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The role of traps in electrical conductivity and optical absorption in phthalocyanine-doped anthracene co-crystals
Journal of Crystal Growth 233 (2001) 583–590
The role of traps in electrical conductivity and optical absorption in phthalocyanine-doped anthracene co-crystals S. Lawrence Selvaraj*, Francis P. Xavier Physics Department, Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai 600 034, India Received 12 May 2001; accepted 9 July 2001 Communicated by M. Schieber
Abstract Anthracene and metal-free phthalocyanine (H2Pc) are well-known organic semiconductors, and are reported to be photoconductive in the UV–VIS and IR regions, respectively. Hence H2Pc-doped anthracene crystal formed is expected to be active in the entire region UV–VIS–IR, and the compound material could be a viable alternative for a possible solar cell material for the entire range of UV, VIS and IR regions of solar radiation. In the present study, cocrystals of H2Pc-doped anthracene are formed using DMSO as the common solvent, and subjected to conductivity studies. The activation energy decreases as the concentration of H2Pc increases up to 1% and decreases beyond 1%. In the UV spectrum, the absorption decreases when the concentration of H2Pc exceeds 1%. Though the traps play a major role in the electrical and optical conductivity of organic molecular solids, their presence beyond a limit decreases the conductivity. The samples are found to be photoactive and could be a possible alternative material for a semiconductorbased solar cell. r 2001 Elsevier Science B.V. All rights reserved. PACS: 81.10; 61.72; 72.20.J; 72.80.L Keywords: A1. Doping; A2. Growth from solutions; B3. Solar cells
1. Introduction Anthracene is an organic semiconductor [1–4], extensively investigated over many decades and it consists of three linear phenyl rings in its crystal structure. The electronic conduction of anthracene is due to free electrons and holes present in the *Corresponding author. Tel.: +91-44-827-6749/332; fax: +91-44-823-1684. E-mail address: [email protected] (S.L. Selvaraj).
crystal. The activation energy of anthracene reported in the literature ranges from 0.9 to 1.6 eV based on the type of the sample and the technique involved [5–12]. Conductivity in anthracene is by two mechanisms, namely, by hopping (production of charge carriers) and or by tunnelling (mobility of charge carriers) [13]. Kepler [14] reports that anthracene exhibits two kinds of photoconductivity, namely, bulk and surface, based on his observations. The current density is larger in the central phenyl ring of anthracene based on theoretical calculations [15] and its
0022-0248/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 6 1 7 - 7
584
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location using NMR spectrum [16]. It is reported that free carriers are further generated when the crystal is illuminated by light of wavelength about ( [17–19] and hence, it is active in the UV 4000 A region of the electromagnetic spectrum. On the other hand, metal-free phthalocyanine (unsubstituted form of phthalocyanineFH2Pc) is active from visible through IR regions of the electromagnetic spectrum. Phthalocyanines and perylene pigments [20] have been used in photovoltaics and organic solar cells [21]. Hence, H2Pc-doped anthracene is expected to be active in the entire region UV–VIS–IR, and hence, the compound material could be tested for a possible solar cell material. We have already reported the band-gap calculations using electroreflectance (ER) spectrum on H2Pc-doped anthracene co-crystals [22] and it shows a decrease in band gap as the concentration of H2Pc increases.
