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Enhanced radiative decay in organic light-emitting diodes by combination of cesium carbonate and silver nanocluster
Journal of Luminescence 166 (2015) 248–252
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Enhanced radiative decay in organic light-emitting diodes by combination of cesium carbonate and silver nanocluster Ying-Chung Chen a, Chia-Yuan Gao a, Kan-Lin Chen b, Chien-Jung Huang c,n a
Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan Department of Electronic Engineering, Fortune Institute of Technology, Kaohsiung, Taiwan c Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan b
art ic l e i nf o
a b s t r a c t
Article history: Received 7 January 2015 Received in revised form 24 April 2015 Accepted 21 May 2015 Available online 29 May 2015
The influence of the cesium carbonate/silver nanocluster/cesium carbonate electron-injection structure (CSC-EIS) is investigated in this study. When the CSC-EIS replaces the cesium carbonate electron-injection structure (CEIS), the shorter response time and rising time is obtained by measuring transient electroluminescence. In addition, the radiative decay rate for the device with the CSC-EIS is enhanced by 43% compared with that of the device with the CEIS because the localized electric field around SNCs is enhanced and the electron-injection barrier between the cathode and the electron-transport layer is remarkably reduced, resulting in the increase in the electron injection from the cathode to emitting layer. & 2015 Elsevier B.V. All rights reserved.
Keywords: Electron injection Transient electroluminescence Radiative decay rate Electric field
1. Introduction Organic light-emitting diode (OLED) has exhibited significant potentiality in the application of display and lighting because of its low power consumption. Thus far, the efficiency of the device is still not optimal. Generally, the mobility of electrons in the electron-transport layer is a few orders of magnitude lower than that of holes in the hole-transport layer [1,2], resulting in the imbalance between electrons and holes within the emitting layer. Moreover, the imbalance between electrons and holes within the emitting layer leads to a decrease in the efficiency of device. The metals of low work function or metal alloys such as Mg:Ag, Ca, and Li are used as cathodes to decrease the electron-injection barrier between the cathode and the electron-transport layer. However, the above-mentioned metals are neither simple fabrication nor stable. Cesium carbonate (Cs2CO3) is described to be the best electron injection material because the electron-injection barrier between the cathode and the electron-transport layer can be remarkably reduced when Cs2CO3 is inserted between the cathode and the electron-transport layer [3]. In addition, it has attracted a lot of attention that the surface plasmon resonance effect (SPRE) of metal nanoparticle and nanocluster is applied to OLED and inorganic light-emitting diode [4–7]. It is well known that the SPRE is caused by interaction between the surface charge n
Corresponding author. Tel.: þ 886 7 5919475; fax: þ886 7 5919357. E-mail address: [email protected] (C.-J. Huang).
http://dx.doi.org/10.1016/j.jlumin.2015.05.046 0022-2313/& 2015 Elsevier B.V. All rights reserved.
of metal and electromagnetic field of the incident light [8]. When the overlap is excellent between the absorption spectrum of metal nanocluster and the PL spectrum of Alq3, the characteristic of the device can be significantly enhanced [9]. However, no one has combined the advantages of metal nanocluster and Cs2CO3, nor anyone has applied it to electron injection structure of OLED. In this article, the silver nanoclusters (SNCs) are introduced between the Cs2CO3 to enhance the characteristic of device by the SPRE of the SNCs and the excellent electron injection ability of the Cs2CO3. Furthermore, a detailed mechanism of the increase in the characteristic of the device by transient electroluminescence measurements is presented.
2. Experimental The schematic structure of OLEDs with the cesium carbonate/ silver nanocluster/cesium carbonate electron-injection structure (CSC-EIS) is shown in Fig. 1. Indium tin oxide (ITO) coated on glass with a sheet resistance of 10 Ω/sq was used as the starting substrate. The substrate was cleaned with acetone, methanol and deionized water, and then dried with nitrogen gas. After cleaning process, the substrates were loaded into a thermal evaporator. Afterward, N,N'-bis-(1-naphthyl)-N,N-dipheny1,1'-bipheny1-4diamine (NPB; 35 nm), tris-(8-hydroxyquinoline) aluminum (Alq3; 40 nm), 4,7-dipheny1-1,10-phenanthroline (BPhen; 10 nm), Cs2CO3 (1 nm), SNCs, Cs2CO3 (1 nm), and aluminum (Al; 100 nm)
Y.-C. Chen et al. / Journal of Luminescence 166 (2015) 248–252
249
Fig. 1. Schematic device structure of the OLED with the CSC-EIS.
