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Photoluminescence efficiency and absorption of aluminum-tris-quinolate (Alq3) thin films
CHEMICAL
9 February 1996
PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 249 (1996) 433-437
Photoluminescence efficiency and absorption of aluminum-tris-quinolate (Alq3) thin films D.Z. Garbuzov, V. Bulovi6, P.E. Burrows, S.R. Forrest Advanced Technology Center jbr Photonics and Optoeleetronie Materials. Department of Electrical Engineering and the Princeton Materials Institute. Princeton University, Princeton, NJ 08544, USA
Received 28 September 1995; in final form 23 November 1995
Abstract Absorption, photoexcitation and internal and external luminescence efficiencies for Alq3 films grown by vacuum deposition have been measured accurately for the first time. The absorption spectrum measured at wavelengths between A = 250 and 450 nm is due to the overlap of the ~L a (385 nm) and IB b (260 nm) transition bands found in the solution spectrum of AIq3. The efficiency of the prominent luminescence band centered at A = 530 nm was observed to be independent of the excitation wavelength in the spectral range between A = 250 and 450 nm. The internal quantum efficiency was found to be (32 + 2)% independent of film thickness from 100 A. to 1.35 ~m, indicating a very short diffusion length for the IL a and ~B b excited states.
Tris (8-hydroxyquinoline)aluminum ( A l q 3 ) h a s been widely used as the active layer in green organic light-emitting devices (OLEDs) [1,2]. Most studies to date have focused on understanding the mechanism of charge transport and the specifics of electroluminescence (EL) of Alq3-based OLEDs [1-3]. However, data on the optical properties of solid films of Alq 3, which are crucial for the in-depth understanding of the operation of OLEDs and other optoelectronic devices are, to our knowledge, not available, For example, the data on optical absorption are apparently limited to Ref. [2] where only a narrow spectral range was studied, while to the best of our knowledge there is as yet no data published on the photoexcitation spectra of Alq3. Also, the absolute values of external and internal quantum efficiencies (tie and ,7~, respectively) of Alq3 thin film photolu-
minescence have not been quantified, although Ref. [1] cites an efficiency of 8% from unpublished experimental data. In this Letter we present results on the absorption, photoexcitation, external and internal quantum efficiencies of Alq3 thin films grown by thermal evaporation. The absorption measurements demonstrate that the overlap of the ]L~- and ~Bb-electronic transition bands of Alq 3 molecules results in an absorption coefficient (o~) in solid films higher than 104 c m - t in the spectral range between A = 250 and 425 nm. Investigation of the photoluminescence excitation (PLE) spectra shows that over this spectral range, the external efficiency for the prominent green luminescence band (with a center wavelength at A = 530 nm) does not depend on the excitation radiation wavelength. Direct measurements of r/e suggest that
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-261 4 ( 9 5 ) 0 1424- I
D.Z, Garbuzov et al. / Chemical Physics Letters 249 (1996) 433-437
434
the efficiency of radiative transitions centered at 530 nm is much higher than previously assumed [1], exceeding 30% at room temperature. Prepurified Alq3 films with thicknesses ranging from 50 ,~ to 1.35 I~m were deposited under vacuum by thermal evaporations onto glass, silicon, and sapphire substrates as described previously [2]. Prior to growth, glass and sapphire substrates were cleaned by three successive rinses in boiling 1,1,1-trichloro-
)\' " 1 o\
n = 1.73 + 0.05 at A = 633 rim. The experimental setup for measuring the film spectral properties (Fig. 1) consists of a 0.25 m monochromator used to select light with a full width at half maximum of A A = 15 nm from the broad spectral band of a Xe-arc lamp. After passing through a 165 Hz chopper, the excitation light was focused to a 1 × 2 mm spot using a CaF 2 lens system and a 45 ° angle tilted mirror. Calibrated, UV-enhanced 1 cm diameter Si-photodiodes, and 4 × 2 cm Si solar cells were used to detect PL and excitation radiation in conjunction with a current and lock-in amplifier. All measurements were done at room temperature, For the absorption measurements, a band-pass filter with transmission between A = 250 and 420 nm was introduced between the sample and the detector to eliminate the Alq3 photoluminescence
x~t~ ~ u o , ~ ot . ~ G ~ .2"~~
~v,.m~ I~_,_~ i
~,~rot~-rEero~ ,
G~SO~SA~'~,,RE ~ ~
~ ~
'
/ ~./ ' /" ~
Si ~DI.k~ CEtl.
