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Molecular orientation dependent photoconductivity of liquid crystalline oligosilanes
ELSEVIER
Synthetic Metals 101 (1999) 461-462
3,lolecular
orientation
dependent
photoconductivit,y
R. Okumoto, T. Yatabe, M. Shimomura, Dfpnrtmcnt
of Polymer
4. Iiaito,
of liquid
crystalline
oligosilanes
N. Minami: Y. Tanabe and Chemical Research,
Physics, h’ational Institute of Materials 1-i Hignshi, Tsukuba, Ibaraki 305-856.5:
AIST
Japan
Abstract Polysilanes are known to be promising hole transport materials among conducting polymers. Their structure can be Recently an oligosilane (Me(SiMea)isMe) is found to show a better controlled by using single dispersed oligomers. This mesophase (SB). In the mesophase, we have observed unusual negative temperature dependence of photocurrent. can be explained by temperature dependent molecular orientation, which has been confirmed by polarized UV absorption spectroscopy. licywords: Photoconductivity, UV-Vis-NIR absorption, Liquid mers, Organic semiconductors based on conjugated molecules
1
Introduction
n-conjugated electrons in polysilanes have attracted much attention due to their unique properties compared with rr-conjugated electrons. The o-conjugation along saturated bonds gives rise to its characteristic photoconductivity [l], phot~oreactivity [2] and coupling of these two effect,s [3]. With photoconductivity, the high hole mobility compared with other n-conjugated conducting polymers has been reported [I], which makes polysilanes promising hole conductors. To improve the hole mobility, controlling the structure of samples should be crucial. In polysilanes, however, unavoidable dispersion of molecular weight may cause st,ructural disorder when processing films. On the other hand, single dispersed oligosilanes are expected to reduce such uncertainty. In fact, films of oligosilanes have well-defined structures. Recently, a mesophase of linear oligosilanes has been discovered in a permethyloligosilane Me(SiMez)ieMe (MSlOj [a]. Its phase diagram is as follows: 114oc
82Oc C
F
‘B 74oc
where C, Se and I indicate a crystalline phase, a mesophase of Smectic I3 and an isotropic phase, respectively. The SB phase has a multi-layer structure with molecular long axes perpendicular to the layer planes. Moreover, G-fold rotational symmetry exists within the planes. For molecules with r-conjugated electrons, high carrier mobility in mesophases has been reported: in a discotic hexagonal phase [5] and in a smectic A phase [G]. Thus controlling the order of molecules in mesophases is proved to be efficient for im-
crystalline
phase transitions,
Other conducting
poly-
proving photoconductivity. In this paper, we will report the temperature dependence of photocurrent in MS10 capillary-filled films. To focus on the transport mechanism in MSlO, carrier generation layer of selenium (Se) has been employed. We found that, in the Sn phase, the photocurrent has unusual temperature dependence that can be attributed to a change in the molecular orientation. We also studied the effect of the orientation in terms of polarized LTV absorption spectroscopy. 2
Experimental
MS10 was synthesized from a mixture of l-chlorotrisilane and 1,3-dichlorotrisilane with sodium in toluene and purified by preparative HPLC [q]. For photocurrent measurements, MS10 powder was melted and capillary-filled at 120°C into a gap (10pm) between two quartz substrates coated with indium-tin oxide (ITO) electrodes. Se was vacuum-deposited beforehand onto an IT0 electrode as a carrier generation layer. Since as deposited Se was in amorphous form and structurally unstable, the substrate was pre-heated above SO”C, which transformed Se into the most stable state (gray Se). After capillary-filling, the sample cells were annealed at 120°C for 1 hour and cooled to room Throughout photocurtemperature at a rate of l’C/min. rent measurements, the temperature of samples was controlled within a stability of O.Z’C by a controller. Orientation of molecules was monitored by polarized UV absorption spect,roscopy using a spectrophotometer with a temperature controlled sample holder. Incident angles of polarized light beam were changed from O0 (normal to a substrate) to 30°, which corresponds to an angle between an
0379-6779/99/$- seefront matter 0 1999 Elsevier ScienceS.A. All rights reserved. PII: so379-6779(98)01198-9
H. Okumoto et al. i Synthetic Metals 101 (1999) 461-462
462
(“C)
- (% 5
800
600 Wavelength
(nm)
Fig. 1. Temperature dependence of photocurrent spectra in capillary-filled MS10 with a carrier generation layer of gray Se. [E=5 x 103V/c,m, photon flux=1 x lO”photonsl~mm’s)
116
5
116
$
109
5
104
3 jj Q
94 84 35
109 104 2 35
.
