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Green electroluminescence from azafluoranthene derivatives in polymer based electroluminescent devices
Optical Materials xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Short Communication
Green electroluminescence from azafluoranthene derivatives in polymer based electroluminescent devices P. Gaß siorski a, E. Gondek b, K.S. Danel c, M. Pokladko-Kowar b, P. Armatys d, A.V. Kityk a,⇑ a
Faculty of Electrical Engineering, Czeßstochowa University of Technology, Armii Krajowej 17, 42-200 Czeßstochowa, Poland Institute of Physics, Cracow University of Technology, Podhora ß z_ ych 1, 30-084 Kraków, Poland c University of Agriculture, Balicka 122, 30-149 Kraków, Poland d Faculty of Physics and Applied Computer Science, AGH-University of Science and Technology, Al. A. Mickiewicza 30, Kraków, Poland b
a r t i c l e
i n f o
Article history: Received 31 July 2012 Received in revised form 10 September 2012 Accepted 11 September 2012 Available online xxxx Keywords: Azafluoranthene dyes Electroluminescence spectra Luminance-voltage characteristic Luminance-current characteristic
a b s t r a c t Paper demonstrates a series of novel electroluminescent devices (OLEDs) with poly (N-vinylcarbazole) active emitting layer doped by the azafluoranthene dyes. Measured steady state electroluminescence spectra, luminance-voltage and luminance-current characteristics provide basic characterization of the designed OLEDs. All the devices exhibit sharp green electroluminescence with the emission maximum being rather weakly dependent on the type of the fluorescent dopant. The obtained results show that a series of recently synthesized azafluoranthene dyes may be considered as efficient green fluorescent emitters for electroluminescent applications. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Over the past decade, organic dyes have attracted a considerable interest due to a number of potential applications in organic electronics among which the electroluminescence and relevant to it displays technology may be considered as prevailing [1–4]. In fact, the electronic properties of guest dye molecules, embedded as dopants into a host polymer layer, define the efficiency of electroluminescent radiation and its color gamut representing the basic characteristics of electroluminescent devices. A set of red, green, and blue emitters with high efficiency and color purity remain the key problems of the full-color display technology. This particularly explains considerable efforts of recent years being particularly undertaken in the organic synthesis and spectroscopic studies of novel fluorescent organic dyes [5–19] likewise the quantum-chemical modeling [18–25] aimed to understand the mechanisms of optical absorption and emission processes as well as the trends in their changes depending on molecular structure modification. Among the latter ones, the pyrazoloquinoline (PQ), likewise an extended number of its derivatives [17–19] are worth to be mentioned. They represent efficient fluorescent emitters, however, relevant electroluminesce of these dyes is limited by the light emission preferably in the blue spectral region [26–29]. Quite
⇑ Corresponding author. E-mail address: [email protected] (A.V. Kityk).
recently, Danel et al. [16] have cyclized several phenyl containing PQs into five membered regioisomers of azafluoranthene derivatives. Spectroscopic studies of these dyes have revealed a significant red shift of both excitation and emission spectra [30–33]. Relevant trends have been also reproduced within the semiempirical and DFT/TDDFT calculations [23–25,30–33]. Eventual fundamental interest to such phenomena is justified by practical needs in novel efficient fluorescent emitters in the green-yellow region of the visible spectra. In the present paper we demonstrate a series of novel electroluminescent devices based on the poly(N-vinylcarbazole) (PVK) active emitting layer doped with the azafluoranthene dyes.
