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Absorption spectra of organic semiconductors in IR-range measured by constant photocurrent method
Journal of Non-Crystalline Solids 352 (2006) 1668–1670 www.elsevier.com/locate/jnoncrysol
Absorption spectra of organic semiconductors in IR-range measured by constant photocurrent method A.V. Fenukhin a, A.G. Kazanskii a
a,*
, A.G. Kolosko b, E.I. Terukov b, A.V. Ziminov
b
Department of Physics, M.V. Lomonosov Moscow State University, Lenisky gory, 119992 Moscow, Russia b A.F. Ioffe Physikotechnical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia Available online 27 March 2006
Abstract Constant photocurrent method has been used to study spectral dependence of the absorption coefficient in copper phthalocyanine thin films formed by the molecules with different peripheral groups. Spectral dependence of the absorption coefficient reveals exponential growth in the energy range 1.4–1.6 eV and a series of three bands in the energy below 1.4 eV. The maximum observed at 1.12 eV is attributed to optical transition from ground state S0 to excited triplet state T1 of a molecule. This transition results in creation of triplet excitons. The correspondence between photoluminescence and absorption spectra measured by constant photocurrent method both with respect to peak positions and relative intensities has been found. Ó 2006 Elsevier B.V. All rights reserved. PACS: 71.35.Aa; 71.35.Cc; 78.30.Jw; 78.66.Qn Keywords: Absorption; Luminescence; Photoconductivity; Polymers and organics
1. Introduction The organic semiconductors have attracted lots of attention for applications in optoelectronic devices, in particular flat flexible displays. For elaboration of such devices and improvement of their characteristics, investigations of correlation between molecule structures and ordering, and optical and electronic properties of the corresponding organic semiconductors are necessary. In spite of wide use of thin films of organic semiconductor materials – metal phthalocyanines – in recent years for producing optoelectronic devices, the mechanisms of generation, recombination and transport of charge carriers in these materials are still under discussion. The investigations of photoconductivity and spectral dependence of absorption coefficient (a) may contribute to understanding of these processes.
*
Corresponding author. Tel.: +7 095 9394118; fax: +7 095 9393731. E-mail address: [email protected] (A.G. Kazanskii).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.113
We have investigated absorption spectra in IR-range of thin films of copper phthalocyanine (CuPc) with different modification structure of molecule and metal-free phthalocyanine (H2Pc) at room temperature. High photocurrent response of these materials has allowed us to use constant photocurrent method (CPM) [1] to measure spectral dependence of product a Æ g, where g is quantum efficiency. The correlations between absorption spectra, photoluminescence and photoconductivity of copper phthalocyanine thin films with different molecular structure have been studied. 2. Experimental All the samples utilized in our investigation were thin films vacuum-deposited by thermal evaporation using Knudsen cell. Pure CuPc powder (sample 1), CuPc with peripheral substitutes of metilenphtalimidal group (sample 2) and CuPc oxidized l-peroxodimer (sample 3) were used as the source materials to create CuPc films with different molecular structures. The films with the thickness of
A.V. Fenukhin et al. / Journal of Non-Crystalline Solids 352 (2006) 1668–1670
3. Results The optical transmission spectra of the films investigated showed Q absorption band or band gap absorption at the energy of 1.77 eV. This absorption band is determined by the optical transitions from ground state S0 to excited singlet state S1 of molecules [4]. Fig. 1 shows CPM results obtained for the films 1–3 for the energy below 1.8 eV. In organic films, the condition that the quantum efficiency g is wavelength independent is not met [3]. Therefore CPM spectra in Fig. 1 present the energy dependence of optical absorption coefficient a, multiplied by the quantum efficiency g, in arbitrary units ((a Æ g)/(a Æ g) (1.8 eV)). Below 1.6 eV, the CPM curves can be divided into two energy regions: first – exponential increase of absorption in the energy range 1.4–1.6 eV; second – low absorption for energy below 1.4 eV. The slope of the curve a Æ g (hm) in the energy range 1.4– 1.6 eV depends on films investigated and apparently on molecular structure of the film. The slope value is 53 meV for sample 2 and 65–68 meV for sample 1 and sample 3. The exponential dependence of a Æ g curves indicates the exponential distribution of localized states of the density of states distribution in metal phthalocyanines [5]. According to [5], the fluctuations of electrostatic potential due to the presence of charged ions in the films, in particular
αη/αη (1.8 eV)
100
1 2 3
10-1
10-2
10-3 0.8
1.0
1.2
1.4
1.6
1.8
2.0
hν (eV) Fig. 1. CPM spectra of CuPc films investigated. The numbers of curves correspond to the sample numbers.
