- Email: [email protected]
Optical Absorption in AlQ
Synthetic Metals 114 Ž2000. 133–137 www.elsevier.comrlocatersynmet
Optical Absorption in AlQ Anver Aziz, K.L. Narasimhan) Tata Institute of Fundamental Research, National Centre of the GoÕernment of India for Nuclear Science and Maths, Homi Bhabha Road, Colaba, Bombay 400005, India Received 9 November 1999; received in revised form 1 March 2000; accepted 3 March 2000
Abstract In this paper, we report on optical absorption in thin films of trisŽ8-hydroxyquinoline.aluminium ŽAlQ. between 0.7 and 5 eV. We report on the first measurement of the sub-bandgap absorption in AlQ using Photothermal Deflection Spectroscopy ŽPDS. for different deposition conditions. The sub-bandgap absorption is not very sensitive to sample deposition temperature but increases on exposure to atmosphere. We present a simple model to explain the absorption data. We calculate the density of states in the gap to be about 6.10 17rcm3 and present evidence that the fermi level lies in the upper half of the band gap close to the LUMO level in AlQ. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Photothermal deflection spectroscopy; Sub-bandgap states; TrisŽ8-hydroxyquinoline.aluminium; Optical absorption
1. Introduction The discovery of efficient electroluminescence in trisŽ8-hydroxyquinoline.aluminium ŽAlQ. and Poly Phenylene Vinylene ŽPPV. has triggered a lot of interest in these compounds w1,2x. However, many aspects of optical and electrical properties still remain the subject of research. Not much is known about the gap states in AlQ in particular. A simple way to probe the gap states is to use optical absorption as it is a measure of the joint density of states in the system. Photothermal deflection spectroscopy ŽPDS. has played an important role in understanding defects in inorganic semiconductors w3x. In this paper, we report on absorption measurements from 0.7–5 eV Ž1700– 250 nm. in AlQ. We compare our results with AlQ in solution and highlight the differences and similarities between the solution and the solid state spectrum. Finally, we discuss the implications of our results on the electronic structure and transport in AlQ films. 2. Experimental Films of AlQ were deposited on quartz or Corning 7059 substrates in a pressure of less than 10y6 Torr. The substrate temperature was varied from 100 to 475 K to )
Corresponding author. Fax: q91-22-2152110.
investigate the effect of deposition conditions on film quality. Films were made using high purity Aldrich AlQ and purified by repeated sublimation or synthesised and purified by sublimation. The films were characterised by optical absorption and luminescence. No difference was found between these two sources of AlQ. The optical absorption in these films was measured between 250–1700 nm. The absorption in the range 250– 450 nm was carried out using a Shimadzu UV2100 spectrophotometer and by PDS in the range 400–1700 nm. The film thickness was about 0.1–1 m for the range 250–450 nm and 1 m for the PDS measurements. The PDS measurements require a liquid deflection medium. The liquid should not affect the film properties. AlQ is soluble in most organic liquids. After many experiments, we found cyclohexane and freon Žboiling point, 378C. acceptable as the deflection medium. Even these liquids tend to attack the surface of the film after some time. To protect the film from the liquid, we deposited a ˚ thick LiF on the AlQ film. To ensure that the LiF 200 A film did not influence the absorption, we measured the ˚ thick LiF film on glass using PDS. absorption of a 200 A We found that the absorption of the LiF film was an order of magnitude smaller than the smallest absorption measured in the AlQ q LiF films. As an extra check on our ˚ results, we varied the thickness of LiF from 25 to 300 A on the AlQ film. The measured absorption Žof the AlQ q LiF film. was independent of the LiF thickness. The PDS
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 0 0 . 0 0 2 3 2 - 0
134
A. Aziz, K.L. Narasimhanr Synthetic Metals 114 (2000) 133–137
measurement is limited to 1000 nm using cyclohexane as the deflecting medium. Beyond this range, the self absorption of cyclohexane becomes important. Freon is satisfactory at least up to 1700 nm. The absorption results obtained with freon were identical to that obtained using cyclohexane in the range 400–1000 nm. The general details of the PDS measurements are described elsewhere w3x. We have also measured the absorption of AlQ in solution. In order to investigate a wide range of absorption, we used different concentrations of AlQ in dichloromethane and had overlapping regions of absorption to enable us to construct the whole curve. To measure absorption in the low absorption range, we used a 10 cm path length cell. 3. Results and discussion. Fig. 1 shows the absorption coefficient of an AlQ film between 0.7 and 5 eV. The spectrum is dominated by two peaks at 3.18 eV Ž390 nm. and 4.7 eV Ž264 nm.. The absorption between 2.8 and 5 eV is in good agreement with published results w4x. Below 2.5 eV, the absorption varies slowly with energy up to 0.7 eV. Fig. 1 also shows the molar extinction coefficient of AlQ dissolved in dichloromethane. The solution spectrum is qualitatively similar to that of the thin film and has two peaks at 3.2 and 4.76 eV Ž386 and 258 nm, respectively. along with two weak vibrational bands. This curve is a composite made up by using different concentrations Ž c . of AlQ and path lengths Ž l .. To properly normalise the optical density ŽOD., care was taken to ensure that there were overlapping ranges between two concentrations Žpath lengths.. This was normalised with a concentration that had the same OD ˚ thin in solution Žat the low energy peak. as in a 1000 A film. This enables us to quantitatively compare the absorption data in solution with the film over the entire energy range.