2. Experimental Actual work deals with four azafluoranthene derivatives, referred for the sake of convenience as D1–D4, with a molecular structure as shown below namely 6-Methyl-1,3-diphenyl-3Hindeno[1,2,3-de]pyrazolo[3,4-b]quinoline (D1), 1,5,6-trimethyl3-phenyl-3H-indeno[1,2,3-de]pyrazolo[3,4-b]quinoline (D2), 1, 3-Diphenyl-3H-indeno[1,2,3-de]pyrazolo[3,4-b]quinoline (D3) and 7-methoxy-3-phenyl-1-propyl-3H-indeno[1,2,3-de]pyrazolo[3,4-b] quinoline (D4). Details concerning the synthesis of these dyes are described in Ref. [16,30,31,33]. They basically consist of two consecutive steps, namely a preparation of the starter compounds which represent phenyl containing PQ-derivatives which then by
0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.09.027
Please cite this article in press as: P. Gaß siorski et al., Opt. Mater. (2012), http://dx.doi.org/10.1016/j.optmat.2012.09.027
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P. Gaß siorski et al. / Optical Materials xxx (2012) xxx–xxx
means of cyclization reaction under special conditions are transferred into azafluoranthene dyes. Commercially available reagents (Aldrich, Fluka, Merck) were used without further purification. The purity of the obtained compounds was checked by TLC. Elementary analysis were performed on a Perkin-Elmer 2400 CHN analyzer.
(a)
(b) (c) Fig. 1. Panel (a): sketch of the electroluminescent device (OLED) of ITO/PEDOT:PSS/ PVK:Dye/Ca/Al configuration. Panel (b): designed OLED with the active PVK:D1 layer emitting the green light (turn-on state). Panel (c): C.I.E. 1931 color coordinates X,Y and Z of electroluminescent radiation emitted by the PVK active layer doped with azafluoranthene dyes D1–D4 (concentration 1.2 mol.%).
The basic spectroscopic characterization of these dyes, i.e. their optical absorption and fluorescence spectra, may be found in a series of recently published works [30,31,33]. Analysis of these data shows that the first vibronic band (0 ? 00 transition) lies for all the dyes in the region 470–480 nm, although the first absorption maxima in most cases is centered at somewhat shorter wavelengths, 440–450 nm. Moreover, due to strongly overlapping vibronic bands the first absorption band is quite broad and considerably overlaps with the emission band of PVK polymer [34] used in the current work as the host material in OLED fabrication. Such feature of azafluoranthene dyes appear to be crucial in a sense of the efficiency of the energy transfer from the host to guest molecules which takes place in light emitting layer. The OLEDs were fabricated in accordance with the single active layer configuration as it is sketched in Fig. 1a. The overall procedure is similar to the one described in recently published work [35]. The anode substrate represents commercial glass plates with a deposited transparent ITO film (Aldrich, resistance 150 Ohm/sq). Slides were ultrasonically cleaned in detergent solution, afterwards rinsed first in the deionized water and next in THF for 80 min. Subsequently it was dried under technical vacuum at 100 °C for 30 min, coated by spin-coating with the hole injection layer of poly (3,4-ethylene dioxythiophene)-poly-(styrene sulfonate) (PEDOT:PSS) in dry argon atmosphere and then again dried in vacuum for 30 min and 100 °C. Organic dyes D1–D4 and PVK were taken in a molar ratio 1.2:100 which provides the best electroluminescence efficiency of the fabricated OLEDs. They were dissolved and carefully blended in THF, afterwards deposited on the substrates by spin-coating (speed 1000 rpm) in argon to remove dissolved air. Finally, the samples were then annealed at 70 °C for 30 min, afterwards the Ca cathode film (10 nm) was deposited by vacuum evaporation and protected with the aluminium film (100 nm). Following the description mentioned above the electroluminescence devices of ITO/PEDOT:PSS/PVK:Dye/Ca/Al configuration (Dye D1,D2,D3 or D4) have been designed, see Fig. 1b.
The electroluminescence spectra of ITO/PEDOT:PSS/PVK:Dye/ Ca/Al OLEDs were recorded using Shimadzu UVVIS 2101 scanning spectrophotometer applying the dc-bias voltage in a forward direction (ITO, positive; Ca/Al, negative). All the measurements were carried out at room temperature and all the devices were exposed in air without encapsulation.
