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Small-sized Al nanoparticles as electron injection hotspots in inverted organic light-emitting diodes
Organic Electronics 28 (2016) 88e93
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Small-sized Al nanoparticles as electron injection hotspots in inverted organic light-emitting diodes Xia Lou, Xin-Xin Wang, Chang-Hai Liu, Jie Liu, Ze-Qun Cui, Zhi-Hao Lu, Xu Gao, Sui-Dong Wang* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu, 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2015 Received in revised form 11 October 2015 Accepted 11 October 2015 Available online xxx
Al nanoparticles, with small size and ultralow coverage on ITO, can play a key role as the electron injection hotspots in both the inverted fluorescent and phosphorescent organic light-emitting diodes. The presence of the hotspots greatly reduces the operational voltage and improves the current efficiency of the devices, which are strongly dependent on the hotspot size. The microscopic and spectroscopic characterization demonstrate that the small-sized hotspots have a minor influence on the surface roughness, transparency and work function of ITO. The hotspot effect is ascribed to the highly efficient electron injection at the Al nanoparticles enhanced by the local electric field, and a physical model is proposed to clarify this mechanism. The finding indicates a promising strategy by design and craft of the injection hotspots in nanoscale to facilitate carrier injection in organic thin film devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Inverted organic light-emitting diodes Al nanoparticles Electron injection Field emission
1. Introduction Organic light-emitting diodes (OLEDs) have been greatly developed since their first demonstration by Tang and VanSlyke in 1987 [1e4], and now are booming in the markets of solid-state lighting and active-matrix (AM) flat-panel displays. However, the fundamental issues such as carrier injection mechanism and exciton processes in OLEDs [5e9], and the technical challenges such as compatibility with thin film transistor (TFT) backplanes [10,11], still need to be further addressed. In particular, commercial backplanes for AM-OLEDs consist of amorphous Si or oxide TFT arrays, which are typically n-type transistors [12,13]. When conventional OLEDs with indium tin oxide (ITO) as the bottom anode are driven by n-type TFTs, the ITO anode is connected to the source electrode of TFTs. This configuration may cause the instability of the source voltage in TFTs and accordingly the image sticking issue in displays [10,11]. Thus, it is highly desired to adopt inverted bottom-emission OLEDs with ITO as the bottom cathode for practical display applications [14e21], where the ITO cathode is connected to the drain electrode of n-type TFTs. ITO possesses high surface work function of about 4.8 eV, and
* Corresponding author. E-mail address: [email protected] (S.-D. Wang). http://dx.doi.org/10.1016/j.orgel.2015.10.012 1566-1199/© 2015 Elsevier B.V. All rights reserved.
the lowest unoccupied molecular orbitals (LUMO) of most organic electron transporting layers (ETL) used in OLEDs range from 2.5 eV to 3.5 eV [9]. It thus implies a very high electron injection barrier at the ITO/ETL interfaces, which can be the bottleneck to hinder the realization of high-performance inverted OLEDs. The reported approaches toward efficient electron injection from ITO include: (1) surface coating with low work function materials such as Mg- and Li-doped ETL [14,15], which faces the ambient instability issue; and (2) introducing an interface dipole layer such as an appropriate metal oxide layer [16e19], which is not compatible with vacuum sublimation and often needs annealing treatments. Both the attempts are to lower the effective work function of ITO, and consequently reduce the electron injection barrier from the bottom cathode. However, we herein demonstrate that the work function reduction is not necessarily the case to realize efficient electron injection in inverted OLEDs. Instead, the electron injection hotspots in nanoscale can be designed and achieved to efficiently emit electrons from ITO while maintaining its high transparency. In this report, the small-sized Al nanoparticles (NPs) prepared simply by thermal evaporation are utilized as the electron injection hotspots on ITO, which greatly reduce the operational voltage and improve the current efficiency for both the inverted fluorescent and phosphorescent OLEDs (F-OLEDs and P-OLEDs, respectively). The combination of microscopic and electronic characterization indicates that the electron injection enhancement is determined
X. Lou et al. / Organic Electronics 28 (2016) 88e93
neither by the surface coverage of the Al NPs, nor by the effective work function of the modified ITO surface. The size of the Al NPs turns out to be critical to the hotspot effect, and thus the reinforced local electric field around the hotspots is proposed to be responsible for the electron injection enhancement in the inverted OLEDs. It can be a general mechanism and may offer new opportunities for the facilitation of carrier injection in organic electronic/optoelectronic devices. 2. Experimental 2.1. Device fabrication The device structures of the inverted F-OLEDs and P-OLEDs are illustrated in Fig. 1a and b, respectively, where the nominal thickness of thermally evaporated Al on ITO varies from 0 to 2 nm. Al is selected due to the advantages of low cost and simple preparation, and its deposition is compatible with the device fabrication process. The fluorescent unit is composed of LiF-doped 4,7-diphenyl-1,10phenanthroline (Bphen:LiF, 5 nm)/tris(8-hydroxyquinoline) aluminum (Alq3, 30 nm)/N,N0 -bis(naphthalen-1-yl)-N,N0 bis(phenyl)-benzidine (NPB, 30 nm), where Bphen:LiF has an optimal doping ratio of 30 wt%. The phosphorescent unit is composed of Bphen:LiF (5 nm)/Bphen (15 nm)/bis(2phenylpyridine)-iridium-acetylacetonate-doped 4,40 -N,N0 -dicarbazole-biphenyl [CBP:Ir(ppy)2(acac), 15 nm]/4,40 ,400 -tris(N-carbazolyl)-triphenyl amine (TCTA, 30 nm), where CBP:Ir(ppy)2(acac) has an optimal doping ratio of 8 wt%. Each category of devices have identical top anode of MoO3 (5 nm)/Al (100 nm), and the only variable is the Al quantity on the bottom cathode, which can be simply controlled by the deposited thickness of Al. After routine cleaning and drying, the ITO-coated glass substrates were treated by UV-ozone for 15 min. Subsequently, all the layers were successively deposited in a high-vacuum deposition system (Trovato MFG, ~1 106 mbar) equipped with multiple thermal sublimation sources, where the film thickness was controlled by a precalibrated quartz microbalance and a switchable shutter. Al NPs spontaneously formed upon a sub-monolayer deposition of Al on ITO, which was obtained by thermal evaporation at a deposition rate of 0.01e0.02 nm/s. All the devices for comparison were fabricated in a same experimental round, and were encapsulated in a glove box after the fabrication. 2.2. Device characterization The electrical and optoelectronic characteristics of the inverted OLEDs were measured using a programmable spectroradiometer (Photo Research PR-655), which is connected to a sourcemeter
Fig. 1. Device structures of (a) inverted fluorescent OLEDs and (b) inverted phosphorescent OLEDs, where Al NPs are deposited on ITO by thermal evaporation.
