ZnO/ ITO/a-NPB (40 nm)/BCP (15 nm)/Alq3 (20 nm)/Al structure.
Fig. 5. Current responses of pentacene under constant luminance (of about 200 cd/m2 at 450 nm) and for different bias voltages (Vds varying from 25 V to 200 V).
was enhanced to 1.87% with a relatively smaller OLED area [17]. Indeed, we noticed that, for IOLED (10 mm 4 mm) and Ids-Illum (50 nm 10 mm), the maximum current density transfer ratio (CDTR) reached about 2.5 101. The Ids-Illum/Ids-Dark current ratio of about 7 was obtained at Vds = 100 V for 200 cd/m2 (Fig. 5). Thus, Fig. 5 shows the response times of photocurrent for different voltages (of 25 V to 200 V); the equilibrium regime is reached more quickly as evidenced by the fast current response for high applied voltages. The rise process as well as the decay process of the current are consistent with an exponential decay. When the light is switched off, the relaxation of the current is accompanied with a less persistent photocurrent, which indicates an important recombination processes of the charge carriers. This behavior is also associated to the minority carrier lifetime, which leads to the come back to the initial state. 4. Conclusion We report the optoelectronic coupling of a blue OLED with a pentacene based photoconductor. The important parameters in term of supplying voltage and luminance in blue were obtained for ZnO (120 nm)/ITO (150 nm)/a-NPB (40 nm)/BCP
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