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Highly fluorescent molecular organic composites for light-emitting diodes
ELSEVIER
Synthetic
Metals
85 (1997)
Highly Fluorescent Molecular Light-Emitting
1225-1228
Organic Composites for Diodes
D. J. Fatemi, H. Murata, C. D. Merritt, and Z. H. Kafafi U.S. Naval Research Laboratory, Washington, DC 20375, USA Abstract Organic films of tris-(8hydroxyquinolinato) aluminum (III) and N,N’-diphenyl-N,N’-bis(3-methylphenyI)-l,l’-biphenyl-4.4’diamine doped with highly fluorescent molecules were prepared by vacuum deposition. Optical absorption and photoluminescence spectra of the composites were measured as a function of dopant concentration, These films were also used as the emitting layer in light-emitting diodes, where their electroluminescence spectra was studied as a function of dopant concentration, Color tunability from the blue-green to the red-orange based on variation in the fluorescent molecule and dopant concentration was attained, and quantum efficiencies were found to be enhanced upon doping of the emitter layer. Keywords:
Photoluminescence,
electroluminescence,
organic light-emitting
1. Introduction Molecular organic light-emitting diodes (MOLEDs) have been the subject of both academic and practical interest in Investigators have sought a detailed recent years [l-9]. understanding of the mechanism for electroluminescence (EL) while at the same time attempting to develop materials useful for multicolored, flat-panel displays. the promise offered by the high Historically, fluorescence quantum yield of organic materials has been countered by the large voltages required for generating EL [lo]. However, Tang and VanSlyke succeeded in developing a MOLED with a luminance over lOOOcd/m* at an operating voltage below 1ov [l]. This breakthrough was accomplished through the inclusion of a hole-transport layer (HTL), l,l-bis(Cdi-ptolylaminophenyl)cyclohexane, in addition to an emission layer (EML), tris-(8-hydroxyquinolinato) aluminum (III) (Alqa), that served the dual role of an electron transport layer (ETL). By reduction of the voltage requirements for charge carrier injection and through containment of the charge carriers within the EML, a MOLED with a high external quantum, efficiency (photons/electrons) of about 1% and a luminous efficiency of 1.5 1mlW was fabricated. Since Alqs is a modest fluorescent material [2,11], Tang ef al. thought about improving the performance of MOLEDs based on Alqs by doping the EML with laser dyes such as coumarin 540 or DCM [2]. Doping at low molarity (0.251%) was found to enhance the quantum efficiency of these devices by a factor of two. In addition, selection of the proper dopant and the dopant concentration enabled tuning of emission between 500nm and 650nm. Using DCM as the dopant and a 60nm Alqa spacer layer between the doped Alqa film and the cathode [4], Littman and Martic were able to increase the dopant concentration to 2% without a decrease in quantum efficiency. This was a substantial increase from the optimum DCM concentration, 0.4%, seen by Tang et al. [2] and led to an enhanced quantum efficiency of 4.2%. These results suggested that quenching of the radiative centers occurred near the cathode, thus limiting the upper quantum efficiency of Alqs-based devices lacking the spacer layer. More recently, Hamada et al. [9] doped TPD with 5 wt% 5,6,11,12-tetraphenylnapthacene (rubrene) and sandwiched it 0379-6779/97/$17.00
0 1997
PII SO379-6779(96)04332-9
Elsevier
Science
S.A
All rights
reserved
diodes, organic composites
between an ETL of Alqs and an HTL of 4,4’,4”-tris(3methylphenylphenylamino)triphenylarnine(MTDATA). MOLEDs based on these organic heterostructures had a luminance of 60.600 cd/m* at approximately 13V. Kido et al. [S] developed a MOLED providing white light by doping a 5nm zone of the Alqa layer with Nile Red. Using a 3nm thick 1,2,4-triazole derivative (p-EtTAZ) between Alqs and the HTL, N,N’-diphenyl-N,N’-bis(3-methylphenyl)1,l ‘-biphenyl4,4’-diamine (TPD), EL from TPD, Alqa, and Nile Red was obtained, producing a broad spectrum with peaks at 410,520, and 600 nm. Despite this great progress in MOLED development. much improvement in device performance is required for commercial use. In this report, the preparation and spectroscopic characterization of highly luminescent organic composites are discussed. These composites were prepared by doping Alq, or TPD with highly fluorescent anthracene and naphthacene derivatives using high vacuum deposition techniques. The goal is to develop highly luminescent materials with enhanced thermal stability and good film morphology for MOLEDs. Absorbance and fluorescence of doped films were studied as a function of MOLEDs based on these films dopant molecule concentration. were fabricated and characterized by EL spectra and, quantum and luminous efficiencies. 2. Experiments A. Preparation
of Organic
and Metal Alloy Thin Films
The chemical structures of the organic materials used in the experiments are shown in Fig. 1. TPD (99.8% purity) was obtained from H. W. Sands Corp. Rubrene (98% purity) and 9,10-bis(phenylethyny1) anthracene (BPEA) (97% purity) were purchased from Aldrich Chemical Co. Alq, (95% purity) was supplied by Tokyo Kasei (TCI) and purified by vacuum sublimation. Magnesium (Aldrich Chemical Co., 99.9% purity) and silver (Hoover and Strong, 99.9% purity) were used as the cathode materials. Indium tin oxide (ITO) deposited and patterned onto a fused silica substrate, with a sheet resistance of lOR/square and a thickness of 300nm, was supplied by Thin Film Devices and used as the transparent anode.