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(Keithley 2400). The inverted OLEDs were biased with the bottom ITO and top Al electrodes as the cathode and anode, respectively. The microscopic morphology of the Al NPs was characterized by transmission electron microscopy (TEM, FEI Quanta FRG 200F) combined with energy dispersive spectroscopy (EDS). The surface roughness of bare and Al-modified ITO was characterized by atomic force microscopy (AFM, Bruker Dimension Icon) in peak-force tapping mode. The work function of bare and Al-modified ITO was measured by ultraviolet photoelectron spectroscopy (UPS, He I, Kratos Axis Ultra DLD) in ultrahigh vacuum. The transmission measurements of bare and Al-modified ITO were carried out with a UVeviseNIR spectrophotometer (PerkinElmer, Lambda 750). 3. Results and discussion The current-densityevoltage (JeV) characteristics of the inverted F-OLEDs are shown in Fig. 2a, demonstrating the evolution in operational voltage with the Al modification on ITO. The operational voltage for the ITO-only device is rather high as expected, due to a high electron injection barrier. Upon the deposition of Al on ITO, a prominent voltage reduction occurs even at Al of only 0.05 nm. With increasing Al thickness, it reaches the lowest voltage at Al of 0.2 nm. However, surprisingly and significantly, the further deposition of Al leads to a continuous increase rather than a further decrease in operational voltage. As a consequence, the device with Al of 0.2 nm exhibits the highest current efficiency over two times compared with the ITO-only device, as demonstrated in Fig. 2b. The remarkable improvement of the device performance suggests that ITO treated by sub-monolayer Al can enable efficient electron injection in the inverted F-OLEDs. Furthermore, as shown in Fig. 2c, the electroluminescence spectra are very similar for all the devices. On the other hand, the device behaviors shown in Fig. 2aec are present as well for the inverted P-OLEDs, and the results are demonstrated in Fig. 2def. In particular, the device with Al of 0.2 nm shows superior performance than the one with Al of 2 nm. It indicates the importance of the low coverage of Al on ITO, whose effect on the electron injection should be driven by a general mechanism. The interesting phenomenon is highly reproducible in 20 groups of inverted F-OLEDs and 8 groups of inverted P-OLEDs. To figure out the morphology of Al with different thickness, Al was correspondingly deposited onto TEM grids in the same chamber under same conditions. Fig. 3aed shows the TEM images of Al at 0.2, 0.6, 1 and 2 nm, respectively, where the EDS data suggest a relative increase in Al quantity with its deposited thickness. When Al thickness is at 0.2 nm, Al NPs with small and uniform size of 1.21 ± 0.14 nm form (Fig. 3a), and the surface density of Al NPs is about 5 1011/cm2. Note that in this case the surface coverage of such small-sized Al NPs is as low as about 0.6%, whereas the corresponding device shows the lowest voltage. Upon increasing Al thickness to 0.6 nm, larger Al NPs with size of 2.48 ± 0.37 nm are present (Fig. 3b). The surface density of Al NPs raises to about 8 1011/cm2, and the total surface coverage is enlarged to about 3.6%. However, the voltage is slightly increased even if more ITO surface is covered by Al. A further increase in Al thickness results in the gradual formation of a continuous Al thin film (Fig. 3c and d). Therefore, the size of the Al NPs, rather than the coverage of Al, plays a key role for the electron injection from the bottom cathode. As demonstrated by the AFM results in Fig. 4aed, the deposition of the Al NPs does not significantly increase the surface roughness of ITO, and thus the large electron injection enhancement cannot be attributed to the morphological change. Instead, we propose that the small-sized Al NPs can be regarded as the electron injection hotspots, at which the local electric field is reinforced and electrons are injected efficiently into the organic active layer.
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Fig. 2. (a) Current density vs voltage characteristics, (b) current efficiency vs current density characteristics, and (c) electroluminescence spectra (intensity being normalized) of inverted fluorescent OLEDs as depicted in Fig. 1a. (d) Current density vs voltage characteristics, (e) current efficiency vs current density characteristics, and (f) electroluminescence spectra (intensity being normalized) of inverted phosphorescent OLEDs as depicted in Fig. 1b. Different symbols refer to different deposited thickness of Al on ITO.