3. Results and discussion Fig. 2(a) demonstrates the SEM image of SNCs on glass substrate. After calculation, the mean cluster size of SNCs in Fig. 2 (a) was found to be 34 nm. Fig. 2(b) demonstrates the absorption spectrum of SNCs with mean cluster size of 34 nm as well as the PL spectrum of Alq3. The result shows that the absorption spectrum of SNCs with mean cluster size of 34 nm is about 525 nm and that the PL spectrum of Alq3 is 523 nm or so. In addition, a significant overlap exists between the absorption spectrum of SNCs and the PL spectrum of Alq3. It is fully understood that the excellent overlap between the two spectra suggests that they can be used for SP-exciton coupling. In other words, this overlap implied that these SNCs can be used to increase the characters of OLEDs. The resulting J–V curves of the devices were plotted in Fig. 3(a). It can be clearly seen that the current density of the device with the CSC-EIS is remarkably increased due to the SPRE of the SNCs, resulting in that the localized electric field around SNCs is enhanced [10]. In addition, it is observed that the operation voltage of the device with the CSC-EIS is lower than that of the device with the CEIS because the electron-injection barrier between the cathode and the electron-transport layer is remarkably reduced by
1.2
1.2
PL spectrum of Alq 3
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0 350
400
450
500
550
600
Absorption (a.u.)
Absorption spectrum of SNCs
Emission intensity (a.u.)
were deposited. In addition, the Cs2CO3 thickness for Cs2CO3 electron-injection structure (CEIS) is 1 nm. NPB and BPhen were used as the hole and electron transport layers, respectively. Alq3 was used as an emitting layer. Cs2CO3, Al and ITO were used as electron-injection layer, cathode and anode, respectively. In addition, the electron-only devices with CSC-EIS and CEIS were fabricated in order to further confirm the relationship between the electric injection and current density. The electron-only device with CSC-EIS and CEIS consist of ITO/Alq3 (40 nm)/BPhen (10 nm)/ Cs2CO3 (1 nm)/SNCs/Cs2CO3 (1 nm)/Al (100 nm) and ITO/Alq3 (40 nm)/BPhen (10 nm)/Cs2CO3 (1 nm)/Al (100 nm). During the deposition, the base pressure of the chamber was maintained as low as 2.4 10 6 Torr. The active area of the device was 6 6 mm2. The deposition rates of all organic materials were maintained at 0.01 nm/s, except for the Al where deposition rate was 0.5 nm/s. The thickness of the layers is controlled by using a quartz-crystal monitor. In addition, the SNCs were fabricated by thermal evaporation of silver slug. The deposition rate and the deposition time are 0.01 nm/s and 300 s respectively. The current density–voltage (J–V) and luminance–voltage (L–V) characteristics of the devices were measured by using a Keithley 2400 (Keithley instruments Inc., USA) and a PR-655 (Photo Research Inc., USA), respectively. All measurements were made at room temperatures under air ambient. In terms of the transient electroluminescence (EL) measurements, the repetition rate and width of the pulse were 1 kHz and 5 μs, respectively. The light output was detected by a fast-biased silicon photodiode ET-2020 (Electro-Optics Technology Inc., USA).
0.0 650
Wavelength (nm) Fig. 2. (a) The SEM image of SNCs on the glass surface. (b) Absorption spectrum of SNCs and PL spectrum of Alq3.
CSC-EIS. It is well known that the electron-injection barrier between the cathode and the electron-transport layer can be remarkably reduced when Cs2CO3 is inserted between the cathode and the electron-transport layer. In addition, the electron injection is enhanced and operation voltage is decreased for the device with CSC-EIS because the SNCs adhered to the cathode, resulting in numerous sharp tips to produce a stronger electric field. Furthermore, the stronger electric field assists electrons to tunnel through the triangular potential barrier [5]. The resulting J–V curves of electron-only devices were plotted in the inset of Fig. 3(a). It is observed that the current density of device with CSC-EIS is higher than that of the device with CEIS, which suggests that the CSC-EIS is useful for enhancement of electron injection. Fig. 3(b) shows the L–V curves of devices with CEIS and CSC-EIS. It is observed that the maximum luminance of devices with CSC-EIS is higher than that of the device with CEIS because the device with CSC-EIS not only enhanced the electron-injection ability from the cathode to the emitting layer but also enhanced the spontaneous recombination of excitons. In addition, the power efficiency-luminance curves of devices with CEIS and CSC-EIS were plotted in the inset of Fig. 3(b). The power efficiency of the device with CSC-EIS is quite high compared with that of the device with CEIS. Furthermore, the increasing rate for power efficiency is even more at higher operation voltage. Perhaps it is due to the fact that the SPRE caused by interaction between the surface charge of metal and the incident light of emitting layer is not obviously shown when the luminance is lower; contrastly, when luminance is higher, the SPRE is obviously shown. With regard to the analysis of carriertransport behaviors, the dynamic behavior of EL is important.
