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Photothermal deflection spectroscopy of a poIy(Para-Phenylene)-type ladder polymer
ELSEVIER
Photothermal
Synthrtic Metals 84 (1997) 651-652
deflection spectroscopy of a Poly(Para-Phenylene)-Type
Ladder
Polymer
M. Moser, S. Tasch and G. Leising Institut ftir Fes&rperphysik, Technische Universitat Graz,Petersgasse l~,~~S~tIlOGraz, Austria
Abstract Photothermal Deflection Spectroscopy (PDS) is briefly described. We report on the absorption coefficient of methyl-substituted poly(para-phenylene),-type ladder polymer and the hyperlinear luminescence of m-LPPP. With these qualities we estimate a laser effect of m-LPPP. A combined curve from PDS and UV-VIS is shown and the resolution compared with a standard UV-VIS measurement. Keywords: Optical absorption and emission spectroscopy: Photothemal deflection spectroscopy
1. Introduction Photothermal deflection spectroscopy (PDS) is a technique to characterize the subbandgap absorption of semiconductors. It is commonly used to determine the optoelectronic quality of inorganic semiconductors “’ for various applications like e.g. solar cells. PDS is of special use for thin, rough, semi-transparent films, because it only detects the amount of absorbed light and not the part of the probe beam which is diffusive scattered by the rough surface. With PDS we measure the absorption versus the wavelength. Because the range of the measured cxd is between 1 and IO-” , where CY.is the absorption coefficient and d the filmthickness, the absorption coefficient should be low in the examined spectral region. PDS is a relative method, this means it is not possible to determine the absolute value of the absorption coefficient even if one knows the sample thickness. To determine the absolute value of the absorption coefficient one needs to perform standard transmission and reflection uv-VIS measurement to calibrate the PDS data. With a set of samples of different thickness and by combining the result curves one could measure the absorption coefficient over a large wavelength region very precisely. The same is possible by combining the PDS data with UV-VIS. The achieved data are of special use to determine unwanted electronic defects (doping,...) in our samples. To build a polymer laser one needs a material with a very low absorption coefficient in the emitting wavelength region with a high photoluminescent quantum yield and a spectral separation of triplet-triplet absorption from the photoluminecence emission such as m-LPPP. To determine the very low absorption coefficient of m-LPPP in the region around 495nm different methods where used such as wave guiding ‘*’ and PDS. Both methods revealed that the coefficient is very low (cl2 cm-‘). mLPPP shows a hyperlinear behavior of its luminescence at high illumination intensities13’. The luminescence is also not limited by saturation effects, which is essential for laser application, 0379-6779197B17.00 0 1997 Elsevier Science S.A All rights reserved PII SO379-6779(96yMO92-1
Outical
Setup
- probebeam (He-Ne laser) . . .. .__.__..... ~~~::........._..____._._.__._ b sensor 2 in detail: :::: :: , segment B :: :: probe laser beam :: -FT :: segment A
pGG-t
pump beam
C-L-L
i
optical
table
: :
: :
:
:
a--
chopper
Fig. 1. Optical setup for PDS measurements
2. Experimental
Setup
The sensor 1 in Fig. 1 is used to correct the intensity of the pump beam. The chopper is mounted off the table to keep the setup vibration free.
652
M. Moser
1.0
1.5
2.0
2.5
3.0
et al. /SyntheticMetals
3.5
4. Conclusion
Fig. 2 The absorption coefficient ~1of m-LPPP versus energy
3. Measurements In Figure 2 the absorption coefficient of m-LPPP determined by transmission and reflection measurements is shown. The dark area represents the error bars of the absolute value for a. The circled cross at 1.55eV shows a result for ~1 obtained from waveguiding experiments. To measure the absorption near 495nm a method with a high resolution like PDS is needed.
s x
.z z s .5
300
400
500
600
700
800
10
10
1
1
& 93 0,l Eg, 0.01
2s
6.51-652
The thickness of the sample is about 10pm which means that the resolution of the measurement is about 1m6. Because of the high purity of the m-LPPP the subgap absorption is below the detection limit of PDS. m-LPPP has also no vibrational transitions in the subbandgap area as reported for other materials (e.g. polycarbonate r4’), With PDS measurements it is also possible, by looking at the steepness of the absorption edge, to identify unwanted doping, which could occur during the preparation of the sample. The very low absorption of m-LPPP below the absorption edge is essential for laser application, because even small absorption losses can have a disastrous effect on potential laser action since the optical path through the active material is very long.