MPc+ and O2 can be the reason for exponential tail distribution. In the energy range below 1.4 eV, our CPM spectra exhibit three maxima at 1.12, 1.21 and 1.36 eV. The positions of these peaks are in close agreement with peak positions obtained in [6] from the measurements of optical absorption spectrum in thick (10–30 lkm) monocrystal flakes of CuPc and maxima obtained in [7,8] from measurements of CuPc photoconductivity spectrum. According to [8], the maximum observed in photoconductivity spectrum at 1.12 eV, originates from optical transition from ground state S0 to excited triplet state T1 and results in creation
100
10-1 αη/αη (1.8 eV)
0.7 lm were evaporated onto pre-cleaned pyroceram and quartz substrates. Usually optical transmission and reflection measurements are used to obtain spectral dependence of absorption coefficient in organic semiconductors [2]. At the same time, this method does not allow to obtain a spectral dependence in thin film of organic semiconductors if a Æ d < 1, where d is the thickness of the film. High photoconductivity of phthalocyanines films have allowed us to use CPM in dc mode for measurements of a spectral dependence in the energy range of about 0.8–2.0 eV. This method was successfully used to resolve sub-band gap absorption features in amorphous semiconductors thin films [1] and was shown recently to be useful for investigation of organic semiconductors [3]. The couples of Al-contacts were evaporated on a surface of films for the purpose of electric and photoelectric measurements. The length of contacts was 4 mm and the distance between contacts was 0.5 mm. The value of an electric field in the sample was 200 V/cm. The measurements were performed in residual vacuum 10 3 Pa. These conditions have allowed us to avoid influence of an atmosphere on results of conductivity and photoconductivity measurements. Photoluminescence measurements have been performed with illumination of the films by argon laser (514.5 nm). Photoluminescence signal was passed trough a grating monochromator and was detected by a cooled germanium photodetector. All the measurements have been performed at room temperature.
1669
CuPc H2Pc
10-2
10-3
10-4 0.8
1.0
1.2
1.4
1.6
1.8
hν (eV) Fig. 2. CPM spectra of CuPc film (sample 1) and H2Pc film.
2.0
1670
A.V. Fenukhin et al. / Journal of Non-Crystalline Solids 352 (2006) 1668–1670
of triplet excitons. The absorption, corresponded to this transition, is expected to be much greater in CuPc than in H2Pc because of the spin-orbit coupling of the Cu ion. The CPM spectra shown in Fig. 2 confirm this supposition. The 1.12 eV maximum intensity for CuPc is much greater than that for our H2Pc film. We have to point out that H2Pc is extremely reactive with respect to metal contamination. Therefore, we cannot exclude that some metal impurities might have been build in our H2Pc film during preparation process. 4. Discussion The constant photocurrent method measures only optical transitions giving rise to free charge carriers. Therefore the maximum, observed at 1.12 eV in CPM spectra, is a strong indication that Frenkel type excitons are the primary photo excitations in organic semiconductors. Subsequent to their formation and random walk within an energetically and spatially disordered array of hopping sites, excitons dissociate into pairs of free charge carriers at a charge transfer state, an impurity or defect [8]. Our measurements reveal the correlation between CPM absorption spectra and photoluminescence spectra of the films, under investigation. The photoluminescence spectra of copper phthalocyanine films with different structure are displayed in Fig. 3. A series of three maxima is visible in photoluminescence spectra with a greater one at wavelength of 1.11 lm (corresponding to energy 1.12 eV). According to [9], phosphorescence emission at 1.1 lm of CuPc powder at room temperature is due to transition from excited triplet state T1 to ground state S0 of a molecule. Thus our CPM data are consistent with the hypothesis of infrared singlet–triplet absorption at hm = 1.12 eV. The curves in Fig. 3 reveal that intensity of a luminescence, in particular at 1.11 lm, depends on films investigated. There is correspondence between photoluminescence and CPM spectra both with respect to peak positions and relative intensities. The highest luminescence is observed in the sample 2, which shows the lowest CPM absorption in the energy range below 1.4 eV. This result, in part, might have its origin in different traps concentration in the films with different structure. The excitons produce photocarriers upon dissociation at a charge transfer state, an impurity or defect. The effectiveness of the photocarriers production by triplet excitons should depend on density of these charge transfer states. Thus the reduction in the density of these states has to result in decrease of relative absorption at 1.12 eV, detected by CPM. At the same time, this reduction can lead to enhancement of the role of radiative recombination of exciton and results in rise of photoluminescence intensity at 1.11 lm. Sample 2 exhibits
Fig. 3. Photoluminescence spectra of CuPc films investigated. The numbers of curves correspond to the sample numbers.
