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White light-emitting organic device with electroluminescent quantum dots and organic molecules
Organic Electronics 10 (2009) 1454–1458
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
White light-emitting organic device with electroluminescent quantum dots and organic molecules Badr Torriss, Alain Haché *, Serge Gauvin * Thin Films and Photonics Research Group, Département de Physique et d’astronomie, Université de Moncton, Moncton, NB, Canada E1A 3E9
a r t i c l e
i n f o
Article history: Received 17 June 2009 Received in revised form 5 August 2009 Accepted 11 August 2009 Available online 18 August 2009 PACS: 85.60.jb
a b s t r a c t Double-layer organic light-emitting devices are used in combination with quantum dots and blue-light-emitting organic molecules to produce electroluminescence covering the entire visible spectrum. Charges injection is shown to be an important mechanism of excitation in quantum dots. Applications to light-emitting devices with greater spectral flexibility are discussed. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Electroluminescence organic Quantum dots Förster energy transfer Injection of electron-hole pairs
1. Introduction Self-assembled semiconductor quantum dots exhibit luminescence that is easily adjustable at the fabrication level. When the surface is capped with certain molecules, the particles become miscible in a variety of organic materials, thereby making possible their integration into organic photonic devices. In recent years, semiconductor quantum dots have been used in conjunction with organic lightemitting devices (OLEDs) to produce white light or emission spectra with adjustable properties [1–3]. In this article, we report a device based on poly(N-vinylcarbazole), abbreviated as PVK, and tris-(8-hydroxyquinoline)aluminum salt, abbreviated as Alq3, that uses red-emitting quantum dots (QDs) as well as blue-emitting organic molecules (iridium(III) bis(2-(4,6-difluorephenyl) pyridinato-N,C2), here-
* Corresponding authors. Tel.: +1 506 858 4938; fax: +1 506 858 4541 (A. Haché), tel.: +1 506 858 4324; fax: +1 506 858 4541 (S. Gauvin). E-mail addresses: [email protected] (A. Haché), serge.gauvin@ umoncton.ca (S. Gauvin). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.08.007
after referred to as Ir(III)DP) to produce white light. Detailed spectral analysis and the mechanisms of luminescence are presented. The experimental results show that the excitation mechanism of QDs via charge injection cannot be neglected (in addition to other excitation mechanisms such as exciton transfer and photoluminescence). It appears that charge injection is the dominant mechanism in our device. It is found that greater flexibility is attained in terms of device spectral properties when using three types of photoemitters. 2. Experimental The structure of the reference device consists of ITO/ PEDOT: PSS/PVK/Alq3/Al. A hole injection polymer poly(3, 4-ethylenedioxythiophene) poly(styrenesulfonate) (known as PEDOT: PSS) was spin-deposited onto an indium tin oxide (ITO) film covering a glass substrate. The layer was heated for 10 min at 200 °C. A 50 nm-thick hole-transporting layer of PVK (Sigma–Aldrich, Mw = 42000) was dipcoated from a PVK:toluene solution with a 0.1:10 weight
B. Torriss et al. / Organic Electronics 10 (2009) 1454–1458
ratio. A 30 nm-thick electron-transporting layer of Alq3 (Sigma–Aldrich, 99.995%) was sublimated at 6.66 104 Pa pressure. Finally, a 200 nm aluminum cathode was deposited by thermal evaporation in the same vacuum to complete the device. Fig. 1 shows the energy band diagram of the overall structure. The PVK/Alq3 bi-layer yields a greenish, 50 nm-wide electroluminescence spectrum. To widen the spectrum and achieve white light emission, red-emitting quantum dots were incorporated in PVK. CdSe/ZnS core–shell quantum dots (Evident Technologies Ltd.) of 5.8 nm in diameter were dispersed in the PVK:toluene solution. The weight ratio (R) of CdSe/ZnS to PVK was set at 0.9:10. To add spectral components to the blue side of the electroluminescence spectrum, it is necessary to use quantum dots with a smaller diameter. However, we found that such particles only emit weakly and tend to be unstable because of their large surface/volume ratio. A different approach is to use phosphorescent molecules. To this end, we dissolved bluelight-emitting molecules Ir(III)DP into PVK to enhance the spectrum in the 450–500 nm region. The weight ratios of CdSe/ZnS to Ir(III)DP and to PVK are 0.9:10 and 8.3:10, respectively. 3. Results and discussion Fig. 2 shows the normalized electroluminescence spectra of the device with and without the addition of quantum dots or Ir(III)DP. The luminescence of the reference structure (without quantum dots or molecules) peaks at 526 nm and takes a greenish appearance, whereas quantum dots produce a luminescence spectrum centered at 620 nm with a 30 nm FWHM. The color pictures in Fig. 3 show color coordinates that gradually approach that of pure white light (0.33, 0.33). For devices without Ir(III)DP, the external quantum yield is 0.45% (at a bias of 14 V) without QDs and 0.40% (at a bias of 14 V) when QDs are included into the device at weight ratio R = 9%. Here, we stress that our main goal was not to optimize the device but rather to understand how the QDs affect the operating mechanisms of OLEDs. For this task the measurement of quantum yield is especially valuable.