240260280300320 Wavelength (nm)
240260280300320 Wavelength (nm)
Fig. 3. Temperature dependence of polarized Uy absorption spectra of h4SlO. Angles of incident lights are 0” (normal to the substrate) (a) and 30” (b), respectively.
b E
c 0
1S,!I
I
I
I
20
40
60
I
80 T (“Cl
I
I
I
100
120
Fig. 2. Different temperature dependencies of photocurrent in the phases C, SB and I of MSlO. The incident light is fixed at 500nm. The other conditions are the same as in Fig. 1. electric field vector (in a polarized light plane) and substrate surface. Consequently, if transition dipoles in molecules align perpendicular to the substrate, the absorbance at O3 should be smaller than that at 30”. 3
Results
and discussion
Figure 1 shows the photocurrent spectra of capillaryfilled MS10 with a carrier generation layer of gray Se, with a bias electric field of 5 x lo3 I’/cm and an incident photon flux of 1 Y 10’2photons/mm2s. The spectra were measured at ‘3°C (in the C phase), at 90°C (in the SB phase) and ai, 12O’C (in the I phase). Each spectrum has a very broad peak corresponding to an absorption spectrum of gral Se. Since these spectral profiles were almost temperatureindependent, the gray Se was stable within the temperat,ure range used and electronic interaction between Se and MS10 may not be thermally enhanced. At a fixed wavelength of 500nm, the temperature dependence of the photocurrent was measured as displayed in Fig. 2. In the C and I phases, the positive temperature dependence of the photocurrent indicates that, the carrier transport, is cont,rolIed by thermally activated hopping processes. On the other hand, in the SB phase, unusual negative dependence was observed. Some change of structure in the SB phase seems to affect the photocurrent. To investigate the structural change induced by heating, polarized UV absorption spectra were measured. In Fig. 3, temperature dependent spectra are displayed with two dif-
ferent incident light, angles of O0 [a) and 30” (b). The spectrum (a) had a peak at BOnm in the C phase (at 35°C). Upon heating, in the SB phase, the peak once disappeared ai 84*C and a broadened peak again became significant at 109’C. The peak was stable in the 1 phase (from 11G to ~~ 13O“C). On the other hand. the spect,ra (L) showed quite different temperature dependence. The peak at Z3012m observed in the C phase wasbroadened in the SB phase but never disappeared upon heating. These spectral changes, especially in the Sg phase, can bc interpreted as follows: average orientation of rod-like hiSlO molectiles, which are parallel to transition dipoles, is normal to the substrate in the SB phase at lower temperature; such orientational order significantly decreases upon heating even in the SB phase. This temperature dependent orientation explains the negative temperature dependence of photocurrent in the Sg phase. The normal orientation of 31SlO molecules to the substrate, which means molecules are parallel to the applied bias field, enhances carrier transport due to increase in ~ contribution of intra-molecular (not inter-molecular) conduction. Thus ihe decrease of orientational order in the Sg phase upon heating causes the decrease of photocurrent. In conclusion, we have shown that the molecular orientation of MS10 considerably affects photocurrent. Photoconductivity in mesophases of a-conjugated molecules is a new field of interest to elucidate their carrier t,ransport mechanism. References [I] R. G. Kepler, J. M. Zeigler, L. A. Harrah, Eiurtz, Phys. Rev. B 35, ‘7818 (1987).
and S. R.
[?I P. Trefonas In, R. West, R. D. AIiller, and D. Hofer, J. Polym. Sci., Polym. Lett. Ed. 21, 523 (lj83). [3] H. Okumoto, M. Shimo_niura, N. hlinami, and Y. Tanabe, Solid State Commun. 104, 131 (1997). [4] T. Yatabe, A. f&to, (1997).
and Y. Tanabe, Chem. Lett. 799
[5] D. Adam et ni., Nature 371, 141 (1991). [6] M. Funabashi and J. Hanna, Phys. Rev. Lett. 78, 2184 (1997).