D.J. Fatemi
et al./Synthetic
Metals
85 (1997)
1225-1228
cooled photodiode array detector. Ex-situ UV-Vis absorption spectra of the films were subsequently recorded using a PerkinElmer Lambda 9 UVNWNIR Spectrophotometer. Ex-situ PL and EL spectra of the emitting materials in the MOLEDs were carried out inside a glove box (M. Braun Labmaster 130). The excitation laser beam was brought to the chamber through an optical fiber. Luminescence was also collected through an optical fiber. Voltage-current-luminance (V-I-L) measurements of the LEDs were conducted using a Keithley 238 High Current Source Measure Unit and a Minolta LS-110 Luminance Meter,
cc>
3. Resultsand Discussion
(4
Fig. 1. Chemical structure of (a) TPD (b) Alqs (c) BPEA, and (d) Rubrene Prior to use, fused silica and IT0 patterned glass substrates were washed in a detergent solution and rinsed in deionized water. They were then sequentially sonicated in a detergent solution, deionized water, acetone, and isopropyl alcohol, followed by another sonication in deionized water. Finally, the substrates were exposed to vapors of isopropyl alcohol. The IT0 patterned glass substrates were also ashed in a Plasmatic Systems Plasma-Preen I Asher. The substrates were loaded onto a deposition wheel inside a vacuum chamber. This six-inch wheel can hold up to fourteen substrates and four quartz crystal microbalances. Alq,, TPD, and either rubrene or BPEA were separately loaded inside Pyrex crucibles. I Two alumina crucibles were filled with magnesium and silver, respectively. The crucibles were then placed in their respective resistive heating furnaces. After evacuating the &amber (- 5 x 10.’ Tot-r), the samples were outgassed for a few hours before deposition of the films was started. The rate of evaporation is monitored by one of four quartz crystal microbalances that are directly mounted on the wheel. Deposition rates of 1 A/s and 4 &s were used for TPD and Alqs, respectively. Typical thicknesses used for the TPD and Alqs layers were 50nm and 40nm, respectively. The molar concentration of the dopant (rubrene or BPEA) ranged from O16%. Pure and composite films of Alqs or TPD were prepared on silica substrates for in-situ spectroscopic characterization. The composite films were prepared by simultaneous deposition of molecular beams of guests and hosts. For the fabrication of MOLEDs, an HTL or ETL was deposited before or after the EML. After deposition of the organic films, Mg andAg were codeposited through a patterned shutter. The weight ratio of magnesium to silver typically used was 12: 1. The thickness of the Mg:Ag films was measured using a stylus profile-meter (Sloan Dektak II) and was determined to be 1COnm.
