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Organic light-emitting devices with triphenylphosphine oxide layer
Synthetic Metals 158 (2008) 617–619
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Organic light-emitting devices with triphenylphosphine oxide layer M.Y. Ha, D.G. Moon ∗ Department of Materials Engineering, Soonchunhyang University, 646 Eupnae-ri, Shinchang-myeon, Asan-si, Chungcheongnam-do 336-745, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 March 2008 Accepted 14 April 2008 Available online 2 June 2008 Keywords: Organic light-emitting device Low voltage driving Electron transport layer Triphenylphosphine oxide Ph3 PO
a b s t r a c t We have developed organic light-emitting devices using triphenylphosphine oxide (Ph3 PO) layers. The operating voltage of device is substantially reduced by using a Ph3 PO layer. For example, the required voltages for a current density of 20 mA/cm2 are 3.5 and 9.7 V for the devices with Ph3 PO and Alq3 layers, respectively. Good electron transporting property of Ph3 PO results in a high luminance of 1000 cd/m2 at a low driving voltage of 4.1 V in a device with a structure of ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 10 nm)/␣-NPD (30 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the first report on efficient organic light-emitting devices (OLEDs) by Tang and VanSlyke, extensive researches have been conducted to reduce the driving voltage [1]. The driving voltage is lowered by using a low work function cathode, or by inserting a very thin Li-compound between the cathode and the organic layer. These methods improve the injection of electrons into the organic layer from the cathode, resulting in the driving voltage of 6–10 V at a luminance of 1000 cd/m2 [2–4]. However, further lowering of the driving voltage is limited by the low mobility of the electron transport layer. For example, the electron mobility of a typical electron transport material, tris-(8hydroxyquinoline)aluminum (Alq3 ) is about two orders of magnitude lower than the hole mobility (∼10−3 cm2 /Vs) of a typical hole transport layer, 4,4 -bis-[N-(1-naphtyl)-N-phenyl-amino]biphenyl (␣-NPD) [5,6]. Many authors have researched high electron mobility materials such as 4,7-diphenyl-1,10-phenanthroline (BPhen) [7], bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2 ) [8]. Although these materials show better electron transport property than Alq3 , high voltage is still required to generate a high luminance. Another approach is to dope low work function metals such as Li, Sr into the electron transport layer [9]. These dopants create free electrons in the electron transport layer, resulting in low electron injection barrier and high conductivity. However, these
∗ Corresponding author. Tel.: +82 42 530 1312; fax: +82 42 530 1722. E-mail address: [email protected] (D.G. Moon). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.04.009
metals may diffuse into the emission layer, causing a quenching of luminescence [10,11]. Recently, organic phosphorous compounds have been used for obtaining low voltage driving devices [12–14]. Oyamada et al. reported Cs-doped phenyldipyrenylphosphine oxide (POPy2 ) as an electron injection material [12]. Other organic phosphorous materials such as 4,4 -bis(diphenylphosphine oxide)biphenyl (PO1) [13] and N-(4-diphenylphosphoryl phenyl)carbazole (MPO12) [14] have been proposed as electron transporting host layers for the blue electrophosphorescent dopants. In this paper, we report low voltage driving OLEDs with triphenylphosphine oxide (Ph3 PO) layer. Ph3 PO has been used to form complexes with lanthanide metal ions for using an emitter in OLEDs [15,16]. However, it has not been used as an electron transport layer. We demonstrate a significant reduction of the driving voltage by using a Ph3 PO layer.