250
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20
CEIS CSC-EIS
250
EL Intensity (a. u.)
Current density (mA/cm 2)
300
200 150 100
4V 5V 6V 7V 8V 9V
15
10
5
50 0
0
2
4
6
8
10
0
2
4
6
8
Time (µs)
12
12000 11000 10000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0
20
CEIS CSC-EIS
EL Intensity (a. u.)
Luminance (cd/m 2)
Voltage (V)
15
10
5
0
0
2
4
6
8
10
12
Voltage (V)
4V 5V 6V 7V 8V 9V
0
2
4
6
8
Time (µs) Fig. 4. The transient EL as a function of time with (a) the CEIS and (b) the CSC-EIS by various voltage pulses.
Fig. 3. (a) The current density–voltage curves and (b) the luminance–voltage curves of devices with the CEIS and the CSC-EIS. Inset of (a) and (b) shows the current density–voltage curves of electron-only devices and power efficiencyluminance characteristics of devices, respectively.
2.2 CEIS CSC-EIS
2.0 1.8
Response Time (µs)
Therefore, the transient EL was also measured in this article to further understand the carrier-transport behaviors. Fig. 4(a) and (b) shows the transient EL as a function of time with the CEIS and the CSC-EIS by various voltage pulses. It is observed that response time was shortened with increase in applied voltages. The response time is defined as the time required before light emission onset after the pulse voltage switched on. The shorter response time suggests that holes and electrons were faster injection to the emitting layer from the electrode. In addition, it is also observed that rising time was shorter with increase in applied voltages. The rising time is defined as the time required for the response to rise from 10% to 90% of maximum EL intensity. The shorter rising time suggests that the maximum number of excitons was faster achieved. Fig. 5 shows the response time–voltage curves of the OLEDs with the CEIS and the CSC-EIS by various voltage pulses. It is observed that response time of the device with the CEIS was shortened with increase in voltage pulses because the carrier mobility was faster from the electrode to the emitting layer. Moreover, response time of the device with the CSC-EIS became a constant within the large voltage pulses range (Vpulses Z7 V). The constant response time suggests that the saturation of carrier mobility can be faster achieved than that of the device with the CEIS [11]. In addition, response time of the devices with the CEIS and the CSC-EIS at 4 V is 2.02 and 0.91 μs, respectively. It is well
1.6 1.4 1.2 1.0 0.8 0.6 0.4
4
5
6
7
8
9
Voltage (V) Fig. 5. The response time–voltage curves of devices with the CEIS and the CSC-EIS.
known, the carrier mobility is inversely proportional to the time. Furthermore, the CSC-EIS only changed the electron-injection structure. Therefore, it can be extrapolated that the carrier mobility was enhanced due to the increase in the electron mobility. Moreover, the electron mobility of the device with the CSC-EIS was enhanced by about 122% compared with that of the device with the CEIS.
Y.-C. Chen et al. / Journal of Luminescence 166 (2015) 248–252
24
1.0 CEIS CSC-EIS
22 18 16 14 12 10 8
CEIS CSC-EIS
0.8
EL Intensity (a. u.)
20
EL Intensity (a. u.)
251
0.6 0.4 0.2
6 4 2 0
0.0 0
1
2
3
4
5
6
7
8
5.2
5.6
9
6.0
6.4
6.8
Time (µs)
Time (µs)
Fig. 7. The transient EL decay as a function of time at 9 V for devices with the CEIS and the CSC-EIS.
4.0
CEIS CSC-EIS
Rising Time (µs)
3.5
expressed as follows [12]:
κ = κr + κnr =
3.0
1 τ
(1)
2.5
where κ is the total decay rate, κr is the radiative decay rate, κnr is the nonradiative, and τ is the decay time. Furthermore, the external quantum efficiency can be expressed as follows [13]:
2.0
ηext =
1.5 1.0
4
5
6
7
8
9
Voltage (V) Fig. 6. (a) The transient EL of devices by 9 V voltage pulse and (b) the rising time– voltage curves of devices with the CEIS and the CSC-EIS.