B. Spectroscopicand DeviceCharacterization In-situ photoluminescence (PL) spectra of freshly prepared films were measured using the 325nm line from a HeCd laser (Omnichrome 20562/10) as the excitation wavelength. Fluorescence from the photoexcited film is collected by an achromatic lens and directed into a I/I m spectrograph with a
A. Dopingof BPEA into Alq3 Fig. 2(a) illustrates the dependence of the fluorescence spectra of films of BPEA:Alqs on dopant concentration. For the lightly doped composites, the fluorescence spectra are characteristic of BPEA molecules and are quite similar to a BPEA solution spectrum. At high BPEA concentrations, a broad, new, substantially red-shifted PL band centered at -560nm is observed. This spectral change in going from low to high BPEA concentration suggests that emission arises from two different species. It is doubtful that the fluorescence originates from any BPEA-Aiqs exciplexes, however. Fluorescence of dilute composites is most likely due to BPEA molecules whereas that of concentrated solutions may be due to a dimer, trimer, or higher aggregate of BPEA. These conclusions are supported by absorption spectra of BPEA:Alqs films (not shown). Absorbance due to BPEA molecules increases linearly with dopant concentration, in accordance with Beer’s law, and indicates that the composites are well-behaved solid solutions. However, at higher BPEA concentrations, deviation from linearity is observed. Fig. 2(b) shows the EL spectra of BPEA:Alqs as a function of BPEA molarity. Qualitatively, the EL spectra are similar to the PL spectra shown in Fig. 2(a). Narrowing in some of the EL bands is observed which may be due to microcavity effects [12]. In addition, the red shift that occurs upon increase of the BPEA concentration is more pronounced in the EL spectra than in the PL of the composites. The wavelength of maximum emission varies from 490nm to 580nm when the BPEA concentration is varied from 1.62% to 14.5%, showing promise for color-tunability of MOLEDs by changing dopant These results may be compared to those of concentration. Takeuchi et al. [3], in which MOLEDs containing an anthracene:Alqs layer were investigated. EL was observed to arise from Alqs, with no emission from the dopant. This contrasts sharply with the result of the present study, in which BPEA emission is dominant even at low dopant concentrations. Reconciliation between these two studies is accomplished by recognizing that anthracene absorption is blue-shifted relative to Alqs emission. Hence, energy transfer from Alqs to anthracene is not expected to occur. The fact that dilute BPEA:Alqs composites produce PL spectra similar to BPEA in solution (Fig. 2(a)), however, shows that energy transfer from host, Alqs, to guest. BPEA, occurs quite efficiently. The PL quantum yield of BPEA:AIqs composite films, relative to that of pure Alqs, is plotted as a function of BPEA concentration in Fig. 3. The quantum yield rises quickly upon
D.J. Fatemi
et al./Synthetic
doping with BPEA and is a factor of three larger than that of a pure Alqs film at BPEA molarities above 2%. It is interesting to note that the enhanced fluorescence efficiency remains almost constant at high BPEA concentrations. This observation is striking in view of the fact that emission at these high concentrations appears to be originating from dimers, trimers, or higher aggregates.
Metals
85 (1997)
1225-1228
1227
The EL quantum efficiency of the MOLEDs was found to be dependent on the BPEA concentration, with a maximum external quantum efficiency of 1.2% measured for a composite with a BPEA molarity of 0.18%.
B. Doping of Rubreneinto Alq, Optical spectra (not shown) of rubrene:Alq, composites show absorption bands due to rubrene and Alqa. The absorbance due to rubrene increases monotonically with dopant concentration, in agreement with Beer’s law. This indicates that rubrene molecules are well dispersed in the Alqa host matrix and that the composites are well-behaved solid solutions. The PL and EL spectra of rubrene:Alqs films depicted in Fig. 4 are quite similar and are red-shifted relative to the PL of an Alqs film. The spectra are characteristic of rubrene molecules and resemble the rubrene solution spectrum, indicating efficient energy transfer from host to guest. In addition to energy transfer from Alqs to rubrene, MOLEDs based on these composites are expected to benefit from rubrene’s carrier trapping ability in Alq3. The ionization potential (IP) of mbrene is 5.4 eV [9b], whereas Alqa’s IP is in the range 5.6-5.8 eV [9a,13]. Thus, rubrene is expected to act as a hole trap in Alqs, enhancing the probability for electron-hole recombination and creating highly fluorescent centers. In a MOLED with only 0.55% rubrene in Alq3, a luminous efficiency of 2.1 lm/W was observed, compared to 1.7 lm/W measured for the device with undoped Alqs.