3. Results and discussion Fig. 2 shows the luminance-voltage characteristics for a series of OLEDs ITO/PEDOT:PSS/PVK:Dye/Ca/Al. They are typical for the
Fig. 2. The luminance-voltage characteristic for the electroluminescent devices (OLEDs) of ITO/PEDOT:PSS/PVK:Dye/Ca/Al (Dye D1–D4) configuration. The curves are labeled according to the type of OLED active layer.
Please cite this article in press as: P. Gaß siorski et al., Opt. Mater. (2012), http://dx.doi.org/10.1016/j.optmat.2012.09.027
P. Ga ß siorski et al. / Optical Materials xxx (2012) xxx–xxx
3
Fig. 3. The normalized steady state electroluminescence spectra of ITO/PEDOT:PSS/ PVK:Dye/Ca/Al (Dye D1–D4) OLEDs recorded at forward dc-bias voltage of 10 V. The curves are labeled according to the type of OLED active layer.
devices of that type. Using these dependences one may estimate the turn-on voltages of relevant OLEDs. In particular, by extracting the voltage value at which the measured electroluminescence emission just exceeded the noise magnitude being detected at U = 0 we arrive with the magnitude for the turn-on voltage of about 5.1 ± 0.2 V in the case of OLED with PVK:D3 active layer and about 6.0 ± 0.2 V for the OLEDs with PVK:D1, PVK:D2 or PVK:D4 layers. Accordingly, there is no evident indication for a substituent effect on the turn-on voltage in this case. Fig. 3 shows the normalized steady state spectra of electroluminescent devices ITO/PEDOT:PSS/PVK:Dye/Ca/Al recorded at forward dc-bias voltage of 10 V. All OLEDs exhibit green electroluminescence, see also Fig. 1b, with nearly the same shape of the electroluminescent band being unstructured and narrow. The full width at half maximum (FWHM) equals about 40 nm in each case whereas the emission maximum slightly varies depending on the fluorescent dopant. For PVK:D1, PVK:D2, PVK:D3 and PVK:D4 active layers it corresponds to 510 nm, 512 nm, 515 nm and 511 nm, respectively. Compared to the OLEDs based on trimethyl derivatives of methoxyphenyl indenopyrazoloquinoline [35] the maxima of the electroluminescence spectra for PVK doped by D1–D4 dyes appear to be slightly blue shifted. More precise characterization of the electroluminescence color may be given within the C.I.E. (1931) standard, see color coordinates listed in Fig. 1c. We may compare now the electroluminescence spectra with the fluorescence ones measured in several organic solvent [30,31,33]. According to these works the fluorescence maxima of D1–D4 dyes appear in the region 500–570 nm and their spectral position considerably depends on the solvent polarity. While the solvent polarity changes from nonpolar cyclohexane to strongly polar acetonitrile all the dyes exhibit the hypsochromic (blue) shift of the first absorption band and the bathochromic (red) shift of the fluorescence band. Such opposite trends appear to be consistent with existing solvatochromic models based on semiempirical [30,31,33] and DFT/TDDFT [23,24] evaluations explaining it by a specific orientation of the dipole moments in the ground and excited states. The Lippert-Mataga polarity function F of PVK is equal approximately 0.148 (static dielectric constant e = 3.0, refractive index n = 1.69). In accordance with the Lippert-Mataga equation [36,37] the spectral position of the fluorescence maximum for the doped PVK: (D1–D4) layer is then expected in the region 510–520 nm. This pretty well agrees with actual experimental observation. Fig. 4 shows the log–log plots of the luminance-current characteristics of the electroluminescent devices ITO/PEDOT:PSS/
Fig. 4. Log–log luminance-current characteristic for the electroluminescent devices (OLEDs) of ITO/PEDOT:PSS/PVK:Dye/Ca/Al (Dye D1–D4) configuration. The solids lines are best linear fits in log–log scale; relevant labels near the lines represent the exponents extracted from the fits. The curves are labeled according to the type of OLED active layer.
