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Optical absorption in Alq
Synthetic Metals 122 (2001) 53±54
Optical absorption in Alq Anver Aziz, K.L. Narasimhan* Solid State Electronics, Tata Institute of Fundamental Research, Colaba, Mumbai 400005, India
Abstract In this paper, we report on the optical absorption of tris(8-hydroxyquinolato) aluminium (Alq) sublimed thin ®lms between 0.7 and 5 eV and compare it with absorption in solution between 1.5 and 5 eV. Speci®cally, we show that above 2.5 eV the absorption in solution and in the thin ®lm are very similar to each other. The absorption falls off exponentially with energy from 3.18 eV in both thin ®lm and in solution. Below 2.5 eV the absorption spectrum of the thin ®lm changes qualitatively and is nearly independent of energy down to 0.7 eV. Exposure of the ®lm to atmosphere signi®cantly affects the subband gap absorption and causes an increase in the defect density. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Alq; Organic semiconductors; Optical absorption; Photo-thermal de¯ection spectroscopy
1. Introduction The discovery of ef®cient electroluminescence in tris(8hydroxyquinolato) aluminium (Alq) has triggered a great deal of interest in the optical properties of organic semiconductors [1]. The study of defects is very important to understand the transport properties. In this paper, we report the ®rst subband gap absorption measurements in thin ®lms of Alq using photo-thermal de¯ection spectroscopy (PDS) and compare it with the absorption of Alq in solution. The normalised subband gap absorption is larger in the solid state than in solution suggesting that defects in the solid state dominate the subband gap absorption. The subband gap absorption increases when the sample is exposed to atmosphere. From the subband absorption we estimate the defect density in the ®lms. 2. Experimental details Thin ®lms of Alq were evaporated on 7059 and quartz glass at a pressure of 10ÿ6 Torr at room temperature. Prior to evaporation, Alq was puri®ed by vacuum sublimation. For the PDS measurements, the samples were about 1 m thick. To measure the solution spectrum, Alq was dissolved in methylene chloride. To cover a wide absorption range, the concentration and the path length were both varied with large overlapping regions to correctly match the spectrum over the * Corresponding author. Fax: 91-22-215-2110. E-mail address: [email protected] (K.L. Narasimhan).
whole range. Details of these measurements will be published elsewhere [2]. 3. Results and discussion Fig. 1 compares the optical absorption of Alq in thin ®lm with Alq in solution. The absorption is characterised by two peaks at 3.18 and 4.7 eV in both the thin ®lm and in solution. We veri®ed that the absorptivity per molecule at 390 nm is the same in the thin ®lm and in solution. This enables us to make a quantitative comparison between the solid state and solution spectra. Using the perimeter free electron theory model [3], we identify these transitions with 1La and 1Bb transitions. This is in agreement with the results of Garbuzov et al. [4]. In solution one also sees two transitions at 3.71 and 3.9 eV, respectively. Although the absorption spectrum in solution and thin ®lm are qualitatively similar between 2.5 and 3.2 eV, the transitions in solution are intramolecular as Ê . Below the separation between molecules is about 500 A 3.18 eV, the absorption falls off exponentially with energy till about 2.5 eV. Below this energy, the absorption (subband absorption) is almost constant in the solid state down to 0.7 eV and is larger than in solution. The subband gap absorption depends on the ®lm deposition temperature, increasing as the substrate temperature is lowered. This suggests that the subband absorption is due to defects in the solid state. Fig. 2 shows the subband gap absorption of a thin ®lm before and after exposure to air. The subband gap absorption is enhanced by exposure to air. We have veri®ed that the
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 6 0 - 6
54
A. Aziz, K.L. Narasimhan / Synthetic Metals 122 (2001) 53±54
forces. We are dealing with narrow band semiconductors. We can account for the broad absorption by assuming that the subband gap levels form a continuum and transitions take place from these levels to a ®nal state whose wave function extends to more than one molecule. We can estimate the defect state density using the formula [5] Z Nd 8 1015 a dE (1)
Fig. 1. A plot of a as a function of energy for a thin film of Alq (*) and extinction coefficient of Alq in solution (Ð).