0.8
1
““,,,‘,,,,‘,,,,,,,,‘,,,“,‘,““,“‘, A
0.2
0 400
450
500
550 600 Wavelength
650 (nm)
700
Fig. 2. (a) PL and (b) EL spectra of BPEA:Alqs function of dopant concentration in mol%.
0
2
4 BPEA
6
8 Concentration
10
12 (mol%)
750
0.8 t-
ca)
‘kO0
450
-
0%
800
composites as a
14
16
Fig. 3. PL quantum yield of BPEA:Alqs composites, relative to that of pure Alqa, as a function of dopant concentration,
500
550 600 Wavelength
650 (nm)
700
Fig. 4. (a) PL and (b) EL spectra of rubrene:Alqs function of dopant concentration in mol%.
750
800
composites as a
1228
D.J. Futemi
a& al./Syntfletic
C. Doping of Rubrene into TPD Fig. 5 shows PL and EL spectra of composite films in which rubrene was doped into TPD, the HTL. For comparison, the PL spectrum from undoped TPD is also depicted. Strong contrast exists between the PL spectra of pure and doped TPD films. In the pure film, the PL peak is at 425nm, characteristic of TPD emission, whereas the PL of the composite film contains no TPD character and is similar to that of rubrene in solution, indicating very efficient energy transfer from host to guest molecules. Similarly, the EL spectrum of rubrene-doped TPD arises primarily from rubrene. Rubrene is believed to act as an electron trap in TPD, since its electron affinity is believed to be 0.9 eV greater than that of TPD [9b], thus enhancing the probability of electron-hole recombination on this highly luminescent site. However, some contributions to the EL spectrum from the ETL, Alq3, was observed. The dependence of luminance on current density, shown in Fig. 6, contrasts the performance of two devices where doping of the emitting layer was done in the ETL, Alqs, and HTL, TPD,
Metals
85 (1997)
1225-1225
respectively. For a given current density, the luminance of the rubrene:TPD device is considerably higher than that of the rubrene:Alq3 device. showing that it has a larger external quantum efficiency. For example, at a current density of 2400A/m’, the luminance of the rubrene:TPD device is 8500 cd/m’, whereas that of a rubrene:Alqs device is only 1500 cd/m2. This result may indicate more efficient hole-electron recombination on the rubrene luminescent centers in TPD versus Alqa-tubrene doped films. Alternatively, quenching of the excitons may occur more readily near the cathode at the metal/organic interface.
4. Conclusion Highly fluorescent organic composites with good film morphology for use in light-emitting devices were successfully prepared using vacuum deposition techniques. Optical spectra showed that dopants were well dispersed in host matrices. PL and EL spectra arose primarily from the dopant molecules signaling efficient energy transfer from host to guest molecules. Colortunability from blue-green to orange-red was achieved by changing the choice of the fluorescent molecule and the dopant concentration. In MOLEDs where rubrene was used as the dopant molecule, a luminous efficiency of 2.1 Im/W was obtained. This work was supported by ONR and DARPA.
References [l] [2] 350
400
450
500 Wavelength
550 (nm)
600
650
700
[3]
Fig. 5. PL (long hash) and EL (short hash) spectra of rubrene:TPD composites for rubrene molar@ of 4.3%. PL of pure TPD is shown by the solid line.
[4] [5] [6] [7] [8] [9]
[lo] [ 1 l] - 0
1000
2000
3000
Current
4000 Density
5000
6000
7000
(A/n?)
6. Luminance-current characteristics of (a) rubrene:TPD and (b) rubrene:Alq3 devices with rubrene molarities of 4.4% and 5.2%, respectively.
