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Ambient effect on the electronic structures of tris-(8-hydroxyquinoline) aluminum films investigated by photoelectron spectroscopy
12 January 2001
Chemical Physics Letters 333 (2001) 212±216
www.elsevier.nl/locate/cplett
Ambient eect on the electronic structures of tris-(8-hydroxyquinoline) aluminum ®lms investigated by photoelectron spectroscopy L.S. Liao a,b, X.H. Sun a,c, L.F. Cheng a, N.B. Wong a,c, C.S. Lee a,*, S.T. Lee a a
Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People's Republic of China b Surface Physics Laboratory, Fudan University, Shanghai 200433, People's Republic of China c Department of Biology and Chemistry, City University of Hong Kong, Hong Kong, People's Republic of China Received 16 March 2000; in ®nal form 07 November 2000
Abstract Thin ®lms of tris-(8-hydroxyquinoline) aluminum (Alq3 ) were exposed to trace amounts of O2 , CO2 , H2 O, or to ambient air. Evolution of electronic structures of Alq3 ®lms with increasing gas exposure was measured using ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS). The vacuum energy level, the highest occupied molecular orbital, and XPS core levels of the constituting elements in Alq3 shifted according to the kind of gas exposure. Chemical reaction between oxygen and the Alq3 ®lms was observed upon oxygen exposure. Moreover, it was found that the dominant in¯uence of ambient conditions on the electronic structures of the Alq3 ®lms was from H2 O. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Since ecient electroluminescence from organic devices was reported by Tang and Van Slyke in 1987 [1], organic light-emitting diodes (OLEDs) have been developed into a stage ready for commercial applications [2,3]. However, limited lifetime of OLEDs is still a major problem. Ambient gases, containing moisture and oxygen, have great in¯uence on the lifetime of OLEDs [4±8]. The ambient gases can aect both organic ®lms and/or electrodes in the devices. Considering the electrodes, it is understandable that the ambient gases
*
Corresponding author. Fax: +852-2788-7830. E-mail address: [email protected] (C.S. Lee).
are more detrimental to the metal cathode (such as Mg:Ag or Al cathode) than to the indium tin oxide anode [4,5,8]. For the organic ®lms, Papadimitrakopoulos et al. have reported that tris-(8-hywould droxyqunoline) aluminum (Alq3 ) decompose into free 8-hydroxyquinoline (8-Hq) and other byproducts in the presence of water. The free 8-Hq can then undergo an oxidative condensation to yield a non-emissive polymeric byproduct and water when oxygen is available [6]. However, Aziz et al. pointed out that the above occurrence in OLEDs had not been con®rmed [7]. Instead, they found that exposure to humidity for more than several hours led to obvious morphological changes due to the crystallization. In our own experiments, we found that Alq3 ®lms when heavily exposed to humidity will result in obvious
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 3 6 0 - 9
L.S. Liao et al. / Chemical Physics Letters 333 (2001) 212±216
morphological changes which are consistent with the results reported by Aziz et al. [7]. The above mentioned investigations indicated that ambient gases have signi®cant eect on the performance of OLEDs, though some dispute still remains. However, these experiments were done in some extreme conditions, e.g. by subjecting Alq3 ®lms to a very humid environment. As is well known, encapsulation is essential for any practical OLED. Thus the extreme humid condition does not normally exist in a real device environment. Therefore, the in¯uence of trace amounts of ambient gases on the long-term performance of OLEDs is a more crucial and relevant issue to be considered. This type of work needs both a good control of ambient exposure as well as sensitive characterization methods. Electronic structures of the Alq3 layer in OLEDs are directly related to various physical processes which control device performance such as carrier injection and transport, and light emission, etc. As the electronic structures of Alq3 are sensitive to exposure to ambient gases, we investigate this eect by using photoelectron spectroscopy and report the results in this Letter. 