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Photoelectric evidence for a neutral excited state in fully polymerized polydiacetylene-p-(toluenesulfonate) single crystals
Volume 55, number 2
PHQTOELECTRIC
15 April 1978
CHEMICAL PHYSICS LE’iTERS
EVIDENCE FOR A NEUTRAL
EXCITED §TATE
IN FULLY POLYMERIZED POLYDLACElYLENE-p~TOLUENESULFONATE)
SINGLE CRYSTALS
B. REiMER and H. BÄSSLER Fachbereich Physikoìwhe Chemie der Universitat Marburg. 0-3550 Marbuq/Lahn.
Germany
Received 5 October 1977
The intensity dependence of photocurrent transients excited by the 1.06 JIline of a Nd laser bas been measured. Observation of an 12 and an 1’ branch indïcate that charge carriers can be generated via twoquantum absorption and that at high intensities photoionïzation of a neutral excited state with a lïfetime of the order 10-r” s dominates.
Based on theoretical arguments [ 11 it has been suggested that the streng optical absorption observed witb certain polydiacetylenes near 2 eV originates from a transition between valence and conduction band of the polymer chain. This assignment bas been supported by an analysís of the absorption band profile [2] although it is known from the literature that absorption lines resulting from an exciton transition can also be asymmetrie [3] _Stationary photoconductivity measurements, on the other hand, have demonstrated that the number of charge carrier pairs generated per absorbed photon is only 10-4 or less [4] _ This obsewation and the photoelectric action spectrum [4,5] led to the conclusion that the 2 eV transítion is rather an exciton than a band-to-band transition. A recent exciton theory worked out by Philpott [6] supports this assignment which, on the other hand, implies that a neutral excited state with probably short yet finte lifetime must exist. This letter reports on an indirect detection of such a state. From the absente of fluorescente of fully polymerized polydiacetylene single crystals one bas to conclude that such a state if it does exist, would not emit light, at least not in the spectral range of common multipliers*. Therefore an altemative detection method had to be employed. Such a method is pulsed * Fiuorescence from polydiicetybnes
bas only been observed polymerïzcd samples. It originates from defects and is blue-shifkd compared with the origin of the 2
wïth parklly
eV transition. See refs. (7,81.
photoconductivity under high light intensities at which bimolecular exciton reactions become possible. It proved to be a powerful tool in probing exciton dynamics in conventional molecular crystals like anthracene, where-neither the singlet nor the triplet exciton carries sufficient energy to ionize a bulk molecule but where collisions between excitons or photoexcitation of singlet or triplet excitons can lead to charge carrier production [9]. In the present investigationpolydiacetylenebis(toluenesulfonate) (PTS) crystals were irradiated wïth high intensity flashes of a Nd laser which emits at 1.06 p where PTS behaves transparent in conventional transmission measurements. The reason for thís choice was a twofold: (i) It was hoped to fmd evidente for two-photon absorption leading (ii) to homogeneous excitation of the crystal volume thus avoiding surface effects which may be severe upon one-photon excitation at hu 2 2 eV where the penetration depth of light can be as low as 100 A [ 101. This was particularly important with type A crystal geometry (see below) where the surface area certainly contained a high number of defect sites as a result of polishing. Two different types of samples were used. Type A was approximately 100 cc thick. Within an accuracy of about 10” the polymer chain axis @) was perpendicular to the surfaces which were contacted by Ag electrodes of about 20% transparency. They were prepared by polishing thick solution-grown monomer crystals with a solvent-soaked tissue and subsequent 315
Volume 55, number 2
CHEMICAL. PHYSICS LETTERS
15 April 1978