PVK:Dye/Ca/Al (Dye D1–D4) recorded at slowly ramping dc-bias voltage. Detailed analysis of these dependences let us specify the mechanism of charge carrier injection. According to Ref. [38], in the injection controlled electroluminescence (ICEL) the emission intensity IEL is expected to be a power-type function of a dominating hole (Jh) or electron (Je) current, i.e. IEL / (Je,h)n. In the simplest approximation n 2, however the exponent may indeed somewhat deviate from this value taking into account the field dependence of the charge carrier mobilities. Alternatively, in the case of volume-controlled electroluminescence (VCEL) the positive and negative space charges are assumed to be largely canceled each other what implies that Jh = Je = J. This means that their decay is dominated by the second-order process of carrier recombination and the electroluminescent emission is always a linear function of the current [38]. Indeed, a linear character of IEL vs J in Fig. 4 evidently indicates on the power-like type of the measured dependences. Moreover, in all the cases the exponent, determined from the slopes of linear fits in the log–log plot, is quite close to the one. Such finding suggests that the electroluminescence is largely dominated by the VCEL mechanism. The photon emission may be caused by the two well known processes: (i) carrier recombination at a host molecule with a subsequent energy transfer to a guest molecule and followed by that quant emission or (ii) carrier trapping at the guest molecules leading to a direct exciton formation on the guest molecules with subsequent photon emission [39]. The carrier trapping mechanism can be in principle excluded since the HOMO energy level for D1–D4 dyes used in OLEDs fabrication does not exceed 5.85 eV [24,40] i.e. appears evidently below the HOMO–LUMO gap of the PVK polymer matrix ðE0H ¼ 5:6 eV; E0L ¼ 2:0 eV[41]). On the other hand, pure PVK layer is characterized by blue electroluminescence [42] with the emission band centered at about 415 nm and FWHW of about 83–86 nm. This blue emission appears to be completely quenched in the doped PVK layers giving rise of the emission in the green spectral region. The latter one indicates that efficient radiative energy transfer mechanism is quite likely although it does not exclude the electroluminescence emission originating from radiationless energy transfer mechanisms. Available data set is not sufficient to discriminate these processes. Fig. 5 presents the luminance vs the consumed electric power for the ITO/PEDOT:PSS/PVK:Dye/Ca/
Please cite this article in press as: P. Gaß siorski et al., Opt. Mater. (2012), http://dx.doi.org/10.1016/j.optmat.2012.09.027
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P. Gaß siorski et al. / Optical Materials xxx (2012) xxx–xxx
References
Fig. 5. The luminance vs the consumed electric power for the electroluminescent devices (OLEDs) of ITO/PEDOT:PSS/PVK:Dye/Ca/Al (Dye D1–D4) configuration. The power is referred to the ITO-electrode area S = 0.24 cm2. The curves are labeled according to the type of OLED active layer.
Al OLEDs which allows immediately compare the luminous efficiency of these devices. The most efficient appears to be the electroluminescent device with PVK:D4 active layer which by the factor of 1.5–2.0 is larger compared to other OLEDs. However, there is no evident correlation between this result and the fluorescence quantum yield reported in Refs. [30,31,33]. This suggests that crucial influence on the luminous efficiency should likely have the energy transfer mechanisms. 4. Conclusion In the present paper we have demonstrated the electroluminescent devices based on the PVK active emitting layer doped with several azafluoranthene derivatives. Measured steady state electroluminescence spectra, luminance-voltage and luminance-current characteristics provide basic characterization of the designed OLEDs. All the devices exhibit sharp green electroluminescence with the emission maximum being rather weakly dependent on the type of the fluorescent dopant. Our analysis reveals that the light emission is essentially dominated by the volume-controlled electroluminescence whereas the carrier trapping mechanism should be excluded from the consideration. Crucial influence on the luminous efficiency should be likely attributed to the energy transfer mechanisms. The obtained results suggest a series of recently synthesized azafluoranthene dyes as efficient sharp green fluorescent emitters for potential electroluminescent applications. Acknowledgement This work is sponsored by the National Science Centre within Grant 2011/03/BST7/0358.
