Hopping conduction and persistent photoconductivity in Cu2ZnSnS4 thin films

The temperature dependence of electrical conductivity and the photoconductivity of polycrystalline Cu2ZnSnS4 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 Cu2ZnSnS4. 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 Cu2ZnSnS4 thin film. These random local potential fluctuations cannot be attributed to grain boundaries but to the large disorder in Cu2ZnSnS4.

Admittance spectroscopy of Cu2ZnSnS4 based thin film solar cells

In this report, we propose an AC response equivalent circuit model to describe the admittance measurements of Cu2ZnSnS4 thin film solar cell grown by sulphurization of stacked metallic precursors. This circuit describes the contact resistances, the back contact, and the heterojunction with two trap levels. The study of the back contact resistance allowed the estimation of a back contact barrier of 246 meV. The analysis of the trap series with varying temperature revealed defect activation energies of 45 meV and 113 meV. The solar cell’s electrical parameters were obtained from the J-V curve: conversion efficiency, 1.21%; fill factor, 50%; open circuit voltage, 360 mV; and short circuit current density, 6.8 mA/cm2.

A study of ternary Cu2SnS3 and Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors

Thin films of Cu2SnS3 and Cu3SnS4 were grown by sulfurization of dc magnetron sputtered Sn–Cu metallic precursors in a S2 atmosphere. Different maximum sulfurization temperatures were tested which allowed the study of the Cu2SnS3 phase changes. For a temperature of 350 ◦C the films were composed of tetragonal (I -42m) Cu2SnS3. The films sulfurized at a maximum temperature of 400 ◦C presented a cubic (F-43m) Cu2SnS3 phase. On increasing the temperature up to 520 ◦C, the Sn content of the layer decreased and orthorhombic (Pmn21) Cu3SnS4 was formed. The phase identification and structural analysis were performed using x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analysis. Raman scattering analysis was also performed and a comparison with XRD and EBSD data allowed the assignment of peaks at 336 and 351 cm−1 for tetragonal Cu2SnS3, 303 and 355 cm−1 for cubic Cu2SnS3, and 318, 348 and 295 cm−1 for the Cu3SnS4 phase. Compositional analysis was done using energy dispersive spectroscopy and induced coupled plasma analysis. Scanning electron microscopy was used to study the morphology of the layers. Transmittance and reflectance measurements permitted the estimation of absorbance and band gap. These ternary compounds present a high absorbance value close to 104 cm−1. The estimated band gap energy was 1.35 eV for tetragonal (I -42m) Cu2SnS3, 0.96 eV for cubic (F-43m) Cu2SnS3 and 1.60 eV for orthorhombic (Pmn21) Cu3SnS4. A hot point probe was used for the determination of semiconductor conductivity type. The results show that all the samples are p-type semiconductors. A four-point probe was used to obtain the resistivity of these samples. The resistivities for tetragonal Cu2SnS3, cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4 are 4.59 × 10−2 cm, 1.26 × 10−2 cm, 7.40 × 10−4 cm, respectively.

Mo bilayer for thin film photovoltaics revisited

Thin film solar cells based on Cu(In,Ga)Se2 as an absorber layer use Mo as the back contact. This metal is widely used in research and in industry but despite this, there are only a few published studies on the properties of Mo. Properties such as low resistivity and good adhesion to soda lime glass are hard to obtain at the same time. These properties are dependent on the deposition conditions and are associated with the overall stress state of the film. In this report, a study of the deposition of a Mo bilayer is carried out by analysing first single and then bilayers. The best properties of the bilayer were achieved when the bottom layer was deposited at 10 × 10−3 mbar with a thickness of 500 nm and the top layer deposited at 1 × 10−3 mbar with a thickness of 300 nm. The films deposited under these conditions showed good adhesion and a sheet resistivity lower than 0.8 .

Bifacial dye-sensitized solar cells: a strategy to enhance overall efficiency based on transparent polyaniline electrode

Dye-sensitized solar cell (DSSC) is a promising solution to global energy and environmental problems because of its clean, low-cost, high efficiency, good durability, and easy fabrication. However, enhancing the efficiency of the DSSC still is an important issue. Here we devise a bifacial DSSC based on a transparent polyaniline (PANI) counter electrode (CE). Owing to the sunlight irradiation simultaneously from the front and the rear sides, more dye molecules are excited and more carriers are generated, which results in the enhancement of short-circuit current density and therefore overall conversion efficiency. The photoelectric properties of PANI can be improved by modifying with 4-aminothiophenol (4-ATP). The bifacial DSSC with 4-ATP/PANI CE achieves a light-to-electric energy conversion efficiency of 8.35%, which is increased by ,24.6% compared to the DSSC irradiated from the front only. This new concept along with promising results provides a new approach for enhancing the photovoltaic performances of solar cells.

Using VISIR in a large undergraduate course: preliminary assessment results

The use of remote labs in undergraduate courses has been reported in literature several times since the mid 90's. Nevertheless, very few articles present results about the learning gains obtained by students using them, especially with a large number of students, thus suggesting a lack of data concerning their pedagogical effectiveness. This paper addresses such a gap by presenting some preliminary results concerning the use of a remote laboratory, known as VISIR, in a large undergraduate course on Applied Physics, with over 500 students enrolled.