the largest photoconductivity at 1.8 eV, compared to that in other samples investigated. This result supports our hypothesis. 5. Conclusions We have studied spectral dependence of absorption coefficient with the help of constant photocurrent method in copper phthalocyanine thin films formed by the molecules with different peripheral groups. The CPM spectra have revealed a series of absorption maxima. The maximum, observed at the energy 1.12 eV have been attributed to optical transition from ground state S0 to excited triplet state T1 of a molecule. This transition results in creation of triplet excitons. The correspondence between the photoluminescence and CPM spectra both as to peak positions and relative intensities has been found. The results obtained support the concept that triplet exciton generate free charge carriers in organic semiconductors as well as singlet one. References [1] M. Vanecek, J. Kocka, J. Stuchlik, A. Triska, Solid State Commun. 39 (1981) 1199. [2] Z.G. Ji, K.W. Wong, P.K. Tse, et al., Thin Solid Films 402 (2002) 79. [3] L. Goris, K. Heinen, M. Nesladek, et al., Proc. SPIE 5464 (2004) 372. [4] J. Simon, J.-J. Andre, Molecular Semiconductors. Photoelectrical Properties and Solar Cells, Springer, Berlin, 1985. [5] H. Naito, K.-H. Kishimoto, T. Nagase, Thin Solid Films 331 (1998) 82. [6] M. Schott, J. Chem. Phys. 44 (1966) 429. [7] P. Day, R.J.P. Williams, J. Chem. Phys. 42 (1965) 4049. [8] S.E. Harrison, J. Chem. Phys. 50 (1969) 4739. [9] P.S. Vincett, E.M. Voigt, K.E. Rieckhoff, J. Chem. Phys. 55 (1971) 4131.
Absorption spectra of organic semiconductors in IR-range measured by constant photocurrent method A.V. Fenukhin a, A.G. Kazanskii a
a,*
, A.G. Kolosko b, E.I. Terukov b, A.V. Ziminov
b
Department of Physics, M.V. Lomonosov Moscow State University, Lenisky gory, 119992 Moscow, Russia b A.F. Ioffe Physikotechnical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia Available online 27 March 2006
Abstract Constant photocurrent method has been used to study spectral dependence of the absorption coefficient in copper phthalocyanine thin films formed by the molecules with different peripheral groups. Spectral dependence of the absorption coefficient reveals exponential growth in the energy range 1.4–1.6 eV and a series of three bands in the energy below 1.4 eV. The maximum observed at 1.12 eV is attributed to optical transition from ground state S0 to excited triplet state T1 of a molecule. This transition results in creation of triplet excitons. The correspondence between photoluminescence and absorption spectra measured by constant photocurrent method both with respect to peak positions and relative intensities has been found. Ó 2006 Elsevier B.V. All rights reserved. PACS: 71.35.Aa; 71.35.Cc; 78.30.Jw; 78.66.Qn Keywords: Absorption; Luminescence; Photoconductivity; Polymers and organics
1. Introduction The organic semiconductors have attracted lots of attention for applications in optoelectronic devices, in particular flat flexible displays. For elaboration of such devices and improvement of their characteristics, investigations of correlation between molecule structures and ordering, and optical and electronic properties of the corresponding organic semiconductors are necessary. In spite of wide use of thin films of organic semiconductor materials – metal phthalocyanines – in recent years for producing optoelectronic devices, the mechanisms of generation, recombination and transport of charge carriers in these materials are still under discussion. The investigations of photoconductivity and spectral dependence of absorption coefficient (a) may contribute to understanding of these processes.