We estimate Nd to be 6 1017 cmÿ3 for the as deposited ®lms and 3 1018 cmÿ3 after exposure to atmosphere. From the absorption measurements we conclude that the Fermi level in Alq is no more than 0.7 eV from the LUMO level. This result is in good agreement with recent ESCA measurements that suggest the Fermi level is 0.5 eV below the LUMO level [6]. In conclusion, we have measured the absorption of Alq in solution (1.5±5 eV) and in thin ®lms (0.7±5 eV). We ®nd that below 2.5 eV the absorption of the thin ®lm is larger than that in solution. We attribute this to defect states. Exposure of the thin ®lm to air increases the subband gap absorption. The defect density is also in¯uenced by exposure to air. Acknowledgements
Fig. 2. Absorption coefficient a as a function of energy for fresh Alq film (*) and that exposed to air for 3 days (&).
subband gap absorption of a sample stored in a dry box for months is similar to that of a freshly made sample. The enhanced defect density is hence not due to any intrinsic sample ageing effects. We now try to understand the subband gap absorption in Alq. Defects in inorganic semiconductors can easily be understood as arising from broken bonds. In these solids, a constant subband gap absorption can be interpreted as absorption arising from transitions from a defect level into a continuum of extended states. The bonding between molecules in organic semiconductors is due to van der Walls
We thank Prof. N. Periasamy for useful discussions, Meghan Patankar and Bindu for their help with the measurements.
References [1] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [2] A. Aziz, K.L. Narasimhan, Synth. Metals, in press. [3] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals, Clarendon Press, Oxford, 1982, p. 7. [4] D.Z. Garbuzov, V. Bulovic, P.E. Burrows, S.R. Forrest, Chem. Phys. Lett. 249 (1996) 433. [5] N.M. Amer, W.B. Jackson, Semiconductors and Semimetals, in: J.I. Pankove (Ed.), Academic Press, NY, 1984, p. 83. [6] 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.
Optical absorption in Alq Anver Aziz, K.L. Narasimhan* Solid State Electronics, Tata Institute of Fundamental Research, Colaba, Mumbai 400005, India
Abstract In this paper, we report on the optical absorption of tris(8-hydroxyquinolato) aluminium (Alq) sublimed thin ®lms between 0.7 and 5 eV and compare it with absorption in solution between 1.5 and 5 eV. Speci®cally, we show that above 2.5 eV the absorption in solution and in the thin ®lm are very similar to each other. The absorption falls off exponentially with energy from 3.18 eV in both thin ®lm and in solution. Below 2.5 eV the absorption spectrum of the thin ®lm changes qualitatively and is nearly independent of energy down to 0.7 eV. Exposure of the ®lm to atmosphere signi®cantly affects the subband gap absorption and causes an increase in the defect density. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Alq; Organic semiconductors; Optical absorption; Photo-thermal de¯ection spectroscopy
1. Introduction The discovery of ef®cient electroluminescence in tris(8hydroxyquinolato) aluminium (Alq) has triggered a great deal of interest in the optical properties of organic semiconductors [1]. The study of defects is very important to understand the transport properties. In this paper, we report the ®rst subband gap absorption measurements in thin ®lms of Alq using photo-thermal de¯ection spectroscopy (PDS) and compare it with the absorption of Alq in solution. The normalised subband gap absorption is larger in the solid state than in solution suggesting that defects in the solid state dominate the subband gap absorption. The subband gap absorption increases when the sample is exposed to atmosphere. From the subband absorption we estimate the defect density in the ®lms. 2. Experimental details Thin ®lms of Alq were evaporated on 7059 and quartz glass at a pressure of 10ÿ6 Torr at room temperature. Prior to evaporation, Alq was puri®ed by vacuum sublimation. For the PDS measurements, the samples were about 1 m thick. To measure the solution spectrum, Alq was dissolved in methylene chloride. To cover a wide absorption range, the concentration and the path length were both varied with large overlapping regions to correctly match the spectrum over the * Corresponding author. Fax: 91-22-215-2110. E-mail address: [email protected] (K.L. Narasimhan).