[ 121 [13]
C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987). C. W. Tang, S. A. VanSlyke, and C. H. Chen, J. Appl. Phys. 65, 3610 (1989). M. Takeuchi, H. Masui, I. Kikuma, M. Masui, T. Muranoi, and T. Wada, Jpn. J. Appl. Phys. 31, IA98 (1992). J. Littman and P. Martic, J. Appl. Phys. 72, 1957 (1992). T. Kichimi, T. Mori, T. Mizutani, Technical Report of IEICE, OME92-54,69 (1993) J, Kido, K. Nagai, and Y. Okamoto, IEEE Transactions on Electron Devices 40, 1342 (1993). S. Naka, K. Shinno, H. Okada, H. Onnagawa, and K. Miyashita, Jpn, J. Appl. Phys. 33, L1772 (1994). J. Kldo, M. Kimura, and K. Nagai, Science 267, 1332 (1995). (a) Y. Hamada, T. Sano, K. Shibata, and K. Kuroki, Jpn. J. Appl. Phys. 34, L824 (1995); (b) Y. Hamada, T. Sano, Y I Nisho, and K. Shibata, Technical Report of IEICE, OME9572, 13 (1996). R. E. Kellog, J. Chem. Phys. 44, 411 (1966). D. Z. Garbuzov, V. Bulovic, P. E. Burrows, S. R. Forrest, Chem. Phys. Lett. 249,433 (1996). N. Takada, T. Tsutsui, and S. Saito, Appl. Phys. Lett. 63, 2032 (1993). Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y. Yonemoto, S. Kawami, and K. Imai, Appl. Phys. Lett. 65, 807 (1994).
Synthetic
Metals
85 (1997)
Highly Fluorescent Molecular Light-Emitting
1225-1228
Organic Composites for Diodes
D. J. Fatemi, H. Murata, C. D. Merritt, and Z. H. Kafafi U.S. Naval Research Laboratory, Washington, DC 20375, USA Abstract Organic films of tris-(8hydroxyquinolinato) aluminum (III) and N,N’-diphenyl-N,N’-bis(3-methylphenyI)-l,l’-biphenyl-4.4’diamine doped with highly fluorescent molecules were prepared by vacuum deposition. Optical absorption and photoluminescence spectra of the composites were measured as a function of dopant concentration, These films were also used as the emitting layer in light-emitting diodes, where their electroluminescence spectra was studied as a function of dopant concentration, Color tunability from the blue-green to the red-orange based on variation in the fluorescent molecule and dopant concentration was attained, and quantum efficiencies were found to be enhanced upon doping of the emitter layer. Keywords:
Photoluminescence,
electroluminescence,
organic light-emitting
1. Introduction Molecular organic light-emitting diodes (MOLEDs) have been the subject of both academic and practical interest in Investigators have sought a detailed recent years [l-9]. understanding of the mechanism for electroluminescence (EL) while at the same time attempting to develop materials useful for multicolored, flat-panel displays. the promise offered by the high Historically, fluorescence quantum yield of organic materials has been countered by the large voltages required for generating EL [lo]. However, Tang and VanSlyke succeeded in developing a MOLED with a luminance over lOOOcd/m* at an operating voltage below 1ov [l]. This breakthrough was accomplished through the inclusion of a hole-transport layer (HTL), l,l-bis(Cdi-ptolylaminophenyl)cyclohexane, in addition to an emission layer (EML), tris-(8-hydroxyquinolinato) aluminum (III) (Alqa), that served the dual role of an electron transport layer (ETL). By reduction of the voltage requirements for charge carrier injection and through containment of the charge carriers within the EML, a MOLED with a high external quantum, efficiency (photons/electrons) of about 1% and a luminous efficiency of 1.5 1mlW was fabricated. Since Alqs is a modest fluorescent material [2,11], Tang ef al. thought about improving the performance of MOLEDs based on Alqs by doping the EML with laser dyes such as coumarin 540 or DCM [2]. Doping at low molarity (0.251%) was found to enhance the quantum efficiency of these devices by a factor of two. In addition, selection of the proper dopant and the dopant concentration enabled tuning of emission between 500nm and 650nm. Using DCM as the dopant and a 60nm Alqa spacer layer between the doped Alqa film and the cathode [4], Littman and Martic were able to increase the dopant concentration to 2% without a decrease in quantum efficiency. This was a substantial increase from the optimum DCM concentration, 0.4%, seen by Tang et al. [2] and led to an enhanced quantum efficiency of 4.2%. These results suggested that quenching of the radiative centers occurred near the cathode, thus limiting the upper quantum efficiency of Alqs-based devices lacking the spacer layer. More recently, Hamada et al. [9] doped TPD with 5 wt% 5,6,11,12-tetraphenylnapthacene (rubrene) and sandwiched it 0379-6779/97/$17.00
0 1997
PII SO379-6779(96)04332-9
Elsevier
Science
S.A
All rights
reserved
diodes, organic composites