Fig. 6(a) shows the transient EL of OLEDs with the CEIS and the CSC-EIS by 9 V voltage pulse. It can be clearly seen that the rising time of the device with the CSC-EIS is shorter than that of the device with the CEIS. The rising time of devices with the CEIS and the CSC-EIS at 9 V is 1.52 and 1.22 μs, respectively. It suggests that the time of excitons achieving the maximum number within the emitting layer for the CSC-EIS is faster by 20% compared with that of device with the CEIS. Fig. 6(b) shows the rising time–voltage curves of OLEDs with the CEIS and the CSC-EIS by various voltage pulses. It is observed that the rising time is shortened with increase in the voltage pulses because the electron mobility is enhanced with increase in the voltage pulses. The transient EL decay as a function of time at 9 V for devices with the CEIS and the CSC-EIS is shown in Fig. 7. The transient EL decay can be fitted with a single exponential to obtain decay time. The decay time of devices with the CEIS and the CSC-EIS is 0.3 and 0.28 μs, respectively. The decay time of the device with the CSCEIS is shorter than that of the CEIS due to the surface plamons of SNCs. The decay time is defined as the time for exciton from excited state to ground state. Since the coupling process between surface plasmons and radiated light is rapider than spontaneous recombination of excitons, the decay time of the exciton in surface plasmon resonance device is reduced. In addition, the decay time is the reciprocal of the decay rate. Moreover, the decay rate is the sum of the radiative decay rate and the nonradiative decay rate. The relationship between the decay time and the decay rate can be
ηpower × IV /(hν) P /(hν) = = ηint × ηextr I /e I /e
(2)
where ηext is the external quantum efficiency, P is the photon power, hν is the photon energy, I is the input current, V is the input voltage, e is the quantity of electric charge, ηpower is the power efficiency, ηint is the internal quantum efficiency, and ηextr is the extraction efficiency. Because the extraction efficiencies of devices with the CEIS and the CSC-EIS are very close, the ηpower is proportional to the ηint. Moreover, the relationship between the internal quantum efficiency and the radiative decay rate can be expressed as follows:
ηint =
κr κ
(3)
By above-mentioned formula the relationship between the external quantum efficiency and the radiative decay rate can be expressed as follows:
ηint2 ηint1
=
ηpower2 ηpower1
=
κr2 κ × 1 κr1 κ2
(4)
Subscript 2 is the device with the CSC-EIS, and subscript 1 is the device with the CEIS. The parameters, ηpower2 ¼1.43, ηpower1 ¼1.07, κ2 ¼3.57, and κ1 ¼ 3.33, is used to calculate the ratio of the κr2 to κr1. Therefore, the radiative decay rate of the device with the CSC-EIS is enhanced by 43% compared with that of the device with the CEIS.
4. Conclusion In summary, the CSC-EIS is introduced between the electrontransport layer and the cathode. The current density of the device with the CSC-EIS is remarkably increased compared with that of the device with the CEIS because the localized electric field around SNCs is enhanced. Furthermore, the operation voltage of the device with the CSC-EIS is lowered than that of the CEIS because the electron-injection barrier between the cathode and the electron-transport layer is remarkably reduced by CSC-EIS. In addition,
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the electron mobility of the device with the CSC-EIS is enhanced by about 122% compared with that of the device with the CEIS. Moreover, the radiative decay rate of the device with the CSC-EIS is enhanced by 43% compared with that of the device with the CEIS.
Acknowledgments This work was partially supported by the Ministry of Science and Technology of Taiwan under Contract no. NSC-102-2221-E390-019-MY2.
References [1] T. Matsushima, M. Takamori, Y. Miyashita, Y. Honma, T. Tanaka, H. Aihara, H. Murata, Org. Electron. 11 (2010) 16. [2] S. Naka, H. Okada, H. Onnagawa, Appl. Phys. Lett. 76 (2000) 197.