whole range. Details of these measurements will be published elsewhere [2]. 3. Results and discussion Fig. 1 compares the optical absorption of Alq in thin ®lm with Alq in solution. The absorption is characterised by two peaks at 3.18 and 4.7 eV in both the thin ®lm and in solution. We veri®ed that the absorptivity per molecule at 390 nm is the same in the thin ®lm and in solution. This enables us to make a quantitative comparison between the solid state and solution spectra. Using the perimeter free electron theory model [3], we identify these transitions with 1La and 1Bb transitions. This is in agreement with the results of Garbuzov et al. [4]. In solution one also sees two transitions at 3.71 and 3.9 eV, respectively. Although the absorption spectrum in solution and thin ®lm are qualitatively similar between 2.5 and 3.2 eV, the transitions in solution are intramolecular as Ê . Below the separation between molecules is about 500 A 3.18 eV, the absorption falls off exponentially with energy till about 2.5 eV. Below this energy, the absorption (subband absorption) is almost constant in the solid state down to 0.7 eV and is larger than in solution. The subband gap absorption depends on the ®lm deposition temperature, increasing as the substrate temperature is lowered. This suggests that the subband absorption is due to defects in the solid state. Fig. 2 shows the subband gap absorption of a thin ®lm before and after exposure to air. The subband gap absorption is enhanced by exposure to air. We have veri®ed that the
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 6 0 - 6
54
A. Aziz, K.L. Narasimhan / Synthetic Metals 122 (2001) 53±54
forces. We are dealing with narrow band semiconductors. We can account for the broad absorption by assuming that the subband gap levels form a continuum and transitions take place from these levels to a ®nal state whose wave function extends to more than one molecule. We can estimate the defect state density using the formula [5] Z Nd 8 1015 a dE (1)
Fig. 1. A plot of a as a function of energy for a thin film of Alq (*) and extinction coefficient of Alq in solution (Ð).
We estimate Nd to be 6 1017 cmÿ3 for the as deposited ®lms and 3 1018 cmÿ3 after exposure to atmosphere. From the absorption measurements we conclude that the Fermi level in Alq is no more than 0.7 eV from the LUMO level. This result is in good agreement with recent ESCA measurements that suggest the Fermi level is 0.5 eV below the LUMO level [6]. In conclusion, we have measured the absorption of Alq in solution (1.5±5 eV) and in thin ®lms (0.7±5 eV). We ®nd that below 2.5 eV the absorption of the thin ®lm is larger than that in solution. We attribute this to defect states. Exposure of the thin ®lm to air increases the subband gap absorption. The defect density is also in¯uenced by exposure to air. Acknowledgements
Fig. 2. Absorption coefficient a as a function of energy for fresh Alq film (*) and that exposed to air for 3 days (&).
subband gap absorption of a sample stored in a dry box for months is similar to that of a freshly made sample. The enhanced defect density is hence not due to any intrinsic sample ageing effects. We now try to understand the subband gap absorption in Alq. Defects in inorganic semiconductors can easily be understood as arising from broken bonds. In these solids, a constant subband gap absorption can be interpreted as absorption arising from transitions from a defect level into a continuum of extended states. The bonding between molecules in organic semiconductors is due to van der Walls
We thank Prof. N. Periasamy for useful discussions, Meghan Patankar and Bindu for their help with the measurements.
References [1] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [2] A. Aziz, K.L. Narasimhan, Synth. Metals, in press. [3] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals, Clarendon Press, Oxford, 1982, p. 7. [4] D.Z. Garbuzov, V. Bulovic, P.E. Burrows, S.R. Forrest, Chem. Phys. Lett. 249 (1996) 433. [5] N.M. Amer, W.B. Jackson, Semiconductors and Semimetals, in: J.I. Pankove (Ed.), Academic Press, NY, 1984, p. 83. [6] 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.