~u,,o~mm~-ot~,~
"
/ a
105' y-" E _o ~ I°4
ethane, followed by three cycles in acetone, and a final rinse in boiling methanol. Silicon substrates were used as received from the manufacturer, with = 20 A-thick native oxide layer on the surface. Alq 3 films grown on Si Substrates were used for ellipsometric determination of the layer thickness and refractive index (n), which was determined to be
'
~
0
' Al%FilmThi£knes's ~a4ooA o 2400A A
1r'°.i 3'0nml ~ °1t \'.. t t~. .0.a01 '~ ...................] [ . '"~l 0.0 0.4 0.8 1.2
I
300
350
\ \
\
Film thickness [jan]
250
\
400
4;0
Wavelength [am] Fig. 2. Absorption spectra of AIq3 films obtained from three differentfilm thicknesses. Inset: Light attenuation versus film thicknesses at A= 370 rim. The slope gives the absorption coefficient.
signal, as well as scattered light at A > 420 nm. Samples grown on sapphire substrates were used for the absorption experiments. The low intensity of the Xe-arc lamp as well as the low sensitivity of the Si-detector at A < 400 nm results in a small signalto-background (scattered light) ratio for thick film samples ( > 5000 ,~), and hence thinner samples (510 and 2400 ,~) were used for absorption measurements in the short wavelength range. The results of the absorption measurements for three films of different thicknesses are given in Fig. 2. The absolute value of the absorption coefficient was calibrated at A = 370 nm by measuring transmission through several samples of different thicknesses (see inset, Fig. 2), and is a = (4.4 + 0.1) × 104 c m - ~. Fig. 2 shows that the overlap of two broad absorption bands with maxima at A = 3 8 5 nm and h = 2 6 0 n m (indicated by arrows) results in a > 104 c m - ~ over the entire spectral range from h = 425 nm down to A = 250 rim, which is the short wavelength detection limit of our setup. The h = 530 nm PLE spectra for thin (150 A) and thick (1.35 Ixm) Alq3 films are shown in Fig. 3. For thin samples, the PLE spectral maximum corresponds to the short wavelength absorption maximum at h = 260 nm. The 1.35 Ixm thick sample, however,
Fig. 1. Schematic diagram of the setup used for measuring film
absorbs
spectral properties,
lengths shorter than the absorption 'edge' at A = 420
> 95% of the incident light at all wave-
D.Z. Garbuzov et al. / Chemical Physics Letters 249 (1996) 433-437 '-72
-,
.
o ~. .~:
1 0 O=
.
.
.
.
.
Aiq3filmlhickness:
[
. . . . 1.35.m --=-- 150A • r.-= .-.
I
• • • In rLrU
oq~, =' ~
g 10-1 ._= E = J 250
, ,~. -2--T--'~. ~ == • •.,
'~'~ ,, ~ ", q:~~ ~ ", " 300 350 400 ' 450 ' 500 Excitation Wavelength Into]
Fig. 3. Photoluminescence excitation spectra of the 530 nm band for film thicknesses of 150 ,~, and 1.35 p,m. Inset: Schematic of
A1.
nm. The plateau at A < 430 nm in the PLE spectrum of this sample indicates that the external efficiency for green luminescence does not depend on the excitation photon energy, and has the same value for both the A = 385 and 260 nm absorption bands, For determination of the absolute value of the external PL efficiency, "O~, a band-pass filter with peak transmission at A = 370 nm was placed immediately above the sample (Fig. 1) to reduce scattered light from the monochromator, while a 1.35 ~ m thick film on a glass substrate was set directly onto the silicon solar cell surface. Using the values obtained for a , and taking into account the decreased UV photoresponse of the Si solar cell as compared to that at longer ( -- 500 nm) wavelengths, we find that A = 370 nm radiation transmitted through the 1.35 I~m thick film does not contribute appreciably to the diode photoresponse, and hence can be neglected, The PL efficiency, integrated over a 27r solid angle (estimated as discussed in Ref. [4]), gives r/~ from 2.8% to 10%. This measured external PL efficiency depends on the excitation area, and the thickness and area of the glass slide, indicating that the measured value of r/~ is strongly affected by scattering, losses and waveguiding of PL in the film and glass substrate. Note that waveguiding effects, substrate losses and other experimental artifacts have also been observed to distort the results of r/e measurements performed using an integrating sphere.