Electron energy (eV)
Vacuum level (0 eV)
2.2 3.2 ITO
PEDOT: PSS
EA = 4.4 QD
5.2
PVK 50 nm
Alq3 30 nm
5.8
5.9
4.8
4.25 Al 200 nm
IE = 6.5 Fig. 1. Energy band diagram of the PVK/Alq3 reference organic device [4,5]. Electron affinity (EA) and ionization energy (IE) values of QDs are taken from Ref. [11].
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Fig. 2. Electroluminescence spectrum of (a) the reference PVK/Alq3 device, (b) with red-emitting quantum dots and (c) with Ir(III)DP plus red-emitting quantum dots. Device (c), the only one with blue-emitting molecules, shows a blue shift and produces white light of better quality.
The arrangement offers a high degree of flexibility. The characteristic of the device’s electroluminescence spectrum can be controlled by choosing various ratios of Ir(III)DP and quantum dot to PVK, and by using different quantum dot sizes. However, because the three types of light emitters interact in a non-trivial way, the relative emission efficiency is not a simple function of concentration or voltage. To fully exploit and tailor the device to a specific application, it is important to understand the luminescence mechanisms involved. For example, quantum dots can luminesce via Förster energy transfer of excitons [6–9] and/or injection of electron–hole pairs [10–13]. Having a large absorption cross section, these particles can also, in theory, be excited by absorbing photons generated inside the device with energies above the CdSe band gap (<700 nm). The rest of this paper focuses on exploring these mechanisms. Fig. 4 shows the surface morphology of various PVK films with and without quantum dots. The microscope pictures show that the surface of pure PVK is homogeneous on a small scale. On the other hand, PVK with quantum dots (R = 3%, image b), shows small aggregates dispersed on the surface. At higher concentration, R = 9%, the surface morphology suggests aggregates, on the same scale as reported by Son et al. in Ref. [15]. In their work on PVK films with quantum dots, TEM images reveal PVK clusters enveloped with quantum dots. Aggregation, it seems, takes the form of a blend of the two materials, making the film effectively uniform on a scale above 300 nm. The fact that quantum dots would be thus embedded into the film (and not just concentrated at the surface) is an important element in explaining the operation of the OLED. We now proceed with an analysis of the emission profiles of devices with and without quantum dots. It is significant that the EL spectrum of the reference OLED (Fig. 2a) shows no evidence of light emission by the PVK. Since excitons are mainly formed at PVK/Alq3 interface, and since holes easily migrate from PVK to Alq3 (the potential barrier
1456
B. Torriss et al. / Organic Electronics 10 (2009) 1454–1458
Fig. 3. Pictures of the devices (a–c) of Fig. 2. The color coordinates of (a), (b) and (c) are (0.3, 0.55), (0.43, 0.45) and (0.24, 0.415), respectively.
Fig. 4. Atomic force microscope images the surface morphology of various PVK: QDs layers.
is only 0.1 eV versus the 1 eV barrier for electrons going from Alq3 into PVK), electron–hole recombination occurs principally in the Alq3 layer, in accordance with previous works [16,17]. Fig. 5 shows the electroluminescence spectrum of the PVK/Alq3 device with quantum dots. At low voltages, near the threshold voltage of 12 V, the spectrum is dominated by Alq3 emission, but as the voltage increases, the contribution from quantum dots exceeds that of Alq3.