between an ETL of Alqs and an HTL of 4,4’,4”-tris(3methylphenylphenylamino)triphenylarnine(MTDATA). MOLEDs based on these organic heterostructures had a luminance of 60.600 cd/m* at approximately 13V. Kido et al. [S] developed a MOLED providing white light by doping a 5nm zone of the Alqa layer with Nile Red. Using a 3nm thick 1,2,4-triazole derivative (p-EtTAZ) between Alqs and the HTL, N,N’-diphenyl-N,N’-bis(3-methylphenyl)1,l ‘-biphenyl4,4’-diamine (TPD), EL from TPD, Alqa, and Nile Red was obtained, producing a broad spectrum with peaks at 410,520, and 600 nm. Despite this great progress in MOLED development. much improvement in device performance is required for commercial use. In this report, the preparation and spectroscopic characterization of highly luminescent organic composites are discussed. These composites were prepared by doping Alq, or TPD with highly fluorescent anthracene and naphthacene derivatives using high vacuum deposition techniques. The goal is to develop highly luminescent materials with enhanced thermal stability and good film morphology for MOLEDs. Absorbance and fluorescence of doped films were studied as a function of MOLEDs based on these films dopant molecule concentration. were fabricated and characterized by EL spectra and, quantum and luminous efficiencies. 2. Experiments A. Preparation
of Organic
and Metal Alloy Thin Films
The chemical structures of the organic materials used in the experiments are shown in Fig. 1. TPD (99.8% purity) was obtained from H. W. Sands Corp. Rubrene (98% purity) and 9,10-bis(phenylethyny1) anthracene (BPEA) (97% purity) were purchased from Aldrich Chemical Co. Alq, (95% purity) was supplied by Tokyo Kasei (TCI) and purified by vacuum sublimation. Magnesium (Aldrich Chemical Co., 99.9% purity) and silver (Hoover and Strong, 99.9% purity) were used as the cathode materials. Indium tin oxide (ITO) deposited and patterned onto a fused silica substrate, with a sheet resistance of lOR/square and a thickness of 300nm, was supplied by Thin Film Devices and used as the transparent anode.
D.J. Fatemi
et al./Synthetic
Metals
85 (1997)
1225-1228
cooled photodiode array detector. Ex-situ UV-Vis absorption spectra of the films were subsequently recorded using a PerkinElmer Lambda 9 UVNWNIR Spectrophotometer. Ex-situ PL and EL spectra of the emitting materials in the MOLEDs were carried out inside a glove box (M. Braun Labmaster 130). The excitation laser beam was brought to the chamber through an optical fiber. Luminescence was also collected through an optical fiber. Voltage-current-luminance (V-I-L) measurements of the LEDs were conducted using a Keithley 238 High Current Source Measure Unit and a Minolta LS-110 Luminance Meter,
cc>
3. Resultsand Discussion
(4
Fig. 1. Chemical structure of (a) TPD (b) Alqs (c) BPEA, and (d) Rubrene Prior to use, fused silica and IT0 patterned glass substrates were washed in a detergent solution and rinsed in deionized water. They were then sequentially sonicated in a detergent solution, deionized water, acetone, and isopropyl alcohol, followed by another sonication in deionized water. Finally, the substrates were exposed to vapors of isopropyl alcohol. The IT0 patterned glass substrates were also ashed in a Plasmatic Systems Plasma-Preen I Asher. The substrates were loaded onto a deposition wheel inside a vacuum chamber. This six-inch wheel can hold up to fourteen substrates and four quartz crystal microbalances. Alq,, TPD, and either rubrene or BPEA were separately loaded inside Pyrex crucibles. I Two alumina crucibles were filled with magnesium and silver, respectively. The crucibles were then placed in their respective resistive heating furnaces. After evacuating the &amber (- 5 x 10.’ Tot-r), the samples were outgassed for a few hours before deposition of the films was started. The rate of evaporation is monitored by one of four quartz crystal microbalances that are directly mounted on the wheel. Deposition rates of 1 A/s and 4 &s were used for TPD and Alqs, respectively. Typical thicknesses used for the TPD and Alqs layers were 50nm and 40nm, respectively. The molar concentration of the dopant (rubrene or BPEA) ranged from O16%. Pure and composite films of Alqs or TPD were prepared on silica substrates for in-situ spectroscopic characterization. The composite films were prepared by simultaneous deposition of molecular beams of guests and hosts. For the fabrication of MOLEDs, an HTL or ETL was deposited before or after the EML. After deposition of the organic films, Mg andAg were codeposited through a patterned shutter. The weight ratio of magnesium to silver typically used was 12: 1. The thickness of the Mg:Ag films was measured using a stylus profile-meter (Sloan Dektak II) and was determined to be 1COnm.