Fig. 3. TEM images with corresponding EDS spectra of Al with different deposited thickness: (a) 0.2 nm, (b) 0.6 nm, (c) 1 nm and (d) 2 nm.
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Fig. 4. AFM images of (a) bare ITO and Al/ITO surfaces: (b) 0.2 nm Al, (c) 0.6 nm Al and (d) 2 nm Al on ITO, where root-mean-square (RMS) surface roughness is shown and no significant change is observed upon Al deposition.
Fig. 5a illustrates the UPS spectra of the bare and Al-modified ITO surfaces, which show the surface work function change upon the Al deposition. The work function reduction is only 0.2 eV (from about 4.8 to 4.6 eV) with Al thickness of 0.2 nm, due to a very low coverage of Al in this case. When Al thickness is at 2 nm, a continuous Al thin film is formed and thus the work function is reduced to about 4.1 eV, which is close to that of bulk Al [18]. Normally, using same ETL in OLEDs, a cathode with lower work function should correspond to more efficient electron injection owing to a lower electron injection barrier. It is therefore interesting that the small-sized Al NPs, without largely reducing the work function of ITO, result in the lowest operational voltage in the inverted F-OLEDs and P-OLEDs. The UPS feature supports our argument on the electron injection hotspots, which possess high electron injection efficiency despite a tiny effective injection area. The low coverage of Al is in favor of minimizing the metal absorption effect, which is characterized by the optical transmission spectra shown in Fig. 5b. Compared with that of bare ITO, the transmission spectrum for 0.2-nm-thick Al on ITO is quite similar. On the other hand, the transmission spectrum for 2-nm-thick Al on ITO shows a small enhancement in light absorption. Focusing on the light emission region from 450 to 650 nm, the transmission decrease is about 2.3% at 530 nm. The results are in good agreement with the different coverage of Al in the two samples. The relatively weak absorption in the case of thinner Al can contribute to the higher current efficiency in the inverted F-OLEDs and P-OLEDs, as shown in Fig. 2b and e, respectively. Hence, the small-sized Al NPs have the advantages of not only facilitating the electron injection but also inducing the negligible metal absorption. Furthermore, although the lateral electron diffusion is expected arising from the electron pumping at the hotspots with low surface coverage, the light emission of the devices is observed to be still spatially uniform. It is reasonable because the surface density of Al NPs is high
and the average distance of neighboring Al NPs is only a few nm, which is much smaller than the thickness of the organic active layer. The above hotspot effect can be rationalized by a physical picture of local electric-field-enhanced electron emission at the Al NPs, as schematically illustrated in Fig. 6. This model is based on the fact that the ITO cathode and Al NPs on top should have same electric potential (F) in device operation, but distinct local electric field (E) due to a large difference in local surface curvature. According to Gauss's Law, E at the ITO/ETL interface (E0) is expressed as below:
E0 ¼
s0 ; ε
(1)
where s0 is the surface charge density on bare ITO and ε is the dielectric constant of ETL. For a small-sized Al NP on ITO with a radius of R1, the surface charge density (s1) becomes very high because of a large local surface curvature. In this case, E at the AlNP/ETL interface (E1) and F can be given by Eqs. (2) and (3) in the following, respectively.
E1 ¼
Q1 s ¼ 1 ε 4pεR21
(2)
and
F¼
Q1 ¼ E1 R1 ; 4pεR1
(3)
where Q1 is the charge quantity in the small-sized Al NP, and s1 is herein approximated to be uniformly distributed on the NP surface. The format of Eqs. (2) and (3) is applicable as well to a relatively large-sized Al NP with corresponding R2, s2 and E2 (Fig. 6). Eq. (4)
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X. Lou et al. / Organic Electronics 28 (2016) 88e93
Fig. 7. Current density vs voltage characteristics of electron-only devices, and inset is the same plot with exponential y-coordinate. Different symbols refer to different deposited thickness of Al on ITO.
Fig. 5. (a) UPS spectra and (b) transmission spectra of bare and Al-modified ITO, and insets are magnification of the selected regions. Numbers in inset of (a) refer to work function increase in eV, and number in inset of (b) refers to transmission decrease in percentage at 530 nm.
electron injection hotspots. Upon raising the thickness of Al on ITO, the number of the hotspots is increased, but the hotspot effect is weakened due to a gradual increase in size of the Al NPs. Therefore, the physical picture in Fig. 6 can well interpret the device results shown in Fig. 2aef and the observation reported in literature [22,23]. The results highlight that the hotspot effect is such remarkable even if the hotspot coverage is rather low. In addition, the hotspot effect is physically a general effect which is not limited by Al NPs, and any conductive nanostructure with a sufficiently small size could be expected to act as the injection hotspots. In order to further verify the physical picture, the corresponding electron-only devices, with the device structure of ITO/Al (0, 0.2 or 2 nm)/Bphen:LiF (5 nm)/Alq3 (100 nm)/Al (100 nm), were also fabricated and tested. Fig. 7 shows the JeV characteristics of the electron-only devices using bare or Al-modified ITO as the bottom cathode. Similar to the results in Fig. 2a, the presence of ultrathin Al on ITO greatly reduces the operational voltage, and J for Al of 0.2 nm is larger than that for Al of 2 nm at same V. The trend in Fig. 7 is well consistent with the finding in the inverted OLEDs, supporting the electron injection hotspot mechanism discussed above. Furthermore, our previous report on the tandem OLEDs indicates that an insertion of ultrathin Al in the charge generation layer is critical to achieving efficient charge separation [24], which could be associated with the hotspot effect as well. 4. Conclusion
Fig. 6. Schematic illustrating the electron injection hotspot effect, where a small-sized and a large-sized Al NPs on ITO are drawn for comparison. Short lines denote surface negative charges on bottom cathode in device operation, smaller NP corresponding to higher local surface charge density and accordingly stronger local electric field.