2. Experimental P-type h1 1 1i Si wafers were used as the substrates in this work. Cleaned Si substrates were transferred into an evaporation chamber, which is attached to a VG ESCALAB 220I-XL photoelectron spectroscopy system. The base pressures in the evaporation chamber and the analysis chamber were 1:0 10ÿ9 and 8:0 10ÿ11 mbar, respectively. Prior to ®lm deposition, the Si surface was in situ cleaned by annealing at 750°C for 10 min followed by Ar ion sputtering at 2 keV for 5 min. Twicesublimated Alq3 powder was used as the source material in the evaporation chamber. A 6 nm thick Alq3 ®lm was formed by thermal deposition on the Si substrate. After ®lm deposition, samples were transferred into an environmental chamber attached to the analysis chamber for gas exposures (the base pressure in the environmental chamber was about 2:0 10ÿ7 mbar). Water, O2 , CO2 , or atmospheric air was used as the exposure gas
213
source, respectively. The water vapor source was prepared by loading deionized water into a quartz tube connected to the environmental chamber. The tube was ®rst cooled down to 0°C. Residual gases in the tube were then pumped out from the tube. The tube was then brought back to room temperature. These cooling±degasing±warming procedures were repeated twice to minimize the non-H2 O molecules present in the tube. Highpurity O2 and CO2 (> 99:98%) were provided from gas cylinders. Exposure dosage of gases was controlled manually and calculated by multiplying pressure with exposure time (1 LM 10ÿ6 Torr s). The pressure was calibrated according to the corresponding gas sensitivity factors. Each exposure step was followed by in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements in the analysis chamber. We used the He I excitation line (21.2 eV) from a He discharge lamp for the UPS measurements and a monochromatic Al Ka excitation line (1486.6 eV) for the XPS measurements. The UPS spectra were recorded with a sample bias of )4.00 V to allow observation of the inelastic electron cut-o. The energy resolution of UPS is within 0.04 eV. Before the experiment, the possible spectral changes due to irradiation of X-ray and ultraviolet sources on Alq3 ®lm were monitored, but no discernible spectral change was detected [9]. A nickel foil and a silver foil were used for the Fermi level (EF ) and the binding energy calibration. 3. Results and discussion Fig. 1 shows evolution of the UPS spectra with increasing H2 O exposure. The as-deposited Alq3 ®lm (the bottom curve) has well resolved peaks, labelled as A, B, C, D, E, and F, respectively. Peak A represents the highest occupied molecular orbital (HOMO) of Alq3 . After being exposed to H2 O for 103 LM, all the peaks as well as the low energy electron cut-o shift towards the lower kinetic energy side by 0.17 eV. Moreover, the peaks are slightly broadened. Upon more than 104 LM H2 O exposure, the peaks and the cuto do not shift additionally, but the peaks are
214
L.S. Liao et al. / Chemical Physics Letters 333 (2001) 212±216
Fig. 1. Evolution of the UPS spectra of Alq3 ®lm with increased exposure of H2 O from 103 to 106 LM, respectively.
further broadened. As a result, peak B was immersed in the spectrum, peak D became a shoulder, and the highest occupied state (HOS) of the HOMO extended towards the EF slightly. Further exposure to H2 O induces continual peak broadening. Alq3 ®lms were also exposed to O2 and CO2 . Fig. 2 shows evolution of the vacuum energy level (Evac ) and the HOS of the Alq3 ®lms as a function of the exposure dose of O2 , CO2 and H2 O. The variation of Evac is obviously dierent for exposure to dierent gases. Unlike the H2 O exposure (curve c) which results in a ®xed decrement of Evac (also shown in Fig. 1), the Evac increases gradually with increasing exposure dose of O2 or CO2 (curve a or b), and then the increment is saturated at 106 LM. Similar to the case of H2 O exposure (curve f), the peaks representing the molecular orbitals were broadened upon exposure to O2 or CO2 (curve d or e), and the HOSs shifted towards the EF . In Fig. 2, we set EF 0 and the ionization potential (IP) can then be easily calculated as IP Evac ÿ HOS.