Synthetic Metals 101 (1999) 461-462
3,lolecular
orientation
dependent
photoconductivit,y
R. Okumoto, T. Yatabe, M. Shimomura, Dfpnrtmcnt
of Polymer
4. Iiaito,
of liquid
crystalline
oligosilanes
N. Minami: Y. Tanabe and Chemical Research,
Physics, h’ational Institute of Materials 1-i Hignshi, Tsukuba, Ibaraki 305-856.5:
AIST
Japan
Abstract Polysilanes are known to be promising hole transport materials among conducting polymers. Their structure can be Recently an oligosilane (Me(SiMea)isMe) is found to show a better controlled by using single dispersed oligomers. This mesophase (SB). In the mesophase, we have observed unusual negative temperature dependence of photocurrent. can be explained by temperature dependent molecular orientation, which has been confirmed by polarized UV absorption spectroscopy. licywords: Photoconductivity, UV-Vis-NIR absorption, Liquid mers, Organic semiconductors based on conjugated molecules
1
Introduction
n-conjugated electrons in polysilanes have attracted much attention due to their unique properties compared with rr-conjugated electrons. The o-conjugation along saturated bonds gives rise to its characteristic photoconductivity [l], phot~oreactivity [2] and coupling of these two effect,s [3]. With photoconductivity, the high hole mobility compared with other n-conjugated conducting polymers has been reported [I], which makes polysilanes promising hole conductors. To improve the hole mobility, controlling the structure of samples should be crucial. In polysilanes, however, unavoidable dispersion of molecular weight may cause st,ructural disorder when processing films. On the other hand, single dispersed oligosilanes are expected to reduce such uncertainty. In fact, films of oligosilanes have well-defined structures. Recently, a mesophase of linear oligosilanes has been discovered in a permethyloligosilane Me(SiMez)ieMe (MSlOj [a]. Its phase diagram is as follows: 114oc
82Oc C
F
‘B 74oc
where C, Se and I indicate a crystalline phase, a mesophase of Smectic I3 and an isotropic phase, respectively. The SB phase has a multi-layer structure with molecular long axes perpendicular to the layer planes. Moreover, G-fold rotational symmetry exists within the planes. For molecules with r-conjugated electrons, high carrier mobility in mesophases has been reported: in a discotic hexagonal phase [5] and in a smectic A phase [G]. Thus controlling the order of molecules in mesophases is proved to be efficient for im-
crystalline
phase transitions,
Other conducting
poly-
proving photoconductivity. In this paper, we will report the temperature dependence of photocurrent in MS10 capillary-filled films. To focus on the transport mechanism in MSlO, carrier generation layer of selenium (Se) has been employed. We found that, in the Sn phase, the photocurrent has unusual temperature dependence that can be attributed to a change in the molecular orientation. We also studied the effect of the orientation in terms of polarized LTV absorption spectroscopy. 2
Experimental
MS10 was synthesized from a mixture of l-chlorotrisilane and 1,3-dichlorotrisilane with sodium in toluene and purified by preparative HPLC [q]. For photocurrent measurements, MS10 powder was melted and capillary-filled at 120°C into a gap (10pm) between two quartz substrates coated with indium-tin oxide (ITO) electrodes. Se was vacuum-deposited beforehand onto an IT0 electrode as a carrier generation layer. Since as deposited Se was in amorphous form and structurally unstable, the substrate was pre-heated above SO”C, which transformed Se into the most stable state (gray Se). After capillary-filling, the sample cells were annealed at 120°C for 1 hour and cooled to room Throughout photocurtemperature at a rate of l’C/min. rent measurements, the temperature of samples was controlled within a stability of O.Z’C by a controller. Orientation of molecules was monitored by polarized UV absorption spect,roscopy using a spectrophotometer with a temperature controlled sample holder. Incident angles of polarized light beam were changed from O0 (normal to a substrate) to 30°, which corresponds to an angle between an
0379-6779/99/$- seefront matter 0 1999 Elsevier ScienceS.A. All rights reserved. PII: so379-6779(98)01198-9
H. Okumoto et al. i Synthetic Metals 101 (1999) 461-462
462
(“C)
- (% 5
800
600 Wavelength
(nm)
Fig. 1. Temperature dependence of photocurrent spectra in capillary-filled MS10 with a carrier generation layer of gray Se. [E=5 x 103V/c,m, photon flux=1 x lO”photonsl~mm’s)
116
5
116
$
109
5
104
3 jj Q
94 84 35
109 104 2 35
.