can then be derived from Eq. (3):
E1 ¼
R2 E : R1 2
(4)
Significantly, E1 > E2 >> E0 is obvious as s1 > s2 >> s0. It indicates that E is strongly reinforced by the Al NPs, and smaller Al NPs induce larger enhancement in E. Since a sufficiently strong E will enable electron field emission via tunneling across the injection barrier, the local electron injection at the Al NPs should be highly efficient. In other words, the Al NPs on ITO, especially the small-sized ones, could behave as the
In conclusion, it is demonstrated that the small-sized Al NPs of 1e2 nm on ITO can act as the electron injection hotspots, which allow efficient electron injection from the bottom cathode in both the inverted F-OLEDs and P-OLEDs. Although the surface coverage of the small-sized Al NPs is as low as about 0.6%, the devices show the most superior performance compared with those based on the large-sized Al NPs or a continuous Al thin film. The hotspot effect is attributed to the large enhancement of local electric field depending on the hotspot size, arising from the surface charge crowding at the Al NPs. The electron injection hotspots can be easily prepared, and do not induce significant changes in the surface roughness, light absorption or work function of ITO. The present approach may open up an alternative path to achieve highly efficient carrier injection in diverse organic electronic/optoelectronic devices. Acknowledgments This work was supported by the National Natural Science
X. Lou et al. / Organic Electronics 28 (2016) 88e93
Foundation of China (No. 61274019), the Collaborative Innovation Center of Suzhou Nano Science & Technology, the Qing Lan Project of Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750. [3] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, Nature 459 (2009) 234. [4] J.S. Chen, F.C. Zhao, D.G. Ma, Mater. Today 17 (2014) 175. [5] M.G. Helander, Z.B. Wang, J. Qiu, M.T. Greiner, D.P. Puzzo, Z.W. Liu, Z.H. Lu, Science 332 (2011) 944. [6] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature 492 (2012) 234. [7] J.H. Jou, W.B. Wang, M.F. Hsu, J.J. Shyue, C.H. Chiu, I.M. Lai, S.Z. Chen, P.H. Wu, C.C. Chen, C.P. Liu, S.M. Shen, ACS Nano 4 (2010) 4054. [8] W.E. Ford, D.Q. Gao, N. Knorr, R. Wirtz, F. Scholz, Z. Karipidou, K. Ogasawara, S. Rosselli, V. Rodin, G. Nelles, F.V. Wrochem, ACS Nano 8 (2014) 9173. [9] Q.H. Wu, Cri. Rev. Solid State Mater. 38 (2013) 318. [10] C.C. Wu, C.W. Chen, C.L. Lin, C.J. Yang, IEEE J. Disp. Technol. 1 (2005) 248. [11] H.H. Hsieh, T.T. Tsai, C.Y. Chang, H.H. Wang, J.Y. Huang, S.F. Hsu, Y.C. Wu, T.C. Tsai, C.S. Chuang, L.H. Chang, Y.H. Lin, SID Int. Symp. Dig. Tech. 41 (2010)
93
140. [12] Y. He, R. Hattori, J. Kanicki, IEEE Trans. Electron Devices 48 (2001) 1322. [13] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488. [14] T.Y. Chu, J.F. Chen, S.Y. Chen, C.J. Chen, C.H. Chen, Appl. Phys. Lett. 89 (2006) 053503. [15] J. Meyer, T. Winkler, S. Hamwi, S. Schmale, H.H. Johannes, T. Weimann, P. Hinze, W. Kowlasky, T. Riedl, Adv. Mater. 20 (2008) 3839. [16] N. Tokmoldin, N. Griffiths, D.D.C. Bradley, S.A. Haque, Adv. Mater. 21 (2009) 3475. [17] H. Lee, C.M. Kang, M. Park, J. Kwak, C. Lee, ACS Appl. Mater. Interfaces 5 (2013) 1977. [18] J. Peng, X.X. Wang, J. Liu, X.D. Huang, J. Xiao, S.D. Wang, H.Q. Wang, W.L. Ma, J. Mater. Chem. C 2 (2014) 864. € fle, A. Schienle, M. Bruns, U. Lemmer, A. Colsmann, Adv. Mater. 26 (2014) [19] S. Ho 2750. [20] J.H. Lee, P.S. Wang, H.D. Park, C.I. Wu, J.J. Kim, Org. Electron 12 (2011) 1763. [21] W.B. Liu, S.H. Liu, J. Yu, W. Zhang, X.M. Wen, Y.M. Yin, L.T. Zhang, P. Chen, W.F. Xie, Appl. Phys. Lett. 104 (2014) 093305. [22] T.Y. Chu, S.Y. Chen, J.F. Chen, C.H. Chen, Jpn. J. Appl. Phys. 45 (2006) 4948. de, F. Martinez, G. Neculqueo, L. Cattin, Phys. Stat. Sol. 2 (2008) 10. [23] J.C. Berne [24] J. Xiao, X.X. Wang, H. Zhu, X. Gao, X.H. Yang, X.H. Zhang, S.D. Wang, Appl. Phys. Lett. 101 (2012) 013301.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Small-sized Al nanoparticles as electron injection hotspots in inverted organic light-emitting diodes Xia Lou, Xin-Xin Wang, Chang-Hai Liu, Jie Liu, Ze-Qun Cui, Zhi-Hao Lu, Xu Gao, Sui-Dong Wang* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu, 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2015 Received in revised form 11 October 2015 Accepted 11 October 2015 Available online xxx
Al nanoparticles, with small size and ultralow coverage on ITO, can play a key role as the electron injection hotspots in both the inverted fluorescent and phosphorescent organic light-emitting diodes. The presence of the hotspots greatly reduces the operational voltage and improves the current efficiency of the devices, which are strongly dependent on the hotspot size. The microscopic and spectroscopic characterization demonstrate that the small-sized hotspots have a minor influence on the surface roughness, transparency and work function of ITO. The hotspot effect is ascribed to the highly efficient electron injection at the Al nanoparticles enhanced by the local electric field, and a physical model is proposed to clarify this mechanism. The finding indicates a promising strategy by design and craft of the injection hotspots in nanoscale to facilitate carrier injection in organic thin film devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Inverted organic light-emitting diodes Al nanoparticles Electron injection Field emission