Fig. 2. The change of vacuum energy level, (Evac , solid lines), and highest occupied molecular state, (HOS, dashed lines), with increased exposure to O2 , CO2 , or H2 O, respectively. (The asprepared ®lm is denoted as exposure to 100 LM for the convenience to draw the ®gure.)
From the IP, we can ®nd that all the exposures studied here resulted in the decrease of IP. We measured the XPS O1s, C1s, N1s, and Al2p core levels of the Alq3 ®lms after each exposure step. As an example, Fig. 3 shows the XPS Al2p core level of the Alq3 ®lm after exposure to O2 , CO2 and H2 O, respectively. Chemical reaction was detectable after the Alq3 ®lm was exposed to 106 LM O2 . In Fig. 3a, the curve representing the exposure to 106 LM O2 can be ®tted with two subcurves (denoted as dotted lines). Curve 1 has the same full width at half maximum as that of the asprepared ®lm. Thus curve 2 is in fact a new component, which indicates the new bonding of Al±O after the exposure. The area ratio of curve 2 to curve 1 is about 1:4. This means that about 20% Al in the Alq3 ®lm has been attacked by oxygen.
L.S. Liao et al. / Chemical Physics Letters 333 (2001) 212±216
215
core levels of O1s, C1s, N1s and Al2p shifted towards higher BE as the exposure dose increased. It suggests that the energy levels at the surface were moved down upon H2 O exposure. Since energy level shifting occurred after O2 or H2 O adsorption on the surface of Alq3 , we can reasonably deduce that the lowest unoccupied molecular orbital (LUMO) of Alq3 is increased after O2 exposure but decreased after H2 O exposure. As a lower LUMO is expected to lower the barrier for electron injection, the present observations tentatively suggest that a trace amount of H2 O adsorbed on Alq3 may be bene®cial to electron injection in the OLEDs. However, further investigations are in progress to con®rm the above speculation. To determine which kind of gas in air is in¯uential on the electronic structures of Alq3 ®lms, we exposed an as-prepared Alq3 ®lm to air for different durations and obtained the UPS spectra as shown in Fig. 4. After being exposed to air at 1 atm for 1 min (the exposure dose 4:56 1010 LM), all the peaks, as well as the lower energy
Fig. 3. The XPS Al2p core level before and after 106 LM exposure of (a) O2 ; (b) CO2 ; and (c) H2 O.
However, no chemical reaction was detectable after the Alq3 ®lm was exposed to CO2 or H2 O up to 106 LM, since the pro®le of the curve was basically identical before and after the exposure. Moreover, Fig. 3a and c indicate a chemical shifting after the exposure. In fact, when the Alq3 ®lm was exposed to O2 , all the core levels of O1s, C1s, N1s and Al2p shifted towards lower BE as the exposure dose increased (not shown here). This suggests that the energy levels at the surface were moved upwards after O2 was adsorbed on the surface. But energy levels were not changed upon CO2 exposure because no shifting in the core level of O1s, C1s, N1s or Al2p was observed. However, when the Alq3 ®lm was exposed to H2 O, all the
Fig. 4. Evolution of the UPS spectra of Alq3 ®lm with increased exposure to ambient air at 1 atm. The exposure time from the bottom spectrum to the top spectrum is 0, 1, 5, 10, 20, 30, and 60 min, respectively.
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L.S. Liao et al. / Chemical Physics Letters 333 (2001) 212±216
cut-o, shifted to lower kinetic energy by 0.16 eV. Moreover, all the peaks were slightly broadened. Upon further exposure, the peaks and the cut-o did not shift any further, but the peaks were further broadened. This indicates that the evolution of the UPS spectra in Fig. 4 is almost the same as that in Fig. 1. This suggests that the dominant eect of the ambient air on Alq3 ®lm comes from H2 O, which is consistent with the observations performed under some extreme conditions [4±8].
Alq3 ®lm may be attributed to moisture. A trace amount of exposure to H2 O might be bene®cial to electron injection in OLEDs because of a lower electron injection barrier.
Acknowledgements This work was supported by the Research Grant Council of Hong Kong (Nos. 9040430 and 8730009).