energy (eV)
2
84 (1997)
O,l
Photothermal deflection spectroscopy is a very sensitive method to measure low absorption coefficients. It is not influenced by scattering or reflection and does only measure the true absorption. With PDS it is possible to measure various types of defects in thin semi transparent films. With samples varied in thickness one can measure over a large range of the absorption coefficient and determine low subbandgap absorption coefficients. Photothermal deflection spectroscopy is a relative method with a possible rang of ad from 1 to l0’h where u is the absorption coefficient and d the sample thickness. With PDS one can quickly estimate if a new sample meets quality standards, by checking the steepness of the change of the absorption coefficient and the subbandgap region. m-LPPP has a very low absorption below the absorption edge. In the region around 495nm a laser effect of m-LPPP is predicted15’, because of the hyperlinear behavior of the luminescence and the low absorption. M-LPPP has such low absorption below the absorption edge that it is not possible to measure the absorption coefficient even with PDS.
0,Ol
IE-3
1E-3
$ lE-4 2 300
1E-4 400
2 Fig. 3. Absorption
500
600
700
800
wavelength [nm] spectrum of high purity m-LPPP
In Figure 3 the absorption edge of m-LPPP at room temperature is shown. The PDS data are combined with data acquired with a standard UV-VIS measurement. The data are normalized to the saturation of PDS. The PDS data are averaged over three measurement cycles and corrected with the pump beam intensity in the absorbing region below 500nm. The noise in the PDS measurement is due to electronic noise sources and independent of the pump beam intensity. The PDS datapoints (solid line, Fig.3), below 475nm are not reliable due to the saturation effect. The saturation effect limits the measurable ad , the limitation depends on the setup but is generally around one.
Acknowledgments I want to thank MStutzmann from Munich for his help and his advices during the time I was building my PDS setup.
References [l] E. Sauvaine, A. Mettler, N. Wyrsch, A. Shah, Solid State Comm. Vol. 85, No. 3 (1993) [2] JSturm, S.Tasch, A.Niko, GLeising, E.Toussaere, J.Zyss, T. Kowalszyk, KSinger, U.Scherf, J.Huber, subm. to Thin Solid Films. [3] G.Kranzelbinder, H.Byrne, S.Hallstein, S.Roth, G.Leising, these proceedings. [4] A. Skumanich, J.C. Scott, Mol. Cryst. Liq. Cryst. Vol. 183 (1990) [.5] W.Graupner, GLeising, G.Lanzani, M.Nisoli, S. De Silvestri and UScherf, Physical Review Letters, Vol.76, Nr.5 (1996)
Photothermal
Synthrtic Metals 84 (1997) 651-652
deflection spectroscopy of a Poly(Para-Phenylene)-Type
Ladder
Polymer
M. Moser, S. Tasch and G. Leising Institut ftir Fes&rperphysik, Technische Universitat Graz,Petersgasse l~,~~S~tIlOGraz, Austria
Abstract Photothermal Deflection Spectroscopy (PDS) is briefly described. We report on the absorption coefficient of methyl-substituted poly(para-phenylene),-type ladder polymer and the hyperlinear luminescence of m-LPPP. With these qualities we estimate a laser effect of m-LPPP. A combined curve from PDS and UV-VIS is shown and the resolution compared with a standard UV-VIS measurement. Keywords: Optical absorption and emission spectroscopy: Photothemal deflection spectroscopy
1. Introduction Photothermal deflection spectroscopy (PDS) is a technique to characterize the subbandgap absorption of semiconductors. It is commonly used to determine the optoelectronic quality of inorganic semiconductors “’ for various applications like e.g. solar cells. PDS is of special use for thin, rough, semi-transparent films, because it only detects the amount of absorbed light and not the part of the probe beam which is diffusive scattered by the rough surface. With PDS we measure the absorption versus the wavelength. Because the range of the measured cxd is between 1 and IO-” , where CY.is the absorption coefficient and d the filmthickness, the absorption coefficient should be low in the examined spectral region. PDS is a relative method, this means it is not possible to determine the absolute value of the absorption coefficient even if one knows the sample thickness. To determine the absolute value of the absorption coefficient one needs to perform standard transmission and reflection uv-VIS measurement to calibrate the PDS data. With a set of samples of different thickness and by combining the result curves one could measure the absorption coefficient over a large wavelength region very precisely. The same is possible by combining the PDS data with UV-VIS. The achieved data are of special use to determine unwanted electronic defects (doping,...) in our samples. To build a polymer laser one needs a material with a very low absorption coefficient in the emitting wavelength region with a high photoluminescent quantum yield and a spectral separation of triplet-triplet absorption from the photoluminecence emission such as m-LPPP. To determine the very low absorption coefficient of m-LPPP in the region around 495nm different methods where used such as wave guiding ‘*’ and PDS. Both methods revealed that the coefficient is very low (cl2 cm-‘). mLPPP shows a hyperlinear behavior of its luminescence at high illumination intensities13’. The luminescence is also not limited by saturation effects, which is essential for laser application, 0379-6779197B17.00 0 1997 Elsevier Science S.A All rights reserved PII SO379-6779(96yMO92-1
Outical
Setup
- probebeam (He-Ne laser) . . .. .__.__..... ~~~::........._..____._._.__._ b sensor 2 in detail: :::: :: , segment B :: :: probe laser beam :: -FT :: segment A
pGG-t
pump beam
C-L-L
i
optical
table
: :
: :
:
:
a--
chopper
Fig. 1. Optical setup for PDS measurements
2. Experimental
Setup
The sensor 1 in Fig. 1 is used to correct the intensity of the pump beam. The chopper is mounted off the table to keep the setup vibration free.