1. Introduction Organic light-emitting diodes (OLEDs) have been greatly developed since their first demonstration by Tang and VanSlyke in 1987 [1e4], and now are booming in the markets of solid-state lighting and active-matrix (AM) flat-panel displays. However, the fundamental issues such as carrier injection mechanism and exciton processes in OLEDs [5e9], and the technical challenges such as compatibility with thin film transistor (TFT) backplanes [10,11], still need to be further addressed. In particular, commercial backplanes for AM-OLEDs consist of amorphous Si or oxide TFT arrays, which are typically n-type transistors [12,13]. When conventional OLEDs with indium tin oxide (ITO) as the bottom anode are driven by n-type TFTs, the ITO anode is connected to the source electrode of TFTs. This configuration may cause the instability of the source voltage in TFTs and accordingly the image sticking issue in displays [10,11]. Thus, it is highly desired to adopt inverted bottom-emission OLEDs with ITO as the bottom cathode for practical display applications [14e21], where the ITO cathode is connected to the drain electrode of n-type TFTs. ITO possesses high surface work function of about 4.8 eV, and
* Corresponding author. E-mail address: [email protected] (S.-D. Wang). http://dx.doi.org/10.1016/j.orgel.2015.10.012 1566-1199/© 2015 Elsevier B.V. All rights reserved.
the lowest unoccupied molecular orbitals (LUMO) of most organic electron transporting layers (ETL) used in OLEDs range from 2.5 eV to 3.5 eV [9]. It thus implies a very high electron injection barrier at the ITO/ETL interfaces, which can be the bottleneck to hinder the realization of high-performance inverted OLEDs. The reported approaches toward efficient electron injection from ITO include: (1) surface coating with low work function materials such as Mg- and Li-doped ETL [14,15], which faces the ambient instability issue; and (2) introducing an interface dipole layer such as an appropriate metal oxide layer [16e19], which is not compatible with vacuum sublimation and often needs annealing treatments. Both the attempts are to lower the effective work function of ITO, and consequently reduce the electron injection barrier from the bottom cathode. However, we herein demonstrate that the work function reduction is not necessarily the case to realize efficient electron injection in inverted OLEDs. Instead, the electron injection hotspots in nanoscale can be designed and achieved to efficiently emit electrons from ITO while maintaining its high transparency. In this report, the small-sized Al nanoparticles (NPs) prepared simply by thermal evaporation are utilized as the electron injection hotspots on ITO, which greatly reduce the operational voltage and improve the current efficiency for both the inverted fluorescent and phosphorescent OLEDs (F-OLEDs and P-OLEDs, respectively). The combination of microscopic and electronic characterization indicates that the electron injection enhancement is determined
X. Lou et al. / Organic Electronics 28 (2016) 88e93
neither by the surface coverage of the Al NPs, nor by the effective work function of the modified ITO surface. The size of the Al NPs turns out to be critical to the hotspot effect, and thus the reinforced local electric field around the hotspots is proposed to be responsible for the electron injection enhancement in the inverted OLEDs. It can be a general mechanism and may offer new opportunities for the facilitation of carrier injection in organic electronic/optoelectronic devices. 2. Experimental 2.1. Device fabrication The device structures of the inverted F-OLEDs and P-OLEDs are illustrated in Fig. 1a and b, respectively, where the nominal thickness of thermally evaporated Al on ITO varies from 0 to 2 nm. Al is selected due to the advantages of low cost and simple preparation, and its deposition is compatible with the device fabrication process. The fluorescent unit is composed of LiF-doped 4,7-diphenyl-1,10phenanthroline (Bphen:LiF, 5 nm)/tris(8-hydroxyquinoline) aluminum (Alq3, 30 nm)/N,N0 -bis(naphthalen-1-yl)-N,N0 bis(phenyl)-benzidine (NPB, 30 nm), where Bphen:LiF has an optimal doping ratio of 30 wt%. The phosphorescent unit is composed of Bphen:LiF (5 nm)/Bphen (15 nm)/bis(2phenylpyridine)-iridium-acetylacetonate-doped 4,40 -N,N0 -dicarbazole-biphenyl [CBP:Ir(ppy)2(acac), 15 nm]/4,40 ,400 -tris(N-carbazolyl)-triphenyl amine (TCTA, 30 nm), where CBP:Ir(ppy)2(acac) has an optimal doping ratio of 8 wt%. Each category of devices have identical top anode of MoO3 (5 nm)/Al (100 nm), and the only variable is the Al quantity on the bottom cathode, which can be simply controlled by the deposited thickness of Al. After routine cleaning and drying, the ITO-coated glass substrates were treated by UV-ozone for 15 min. Subsequently, all the layers were successively deposited in a high-vacuum deposition system (Trovato MFG, ~1 106 mbar) equipped with multiple thermal sublimation sources, where the film thickness was controlled by a precalibrated quartz microbalance and a switchable shutter. Al NPs spontaneously formed upon a sub-monolayer deposition of Al on ITO, which was obtained by thermal evaporation at a deposition rate of 0.01e0.02 nm/s. All the devices for comparison were fabricated in a same experimental round, and were encapsulated in a glove box after the fabrication. 2.2. Device characterization The electrical and optoelectronic characteristics of the inverted OLEDs were measured using a programmable spectroradiometer (Photo Research PR-655), which is connected to a sourcemeter
Fig. 1. Device structures of (a) inverted fluorescent OLEDs and (b) inverted phosphorescent OLEDs, where Al NPs are deposited on ITO by thermal evaporation.
