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Partial carbonization of aromatic polyimide films—aiming at solar cells
Materials Science and Engineering B99 (2003) 131 /133 www.elsevier.com/locate/mseb
Partial carbonization of aromatic polyimide films* aiming at solar cells /
M. Doyama *, Y. Takahashi, H. Abe, Y. Inoue, Y. Kogure, T. Nozaki, S. Yamada Teikyo University of Science and Technology, Uenohara, Yamanashi 409-0193, Japan Received 14 June 2002; received in revised form 2 October 2002; accepted 21 October 2002
Abstract Aromatic polyimide films are partially carbonized between 700 and 1000 8C in an atmosphere of hydrogen. Electrical conductivity and Hall coefficient have been measured. Electrical conductivity increases with measuring temperatures. The electrical conductivity s can be expressed as s/s0 exp(/Es /k T), where k is the Boltzmann constant.T is the measuring temperature in K . E depends upon the carbonized temperature. The experimental data show the Hall coefficient RH is negative, and this implies the majority carriers are negatively charged, i.e. electrons. Thus the specimens are n-type semiconductors. The carrier density h can be expressed by h/Ah exp (/Eh /kT ) and carrier mobility m can be expressed by m/Am exp(Em /kT ). Es , Eh and Em depend upon the carbonized temperature. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Aromatic polyimide; Carbonization of polyimide; Semiconductors; p-Type semiconductors; Hall coefficient; Electrical conductivity
1. Introduction Development of silicon in electronic industries has been tremendously growing in the 20th century. It is natural to consider carbon, which is also in the same column in the periodic table as silicon, to be a candidate of an electronic material. In general industrial carbon is made from pitch. Such carbon normally contains much of impurities. We have chosen aromatic polyimide as a raw material and partially carbonized it and studied its electrical properties. It is known that an abrupt release of oxygen associated with remarkable weight loss and shrinkage occurred over a rather narrow temperature range of 550 /650 8C in Kapton (trade name of DuPont Co.). Above 700 8C, nitrogen and the remaining oxygen were released gradually above 700 8C in Kapton [1]. On X-ray powder pattern of carbonized film suggests amorphous carbon with carbonization temperature below 1000 8C. A graph of Spain’s paper [2] suggests
* Corresponding author. Tel.: /81-554-63-4411; fax: 81-554-634431 E-mail address: [email protected] (M. Doyama).
that a Kapton film becomes amorphous carbon with a band gap when carbonized below 1000 8C. Aromatic polyimide Upilex-S (trade name of the Ube Industries Ltd.) shows similar properties but occurs at higher temperature.
2. Experiment Upilex-S films (trade name of the Ube Industries Ltd.) of 25 mm were cut into 60/25 mm. The films were sandwiched between high purity graphite blocks and bound by a wire. The assembly was put in a fused quartz tube, hydrogen gas was flown and heated in a split furnace. After the temperature was reached to a carbonization temperature, it was kept constant for 30 min. The heating current was cut and the gas was continuously flown until the furnace temperature decreased to 200 8C. Gold was sputtered on the film as the electrodes. Aluminum foils were pasted on the gold sputtered electrodes by silver solder. The carbonized polyimide foil and the electrodes were sandwiched with insulating boards. Heaters were mounted near the specimen. The whole assembly was cooled by ice water
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 5 2 8 - 7
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M. Doyama et al. / Materials Science and Engineering B99 (2003) 131 /133
passing through a copper pipe. An alumel/chromel thermocouple was set in the assembly. The four point method was used to avoid the contact voltage and the leak for the voltmeter. All the readings were automatically taken by a personal computer. The assembly was sandwiched with a pair of Sa /Co magnets. The Hall voltage was measured by a multi-meter.
3. Results and discussions The polyimide films were carbonized between 700 and 1000 8C. The electrical conductivity s can be expressed as ss0 exp(Es =kT )
(1)
where k is the Boltzmann’s constant. T is the absolute temperature. s0 and Es were found to be a function of the carbonized temperature. Natural logarithm of s versus T1 is plotted in Fig. 1. The values of s is also shown. The logarithm of the activation energy Es was plotted against the reciprocal of the carbonizing temperature Tc in Fig. 2. Here the activation energy was found to be 0.93 eV. The activation energy Es ? is the activation energy for the carbonization including many processes.