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Please cite this article in press as: P. Gaß siorski et al., Opt. Mater. (2012), http://dx.doi.org/10.1016/j.optmat.2012.09.027
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Short Communication
Green electroluminescence from azafluoranthene derivatives in polymer based electroluminescent devices P. Gaß siorski a, E. Gondek b, K.S. Danel c, M. Pokladko-Kowar b, P. Armatys d, A.V. Kityk a,⇑ a
Faculty of Electrical Engineering, Czeßstochowa University of Technology, Armii Krajowej 17, 42-200 Czeßstochowa, Poland Institute of Physics, Cracow University of Technology, Podhora ß z_ ych 1, 30-084 Kraków, Poland c University of Agriculture, Balicka 122, 30-149 Kraków, Poland d Faculty of Physics and Applied Computer Science, AGH-University of Science and Technology, Al. A. Mickiewicza 30, Kraków, Poland b
a r t i c l e
i n f o
Article history: Received 31 July 2012 Received in revised form 10 September 2012 Accepted 11 September 2012 Available online xxxx Keywords: Azafluoranthene dyes Electroluminescence spectra Luminance-voltage characteristic Luminance-current characteristic
a b s t r a c t Paper demonstrates a series of novel electroluminescent devices (OLEDs) with poly (N-vinylcarbazole) active emitting layer doped by the azafluoranthene dyes. Measured steady state electroluminescence spectra, luminance-voltage and luminance-current characteristics provide basic characterization of the designed OLEDs. All the devices exhibit sharp green electroluminescence with the emission maximum being rather weakly dependent on the type of the fluorescent dopant. The obtained results show that a series of recently synthesized azafluoranthene dyes may be considered as efficient green fluorescent emitters for electroluminescent applications. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Over the past decade, organic dyes have attracted a considerable interest due to a number of potential applications in organic electronics among which the electroluminescence and relevant to it displays technology may be considered as prevailing [1–4]. In fact, the electronic properties of guest dye molecules, embedded as dopants into a host polymer layer, define the efficiency of electroluminescent radiation and its color gamut representing the basic characteristics of electroluminescent devices. A set of red, green, and blue emitters with high efficiency and color purity remain the key problems of the full-color display technology. This particularly explains considerable efforts of recent years being particularly undertaken in the organic synthesis and spectroscopic studies of novel fluorescent organic dyes [5–19] likewise the quantum-chemical modeling [18–25] aimed to understand the mechanisms of optical absorption and emission processes as well as the trends in their changes depending on molecular structure modification. Among the latter ones, the pyrazoloquinoline (PQ), likewise an extended number of its derivatives [17–19] are worth to be mentioned. They represent efficient fluorescent emitters, however, relevant electroluminesce of these dyes is limited by the light emission preferably in the blue spectral region [26–29]. Quite
⇑ Corresponding author. E-mail address: [email protected] (A.V. Kityk).
recently, Danel et al. [16] have cyclized several phenyl containing PQs into five membered regioisomers of azafluoranthene derivatives. Spectroscopic studies of these dyes have revealed a significant red shift of both excitation and emission spectra [30–33]. Relevant trends have been also reproduced within the semiempirical and DFT/TDDFT calculations [23–25,30–33]. Eventual fundamental interest to such phenomena is justified by practical needs in novel efficient fluorescent emitters in the green-yellow region of the visible spectra. In the present paper we demonstrate a series of novel electroluminescent devices based on the poly(N-vinylcarbazole) (PVK) active emitting layer doped with the azafluoranthene dyes.