[3] Y. Li, D.Q. Zhang, L. Duan, R. Zhang, L.D. Wang, Y. Qiu, Appl. Phys. Lett. 90 (2007) 012119. [4] C.Y. Cho, S.J. Lee, J.H. Song, S.H. Hong, S.M. Lee, Y.H. Cho, S.J. Park, Appl. Phys. Lett. 98 (2011) 051106-1. [5] F. Liu, J.M. Nunzi, Org. Electron. 13 (2012) 1623. [6] Y. Xiao, J.P. Yang, P.P. Cheng, J.J. Zhu, Z.Q. Xu, Y.H. Deng, S.T. Lee, Y.Q. Li, J. X. Tang, Appl. Phys. Lett. 100 (2012) 013308-1. [7] K.C. Tien, M.S. Lin, Y.H. Lin, C.H. Tsai, M.H. Shiu, M.C. Wei, H.C. Cheng, C.L. Ling, H.W. Lin, C.C. Wu, Org. Electron. 11 (2010) 397. [8] W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424 (2003) 824. [9] Y.C. Chen, C.Y. Gao, K.L. Chen, T.H. Meen, C.J. Huang, J. Nanomater. 2013 (2013) 2 (article ID 841436). [10] Y.C. Chen, C.Y. Gao, K.L. Chen, C.J. Huang, Appl. Surf. Sci. 295 (2014) 266. [11] S.H. Feng, M.C. Shih, C.J. Huang, C.T. Chung, Thin Solid Films 517 (2009) 2719. [12] E.F. Schubert, Light-Emitting Diodes, Cambridge University, England, 2006. [13] H.C. Kuo, F.Y. Lai, S.Y. Kuo, Principles and Applications of Light-Emitting Diode, Wunan, Taipei, 2011.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Enhanced radiative decay in organic light-emitting diodes by combination of cesium carbonate and silver nanocluster Ying-Chung Chen a, Chia-Yuan Gao a, Kan-Lin Chen b, Chien-Jung Huang c,n a
Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan Department of Electronic Engineering, Fortune Institute of Technology, Kaohsiung, Taiwan c Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan b
art ic l e i nf o
a b s t r a c t
Article history: Received 7 January 2015 Received in revised form 24 April 2015 Accepted 21 May 2015 Available online 29 May 2015
The influence of the cesium carbonate/silver nanocluster/cesium carbonate electron-injection structure (CSC-EIS) is investigated in this study. When the CSC-EIS replaces the cesium carbonate electron-injection structure (CEIS), the shorter response time and rising time is obtained by measuring transient electroluminescence. In addition, the radiative decay rate for the device with the CSC-EIS is enhanced by 43% compared with that of the device with the CEIS because the localized electric field around SNCs is enhanced and the electron-injection barrier between the cathode and the electron-transport layer is remarkably reduced, resulting in the increase in the electron injection from the cathode to emitting layer. & 2015 Elsevier B.V. All rights reserved.
Keywords: Electron injection Transient electroluminescence Radiative decay rate Electric field
1. Introduction Organic light-emitting diode (OLED) has exhibited significant potentiality in the application of display and lighting because of its low power consumption. Thus far, the efficiency of the device is still not optimal. Generally, the mobility of electrons in the electron-transport layer is a few orders of magnitude lower than that of holes in the hole-transport layer [1,2], resulting in the imbalance between electrons and holes within the emitting layer. Moreover, the imbalance between electrons and holes within the emitting layer leads to a decrease in the efficiency of device. The metals of low work function or metal alloys such as Mg:Ag, Ca, and Li are used as cathodes to decrease the electron-injection barrier between the cathode and the electron-transport layer. However, the above-mentioned metals are neither simple fabrication nor stable. Cesium carbonate (Cs2CO3) is described to be the best electron injection material because the electron-injection barrier between the cathode and the electron-transport layer can be remarkably reduced when Cs2CO3 is inserted between the cathode and the electron-transport layer [3]. In addition, it has attracted a lot of attention that the surface plasmon resonance effect (SPRE) of metal nanoparticle and nanocluster is applied to OLED and inorganic light-emitting diode [4–7]. It is well known that the SPRE is caused by interaction between the surface charge n
Corresponding author. Tel.: þ 886 7 5919475; fax: þ886 7 5919357. E-mail address: [email protected] (C.-J. Huang).
http://dx.doi.org/10.1016/j.jlumin.2015.05.046 0022-2313/& 2015 Elsevier B.V. All rights reserved.
of metal and electromagnetic field of the incident light [8]. When the overlap is excellent between the absorption spectrum of metal nanocluster and the PL spectrum of Alq3, the characteristic of the device can be significantly enhanced [9]. However, no one has combined the advantages of metal nanocluster and Cs2CO3, nor anyone has applied it to electron injection structure of OLED. In this article, the silver nanoclusters (SNCs) are introduced between the Cs2CO3 to enhance the characteristic of device by the SPRE of the SNCs and the excellent electron injection ability of the Cs2CO3. Furthermore, a detailed mechanism of the increase in the characteristic of the device by transient electroluminescence measurements is presented.
