Eu-β-diketonate complex OLED as UV portable dosimeter

Synthetic Metals 161 (2011) 964–968

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Eu-␤-diketonate complex OLED as UV portable dosimeter W. Quirino a , R. Reyes b , C. Legnani a , P.C. Nóbrega c , P.A. Santa-Cruz c , M. Cremona a,d,∗ a

DIMAT – Divisão de Metrologia de Materiais, Instituto Nacional de Metrologia, Normalizac¸ão e Qualidade Industrial, INMETRO, Duque de Caxias, RJ, Brazil Facultad de Ingeniería Química y Manufacturera, Universidad Nacional de Ingeniería, Av. Tupac Amaru SN, Lima, Peru c Departamento de Química Fundamental, Universidade Federal de Pernambuco, Cidade Universitária, Recife, PE, CEP 50740-540, Brazil d Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, Rio de Janeiro, RJ, CEP 22453-970, Brazil b

a r t i c l e

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Article history: Received 11 July 2010 Received in revised form 14 January 2011 Accepted 1 March 2011 Available online 6 May 2011 Keywords: OLED UV dosimeter Europium complexes Electroluminescence Photo-degradation

a b s t r a c t The fabrication and the characterization results of a triple-layer organic light emitting diode (OLED) using a fluorinated Eu3+ -␤-diketone complex particularly sensitive to UV radiation are presented. The electroluminescence spectra of the devices, fabricated by thermo evaporation carried out in a high vacuum environment, present narrow emission bands characteristics of the Eu3+ ion. The intensity of these peaks decrease as a function of the UV radiation impinging on the device, and is strongly dependent from the UV radiation cumulative dose. The results show that measuring the main Eu3+ emission intensity it is possible to fabricate a portable UV personal dosimeter using this kind of complexes as emitting layer in an OLED device. The OLED emission will be inversely proportional to the UV irradiation exposure. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Ultraviolet radiation (UVR) exists in the environment due to the natural source of the Sun and/or artificial light sources. The skin and eyes of humans are inevitably exposed to several ultraviolet radiation doses throughout normal life. Small amounts of UV radiation in specific range of wavelengths are beneficial for people and essential as a catalyst agent in dermal vitamin D formation. UV radiation is also used for threat several diseases, e.g. dynamical photo therapy (DPT) techniques. However, prolonged human exposure to harmful solar UV radiation may result in acute and chronic health effects on the skin, eye and immune system [1,2]. The concerns with the increase of ambient UV radiation were worsened with the fact that the natural protection from most harmful solar UV radiation provided by the atmosphere has deteriorated over the last decades. As a consequence, many research for effective UV protection and for developing UV quantification methods has been increased over the years [3,4]. Organic fluorescent compounds have optical characteristics that are strongly dependent from photo irradiation. These compounds can be used as passive or active components in optoelectronic devices for measurement of total dose related to the UV irradia-

∗ Corresponding author at: Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, Rio de Janeiro, RJ, CEP 22453-970, Brazil. Tel.: +55 21 3527 1260; fax: +55 21 3527 1271. E-mail address: cremona@fis.puc-rio.br (M. Cremona). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.03.001

tion exposure (dosimeters). OLEDs (organic light emitting diodes) are a novel class of devices which use these compounds and can be used for this purpose. They are thin film devices constituted by a heterostructure that can be deposited on several types of substrates including plastic ones [5]. However, the degradation of the organic materials, which are commonly used in the OLED fabrication, is still the principal weakness of these devices. Many efforts have been made in order to understand the factors that influenced the different degradation mechanisms of the OLEDs and their organic materials. Another important aspect of the OLED technology, especially for the implementation of portable devices, is their operational stability against radiation originated from the environment, such as intense sunlight [6–9]. Nevertheless, it is also possible to take advantage of the OLED degradation to obtain a radiometric dosimeter when a specific OLED is submitted to a UV radiation source. Normally is very difficult to detect slight changes in radiation intensity when the source has a wide emission band. In opposite, small changes in the Rare-earth (RE) emission intensity are easily detectable due to their very narrow emissions. Rare-earth complexes have characteristic luminescence properties due to the intra and intermolecular energy transfer process. It is known that lanthanide complexes are well protected from environmental perturbations by the overlying 5s2 and 5p6 orbitals. The various states arising from 4f configurations are split by external fields only to the extent of about 100 cm−1 [3]. Therefore, emission bands are extremely sharp when electronic transitions occur, resulting in almost monochromatic and long-lifetime emission. In the efficient lanthanide complexes, the excited state of a lumines-