2. Experimental Actual work deals with four azafluoranthene derivatives, referred for the sake of convenience as D1–D4, with a molecular structure as shown below namely 6-Methyl-1,3-diphenyl-3Hindeno[1,2,3-de]pyrazolo[3,4-b]quinoline (D1), 1,5,6-trimethyl3-phenyl-3H-indeno[1,2,3-de]pyrazolo[3,4-b]quinoline (D2), 1, 3-Diphenyl-3H-indeno[1,2,3-de]pyrazolo[3,4-b]quinoline (D3) and 7-methoxy-3-phenyl-1-propyl-3H-indeno[1,2,3-de]pyrazolo[3,4-b] quinoline (D4). Details concerning the synthesis of these dyes are described in Ref. [16,30,31,33]. They basically consist of two consecutive steps, namely a preparation of the starter compounds which represent phenyl containing PQ-derivatives which then by
0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.09.027
Please cite this article in press as: P. Gaß siorski et al., Opt. Mater. (2012), http://dx.doi.org/10.1016/j.optmat.2012.09.027
2
P. Gaß siorski et al. / Optical Materials xxx (2012) xxx–xxx
means of cyclization reaction under special conditions are transferred into azafluoranthene dyes. Commercially available reagents (Aldrich, Fluka, Merck) were used without further purification. The purity of the obtained compounds was checked by TLC. Elementary analysis were performed on a Perkin-Elmer 2400 CHN analyzer.
(a)
(b) (c) Fig. 1. Panel (a): sketch of the electroluminescent device (OLED) of ITO/PEDOT:PSS/ PVK:Dye/Ca/Al configuration. Panel (b): designed OLED with the active PVK:D1 layer emitting the green light (turn-on state). Panel (c): C.I.E. 1931 color coordinates X,Y and Z of electroluminescent radiation emitted by the PVK active layer doped with azafluoranthene dyes D1–D4 (concentration 1.2 mol.%).
The basic spectroscopic characterization of these dyes, i.e. their optical absorption and fluorescence spectra, may be found in a series of recently published works [30,31,33]. Analysis of these data shows that the first vibronic band (0 ? 00 transition) lies for all the dyes in the region 470–480 nm, although the first absorption maxima in most cases is centered at somewhat shorter wavelengths, 440–450 nm. Moreover, due to strongly overlapping vibronic bands the first absorption band is quite broad and considerably overlaps with the emission band of PVK polymer [34] used in the current work as the host material in OLED fabrication. Such feature of azafluoranthene dyes appear to be crucial in a sense of the efficiency of the energy transfer from the host to guest molecules which takes place in light emitting layer. The OLEDs were fabricated in accordance with the single active layer configuration as it is sketched in Fig. 1a. The overall procedure is similar to the one described in recently published work [35]. The anode substrate represents commercial glass plates with a deposited transparent ITO film (Aldrich, resistance 150 Ohm/sq). Slides were ultrasonically cleaned in detergent solution, afterwards rinsed first in the deionized water and next in THF for 80 min. Subsequently it was dried under technical vacuum at 100 °C for 30 min, coated by spin-coating with the hole injection layer of poly (3,4-ethylene dioxythiophene)-poly-(styrene sulfonate) (PEDOT:PSS) in dry argon atmosphere and then again dried in vacuum for 30 min and 100 °C. Organic dyes D1–D4 and PVK were taken in a molar ratio 1.2:100 which provides the best electroluminescence efficiency of the fabricated OLEDs. They were dissolved and carefully blended in THF, afterwards deposited on the substrates by spin-coating (speed 1000 rpm) in argon to remove dissolved air. Finally, the samples were then annealed at 70 °C for 30 min, afterwards the Ca cathode film (10 nm) was deposited by vacuum evaporation and protected with the aluminium film (100 nm). Following the description mentioned above the electroluminescence devices of ITO/PEDOT:PSS/PVK:Dye/Ca/Al configuration (Dye D1,D2,D3 or D4) have been designed, see Fig. 1b.