thermal polymerïzation. Irradiation occurred through the contacts, the propagation vector s being parallel to the chain ax&. Type B was a freshly cleaved poIymer crystal, about 1 mm thïck, contacted at the side faces to give an electrode separation of about 2 mm. It was irradiated perpendicukr to the bc cxystal plane, i.e. s 1 b with light polarized perpendicular to 0. All measurements were performed under vacuum (< lO-4 torr). The Nd laser used for excitation gave a maximum photon flux of 1.6 X 1026 photons cm-* s-l at 1.06 p (’ 30 MWfcm*) and had a pulse halfwidth of about 25 ns. Absolute intensities were measured with a calorimeter, relative întensities with a fast photpdiode. The time constant of the circuit for photocurrent measurements was < 10 ns. A characteristic photocurrent transient recorded with a type A crystal is shown in fig. 1 - The current reaches a maximum by the end of the laser pulse and decays within a time Interval of the order of 100 ns. In fig. 2 the dependence of the maximum current amplitude ip on lïght intensity 1 is plotted for two samples at different electric fields. Clearly two ranges can be distinguished: For I> 7 X 1025 photons cmW s-l ip follows an I3 relationship. At lower I either an fi or @ dependence is obtaïned. In the followìng the various photocurrent components are designated as fl). ~4~)or P3), respectively. Component P1) is nat always obsertied. It seems to depend on electrode properties and displays a different tïme decay than zf*) or zf3). It is attributed to injection from the contact and shall not be considered here further. The @)(t) and P3)(t) follow a hyperbolic decay law,
$~.(3)(~) = ji2)*(3)/(1
+
Fig. 1. Photocurrent puk
observedupon irradiationof a
120 JLthïckFTS crystaì wïth a Nd laser flash of an intensity
1.5 X 10z6 photons cm” s-l. The appliedvoltagewas 300 V, the electrode area 5 X 10ez cm*, the load resìstor 59r-L
i=
where [ll] . The fact that 7r >rL ïndïcates that the peak photocurrent iP = en,pFq
(1) is a measure for the total carrier density n. built-up
t/Tr),
at least over 50 to 100 ns after the peak maximum. For longer times the decay curve approaches an exponential. This demonstrates that irradiation produces pairs of charge carriers which recombine. The recombination time rr exceeds the laser pulse width rL but must be shorter than the transit time. At ïow carrier density trapping becomes the dominating carrier loss mechanism. Details will be published else* me passííïlity cannot be excluded that due to surface írr~&&ties ïntroducedby crystal polishingthe incident light is scattered near the contact causingrandom light propagationinsidethe sample. 316
Fii. 2. Intensity dependenceof the maximumphotocurrent amplitudefor two different samples.Samplea: d = 80 gm, F = 4 X 104 Vjcm, sample b: d = 150 Pm, F = 6.7 X 103 Vf cm, electrode area in botb cases 2 x 10” cm*.
Volume 55, number2
CHEMICAL PYYSICS LETTERS
during the laser flash, cr being the sum of electron and hole mobility and q the electrode area. Therefore the dependence of ip on light intensity, i&), reflects the kinetics of the charge-carrier generation process. The fa& (i) that 112)a Z2 and (ii) that the photon energy is belGw the optical threshold strongly suggest that fi2) is due to photocarrier production via two-photon absorption. In the present context, however, the essential result is the observation of a cubic $, versus Z relationship at high intensities. Although no estimate exists it seems rather unlïkely that at the laser intensities employed three-photon absorption can dominate over the two-photon process. Therefore the change from ip a Z2 to $_,a I3 must reflect onset of an additional interaction mechanism. In principle this could be optical release of carriers trapped durïng a preceding pbotocurrent pulse. However, such an effect should depend on sample history and is unable to explain the high current densitïes observed. Therefore the following model is proposed: At a rate 6f2 two-photon absorption occurs, 6 being the two-photon absorption cross section. Only the fraction cpof all absorption events contributes to charge carrier production, the rest leads to generation of an unspecifïed excited state X. This state can either decay non-radiatively with a rate constant fi or can be photoionized by absorption of an additional 1.17 eV photon. If G designates the absorption cross section and
(21
where it has been assumed tbat the photoiouization efficiency of X* is small, i.e. 0 & crl;pX. The condition & = cpxuZt defimes the intens@ Zt at which carrier production via primary two-photon absorption equals carrier production by photoionization of the excited state X. Inserting the experimental value Zt = 1.3 X 1025 photons cm-2 s-1 and e/p = 30 which follows from tbe increase of the one-photon photoiouization cross section when going from 2.34 (twophoton ene& to 3.50 eV (three-photon energy) 1443 yields /3 = 4 X 1026 u [s-l ] . Under the assumption that u is between 10-16 and 10-17 cm2, i.e. X + X* isanallGwed transition, a lifetime 0-l for the state X between 25 and 250 ps follows.