Fig. 1. A plot of Ž1. absorption coefficient of AlQ film as a function of energy Ždashed line., Ž2. molar extinction co-efficient of AlQ in CH 2 Cl 2 as a function of energy Žsolid line. Žsee text.. The inset shows the energy level diagram for a single molecule.
Fig. 2. Absorption of Alq in Ž1. Ž –PP–PP– . thin film and in the following solutions Ž2. Ž – – – . CCl 4 , Ž3. ŽPPPPP. CH 2 Cl 2 , and Ž4. Ž –P–P– . CH 3 OH. The OD is normalised to unity to facilitate comparison between various spectra. The inset shows the normalised OD for Ž1. v a thin film and Ž2. I for AlQ in CH 3 OH solution. The thin film spectrum is rigidly shifted in energy so that the peak position is the same for both samples.
Fig. 2 shows the absorption of Ža. 1 m thin film on glass and Žb. solution spectra of AlQ dissolved in various solvents. ŽTo facilitate comparison among the various spectra, the OD is normalised to unity.. The low energy peak moves to higher energy as the polarity Ždielectric constant. of the solvent increases and the whole spectrum undergoes a rigid shift in energy. The inset in Fig. 2 shows the spectrum of the AlQ in methanol and AlQ thin film Žrigidly shifted in energy so as to make the low energy peaks coincide.. The spectra are identical even though the concentration of AlQ is different by more than four orders of magnitude in the two cases. We first attempt to understand the solution spectra. In general, in the vapour phase, the observed spectrum of a molecule is that of isolated molecules with its rotational, vibrational, and electronic excitations. In solution, even at low concentration, due to the interaction with the solvent, the spectra of the solute molecules is inhomogeneously broadened. This accounts for the large widths seen in the solution spectra in Figs. 1 and 2 w5x. The position of the bands also depends on the nature of the solvent. Generally polar solvents tend to shift the position of the bands w5x. This is seen clearly for AlQ in Fig. 2. We estimated the number of molecules in the film from the density of AlQ and the film thickness and also calculated the number of molecules in solution. The number of molecules in the path of the beam was the same in solution and in the thin film when the OD was the same in both cases. For the low energy peak, the absorption per molecule is the same in both the solid and in solution.