652
M. Moser
1.0
1.5
2.0
2.5
3.0
et al. /SyntheticMetals
3.5
4. Conclusion
Fig. 2 The absorption coefficient ~1of m-LPPP versus energy
3. Measurements In Figure 2 the absorption coefficient of m-LPPP determined by transmission and reflection measurements is shown. The dark area represents the error bars of the absolute value for a. The circled cross at 1.55eV shows a result for ~1 obtained from waveguiding experiments. To measure the absorption near 495nm a method with a high resolution like PDS is needed.
s x
.z z s .5
300
400
500
600
700
800
10
10
1
1
& 93 0,l Eg, 0.01
2s
6.51-652
The thickness of the sample is about 10pm which means that the resolution of the measurement is about 1m6. Because of the high purity of the m-LPPP the subgap absorption is below the detection limit of PDS. m-LPPP has also no vibrational transitions in the subbandgap area as reported for other materials (e.g. polycarbonate r4’), With PDS measurements it is also possible, by looking at the steepness of the absorption edge, to identify unwanted doping, which could occur during the preparation of the sample. The very low absorption of m-LPPP below the absorption edge is essential for laser application, because even small absorption losses can have a disastrous effect on potential laser action since the optical path through the active material is very long.
energy (eV)
2
84 (1997)
O,l
Photothermal deflection spectroscopy is a very sensitive method to measure low absorption coefficients. It is not influenced by scattering or reflection and does only measure the true absorption. With PDS it is possible to measure various types of defects in thin semi transparent films. With samples varied in thickness one can measure over a large range of the absorption coefficient and determine low subbandgap absorption coefficients. Photothermal deflection spectroscopy is a relative method with a possible rang of ad from 1 to l0’h where u is the absorption coefficient and d the sample thickness. With PDS one can quickly estimate if a new sample meets quality standards, by checking the steepness of the change of the absorption coefficient and the subbandgap region. m-LPPP has a very low absorption below the absorption edge. In the region around 495nm a laser effect of m-LPPP is predicted15’, because of the hyperlinear behavior of the luminescence and the low absorption. M-LPPP has such low absorption below the absorption edge that it is not possible to measure the absorption coefficient even with PDS.
0,Ol
IE-3
1E-3
$ lE-4 2 300
1E-4 400
2 Fig. 3. Absorption
500
600
700
800
wavelength [nm] spectrum of high purity m-LPPP
In Figure 3 the absorption edge of m-LPPP at room temperature is shown. The PDS data are combined with data acquired with a standard UV-VIS measurement. The data are normalized to the saturation of PDS. The PDS data are averaged over three measurement cycles and corrected with the pump beam intensity in the absorbing region below 500nm. The noise in the PDS measurement is due to electronic noise sources and independent of the pump beam intensity. The PDS datapoints (solid line, Fig.3), below 475nm are not reliable due to the saturation effect. The saturation effect limits the measurable ad , the limitation depends on the setup but is generally around one.
Acknowledgments I want to thank MStutzmann from Munich for his help and his advices during the time I was building my PDS setup.
References [l] E. Sauvaine, A. Mettler, N. Wyrsch, A. Shah, Solid State Comm. Vol. 85, No. 3 (1993) [2] JSturm, S.Tasch, A.Niko, GLeising, E.Toussaere, J.Zyss, T. Kowalszyk, KSinger, U.Scherf, J.Huber, subm. to Thin Solid Films. [3] G.Kranzelbinder, H.Byrne, S.Hallstein, S.Roth, G.Leising, these proceedings. [4] A. Skumanich, J.C. Scott, Mol. Cryst. Liq. Cryst. Vol. 183 (1990) [.5] W.Graupner, GLeising, G.Lanzani, M.Nisoli, S. De Silvestri and UScherf, Physical Review Letters, Vol.76, Nr.5 (1996)