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(Keithley 2400). The inverted OLEDs were biased with the bottom ITO and top Al electrodes as the cathode and anode, respectively. The microscopic morphology of the Al NPs was characterized by transmission electron microscopy (TEM, FEI Quanta FRG 200F) combined with energy dispersive spectroscopy (EDS). The surface roughness of bare and Al-modified ITO was characterized by atomic force microscopy (AFM, Bruker Dimension Icon) in peak-force tapping mode. The work function of bare and Al-modified ITO was measured by ultraviolet photoelectron spectroscopy (UPS, He I, Kratos Axis Ultra DLD) in ultrahigh vacuum. The transmission measurements of bare and Al-modified ITO were carried out with a UVeviseNIR spectrophotometer (PerkinElmer, Lambda 750). 3. Results and discussion The current-densityevoltage (JeV) characteristics of the inverted F-OLEDs are shown in Fig. 2a, demonstrating the evolution in operational voltage with the Al modification on ITO. The operational voltage for the ITO-only device is rather high as expected, due to a high electron injection barrier. Upon the deposition of Al on ITO, a prominent voltage reduction occurs even at Al of only 0.05 nm. With increasing Al thickness, it reaches the lowest voltage at Al of 0.2 nm. However, surprisingly and significantly, the further deposition of Al leads to a continuous increase rather than a further decrease in operational voltage. As a consequence, the device with Al of 0.2 nm exhibits the highest current efficiency over two times compared with the ITO-only device, as demonstrated in Fig. 2b. The remarkable improvement of the device performance suggests that ITO treated by sub-monolayer Al can enable efficient electron injection in the inverted F-OLEDs. Furthermore, as shown in Fig. 2c, the electroluminescence spectra are very similar for all the devices. On the other hand, the device behaviors shown in Fig. 2aec are present as well for the inverted P-OLEDs, and the results are demonstrated in Fig. 2def. In particular, the device with Al of 0.2 nm shows superior performance than the one with Al of 2 nm. It indicates the importance of the low coverage of Al on ITO, whose effect on the electron injection should be driven by a general mechanism. The interesting phenomenon is highly reproducible in 20 groups of inverted F-OLEDs and 8 groups of inverted P-OLEDs. To figure out the morphology of Al with different thickness, Al was correspondingly deposited onto TEM grids in the same chamber under same conditions. Fig. 3aed shows the TEM images of Al at 0.2, 0.6, 1 and 2 nm, respectively, where the EDS data suggest a relative increase in Al quantity with its deposited thickness. When Al thickness is at 0.2 nm, Al NPs with small and uniform size of 1.21 ± 0.14 nm form (Fig. 3a), and the surface density of Al NPs is about 5 1011/cm2. Note that in this case the surface coverage of such small-sized Al NPs is as low as about 0.6%, whereas the corresponding device shows the lowest voltage. Upon increasing Al thickness to 0.6 nm, larger Al NPs with size of 2.48 ± 0.37 nm are present (Fig. 3b). The surface density of Al NPs raises to about 8 1011/cm2, and the total surface coverage is enlarged to about 3.6%. However, the voltage is slightly increased even if more ITO surface is covered by Al. A further increase in Al thickness results in the gradual formation of a continuous Al thin film (Fig. 3c and d). Therefore, the size of the Al NPs, rather than the coverage of Al, plays a key role for the electron injection from the bottom cathode. As demonstrated by the AFM results in Fig. 4aed, the deposition of the Al NPs does not significantly increase the surface roughness of ITO, and thus the large electron injection enhancement cannot be attributed to the morphological change. Instead, we propose that the small-sized Al NPs can be regarded as the electron injection hotspots, at which the local electric field is reinforced and electrons are injected efficiently into the organic active layer.
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Fig. 2. (a) Current density vs voltage characteristics, (b) current efficiency vs current density characteristics, and (c) electroluminescence spectra (intensity being normalized) of inverted fluorescent OLEDs as depicted in Fig. 1a. (d) Current density vs voltage characteristics, (e) current efficiency vs current density characteristics, and (f) electroluminescence spectra (intensity being normalized) of inverted phosphorescent OLEDs as depicted in Fig. 1b. Different symbols refer to different deposited thickness of Al on ITO.