B. Spectroscopicand DeviceCharacterization In-situ photoluminescence (PL) spectra of freshly prepared films were measured using the 325nm line from a HeCd laser (Omnichrome 20562/10) as the excitation wavelength. Fluorescence from the photoexcited film is collected by an achromatic lens and directed into a I/I m spectrograph with a
A. Dopingof BPEA into Alq3 Fig. 2(a) illustrates the dependence of the fluorescence spectra of films of BPEA:Alqs on dopant concentration. For the lightly doped composites, the fluorescence spectra are characteristic of BPEA molecules and are quite similar to a BPEA solution spectrum. At high BPEA concentrations, a broad, new, substantially red-shifted PL band centered at -560nm is observed. This spectral change in going from low to high BPEA concentration suggests that emission arises from two different species. It is doubtful that the fluorescence originates from any BPEA-Aiqs exciplexes, however. Fluorescence of dilute composites is most likely due to BPEA molecules whereas that of concentrated solutions may be due to a dimer, trimer, or higher aggregate of BPEA. These conclusions are supported by absorption spectra of BPEA:Alqs films (not shown). Absorbance due to BPEA molecules increases linearly with dopant concentration, in accordance with Beer’s law, and indicates that the composites are well-behaved solid solutions. However, at higher BPEA concentrations, deviation from linearity is observed. Fig. 2(b) shows the EL spectra of BPEA:Alqs as a function of BPEA molarity. Qualitatively, the EL spectra are similar to the PL spectra shown in Fig. 2(a). Narrowing in some of the EL bands is observed which may be due to microcavity effects [12]. In addition, the red shift that occurs upon increase of the BPEA concentration is more pronounced in the EL spectra than in the PL of the composites. The wavelength of maximum emission varies from 490nm to 580nm when the BPEA concentration is varied from 1.62% to 14.5%, showing promise for color-tunability of MOLEDs by changing dopant These results may be compared to those of concentration. Takeuchi et al. [3], in which MOLEDs containing an anthracene:Alqs layer were investigated. EL was observed to arise from Alqs, with no emission from the dopant. This contrasts sharply with the result of the present study, in which BPEA emission is dominant even at low dopant concentrations. Reconciliation between these two studies is accomplished by recognizing that anthracene absorption is blue-shifted relative to Alqs emission. Hence, energy transfer from Alqs to anthracene is not expected to occur. The fact that dilute BPEA:Alqs composites produce PL spectra similar to BPEA in solution (Fig. 2(a)), however, shows that energy transfer from host, Alqs, to guest. BPEA, occurs quite efficiently. The PL quantum yield of BPEA:AIqs composite films, relative to that of pure Alqs, is plotted as a function of BPEA concentration in Fig. 3. The quantum yield rises quickly upon
D.J. Fatemi
et al./Synthetic
doping with BPEA and is a factor of three larger than that of a pure Alqs film at BPEA molarities above 2%. It is interesting to note that the enhanced fluorescence efficiency remains almost constant at high BPEA concentrations. This observation is striking in view of the fact that emission at these high concentrations appears to be originating from dimers, trimers, or higher aggregates.
Metals
85 (1997)
1225-1228
1227
The EL quantum efficiency of the MOLEDs was found to be dependent on the BPEA concentration, with a maximum external quantum efficiency of 1.2% measured for a composite with a BPEA molarity of 0.18%.