The overlap between the QD absorption profile and the photoluminescence spectrum of Alq3 [9] suggests the possibility of exciton transfer from Alq3 to QDs via the Förster mechanism [18]. Coe et al. [7] demonstrated that, for a trilayer hybrid QDs/organic LED, the Förster energy transfer of excitons from organic materials to the QDs dominates the EL process rather than charge injection into QDs. As Fig. 6 illustrates, at low voltages the majority of excitons recombining in Alq3 do so very close to the interface with
Fig. 5. Emission spectra profiles of ITO/PEDOT: PSS/PVK: QD/Alq3/Al devices with quantum dots. The relative contribution of QDs and Alq3 varies with voltage, as shown in inset.
B. Torriss et al. / Organic Electronics 10 (2009) 1454–1458
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Fig. 6. Exciton generation and luminescence processes in a quantum dotdoped OLED.
PVK. This favours excitonic pumping of QDs, which are located in the PVK. Increasing voltage causes excitons to recombine further into Alq3 and farther away from QDs, which does not favour the Förster mechanism. (Exciton energy transfer to quantum dots occurs within the Förster radius df). In our devices, however, quantum dots consistently emit more efficiently at higher voltages, in spite of the fact that electroluminescence from Alq3 saturates (see Fig. 5). Furthermore, QDs emit steadily even when emission from Alq3 declines with time. This trend is clearly shown in Fig. 7, whereby the red emission of QDs exhibits remarkable stability for over 15 s. These data suggests that Förster energy transfer is not the dominant excitation process of QDs in our OLEDs. Instead, charge injection appears to be an important additional mechanism of electroluminescence in QDs. Fig. 8 shows how doping PVK with QDs does not change the shape of the I–V curve. In both concentrations shown, the curves are best fitted by a space-charge-limited conduction model (SCLC) with an exponential distribution of traps [20]. The current density J is well described by the equation [19]
Fig. 8. Current–voltage curves of ITO/PEDOT: PSS/PVK: (CdSe/ZnS)/Alq3/ Al devices with CdSe/ZnS mass loadings of 0% and 9%.
J ¼ qð1mÞ em lef
m mþ1 mþ1 Nef m 2m þ 1 V m 2mþ1 mþ1 Ntr m þ 1 d
where q is the electron charge, m is a characteristic energy distribution parameter of traps, e is the electrical permittivity, lef is the effective mobility, Nef is the density of states in the transport band, Ntr is the total concentration of traps, V is the applied voltage and d is the sample thickness. However, doping does induce a decrease of current density and an increase of m from 7 to 12. According to SCLC theoretical framework, this variation in m indicates that traps are deeper in the doped structure. As it should be and shown in Fig. 1, the electron affinity level of the QDs is well below the PVK LUMO level. This suggests that electrons could easily be trapped by QDs. Then, doping PVK with QDs results in, (1) an increase in traps concentration and thus a decrease in current density and (2) an increase in m because QDs act as deep traps. In contrast, the PVK HOMO level is well above the QDs ionization energy level. The holes injected in the PVK/QDs layer then diffuse through the PVK molecules. Because holes can tunnel through the Zn shell into the valence band of the CdSe quantum dots (attracted by the trapped electron), the QDs can thus emit light via electron–hole pair recombination. Therefore, the charge injection into QDs appears to be an important mechanism of excitation in quantum dots which adds to the Förster energy transfer. On the other hand, the mechanism of photoemission from Ir(III)DP also appears to be from current excitation. Indeed, photoexcitation would require absorption of light at wavelengths below 450 nm, whereas very little light is emitted by Alq3 bellow 500 nm. 4. Conclusions
Fig. 7. The temporal evolution of the electroluminescence ITO/PEDOT: PSS/PVK: (CdSe/ZnS)/Alq3/Al device a constant voltage of 30 V applied.
In conclusion, we have demonstrated an hybrid organic device for generation of electroluminescence with adjustable properties. By using different excitation mechanisms for light emission, quantum dots add flexibility to OLED composition and can be used towards the manipulation
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B. Torriss et al. / Organic Electronics 10 (2009) 1454–1458
of their spectral emission profile in order to produce, among others, white light of high quality. Acknowledgements This work was supported by NSERC, the Canada Research Chair program, the New Brunswick Innovation Fund and the Atlantic Innovation Fund. References [1] J.H. Park, J.Y. Kim, B.D. Chin, Y.Ch. Kim, J.K. Kim, O.O. Park, Nanotechnology 15 (2004) 1217. [2] Y. Xuan, D. Pan, N. Zhao, X. Ji, Dongge Ma, Nanotechnology 17 (2006) 4966. [3] P.O. Anikeeva, J.E. Halpert, M.G. Bawendi, V. Bulovic, Nano Letters 7 (2007) 2196. [4] R.K. Ligman, L. Mangolini, U.R. Kortshagen, S.A. Campbella, Applied Physics Letters 90 (2007) 061116. [5] K. Seki, E. Ito, H. Ishii, Synthetic Metals 91 (1997) 137.