Fig. 3. TEM images with corresponding EDS spectra of Al with different deposited thickness: (a) 0.2 nm, (b) 0.6 nm, (c) 1 nm and (d) 2 nm.
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Fig. 4. AFM images of (a) bare ITO and Al/ITO surfaces: (b) 0.2 nm Al, (c) 0.6 nm Al and (d) 2 nm Al on ITO, where root-mean-square (RMS) surface roughness is shown and no significant change is observed upon Al deposition.
Fig. 5a illustrates the UPS spectra of the bare and Al-modified ITO surfaces, which show the surface work function change upon the Al deposition. The work function reduction is only 0.2 eV (from about 4.8 to 4.6 eV) with Al thickness of 0.2 nm, due to a very low coverage of Al in this case. When Al thickness is at 2 nm, a continuous Al thin film is formed and thus the work function is reduced to about 4.1 eV, which is close to that of bulk Al [18]. Normally, using same ETL in OLEDs, a cathode with lower work function should correspond to more efficient electron injection owing to a lower electron injection barrier. It is therefore interesting that the small-sized Al NPs, without largely reducing the work function of ITO, result in the lowest operational voltage in the inverted F-OLEDs and P-OLEDs. The UPS feature supports our argument on the electron injection hotspots, which possess high electron injection efficiency despite a tiny effective injection area. The low coverage of Al is in favor of minimizing the metal absorption effect, which is characterized by the optical transmission spectra shown in Fig. 5b. Compared with that of bare ITO, the transmission spectrum for 0.2-nm-thick Al on ITO is quite similar. On the other hand, the transmission spectrum for 2-nm-thick Al on ITO shows a small enhancement in light absorption. Focusing on the light emission region from 450 to 650 nm, the transmission decrease is about 2.3% at 530 nm. The results are in good agreement with the different coverage of Al in the two samples. The relatively weak absorption in the case of thinner Al can contribute to the higher current efficiency in the inverted F-OLEDs and P-OLEDs, as shown in Fig. 2b and e, respectively. Hence, the small-sized Al NPs have the advantages of not only facilitating the electron injection but also inducing the negligible metal absorption. Furthermore, although the lateral electron diffusion is expected arising from the electron pumping at the hotspots with low surface coverage, the light emission of the devices is observed to be still spatially uniform. It is reasonable because the surface density of Al NPs is high
and the average distance of neighboring Al NPs is only a few nm, which is much smaller than the thickness of the organic active layer. The above hotspot effect can be rationalized by a physical picture of local electric-field-enhanced electron emission at the Al NPs, as schematically illustrated in Fig. 6. This model is based on the fact that the ITO cathode and Al NPs on top should have same electric potential (F) in device operation, but distinct local electric field (E) due to a large difference in local surface curvature. According to Gauss's Law, E at the ITO/ETL interface (E0) is expressed as below:
E0 ¼
s0 ; ε
(1)
where s0 is the surface charge density on bare ITO and ε is the dielectric constant of ETL. For a small-sized Al NP on ITO with a radius of R1, the surface charge density (s1) becomes very high because of a large local surface curvature. In this case, E at the AlNP/ETL interface (E1) and F can be given by Eqs. (2) and (3) in the following, respectively.
E1 ¼
Q1 s ¼ 1 ε 4pεR21
(2)
and
F¼
Q1 ¼ E1 R1 ; 4pεR1
(3)
where Q1 is the charge quantity in the small-sized Al NP, and s1 is herein approximated to be uniformly distributed on the NP surface. The format of Eqs. (2) and (3) is applicable as well to a relatively large-sized Al NP with corresponding R2, s2 and E2 (Fig. 6). Eq. (4)
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Fig. 7. Current density vs voltage characteristics of electron-only devices, and inset is the same plot with exponential y-coordinate. Different symbols refer to different deposited thickness of Al on ITO.
Fig. 5. (a) UPS spectra and (b) transmission spectra of bare and Al-modified ITO, and insets are magnification of the selected regions. Numbers in inset of (a) refer to work function increase in eV, and number in inset of (b) refers to transmission decrease in percentage at 530 nm.
electron injection hotspots. Upon raising the thickness of Al on ITO, the number of the hotspots is increased, but the hotspot effect is weakened due to a gradual increase in size of the Al NPs. Therefore, the physical picture in Fig. 6 can well interpret the device results shown in Fig. 2aef and the observation reported in literature [22,23]. The results highlight that the hotspot effect is such remarkable even if the hotspot coverage is rather low. In addition, the hotspot effect is physically a general effect which is not limited by Al NPs, and any conductive nanostructure with a sufficiently small size could be expected to act as the injection hotspots. In order to further verify the physical picture, the corresponding electron-only devices, with the device structure of ITO/Al (0, 0.2 or 2 nm)/Bphen:LiF (5 nm)/Alq3 (100 nm)/Al (100 nm), were also fabricated and tested. Fig. 7 shows the JeV characteristics of the electron-only devices using bare or Al-modified ITO as the bottom cathode. Similar to the results in Fig. 2a, the presence of ultrathin Al on ITO greatly reduces the operational voltage, and J for Al of 0.2 nm is larger than that for Al of 2 nm at same V. The trend in Fig. 7 is well consistent with the finding in the inverted OLEDs, supporting the electron injection hotspot mechanism discussed above. Furthermore, our previous report on the tandem OLEDs indicates that an insertion of ultrathin Al in the charge generation layer is critical to achieving efficient charge separation [24], which could be associated with the hotspot effect as well. 4. Conclusion
Fig. 6. Schematic illustrating the electron injection hotspot effect, where a small-sized and a large-sized Al NPs on ITO are drawn for comparison. Short lines denote surface negative charges on bottom cathode in device operation, smaller NP corresponding to higher local surface charge density and accordingly stronger local electric field.