4. Conclusions In summary, exposure to a trace amount of O2 , CO2 or H2 O will induce a change in the electronic structures of the Alq3 ®lm. However, the changes are dierent when dierent species are adsorbed on its surface. O2 adsorption gradually increases the Evac and raises the energy levels at the surface of the Alq3 ®lm; CO2 adsorption increases the Evac but does not move the core levels of Alq3 ; and H2 O adsorption lowers the Evac and moves down the energy levels at the surface of the Alq3 ®lm. Moreover, as the exposure to O2 , CO2 or H2 O is increased, the molecular orbitals broadened gradually, which results in shifting the HOS towards the EF . Moreover, chemical reaction between Al and O has been observed after O2 exposure. In addition, when the Alq3 ®lm is exposed to air, the changes in electronic structures are similar to those observed upon H2 O exposure. Therefore, the dominant in¯uence of ambient conditions on the
References [1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [2] C.W. Tang, Digest SID 1996 International Symposium, SID, San Diego, CA, 1996, p. 181. [3] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, A. Stocking, Science 273 (1996) 884. [4] P.E. Burrows, V. Bulovic, S.R. Forrest, L.S. Sapochak, D.M. McCarty, M.E. Thomposon, Appl. Phys. Lett. 65 (1994) 2922. [5] L.M. Do, M. Oyamada, A. Koike, E.M. Han, N. Yamamoto, M. Fujihira, Thin Solid Films 273 (1996) 209. [6] K.A. Higginson, X.M. Zhang, F. Papadimitrakopoulos, Chem. Mater. 10 (1998) 1017. [7] H. Aziz, Z. Popovic, S. Xie, A.M. Hor, N.X. Hu, C. Tripp, G. Xu, Appl. Phys. Lett. 72 (1998) 756. [8] H. Aziz, Z. Popovic, C.P. Tripp, N.X. Hu, A.M. Hor, G. Xu, Appl. Phys. Lett. 72 (1998) 2642. [9] L.S. Liao, L.S. Hung, W.C. Chan, X.M. Ding, T.K. Sham, I. Bello, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 75 (1999) 1619.
Chemical Physics Letters 333 (2001) 212±216
www.elsevier.nl/locate/cplett
Ambient eect on the electronic structures of tris-(8-hydroxyquinoline) aluminum ®lms investigated by photoelectron spectroscopy L.S. Liao a,b, X.H. Sun a,c, L.F. Cheng a, N.B. Wong a,c, C.S. Lee a,*, S.T. Lee a a
Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People's Republic of China b Surface Physics Laboratory, Fudan University, Shanghai 200433, People's Republic of China c Department of Biology and Chemistry, City University of Hong Kong, Hong Kong, People's Republic of China Received 16 March 2000; in ®nal form 07 November 2000
Abstract Thin ®lms of tris-(8-hydroxyquinoline) aluminum (Alq3 ) were exposed to trace amounts of O2 , CO2 , H2 O, or to ambient air. Evolution of electronic structures of Alq3 ®lms with increasing gas exposure was measured using ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS). The vacuum energy level, the highest occupied molecular orbital, and XPS core levels of the constituting elements in Alq3 shifted according to the kind of gas exposure. Chemical reaction between oxygen and the Alq3 ®lms was observed upon oxygen exposure. Moreover, it was found that the dominant in¯uence of ambient conditions on the electronic structures of the Alq3 ®lms was from H2 O. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Since ecient electroluminescence from organic devices was reported by Tang and Van Slyke in 1987 [1], organic light-emitting diodes (OLEDs) have been developed into a stage ready for commercial applications [2,3]. However, limited lifetime of OLEDs is still a major problem. Ambient gases, containing moisture and oxygen, have great in¯uence on the lifetime of OLEDs [4±8]. The ambient gases can aect both organic ®lms and/or electrodes in the devices. Considering the electrodes, it is understandable that the ambient gases
*
Corresponding author. Fax: +852-2788-7830. E-mail address: [email protected] (C.S. Lee).