[6] S. Coe-Sullivan, W.K. Woo, M. Bawendi, V. Bulovic, Nature 420 (2002) 800. [7] S. Coe-Sullivan, W.K. Woo, T.S. Steckel, M. Bawendi, V. Bulovic, Organic Electronics 4 (2003) 123. [8] Y. Li, A. Rizzo, M. Mazzeo, L. Carbone, L. Manna, R. Cingolani, G. Gigli, Applied Physics 97 (2005) 113501. [9] P.O. Anikeea, C.F. Madigan, J.E. Halpert, M.G. Bawendi, V. Bulovic, Physical Review B 78 (2008) 085434. [10] H. Mattoussi, L.H. Radzilowski, B.O. Dabbousi, M.F. Rubner, Journal of Applied Physics 83 (1998) 7965. [11] R.A.M. Hikmet, D.V. Talapin, H. Weller, Journal of Applied Physics 93 (2003) 3509. [12] H. Yang, P.H. Holloway, B.B. Ratna, Journal of Applied Physics 93 (2003) 586. [13] S. Chaudhary, M. Ozkan, Warren C.W. Chan, Applied Physics Letters 84 (2004) 2925. [15] D.-I. Son, J.-H. Kim, D.-H. Park, W.K. Choi, F. Li, J.H. Ham, T.W. Kim, Nanotechnology 19 (2008) 055204. [16] C.W. Tang, S.A. VanSlyke, Applied Physics Letters 51 (1987) 913. [17] P.E. Burrows, Z. Shen, V. Bulovic, D.M. McCarty, S.R. Forrest, Applied Physics 79 (1997) 7991. [18] Th. Förster, Annals of Physics 437 (1948) 55. [19] J. Kalinowski, Organic Light-Emitting Diodes, CRC Press, 2005. [20] B. Torriss, M.Sc. thesis, Université de Moncton, Canada, 2009.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
White light-emitting organic device with electroluminescent quantum dots and organic molecules Badr Torriss, Alain Haché *, Serge Gauvin * Thin Films and Photonics Research Group, Département de Physique et d’astronomie, Université de Moncton, Moncton, NB, Canada E1A 3E9
a r t i c l e
i n f o
Article history: Received 17 June 2009 Received in revised form 5 August 2009 Accepted 11 August 2009 Available online 18 August 2009 PACS: 85.60.jb
a b s t r a c t Double-layer organic light-emitting devices are used in combination with quantum dots and blue-light-emitting organic molecules to produce electroluminescence covering the entire visible spectrum. Charges injection is shown to be an important mechanism of excitation in quantum dots. Applications to light-emitting devices with greater spectral flexibility are discussed. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Electroluminescence organic Quantum dots Förster energy transfer Injection of electron-hole pairs
1. Introduction Self-assembled semiconductor quantum dots exhibit luminescence that is easily adjustable at the fabrication level. When the surface is capped with certain molecules, the particles become miscible in a variety of organic materials, thereby making possible their integration into organic photonic devices. In recent years, semiconductor quantum dots have been used in conjunction with organic lightemitting devices (OLEDs) to produce white light or emission spectra with adjustable properties [1–3]. In this article, we report a device based on poly(N-vinylcarbazole), abbreviated as PVK, and tris-(8-hydroxyquinoline)aluminum salt, abbreviated as Alq3, that uses red-emitting quantum dots (QDs) as well as blue-emitting organic molecules (iridium(III) bis(2-(4,6-difluorephenyl) pyridinato-N,C2), here-
* Corresponding authors. Tel.: +1 506 858 4938; fax: +1 506 858 4541 (A. Haché), tel.: +1 506 858 4324; fax: +1 506 858 4541 (S. Gauvin). E-mail addresses: [email protected] (A. Haché), serge.gauvin@ umoncton.ca (S. Gauvin). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.08.007