2. Experimental OLEDs were fabricated on indium tin oxide (ITO) coated glass substrates. The sheet resistance of ITO film was about 10 /. After defining the ITO anode patterns using standard photolithography process, the substrates were cleaned with isopropyl alcohol and deionized water followed by exposing to oxygen plasma. All organic and metal layers were deposited by using a thermal evaporation method in a base pressure of about 1 × 10−6 mbar. A 15-nm thick 4,4 ,4 -tris[N-(2-naphthyl)-Nphenyl-amino]-triphenylamine (2-TNATA) layer was deposited on the patterned ITO substrate, followed by the deposition of a 40nm thick ␣-NPD layer. In some devices, rubrene molecules were
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M.Y. Ha, D.G. Moon / Synthetic Metals 158 (2008) 617–619
doped into the ␣-NPD layer. After then, a 60 nm Ph3 PO or Alq3 layer was deposited. Here, Alq3 layer was used to be a referenced one. The deposition rates of 2-TNATA, ␣-NPD, and Alq3 layers were maintained to be 0.1 nm/s. The Ph3 PO layer was evaporated with a rate of 0.05 nm/s. After depositions of the organic layers, a 0.5-nm thick LiF and a 100-nm thick Al layers were sequentially evaporated through a shadow mask. All the completed devices were encapsulated without exposing to air in a nitrogen atmosphere glove box. Current density–voltage–luminance (J–V–L) characteristics of the devices were measured using computer controlled Keithley 2400 source-measure units and a luminance meter (Minolta LS100). Electroluminescence (EL) and photoluminescence (PL) spectra were measured with a spectroradiometer (Minolta CS1000). 3. Result and discussion Fig. 1 shows the J–V–L curves of the devices with Ph3 PO and Alq3 layers. Device structure was ITO/2-TNATA (15 nm)/␣-NPD (40 nm)/Ph3 PO (60 nm) or Alq3 (60 nm)/LiF (0.5 nm)/Al. The voltage for the current density is substantially reduced by using a Ph3 PO layer. For example, the voltages for achieving the current density of 20 mA/cm2 are 3.5 and 9.7 V for the Ph3 PO and Alq3 devices, respectively. The higher current density in the Ph3 PO devices means that Ph3 PO has a good electron transporting property as the other organic phosphorous compounds [12–14]. In order to investigate the effect of Ph3 PO layer on the J–V curves, the thickness of Ph3 PO layer was varied from 20 to 100 nm. The J–V curves are almost independent of the thickness of Ph3 PO layer, indicating that most of the applied voltage is across the 2-TNATA and ␣-NPD layers. These results support that Ph3 PO has a good capability of transporting electrons. The driving voltage for luminance output is also reduced by using a Ph3 PO layer. The turn-on voltage (a voltage for a luminance of 1 cd/m2 ) is 2.8 V in the Ph3 PO device. The luminance increases rapidly as the driving voltage increases above the turn-on voltage, being a 100 cd/m2 at a voltage of 3.7 V. The EL spectrum of the Ph3 PO device is shown in Fig. 2. The EL spectrum shows the peak intensity at 450 nm, which coincides with the PL spectrum of ␣-NPD. The figure indicates that the ␣-NPD layer in the Ph3 PO devices acts as an emitting material. Any visible light was not observed in the Ph3 PO film or solution under the UV irradiation. Since the emission from the ␣-NPD layer is not efficient, the rubrene molecules were doped into the ␣-NPD layer. Fig. 3 shows the J–V–L curves of the rubrene-doped device with a Ph3 PO layer. The device has a structure of ITO/2-TNATA (15 nm)/␣NPD:rubrene (1%, 40 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al. The
Fig. 1. Current density (solid)–voltage–luminance (open) curves of devices with Ph3 PO (circle) and Alq3 (square) layers. Device structure: ITO/2-TNATA (15 nm)/␣NPD (40 nm)/Ph3 PO or Alq3 (60 nm)/LiF (0.5 nm)/Al. Inset: chemical structure of Ph3 PO.
Fig. 2. EL spectrum of device ITO/2-TNATA (15 nm)/␣-NPD (40 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al.
voltage for the same current density increases by doping of rubrene molecules into the ␣-NPD layer. For example, the voltages for 20 mA/cm2 are 3.5 and 5.1 V for the undoped (seen in Fig. 1) and doped devices, respectively. The increase of an applied voltage in a rubrene-doped device may be due to the change of the carrier mobility by the rubrene molecules. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of rubrene are 5.4 and 3.2 eV, respectively [17]. The HOMO and LUMO levels of ␣-NPD are 5.7 and 2.6 eV, respectively [18]. Since the HOMO and LUMO levels of rubrene are located inside the band gap of ␣-NPD, rubrene molecules act as traps for the carriers, reducing the carrier mobility. The turn-on voltage of the rubrene-doped device is 3.3 V, and the required voltages for the luminances of 100 and 1000 cd/m2 are 4.1 and 5.1 V, respectively. Although the driving voltage increases slightly by rubrene, the EL efficiency substantially increases from 0.5 cd/A for the undoped device to 5.7 cd/A for the doped device. The inset of Fig. 3 shows the EL spectrum of rubrene-doped device. The spectrum shows strong emission at 557 nm originating from rubrene molecular sites and weak emission due to the ␣-NPD host. For reducing the driving voltage of rubrene-doped device, the emission zone has been investigated by preparing two kinds of devices; ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 10 nm)/␣-NPD (30 nm)/Ph3 PO (60 nm)/LiF/Al (device A) and ITO/2TNATA (15 nm)/␣-NPD (30 nm)/␣-NPD:rubrene (1%, 10 nm)/Ph3 PO (60 nm)/LiF/Al (device B). In device A, the rubrene molecules were doped into the ␣-NPD layer adjacent to the 2-TNATA interface. In device B, the doped region was located to be contacted with the
Fig. 3. Current density (solid)–voltage–luminance (open) curves and EL spectrum (inset) of the rubrene-doped device with Ph3 PO layer. Device structure: ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 40 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al.