Fig. 2. Es was plotted against the reciprocal of the carbonizing temperature Tc
The Hall coefficient RH is given by RH /Ey /(Bz Jx ), where Ey , Bz , and Jx are electric field in the y direction, magnetic field in the z direction and electric current density in the x direction, respectively. x, y , and z are rectangular coordinates. The experimental data show RH is negative, and this implies the carriers are negatively charged, i.e. electrons. The specimens are ntype semiconductors. The carrier density h is given by h /(je jRH)1 and the mobility m is m/s/(h je j), where jej is the absolute value of the electron charge and s is the electrical conductivity. For Upilex S, fitting h / Ah exp(/Eh /kT ) and m/Am exp(Em /kT ). For specimens carbonized between 700 and 1000 8C, natural logarithm of h is plotted against the reciprocal of the measuring temperature in Fig. 3. Natural logarithm of Eh is plotted against the reciprocal of the carbonized temperature in Fig. 4. One finds Eh /0.80 eV. The natural logarithm of m versus reciprocal measuring temperature for specimens carbonized at 750 and 800 8C are plotted in Fig. 5. Preliminary experiments show that positive Hall coefficients were obtained in partially carbonized Upilex S with boron [3]. This implies majority carriers are positive holes in this partially carbonized polyimide The upilex S as received from Ube Industries contains phosphorus, so that the partially carbonized Upilex S shows negative Hall coefficient. This suggests that the p /n junction films made by carbon can be used as solar cells in the future dream.
4. Conclusions
Fig. 1. ln s in Eq. (1) versus reciprocal of measuring temperature for Upiles S, carbonized temperatures are also shown.
The polyimide films containing phosphorous are partially carbonized between 700 and 1000 8C. The partially carbonized polyimide is amorphous and an n type donor. The band gap depends on the carbonized temperature. When the carbonized temperature is
M. Doyama et al. / Materials Science and Engineering B99 (2003) 131 /133
133
Fig. 5. Plot of ln m versus the reciprocal of measuring temperature for Upilex S carbonized at 750 and 800 8C.
higher, the band gap between the impurity level and the conduction band is smaller. Further investigation is useful.
Acknowledgements Fig. 3. Plot of ln h versus the reciprocal of the measuring temperature for Upilex S.
The research is partially supported by a Grant-in-Aid for the Advanced Materials Research Center at Teikyo University of Science and Technology (TUST) supported by the Ministry of Education, Sports and Culture and Science and Technology, TUST and Ube Industries, Ltd. We are particularly thankful to Messrs. R. Sato and S. Hashiguchi at Ube Industries, Ltd. Samariumcobalt magnets were donated by Epson Co. and the authors are thankful to T. Shimoda for his kind arrangemnent.
References
Fig. 4. Plot of ln h versus the reciprocal of carbonized temperature for Upilex S.
[1] M. Inagaki, S. Harada, T. Sato, T. Nakajima, Y. Horino, K. Morita, Carbon 27 (1989) 253. [2] I.L. Spain, in: P.L. Walker, P.A. Thrower (Eds.), Chemistry and Physics of Carbon, vol. 16, Marcel Dekker, New York, 1982, pp. 119 /322. [3] M. Doyama, Y. Takahashi, H. Abe, T. Nozaki, Y. Inoue, Y. Kogure, S. Yamada, to be published.
Partial carbonization of aromatic polyimide films* aiming at solar cells /
M. Doyama *, Y. Takahashi, H. Abe, Y. Inoue, Y. Kogure, T. Nozaki, S. Yamada Teikyo University of Science and Technology, Uenohara, Yamanashi 409-0193, Japan Received 14 June 2002; received in revised form 2 October 2002; accepted 21 October 2002
Abstract Aromatic polyimide films are partially carbonized between 700 and 1000 8C in an atmosphere of hydrogen. Electrical conductivity and Hall coefficient have been measured. Electrical conductivity increases with measuring temperatures. The electrical conductivity s can be expressed as s/s0 exp(/Es /k T), where k is the Boltzmann constant.T is the measuring temperature in K . E depends upon the carbonized temperature. The experimental data show the Hall coefficient RH is negative, and this implies the majority carriers are negatively charged, i.e. electrons. Thus the specimens are n-type semiconductors. The carrier density h can be expressed by h/Ah exp (/Eh /kT ) and carrier mobility m can be expressed by m/Am exp(Em /kT ). Es , Eh and Em depend upon the carbonized temperature. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Aromatic polyimide; Carbonization of polyimide; Semiconductors; p-Type semiconductors; Hall coefficient; Electrical conductivity
1. Introduction Development of silicon in electronic industries has been tremendously growing in the 20th century. It is natural to consider carbon, which is also in the same column in the periodic table as silicon, to be a candidate of an electronic material. In general industrial carbon is made from pitch. Such carbon normally contains much of impurities. We have chosen aromatic polyimide as a raw material and partially carbonized it and studied its electrical properties. It is known that an abrupt release of oxygen associated with remarkable weight loss and shrinkage occurred over a rather narrow temperature range of 550 /650 8C in Kapton (trade name of DuPont Co.). Above 700 8C, nitrogen and the remaining oxygen were released gradually above 700 8C in Kapton [1]. On X-ray powder pattern of carbonized film suggests amorphous carbon with carbonization temperature below 1000 8C. A graph of Spain’s paper [2] suggests
* Corresponding author. Tel.: /81-554-63-4411; fax: 81-554-634431 E-mail address: [email protected] (M. Doyama).
that a Kapton film becomes amorphous carbon with a band gap when carbonized below 1000 8C. Aromatic polyimide Upilex-S (trade name of the Ube Industries Ltd.) shows similar properties but occurs at higher temperature.