435
Two techniques have been applied to decrease the influence of waveguiding effects and to accurately estimate r/e. In the first method, the samples were placed film-side-down in index-matching microscope immersion oil with n = 1.524 dropped onto the surface of the solar cell. Under these conditions, we obtained T~e= 28% for the radiation emitted towards the solar cell. In order to evaluate the PL intensity in the opposite direction (through the glass slide), an identical sample was placed on the solar cell without the immersion oil, and the PL signals of the two samples were compared using a second photodetector placed above the samples (Fig. 1). The PL efficiency of the immersed sample was 0.8 times as large as that of the non-immersed sample, whose absolute efficiency was measured to be --- 3.6%. Therefore, the total efficiency of the immersed samples was 28% + (0.8) × 3.6% --- 31%. This number can be considered as a lower limit for "qe since the refractive index of the immersion fluid was less than n for Alq 3, and hence waveguiding losses to the Alq3 film were not completely eliminated. In the second measurement method, the luminescence intensities of the 1.35 Ixm thick film grown on Si substrates was compared with that of the film grown on a glass substrate. Using the previous calibration of the efficiency of the film on glass, it was found that the absolute value of r/e for the film on the Si substrate equals to (3.2 + 0.2)%. Since the refractive index of Si (4.16 at A = 530 nm) is much higher than that of Alq3, waveguide effects are eliminated and only photons emitted at angles less than that for total internal reflection (35 °) can contribute to "qe" In this case, a good approximation to tie is given by
rle = ( 1 - R ~) ( 1 + R 2 )
n - ~n2 - 1 2n
( 1)
where n is the Alq3 refractive index and R~ and R e are the Fresnel reflection coefficients for the PL radiation at the Alq-air and Si-Alq3 interfaces, respectively, averaged over angles < 35 °. The product of the first two factors in Eq. (1) is 1.09, while (n - ~ n 2 - 1 ) / 2 n =- 0.093. This gives a > 10-fold difference between ~e and 71i caused by the total internal reflection at the Alq3-air interface. Using Eq. (1), we calculate from r/e that the internal effi-
D.Z. Garbuzov et a l . / Chemical Physics Letters 249 (1996) 433-437
436
ciency for the green luminescence band centered at h = 530 nm is 7/i = (32 + 2)%. The difference between the calculated ~ and the measured r/e of the immersed samples is unexpectedly small if one considers the large fraction of luminescence waveguided in the Alq3 film sandwiched between the immersion oil and the glass, However, a considerable fraction of this waveguided radiation is also collected by the solar cell due to total internal reflection at the oil-air interface, as shown in Fig. 1. Thus, the results obtained by both methods are in good agreement, The dependence of the internal luminescence efficiency on film thickness for samples grown on glass substrates is shown in Fig. 4. To obtain these data, the luminescence from both the thin and previously calibrated thick samples were measured with filters placed over the photodetector to eliminate any nonabsorbed excitation (A = 370 nm) and scattered light from the monochromator. The amount of absorbed excitation light was directly measured for films with thickness > 500 ,~, and was calculated for thinner films using the data in the inset of Fig. 2. Data shown in Fig. 4 correspond to freshly prepared sampies, with air exposure times < 5 h. We have not observed any efficiency oscillations with film thickness which could be attributed to microcavity or interference effects [5] due to the low reflectance of the Alq3-glass interface and coordinate-angle averaging of these effects. Fig. 4 demonstrates that the luminescence efficiency of Aiq3 films remains constant over two orders of magnitude of thickness, with 40
...... , ........ , ........ , ~! 35~-~"x"--~ . . . . . . . . . . ~ ~ -~
30-
efficiency, r/i = (32 + 2)%, w i t h the electrolumines-
,"
= 25°>" ~' '3 ,, ~E 20. ,, tu ,, 15. ~ _-c lO.J o_ 50 ....... 0.01