B. Doping of Rubreneinto Alq, Optical spectra (not shown) of rubrene:Alq, composites show absorption bands due to rubrene and Alqa. The absorbance due to rubrene increases monotonically with dopant concentration, in agreement with Beer’s law. This indicates that rubrene molecules are well dispersed in the Alqa host matrix and that the composites are well-behaved solid solutions. The PL and EL spectra of rubrene:Alqs films depicted in Fig. 4 are quite similar and are red-shifted relative to the PL of an Alqs film. The spectra are characteristic of rubrene molecules and resemble the rubrene solution spectrum, indicating efficient energy transfer from host to guest. In addition to energy transfer from Alqs to rubrene, MOLEDs based on these composites are expected to benefit from rubrene’s carrier trapping ability in Alq3. The ionization potential (IP) of mbrene is 5.4 eV [9b], whereas Alqa’s IP is in the range 5.6-5.8 eV [9a,13]. Thus, rubrene is expected to act as a hole trap in Alqs, enhancing the probability for electron-hole recombination and creating highly fluorescent centers. In a MOLED with only 0.55% rubrene in Alq3, a luminous efficiency of 2.1 lm/W was observed, compared to 1.7 lm/W measured for the device with undoped Alqs.
0.8
1
““,,,‘,,,,‘,,,,,,,,‘,,,“,‘,““,“‘, A
0.2
0 400
450
500
550 600 Wavelength
650 (nm)
700
Fig. 2. (a) PL and (b) EL spectra of BPEA:Alqs function of dopant concentration in mol%.
0
2
4 BPEA
6
8 Concentration
10
12 (mol%)
750
0.8 t-
ca)
‘kO0
450
-
0%
800
composites as a
14
16
Fig. 3. PL quantum yield of BPEA:Alqs composites, relative to that of pure Alqa, as a function of dopant concentration,
500
550 600 Wavelength
650 (nm)
700
Fig. 4. (a) PL and (b) EL spectra of rubrene:Alqs function of dopant concentration in mol%.
750
800
composites as a
1228
D.J. Futemi
a& al./Syntfletic
C. Doping of Rubrene into TPD Fig. 5 shows PL and EL spectra of composite films in which rubrene was doped into TPD, the HTL. For comparison, the PL spectrum from undoped TPD is also depicted. Strong contrast exists between the PL spectra of pure and doped TPD films. In the pure film, the PL peak is at 425nm, characteristic of TPD emission, whereas the PL of the composite film contains no TPD character and is similar to that of rubrene in solution, indicating very efficient energy transfer from host to guest molecules. Similarly, the EL spectrum of rubrene-doped TPD arises primarily from rubrene. Rubrene is believed to act as an electron trap in TPD, since its electron affinity is believed to be 0.9 eV greater than that of TPD [9b], thus enhancing the probability of electron-hole recombination on this highly luminescent site. However, some contributions to the EL spectrum from the ETL, Alq3, was observed. The dependence of luminance on current density, shown in Fig. 6, contrasts the performance of two devices where doping of the emitting layer was done in the ETL, Alqs, and HTL, TPD,
Metals
85 (1997)
1225-1225
respectively. For a given current density, the luminance of the rubrene:TPD device is considerably higher than that of the rubrene:Alq3 device. showing that it has a larger external quantum efficiency. For example, at a current density of 2400A/m’, the luminance of the rubrene:TPD device is 8500 cd/m’, whereas that of a rubrene:Alqs device is only 1500 cd/m2. This result may indicate more efficient hole-electron recombination on the rubrene luminescent centers in TPD versus Alqa-tubrene doped films. Alternatively, quenching of the excitons may occur more readily near the cathode at the metal/organic interface.
4. Conclusion Highly fluorescent organic composites with good film morphology for use in light-emitting devices were successfully prepared using vacuum deposition techniques. Optical spectra showed that dopants were well dispersed in host matrices. PL and EL spectra arose primarily from the dopant molecules signaling efficient energy transfer from host to guest molecules. Colortunability from blue-green to orange-red was achieved by changing the choice of the fluorescent molecule and the dopant concentration. In MOLEDs where rubrene was used as the dopant molecule, a luminous efficiency of 2.1 Im/W was obtained. This work was supported by ONR and DARPA.
References [l] [2] 350
400
450
500 Wavelength
550 (nm)
600
650
700
[3]
Fig. 5. PL (long hash) and EL (short hash) spectra of rubrene:TPD composites for rubrene molar@ of 4.3%. PL of pure TPD is shown by the solid line.
[4] [5] [6] [7] [8] [9]
[lo] [ 1 l] - 0
1000
2000
3000
Current
4000 Density
5000
6000
7000
(A/n?)
6. Luminance-current characteristics of (a) rubrene:TPD and (b) rubrene:Alq3 devices with rubrene molarities of 4.4% and 5.2%, respectively.
[ 121 [13]
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