2. Experimental procedures 2.1. Co-crystallization A brief procedure for the preparation of the H2Pc-doped co-crystals is already reported [22]. As the growth of organic crystals mainly depends on the purity of the chemical [23], anthracene used for the present study is purified [24] by recrystallization, column chromatography, vacuum sublimation and zone refining [25]. H2Pc used for the study was purchased from Sigma-Aldrich (99%). Dimethyl sulphoxide (DMSO), which is a good solvent for anthracene, dissolves H2Pc sparingly. Hence, DMSO was selected as a common solvent to prepare H2Pc-doped anthracene co-crystals. 40 ml of DMSO in 100 ml beaker was kept in a hot water bath (901C) for 20 min and anthracene was added slowly till saturation is reached. Knowing the weight of anthracene added, H2Pc saturation (0.12%) was calculated, added to the hot solution (DMSO þ anthracene) and stirred well. When all H2Pc has dissolved, heating was stopped and the mixture was allowed to cool slowly in the water bath itself. When the room temperature is reached, the beaker is removed and left for a day for full co-crystallization. Blue
coloured co-crystals in the form of flakes (thickness about 500 mm) were formed, which were powdered and made into pellets of 2 mm thickness using 10 ton pressure. The above procedure was repeated to prepare co-crystals with the varying concentration of H2Pc, namely 0.47%, 0.62%, 0.9%, 1.0%, 1.5% and 2.0%. Pure anthracene (with 0% H2Pc) was used as the control sample. The percentages of H2Pc were finalized after C, H and N analysis (Heraeus CHN-O-RAPID analyser) of the samples were prepared. The samples prepared were subjected to X-ray analysis and it was found that H2Pc does not alter the crystal structure of anthracene. Since H2Pc doped is less than 2%, no crystal structure change is expected. The X-ray diffractograms of: (a) pure H2Pc, (b) anthracene and (c) H2Pc-doped anthracene are shown in Fig. 1. XRD pattern of H2Pc-doped anthracene co-crystals and that of pure anthracene are identical with respect to peak positions. However, the relative intensities of the two patterns differ. For example, corresponding peak intensity ratios (I1 =I2 ) of pure anthracene are markedly different from (I10 =I20 ) of H2Pc-doped anthracene. 2.2. Conductivity studies The conductivity studies on the H2Pc-doped anthracene pellets were carried out by connecting the sample in series to a DC power supply and a picoammeter (Keithley 480). Electrode contacts were made on to the sample using the silver paint [26] and the electrode distance measured. When the sample is kept in the dark, the energy bands adjust themselves relative to the Fermi level and reach an equilibrium state. This equilibrium is disturbed when an electric field is applied thereby bringing changes in barrier widths and heights both inside the crystal and at the crystal electrode boundaries. This results in a flow of charges called ‘dark current’. The field (E) vs. dark current (Id ) plot for H2Pc-doped anthracene pellets in Fig. 2 shows that dark current increases as the concentration of H2Pc increases up to 1% and there is no indication of conductivity for doping more than 1%. The space between the electrodes was illuminated with a halogen lamp (100 W) and
S.L. Selvaraj, F.P. Xavier / Journal of Crystal Growth 233 (2001) 583–590
585
Fig. 1. X-ray diffraction pattern of: (a) H2Pc (b) anthracene and (c) H2Pc-doped anthracene co-crystal.
from E vs. Iph plot (Fig. 3), it was observed that photocurrent (Iph ) is higher than the dark current for all concentrations up to 1% of H2Pc. The resistance of the material is very high (order of 1012) and hence, the noise current is assumed to be
negligible. Due to high resistivity of the organic materials under investigation, the potential inaccuracy due to contact resistance is negligible [27]. The photocurrent increases with the concentration as it was observed in the case of dark conductivity.
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Fig. 2. Field (E) vs. dark current (Id ) for H2Pc-doped anthracene co-crystals.
The temperature-dependent studies on the samples were carried out by keeping the samples in a temperature-controlled oven. The dark current and photocurrent were found to increase with the temperature. The plot of ln I vs. 1=kT is shown in Fig. 4 for various concentrations of H2Pc-doped anthracene samples. The activation energy is calculated from ln I vs. 1=kT and is given in Table 1. 2.3. Visible and ultra-violet spectrum The ultraviolet spectra for the samples were taken in the range 300–600 nm and are shown in Fig. 5. The absorption is large for pure anthracene, 0.47%, 0.62%, and 1.0% of H2Pc. But there is a dip in the absorption of the samples when the concentration exceeds 1.0% of H2Pc. The absorption decreases for the sample with 1.5% and finally, the absorption quenches to minimum for 2.0% of H2Pc.