a noticeable drop in efficiency observed only for films thinner than 100 ,~ [6]. A comparison of the absorption spectrum shown in Fig. 2 with the absorption features of solutions of compounds similar to Alq3 [7] allows one to unambiguously attribute the longer and shorter-wavelength absorption bands to transitions from the ground state to the ~La and ~B b excited states, respectively [8]. The plateau of the spectral dependence of the photoluminescence excitation ( P L E ) o v e r the wavelength range that incorporates both absorption lines is due to fast non-radiative relaxation from the ~B b to the ~L a state, with subsequent radiative recombination to the ground state (Fig. 3, inset). This implies I that the ~B b - L a relaxation time is substantially shorter than the radiative lifetime of the ~B b state. This is consistent with previous observations for many dye molecules in solution [9]. Another important conclusion which may be drawn from the presence of the PLE plateau is that the diffusion length, l X, of both excited states is very short. That is, l~ < a - ~ = 250 .~, where a = 4 × 105 cm-~ for the short excitation wavelength of h = 260 nm. If I x were longer than 250 A, one would expect a decrease in PL efficiency at excitation wavelengths of h -=- 260 nm due to surface recombination. This conclusion is supported by measurements of the dependence of quantum efficiency on film thickness (Fig. 4), from which it follows that l x is less than 100 ,~. It should be noted that the observed decrease of ~h with film thickness is much weaker than previously reported [2] for Alq 3-based OLEDs, indicating that there are additional mechanisms determining the quantum efficiency dependence on layer thickness in multilayer, heterojunction devices. A comparison of the internal photoluminescence
. ............... 011 Film Thickness [~m] Fig. 4.
, 1
cence efficiency of OLEDs (1%-2%) indicates that there is still considerable potential for further improvement in OLED performance. This improvement may be obtained by increasing the number of injected electron-hole pairs recombining through ~L, and I B b states, as well as by improving the radiative coupling out of the device by decreasing waveguiding in the substrate. In conclusion, the absorption, photoexcitation, external and internal photoluminescence efficiencies of Alq 3 thin films grown by thermal evaporation have
D.Z. Garbuzov et al. / Chemical Physics Letters 249 (1996) 433-437
437
been measured for the first time over the wavelength range 200 < h < 600 nm. The absorption measurements demonstrate that the overlap of the 1L~ and ~B b electronic transition bands of Alq 3 results in an absorption coefficient in solid films of a < 10 4 c m - 1 in the spectral range from A = 250 to 425 nm. Furthermore, investigation of the photoluminescence excitation spectra over this spectral range indicates that the efficiency for the green luminescence band centered at h = 530 nm does not depend on the
and the Air Force Office of Scientific Research (C. Lee and G. Pomrenke) for their generous support of this research.
excitation radiation wavelength. Direct measurements of the absolute value of r/g suggest that the total external efficiency and the internal efficiency for radiative transitions centered at h = 530 nm is much higher than previously assumed [1], and is ~/i = (32 + 2)% at room temperature. A comparison
[2] P.E. Burrows and S.R. Forrest, Appl. Phys. Letters 64 (1993) 2285. [3] Z. Shen, P.E. Burrows, V. Bulovi~, S.R. Forrest, D.M. McEarthy and M.E. Thompson, submitted for publication. [4] D.Z. Garbuzov, J. Luminescence 27 (1982)109. [5] s. Saito, T. Tsutsui, M. Era, N. Takada, C. Adachi, Y.
of this result with the best published value of r/e (1.3%) for Alq3 electroluminescence [1 ] shows that there remains large room for further improvements in the efficiency of green OLEDs.
navia, Applied physics series, Vol. 170 (Helsinki, 1990) p. 215.
--
The authors thank Universal Displays Corp., the Advanced Research Projects Agency (D. Slobodin),
References
[1] c.w. Tang, S.A. VanSlyke and C.H. Chen, J. AppL Phys. 65 (1989) 3610.
Hamada and T. Wakimoto, SPIE Proc. 1910 (1993) 212. [6] C. Adachi, T. Tsutsui and S. Saito, Acta polytechnica scandi-
[7] H.-H. Perkampus and K. Kortfim, Z. Anal. Chem. 190 (1962) 111. [8] H.B. Klevens and J.R. Platt, J. Chem. Phys. 17 (1949) 470. [9] F.P. Schafer, in: Topics in applied physics, Vol. 1, Dye lasers, ed. F.P. Schafer (Spring, Berlin, 1973) p. 29.
