Sol-Gel processing64 is a low temperature chemical route for the production of inorganic oxide materials. The process can be used to produce a wide range of single and multi- component oxides in crystalline or amorphous (glass) form. The process can be used to
Some of the earliest literature relating to the Sol-Gel method appears in 1943 in the form of a patent by Gefficken and Berger74 on oxide coatings, particularly silica. The first major industrial use of Sol-Gel appeared in 1959 for the volume production of coated rear view mirrors for the automotive industry, the system being T i02-Si02-T i0274. An important review appeared in 1969 by Schroeder75 which explains oxide layers deposited from organic solutions. Schroeder also reviews the deposition techniques available to the Sol-Gel technologist, and the production of both single and multioxide coatings. By 1970, dip-coating had been applied by Schroeder64 to glasses in order to modify their optical properties. From this time Sol-Gel was fully recognised as a viable way of producing inorganic materials, and in particular metal oxides. The last decade has seen intensive interest in Sol-Gel technology, due to possibilities for engineering the properties of glassy and ceramic materials.
Sol-Gel offers many attractive advantages over conventional ways of producing oxide materials. These include the possibility of varying the film properties extensively by changing the composition of the solution (to produce change in film microstructure) and a relatively low process cost. In addition Sol-Gel overcomes the difficulties of producing a high quality dielectric-semiconductor interface, and obtaining a stoichiometric ratio of elements and molecular homogeneity in multicomponent oxide films76. There are many forms in which the oxide gel product can be produced. These include powders, coatings, monoliths and fibres63,64,76 which often require very low processing temperatures. The possibility of the use of high purity starting materials and the ease with which large and complex shaped substrates can be coated has meant that this technique is becoming increasingly attractive to optoelectronic specialists. The need for low-cost thin film
production processes has increased the interest in Sol-Gel (and other non-vacuum deposition techniques) capable of deposition at high rates and over larger areas.
2.7 Summary
Solar power has not become the major alternative energy source it was once anticipated to be. This is mainly due to the high material and processing costs associated with conventional PV cells. While there is considerable research effort into the development of amorphous silicon (a-Si), and cadmium technologies, there is an interest to develop PV cells constructed using less traditional materials and methods that may offer a cost effective alternative.
MPcs are organic dyes that are sensitive to various regions of the visible spectrum and have therefore been intensively investigated for PV applications. Added to this they are chemically and thermally stable, abundant, cheap and easy to fabricate. But as PV cells they have not reached the required high conversion efficiencies. This limitation is due to the materials’ poor ability to collect and transport charge carriers.
Recent research has emphasised the potential of using organic dyes to spectrally sensitise wide band gap semiconductors for use in PV applications, an approach which offers relatively low cost materials and fabrication methods. It is anticipated that with the addition of the MPc sensitising dyes; ClAlPc, CuPc, and PbPc the absorption of light in the PV cell is modified to the dyes spectral sensitive region resulting in a much broader range of the solar spectrum being utilised therefore in contrast to traditional PV cells the
photoeffect (or production of photogenerated carriers) does not take place inside the semiconductor but in the sensitising dye on its surface.
MPcs are organic based macromolecules which have potential as the dye species, they are inherently p-type semiconductors with an energy gap of ~2eV. It is expected that the photoinduced electrons created in the MPc layer are transferred to the conduction band of the inorganic semiconductor (Ti02), which has a higher mobility, a larger energy gap and exhibits no photoconduction sensitivity over most of the visible region.
Although it is the principal intention of the author to investigate the feasibility of spectrally sensitising Sol-Gel derived T i02, the constituent materials from which the devices are constructed are optically, structurally and electrically characterised. This will assist in reproducibility and future optimisation. It is recognised that Sol-Gel process parameters have a major influence on factors such as refractive index and structure of Sol-Gel products. The full extent to which Sol-Gel processing parameters interact to affect the properties of T i0 2 films are not known, and could have serious implications for the PV cell performance.
It is hoped that this work will help in the identification of the most appropriate dyes and demonstrate the versatility and advantages of using Sol-Gel technology in the manufacture of the T i02 transporting layer.