3. Results and discussion The percentage of H2Pc in anthracene was determined using C, H and N analysis. X-ray diffraction done on the samples does not show any crystal structure change in the doped sample. Powdered XRD pattern of pure anthracene and anthracene doped with 1.0% H2Pc in Fig. 1, shows that all the peaks present in H2Pc-doped anthracene co-crystal belong to that of anthracene, however, the relative intensities of the peaks have altered. The percentage of H2Pc, along with the activation energy for both dark conductivity (Ed ) and photoconductivity (Eph ), that are calculated from the temperature-dependent conductivity studies, are given in Table 1. From Table 1, it is understood that activation energy decreases for both dark conductivity and photoconductivity as the concentration of H2Pc increases. In organic semiconductors, the trap levels play a predominant role in the conductivity. It is, therefore, suggested
S.L. Selvaraj, F.P. Xavier / Journal of Crystal Growth 233 (2001) 583–590
587
Fig. 3. Field (E) vs. photocurrent (Iph ) for H2Pc-doped anthracene co-crystals.
that as the concentration of H2Pc increases, the trap level also increases, leading to a decrease in the activation energy. The doping of H2Pc onto anthracene creates an easy passage for the flow of electrons between its molecules, thereby decreasing its dark and photo activation energies. Beyond 1% of H2Pc, the number of trap levels continues to increase, but the conductivity decreases, because when large number of traps being present leads to decrease in conductivity as the electrons are trapped and retrapped in travelling between the traps without reaching the conduction band. The same reason is also found to hold good for the absorption of the samples in the VIS and UV region. The UV spectrum shows that there is a very good absorption for the samples with lower concentration of H2Pc and the absorption decreases for higher concentration; finally, the absorption quenches to a minimum for 2%. Thus, at high concentration, the applied optical energy is used in moving between the traps, which are largely present.
The currents obtained for both dark conductivity and photoconductivity of the doped compounds are greater than the currents obtained for pure anthracene. Anthracene and H2Pc (with 0.5% and 1.0%) is mixed physically and after grinding the mixture, pellets were made and conductivity studies were carried out, which yielded very low dark current and photocurrent. The conductivity response in the physical mixture is very low when compared with the co-crystals, as physical mixture does not involve the phenomenon of doping, leading to a large number of grain boundaries. On the other hand, H2Pc in the cocrystals is doped, minimizing the grain boundaries. We have already reported that doping H2Pc onto anthracene decreases the band gap of anthracene [22] and at present, we see that activation energy of anthracene decreases with the concentration of H2Pc, thereby increasing the conductivity. Hence, H2Pc could be a suitable dopant for anthracene to find its application in the semiconductor-based solar cell devices.
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Fig. 4. ln I vs. 1=kT for anthracene-doped with H2Pc (0%, 0.47%, 1.5% and 2.0%).
Table 1 Activation energy for the co-crystals of anthracene with varying concentration of H2Pc % of H2Pc
0 0.12 0.47 0.62 0.92 1.0 1.5 2.0
Activation energy (eV) Ed
Eph
0.2511 0.2403 0.2301 0.2168 0.2052 0.1985 0.4409 0.4589
0.2343 0.2147 0.2041 0.1882 0.1735 0.1629 0.4016 0.4143
4. Conclusions Anthracene doped with metal-free phthalocyanine is prepared by the co-crystallization method
and its activation energy is determined. The activation energies for dark conductivity and photoconductivity of the H2Pc-doped anthracene samples decrease as the concentration of H2Pc increases. The trap level increases with the concentration of H2Pc, leading to the decrease in activation energy. But beyond 1% of H2Pc, the trap levels are present in large numbers and they obstruct the flow of electrons from valence band to the conduction band, as the excitation energy is lost in travelling between the traps. Though traps are necessary for better conductivity in solids, their number should be controlled and energy level altered so that they do not obstruct the passage of electrons to the conduction band. Thus, we see that traps which decrease the activation energy may lead to increase in activation energy if they are present in large number beyond need. The samples are photosensitive and UV active and could be tested for a solar cell application.
S.L. Selvaraj, F.P. Xavier / Journal of Crystal Growth 233 (2001) 583–590
Fig. 5. Visible and UV spectrum for H2Pc-doped anthracene co-crystals (0%, 0.47%, 1.0%, 1.5% and 2.0%).