A. Aziz, K.L. Narasimhanr Synthetic Metals 114 (2000) 133–137
135
The optical absorption in conjugated systems is easily understood using a free electron model w6x. In a ring molecule, the p electrons are considered free to move in the plane of the molecule. The energy eigen-values are given by Eq s h2 q 2r Ž 8p 2 mr 2 . , where r is the radius of the ring. In a catacondensed system of n rings, the highest filled level is q s n. The change in the ring Žangular momentum. quantum number for a transition q to q q 1 is either 1 Ž S . or 2 n q 1 Ž Q .. It is easy to see that transitions from G to Q would be forbidden. However, the Q and S states are usually mixed and so transitions to Q become allowed. Transitions to Q and S are labeled L and B, respectively w6x. We associate the low energy peak with the 1 La transition and the high energy peak with the 1 B b transition where the subscripts a and b relate to different polarisations w4x. The inset in Fig. 1 is a schematic of the energy levels and transitions. The broad absorption spectrum between 3.5 and 4.3 eV in solution arises Ža. because of a spread in the GQ energy due to solvent interaction, and Žb. excitation to higher vibrational states of the molecule ŽFig. 1.. The separation of AlQ molecules in the solution is ˚ In solution, these transitions are intramolecabout 250 A. ular transitions. We now discuss the absorption of AlQ in the solid state in light of our understanding of solution spectra. The peaks at 3.18 and 4.7 eV Ž390 and 264 nm, respectively. in the solid originate from the 1 La and 1 B b transitions as in solution. Evaporated films of AlQ are amorphous. There exist both short range and long range potential fluctuations in the material. We now briefly discuss as to how these fluctuations affect the electronic structure in these materials. If U is the strength of the short range potential fluctuations and W is the bandwidth, it is well known from Anderson’s theorem that potential fluctuations will lead to localisation of the wavefunctions w7x. Anderson defined a parameter P s UrW and showed that for P ) Pc all states in the band are localised. As P Pc from below, localisation sets in at the band edges and moves towards the band centre. We hence model the electronic structure as shown in Fig. 3. The density of states at the band edges tails off into the gap. The electronic wavefunctions in the material are localised Žshown hatched in Fig. 3. except for a small region in energy where they are extended. There exist in the material also long range potential fluctuations Ž V . w8x. Each Fourier component of V, in general, will have both a symmetric and an antisymmetric component. The antisymmetric component will locally change the band gap.
™
Fig. 3. A schematic band energy diagram of AlQ in solid. The hatched regions depict localised states. T represents a transition from a localised state to an extended state.
The total absorption at energy E comprises of intermolecular and intramolecular absorption. For intermolecular excitations, the important transitions that contribute to the absorption are 1. from an extended state to an extended state, and 2. from an extended state to a localised state. This is shown as T in Fig. 3. The matrix element from a localised state to another localised state is very small unless both the ground state and the excited state belong to the same molecule Žintramolecular.. We see from Fig. 2 that the broad absorption between 3.5 and 4.3 eV is larger in solid than in solution. We conclude that this increase is due to the additional intermolecular absorption that takes place in the solid state.
4. Sub-bandgap absorption Below 2.5 eV, the absorption in the solid state is larger than in solution. It also changes slowly with energy. A simple implication of this result is that there is a pseudoconstant density of states in the gap of AlQ at least up to 0.7 eV from the band edge. In inorganic semiconductors, such an absorption would be characteristic of structural defects. These defects are influenced by deposition conditions. As the deposition temperature increases, the adatom mobility of the evaporated atoms is expected to increase. This can result in a reduction of structural defects and lower sub-bandgap absorption. In an attempt to see if the sub-bandgap absorption in AlQ is due to structural defects, we deposited AlQ at different substrate temperatures. Fig. 4 shows the absorption of AlQ films made at 100, 300, and 475 K. We see that the sub-bandgap absorption for samples deposited at 300 and 475 K are similar to each other and increases by a factor of about 2 for samples deposited at 100 K.
136
A. Aziz, K.L. Narasimhanr Synthetic Metals 114 (2000) 133–137
Fig. 4. Absorption coefficient as a function of energy for AlQ films made at Ž1. ' 100 K, Ž2. v 300 K and Ž3. B 475 K. The lines in the figure have been drawn through the data points as an aid to the eye.
The sub-bandgap absorption is influenced by exposure to air. Fig. 5 shows the absorption of an AlQ film made at room temperature and measured immediately after deposition and again after exposure to ambient air for 3 days. We see enhanced absorption beginning at 2.6 eV. This does not decrease even on annealing to 2008C. A control sample left in vacuum for a similar period does not show the enhanced absorption. The sub-bandgap absorption in AlQ can be understood from Fig. 3. In the framework of the free electron model associated with each ring, we see that potential fluctuations in the material will shift the energy associated with a ring by the magnitude of the potential fluctuation. This gives rise to localised gap states. Excitation from a pseudo-constant density of states to an extended state gives rise to the measured sub-bandgap absorption. Since the absorption is approximately constant to at least 0.7 eV in energy, the localised states must extend to at least up to 0.7 eV from one of the band edges. We can estimate the density of gap states using the relation w3x
difference between the two mobilities is as follows. If the localised states measured in sub-gap absorption measurements arise from states derived from the G level, then a hole injected into G will be trapped by the localised states and reduce the hole mobility. These states will not act as electron traps. The hole mobility will hence be much smaller than the electron mobility. The electron drift mobility is dispersive-suggesting a distribution Žin energy. of trap states for electrons. Forsythe et al. w10x have measured thermally stimulated luminescence measurements in AlQ. From their experiments, they conclude that the deepest trap level in AlQ is 0.25 eV. In a thermally stimulated luminescence measurement, the sample is irradiated with light at a low temperature. Electrons and holes are trapped in trap states. On heating the sample, if one of the carriers is excited into an extended state, it can combine radiatively with the trapped carrier of the opposite sign. From their measurements, the authors concluded that there is a distribution of states in the gap and the deepest level is 0.25 eV. This experiment cannot distinguish the sign of the charge carrier associated with this trap level. The drift mobility experiments suggest that holes are more deeply trapped than electrons. Hence we suggest that the trap in Thermally Stimulated Luminescence measurements is related to an electron trap. The deepest electron trap is only 0.25 eV from the Q level. In order to be an electron trap, the level has to be empty. This suggests that the levels, which give rise to sub-bandgap absorption in AlQ, must be occupied. These levels must then originate from the G level. It then follows that the Fermi level in AlQ is - 0.7 eV from the Q level. A recent XPS measurement of the fermi level in AlQ estimates it to be 0.57 eV below the lowest unoccupied state w11x. This is in very good agreement with the results reported here.