are more detrimental to the metal cathode (such as Mg:Ag or Al cathode) than to the indium tin oxide anode [4,5,8]. For the organic ®lms, Papadimitrakopoulos et al. have reported that tris-(8-hywould droxyqunoline) aluminum (Alq3 ) decompose into free 8-hydroxyquinoline (8-Hq) and other byproducts in the presence of water. The free 8-Hq can then undergo an oxidative condensation to yield a non-emissive polymeric byproduct and water when oxygen is available [6]. However, Aziz et al. pointed out that the above occurrence in OLEDs had not been con®rmed [7]. Instead, they found that exposure to humidity for more than several hours led to obvious morphological changes due to the crystallization. In our own experiments, we found that Alq3 ®lms when heavily exposed to humidity will result in obvious
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 3 6 0 - 9
L.S. Liao et al. / Chemical Physics Letters 333 (2001) 212±216
morphological changes which are consistent with the results reported by Aziz et al. [7]. The above mentioned investigations indicated that ambient gases have signi®cant eect on the performance of OLEDs, though some dispute still remains. However, these experiments were done in some extreme conditions, e.g. by subjecting Alq3 ®lms to a very humid environment. As is well known, encapsulation is essential for any practical OLED. Thus the extreme humid condition does not normally exist in a real device environment. Therefore, the in¯uence of trace amounts of ambient gases on the long-term performance of OLEDs is a more crucial and relevant issue to be considered. This type of work needs both a good control of ambient exposure as well as sensitive characterization methods. Electronic structures of the Alq3 layer in OLEDs are directly related to various physical processes which control device performance such as carrier injection and transport, and light emission, etc. As the electronic structures of Alq3 are sensitive to exposure to ambient gases, we investigate this eect by using photoelectron spectroscopy and report the results in this Letter. 2. Experimental P-type h1 1 1i Si wafers were used as the substrates in this work. Cleaned Si substrates were transferred into an evaporation chamber, which is attached to a VG ESCALAB 220I-XL photoelectron spectroscopy system. The base pressures in the evaporation chamber and the analysis chamber were 1:0 10ÿ9 and 8:0 10ÿ11 mbar, respectively. Prior to ®lm deposition, the Si surface was in situ cleaned by annealing at 750°C for 10 min followed by Ar ion sputtering at 2 keV for 5 min. Twicesublimated Alq3 powder was used as the source material in the evaporation chamber. A 6 nm thick Alq3 ®lm was formed by thermal deposition on the Si substrate. After ®lm deposition, samples were transferred into an environmental chamber attached to the analysis chamber for gas exposures (the base pressure in the environmental chamber was about 2:0 10ÿ7 mbar). Water, O2 , CO2 , or atmospheric air was used as the exposure gas
213
source, respectively. The water vapor source was prepared by loading deionized water into a quartz tube connected to the environmental chamber. The tube was ®rst cooled down to 0°C. Residual gases in the tube were then pumped out from the tube. The tube was then brought back to room temperature. These cooling±degasing±warming procedures were repeated twice to minimize the non-H2 O molecules present in the tube. Highpurity O2 and CO2 (> 99:98%) were provided from gas cylinders. Exposure dosage of gases was controlled manually and calculated by multiplying pressure with exposure time (1 LM 10ÿ6 Torr s). The pressure was calibrated according to the corresponding gas sensitivity factors. Each exposure step was followed by in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements in the analysis chamber. We used the He I excitation line (21.2 eV) from a He discharge lamp for the UPS measurements and a monochromatic Al Ka excitation line (1486.6 eV) for the XPS measurements. The UPS spectra were recorded with a sample bias of )4.00 V to allow observation of the inelastic electron cut-o. The energy resolution of UPS is within 0.04 eV. Before the experiment, the possible spectral changes due to irradiation of X-ray and ultraviolet sources on Alq3 ®lm were monitored, but no discernible spectral change was detected [9]. A nickel foil and a silver foil were used for the Fermi level (EF ) and the binding energy calibration. 3. Results and discussion Fig. 1 shows evolution of the UPS spectra with increasing H2 O exposure. The as-deposited Alq3 ®lm (the bottom curve) has well resolved peaks, labelled as A, B, C, D, E, and F, respectively. Peak A represents the highest occupied molecular orbital (HOMO) of Alq3 . After being exposed to H2 O for 103 LM, all the peaks as well as the low energy electron cut-o shift towards the lower kinetic energy side by 0.17 eV. Moreover, the peaks are slightly broadened. Upon more than 104 LM H2 O exposure, the peaks and the cuto do not shift additionally, but the peaks are
214
L.S. Liao et al. / Chemical Physics Letters 333 (2001) 212±216
Fig. 1. Evolution of the UPS spectra of Alq3 ®lm with increased exposure of H2 O from 103 to 106 LM, respectively.