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entire ␣-NPD layer (device of Fig. 3). On the other hand, the driving voltage for the luminance output is higher in device B, because of the low current density resulting from rubrene traps and the low EL efficiency owing to the emission from an undoped ␣-NPD layer. 4. Conclusion
Fig. 4. EL spectra of devices A and B. Device A: ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 10 nm)/␣-NPD (30 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al, device B: ITO/2-NTATA (15 nm)/␣-NPD (30 nm)/␣-NPD:rubrene (10 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al.
Ph3 PO layer. In both devices, the thickness of the doped ␣-NPD layer was fixed to be 10 nm. Fig. 4 shows the EL spectra of devices A and B. Device A shows strong rubrene and weak ␣-NPD emissions. On the other hand, device B shows strong ␣-NPD and weak rubrene emissions. These results indicate that most of emission sites are placed in the ␣NPD layer adjacent to the 2-TNATA interface. Since the emission in rubrene-doped devices is mainly due to the direct recombination on the rubrene molecules [17], the electrons injected from the cathode are recombined with holes in the emission layer adjacent to the 2-TNATA interface. In addition, although the electron mobility in ␣-NPD is expected to be extremely low, the electrons transport several hundreds of angstroms inside the ␣-NPD layer before recombining with holes. The voltage for the same current density is lower in device A. For example, the voltages for a current density of 20 mA/cm2 are 4.1 and 4.9 V for devices A and B, respectively. The higher operating voltage in device B suggests that the electrons injected from Ph3 PO layer are trapped by rubrene molecules before being transported to the recombination zone. The L–V curves show the same tendency as the J–V curves. In device A, the turn-on voltage is 2.8 V, and the luminance of 1000 cd/m2 is achieved at 4.1 V. Therefore, low voltage operating devices can be realized by controlling the doped zone and by using a Ph3 PO layer. The EL efficiency of device A is 5 cd/A at 1000 cd/m2 , being almost same as that of the device doped with rubrene molecules into the
We have fabricated low voltage driving OLEDs using Ph3 PO layers. The Ph3 PO layer has a high electron transporting property, resulting in the light output from ␣-NPD layer adjacent to the 2-TNATA interface, in a device with a structure of ITO/2-TNATA/␣NPD/Ph3 PO/LiF/Al. In this device, when the rubrene molecules are doped in the entire ␣-NPD layer, the operating voltage increases as the dopants act as traps for electrons. By doping the rubrene molecules into the emission zone of the ␣-NPD layer, the voltage rising due to the rubrene traps can be reduced. With a structure of ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 10 nm)/␣NPD (40 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al, a high luminance of 1000 cd/m2 is achieved at a low driving voltage of 4.1 V. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] T. Wakimoto, Y. Fukuda, K. Nagayama, A. Yokoi, H. Nakada, M. Tsuchida, IEEE Trans. Elect. Dev. 44 (1997) 1245. [3] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152. [4] Z. Liu, O.V. Salata, N. Male, Synth. Met. 128 (2002) 211. [5] S. Naka, H. Okada, H. Onnagawa, Y. Yamaguchi, T. Tsutsui, Synth. Met. 111 (2000) 331. [6] P.G. Kepler, P.M. Beeson, S.J. Jacobs, R.A. Anderson, M.B. Sinciair, V.S. Valencia, P.A. Cahill, Appl. Phys. Lett. 66 (1995) 3618. [7] S. Naka, H. Okada, H. Onnagawa, Appl. Phys. Lett. 76 (2000) 197. [8] Y. Hamada, T. Sano, K. Shibata, K. Kuroki, Jpn. J. Appl. Phys. 34 (1995) L824. [9] J. Kido, T. Matsumoto, Appl. Phys. Lett. 73 (1998) 2866. ¨ [10] V. Choong, Y. Park, Y. Gao, T. Wehrmeister, K. Mullen, B.R. Hsieh, C.W. Tang, Appl. Phys. Lett. 69 (1996) 1492. [11] E.I. Haskal, A. Curioni, P.F. Seidler, W. Andreoni, Appl. Phys. Lett. 71 (1997) 1151. [12] T. Oyamada, H. Sasabe, C. Adachi, S. Murase, T. Tominaga, C. Maeda, Appl. Phys. Lett. 86 (2005) 033503. [13] P.E. Burrows, A.B. Padmaperuma, L.S. Sapochak, P. Djurovich, M.E. Thompson, Appl. Phys. Lett. 88 (2006) 183503. [14] X. Cai, A.B. Padmaperuma, L.S. Sapochak, P.A. Vecchi, P.E. Burrows, Appl. Phys. Lett. 92 (2008) 083308. [15] S. Capecchi, O. Renault, D.G. Moon, M. Halim, M. Etchells, P.J. Dobson, O.V. Salata, V. Christou, Adv. Mater. 