The Persistent Photoconductivity (PPC) effect was studied in individual tin oxide (SnO 2 ) nanobelts as a function of temperature, in air, helium, and vacuum atmospheres, and low temperature Photoluminescence measurements were carried out to study the optical transitions and to determine of the acceptor/donors levels and their best representation inside the band gap. Under ultraviolet (UV) illumination and at temperatures in the range of 200 to 400K we observed a fast and strong enhancement of the photoconductivity, and the maximum value of the photocurrent induced increases as the temperature or the oxygen concentration decreases. By turning off the UV illumination the induced photocurrent decays with lifetimes up to several hours. The photoconductivity and the PPC results were explained by adsorption and desorption of molecular oxygen at the surface of the SnO 2 nanobelts. Based on the temperature dependence of the PPC decay an activation energy of 230 meV was found, which corresponds to the energy necessary for thermal ionization of free holes from acceptor levels to the valence band, in agreement with the photoluminescence results presented. The molecular-oxygen recombination with holes is the origin of the PPC effect in metal oxide semiconductors, so that, the PPC effect is not related to the oxygen vacancies, as commonly presented in the literature.

In this work, we investigated structural, morphological, electrical, and optical properties from a set of Cu 2 ZnSnS 4 thin films grown by sulfurization of metallic precursors deposited on soda lime glass substrates coated with or without molybdenum. X-ray diffraction and Raman spectroscopy measurements revealed the formation of single-phase Cu 2 ZnSnS 4 thin films. A good crystallinity and grain compactness of the film was found by scanning electron microscopy. The grown films are poor in copper and rich in zinc, which is a composition close to that of the Cu 2 ZnSnS 4 solar cells with best reported efficiency. Electrical conductivity and Hall effect measurements showed a high doping level and a strong compensation. The temperature dependence of the free hole concentration showed that the films are nondegenerate. Photoluminescence spectroscopy showed an asymmetric broadband emission. The experimental behavior with increasing excitation power or temperature cannot be explained by donor-acceptor pair transitions. A model of radiative recombination of an electron with a hole bound to an acceptor level, broadened by potential fluctuations of the valence-band edge, was proposed. An ionization energy for the acceptor level in the range 29-40 meV was estimated, and a value of 172 ± 2 meV was obtained for the potential fluctuation in the valence-band edge.

The temperature dependence of electrical conductivity and the photoconductivity of polycrystalline Cu 2 ZnSnS 4 were investigated. It was found that at high temperatures the electrical conductivity was dominated by band conduction and nearest-neighbour hopping. However, at lower temperatures, both Mott variable-range hopping (VRH) and Efros-Shklovskii VRH were observed. The analysis of electrical transport showed high doping levels and a large compensation ratio, demonstrating large degree of disorder in Cu 2 ZnSnS 4 . Photoconductivity studies showed the presence of a persistent photoconductivity effect with decay time increasing with temperature, due to the presence of random local potential fluctuations in the Cu 2 ZnSnS 4 thin film. These random local potential fluctuations cannot be attributed to grain boundaries but to the large disorder in Cu 2 ZnSnS 4 .the CZTS absorber layer and the Mo back contact have been pointed out as factors that reduce the solar cell efficiency [4,5]. Strong research efforts have been made in the study of different CZTS synthesis routes [3][4][5][6][7][8][9][10], CZTS structural and optical properties [2,[6][7][8], as well as in the development of CZTS solar cells [3-5, 9, 10]. However, very little is known about the electrical transport and photoconductivity properties of CZTS thin films and further studies are required.The study of the temperature dependence of electrical transport in semiconductor materials is very important for the knowledge of material parameters, the understanding of the transport and microscopic mechanisms of charge transfer, as well as for the optimization and design of solar cells. Very recently, we have shown that the radiative and non-radiative recombination mechanisms in CZTS thin films are strongly

We report the results of a study of the sulphurization time effects on Cu2ZnSnS4 absorbers and thin film solar cells prepared from dc-sputtered stacked metallic precursors. Three different time intervals, 10 min, 30 min and 60 min, at maximum sulphurization temperature were considered. The effects of this parameter' change were studied both on the absorber layer properties and on the final solar cell performance. The composition, structure, morphology and thicknesses of the CZTS layers were analysed. The electrical characterization of the absorber layer was carried out by measuring the transversal electrical resistance of the samples as a function of temperature. This study shows an increase of the conductivity activation energy from 10 meV to 54 meV for increasing sulphurization time from 10 min to 60 min. The solar cells were built with the following structure: SLG/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni:Al grid. Several ac response equivalent circuit models were tested to fit impedance measurements. The best results were used to extract the device series and shunt resistances and capacitances. Absorber layer's electronic properties were also determined using the Mott-Schottky method. The results show a decrease of the average acceptor doping density and built-in voltage, from 2.0×10 17 cm −3 to 6.5×10 15 cm −3 and from 0.71 V to 0.51 V, respectively, with increasing sulphurization time. This results also show an increase of the depletion region width from approximately 90 nm to 250 nm.

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