after referred to as Ir(III)DP) to produce white light. Detailed spectral analysis and the mechanisms of luminescence are presented. The experimental results show that the excitation mechanism of QDs via charge injection cannot be neglected (in addition to other excitation mechanisms such as exciton transfer and photoluminescence). It appears that charge injection is the dominant mechanism in our device. It is found that greater flexibility is attained in terms of device spectral properties when using three types of photoemitters. 2. Experimental The structure of the reference device consists of ITO/ PEDOT: PSS/PVK/Alq3/Al. A hole injection polymer poly(3, 4-ethylenedioxythiophene) poly(styrenesulfonate) (known as PEDOT: PSS) was spin-deposited onto an indium tin oxide (ITO) film covering a glass substrate. The layer was heated for 10 min at 200 °C. A 50 nm-thick hole-transporting layer of PVK (Sigma–Aldrich, Mw = 42000) was dipcoated from a PVK:toluene solution with a 0.1:10 weight
B. Torriss et al. / Organic Electronics 10 (2009) 1454–1458
ratio. A 30 nm-thick electron-transporting layer of Alq3 (Sigma–Aldrich, 99.995%) was sublimated at 6.66 104 Pa pressure. Finally, a 200 nm aluminum cathode was deposited by thermal evaporation in the same vacuum to complete the device. Fig. 1 shows the energy band diagram of the overall structure. The PVK/Alq3 bi-layer yields a greenish, 50 nm-wide electroluminescence spectrum. To widen the spectrum and achieve white light emission, red-emitting quantum dots were incorporated in PVK. CdSe/ZnS core–shell quantum dots (Evident Technologies Ltd.) of 5.8 nm in diameter were dispersed in the PVK:toluene solution. The weight ratio (R) of CdSe/ZnS to PVK was set at 0.9:10. To add spectral components to the blue side of the electroluminescence spectrum, it is necessary to use quantum dots with a smaller diameter. However, we found that such particles only emit weakly and tend to be unstable because of their large surface/volume ratio. A different approach is to use phosphorescent molecules. To this end, we dissolved bluelight-emitting molecules Ir(III)DP into PVK to enhance the spectrum in the 450–500 nm region. The weight ratios of CdSe/ZnS to Ir(III)DP and to PVK are 0.9:10 and 8.3:10, respectively. 3. Results and discussion Fig. 2 shows the normalized electroluminescence spectra of the device with and without the addition of quantum dots or Ir(III)DP. The luminescence of the reference structure (without quantum dots or molecules) peaks at 526 nm and takes a greenish appearance, whereas quantum dots produce a luminescence spectrum centered at 620 nm with a 30 nm FWHM. The color pictures in Fig. 3 show color coordinates that gradually approach that of pure white light (0.33, 0.33). For devices without Ir(III)DP, the external quantum yield is 0.45% (at a bias of 14 V) without QDs and 0.40% (at a bias of 14 V) when QDs are included into the device at weight ratio R = 9%. Here, we stress that our main goal was not to optimize the device but rather to understand how the QDs affect the operating mechanisms of OLEDs. For this task the measurement of quantum yield is especially valuable.
Electron energy (eV)
Vacuum level (0 eV)
2.2 3.2 ITO
PEDOT: PSS
EA = 4.4 QD
5.2
PVK 50 nm
Alq3 30 nm
5.8
5.9
4.8
4.25 Al 200 nm
IE = 6.5 Fig. 1. Energy band diagram of the PVK/Alq3 reference organic device [4,5]. Electron affinity (EA) and ionization energy (IE) values of QDs are taken from Ref. [11].
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Fig. 2. Electroluminescence spectrum of (a) the reference PVK/Alq3 device, (b) with red-emitting quantum dots and (c) with Ir(III)DP plus red-emitting quantum dots. Device (c), the only one with blue-emitting molecules, shows a blue shift and produces white light of better quality.