12 (2000) 1591. [16] D.G. Moon, O.V. Salata, M. Etchells, P.J. Dobson, V. Christou, Synth. Met. 123 (2001) 355. [17] M. Murata, C.D. Merritt, Z. Kafafi, IEEE J. Sel. Top. Quant. Electron. 4 (1998) 119. [18] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Organic light-emitting devices with triphenylphosphine oxide layer M.Y. Ha, D.G. Moon ∗ Department of Materials Engineering, Soonchunhyang University, 646 Eupnae-ri, Shinchang-myeon, Asan-si, Chungcheongnam-do 336-745, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 March 2008 Accepted 14 April 2008 Available online 2 June 2008 Keywords: Organic light-emitting device Low voltage driving Electron transport layer Triphenylphosphine oxide Ph3 PO
a b s t r a c t We have developed organic light-emitting devices using triphenylphosphine oxide (Ph3 PO) layers. The operating voltage of device is substantially reduced by using a Ph3 PO layer. For example, the required voltages for a current density of 20 mA/cm2 are 3.5 and 9.7 V for the devices with Ph3 PO and Alq3 layers, respectively. Good electron transporting property of Ph3 PO results in a high luminance of 1000 cd/m2 at a low driving voltage of 4.1 V in a device with a structure of ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 10 nm)/␣-NPD (30 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the first report on efficient organic light-emitting devices (OLEDs) by Tang and VanSlyke, extensive researches have been conducted to reduce the driving voltage [1]. The driving voltage is lowered by using a low work function cathode, or by inserting a very thin Li-compound between the cathode and the organic layer. These methods improve the injection of electrons into the organic layer from the cathode, resulting in the driving voltage of 6–10 V at a luminance of 1000 cd/m2 [2–4]. However, further lowering of the driving voltage is limited by the low mobility of the electron transport layer. For example, the electron mobility of a typical electron transport material, tris-(8hydroxyquinoline)aluminum (Alq3 ) is about two orders of magnitude lower than the hole mobility (∼10−3 cm2 /Vs) of a typical hole transport layer, 4,4 -bis-[N-(1-naphtyl)-N-phenyl-amino]biphenyl (␣-NPD) [5,6]. Many authors have researched high electron mobility materials such as 4,7-diphenyl-1,10-phenanthroline (BPhen) [7], bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2 ) [8]. Although these materials show better electron transport property than Alq3 , high voltage is still required to generate a high luminance. Another approach is to dope low work function metals such as Li, Sr into the electron transport layer [9]. These dopants create free electrons in the electron transport layer, resulting in low electron injection barrier and high conductivity. However, these
∗ Corresponding author. Tel.: +82 42 530 1312; fax: +82 42 530 1722. E-mail address: [email protected] (D.G. Moon). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.04.009
metals may diffuse into the emission layer, causing a quenching of luminescence [10,11]. Recently, organic phosphorous compounds have been used for obtaining low voltage driving devices [12–14]. Oyamada et al. reported Cs-doped phenyldipyrenylphosphine oxide (POPy2 ) as an electron injection material [12]. Other organic phosphorous materials such as 4,4 -bis(diphenylphosphine oxide)biphenyl (PO1) [13] and N-(4-diphenylphosphoryl phenyl)carbazole (MPO12) [14] have been proposed as electron transporting host layers for the blue electrophosphorescent dopants. In this paper, we report low voltage driving OLEDs with triphenylphosphine oxide (Ph3 PO) layer. Ph3 PO has been used to form complexes with lanthanide metal ions for using an emitter in OLEDs [15,16]. However, it has not been used as an electron transport layer. We demonstrate a significant reduction of the driving voltage by using a Ph3 PO layer.