further broadened. As a result, peak B was immersed in the spectrum, peak D became a shoulder, and the highest occupied state (HOS) of the HOMO extended towards the EF slightly. Further exposure to H2 O induces continual peak broadening. Alq3 ®lms were also exposed to O2 and CO2 . Fig. 2 shows evolution of the vacuum energy level (Evac ) and the HOS of the Alq3 ®lms as a function of the exposure dose of O2 , CO2 and H2 O. The variation of Evac is obviously dierent for exposure to dierent gases. Unlike the H2 O exposure (curve c) which results in a ®xed decrement of Evac (also shown in Fig. 1), the Evac increases gradually with increasing exposure dose of O2 or CO2 (curve a or b), and then the increment is saturated at 106 LM. Similar to the case of H2 O exposure (curve f), the peaks representing the molecular orbitals were broadened upon exposure to O2 or CO2 (curve d or e), and the HOSs shifted towards the EF . In Fig. 2, we set EF 0 and the ionization potential (IP) can then be easily calculated as IP Evac ÿ HOS.
Fig. 2. The change of vacuum energy level, (Evac , solid lines), and highest occupied molecular state, (HOS, dashed lines), with increased exposure to O2 , CO2 , or H2 O, respectively. (The asprepared ®lm is denoted as exposure to 100 LM for the convenience to draw the ®gure.)
From the IP, we can ®nd that all the exposures studied here resulted in the decrease of IP. We measured the XPS O1s, C1s, N1s, and Al2p core levels of the Alq3 ®lms after each exposure step. As an example, Fig. 3 shows the XPS Al2p core level of the Alq3 ®lm after exposure to O2 , CO2 and H2 O, respectively. Chemical reaction was detectable after the Alq3 ®lm was exposed to 106 LM O2 . In Fig. 3a, the curve representing the exposure to 106 LM O2 can be ®tted with two subcurves (denoted as dotted lines). Curve 1 has the same full width at half maximum as that of the asprepared ®lm. Thus curve 2 is in fact a new component, which indicates the new bonding of Al±O after the exposure. The area ratio of curve 2 to curve 1 is about 1:4. This means that about 20% Al in the Alq3 ®lm has been attacked by oxygen.
L.S. Liao et al. / Chemical Physics Letters 333 (2001) 212±216
215
core levels of O1s, C1s, N1s and Al2p shifted towards higher BE as the exposure dose increased. It suggests that the energy levels at the surface were moved down upon H2 O exposure. Since energy level shifting occurred after O2 or H2 O adsorption on the surface of Alq3 , we can reasonably deduce that the lowest unoccupied molecular orbital (LUMO) of Alq3 is increased after O2 exposure but decreased after H2 O exposure. As a lower LUMO is expected to lower the barrier for electron injection, the present observations tentatively suggest that a trace amount of H2 O adsorbed on Alq3 may be bene®cial to electron injection in the OLEDs. However, further investigations are in progress to con®rm the above speculation. To determine which kind of gas in air is in¯uential on the electronic structures of Alq3 ®lms, we exposed an as-prepared Alq3 ®lm to air for different durations and obtained the UPS spectra as shown in Fig. 4. After being exposed to air at 1 atm for 1 min (the exposure dose 4:56 1010 LM), all the peaks, as well as the lower energy
Fig. 3. The XPS Al2p core level before and after 106 LM exposure of (a) O2 ; (b) CO2 ; and (c) H2 O.