can then be derived from Eq. (3):
E1 ¼
R2 E : R1 2
(4)
Significantly, E1 > E2 >> E0 is obvious as s1 > s2 >> s0. It indicates that E is strongly reinforced by the Al NPs, and smaller Al NPs induce larger enhancement in E. Since a sufficiently strong E will enable electron field emission via tunneling across the injection barrier, the local electron injection at the Al NPs should be highly efficient. In other words, the Al NPs on ITO, especially the small-sized ones, could behave as the
In conclusion, it is demonstrated that the small-sized Al NPs of 1e2 nm on ITO can act as the electron injection hotspots, which allow efficient electron injection from the bottom cathode in both the inverted F-OLEDs and P-OLEDs. Although the surface coverage of the small-sized Al NPs is as low as about 0.6%, the devices show the most superior performance compared with those based on the large-sized Al NPs or a continuous Al thin film. The hotspot effect is attributed to the large enhancement of local electric field depending on the hotspot size, arising from the surface charge crowding at the Al NPs. The electron injection hotspots can be easily prepared, and do not induce significant changes in the surface roughness, light absorption or work function of ITO. The present approach may open up an alternative path to achieve highly efficient carrier injection in diverse organic electronic/optoelectronic devices. Acknowledgments This work was supported by the National Natural Science
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Foundation of China (No. 61274019), the Collaborative Innovation Center of Suzhou Nano Science & Technology, the Qing Lan Project of Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750. [3] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, Nature 459 (2009) 234. [4] J.S. Chen, F.C. Zhao, D.G. Ma, Mater. Today 17 (2014) 175. [5] M.G. Helander, Z.B. Wang, J. Qiu, M.T. Greiner, D.P. Puzzo, Z.W. Liu, Z.H. Lu, Science 332 (2011) 944. [6] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature 492 (2012) 234. [7] J.H. Jou, W.B. Wang, M.F. Hsu, J.J. Shyue, C.H. Chiu, I.M. Lai, S.Z. Chen, P.H. Wu, C.C. Chen, C.P. Liu, S.M. Shen, ACS Nano 4 (2010) 4054. [8] W.E. Ford, D.Q. Gao, N. Knorr, R. Wirtz, F. Scholz, Z. Karipidou, K. Ogasawara, S. Rosselli, V. Rodin, G. Nelles, F.V. Wrochem, ACS Nano 8 (2014) 9173. [9] Q.H. Wu, Cri. Rev. Solid State Mater. 38 (2013) 318. [10] C.C. Wu, C.W. Chen, C.L. Lin, C.J. Yang, IEEE J. Disp. Technol. 1 (2005) 248. [11] H.H. Hsieh, T.T. Tsai, C.Y. Chang, H.H. Wang, J.Y. Huang, S.F. Hsu, Y.C. Wu, T.C. Tsai, C.S. Chuang, L.H. Chang, Y.H. Lin, SID Int. Symp. Dig. Tech. 41 (2010)
93
140. [12] Y. He, R. Hattori, J. Kanicki, IEEE Trans. Electron Devices 48 (2001) 1322. [13] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488. [14] T.Y. Chu, J.F. Chen, S.Y. Chen, C.J. Chen, C.H. Chen, Appl. Phys. Lett. 89 (2006) 053503. [15] J. Meyer, T. Winkler, S. Hamwi, S. Schmale, H.H. Johannes, T. Weimann, P. Hinze, W. Kowlasky, T. Riedl, Adv. Mater. 20 (2008) 3839. [16] N. Tokmoldin, N. Griffiths, D.D.C. Bradley, S.A. Haque, Adv. Mater. 21 (2009) 3475. [17] H. Lee, C.M. Kang, M. Park, J. Kwak, C. Lee, ACS Appl. Mater. Interfaces 5 (2013) 1977. [18] J. Peng, X.X. Wang, J. Liu, X.D. Huang, J. Xiao, S.D. Wang, H.Q. Wang, W.L. Ma, J. Mater. Chem. C 2 (2014) 864. € fle, A. Schienle, M. Bruns, U. Lemmer, A. Colsmann, Adv. Mater. 26 (2014) [19] S. Ho 2750. [20] J.H. Lee, P.S. Wang, H.D. Park, C.I. Wu, J.J. Kim, Org. Electron 12 (2011) 1763. [21] W.B. Liu, S.H. Liu, J. Yu, W. Zhang, X.M. Wen, Y.M. Yin, L.T. Zhang, P. Chen, W.F. Xie, Appl. Phys. Lett. 104 (2014) 093305. [22] T.Y. Chu, S.Y. Chen, J.F. Chen, C.H. Chen, Jpn. J. Appl. Phys. 45 (2006) 4948. de, F. Martinez, G. Neculqueo, L. Cattin, Phys. Stat. Sol. 2 (2008) 10. [23] J.C. Berne [24] J. Xiao, X.X. Wang, H. Zhu, X. Gao, X.H. Yang, X.H. Zhang, S.D. Wang, Appl. Phys. Lett. 101 (2012) 013301.