2. Experimental The schematic structure of OLEDs with the cesium carbonate/ silver nanocluster/cesium carbonate electron-injection structure (CSC-EIS) is shown in Fig. 1. Indium tin oxide (ITO) coated on glass with a sheet resistance of 10 Ω/sq was used as the starting substrate. The substrate was cleaned with acetone, methanol and deionized water, and then dried with nitrogen gas. After cleaning process, the substrates were loaded into a thermal evaporator. Afterward, N,N'-bis-(1-naphthyl)-N,N-dipheny1,1'-bipheny1-4diamine (NPB; 35 nm), tris-(8-hydroxyquinoline) aluminum (Alq3; 40 nm), 4,7-dipheny1-1,10-phenanthroline (BPhen; 10 nm), Cs2CO3 (1 nm), SNCs, Cs2CO3 (1 nm), and aluminum (Al; 100 nm)
Y.-C. Chen et al. / Journal of Luminescence 166 (2015) 248–252
249
Fig. 1. Schematic device structure of the OLED with the CSC-EIS.
3. Results and discussion Fig. 2(a) demonstrates the SEM image of SNCs on glass substrate. After calculation, the mean cluster size of SNCs in Fig. 2 (a) was found to be 34 nm. Fig. 2(b) demonstrates the absorption spectrum of SNCs with mean cluster size of 34 nm as well as the PL spectrum of Alq3. The result shows that the absorption spectrum of SNCs with mean cluster size of 34 nm is about 525 nm and that the PL spectrum of Alq3 is 523 nm or so. In addition, a significant overlap exists between the absorption spectrum of SNCs and the PL spectrum of Alq3. It is fully understood that the excellent overlap between the two spectra suggests that they can be used for SP-exciton coupling. In other words, this overlap implied that these SNCs can be used to increase the characters of OLEDs. The resulting J–V curves of the devices were plotted in Fig. 3(a). It can be clearly seen that the current density of the device with the CSC-EIS is remarkably increased due to the SPRE of the SNCs, resulting in that the localized electric field around SNCs is enhanced [10]. In addition, it is observed that the operation voltage of the device with the CSC-EIS is lower than that of the device with the CEIS because the electron-injection barrier between the cathode and the electron-transport layer is remarkably reduced by
1.2
1.2
PL spectrum of Alq 3
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0 350
400
450
500
550
600
Absorption (a.u.)
Absorption spectrum of SNCs
Emission intensity (a.u.)
were deposited. In addition, the Cs2CO3 thickness for Cs2CO3 electron-injection structure (CEIS) is 1 nm. NPB and BPhen were used as the hole and electron transport layers, respectively. Alq3 was used as an emitting layer. Cs2CO3, Al and ITO were used as electron-injection layer, cathode and anode, respectively. In addition, the electron-only devices with CSC-EIS and CEIS were fabricated in order to further confirm the relationship between the electric injection and current density. The electron-only device with CSC-EIS and CEIS consist of ITO/Alq3 (40 nm)/BPhen (10 nm)/ Cs2CO3 (1 nm)/SNCs/Cs2CO3 (1 nm)/Al (100 nm) and ITO/Alq3 (40 nm)/BPhen (10 nm)/Cs2CO3 (1 nm)/Al (100 nm). During the deposition, the base pressure of the chamber was maintained as low as 2.4 10 6 Torr. The active area of the device was 6 6 mm2. The deposition rates of all organic materials were maintained at 0.01 nm/s, except for the Al where deposition rate was 0.5 nm/s. The thickness of the layers is controlled by using a quartz-crystal monitor. In addition, the SNCs were fabricated by thermal evaporation of silver slug. The deposition rate and the deposition time are 0.01 nm/s and 300 s respectively. The current density–voltage (J–V) and luminance–voltage (L–V) characteristics of the devices were measured by using a Keithley 2400 (Keithley instruments Inc., USA) and a PR-655 (Photo Research Inc., USA), respectively. All measurements were made at room temperatures under air ambient. In terms of the transient electroluminescence (EL) measurements, the repetition rate and width of the pulse were 1 kHz and 5 μs, respectively. The light output was detected by a fast-biased silicon photodiode ET-2020 (Electro-Optics Technology Inc., USA).
0.0 650
Wavelength (nm) Fig. 2. (a) The SEM image of SNCs on the glass surface. (b) Absorption spectrum of SNCs and PL spectrum of Alq3.