Ns s 8.10 15Ha d E. From the data of Fig. 3, we find that Ns for as deposited films is 6.10 17 rcm3. On exposure to air, Ns increases to about 3.10 18 rcm3. Optical absorption measurements cannot determine whether the localised states are derived from the G or the Q level. We can infer this from other measurements. We briefly summarise some of these experiments. Kepler et al. w9x have measured the mobility of electrons and holes in AlQ. The electron drift mobility is about 10y6 cm2rV s and the hole drift mobility is at least two orders of magnitude smaller. A simple way to understand the
Fig. 5. Absorption coefficient as a function of energy for a sample Ž1. I as grown and Ž2. v after exposure to air for 3 days.
A. Aziz, K.L. Narasimhanr Synthetic Metals 114 (2000) 133–137
These results have important implications for electrical transport in AlQ films. If we assume that the gap of AlQ has a large density of states, then hopping conductivity can be important in AlQ. Preliminary results in fact indicate the presence of this transport path in AlQw12x. In conclusion, we have investigated the optical absorption in AlQ. We also report the first measurements of sub-bandgap absorption in AlQ films using PDS. We find that the density of gap states varies slowly with energy in the gap and extends to at least 0.7 eV from the Q level. The density of gap states in AlQ is not drastically affected by the deposition temperature Žbetween 100 and 475 K.. Exposure of AlQ to atmosphere for a couple of days increases sub-bandgap absorption. We hence conclude that chemical reactions affect the density of gap states. We explain our results using a simple model for absorption. We also argue from our results that the fermi level in AlQ is close to the Q level.
Acknowledgements We thank Professor N. Periasamy and Dr. L.C. Padhy for many useful discussions and Meghan Patankar for his help.
137
References w1x C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 Ž1989. 3610. w2x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 Ž1990. 539. w3x N.M. Amer, W.B. Jackson, in: J.I. Pankove ŽEd.., Semiconductors and Semimetals 21B Academic Press, Boston, 1984, p. 83. w4x D.Z. Garbuzov, V. Bulovic, P.E. Burrows, S.R. Forrest, Chem. Phys. Lett. 249 Ž1996. 433. w5x H.H. Jaffe, M. Orchin, in: Theory and Applications of Ultraviolet Spectroscopy, Wiley, New York, 1962, p. 186. w6x M. Pope, C.E. Swenberg, in: Electronic Processes in Organic Crystals, Clarendon Press, Oxford, 1982, p. 7. w7x P.W. Anderson, Phys. Rev. 109 Ž1958. 1492. w8x H. Fritzsche, J. Non-Cryst.Solids 6 Ž1971. 49. w9x R.G. Kepler, P.M. Beeson, S.J. Jacobs, R.A. Anderson, M.B. Sinclair, V.S. Valencia, P.A. Cahill, Appl. Phys. Lett. 66 Ž1995. 3618. w10x E.W. Forsythe, D.C. Morton, C.W. Tang, Y. Gao, Appl. Phys. Lett. 73 Ž1998. 1457. w11x 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 Ž1999. 1619. w12x A. Aziz, K.L. Narasimhan, in: Vikram Kumar, S.K. Agarwal ŽEds.., 11 International Workshop on Physics of Semiconductor Devices Allied Publishers, New Delhi, 1999, p. 1341.