2. Experimental OLEDs were fabricated on indium tin oxide (ITO) coated glass substrates. The sheet resistance of ITO film was about 10 /. After defining the ITO anode patterns using standard photolithography process, the substrates were cleaned with isopropyl alcohol and deionized water followed by exposing to oxygen plasma. All organic and metal layers were deposited by using a thermal evaporation method in a base pressure of about 1 × 10−6 mbar. A 15-nm thick 4,4 ,4 -tris[N-(2-naphthyl)-Nphenyl-amino]-triphenylamine (2-TNATA) layer was deposited on the patterned ITO substrate, followed by the deposition of a 40nm thick ␣-NPD layer. In some devices, rubrene molecules were
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M.Y. Ha, D.G. Moon / Synthetic Metals 158 (2008) 617–619
doped into the ␣-NPD layer. After then, a 60 nm Ph3 PO or Alq3 layer was deposited. Here, Alq3 layer was used to be a referenced one. The deposition rates of 2-TNATA, ␣-NPD, and Alq3 layers were maintained to be 0.1 nm/s. The Ph3 PO layer was evaporated with a rate of 0.05 nm/s. After depositions of the organic layers, a 0.5-nm thick LiF and a 100-nm thick Al layers were sequentially evaporated through a shadow mask. All the completed devices were encapsulated without exposing to air in a nitrogen atmosphere glove box. Current density–voltage–luminance (J–V–L) characteristics of the devices were measured using computer controlled Keithley 2400 source-measure units and a luminance meter (Minolta LS100). Electroluminescence (EL) and photoluminescence (PL) spectra were measured with a spectroradiometer (Minolta CS1000). 3. Result and discussion Fig. 1 shows the J–V–L curves of the devices with Ph3 PO and Alq3 layers. Device structure was ITO/2-TNATA (15 nm)/␣-NPD (40 nm)/Ph3 PO (60 nm) or Alq3 (60 nm)/LiF (0.5 nm)/Al. The voltage for the current density is substantially reduced by using a Ph3 PO layer. For example, the voltages for achieving the current density of 20 mA/cm2 are 3.5 and 9.7 V for the Ph3 PO and Alq3 devices, respectively. The higher current density in the Ph3 PO devices means that Ph3 PO has a good electron transporting property as the other organic phosphorous compounds [12–14]. In order to investigate the effect of Ph3 PO layer on the J–V curves, the thickness of Ph3 PO layer was varied from 20 to 100 nm. The J–V curves are almost independent of the thickness of Ph3 PO layer, indicating that most of the applied voltage is across the 2-TNATA and ␣-NPD layers. These results support that Ph3 PO has a good capability of transporting electrons. The driving voltage for luminance output is also reduced by using a Ph3 PO layer. The turn-on voltage (a voltage for a luminance of 1 cd/m2 ) is 2.8 V in the Ph3 PO device. The luminance increases rapidly as the driving voltage increases above the turn-on voltage, being a 100 cd/m2 at a voltage of 3.7 V. The EL spectrum of the Ph3 PO device is shown in Fig. 2. The EL spectrum shows the peak intensity at 450 nm, which coincides with the PL spectrum of ␣-NPD. The figure indicates that the ␣-NPD layer in the Ph3 PO devices acts as an emitting material. Any visible light was not observed in the Ph3 PO film or solution under the UV irradiation. Since the emission from the ␣-NPD layer is not efficient, the rubrene molecules were doped into the ␣-NPD layer. Fig. 3 shows the J–V–L curves of the rubrene-doped device with a Ph3 PO layer. The device has a structure of ITO/2-TNATA (15 nm)/␣NPD:rubrene (1%, 40 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al. The
Fig. 1. Current density (solid)–voltage–luminance (open) curves of devices with Ph3 PO (circle) and Alq3 (square) layers. Device structure: ITO/2-TNATA (15 nm)/␣NPD (40 nm)/Ph3 PO or Alq3 (60 nm)/LiF (0.5 nm)/Al. Inset: chemical structure of Ph3 PO.