The electroluminescence spectra of ITO/PEDOT:PSS/PVK:Dye/ Ca/Al OLEDs were recorded using Shimadzu UVVIS 2101 scanning spectrophotometer applying the dc-bias voltage in a forward direction (ITO, positive; Ca/Al, negative). All the measurements were carried out at room temperature and all the devices were exposed in air without encapsulation.
3. Results and discussion Fig. 2 shows the luminance-voltage characteristics for a series of OLEDs ITO/PEDOT:PSS/PVK:Dye/Ca/Al. They are typical for the
Fig. 2. The luminance-voltage characteristic for the electroluminescent devices (OLEDs) of ITO/PEDOT:PSS/PVK:Dye/Ca/Al (Dye D1–D4) configuration. The curves are labeled according to the type of OLED active layer.
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Fig. 3. The normalized steady state electroluminescence spectra of ITO/PEDOT:PSS/ PVK:Dye/Ca/Al (Dye D1–D4) OLEDs recorded at forward dc-bias voltage of 10 V. The curves are labeled according to the type of OLED active layer.
devices of that type. Using these dependences one may estimate the turn-on voltages of relevant OLEDs. In particular, by extracting the voltage value at which the measured electroluminescence emission just exceeded the noise magnitude being detected at U = 0 we arrive with the magnitude for the turn-on voltage of about 5.1 ± 0.2 V in the case of OLED with PVK:D3 active layer and about 6.0 ± 0.2 V for the OLEDs with PVK:D1, PVK:D2 or PVK:D4 layers. Accordingly, there is no evident indication for a substituent effect on the turn-on voltage in this case. Fig. 3 shows the normalized steady state spectra of electroluminescent devices ITO/PEDOT:PSS/PVK:Dye/Ca/Al recorded at forward dc-bias voltage of 10 V. All OLEDs exhibit green electroluminescence, see also Fig. 1b, with nearly the same shape of the electroluminescent band being unstructured and narrow. The full width at half maximum (FWHM) equals about 40 nm in each case whereas the emission maximum slightly varies depending on the fluorescent dopant. For PVK:D1, PVK:D2, PVK:D3 and PVK:D4 active layers it corresponds to 510 nm, 512 nm, 515 nm and 511 nm, respectively. Compared to the OLEDs based on trimethyl derivatives of methoxyphenyl indenopyrazoloquinoline [35] the maxima of the electroluminescence spectra for PVK doped by D1–D4 dyes appear to be slightly blue shifted. More precise characterization of the electroluminescence color may be given within the C.I.E. (1931) standard, see color coordinates listed in Fig. 1c. We may compare now the electroluminescence spectra with the fluorescence ones measured in several organic solvent [30,31,33]. According to these works the fluorescence maxima of D1–D4 dyes appear in the region 500–570 nm and their spectral position considerably depends on the solvent polarity. While the solvent polarity changes from nonpolar cyclohexane to strongly polar acetonitrile all the dyes exhibit the hypsochromic (blue) shift of the first absorption band and the bathochromic (red) shift of the fluorescence band. Such opposite trends appear to be consistent with existing solvatochromic models based on semiempirical [30,31,33] and DFT/TDDFT [23,24] evaluations explaining it by a specific orientation of the dipole moments in the ground and excited states. The Lippert-Mataga polarity function F of PVK is equal approximately 0.148 (static dielectric constant e = 3.0, refractive index n = 1.69). In accordance with the Lippert-Mataga equation [36,37] the spectral position of the fluorescence maximum for the doped PVK: (D1–D4) layer is then expected in the region 510–520 nm. This pretty well agrees with actual experimental observation. Fig. 4 shows the log–log plots of the luminance-current characteristics of the electroluminescent devices ITO/PEDOT:PSS/
Fig. 4. Log–log luminance-current characteristic for the electroluminescent devices (OLEDs) of ITO/PEDOT:PSS/PVK:Dye/Ca/Al (Dye D1–D4) configuration. The solids lines are best linear fits in log–log scale; relevant labels near the lines represent the exponents extracted from the fits. The curves are labeled according to the type of OLED active layer.