The arrangement offers a high degree of flexibility. The characteristic of the device’s electroluminescence spectrum can be controlled by choosing various ratios of Ir(III)DP and quantum dot to PVK, and by using different quantum dot sizes. However, because the three types of light emitters interact in a non-trivial way, the relative emission efficiency is not a simple function of concentration or voltage. To fully exploit and tailor the device to a specific application, it is important to understand the luminescence mechanisms involved. For example, quantum dots can luminesce via Förster energy transfer of excitons [6–9] and/or injection of electron–hole pairs [10–13]. Having a large absorption cross section, these particles can also, in theory, be excited by absorbing photons generated inside the device with energies above the CdSe band gap (<700 nm). The rest of this paper focuses on exploring these mechanisms. Fig. 4 shows the surface morphology of various PVK films with and without quantum dots. The microscope pictures show that the surface of pure PVK is homogeneous on a small scale. On the other hand, PVK with quantum dots (R = 3%, image b), shows small aggregates dispersed on the surface. At higher concentration, R = 9%, the surface morphology suggests aggregates, on the same scale as reported by Son et al. in Ref. [15]. In their work on PVK films with quantum dots, TEM images reveal PVK clusters enveloped with quantum dots. Aggregation, it seems, takes the form of a blend of the two materials, making the film effectively uniform on a scale above 300 nm. The fact that quantum dots would be thus embedded into the film (and not just concentrated at the surface) is an important element in explaining the operation of the OLED. We now proceed with an analysis of the emission profiles of devices with and without quantum dots. It is significant that the EL spectrum of the reference OLED (Fig. 2a) shows no evidence of light emission by the PVK. Since excitons are mainly formed at PVK/Alq3 interface, and since holes easily migrate from PVK to Alq3 (the potential barrier
1456
B. Torriss et al. / Organic Electronics 10 (2009) 1454–1458
Fig. 3. Pictures of the devices (a–c) of Fig. 2. The color coordinates of (a), (b) and (c) are (0.3, 0.55), (0.43, 0.45) and (0.24, 0.415), respectively.
Fig. 4. Atomic force microscope images the surface morphology of various PVK: QDs layers.
is only 0.1 eV versus the 1 eV barrier for electrons going from Alq3 into PVK), electron–hole recombination occurs principally in the Alq3 layer, in accordance with previous works [16,17]. Fig. 5 shows the electroluminescence spectrum of the PVK/Alq3 device with quantum dots. At low voltages, near the threshold voltage of 12 V, the spectrum is dominated by Alq3 emission, but as the voltage increases, the contribution from quantum dots exceeds that of Alq3.
The overlap between the QD absorption profile and the photoluminescence spectrum of Alq3 [9] suggests the possibility of exciton transfer from Alq3 to QDs via the Förster mechanism [18]. Coe et al. [7] demonstrated that, for a trilayer hybrid QDs/organic LED, the Förster energy transfer of excitons from organic materials to the QDs dominates the EL process rather than charge injection into QDs. As Fig. 6 illustrates, at low voltages the majority of excitons recombining in Alq3 do so very close to the interface with
Fig. 5. Emission spectra profiles of ITO/PEDOT: PSS/PVK: QD/Alq3/Al devices with quantum dots. The relative contribution of QDs and Alq3 varies with voltage, as shown in inset.
B. Torriss et al. / Organic Electronics 10 (2009) 1454–1458
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Fig. 6. Exciton generation and luminescence processes in a quantum dotdoped OLED.
PVK. This favours excitonic pumping of QDs, which are located in the PVK. Increasing voltage causes excitons to recombine further into Alq3 and farther away from QDs, which does not favour the Förster mechanism. (Exciton energy transfer to quantum dots occurs within the Förster radius df). In our devices, however, quantum dots consistently emit more efficiently at higher voltages, in spite of the fact that electroluminescence from Alq3 saturates (see Fig. 5). Furthermore, QDs emit steadily even when emission from Alq3 declines with time. This trend is clearly shown in Fig. 7, whereby the red emission of QDs exhibits remarkable stability for over 15 s. These data suggests that Förster energy transfer is not the dominant excitation process of QDs in our OLEDs. Instead, charge injection appears to be an important additional mechanism of electroluminescence in QDs. Fig. 8 shows how doping PVK with QDs does not change the shape of the I–V curve. In both concentrations shown, the curves are best fitted by a space-charge-limited conduction model (SCLC) with an exponential distribution of traps [20]. The current density J is well described by the equation [19]
Fig. 8. Current–voltage curves of ITO/PEDOT: PSS/PVK: (CdSe/ZnS)/Alq3/ Al devices with CdSe/ZnS mass loadings of 0% and 9%.
