Elaboration of the deposition technology and the investigation of the electrical and photoelectrical properties of CdS/CdTe heteroj unctions
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StatusThe thesis was presented on the 9 June, 2006Approved by NCAA on the 28 September, 2006 Abstract – 1.48 Mb / in romanianThesisCZU 621.315.592
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Keywords
cadmium sulphide, cadmium telluride, thin films, heterojunction, solar cell, current flow mechanism, photoluminescence, optical absorption, optical transmission, reflection, modulation spectroscopy, photocurrent spectral distribution, annealing, lifetime, photocurrent relaxationSummary
This thesis is related to the elaboration of manufacturing technology of CdS/CdTe thin film heteroj unctions and their properties study. Thin film CdS/CdTe heteroj unctions were fabricated by using the close spaced sublimation method, onto SnO2 covered glass plates (2x2 cm2). The investigations of the source and substrates temperatures influence on the morphology and electrophysical properties of CdS and CdTe thin films, has been studied. The optimum source and substrate temperatures have been determined as follows: for CdS Tsource=640°C, TSUbStrate=4450C, for CdTe TSOurce=550°C, Tsubstrate=445°C. The elements distribution in CdS and CdTe thin films are presented. It has been established, that Sn and Si atoms are presented in CdS thin films along with basic elements (Cd and S) due to diffusion processes. For to enhance CdS/CdTe heteroj unctions photosensitivity, it has been annealed (390-420°C) in presence of CdCl2 for 15-60 min.
As it came from the analysis of current-voltage dependencies in the 78-370K temperature range, the current flow mechanism in CdS/CdTe heteroj unction is determined by the tunneling processes through dislocations, which penetrate the space charge region. The concentration of dislocations has been estimated as 2-105 cm"2. The multistep tunnelling processes through local centres in the space charge region predominate at reverse biases.
The investigations of the photoluminescence spectra, optical absorption and reflection have revealed the presence of a layer consisting of CdSxTei_x solid solutions (x=0,03-0,05), which is formed at the heteroj unction interface, after its annealing in the presence of CdCl2. The appearance of the excitonic maxima in the annealed heteroj unctions indicates to an enhancement of the crystalline structure and the decrease of the defects concentration at the heteroj unction interface.
The study of the kinetics of CdTe thin films photoconductivity revealed a 20 meV impurity level, formed as a result of heteroj unction annealing in presence of CdCl2, which leads to the increase of the nonequilibrium charge carriers lifetime up to 200 us.
Thin film CdS/CdTe heteroj unctions annealed in presence of CdCl2 have a steady photosensitivity at 300K in the 0,52-0,83 цт spectral region. The photosensitivity spectrum at low temperatures (78K) is determined by the generation of electron-hole pairs in CdTe layer. As the intensity of integral illumination increases, the open-circuit voltage (Ucd) tends to saturation and the short-circuit current (Isc) increases linearly and at 80 mW/cm2 Ucd=0,74 V, Isc=22,3 mA/cm2, the fill factor is 0,43, and the efficiency - 8,8%.
The main results were published in 30 scientific papers.
The thesis is written in Romanian and consists of 112 text pages, 116 figures, 9 tables and 148 references.
Oficial Reviewers
- Teodor Şişianu
doctor habilitat, professor - Leonid Culiuc
doctor habilitat, professor, Institute of Applied Physics of the
Theses