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Fig. 1. Molecular structures of the compound used in this work: (a) Eu(btfa)3 bipy (btfa = 4,4,4-trifluoro-1-phenyl-2,4-butanedione, bipy = 2,2 -bipyridine); (b) Alq3 (tris(8hydroxyquinoline aluminum)); (c) NPB (N,N -bis(1-naphtyl)-N,N -diphenyl-1,1 -biphenyl-4,4 -diamine). In (d) the OLED architecture showing the thickness (in nm) of each layer is reported.

cent lanthanide(III) ion is generally populated by transfer from the triplet state of the organic ligands [10–12]. Because of these luminescence properties the Rare-earth complexes are widely applied as luminescent probes, in fluoro-immunoassays, in optical markers and most recently as display materials in organic electroluminescent devices (EL) [13–15]. In the present work our effort is mainly devoted to the study of the decrease of the electroluminescent emission and the operational lifetime, at room temperature, when the Eu-complexes based OLED are exposed to UV radiation in comparison with the not irradiated device. Because of the particularly sensitive Eu(btfa)3 bipy (btfa = 4,4,4-trifluoro-1phenyl-2,4-butanedione, bipy = 2,2 -bipyridine) compound when submitted to the UV irradiation, the intensities of these peaks decrease quickly when the device is submitted to this irradiation. The process is irreversible and the behavior is possibly associated with the ligand’s photodegradation [16–18]. Radiation outside the UV range does not affect these ligands, allowing selective UV dosimetry even under sunlight exposure. The Eu(btfa)3 bipy family complex was strategically selected due to its well know UV dosimeter capability [19–21] and, for this reason, can be employed as a visual, real-time and low cost radiometric method for future UV exposure quantification. 2. Experimental The synthesis of the ␤-diketone Eu-complex compound, which chemical structure is shown in Fig. 1a, was obtained in powder form according to synthesis route published previously [17].

This complex was used to fabricate OLED devices as follow: prepatterned anodes of indium tin oxide (ITO) pre-deposited on glass substrates (with a sheet resistance of 8 /sq.) were cleaned by boiling in a detergent solution, then rinsed with deionized water, followed by sonication in acetone and ethanol for 5–10 min in each solvent. After cleaning, the substrates were dried under N2 . ´˚ Organic layers were sequentially deposited at a rate of 1–3 A/s onto room temperature substrates by thermal evaporation from resistively heated tantalum boats in vacuum, at a base pressure of 3–4 × 10−6 Torr. The layer thickness was controlled in situ through a quartz crystal thickness monitor system and confirmed by a successive calibrated perfilometer measurement. The devices were assembled using a heterojunction between three organic molecular materials containing the (NPB) N,N -bis(1-naphtyl)-N,N diphenyl-1,1 -biphenyl-4,4 -diamine as hole-transporting layer; Europium-containing complex as emitting layer and (Alq3 ) tris(8-hydroxyquinoline aluminum) as electron transporting layer (Fig. 1a–c). Finally, without breaking the vacuum, a 150 nm thick aluminum cathode was evaporated from a tungsten wire basket at approximately 0.5 nm/s rate in the same vacuum chamber. The fabricated EL devices presenting an active area of about 10 mm2 were operated in forward bias voltage, with ITO as the positive electrode and Al as the negative one. The devices were characterized in air within 2 h of the fabrication. The device configuration used in this work is presented in Fig. 1d. The absorption spectra of the RE3+ -complex films were recorded on a Perkin-Elmer Lambda 19 spectrophotometer. The photoluminescence and electroluminescence spectra were obtained in a Photon Technology International (PTI) fluorescence spectropho-

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tometer. The device brightness was measured using a calibrated radiometer/photometer by United Detector Technology (UDT-350). For the UVA irradiation experiments a He–Xe light source was used and its power vs. wavelength spectrum is reported in Fig. 2. 3. Results and discussion Fig. 3 shows the absorption spectra measured at room temperature of the Eu(btfa)3 bipy powder (a) and of the same complex evaporated as thin films (b) onto a quartz substrate (thickness = 50 nm). It can be seen that the absorption bands of Eu(btfa)3 bipy after evaporation do not show significant difference in comparison with the original complex in powder form. Furthermore, in both cases, the complex exhibit ultraviolet absorption peak in the range of 200–400 nm, and the maximum absorption peaks are located at 235 and 332 nm, respectively. According to the strong absorption of the complex in a near ultraviolet region, these bands cannot come from f–f or f–d electron transition of the rare earth ions, which are generally too weak and could be coved up easily. Thus, the absorption spectrum of the ␤-diketone is associated with singlet–singlet transitions of the conjugated chromophore due to the chelation between metal ions and ligand with strong ␲–␲* character [22].