J ¼ qð1mÞ em lef
m mþ1 mþ1 Nef m 2m þ 1 V m 2mþ1 mþ1 Ntr m þ 1 d
where q is the electron charge, m is a characteristic energy distribution parameter of traps, e is the electrical permittivity, lef is the effective mobility, Nef is the density of states in the transport band, Ntr is the total concentration of traps, V is the applied voltage and d is the sample thickness. However, doping does induce a decrease of current density and an increase of m from 7 to 12. According to SCLC theoretical framework, this variation in m indicates that traps are deeper in the doped structure. As it should be and shown in Fig. 1, the electron affinity level of the QDs is well below the PVK LUMO level. This suggests that electrons could easily be trapped by QDs. Then, doping PVK with QDs results in, (1) an increase in traps concentration and thus a decrease in current density and (2) an increase in m because QDs act as deep traps. In contrast, the PVK HOMO level is well above the QDs ionization energy level. The holes injected in the PVK/QDs layer then diffuse through the PVK molecules. Because holes can tunnel through the Zn shell into the valence band of the CdSe quantum dots (attracted by the trapped electron), the QDs can thus emit light via electron–hole pair recombination. Therefore, the charge injection into QDs appears to be an important mechanism of excitation in quantum dots which adds to the Förster energy transfer. On the other hand, the mechanism of photoemission from Ir(III)DP also appears to be from current excitation. Indeed, photoexcitation would require absorption of light at wavelengths below 450 nm, whereas very little light is emitted by Alq3 bellow 500 nm. 4. Conclusions
Fig. 7. The temporal evolution of the electroluminescence ITO/PEDOT: PSS/PVK: (CdSe/ZnS)/Alq3/Al device a constant voltage of 30 V applied.
In conclusion, we have demonstrated an hybrid organic device for generation of electroluminescence with adjustable properties. By using different excitation mechanisms for light emission, quantum dots add flexibility to OLED composition and can be used towards the manipulation
1458
B. Torriss et al. / Organic Electronics 10 (2009) 1454–1458
of their spectral emission profile in order to produce, among others, white light of high quality. Acknowledgements This work was supported by NSERC, the Canada Research Chair program, the New Brunswick Innovation Fund and the Atlantic Innovation Fund. References [1] J.H. Park, J.Y. Kim, B.D. Chin, Y.Ch. Kim, J.K. Kim, O.O. Park, Nanotechnology 15 (2004) 1217. [2] Y. Xuan, D. Pan, N. Zhao, X. Ji, Dongge Ma, Nanotechnology 17 (2006) 4966. [3] P.O. Anikeeva, J.E. Halpert, M.G. Bawendi, V. Bulovic, Nano Letters 7 (2007) 2196. [4] R.K. Ligman, L. Mangolini, U.R. Kortshagen, S.A. Campbella, Applied Physics Letters 90 (2007) 061116. [5] K. Seki, E. Ito, H. Ishii, Synthetic Metals 91 (1997) 137.
[6] S. Coe-Sullivan, W.K. Woo, M. Bawendi, V. Bulovic, Nature 420 (2002) 800. [7] S. Coe-Sullivan, W.K. Woo, T.S. Steckel, M. Bawendi, V. Bulovic, Organic Electronics 4 (2003) 123. [8] Y. Li, A. Rizzo, M. Mazzeo, L. Carbone, L. Manna, R. Cingolani, G. Gigli, Applied Physics 97 (2005) 113501. [9] P.O. Anikeea, C.F. Madigan, J.E. Halpert, M.G. Bawendi, V. Bulovic, Physical Review B 78 (2008) 085434. [10] H. Mattoussi, L.H. Radzilowski, B.O. Dabbousi, M.F. Rubner, Journal of Applied Physics 83 (1998) 7965. [11] R.A.M. Hikmet, D.V. Talapin, H. Weller, Journal of Applied Physics 93 (2003) 3509. [12] H. Yang, P.H. Holloway, B.B. Ratna, Journal of Applied Physics 93 (2003) 586. [13] S. Chaudhary, M. Ozkan, Warren C.W. Chan, Applied Physics Letters 84 (2004) 2925. [15] D.-I. Son, J.-H. Kim, D.-H. Park, W.K. Choi, F. Li, J.H. Ham, T.W. Kim, Nanotechnology 19 (2008) 055204. [16] C.W. Tang, S.A. VanSlyke, Applied Physics Letters 51 (1987) 913. [17] P.E. Burrows, Z. Shen, V. Bulovic, D.M. McCarty, S.R. Forrest, Applied Physics 79 (1997) 7991. [18] Th. Förster, Annals of Physics 437 (1948) 55. [19] J. Kalinowski, Organic Light-Emitting Diodes, CRC Press, 2005. [20] B. Torriss, M.Sc. thesis, Université de Moncton, Canada, 2009.