REFERENCES
1. Moller, H. J. Semiconductors for Solar Cells, Artech House, 1993.
2. Chapin, D. M., Fuller, C. S., and Pearson, G. L. J. App. Phys. 25 (1954) 676. 3. Raynolds, D. C. Phys. Rev. 96 (1954)
4. Coutts, T. J. Thin Solid Films. 50 (1978) 99-117.
5. Whitlock, J. B., Panayotatos P., Sharma, G. D., Cox, M. D., Sauers, R. R., and Bird, G. R. Opt. Eng. 32(8) (1993) 1921-1933.
6. Das, S. K., and Morris, G. C. J. Appl. Phys. 72(10) (1992) 4940. 7. Green, M. A. Prog. Photovoltaics: Res. appl. 2 (1994) 87-94.
8. Al-Mohammad, A., and Soulieh, M. Thin Solid Films. 271 (1995) 132-137.
9. Takada, J., Awaji, H., Koshioka, M., Nevin, W. A., Imanishi, M., and Fukada, N. J. Appl. Phys. 75(8) (1994) 4055.
10. Yanagi, H., Kataura, H., and Yasukiyo, U. J. Appl. Phys. 75(1) (1994) 568. 11. O’Regan, B., and Gratzel, M. Nature. 353 (1991) 737-740.
12. Simon, J., and Andre, J.J. Molecular Semiconductors, Springer, 1984. 13. Linstead, R. P. J. Chem. Soc. (1934) 1016, 1031.
14. Robertson, J. M. J. Chem. Soc. (1935) 613.
15. Petty, M. C. Polymer Surfaces and Interfaces. John Wiley & Sons, 1987, p. 173. 16. Vincett, P. S., Popovic, Z. D., and McIntyre, L. Thin Solid Films 82 (1981) 357. 17. Vartanyan, A. T. Zh. Fiz. Khim., 22 (1948) 769.
18. Sussman, A. J. Appl. Phys. 38 (1967) 2738-2752. 19. Antohe, S. Rev. Roum. Phys. 37(3) (1992) 309-313.
22. Frauenheim, T. H., Hamann, C., and Muller, M. Phys. Stat. Sol. (a) 86 (1984) 735. 23. Wynne, K. A. Inorg. Chem. 23 (1984) 4658.
24. Yanagi, H., Imamura, M., and Ashida, M. J. Appl Phys. 75(4) (1994) 2061. 25. Hamann, C. and Mrwa, A. Int. J. Elec. 73(5) (1992) 1039-1040.
26. Machida, Y., Saito, Y., Taomoto, A., Nichogi, K., Waragai, K„ and Asakawa, S.
Jpn. J. Appl. Phys. 28(2) (1989) 297-298.
27. Hassan, A. K., and Gould, R. D. Phys. Stat. Sol (a)132 (1992) 91-101. 28. Yanagi, H. J. Appl. Phys. 96 (1992) 1366.
29. Schechtman, B. H., and Spicer, W. E. J. Mole. Spec. 33 (1970) 28-48. 30. Chadderton, L. T. J. Phys. Chem. Solids. 24 (1963) 751-757.
31. Zhang, H., Pan, D., and Fang, J. Proc. 3rd. Int. Conf. Properties and Applications of Dielectric Materials. July 8-12 (1991) 647.
32. Day, P., and Williams, J. P. J. Chem. Phys. 37(3) (1962) 567. 33. Davidson, A. T. J. Chem. Phys. 77(1) (1982) 168
34. Loutfy, R. O., Hor, A. M., and Rucklldge, A. J. Imag. Sci. 31 (1987) 31-37. 35. Yoneyama, M., Sugi, M., Saito, M., Ikegami, K., Kuroda, S., and Iizima, S. Jap. J.
Appl. Phys. 25(7) (1986) 961-965.
36. Verzimacha, Y. I., Kovalchuk, A. V., Kurik, M. V., Hamann, C., and Mrwa, A.
Phys. Stat. Sol. (a) 82 (1984) K i l l .
37. Yanagi, H., Imamura, M., and Ashida, M. J. Appl. Phys. 75(4) (1994) 2061-2068. 38. Hamann, C. Phys. Stat. Sol. (a) 10 (1972) 83-90.
39. Gould, R. D. Thin Solid Films. 125 (1985) 63-69.
40. Hayashi, S., Kozaru, K, and Yamamoto, K. Sol Stat. Comms. 79(9) (1991) 763-767. 41. Dahne, S. J. Imag. Sci. Tech. 38(2) (1994) 101-117.