15 Aprï! 1978
The two-photon absorption cross section 6 can be calculated by inte&ating eq. (2). In view of the uncertainties involved in numerical analysis the laser pulse shape can be approximated with sufficient accuracy by Z(f) = Zo for 0 d t d 7~ and Z(t) = 0 for t > 7L. Combining eqs. (1) and (2) yields
n=
(3)
r21/eFw=&Z~T~
for the charge carrier density in the low intensity region. Inserting
transition. From the fact, however, that this state is populated before the charge carriers recombine one can exclude the possibility that it is generated in course of electron-hole recombination. This lends further support to the idea that the dominant optical transition in PTS is excitonic in nature. The short lifetime of X indicates existente of an efficient non-
radiative decay Channel leading to rapid deactivation of excited states in PTS. We are indebted to K. Lochner for growing the crystals and to the Deutsche Forschungsgemeinschaft for fmancial support_
317
Voiume 5.5,number 2
CHEMICAL PHYSICS LEITËRS
References [ 1] [2] [3f 141
E.G. Wilson, J. Phys. C8 (1975) 727. D. Bloor, Chem. Phys. Letters42 (1976) 174. D.M. Burhnd. J. Chem. Phys. 59 (1973) 4283. K. Lochner, B. Rehner and H. Bässler,Phys. Stat. Sol. 76b (1976) 533. [S J R.R. Chance and R.H. Baughman.J. Chem. Phys- 64 (1976) 3889[6] M.R. Phïlpott, Chem. Phys. Letters 50 (1977) 18. [7] D. Bloor, D.N. Batchelder and F.H. Preston, Phys. Stat Sol 40a (1977) 279.
318
15 April 1978
[8] H. Eichele and hl. Schwoerer, Phys Stat. Sol a, to be published (91 CE. Swenberg and N.E. Geacïntov, in: Organïc molecuIar photophysïcs, ed. J.B. Birks (Wiley, New York, 1973) p. 489. [lol B. Reimer. H. Bässler.1. Hesse md G. Weiser, Phys. Sta. Sol. 73b (1976) 709. [ 111 B. Reimer and H. Bässier.Phys. Stat. SOL 85b, to be published. 1121 A. Bergman and J. Jortner, Chem- Phyn Letters 26 (1974) 323.
PHQTOELECTRIC
15 April 1978
CHEMICAL PHYSICS LE’iTERS
EVIDENCE FOR A NEUTRAL
EXCITED §TATE
IN FULLY POLYMERIZED POLYDLACElYLENE-p~TOLUENESULFONATE)
SINGLE CRYSTALS
B. REiMER and H. BÄSSLER Fachbereich Physikoìwhe Chemie der Universitat Marburg. 0-3550 Marbuq/Lahn.
Germany
Received 5 October 1977
The intensity dependence of photocurrent transients excited by the 1.06 JIline of a Nd laser bas been measured. Observation of an 12 and an 1’ branch indïcate that charge carriers can be generated via twoquantum absorption and that at high intensities photoionïzation of a neutral excited state with a lïfetime of the order 10-r” s dominates.
Based on theoretical arguments [ 11 it has been suggested that the streng optical absorption observed witb certain polydiacetylenes near 2 eV originates from a transition between valence and conduction band of the polymer chain. This assignment bas been supported by an analysís of the absorption band profile [2] although it is known from the literature that absorption lines resulting from an exciton transition can also be asymmetrie [3] _Stationary photoconductivity measurements, on the other hand, have demonstrated that the number of charge carrier pairs generated per absorbed photon is only 10-4 or less [4] _ This obsewation and the photoelectric action spectrum [4,5] led to the conclusion that the 2 eV transítion is rather an exciton than a band-to-band transition. A recent exciton theory worked out by Philpott [6] supports this assignment which, on the other hand, implies that a neutral excited state with probably short yet finte lifetime must exist. This letter reports on an indirect detection of such a state. From the absente of fluorescente of fully polymerized polydiacetylene single crystals one bas to conclude that such a state if it does exist, would not emit light, at least not in the spectral range of common multipliers*. Therefore an altemative detection method had to be employed. Such a method is pulsed * Fiuorescence from polydiicetybnes
bas only been observed polymerïzcd samples. It originates from defects and is blue-shifkd compared with the origin of the 2
wïth parklly
eV transition. See refs. (7,81.