9 February 1996
PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 249 (1996) 433-437
Photoluminescence efficiency and absorption of aluminum-tris-quinolate (Alq3) thin films D.Z. Garbuzov, V. Bulovi6, P.E. Burrows, S.R. Forrest Advanced Technology Center jbr Photonics and Optoeleetronie Materials. Department of Electrical Engineering and the Princeton Materials Institute. Princeton University, Princeton, NJ 08544, USA
Received 28 September 1995; in final form 23 November 1995
Abstract Absorption, photoexcitation and internal and external luminescence efficiencies for Alq3 films grown by vacuum deposition have been measured accurately for the first time. The absorption spectrum measured at wavelengths between A = 250 and 450 nm is due to the overlap of the ~L a (385 nm) and IB b (260 nm) transition bands found in the solution spectrum of AIq3. The efficiency of the prominent luminescence band centered at A = 530 nm was observed to be independent of the excitation wavelength in the spectral range between A = 250 and 450 nm. The internal quantum efficiency was found to be (32 + 2)% independent of film thickness from 100 A. to 1.35 ~m, indicating a very short diffusion length for the IL a and ~B b excited states.
Tris (8-hydroxyquinoline)aluminum ( A l q 3 ) h a s been widely used as the active layer in green organic light-emitting devices (OLEDs) [1,2]. Most studies to date have focused on understanding the mechanism of charge transport and the specifics of electroluminescence (EL) of Alq3-based OLEDs [1-3]. However, data on the optical properties of solid films of Alq 3, which are crucial for the in-depth understanding of the operation of OLEDs and other optoelectronic devices are, to our knowledge, not available, For example, the data on optical absorption are apparently limited to Ref. [2] where only a narrow spectral range was studied, while to the best of our knowledge there is as yet no data published on the photoexcitation spectra of Alq3. Also, the absolute values of external and internal quantum efficiencies (tie and ,7~, respectively) of Alq3 thin film photolu-
minescence have not been quantified, although Ref. [1] cites an efficiency of 8% from unpublished experimental data. In this Letter we present results on the absorption, photoexcitation, external and internal quantum efficiencies of Alq3 thin films grown by thermal evaporation. The absorption measurements demonstrate that the overlap of the ]L~- and ~Bb-electronic transition bands of Alq 3 molecules results in an absorption coefficient (o~) in solid films higher than 104 c m - t in the spectral range between A = 250 and 425 nm. Investigation of the photoluminescence excitation (PLE) spectra shows that over this spectral range, the external efficiency for the prominent green luminescence band (with a center wavelength at A = 530 nm) does not depend on the excitation radiation wavelength. Direct measurements of r/e suggest that
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-261 4 ( 9 5 ) 0 1424- I
D.Z, Garbuzov et al. / Chemical Physics Letters 249 (1996) 433-437
434
the efficiency of radiative transitions centered at 530 nm is much higher than previously assumed [1], exceeding 30% at room temperature. Prepurified Alq3 films with thicknesses ranging from 50 ,~ to 1.35 I~m were deposited under vacuum by thermal evaporations onto glass, silicon, and sapphire substrates as described previously [2]. Prior to growth, glass and sapphire substrates were cleaned by three successive rinses in boiling 1,1,1-trichloro-
)\' " 1 o\
n = 1.73 + 0.05 at A = 633 rim. The experimental setup for measuring the film spectral properties (Fig. 1) consists of a 0.25 m monochromator used to select light with a full width at half maximum of A A = 15 nm from the broad spectral band of a Xe-arc lamp. After passing through a 165 Hz chopper, the excitation light was focused to a 1 × 2 mm spot using a CaF 2 lens system and a 45 ° angle tilted mirror. Calibrated, UV-enhanced 1 cm diameter Si-photodiodes, and 4 × 2 cm Si solar cells were used to detect PL and excitation radiation in conjunction with a current and lock-in amplifier. All measurements were done at room temperature, For the absorption measurements, a band-pass filter with transmission between A = 250 and 420 nm was introduced between the sample and the detector to eliminate the Alq3 photoluminescence
x~t~ ~ u o , ~ ot . ~ G ~ .2"~~
~v,.m~ I~_,_~ i
~,~rot~-rEero~ ,
G~SO~SA~'~,,RE ~ ~
~ ~
'
/ ~./ ' /" ~
Si ~DI.k~ CEtl.