PVK:Dye/Ca/Al (Dye D1–D4) recorded at slowly ramping dc-bias voltage. Detailed analysis of these dependences let us specify the mechanism of charge carrier injection. According to Ref. [38], in the injection controlled electroluminescence (ICEL) the emission intensity IEL is expected to be a power-type function of a dominating hole (Jh) or electron (Je) current, i.e. IEL / (Je,h)n. In the simplest approximation n 2, however the exponent may indeed somewhat deviate from this value taking into account the field dependence of the charge carrier mobilities. Alternatively, in the case of volume-controlled electroluminescence (VCEL) the positive and negative space charges are assumed to be largely canceled each other what implies that Jh = Je = J. This means that their decay is dominated by the second-order process of carrier recombination and the electroluminescent emission is always a linear function of the current [38]. Indeed, a linear character of IEL vs J in Fig. 4 evidently indicates on the power-like type of the measured dependences. Moreover, in all the cases the exponent, determined from the slopes of linear fits in the log–log plot, is quite close to the one. Such finding suggests that the electroluminescence is largely dominated by the VCEL mechanism. The photon emission may be caused by the two well known processes: (i) carrier recombination at a host molecule with a subsequent energy transfer to a guest molecule and followed by that quant emission or (ii) carrier trapping at the guest molecules leading to a direct exciton formation on the guest molecules with subsequent photon emission [39]. The carrier trapping mechanism can be in principle excluded since the HOMO energy level for D1–D4 dyes used in OLEDs fabrication does not exceed 5.85 eV [24,40] i.e. appears evidently below the HOMO–LUMO gap of the PVK polymer matrix ðE0H ¼ 5:6 eV; E0L ¼ 2:0 eV[41]). On the other hand, pure PVK layer is characterized by blue electroluminescence [42] with the emission band centered at about 415 nm and FWHW of about 83–86 nm. This blue emission appears to be completely quenched in the doped PVK layers giving rise of the emission in the green spectral region. The latter one indicates that efficient radiative energy transfer mechanism is quite likely although it does not exclude the electroluminescence emission originating from radiationless energy transfer mechanisms. Available data set is not sufficient to discriminate these processes. Fig. 5 presents the luminance vs the consumed electric power for the ITO/PEDOT:PSS/PVK:Dye/Ca/
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References
Fig. 5. The luminance vs the consumed electric power for the electroluminescent devices (OLEDs) of ITO/PEDOT:PSS/PVK:Dye/Ca/Al (Dye D1–D4) configuration. The power is referred to the ITO-electrode area S = 0.24 cm2. The curves are labeled according to the type of OLED active layer.
Al OLEDs which allows immediately compare the luminous efficiency of these devices. The most efficient appears to be the electroluminescent device with PVK:D4 active layer which by the factor of 1.5–2.0 is larger compared to other OLEDs. However, there is no evident correlation between this result and the fluorescence quantum yield reported in Refs. [30,31,33]. This suggests that crucial influence on the luminous efficiency should likely have the energy transfer mechanisms. 4. Conclusion In the present paper we have demonstrated the electroluminescent devices based on the PVK active emitting layer doped with several azafluoranthene derivatives. Measured steady state electroluminescence spectra, luminance-voltage and luminance-current characteristics provide basic characterization of the designed OLEDs. All the devices exhibit sharp green electroluminescence with the emission maximum being rather weakly dependent on the type of the fluorescent dopant. Our analysis reveals that the light emission is essentially dominated by the volume-controlled electroluminescence whereas the carrier trapping mechanism should be excluded from the consideration. Crucial influence on the luminous efficiency should be likely attributed to the energy transfer mechanisms. The obtained results suggest a series of recently synthesized azafluoranthene dyes as efficient sharp green fluorescent emitters for potential electroluminescent applications. Acknowledgement This work is sponsored by the National Science Centre within Grant 2011/03/BST7/0358.
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