However, no chemical reaction was detectable after the Alq3 ®lm was exposed to CO2 or H2 O up to 106 LM, since the pro®le of the curve was basically identical before and after the exposure. Moreover, Fig. 3a and c indicate a chemical shifting after the exposure. In fact, when the Alq3 ®lm was exposed to O2 , all the core levels of O1s, C1s, N1s and Al2p shifted towards lower BE as the exposure dose increased (not shown here). This suggests that the energy levels at the surface were moved upwards after O2 was adsorbed on the surface. But energy levels were not changed upon CO2 exposure because no shifting in the core level of O1s, C1s, N1s or Al2p was observed. However, when the Alq3 ®lm was exposed to H2 O, all the
Fig. 4. Evolution of the UPS spectra of Alq3 ®lm with increased exposure to ambient air at 1 atm. The exposure time from the bottom spectrum to the top spectrum is 0, 1, 5, 10, 20, 30, and 60 min, respectively.
216
L.S. Liao et al. / Chemical Physics Letters 333 (2001) 212±216
cut-o, shifted to lower kinetic energy by 0.16 eV. Moreover, all the peaks were slightly broadened. Upon further exposure, the peaks and the cut-o did not shift any further, but the peaks were further broadened. This indicates that the evolution of the UPS spectra in Fig. 4 is almost the same as that in Fig. 1. This suggests that the dominant eect of the ambient air on Alq3 ®lm comes from H2 O, which is consistent with the observations performed under some extreme conditions [4±8].
Alq3 ®lm may be attributed to moisture. A trace amount of exposure to H2 O might be bene®cial to electron injection in OLEDs because of a lower electron injection barrier.
Acknowledgements This work was supported by the Research Grant Council of Hong Kong (Nos. 9040430 and 8730009).
4. Conclusions In summary, exposure to a trace amount of O2 , CO2 or H2 O will induce a change in the electronic structures of the Alq3 ®lm. However, the changes are dierent when dierent species are adsorbed on its surface. O2 adsorption gradually increases the Evac and raises the energy levels at the surface of the Alq3 ®lm; CO2 adsorption increases the Evac but does not move the core levels of Alq3 ; and H2 O adsorption lowers the Evac and moves down the energy levels at the surface of the Alq3 ®lm. Moreover, as the exposure to O2 , CO2 or H2 O is increased, the molecular orbitals broadened gradually, which results in shifting the HOS towards the EF . Moreover, chemical reaction between Al and O has been observed after O2 exposure. In addition, when the Alq3 ®lm is exposed to air, the changes in electronic structures are similar to those observed upon H2 O exposure. Therefore, the dominant in¯uence of ambient conditions on the
References [1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [2] C.W. Tang, Digest SID 1996 International Symposium, SID, San Diego, CA, 1996, p. 181. [3] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, A. Stocking, Science 273 (1996) 884. [4] P.E. Burrows, V. Bulovic, S.R. Forrest, L.S. Sapochak, D.M. McCarty, M.E. Thomposon, Appl. Phys. Lett. 65 (1994) 2922. [5] L.M. Do, M. Oyamada, A. Koike, E.M. Han, N. Yamamoto, M. Fujihira, Thin Solid Films 273 (1996) 209. [6] K.A. Higginson, X.M. Zhang, F. Papadimitrakopoulos, Chem. Mater. 10 (1998) 1017. [7] H. Aziz, Z. Popovic, S. Xie, A.M. Hor, N.X. Hu, C. Tripp, G. Xu, Appl. Phys. Lett. 72 (1998) 756. [8] H. Aziz, Z. Popovic, C.P. Tripp, N.X. Hu, A.M. Hor, G. Xu, Appl. Phys. Lett. 72 (1998) 2642. [9] L.S. Liao, L.S. Hung, W.C. Chan, X.M. Ding, T.K. Sham, I. Bello, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 75 (1999) 1619.