~u,,o~mm~-ot~,~
"
/ a
105' y-" E _o ~ I°4
ethane, followed by three cycles in acetone, and a final rinse in boiling methanol. Silicon substrates were used as received from the manufacturer, with = 20 A-thick native oxide layer on the surface. Alq 3 films grown on Si Substrates were used for ellipsometric determination of the layer thickness and refractive index (n), which was determined to be
'
~
0
' Al%FilmThi£knes's ~a4ooA o 2400A A
1r'°.i 3'0nml ~ °1t \'.. t t~. .0.a01 '~ ...................] [ . '"~l 0.0 0.4 0.8 1.2
I
300
350
\ \
\
Film thickness [jan]
250
\
400
4;0
Wavelength [am] Fig. 2. Absorption spectra of AIq3 films obtained from three differentfilm thicknesses. Inset: Light attenuation versus film thicknesses at A= 370 rim. The slope gives the absorption coefficient.
signal, as well as scattered light at A > 420 nm. Samples grown on sapphire substrates were used for the absorption experiments. The low intensity of the Xe-arc lamp as well as the low sensitivity of the Si-detector at A < 400 nm results in a small signalto-background (scattered light) ratio for thick film samples ( > 5000 ,~), and hence thinner samples (510 and 2400 ,~) were used for absorption measurements in the short wavelength range. The results of the absorption measurements for three films of different thicknesses are given in Fig. 2. The absolute value of the absorption coefficient was calibrated at A = 370 nm by measuring transmission through several samples of different thicknesses (see inset, Fig. 2), and is a = (4.4 + 0.1) × 104 c m - ~. Fig. 2 shows that the overlap of two broad absorption bands with maxima at A = 3 8 5 nm and h = 2 6 0 n m (indicated by arrows) results in a > 104 c m - ~ over the entire spectral range from h = 425 nm down to A = 250 rim, which is the short wavelength detection limit of our setup. The h = 530 nm PLE spectra for thin (150 A) and thick (1.35 Ixm) Alq3 films are shown in Fig. 3. For thin samples, the PLE spectral maximum corresponds to the short wavelength absorption maximum at h = 260 nm. The 1.35 Ixm thick sample, however,
Fig. 1. Schematic diagram of the setup used for measuring film
absorbs
spectral properties,
lengths shorter than the absorption 'edge' at A = 420
> 95% of the incident light at all wave-
D.Z. Garbuzov et al. / Chemical Physics Letters 249 (1996) 433-437 '-72
-,
.
o ~. .~:
1 0 O=
.
.
.
.
.
Aiq3filmlhickness:
[
. . . . 1.35.m --=-- 150A • r.-= .-.
I
• • • In rLrU
oq~, =' ~
g 10-1 ._= E = J 250
, ,~. -2--T--'~. ~ == • •.,
'~'~ ,, ~ ", q:~~ ~ ", " 300 350 400 ' 450 ' 500 Excitation Wavelength Into]
Fig. 3. Photoluminescence excitation spectra of the 530 nm band for film thicknesses of 150 ,~, and 1.35 p,m. Inset: Schematic of
A1.
nm. The plateau at A < 430 nm in the PLE spectrum of this sample indicates that the external efficiency for green luminescence does not depend on the excitation photon energy, and has the same value for both the A = 385 and 260 nm absorption bands, For determination of the absolute value of the external PL efficiency, "O~, a band-pass filter with peak transmission at A = 370 nm was placed immediately above the sample (Fig. 1) to reduce scattered light from the monochromator, while a 1.35 ~ m thick film on a glass substrate was set directly onto the silicon solar cell surface. Using the values obtained for a , and taking into account the decreased UV photoresponse of the Si solar cell as compared to that at longer ( -- 500 nm) wavelengths, we find that A = 370 nm radiation transmitted through the 1.35 I~m thick film does not contribute appreciably to the diode photoresponse, and hence can be neglected, The PL efficiency, integrated over a 27r solid angle (estimated as discussed in Ref. [4]), gives r/~ from 2.8% to 10%. This measured external PL efficiency depends on the excitation area, and the thickness and area of the glass slide, indicating that the measured value of r/~ is strongly affected by scattering, losses and waveguiding of PL in the film and glass substrate. Note that waveguiding effects, substrate losses and other experimental artifacts have also been observed to distort the results of r/e measurements performed using an integrating sphere.