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Acknowledgements The authors are thankful to the members of the research team at Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai for their useful discussions. We specially thank Dr. K. Swaminathan, LIFE and Dr. Babu Varghese, RSIC, IIT Chennai for their help in the experimental studies. One of the authors (SLS) gratefully acknowledges the Jawaharlal Nehru Memorial Fund, Teen Murti House, New Delhi for the research fellowship to carry out his research.
References [1] A. Bree, D.J. Carswell, L.E. Lyons, J. Chem. Soc. (1955) 1728. [2] L.E. Lyons, G.C. Morris, J. Chem. Soc. (1957) 3648. [3] S. Choi, S.A. Rice, J. Chem. Phys. 38 (1963) 366. [4] S.Z. Weisz, R.C. Jarnagin, M. Silver, J. Chem. Phys. 40 (1964) 3365. [5] D.D. Eley, G.D. Parfitt, M.J. Perry, D.H. Taysum, Trans. Faraday Soc. 49 (1953) 79. [6] H. Mette, H. Pick, Z. Physik 134 (1953) 566. [7] H. Inokuchi, Bull. Chem. Soc. Japan 29 (1956) 131. [8] D.C. Northrop, O. Simpson, Proc. Roy. Soc. A 234 (1956) 124.
[9] J. Kommandeur, W.G. Schneider, J. Chem. Phys. 28 (1958) 582. [10] O.H. LeBlanc, J. Chem. Phys. 35 (1961) 1275. [11] H. Kokado, W.G. Schneider, J. Chem. Phys. 40 (1964) 2937. [12] J.V. Joseph, F.P. Xavier, Bull. Mater. Sci. 21 (1997) 1. [13] R.A. Keller, H.E. Rast, J. Chem. Phys. 36 (1962) 2640. [14] R.G. Kepler, Phys. Rev. 119 (1960) 1226. [15] E. Steiner, P.W. Fowler, Int. J. Quantum Chem. 60 (1998) 609. [16] S.L. Selvaraj, F.P. Xavier, Bull. Electrochem. 14 (1998) 430. [17] I. Nakada, K. Ariga, A. Ichimiya, J. Phys. Soc. Japan 19 (1964) 1587. [18] D.J. Carswell, L.E. Lyons, J. Chem. Soc. (1955) 1734. [19] A.G. Chynoweth, W.G. Schneider, J. Chem. Phys. 22 (1954) 1021. [20] K.Y. Law, J. Phys. Chem. 92 (1988) 4226. [21] S. Siebentritt, S. Gunster, D. Meissner, Synth. Met. 41 (1991) 1173. [22] S.L. Selvaraj, F.P. Xavier, J. Crystal Growth 225 (2001) 168. [23] N. Karl, Mol. Cryst. Liq. Cryst. 171 (1989) 157. [24] B. Marciniak, W. Waclawek, J. Crystal Growth 52 (1981) 623. [25] A. Matsui, Y. Ishii, Jpn. J. Appl. Phys. 6 (1967) 127. [26] I. Granacher, Solid State Commun. 2 (1964) 365. [27] F.P. Xavier, A.R. Inigo, G.J. Goldsmith, J. Porphyrins Phthalocyanines 3 (1999) 679.
The role of traps in electrical conductivity and optical absorption in phthalocyanine-doped anthracene co-crystals S. Lawrence Selvaraj*, Francis P. Xavier Physics Department, Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai 600 034, India Received 12 May 2001; accepted 9 July 2001 Communicated by M. Schieber
Abstract Anthracene and metal-free phthalocyanine (H2Pc) are well-known organic semiconductors, and are reported to be photoconductive in the UV–VIS and IR regions, respectively. Hence H2Pc-doped anthracene crystal formed is expected to be active in the entire region UV–VIS–IR, and the compound material could be a viable alternative for a possible solar cell material for the entire range of UV, VIS and IR regions of solar radiation. In the present study, cocrystals of H2Pc-doped anthracene are formed using DMSO as the common solvent, and subjected to conductivity studies. The activation energy decreases as the concentration of H2Pc increases up to 1% and decreases beyond 1%. In the UV spectrum, the absorption decreases when the concentration of H2Pc exceeds 1%. Though the traps play a major role in the electrical and optical conductivity of organic molecular solids, their presence beyond a limit decreases the conductivity. The samples are found to be photoactive and could be a possible alternative material for a semiconductorbased solar cell. r 2001 Elsevier Science B.V. All rights reserved. PACS: 81.10; 61.72; 72.20.J; 72.80.L Keywords: A1. Doping; A2. Growth from solutions; B3. Solar cells
1. Introduction Anthracene is an organic semiconductor [1–4], extensively investigated over many decades and it consists of three linear phenyl rings in its crystal structure. The electronic conduction of anthracene is due to free electrons and holes present in the *Corresponding author. Tel.: +91-44-827-6749/332; fax: +91-44-823-1684. E-mail address: [email protected] (S.L. Selvaraj).