Fig. 2. EL spectrum of device ITO/2-TNATA (15 nm)/␣-NPD (40 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al.
voltage for the same current density increases by doping of rubrene molecules into the ␣-NPD layer. For example, the voltages for 20 mA/cm2 are 3.5 and 5.1 V for the undoped (seen in Fig. 1) and doped devices, respectively. The increase of an applied voltage in a rubrene-doped device may be due to the change of the carrier mobility by the rubrene molecules. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of rubrene are 5.4 and 3.2 eV, respectively [17]. The HOMO and LUMO levels of ␣-NPD are 5.7 and 2.6 eV, respectively [18]. Since the HOMO and LUMO levels of rubrene are located inside the band gap of ␣-NPD, rubrene molecules act as traps for the carriers, reducing the carrier mobility. The turn-on voltage of the rubrene-doped device is 3.3 V, and the required voltages for the luminances of 100 and 1000 cd/m2 are 4.1 and 5.1 V, respectively. Although the driving voltage increases slightly by rubrene, the EL efficiency substantially increases from 0.5 cd/A for the undoped device to 5.7 cd/A for the doped device. The inset of Fig. 3 shows the EL spectrum of rubrene-doped device. The spectrum shows strong emission at 557 nm originating from rubrene molecular sites and weak emission due to the ␣-NPD host. For reducing the driving voltage of rubrene-doped device, the emission zone has been investigated by preparing two kinds of devices; ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 10 nm)/␣-NPD (30 nm)/Ph3 PO (60 nm)/LiF/Al (device A) and ITO/2TNATA (15 nm)/␣-NPD (30 nm)/␣-NPD:rubrene (1%, 10 nm)/Ph3 PO (60 nm)/LiF/Al (device B). In device A, the rubrene molecules were doped into the ␣-NPD layer adjacent to the 2-TNATA interface. In device B, the doped region was located to be contacted with the
Fig. 3. Current density (solid)–voltage–luminance (open) curves and EL spectrum (inset) of the rubrene-doped device with Ph3 PO layer. Device structure: ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 40 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al.
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entire ␣-NPD layer (device of Fig. 3). On the other hand, the driving voltage for the luminance output is higher in device B, because of the low current density resulting from rubrene traps and the low EL efficiency owing to the emission from an undoped ␣-NPD layer. 4. Conclusion
Fig. 4. EL spectra of devices A and B. Device A: ITO/2-TNATA (15 nm)/␣-NPD:rubrene (1%, 10 nm)/␣-NPD (30 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al, device B: ITO/2-NTATA (15 nm)/␣-NPD (30 nm)/␣-NPD:rubrene (10 nm)/Ph3 PO (60 nm)/LiF (0.5 nm)/Al.
Ph3 PO layer. In both devices, the thickness of the doped ␣-NPD layer was fixed to be 10 nm. Fig. 4 shows the EL spectra of devices A and B. Device A shows strong rubrene and weak ␣-NPD emissions. On the other hand, device B shows strong ␣-NPD and weak rubrene emissions. These results indicate that most of emission sites are placed in the ␣NPD layer adjacent to the 2-TNATA interface. Since the emission in rubrene-doped devices is mainly due to the direct recombination on the rubrene molecules [17], the electrons injected from the cathode are recombined with holes in the emission layer adjacent to the 2-TNATA interface. In addition, although the electron mobility in ␣-NPD is expected to be extremely low, the electrons transport several hundreds of angstroms inside the ␣-NPD layer before recombining with holes. The voltage for the same current density is lower in device A. For example, the voltages for a current density of 20 mA/cm2 are 4.1 and 4.9 V for devices A and B, respectively. The higher operating voltage in device B suggests that the electrons injected from Ph3 PO layer are trapped by rubrene molecules before being transported to the recombination zone. The L–V curves show the same tendency as the J–V curves. In device A, the turn-on voltage is 2.8 V, and the luminance of 1000 cd/m2 is achieved at 4.1 V. Therefore, low voltage operating devices can be realized by controlling the doped zone and by using a Ph3 PO layer. The EL efficiency of device A is 5 cd/A at 1000 cd/m2 , being almost same as that of the device doped with rubrene molecules into the
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