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Author: Lidia Ghimpu Degree: doctor of physics and mathematics Speciality:  01.04.10 - Semiconductors physics and engineering Year: 2006 Scientific adviser: Petru Gaşin doctor habilitat, professor, Moldova State University Scientific consultant: Veaceslav Ursachi doctor habilitat, associate professor (docent), Institute of Applied Physics of the Institution:

Status The thesis was presented on the 5 April, 2006 Approved by NCAA on the 29 June, 2006 Abstract – 0.92 Mb / in romanian Thesis CZU 621.315.592 5.97 Mb / in romanian 162 pages

Keywords

Summary

The thesis presents the results of investigation of the electrical and photoelectrical properties of CdSe-ZnTe and CdS-CdTe thin film heterojunctions (HJ).

CdSe-ZnTe heterojunctions were fabricated by successive deposition of CdSe layers using the close space sublimation technique, while ZnTe layers were prepared using the flash method on the glass substrates covered with conductive (~10~3 Q-cm) and transparent (~80%) SnOi layers. CdS-CdTe he teroj onctions were manufactured by deposition of CdS and CdTe layers using the close space sublimation technique. The thin film layers were polycrystalline and the carrier concentration at the temperature of 300 К was equal to (2-8)-1017cm"3 for CdSe, 1018cm"3 for ZnTe, 1016cm"3 for CdS and (1014-1015) cm"3 for CdTe. To increase photosensitivity, the CdS and CdTe layers were chemically treated in CdCl2:CH3OH saturated solution and then thermally treated at (380-400) °C during 35-45 min. This treatment results in the increase of the photosensitivity of CdSe and CdTe layers by a factor of 80 and 10-14, respectively.

The transport mechanism in CdS-CdTe and CdSe-ZnTe heterojunctions is determined by generation-recombination and by tunneling-recombination processes respectively. For both heterojunctions, the transport mechanism under the reverse bias is determined by multistep tunneling process.

The photovoltaic parameters of CdSe-ZnTe heterojunctions under the illumination with integrated light with the power of 54 mW/cm2 are the following: Isc= 4-6 mA/cm2, Uoc=0.3-0.4 V, т] = 4%. The photosensitivity of CdSe-ZnTe HJ corresponds to the region of 0.5-0.9 jam. The spectral distribution of photosensitivity strongly depends on the thickness of CdSe and ZnTe components. For high-sensitive CdSe-based structures, the generation of the electron-hole pairs in CdSe is predominant.

The light and dark current-voltage characteristics of CdS-CdTe HJ intersect due to the high photoconduction of the materials. The Isc = f(Uoc) dependence is exponential and has the same shape as the dependence of dark VCC. The light current-voltage characteristic exhibit an unusual behavior at forward bias (with an inflexion) due to the potential barrier between the CdS material and Ni-contact. The deposition of a thin layer of tellurium between CdTe and Ni leads to the improvement of characteristics. The treatment of samples in different media (CdCb, МпСЬ and CsCb) proves that the medium does not influence on the inflexion and therefore confirms the existents of the interface barrier. Optimal photovoltaic parameters were obtained for CdS-CdTe HJ due to CdCb chemical treatment. The open circuit voltage decreases with increasing temperature. The thermal coefficient of the open circuit voltage is equal to 2.1 mW/K. The short-circuit current slightly depends on the temperature. CdS-CdTe HJ is characterized by a constant photosensitivity in the wavelength region of 0.51-0.9 цт and the quantum efficiency is 0.8 for the maximum photosensitivity. The diffusion length, calculated from the long-wavelength decay of the photosensitivity curve, is equal to 1.4 цт. The following parameters are obtained under the integral light illumination with the power of 100 mW/cm2 at 300 K: Isc= (18-21) mA/cm2, Uoc = (0.8-0.82) V, FF= (0.56-0.6), the efficiency is around (9-10)%. For the confirmation of the obtained results the solar batteries samples with an electric power of 0.45 W and 0.80 W were fabricated.

The originality of the presented results was confirmed by patents MD № 1653, MD № 1308 and the positive decision No. from 15th June, 2005.

Oficial Reviewers

• Eugen Gheorghiţă
  doctor habilitat, professor, Tiraspol State University
• Victoria Trofimov
  doctor, associate professor (docent), Moldova State University

 Theses