crystal. The activation energy of anthracene reported in the literature ranges from 0.9 to 1.6 eV based on the type of the sample and the technique involved [5–12]. Conductivity in anthracene is by two mechanisms, namely, by hopping (production of charge carriers) and or by tunnelling (mobility of charge carriers) [13]. Kepler [14] reports that anthracene exhibits two kinds of photoconductivity, namely, bulk and surface, based on his observations. The current density is larger in the central phenyl ring of anthracene based on theoretical calculations [15] and its
0022-0248/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 6 1 7 - 7
584
S.L. Selvaraj, F.P. Xavier / Journal of Crystal Growth 233 (2001) 583–590
location using NMR spectrum [16]. It is reported that free carriers are further generated when the crystal is illuminated by light of wavelength about ( [17–19] and hence, it is active in the UV 4000 A region of the electromagnetic spectrum. On the other hand, metal-free phthalocyanine (unsubstituted form of phthalocyanineFH2Pc) is active from visible through IR regions of the electromagnetic spectrum. Phthalocyanines and perylene pigments [20] have been used in photovoltaics and organic solar cells [21]. Hence, H2Pc-doped anthracene is expected to be active in the entire region UV–VIS–IR, and hence, the compound material could be tested for a possible solar cell material. We have already reported the band-gap calculations using electroreflectance (ER) spectrum on H2Pc-doped anthracene co-crystals [22] and it shows a decrease in band gap as the concentration of H2Pc increases.
2. Experimental procedures 2.1. Co-crystallization A brief procedure for the preparation of the H2Pc-doped co-crystals is already reported [22]. As the growth of organic crystals mainly depends on the purity of the chemical [23], anthracene used for the present study is purified [24] by recrystallization, column chromatography, vacuum sublimation and zone refining [25]. H2Pc used for the study was purchased from Sigma-Aldrich (99%). Dimethyl sulphoxide (DMSO), which is a good solvent for anthracene, dissolves H2Pc sparingly. Hence, DMSO was selected as a common solvent to prepare H2Pc-doped anthracene co-crystals. 40 ml of DMSO in 100 ml beaker was kept in a hot water bath (901C) for 20 min and anthracene was added slowly till saturation is reached. Knowing the weight of anthracene added, H2Pc saturation (0.12%) was calculated, added to the hot solution (DMSO þ anthracene) and stirred well. When all H2Pc has dissolved, heating was stopped and the mixture was allowed to cool slowly in the water bath itself. When the room temperature is reached, the beaker is removed and left for a day for full co-crystallization. Blue
coloured co-crystals in the form of flakes (thickness about 500 mm) were formed, which were powdered and made into pellets of 2 mm thickness using 10 ton pressure. The above procedure was repeated to prepare co-crystals with the varying concentration of H2Pc, namely 0.47%, 0.62%, 0.9%, 1.0%, 1.5% and 2.0%. Pure anthracene (with 0% H2Pc) was used as the control sample. The percentages of H2Pc were finalized after C, H and N analysis (Heraeus CHN-O-RAPID analyser) of the samples were prepared. The samples prepared were subjected to X-ray analysis and it was found that H2Pc does not alter the crystal structure of anthracene. Since H2Pc doped is less than 2%, no crystal structure change is expected. The X-ray diffractograms of: (a) pure H2Pc, (b) anthracene and (c) H2Pc-doped anthracene are shown in Fig. 1. XRD pattern of H2Pc-doped anthracene co-crystals and that of pure anthracene are identical with respect to peak positions. However, the relative intensities of the two patterns differ. For example, corresponding peak intensity ratios (I1 =I2 ) of pure anthracene are markedly different from (I10 =I20 ) of H2Pc-doped anthracene. 2.2. Conductivity studies The conductivity studies on the H2Pc-doped anthracene pellets were carried out by connecting the sample in series to a DC power supply and a picoammeter (Keithley 480). Electrode contacts were made on to the sample using the silver paint [26] and the electrode distance measured. When the sample is kept in the dark, the energy bands adjust themselves relative to the Fermi level and reach an equilibrium state. This equilibrium is disturbed when an electric field is applied thereby bringing changes in barrier widths and heights both inside the crystal and at the crystal electrode boundaries. This results in a flow of charges called ‘dark current’. The field (E) vs. dark current (Id ) plot for H2Pc-doped anthracene pellets in Fig. 2 shows that dark current increases as the concentration of H2Pc increases up to 1% and there is no indication of conductivity for doping more than 1%. The space between the electrodes was illuminated with a halogen lamp (100 W) and