photoconductivity under high light intensities at which bimolecular exciton reactions become possible. It proved to be a powerful tool in probing exciton dynamics in conventional molecular crystals like anthracene, where-neither the singlet nor the triplet exciton carries sufficient energy to ionize a bulk molecule but where collisions between excitons or photoexcitation of singlet or triplet excitons can lead to charge carrier production [9]. In the present investigationpolydiacetylenebis(toluenesulfonate) (PTS) crystals were irradiated wïth high intensity flashes of a Nd laser which emits at 1.06 p where PTS behaves transparent in conventional transmission measurements. The reason for thís choice was a twofold: (i) It was hoped to fmd evidente for two-photon absorption leading (ii) to homogeneous excitation of the crystal volume thus avoiding surface effects which may be severe upon one-photon excitation at hu 2 2 eV where the penetration depth of light can be as low as 100 A [ 101. This was particularly important with type A crystal geometry (see below) where the surface area certainly contained a high number of defect sites as a result of polishing. Two different types of samples were used. Type A was approximately 100 cc thick. Within an accuracy of about 10” the polymer chain axis @) was perpendicular to the surfaces which were contacted by Ag electrodes of about 20% transparency. They were prepared by polishing thick solution-grown monomer crystals with a solvent-soaked tissue and subsequent 315
Volume 55, number 2
CHEMICAL. PHYSICS LETTERS
15 April 1978
thermal polymerïzation. Irradiation occurred through the contacts, the propagation vector s being parallel to the chain ax&. Type B was a freshly cleaved poIymer crystal, about 1 mm thïck, contacted at the side faces to give an electrode separation of about 2 mm. It was irradiated perpendicukr to the bc cxystal plane, i.e. s 1 b with light polarized perpendicular to 0. All measurements were performed under vacuum (< lO-4 torr). The Nd laser used for excitation gave a maximum photon flux of 1.6 X 1026 photons cm-* s-l at 1.06 p (’ 30 MWfcm*) and had a pulse halfwidth of about 25 ns. Absolute intensities were measured with a calorimeter, relative întensities with a fast photpdiode. The time constant of the circuit for photocurrent measurements was < 10 ns. A characteristic photocurrent transient recorded with a type A crystal is shown in fig. 1 - The current reaches a maximum by the end of the laser pulse and decays within a time Interval of the order of 100 ns. In fig. 2 the dependence of the maximum current amplitude ip on lïght intensity 1 is plotted for two samples at different electric fields. Clearly two ranges can be distinguished: For I> 7 X 1025 photons cmW s-l ip follows an I3 relationship. At lower I either an fi or @ dependence is obtaïned. In the followìng the various photocurrent components are designated as fl). ~4~)or P3), respectively. Component P1) is nat always obsertied. It seems to depend on electrode properties and displays a different tïme decay than zf*) or zf3). It is attributed to injection from the contact and shall not be considered here further. The @)(t) and P3)(t) follow a hyperbolic decay law,
$~.(3)(~) = ji2)*(3)/(1
+
Fig. 1. Photocurrent puk
observedupon irradiationof a
120 JLthïckFTS crystaì wïth a Nd laser flash of an intensity
1.5 X 10z6 photons cm” s-l. The appliedvoltagewas 300 V, the electrode area 5 X 10ez cm*, the load resìstor 59r-L
i=
where [ll] . The fact that 7r >rL ïndïcates that the peak photocurrent iP = en,pFq
(1) is a measure for the total carrier density n. built-up
t/Tr),
at least over 50 to 100 ns after the peak maximum. For longer times the decay curve approaches an exponential. This demonstrates that irradiation produces pairs of charge carriers which recombine. The recombination time rr exceeds the laser pulse width rL but must be shorter than the transit time. At ïow carrier density trapping becomes the dominating carrier loss mechanism. Details will be published else* me passííïlity cannot be excluded that due to surface írr~&&ties ïntroducedby crystal polishingthe incident light is scattered near the contact causingrandom light propagationinsidethe sample. 316
Fii. 2. Intensity dependenceof the maximumphotocurrent amplitudefor two different samples.Samplea: d = 80 gm, F = 4 X 104 Vjcm, sample b: d = 150 Pm, F = 6.7 X 103 Vf cm, electrode area in botb cases 2 x 10” cm*.