2. Experiment Upilex-S films (trade name of the Ube Industries Ltd.) of 25 mm were cut into 60/25 mm. The films were sandwiched between high purity graphite blocks and bound by a wire. The assembly was put in a fused quartz tube, hydrogen gas was flown and heated in a split furnace. After the temperature was reached to a carbonization temperature, it was kept constant for 30 min. The heating current was cut and the gas was continuously flown until the furnace temperature decreased to 200 8C. Gold was sputtered on the film as the electrodes. Aluminum foils were pasted on the gold sputtered electrodes by silver solder. The carbonized polyimide foil and the electrodes were sandwiched with insulating boards. Heaters were mounted near the specimen. The whole assembly was cooled by ice water
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 5 2 8 - 7
132
M. Doyama et al. / Materials Science and Engineering B99 (2003) 131 /133
passing through a copper pipe. An alumel/chromel thermocouple was set in the assembly. The four point method was used to avoid the contact voltage and the leak for the voltmeter. All the readings were automatically taken by a personal computer. The assembly was sandwiched with a pair of Sa /Co magnets. The Hall voltage was measured by a multi-meter.
3. Results and discussions The polyimide films were carbonized between 700 and 1000 8C. The electrical conductivity s can be expressed as ss0 exp(Es =kT )
(1)
where k is the Boltzmann’s constant. T is the absolute temperature. s0 and Es were found to be a function of the carbonized temperature. Natural logarithm of s versus T1 is plotted in Fig. 1. The values of s is also shown. The logarithm of the activation energy Es was plotted against the reciprocal of the carbonizing temperature Tc in Fig. 2. Here the activation energy was found to be 0.93 eV. The activation energy Es ? is the activation energy for the carbonization including many processes.
Fig. 2. Es was plotted against the reciprocal of the carbonizing temperature Tc
The Hall coefficient RH is given by RH /Ey /(Bz Jx ), where Ey , Bz , and Jx are electric field in the y direction, magnetic field in the z direction and electric current density in the x direction, respectively. x, y , and z are rectangular coordinates. The experimental data show RH is negative, and this implies the carriers are negatively charged, i.e. electrons. The specimens are ntype semiconductors. The carrier density h is given by h /(je jRH)1 and the mobility m is m/s/(h je j), where jej is the absolute value of the electron charge and s is the electrical conductivity. For Upilex S, fitting h / Ah exp(/Eh /kT ) and m/Am exp(Em /kT ). For specimens carbonized between 700 and 1000 8C, natural logarithm of h is plotted against the reciprocal of the measuring temperature in Fig. 3. Natural logarithm of Eh is plotted against the reciprocal of the carbonized temperature in Fig. 4. One finds Eh /0.80 eV. The natural logarithm of m versus reciprocal measuring temperature for specimens carbonized at 750 and 800 8C are plotted in Fig. 5. Preliminary experiments show that positive Hall coefficients were obtained in partially carbonized Upilex S with boron [3]. This implies majority carriers are positive holes in this partially carbonized polyimide The upilex S as received from Ube Industries contains phosphorus, so that the partially carbonized Upilex S shows negative Hall coefficient. This suggests that the p /n junction films made by carbon can be used as solar cells in the future dream.
4. Conclusions
Fig. 1. ln s in Eq. (1) versus reciprocal of measuring temperature for Upiles S, carbonized temperatures are also shown.
The polyimide films containing phosphorous are partially carbonized between 700 and 1000 8C. The partially carbonized polyimide is amorphous and an n type donor. The band gap depends on the carbonized temperature. When the carbonized temperature is
M. Doyama et al. / Materials Science and Engineering B99 (2003) 131 /133
133
Fig. 5. Plot of ln m versus the reciprocal of measuring temperature for Upilex S carbonized at 750 and 800 8C.
higher, the band gap between the impurity level and the conduction band is smaller. Further investigation is useful.
Acknowledgements Fig. 3. Plot of ln h versus the reciprocal of the measuring temperature for Upilex S.
The research is partially supported by a Grant-in-Aid for the Advanced Materials Research Center at Teikyo University of Science and Technology (TUST) supported by the Ministry of Education, Sports and Culture and Science and Technology, TUST and Ube Industries, Ltd. We are particularly thankful to Messrs. R. Sato and S. Hashiguchi at Ube Industries, Ltd. Samariumcobalt magnets were donated by Epson Co. and the authors are thankful to T. Shimoda for his kind arrangemnent.
References
Fig. 4. Plot of ln h versus the reciprocal of carbonized temperature for Upilex S.
[1] M. Inagaki, S. Harada, T. Sato, T. Nakajima, Y. Horino, K. Morita, Carbon 27 (1989) 253. [2] I.L. Spain, in: P.L. Walker, P.A. Thrower (Eds.), Chemistry and Physics of Carbon, vol. 16, Marcel Dekker, New York, 1982, pp. 119 /322. [3] M. Doyama, Y. Takahashi, H. Abe, T. Nozaki, Y. Inoue, Y. Kogure, S. Yamada, to be published.