S.L. Selvaraj, F.P. Xavier / Journal of Crystal Growth 233 (2001) 583–590
585
Fig. 1. X-ray diffraction pattern of: (a) H2Pc (b) anthracene and (c) H2Pc-doped anthracene co-crystal.
from E vs. Iph plot (Fig. 3), it was observed that photocurrent (Iph ) is higher than the dark current for all concentrations up to 1% of H2Pc. The resistance of the material is very high (order of 1012) and hence, the noise current is assumed to be
negligible. Due to high resistivity of the organic materials under investigation, the potential inaccuracy due to contact resistance is negligible [27]. The photocurrent increases with the concentration as it was observed in the case of dark conductivity.
586
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Fig. 2. Field (E) vs. dark current (Id ) for H2Pc-doped anthracene co-crystals.
The temperature-dependent studies on the samples were carried out by keeping the samples in a temperature-controlled oven. The dark current and photocurrent were found to increase with the temperature. The plot of ln I vs. 1=kT is shown in Fig. 4 for various concentrations of H2Pc-doped anthracene samples. The activation energy is calculated from ln I vs. 1=kT and is given in Table 1. 2.3. Visible and ultra-violet spectrum The ultraviolet spectra for the samples were taken in the range 300–600 nm and are shown in Fig. 5. The absorption is large for pure anthracene, 0.47%, 0.62%, and 1.0% of H2Pc. But there is a dip in the absorption of the samples when the concentration exceeds 1.0% of H2Pc. The absorption decreases for the sample with 1.5% and finally, the absorption quenches to minimum for 2.0% of H2Pc.
3. Results and discussion The percentage of H2Pc in anthracene was determined using C, H and N analysis. X-ray diffraction done on the samples does not show any crystal structure change in the doped sample. Powdered XRD pattern of pure anthracene and anthracene doped with 1.0% H2Pc in Fig. 1, shows that all the peaks present in H2Pc-doped anthracene co-crystal belong to that of anthracene, however, the relative intensities of the peaks have altered. The percentage of H2Pc, along with the activation energy for both dark conductivity (Ed ) and photoconductivity (Eph ), that are calculated from the temperature-dependent conductivity studies, are given in Table 1. From Table 1, it is understood that activation energy decreases for both dark conductivity and photoconductivity as the concentration of H2Pc increases. In organic semiconductors, the trap levels play a predominant role in the conductivity. It is, therefore, suggested
S.L. Selvaraj, F.P. Xavier / Journal of Crystal Growth 233 (2001) 583–590
587
Fig. 3. Field (E) vs. photocurrent (Iph ) for H2Pc-doped anthracene co-crystals.
that as the concentration of H2Pc increases, the trap level also increases, leading to a decrease in the activation energy. The doping of H2Pc onto anthracene creates an easy passage for the flow of electrons between its molecules, thereby decreasing its dark and photo activation energies. Beyond 1% of H2Pc, the number of trap levels continues to increase, but the conductivity decreases, because when large number of traps being present leads to decrease in conductivity as the electrons are trapped and retrapped in travelling between the traps without reaching the conduction band. The same reason is also found to hold good for the absorption of the samples in the VIS and UV region. The UV spectrum shows that there is a very good absorption for the samples with lower concentration of H2Pc and the absorption decreases for higher concentration; finally, the absorption quenches to a minimum for 2%. Thus, at high concentration, the applied optical energy is used in moving between the traps, which are largely present.