43. Gurney, R. W. and Mott, N. F. Proc. Roy. Soc. A164 (1938) 151-167. 44. West, W., and Gilman, P. B. Photo. Sci. Eng. 13(5) (1969) 221-230.
45. Sakata, T., Hashimoto, K., and Hiramoto, M. J. Phys. Chem. 94 (1990) 3040-3045. 46. Harima, Y., Yamashita, K., and Suzuki, H. Appl. Phys. Letts. 45(10) (1984) 1144-
1145.
47. Tang, C. W.Appl. Phys. Letts. 48(2) (1986) 183-185.
48. Shen, Y., Wang, L., Lu, Z., Wei, Y., Zhou, Q., Mao, H., and Xu, H. Thin Solid Films. 257(1995) 144-146.
49. Borenberger, P. M. J. Appl. Phys. 62(7) (1987) 2942-2945
50. Kudo, K., and Moriizumi, T. Jap. J. Appl. Phys. 19(11) (1980) L683-L685.
51. Hiramoto, M., Fujiwara, H., and Yokoyama, M. J. Appl. Phys. 72(8) (1992) 3781 - 3787.
52. Knodler, R., Sopka, J., Harbach, F., and Grunling, H. W. Solar Energy Materials and Solar Cells. 30 (1993) 277-281.
53. Giraudeau, A. J. Am. Chem. Soc. 102(16) (1980) 5137.
54. Korsunovskii, G. A. Translated from: Elektrokhimiya, 19(5) (1983) 615-620. 55. Green, M. A., and Emery, K. Progress in Photovoltaics: Research, and
Applications.2 (1994) 27-34.
56. Komp, R. J. Practical Photovoltaics: Electricity from Solar Cells.2nd edition, Actec, (1989).
57. Calderon, W. IEEE Potentials. Feb/Mar (1995) 13-16. 58. Cronemeyer, D. C. Phy. Rev. 87(5) (1952) 876-886.
59. Lee, W. G., Woo, S. I., Kim, J. C., Choi, S. H., and Oh, K. H. Thin Solid Films. 237 (1994)105-111.
60. Huusko, J., Lantto, V., and Torvela, H. Sensors and Actuators. B. 15-16 (1993) 245-248.
61. Cantao, M. P., Cisneros, J. I., and Torresi, R. M. Thin Solid Films 259 (1995) VO- 74.
62. Sorek, Y., Reisfeld, R., Finkelstein, I., and Ruschin, S. Appl. Phys. Letts. 63(24) (1993) 3256-3258.
63. Anderson, M. A., Gieselmann, M. J. and Xu, Q. J. Memb. Sci. 39 (1988) 243-258. 64. Brinker, C. J., and Scherer, G. W. Sol-Gel Science, Academic Press, New York
(1990).
65. Takahashi, Y., and Matsuoka, Y. J. Mat. Sci.23 (1988) 2259-2266. 66. Vorotilov, K. A. Thin Solid Films. 207 (1992) 180-184.
67.Feuersanger, A. E. Proc. IEEE. Dec (1964) 1463-1465. 68. Ardakani, H. IC. Thin Solid Films. 248 (1994) 234-239.
69. Leinen, D., Espinos, J. P., and Gonzalez-Elipe, A. R. J. Vac. Sci. Technol. A. 12(5)
(1994) 2728-2732.
70. Chuu, D. S., and Dai, C. M. Phys. Rev. B. 45(20) (1992) 805.
71. Forro. L., Chauvet, O., Emin, D., Zuppiroli, L., Berger, H., and Levy, F. J. Appl. Phys. 75(1) (1994) 633-635.
72. Yoko, T., Yuasa, A., Kamiya, K., and Sakka, S. J. Electrochem. Soc. 138(8) (1991) 2279-2285.
73. Konenkamp, R., and Henninger, R. Appl. Phys. A 58 (1994) 87-90. 74. Dislich, H., and Hinz, P. J. J. Non-Cryst-Solids. 48 (1982) 11-16.
75. Shroeder, H., Physics o f Thin Films. 5 Academic Press New-York (1969) 87. 76. Jones, R. W. Fundamental Principles ofSol-Gel technology Institute of Metals