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In the photoluminescence (PL) spectrum of the [Eu(btfa)3 bipy] film, displayed in Fig. 4 is possible to identify clearly the narrow bands at 533, 576–590, 612, 650 and 702 nm, which are attributed to the characteristic 5 D0 → 7 FJ (J = 0, 1, 2, 3, 4) transitions of the Eu3+ ion [23]. The 5 D0 → 7 F2 transition (the predominant emission line) is approximately ten times more intense than the 5 D0 → 7 F1 transition, resulting in a highly red emission. The small intensity of the 5 D0 –7 F1 transition compared to 5 D0 –7 F2 suggests a strong mixture of the f orbital with the d orbital of the Eu3+ ion. Moreover, compared to those of other transitions, the much stronger intensity of 5 D0 –7 F2 indicates that the local-ligand field splitting is particularly evident for the 5 D0 –7 F1–3 transitions, presenting (2J + 1) Stark components. The presence of only one 5 D0 –7 F0 line indicates that the Eu3+ ion occupies a single site and a single chemical environment exists around it [24]. All this results can be attributed to a strong bond between the central ion and the ligands [25]. The emission spectrum of the europium complex was recorded at room temperature under excitation at  = 330 nm which matches the corresponding absorption spectrum, confirming that energy transfer takes place from the ligand to the Eu ion and does not exhibit the ligand centered transitions, suggesting that there is an efficient intramolecular energy transfer from the ␤-diketone ligand to the Eu3+ ion. The effective excitation energy generated by carrier recombination populates the excited singlet S1 from the ground singlet state S0 in the beta-diketone-ligand. Competing with the fast intersystem crossing (ISC) conversion are organic electrofluorescence and non-radiative deactivation of S1 . Intramolecular energy transfers (ET) from ligands from S1 and T excited states leads a population of 5 D1 level (fast decay ∼4 ␮s [18]) and a long-lived metastable 5 D0 level of Eu3+ ion giving a rise of Eu3+ emission peaks to the ground multiplet 7 FJ (J = 0–4). Fig. 5 exhibits the room temperature EL spectra of the Eu(bfta)3 bipy complex for different values of the bias voltage. Similarly as in the photoluminescence spectra, it is possible to identify the 5 D0 → 7 FJ transition due to the Eu3+ ion. The intensity of the EL emission varied as a function of the bias voltage applied and no emission from the organic ligand was observed. In Fig. 6 it is shown the OLED emission behavior as a function of the time. This is what we call “intrinsic” device degradation because no UV light was applied before recording the EL spectra. For these experiments the selected device was kept in darkness all the time and a 20 V bias voltage was applied at different times just to record the spectrum. Moreover, in order to avoid the electrical device degradation the EL measurements were performed quickly

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recording each spectrum from 520 to 640 nm with 2 nm step and an integration time of 0.2 s. The electroluminescence spectra were recorded until the 612 nm peak intensity (typical of the Eu3+ ion) decreased to the half of the initial value. Then with an exponential fit it is possible to estimate the device’s operational lifetime for a non-encapsulated device. The decrease of the 612 nm EL peak intensity is shown in the inset of Fig. 6. In the absence of any UV irradiation, and for a non-encapsulated device, a lifetime of about 40 min is achieved. This time is mainly due to the degradation of the Eu(bfta)3 bipy based OLED caused by environmental conditions like moisture, O2 among others. However, upon the irradiation with UVA (UV = 360 nm) light from the He–Xe light source of Fig. 2 the marked sensitivity of this Eu-complex to UV exposure is observed by accelerating the decrease of the OLED lifetime, monitored by EL peak intensity decreasing (Fig. 7). In this case, with the device turned off, it was irradiated for about 1 min with 1.4 mW/cm2 fixed power density of UVA light. Then, the UVA light was blocked and a 20 V bias voltage was immediately applied just to record the EL spectrum in the same way described before. Successively the OLED was irradiated again with the same UVA light conditions and the whole procedure was repeated. In this way the device is submitted a cumulative UVA irradiation. The EL spectra were recorded, as before, until the 612 nm peak intensity decreased to the half of the

