Effect of RF Power on the Physical Properties of Sputtered ZnSe Nanostructured Thin Films for Photovoltaic Applications

Zinc selenide (ZnSe) thin films were deposited by RF magnetron sputtering in specific conditions, onto optical glass substrates, at different RF plasma power. The prepared ZnSe layers were afterwards subjected to a series of structural, morphological, optical and electrical characterizations. The obtained results pointed out the optimal sputtering conditions to obtain ZnSe films of excellent quality, especially in terms of better optical properties, lower superficial roughness, reduced micro-strain and a band gap value closer to the one reported for the ZnSe bulk semiconducting material. Electrical characterization were afterwards carried out by measuring the current–voltage (I-V) characteristics at room temperature, of prepared “sandwich”-like Au/ZnSe/Au structures. The analysis of I-V characteristics have shown that at low injection levels there is an Ohmic conduction, followed at high injection levels, after a well-defined transition voltage, by a Space Charge Limited Current (SCLC) in the presence of an exponential trap distribution in the band gap of the ZnSe thin films. The results obtained from all the characterization techniques presented, demonstrated thus the potential of ZnSe thin films sputtered under optimized RF plasma conditions, to be used as alternative environmentally-friendly Cd-free window layers within photovoltaic cells manufacturing.

Electrical Conduction Mechanisms in n-CdS/p-CuFeO2 Heterojunction Diode

In this work, n-CdS/p-CuFeO2 heterojunction diode was fabricated by thermal evaporating CdS thin films on 1 mm thick-CuFeO2 ceramic substrate with substrate temperature kept at 373 K during evaporation process. The forward current-voltage characteristics of n-CdS/p-CuFeO2 heterojunction in a temperature range of 100-300 K were investigated to determine the electrical parameters and conduction mechanism. It was found that, at forward bias below 0.5 V, the conduction mechanism of the diode is dominated by thermionic emission (TE) mechanism. At bias voltage above 0.5 V, the current transport is due to space charge limited current (SCLC) controlled by an exponential trap distribution in the band gap of CdS. The temperature dependence of the saturation current and ideality factor are well described by tunneling enhanced recombination at junction interface with activation and characteristic tunneling energy values as about 1.79 eV and E00 = 86 meV, respectively. The value of interface state density (Nss) evaluated from capacitance spectroscopy increases from 2.09x1011 eV-1cm-2 (at 300 K) to 2.70x1011 eV-1cm-2 (at 363 K). Free carrier concentration of 5.80x1013 cm-3 at room temperature was estimated from capacitance-voltage measurements at 50 kHz.

SCLC theory for a Gaussian trap distribution

A simplified one-carrier space charge limited conduction theory has been developed for a Gaussian trap distribution in. an insulator. It is shown that the distribution behaves much like a discrete trap level at low applied fields, although the current has a temperature dependent activation energy. At higher fields the current-voltage relation becomes superquadratic, but this does not account for the behaviour normally attributed to an exponential trap distribution.

Phosphorescence and electron traps II. The interpretation of long-period phosphorescence

This paper is concerned with the measurement and theory of long-period phosphorescence in solids. The delayed emission is, in such cases, due to the time electrons spend in traps or metastable states before returning to the luminescence centres. The theory shows how the observed decay laws are related to the trap distributions found in Paper I. In alkaline earth phosphors there are equal numbers of traps at all depths over the range considered, and the expression developed shows that the intensity of phosphorescence should be inversely proportional to time. For zinc sulphide phosphors an exponential trap distribution has been found, and this leads to a simple inverse power law for the theoretical decay curve. Measurements of decay on these phosphors utilizing an electron multiplier have confirmed the theory, and put on a quantitative basis the connexion between thermoluminescence and long-period decay. At the same time the work provides the first satisfactory and detailed explanation of longperiod decays. The extent -to which retrapping of electrons modifies the picture given is discussed.