CSC-EIS. It is well known that the electron-injection barrier between the cathode and the electron-transport layer can be remarkably reduced when Cs2CO3 is inserted between the cathode and the electron-transport layer. In addition, the electron injection is enhanced and operation voltage is decreased for the device with CSC-EIS because the SNCs adhered to the cathode, resulting in numerous sharp tips to produce a stronger electric field. Furthermore, the stronger electric field assists electrons to tunnel through the triangular potential barrier [5]. The resulting J–V curves of electron-only devices were plotted in the inset of Fig. 3(a). It is observed that the current density of device with CSC-EIS is higher than that of the device with CEIS, which suggests that the CSC-EIS is useful for enhancement of electron injection. Fig. 3(b) shows the L–V curves of devices with CEIS and CSC-EIS. It is observed that the maximum luminance of devices with CSC-EIS is higher than that of the device with CEIS because the device with CSC-EIS not only enhanced the electron-injection ability from the cathode to the emitting layer but also enhanced the spontaneous recombination of excitons. In addition, the power efficiency-luminance curves of devices with CEIS and CSC-EIS were plotted in the inset of Fig. 3(b). The power efficiency of the device with CSC-EIS is quite high compared with that of the device with CEIS. Furthermore, the increasing rate for power efficiency is even more at higher operation voltage. Perhaps it is due to the fact that the SPRE caused by interaction between the surface charge of metal and the incident light of emitting layer is not obviously shown when the luminance is lower; contrastly, when luminance is higher, the SPRE is obviously shown. With regard to the analysis of carriertransport behaviors, the dynamic behavior of EL is important.
250
Y.-C. Chen et al. / Journal of Luminescence 166 (2015) 248–252
20
CEIS CSC-EIS
250
EL Intensity (a. u.)
Current density (mA/cm 2)
300
200 150 100
4V 5V 6V 7V 8V 9V
15
10
5
50 0
0
2
4
6
8
10
0
2
4
6
8
Time (µs)
12
12000 11000 10000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0
20
CEIS CSC-EIS
EL Intensity (a. u.)
Luminance (cd/m 2)
Voltage (V)
15
10
5
0
0
2
4
6
8
10
12
Voltage (V)
4V 5V 6V 7V 8V 9V
0
2
4
6
8
Time (µs) Fig. 4. The transient EL as a function of time with (a) the CEIS and (b) the CSC-EIS by various voltage pulses.
Fig. 3. (a) The current density–voltage curves and (b) the luminance–voltage curves of devices with the CEIS and the CSC-EIS. Inset of (a) and (b) shows the current density–voltage curves of electron-only devices and power efficiencyluminance characteristics of devices, respectively.
2.2 CEIS CSC-EIS
2.0 1.8
Response Time (µs)
Therefore, the transient EL was also measured in this article to further understand the carrier-transport behaviors. Fig. 4(a) and (b) shows the transient EL as a function of time with the CEIS and the CSC-EIS by various voltage pulses. It is observed that response time was shortened with increase in applied voltages. The response time is defined as the time required before light emission onset after the pulse voltage switched on. The shorter response time suggests that holes and electrons were faster injection to the emitting layer from the electrode. In addition, it is also observed that rising time was shorter with increase in applied voltages. The rising time is defined as the time required for the response to rise from 10% to 90% of maximum EL intensity. The shorter rising time suggests that the maximum number of excitons was faster achieved. Fig. 5 shows the response time–voltage curves of the OLEDs with the CEIS and the CSC-EIS by various voltage pulses. It is observed that response time of the device with the CEIS was shortened with increase in voltage pulses because the carrier mobility was faster from the electrode to the emitting layer. Moreover, response time of the device with the CSC-EIS became a constant within the large voltage pulses range (Vpulses Z7 V). The constant response time suggests that the saturation of carrier mobility can be faster achieved than that of the device with the CEIS [11]. In addition, response time of the devices with the CEIS and the CSC-EIS at 4 V is 2.02 and 0.91 μs, respectively. It is well
1.6 1.4 1.2 1.0 0.8 0.6 0.4
4
5
6
7
8
9
Voltage (V) Fig. 5. The response time–voltage curves of devices with the CEIS and the CSC-EIS.
known, the carrier mobility is inversely proportional to the time. Furthermore, the CSC-EIS only changed the electron-injection structure. Therefore, it can be extrapolated that the carrier mobility was enhanced due to the increase in the electron mobility. Moreover, the electron mobility of the device with the CSC-EIS was enhanced by about 122% compared with that of the device with the CEIS.
Y.-C. Chen et al. / Journal of Luminescence 166 (2015) 248–252
24
1.0 CEIS CSC-EIS
22 18 16 14 12 10 8
CEIS CSC-EIS
0.8
EL Intensity (a. u.)