Fig. 6. EL emission behavior of a non-encapsulated OLED without UVA light irradiation (“intrinsic” degradation). The different colored curves (a–e) are examples of EL emissions taken at different times. In the inset is reported the 612 nm peak intensity behavior vs. time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. EL emission behavior of a non-encapsulated OLED with UVA (UV = 360 nm) light irradiation. The different colored curves are examples of EL emissions taken at different times each one after UVA irradiation: (a) not UVA irradiated; (n) after “n” UVA irradiations. In the inset is reported the 612 nm peak intensity behavior vs. time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

initial value. The inset in Fig. 7 shows the decreasing of the 612 nm peak emission as a function of the time in a period of 90 min. In this figure the black circles are the experimental points (not all taken are shown) and the line is a best fit to estimate the lifetime. The difference with the previous behavior (inset of Fig. 6) is quite clear. The observed behavior is irreversible and can be associated with the ligand’s photodegradation. Indeed, the irradiated device kept in the dark does not recover the same level of EL intensity as the not-irradiated kept in the same conditions. Differently from recent hypothesis of a process involving loss of mass (nitrogen for instance) in other systems [16], the present case may be related to a single photocleavage, without loss of mass [17,19–21]. As noted earlier, biological damage caused by UV light is additive, particularly over a short period, such as a day, over which the damaged cells do not have sufficient time to effect a significant degree of repair. In this way, more useful than a UV indicator is a UV dosimeter: a device which is capable of registering the total amount of UV exposure. The response curve of the Eu(bfta)3 bipy based OLED submitted to UV irradiation indicates clearly that this kind of device can be employed for human exposure measurements due to its portability and suitable optical properties. Although it is still not fully characterized as radiometric method, this particular device shows the versatility and the potentialities that the OLEDs can offer for the fabrication of a simple UV exposure quantification device that may act as a dosimeter. Two important features characterize the Eu(bfta)3 bipy based OLED proposed device: sensitivity, related to the memory effect of the electroluminescence intensity as a function of the integrated amount of UV radiation received, allied to the selectivity, related to the absorption spectrum associated with the ligand’s photodegradation of the Eu complex, that results in a device very selective to UV radiation. Then, the results reported above can be readily used to fabricate a UV dosimeter based on rare earth OLED, by rendering the UV-induced electroluminescence intensity decrease irreversible. Since this OLED-based dosimeter may work passively during the dose storage, the lifetime of the device is assured. For that, the bias voltage may be applied in short periods, acting actively only to read the electroluminescence assigned to the cumulated dose [26].

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4. Conclusion Triple-layer electroluminescent organic devices using NPB as hole transporting layer, [Eu(bfta)3 bipy] complex as emitting layer and Alq3 as electron transporting layer were grown and characterized. The EL spectra of the devices exhibit emission characteristics of the sharp lines from the Eu3+ ion. The [Eu(btfa)3 bipy] organic complex is very sensitive to the UV light showing a decrease of the PL intensity with the irradiation time. Our results show that it is possible to fabricate a portable UV dosimeter using this complex as emitting layer in an OLED device. The OLED emission will be inversely proportional to the UV irradiation exposure, which allows the monitoring of the UV dose absorbed [26]. Acknowledgements This work was supported by RENAMI, CNPq, CAPES, FAPERJ, and FACEPE. References [1] R.A. Lester, A.V. Parisi, M.G. Kimlin, J. Sabburg, Phys. Med. Biol. 48 (2003) 3685–3698. [2] J.C.F. Wong, A.V. Parisi, Internet Photochemistry and Photobiology, http://www.photobiology.com/UVR98/wongrev/index.htm. [3] R. McKenzie, B. Conner, G. Bodeker, Science 285 (1999) 1709–1711. [4] S. Harry, O. Peter, K. Gert, Arctic Alpine Ecosystems and People in a Changing Environment, Springer, Berlin/Heidelberg, 2007, Chapter 16. [5] C. Legnani, C. Vilani, V. Calil, H. Barud, W.G. Quirino, C.A. Achete, S. Ribeiro, M. Cremona, Thin Solid Films 517 (2008) 1016.

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