Optical Absorption in AlQ Anver Aziz, K.L. Narasimhan) Tata Institute of Fundamental Research, National Centre of the GoÕernment of India for Nuclear Science and Maths, Homi Bhabha Road, Colaba, Bombay 400005, India Received 9 November 1999; received in revised form 1 March 2000; accepted 3 March 2000
Abstract In this paper, we report on optical absorption in thin films of trisŽ8-hydroxyquinoline.aluminium ŽAlQ. between 0.7 and 5 eV. We report on the first measurement of the sub-bandgap absorption in AlQ using Photothermal Deflection Spectroscopy ŽPDS. for different deposition conditions. The sub-bandgap absorption is not very sensitive to sample deposition temperature but increases on exposure to atmosphere. We present a simple model to explain the absorption data. We calculate the density of states in the gap to be about 6.10 17rcm3 and present evidence that the fermi level lies in the upper half of the band gap close to the LUMO level in AlQ. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Photothermal deflection spectroscopy; Sub-bandgap states; TrisŽ8-hydroxyquinoline.aluminium; Optical absorption
1. Introduction The discovery of efficient electroluminescence in trisŽ8-hydroxyquinoline.aluminium ŽAlQ. and Poly Phenylene Vinylene ŽPPV. has triggered a lot of interest in these compounds w1,2x. However, many aspects of optical and electrical properties still remain the subject of research. Not much is known about the gap states in AlQ in particular. A simple way to probe the gap states is to use optical absorption as it is a measure of the joint density of states in the system. Photothermal deflection spectroscopy ŽPDS. has played an important role in understanding defects in inorganic semiconductors w3x. In this paper, we report on absorption measurements from 0.7–5 eV Ž1700– 250 nm. in AlQ. We compare our results with AlQ in solution and highlight the differences and similarities between the solution and the solid state spectrum. Finally, we discuss the implications of our results on the electronic structure and transport in AlQ films. 2. Experimental Films of AlQ were deposited on quartz or Corning 7059 substrates in a pressure of less than 10y6 Torr. The substrate temperature was varied from 100 to 475 K to )
Corresponding author. Fax: q91-22-2152110.
investigate the effect of deposition conditions on film quality. Films were made using high purity Aldrich AlQ and purified by repeated sublimation or synthesised and purified by sublimation. The films were characterised by optical absorption and luminescence. No difference was found between these two sources of AlQ. The optical absorption in these films was measured between 250–1700 nm. The absorption in the range 250– 450 nm was carried out using a Shimadzu UV2100 spectrophotometer and by PDS in the range 400–1700 nm. The film thickness was about 0.1–1 m for the range 250–450 nm and 1 m for the PDS measurements. The PDS measurements require a liquid deflection medium. The liquid should not affect the film properties. AlQ is soluble in most organic liquids. After many experiments, we found cyclohexane and freon Žboiling point, 378C. acceptable as the deflection medium. Even these liquids tend to attack the surface of the film after some time. To protect the film from the liquid, we deposited a ˚ thick LiF on the AlQ film. To ensure that the LiF 200 A film did not influence the absorption, we measured the ˚ thick LiF film on glass using PDS. absorption of a 200 A We found that the absorption of the LiF film was an order of magnitude smaller than the smallest absorption measured in the AlQ q LiF films. As an extra check on our ˚ results, we varied the thickness of LiF from 25 to 300 A on the AlQ film. The measured absorption Žof the AlQ q LiF film. was independent of the LiF thickness. The PDS
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 0 0 . 0 0 2 3 2 - 0
134
A. Aziz, K.L. Narasimhanr Synthetic Metals 114 (2000) 133–137
measurement is limited to 1000 nm using cyclohexane as the deflecting medium. Beyond this range, the self absorption of cyclohexane becomes important. Freon is satisfactory at least up to 1700 nm. The absorption results obtained with freon were identical to that obtained using cyclohexane in the range 400–1000 nm. The general details of the PDS measurements are described elsewhere w3x. We have also measured the absorption of AlQ in solution. In order to investigate a wide range of absorption, we used different concentrations of AlQ in dichloromethane and had overlapping regions of absorption to enable us to construct the whole curve. To measure absorption in the low absorption range, we used a 10 cm path length cell. 3. Results and discussion. Fig. 1 shows the absorption coefficient of an AlQ film between 0.7 and 5 eV. The spectrum is dominated by two peaks at 3.18 eV Ž390 nm. and 4.7 eV Ž264 nm.. The absorption between 2.8 and 5 eV is in good agreement with published results w4x. Below 2.5 eV, the absorption varies slowly with energy up to 0.7 eV. Fig. 1 also shows the molar extinction coefficient of AlQ dissolved in dichloromethane. The solution spectrum is qualitatively similar to that of the thin film and has two peaks at 3.2 and 4.76 eV Ž386 and 258 nm, respectively. along with two weak vibrational bands. This curve is a composite made up by using different concentrations Ž c . of AlQ and path lengths Ž l .. To properly normalise the optical density ŽOD., care was taken to ensure that there were overlapping ranges between two concentrations Žpath lengths.. This was normalised with a concentration that had the same OD ˚ thin in solution Žat the low energy peak. as in a 1000 A film. This enables us to quantitatively compare the absorption data in solution with the film over the entire energy range.