240260280300320 Wavelength (nm)
240260280300320 Wavelength (nm)
Fig. 3. Temperature dependence of polarized Uy absorption spectra of h4SlO. Angles of incident lights are 0” (normal to the substrate) (a) and 30” (b), respectively.
b E
c 0
1S,!I
I
I
I
20
40
60
I
80 T (“Cl
I
I
I
100
120
Fig. 2. Different temperature dependencies of photocurrent in the phases C, SB and I of MSlO. The incident light is fixed at 500nm. The other conditions are the same as in Fig. 1. electric field vector (in a polarized light plane) and substrate surface. Consequently, if transition dipoles in molecules align perpendicular to the substrate, the absorbance at O3 should be smaller than that at 30”. 3
Results
and discussion
Figure 1 shows the photocurrent spectra of capillaryfilled MS10 with a carrier generation layer of gray Se, with a bias electric field of 5 x lo3 I’/cm and an incident photon flux of 1 Y 10’2photons/mm2s. The spectra were measured at ‘3°C (in the C phase), at 90°C (in the SB phase) and ai, 12O’C (in the I phase). Each spectrum has a very broad peak corresponding to an absorption spectrum of gral Se. Since these spectral profiles were almost temperatureindependent, the gray Se was stable within the temperat,ure range used and electronic interaction between Se and MS10 may not be thermally enhanced. At a fixed wavelength of 500nm, the temperature dependence of the photocurrent was measured as displayed in Fig. 2. In the C and I phases, the positive temperature dependence of the photocurrent indicates that, the carrier transport, is cont,rolIed by thermally activated hopping processes. On the other hand, in the SB phase, unusual negative dependence was observed. Some change of structure in the SB phase seems to affect the photocurrent. To investigate the structural change induced by heating, polarized UV absorption spectra were measured. In Fig. 3, temperature dependent spectra are displayed with two dif-
ferent incident light, angles of O0 [a) and 30” (b). The spectrum (a) had a peak at BOnm in the C phase (at 35°C). Upon heating, in the SB phase, the peak once disappeared ai 84*C and a broadened peak again became significant at 109’C. The peak was stable in the 1 phase (from 11G to ~~ 13O“C). On the other hand. the spect,ra (L) showed quite different temperature dependence. The peak at Z3012m observed in the C phase wasbroadened in the SB phase but never disappeared upon heating. These spectral changes, especially in the Sg phase, can bc interpreted as follows: average orientation of rod-like hiSlO molectiles, which are parallel to transition dipoles, is normal to the substrate in the SB phase at lower temperature; such orientational order significantly decreases upon heating even in the SB phase. This temperature dependent orientation explains the negative temperature dependence of photocurrent in the Sg phase. The normal orientation of 31SlO molecules to the substrate, which means molecules are parallel to the applied bias field, enhances carrier transport due to increase in ~ contribution of intra-molecular (not inter-molecular) conduction. Thus ihe decrease of orientational order in the Sg phase upon heating causes the decrease of photocurrent. In conclusion, we have shown that the molecular orientation of MS10 considerably affects photocurrent. Photoconductivity in mesophases of a-conjugated molecules is a new field of interest to elucidate their carrier t,ransport mechanism. References [I] R. G. Kepler, J. M. Zeigler, L. A. Harrah, Eiurtz, Phys. Rev. B 35, ‘7818 (1987).
and S. R.
[?I P. Trefonas In, R. West, R. D. AIiller, and D. Hofer, J. Polym. Sci., Polym. Lett. Ed. 21, 523 (lj83). [3] H. Okumoto, M. Shimo_niura, N. hlinami, and Y. Tanabe, Solid State Commun. 104, 131 (1997). [4] T. Yatabe, A. f&to, (1997).
and Y. Tanabe, Chem. Lett. 799
[5] D. Adam et ni., Nature 371, 141 (1991). [6] M. Funabashi and J. Hanna, Phys. Rev. Lett. 78, 2184 (1997).