*
Corresponding author. Tel.: +7 095 9394118; fax: +7 095 9393731. E-mail address: [email protected] (A.G. Kazanskii).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.113
We have investigated absorption spectra in IR-range of thin films of copper phthalocyanine (CuPc) with different modification structure of molecule and metal-free phthalocyanine (H2Pc) at room temperature. High photocurrent response of these materials has allowed us to use constant photocurrent method (CPM) [1] to measure spectral dependence of product a Æ g, where g is quantum efficiency. The correlations between absorption spectra, photoluminescence and photoconductivity of copper phthalocyanine thin films with different molecular structure have been studied. 2. Experimental All the samples utilized in our investigation were thin films vacuum-deposited by thermal evaporation using Knudsen cell. Pure CuPc powder (sample 1), CuPc with peripheral substitutes of metilenphtalimidal group (sample 2) and CuPc oxidized l-peroxodimer (sample 3) were used as the source materials to create CuPc films with different molecular structures. The films with the thickness of
A.V. Fenukhin et al. / Journal of Non-Crystalline Solids 352 (2006) 1668–1670
3. Results The optical transmission spectra of the films investigated showed Q absorption band or band gap absorption at the energy of 1.77 eV. This absorption band is determined by the optical transitions from ground state S0 to excited singlet state S1 of molecules [4]. Fig. 1 shows CPM results obtained for the films 1–3 for the energy below 1.8 eV. In organic films, the condition that the quantum efficiency g is wavelength independent is not met [3]. Therefore CPM spectra in Fig. 1 present the energy dependence of optical absorption coefficient a, multiplied by the quantum efficiency g, in arbitrary units ((a Æ g)/(a Æ g) (1.8 eV)). Below 1.6 eV, the CPM curves can be divided into two energy regions: first – exponential increase of absorption in the energy range 1.4–1.6 eV; second – low absorption for energy below 1.4 eV. The slope of the curve a Æ g (hm) in the energy range 1.4– 1.6 eV depends on films investigated and apparently on molecular structure of the film. The slope value is 53 meV for sample 2 and 65–68 meV for sample 1 and sample 3. The exponential dependence of a Æ g curves indicates the exponential distribution of localized states of the density of states distribution in metal phthalocyanines [5]. According to [5], the fluctuations of electrostatic potential due to the presence of charged ions in the films, in particular
αη/αη (1.8 eV)
100
1 2 3
10-1
10-2
10-3 0.8
1.0
1.2
1.4
1.6
1.8
2.0
hν (eV) Fig. 1. CPM spectra of CuPc films investigated. The numbers of curves correspond to the sample numbers.
MPc+ and O2 can be the reason for exponential tail distribution. In the energy range below 1.4 eV, our CPM spectra exhibit three maxima at 1.12, 1.21 and 1.36 eV. The positions of these peaks are in close agreement with peak positions obtained in [6] from the measurements of optical absorption spectrum in thick (10–30 lkm) monocrystal flakes of CuPc and maxima obtained in [7,8] from measurements of CuPc photoconductivity spectrum. According to [8], the maximum observed in photoconductivity spectrum at 1.12 eV, originates from optical transition from ground state S0 to excited triplet state T1 and results in creation
100
10-1 αη/αη (1.8 eV)
0.7 lm were evaporated onto pre-cleaned pyroceram and quartz substrates. Usually optical transmission and reflection measurements are used to obtain spectral dependence of absorption coefficient in organic semiconductors [2]. At the same time, this method does not allow to obtain a spectral dependence in thin film of organic semiconductors if a Æ d < 1, where d is the thickness of the film. High photoconductivity of phthalocyanines films have allowed us to use CPM in dc mode for measurements of a spectral dependence in the energy range of about 0.8–2.0 eV. This method was successfully used to resolve sub-band gap absorption features in amorphous semiconductors thin films [1] and was shown recently to be useful for investigation of organic semiconductors [3]. The couples of Al-contacts were evaporated on a surface of films for the purpose of electric and photoelectric measurements. The length of contacts was 4 mm and the distance between contacts was 0.5 mm. The value of an electric field in the sample was 200 V/cm. The measurements were performed in residual vacuum 10 3 Pa. These conditions have allowed us to avoid influence of an atmosphere on results of conductivity and photoconductivity measurements. Photoluminescence measurements have been performed with illumination of the films by argon laser (514.5 nm). Photoluminescence signal was passed trough a grating monochromator and was detected by a cooled germanium photodetector. All the measurements have been performed at room temperature.
1669
CuPc H2Pc
10-2
10-3
10-4 0.8
1.0
1.2
1.4
1.6
1.8
hν (eV) Fig. 2. CPM spectra of CuPc film (sample 1) and H2Pc film.