Fig. 1. A plot of Ž1. absorption coefficient of AlQ film as a function of energy Ždashed line., Ž2. molar extinction co-efficient of AlQ in CH 2 Cl 2 as a function of energy Žsolid line. Žsee text.. The inset shows the energy level diagram for a single molecule.
Fig. 2. Absorption of Alq in Ž1. Ž –PP–PP– . thin film and in the following solutions Ž2. Ž – – – . CCl 4 , Ž3. ŽPPPPP. CH 2 Cl 2 , and Ž4. Ž –P–P– . CH 3 OH. The OD is normalised to unity to facilitate comparison between various spectra. The inset shows the normalised OD for Ž1. v a thin film and Ž2. I for AlQ in CH 3 OH solution. The thin film spectrum is rigidly shifted in energy so that the peak position is the same for both samples.
Fig. 2 shows the absorption of Ža. 1 m thin film on glass and Žb. solution spectra of AlQ dissolved in various solvents. ŽTo facilitate comparison among the various spectra, the OD is normalised to unity.. The low energy peak moves to higher energy as the polarity Ždielectric constant. of the solvent increases and the whole spectrum undergoes a rigid shift in energy. The inset in Fig. 2 shows the spectrum of the AlQ in methanol and AlQ thin film Žrigidly shifted in energy so as to make the low energy peaks coincide.. The spectra are identical even though the concentration of AlQ is different by more than four orders of magnitude in the two cases. We first attempt to understand the solution spectra. In general, in the vapour phase, the observed spectrum of a molecule is that of isolated molecules with its rotational, vibrational, and electronic excitations. In solution, even at low concentration, due to the interaction with the solvent, the spectra of the solute molecules is inhomogeneously broadened. This accounts for the large widths seen in the solution spectra in Figs. 1 and 2 w5x. The position of the bands also depends on the nature of the solvent. Generally polar solvents tend to shift the position of the bands w5x. This is seen clearly for AlQ in Fig. 2. We estimated the number of molecules in the film from the density of AlQ and the film thickness and also calculated the number of molecules in solution. The number of molecules in the path of the beam was the same in solution and in the thin film when the OD was the same in both cases. For the low energy peak, the absorption per molecule is the same in both the solid and in solution.
A. Aziz, K.L. Narasimhanr Synthetic Metals 114 (2000) 133–137
135
The optical absorption in conjugated systems is easily understood using a free electron model w6x. In a ring molecule, the p electrons are considered free to move in the plane of the molecule. The energy eigen-values are given by Eq s h2 q 2r Ž 8p 2 mr 2 . , where r is the radius of the ring. In a catacondensed system of n rings, the highest filled level is q s n. The change in the ring Žangular momentum. quantum number for a transition q to q q 1 is either 1 Ž S . or 2 n q 1 Ž Q .. It is easy to see that transitions from G to Q would be forbidden. However, the Q and S states are usually mixed and so transitions to Q become allowed. Transitions to Q and S are labeled L and B, respectively w6x. We associate the low energy peak with the 1 La transition and the high energy peak with the 1 B b transition where the subscripts a and b relate to different polarisations w4x. The inset in Fig. 1 is a schematic of the energy levels and transitions. The broad absorption spectrum between 3.5 and 4.3 eV in solution arises Ža. because of a spread in the GQ energy due to solvent interaction, and Žb. excitation to higher vibrational states of the molecule ŽFig. 1.. The separation of AlQ molecules in the solution is ˚ In solution, these transitions are intramolecabout 250 A. ular transitions. We now discuss the absorption of AlQ in the solid state in light of our understanding of solution spectra. The peaks at 3.18 and 4.7 eV Ž390 and 264 nm, respectively. in the solid originate from the 1 La and 1 B b transitions as in solution. Evaporated films of AlQ are amorphous. There exist both short range and long range potential fluctuations in the material. We now briefly discuss as to how these fluctuations affect the electronic structure in these materials. If U is the strength of the short range potential fluctuations and W is the bandwidth, it is well known from Anderson’s theorem that potential fluctuations will lead to localisation of the wavefunctions w7x. Anderson defined a parameter P s UrW and showed that for P ) Pc all states in the band are localised. As P Pc from below, localisation sets in at the band edges and moves towards the band centre. We hence model the electronic structure as shown in Fig. 3. The density of states at the band edges tails off into the gap. The electronic wavefunctions in the material are localised Žshown hatched in Fig. 3. except for a small region in energy where they are extended. There exist in the material also long range potential fluctuations Ž V . w8x. Each Fourier component of V, in general, will have both a symmetric and an antisymmetric component. The antisymmetric component will locally change the band gap.