435
Two techniques have been applied to decrease the influence of waveguiding effects and to accurately estimate r/e. In the first method, the samples were placed film-side-down in index-matching microscope immersion oil with n = 1.524 dropped onto the surface of the solar cell. Under these conditions, we obtained T~e= 28% for the radiation emitted towards the solar cell. In order to evaluate the PL intensity in the opposite direction (through the glass slide), an identical sample was placed on the solar cell without the immersion oil, and the PL signals of the two samples were compared using a second photodetector placed above the samples (Fig. 1). The PL efficiency of the immersed sample was 0.8 times as large as that of the non-immersed sample, whose absolute efficiency was measured to be --- 3.6%. Therefore, the total efficiency of the immersed samples was 28% + (0.8) × 3.6% --- 31%. This number can be considered as a lower limit for "qe since the refractive index of the immersion fluid was less than n for Alq 3, and hence waveguiding losses to the Alq3 film were not completely eliminated. In the second measurement method, the luminescence intensities of the 1.35 Ixm thick film grown on Si substrates was compared with that of the film grown on a glass substrate. Using the previous calibration of the efficiency of the film on glass, it was found that the absolute value of r/e for the film on the Si substrate equals to (3.2 + 0.2)%. Since the refractive index of Si (4.16 at A = 530 nm) is much higher than that of Alq3, waveguide effects are eliminated and only photons emitted at angles less than that for total internal reflection (35 °) can contribute to "qe" In this case, a good approximation to tie is given by
rle = ( 1 - R ~) ( 1 + R 2 )
n - ~n2 - 1 2n
( 1)
where n is the Alq3 refractive index and R~ and R e are the Fresnel reflection coefficients for the PL radiation at the Alq-air and Si-Alq3 interfaces, respectively, averaged over angles < 35 °. The product of the first two factors in Eq. (1) is 1.09, while (n - ~ n 2 - 1 ) / 2 n =- 0.093. This gives a > 10-fold difference between ~e and 71i caused by the total internal reflection at the Alq3-air interface. Using Eq. (1), we calculate from r/e that the internal effi-
D.Z. Garbuzov et a l . / Chemical Physics Letters 249 (1996) 433-437
436
ciency for the green luminescence band centered at h = 530 nm is 7/i = (32 + 2)%. The difference between the calculated ~ and the measured r/e of the immersed samples is unexpectedly small if one considers the large fraction of luminescence waveguided in the Alq3 film sandwiched between the immersion oil and the glass, However, a considerable fraction of this waveguided radiation is also collected by the solar cell due to total internal reflection at the oil-air interface, as shown in Fig. 1. Thus, the results obtained by both methods are in good agreement, The dependence of the internal luminescence efficiency on film thickness for samples grown on glass substrates is shown in Fig. 4. To obtain these data, the luminescence from both the thin and previously calibrated thick samples were measured with filters placed over the photodetector to eliminate any nonabsorbed excitation (A = 370 nm) and scattered light from the monochromator. The amount of absorbed excitation light was directly measured for films with thickness > 500 ,~, and was calculated for thinner films using the data in the inset of Fig. 2. Data shown in Fig. 4 correspond to freshly prepared sampies, with air exposure times < 5 h. We have not observed any efficiency oscillations with film thickness which could be attributed to microcavity or interference effects [5] due to the low reflectance of the Alq3-glass interface and coordinate-angle averaging of these effects. Fig. 4 demonstrates that the luminescence efficiency of Aiq3 films remains constant over two orders of magnitude of thickness, with 40
...... , ........ , ........ , ~! 35~-~"x"--~ . . . . . . . . . . ~ ~ -~
30-
efficiency, r/i = (32 + 2)%, w i t h the electrolumines-
,"
= 25°>" ~' '3 ,, ~E 20. ,, tu ,, 15. ~ _-c lO.J o_ 50 ....... 0.01