2.0
1670
A.V. Fenukhin et al. / Journal of Non-Crystalline Solids 352 (2006) 1668–1670
of triplet excitons. The absorption, corresponded to this transition, is expected to be much greater in CuPc than in H2Pc because of the spin-orbit coupling of the Cu ion. The CPM spectra shown in Fig. 2 confirm this supposition. The 1.12 eV maximum intensity for CuPc is much greater than that for our H2Pc film. We have to point out that H2Pc is extremely reactive with respect to metal contamination. Therefore, we cannot exclude that some metal impurities might have been build in our H2Pc film during preparation process. 4. Discussion The constant photocurrent method measures only optical transitions giving rise to free charge carriers. Therefore the maximum, observed at 1.12 eV in CPM spectra, is a strong indication that Frenkel type excitons are the primary photo excitations in organic semiconductors. Subsequent to their formation and random walk within an energetically and spatially disordered array of hopping sites, excitons dissociate into pairs of free charge carriers at a charge transfer state, an impurity or defect [8]. Our measurements reveal the correlation between CPM absorption spectra and photoluminescence spectra of the films, under investigation. The photoluminescence spectra of copper phthalocyanine films with different structure are displayed in Fig. 3. A series of three maxima is visible in photoluminescence spectra with a greater one at wavelength of 1.11 lm (corresponding to energy 1.12 eV). According to [9], phosphorescence emission at 1.1 lm of CuPc powder at room temperature is due to transition from excited triplet state T1 to ground state S0 of a molecule. Thus our CPM data are consistent with the hypothesis of infrared singlet–triplet absorption at hm = 1.12 eV. The curves in Fig. 3 reveal that intensity of a luminescence, in particular at 1.11 lm, depends on films investigated. There is correspondence between photoluminescence and CPM spectra both with respect to peak positions and relative intensities. The highest luminescence is observed in the sample 2, which shows the lowest CPM absorption in the energy range below 1.4 eV. This result, in part, might have its origin in different traps concentration in the films with different structure. The excitons produce photocarriers upon dissociation at a charge transfer state, an impurity or defect. The effectiveness of the photocarriers production by triplet excitons should depend on density of these charge transfer states. Thus the reduction in the density of these states has to result in decrease of relative absorption at 1.12 eV, detected by CPM. At the same time, this reduction can lead to enhancement of the role of radiative recombination of exciton and results in rise of photoluminescence intensity at 1.11 lm. Sample 2 exhibits
Fig. 3. Photoluminescence spectra of CuPc films investigated. The numbers of curves correspond to the sample numbers.
the largest photoconductivity at 1.8 eV, compared to that in other samples investigated. This result supports our hypothesis. 5. Conclusions We have studied spectral dependence of absorption coefficient with the help of constant photocurrent method in copper phthalocyanine thin films formed by the molecules with different peripheral groups. The CPM spectra have revealed a series of absorption maxima. The maximum, observed at the energy 1.12 eV have been attributed to optical transition from ground state S0 to excited triplet state T1 of a molecule. This transition results in creation of triplet excitons. The correspondence between the photoluminescence and CPM spectra both as to peak positions and relative intensities has been found. The results obtained support the concept that triplet exciton generate free charge carriers in organic semiconductors as well as singlet one. References [1] M. Vanecek, J. Kocka, J. Stuchlik, A. Triska, Solid State Commun. 39 (1981) 1199. [2] Z.G. Ji, K.W. Wong, P.K. Tse, et al., Thin Solid Films 402 (2002) 79. [3] L. Goris, K. Heinen, M. Nesladek, et al., Proc. SPIE 5464 (2004) 372. [4] J. Simon, J.-J. Andre, Molecular Semiconductors. Photoelectrical Properties and Solar Cells, Springer, Berlin, 1985. [5] H. Naito, K.-H. Kishimoto, T. Nagase, Thin Solid Films 331 (1998) 82. [6] M. Schott, J. Chem. Phys. 44 (1966) 429. [7] P. Day, R.J.P. Williams, J. Chem. Phys. 42 (1965) 4049. [8] S.E. Harrison, J. Chem. Phys. 50 (1969) 4739. [9] P.S. Vincett, E.M. Voigt, K.E. Rieckhoff, J. Chem. Phys. 55 (1971) 4131.