™
Fig. 3. A schematic band energy diagram of AlQ in solid. The hatched regions depict localised states. T represents a transition from a localised state to an extended state.
The total absorption at energy E comprises of intermolecular and intramolecular absorption. For intermolecular excitations, the important transitions that contribute to the absorption are 1. from an extended state to an extended state, and 2. from an extended state to a localised state. This is shown as T in Fig. 3. The matrix element from a localised state to another localised state is very small unless both the ground state and the excited state belong to the same molecule Žintramolecular.. We see from Fig. 2 that the broad absorption between 3.5 and 4.3 eV is larger in solid than in solution. We conclude that this increase is due to the additional intermolecular absorption that takes place in the solid state.
4. Sub-bandgap absorption Below 2.5 eV, the absorption in the solid state is larger than in solution. It also changes slowly with energy. A simple implication of this result is that there is a pseudoconstant density of states in the gap of AlQ at least up to 0.7 eV from the band edge. In inorganic semiconductors, such an absorption would be characteristic of structural defects. These defects are influenced by deposition conditions. As the deposition temperature increases, the adatom mobility of the evaporated atoms is expected to increase. This can result in a reduction of structural defects and lower sub-bandgap absorption. In an attempt to see if the sub-bandgap absorption in AlQ is due to structural defects, we deposited AlQ at different substrate temperatures. Fig. 4 shows the absorption of AlQ films made at 100, 300, and 475 K. We see that the sub-bandgap absorption for samples deposited at 300 and 475 K are similar to each other and increases by a factor of about 2 for samples deposited at 100 K.
136
A. Aziz, K.L. Narasimhanr Synthetic Metals 114 (2000) 133–137
Fig. 4. Absorption coefficient as a function of energy for AlQ films made at Ž1. ' 100 K, Ž2. v 300 K and Ž3. B 475 K. The lines in the figure have been drawn through the data points as an aid to the eye.
The sub-bandgap absorption is influenced by exposure to air. Fig. 5 shows the absorption of an AlQ film made at room temperature and measured immediately after deposition and again after exposure to ambient air for 3 days. We see enhanced absorption beginning at 2.6 eV. This does not decrease even on annealing to 2008C. A control sample left in vacuum for a similar period does not show the enhanced absorption. The sub-bandgap absorption in AlQ can be understood from Fig. 3. In the framework of the free electron model associated with each ring, we see that potential fluctuations in the material will shift the energy associated with a ring by the magnitude of the potential fluctuation. This gives rise to localised gap states. Excitation from a pseudo-constant density of states to an extended state gives rise to the measured sub-bandgap absorption. Since the absorption is approximately constant to at least 0.7 eV in energy, the localised states must extend to at least up to 0.7 eV from one of the band edges. We can estimate the density of gap states using the relation w3x
difference between the two mobilities is as follows. If the localised states measured in sub-gap absorption measurements arise from states derived from the G level, then a hole injected into G will be trapped by the localised states and reduce the hole mobility. These states will not act as electron traps. The hole mobility will hence be much smaller than the electron mobility. The electron drift mobility is dispersive-suggesting a distribution Žin energy. of trap states for electrons. Forsythe et al. w10x have measured thermally stimulated luminescence measurements in AlQ. From their experiments, they conclude that the deepest trap level in AlQ is 0.25 eV. In a thermally stimulated luminescence measurement, the sample is irradiated with light at a low temperature. Electrons and holes are trapped in trap states. On heating the sample, if one of the carriers is excited into an extended state, it can combine radiatively with the trapped carrier of the opposite sign. From their measurements, the authors concluded that there is a distribution of states in the gap and the deepest level is 0.25 eV. This experiment cannot distinguish the sign of the charge carrier associated with this trap level. The drift mobility experiments suggest that holes are more deeply trapped than electrons. Hence we suggest that the trap in Thermally Stimulated Luminescence measurements is related to an electron trap. The deepest electron trap is only 0.25 eV from the Q level. In order to be an electron trap, the level has to be empty. This suggests that the levels, which give rise to sub-bandgap absorption in AlQ, must be occupied. These levels must then originate from the G level. It then follows that the Fermi level in AlQ is - 0.7 eV from the Q level. A recent XPS measurement of the fermi level in AlQ estimates it to be 0.57 eV below the lowest unoccupied state w11x. This is in very good agreement with the results reported here.