20
EL Intensity (a. u.)
251
0.6 0.4 0.2
6 4 2 0
0.0 0
1
2
3
4
5
6
7
8
5.2
5.6
9
6.0
6.4
6.8
Time (µs)
Time (µs)
Fig. 7. The transient EL decay as a function of time at 9 V for devices with the CEIS and the CSC-EIS.
4.0
CEIS CSC-EIS
Rising Time (µs)
3.5
expressed as follows [12]:
κ = κr + κnr =
3.0
1 τ
(1)
2.5
where κ is the total decay rate, κr is the radiative decay rate, κnr is the nonradiative, and τ is the decay time. Furthermore, the external quantum efficiency can be expressed as follows [13]:
2.0
ηext =
1.5 1.0
4
5
6
7
8
9
Voltage (V) Fig. 6. (a) The transient EL of devices by 9 V voltage pulse and (b) the rising time– voltage curves of devices with the CEIS and the CSC-EIS.
Fig. 6(a) shows the transient EL of OLEDs with the CEIS and the CSC-EIS by 9 V voltage pulse. It can be clearly seen that the rising time of the device with the CSC-EIS is shorter than that of the device with the CEIS. The rising time of devices with the CEIS and the CSC-EIS at 9 V is 1.52 and 1.22 μs, respectively. It suggests that the time of excitons achieving the maximum number within the emitting layer for the CSC-EIS is faster by 20% compared with that of device with the CEIS. Fig. 6(b) shows the rising time–voltage curves of OLEDs with the CEIS and the CSC-EIS by various voltage pulses. It is observed that the rising time is shortened with increase in the voltage pulses because the electron mobility is enhanced with increase in the voltage pulses. The transient EL decay as a function of time at 9 V for devices with the CEIS and the CSC-EIS is shown in Fig. 7. The transient EL decay can be fitted with a single exponential to obtain decay time. The decay time of devices with the CEIS and the CSC-EIS is 0.3 and 0.28 μs, respectively. The decay time of the device with the CSCEIS is shorter than that of the CEIS due to the surface plamons of SNCs. The decay time is defined as the time for exciton from excited state to ground state. Since the coupling process between surface plasmons and radiated light is rapider than spontaneous recombination of excitons, the decay time of the exciton in surface plasmon resonance device is reduced. In addition, the decay time is the reciprocal of the decay rate. Moreover, the decay rate is the sum of the radiative decay rate and the nonradiative decay rate. The relationship between the decay time and the decay rate can be
ηpower × IV /(hν) P /(hν) = = ηint × ηextr I /e I /e
(2)
where ηext is the external quantum efficiency, P is the photon power, hν is the photon energy, I is the input current, V is the input voltage, e is the quantity of electric charge, ηpower is the power efficiency, ηint is the internal quantum efficiency, and ηextr is the extraction efficiency. Because the extraction efficiencies of devices with the CEIS and the CSC-EIS are very close, the ηpower is proportional to the ηint. Moreover, the relationship between the internal quantum efficiency and the radiative decay rate can be expressed as follows:
ηint =
κr κ
(3)
By above-mentioned formula the relationship between the external quantum efficiency and the radiative decay rate can be expressed as follows:
ηint2 ηint1
=
ηpower2 ηpower1
=
κr2 κ × 1 κr1 κ2
(4)
Subscript 2 is the device with the CSC-EIS, and subscript 1 is the device with the CEIS. The parameters, ηpower2 ¼1.43, ηpower1 ¼1.07, κ2 ¼3.57, and κ1 ¼ 3.33, is used to calculate the ratio of the κr2 to κr1. Therefore, the radiative decay rate of the device with the CSC-EIS is enhanced by 43% compared with that of the device with the CEIS.
4. Conclusion In summary, the CSC-EIS is introduced between the electrontransport layer and the cathode. The current density of the device with the CSC-EIS is remarkably increased compared with that of the device with the CEIS because the localized electric field around SNCs is enhanced. Furthermore, the operation voltage of the device with the CSC-EIS is lowered than that of the CEIS because the electron-injection barrier between the cathode and the electron-transport layer is remarkably reduced by CSC-EIS. In addition,
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the electron mobility of the device with the CSC-EIS is enhanced by about 122% compared with that of the device with the CEIS. Moreover, the radiative decay rate of the device with the CSC-EIS is enhanced by 43% compared with that of the device with the CEIS.
Acknowledgments This work was partially supported by the Ministry of Science and Technology of Taiwan under Contract no. NSC-102-2221-E390-019-MY2.
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