a noticeable drop in efficiency observed only for films thinner than 100 ,~ [6]. A comparison of the absorption spectrum shown in Fig. 2 with the absorption features of solutions of compounds similar to Alq3 [7] allows one to unambiguously attribute the longer and shorter-wavelength absorption bands to transitions from the ground state to the ~La and ~B b excited states, respectively [8]. The plateau of the spectral dependence of the photoluminescence excitation ( P L E ) o v e r the wavelength range that incorporates both absorption lines is due to fast non-radiative relaxation from the ~B b to the ~L a state, with subsequent radiative recombination to the ground state (Fig. 3, inset). This implies I that the ~B b - L a relaxation time is substantially shorter than the radiative lifetime of the ~B b state. This is consistent with previous observations for many dye molecules in solution [9]. Another important conclusion which may be drawn from the presence of the PLE plateau is that the diffusion length, l X, of both excited states is very short. That is, l~ < a - ~ = 250 .~, where a = 4 × 105 cm-~ for the short excitation wavelength of h = 260 nm. If I x were longer than 250 A, one would expect a decrease in PL efficiency at excitation wavelengths of h -=- 260 nm due to surface recombination. This conclusion is supported by measurements of the dependence of quantum efficiency on film thickness (Fig. 4), from which it follows that l x is less than 100 ,~. It should be noted that the observed decrease of ~h with film thickness is much weaker than previously reported [2] for Alq 3-based OLEDs, indicating that there are additional mechanisms determining the quantum efficiency dependence on layer thickness in multilayer, heterojunction devices. A comparison of the internal photoluminescence
. ............... 011 Film Thickness [~m] Fig. 4.
, 1
cence efficiency of OLEDs (1%-2%) indicates that there is still considerable potential for further improvement in OLED performance. This improvement may be obtained by increasing the number of injected electron-hole pairs recombining through ~L, and I B b states, as well as by improving the radiative coupling out of the device by decreasing waveguiding in the substrate. In conclusion, the absorption, photoexcitation, external and internal photoluminescence efficiencies of Alq 3 thin films grown by thermal evaporation have
D.Z. Garbuzov et al. / Chemical Physics Letters 249 (1996) 433-437
437
been measured for the first time over the wavelength range 200 < h < 600 nm. The absorption measurements demonstrate that the overlap of the 1L~ and ~B b electronic transition bands of Alq 3 results in an absorption coefficient in solid films of a < 10 4 c m - 1 in the spectral range from A = 250 to 425 nm. Furthermore, investigation of the photoluminescence excitation spectra over this spectral range indicates that the efficiency for the green luminescence band centered at h = 530 nm does not depend on the
and the Air Force Office of Scientific Research (C. Lee and G. Pomrenke) for their generous support of this research.
excitation radiation wavelength. Direct measurements of the absolute value of r/g suggest that the total external efficiency and the internal efficiency for radiative transitions centered at h = 530 nm is much higher than previously assumed [1], and is ~/i = (32 + 2)% at room temperature. A comparison
[2] P.E. Burrows and S.R. Forrest, Appl. Phys. Letters 64 (1993) 2285. [3] Z. Shen, P.E. Burrows, V. Bulovi~, S.R. Forrest, D.M. McEarthy and M.E. Thompson, submitted for publication. [4] D.Z. Garbuzov, J. Luminescence 27 (1982)109. [5] s. Saito, T. Tsutsui, M. Era, N. Takada, C. Adachi, Y.
of this result with the best published value of r/e (1.3%) for Alq3 electroluminescence [1 ] shows that there remains large room for further improvements in the efficiency of green OLEDs.
navia, Applied physics series, Vol. 170 (Helsinki, 1990) p. 215.
--
The authors thank Universal Displays Corp., the Advanced Research Projects Agency (D. Slobodin),
References
[1] c.w. Tang, S.A. VanSlyke and C.H. Chen, J. AppL Phys. 65 (1989) 3610.
Hamada and T. Wakimoto, SPIE Proc. 1910 (1993) 212. [6] C. Adachi, T. Tsutsui and S. Saito, Acta polytechnica scandi-
[7] H.-H. Perkampus and K. Kortfim, Z. Anal. Chem. 190 (1962) 111. [8] H.B. Klevens and J.R. Platt, J. Chem. Phys. 17 (1949) 470. [9] F.P. Schafer, in: Topics in applied physics, Vol. 1, Dye lasers, ed. F.P. Schafer (Spring, Berlin, 1973) p. 29.