Ns s 8.10 15Ha d E. From the data of Fig. 3, we find that Ns for as deposited films is 6.10 17 rcm3. On exposure to air, Ns increases to about 3.10 18 rcm3. Optical absorption measurements cannot determine whether the localised states are derived from the G or the Q level. We can infer this from other measurements. We briefly summarise some of these experiments. Kepler et al. w9x have measured the mobility of electrons and holes in AlQ. The electron drift mobility is about 10y6 cm2rV s and the hole drift mobility is at least two orders of magnitude smaller. A simple way to understand the
Fig. 5. Absorption coefficient as a function of energy for a sample Ž1. I as grown and Ž2. v after exposure to air for 3 days.
A. Aziz, K.L. Narasimhanr Synthetic Metals 114 (2000) 133–137
These results have important implications for electrical transport in AlQ films. If we assume that the gap of AlQ has a large density of states, then hopping conductivity can be important in AlQ. Preliminary results in fact indicate the presence of this transport path in AlQw12x. In conclusion, we have investigated the optical absorption in AlQ. We also report the first measurements of sub-bandgap absorption in AlQ films using PDS. We find that the density of gap states varies slowly with energy in the gap and extends to at least 0.7 eV from the Q level. The density of gap states in AlQ is not drastically affected by the deposition temperature Žbetween 100 and 475 K.. Exposure of AlQ to atmosphere for a couple of days increases sub-bandgap absorption. We hence conclude that chemical reactions affect the density of gap states. We explain our results using a simple model for absorption. We also argue from our results that the fermi level in AlQ is close to the Q level.
Acknowledgements We thank Professor N. Periasamy and Dr. L.C. Padhy for many useful discussions and Meghan Patankar for his help.
137
References w1x C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 Ž1989. 3610. w2x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 Ž1990. 539. w3x N.M. Amer, W.B. Jackson, in: J.I. Pankove ŽEd.., Semiconductors and Semimetals 21B Academic Press, Boston, 1984, p. 83. w4x D.Z. Garbuzov, V. Bulovic, P.E. Burrows, S.R. Forrest, Chem. Phys. Lett. 249 Ž1996. 433. w5x H.H. Jaffe, M. Orchin, in: Theory and Applications of Ultraviolet Spectroscopy, Wiley, New York, 1962, p. 186. w6x M. Pope, C.E. Swenberg, in: Electronic Processes in Organic Crystals, Clarendon Press, Oxford, 1982, p. 7. w7x P.W. Anderson, Phys. Rev. 109 Ž1958. 1492. w8x H. Fritzsche, J. Non-Cryst.Solids 6 Ž1971. 49. w9x R.G. Kepler, P.M. Beeson, S.J. Jacobs, R.A. Anderson, M.B. Sinclair, V.S. Valencia, P.A. Cahill, Appl. Phys. Lett. 66 Ž1995. 3618. w10x E.W. Forsythe, D.C. Morton, C.W. Tang, Y. Gao, Appl. Phys. Lett. 73 Ž1998. 1457. w11x 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 Ž1999. 1619. w12x A. Aziz, K.L. Narasimhan, in: Vikram Kumar, S.K. Agarwal ŽEds.., 11 International Workshop on Physics of Semiconductor Devices Allied Publishers, New Delhi, 1999, p. 1341.