Volume 55, number2
CHEMICAL PYYSICS LETTERS
during the laser flash, cr being the sum of electron and hole mobility and q the electrode area. Therefore the dependence of ip on light intensity, i&), reflects the kinetics of the charge-carrier generation process. The fa& (i) that 112)a Z2 and (ii) that the photon energy is belGw the optical threshold strongly suggest that fi2) is due to photocarrier production via two-photon absorption. In the present context, however, the essential result is the observation of a cubic $, versus Z relationship at high intensities. Although no estimate exists it seems rather unlïkely that at the laser intensities employed three-photon absorption can dominate over the two-photon process. Therefore the change from ip a Z2 to $_,a I3 must reflect onset of an additional interaction mechanism. In principle this could be optical release of carriers trapped durïng a preceding pbotocurrent pulse. However, such an effect should depend on sample history and is unable to explain the high current densitïes observed. Therefore the following model is proposed: At a rate 6f2 two-photon absorption occurs, 6 being the two-photon absorption cross section. Only the fraction cpof all absorption events contributes to charge carrier production, the rest leads to generation of an unspecifïed excited state X. This state can either decay non-radiatively with a rate constant fi or can be photoionized by absorption of an additional 1.17 eV photon. If G designates the absorption cross section and
(21
where it has been assumed tbat the photoiouization efficiency of X* is small, i.e. 0 & crl;pX. The condition & = cpxuZt defimes the intens@ Zt at which carrier production via primary two-photon absorption equals carrier production by photoionization of the excited state X. Inserting the experimental value Zt = 1.3 X 1025 photons cm-2 s-1 and e/p = 30 which follows from tbe increase of the one-photon photoiouization cross section when going from 2.34 (twophoton ene& to 3.50 eV (three-photon energy) 1443 yields /3 = 4 X 1026 u [s-l ] . Under the assumption that u is between 10-16 and 10-17 cm2, i.e. X + X* isanallGwed transition, a lifetime 0-l for the state X between 25 and 250 ps follows.
15 Aprï! 1978
The two-photon absorption cross section 6 can be calculated by inte&ating eq. (2). In view of the uncertainties involved in numerical analysis the laser pulse shape can be approximated with sufficient accuracy by Z(f) = Zo for 0 d t d 7~ and Z(t) = 0 for t > 7L. Combining eqs. (1) and (2) yields
n=
(3)
r21/eFw=&Z~T~
for the charge carrier density in the low intensity region. Inserting
transition. From the fact, however, that this state is populated before the charge carriers recombine one can exclude the possibility that it is generated in course of electron-hole recombination. This lends further support to the idea that the dominant optical transition in PTS is excitonic in nature. The short lifetime of X indicates existente of an efficient non-
radiative decay Channel leading to rapid deactivation of excited states in PTS. We are indebted to K. Lochner for growing the crystals and to the Deutsche Forschungsgemeinschaft for fmancial support_
317
Voiume 5.5,number 2
CHEMICAL PHYSICS LEITËRS
References [ 1] [2] [3f 141
E.G. Wilson, J. Phys. C8 (1975) 727. D. Bloor, Chem. Phys. Letters42 (1976) 174. D.M. Burhnd. J. Chem. Phys. 59 (1973) 4283. K. Lochner, B. Rehner and H. Bässler,Phys. Stat. Sol. 76b (1976) 533. [S J R.R. Chance and R.H. Baughman.J. Chem. Phys- 64 (1976) 3889[6] M.R. Phïlpott, Chem. Phys. Letters 50 (1977) 18. [7] D. Bloor, D.N. Batchelder and F.H. Preston, Phys. Stat Sol 40a (1977) 279.
318
15 April 1978
[8] H. Eichele and hl. Schwoerer, Phys Stat. Sol a, to be published (91 CE. Swenberg and N.E. Geacïntov, in: Organïc molecuIar photophysïcs, ed. J.B. Birks (Wiley, New York, 1973) p. 489. [lol B. Reimer. H. Bässler.1. Hesse md G. Weiser, Phys. Sta. Sol. 73b (1976) 709. [ 111 B. Reimer and H. Bässier.Phys. Stat. SOL 85b, to be published. 1121 A. Bergman and J. Jortner, Chem- Phyn Letters 26 (1974) 323.