The currents obtained for both dark conductivity and photoconductivity of the doped compounds are greater than the currents obtained for pure anthracene. Anthracene and H2Pc (with 0.5% and 1.0%) is mixed physically and after grinding the mixture, pellets were made and conductivity studies were carried out, which yielded very low dark current and photocurrent. The conductivity response in the physical mixture is very low when compared with the co-crystals, as physical mixture does not involve the phenomenon of doping, leading to a large number of grain boundaries. On the other hand, H2Pc in the cocrystals is doped, minimizing the grain boundaries. We have already reported that doping H2Pc onto anthracene decreases the band gap of anthracene [22] and at present, we see that activation energy of anthracene decreases with the concentration of H2Pc, thereby increasing the conductivity. Hence, H2Pc could be a suitable dopant for anthracene to find its application in the semiconductor-based solar cell devices.
588
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Fig. 4. ln I vs. 1=kT for anthracene-doped with H2Pc (0%, 0.47%, 1.5% and 2.0%).
Table 1 Activation energy for the co-crystals of anthracene with varying concentration of H2Pc % of H2Pc
0 0.12 0.47 0.62 0.92 1.0 1.5 2.0
Activation energy (eV) Ed
Eph
0.2511 0.2403 0.2301 0.2168 0.2052 0.1985 0.4409 0.4589
0.2343 0.2147 0.2041 0.1882 0.1735 0.1629 0.4016 0.4143
4. Conclusions Anthracene doped with metal-free phthalocyanine is prepared by the co-crystallization method
and its activation energy is determined. The activation energies for dark conductivity and photoconductivity of the H2Pc-doped anthracene samples decrease as the concentration of H2Pc increases. The trap level increases with the concentration of H2Pc, leading to the decrease in activation energy. But beyond 1% of H2Pc, the trap levels are present in large numbers and they obstruct the flow of electrons from valence band to the conduction band, as the excitation energy is lost in travelling between the traps. Though traps are necessary for better conductivity in solids, their number should be controlled and energy level altered so that they do not obstruct the passage of electrons to the conduction band. Thus, we see that traps which decrease the activation energy may lead to increase in activation energy if they are present in large number beyond need. The samples are photosensitive and UV active and could be tested for a solar cell application.
S.L. Selvaraj, F.P. Xavier / Journal of Crystal Growth 233 (2001) 583–590
Fig. 5. Visible and UV spectrum for H2Pc-doped anthracene co-crystals (0%, 0.47%, 1.0%, 1.5% and 2.0%).
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Acknowledgements The authors are thankful to the members of the research team at Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai for their useful discussions. We specially thank Dr. K. Swaminathan, LIFE and Dr. Babu Varghese, RSIC, IIT Chennai for their help in the experimental studies. One of the authors (SLS) gratefully acknowledges the Jawaharlal Nehru Memorial Fund, Teen Murti House, New Delhi for the research fellowship to carry out his research.
References [1] A. Bree, D.J. Carswell, L.E. Lyons, J. Chem. Soc. (1955) 1728. [2] L.E. Lyons, G.C. Morris, J. Chem. Soc. (1957) 3648. [3] S. Choi, S.A. Rice, J. Chem. Phys. 38 (1963) 366. [4] S.Z. Weisz, R.C. Jarnagin, M. Silver, J. Chem. Phys. 40 (1964) 3365. [5] D.D. Eley, G.D. Parfitt, M.J. Perry, D.H. Taysum, Trans. Faraday Soc. 49 (1953) 79. [6] H. Mette, H. Pick, Z. Physik 134 (1953) 566. [7] H. Inokuchi, Bull. Chem. Soc. Japan 29 (1956) 131. [8] D.C. Northrop, O. Simpson, Proc. Roy. Soc. A 234 (1956) 124.
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