O. Vigil-Galán

Development of a Selective Chemical Etch To Improve the Conversion Efficiency of Zn-Rich Cu2ZnSnS4 Solar Cells

Improvement of the efficiency of Cu(2)ZnSnS(4) (CZTS)-based solar cells requires the development of specific procedures to remove or avoid the formation of detrimental secondary phases. The presence of these phases is favored by the Zn-rich and Cu-poor conditions that are required to obtain device-grade layers. We have developed a selective chemical etching process based on the use of hydrochloric acid solutions to remove Zn-rich secondary phases from the CZTS film surface, which are partly responsible for the deterioration of the series resistance of the cells and, as a consequence, the conversion efficiency. Using this approach, we have obtained CZTS-based devices with 5.2% efficiency, which is nearly twice that of the devices we have prepared without this etching process.

SnS-based thin film solar cells: perspectives over the last 25 years

New types of thin film solar cells made from earth-abundant, non-toxic materials and with adequate physical properties such as band-gap energy, large absorption coefficient and p-type conductivity are needed in order to replace the current technology based on CuInGaSe2 and CdTe absorber materials, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). The constituent elements of the SnS film are abundant in the earth’s crust, and non-toxic. If this compound is used as the absorber layer in solar cells, high efficient devices should be fabricated with relative low cost technologies. Despite these properties, low efficiency SnS-based solar cells have been reported up to now. In this work, we present a review about the state of the art of SnS films and devices. Finally, an analysis about different factors that are limiting high efficiency solar cells is presented.

Physical properties of Bi2Te3 and Sb2Te3 films deposited by close space vapor transport

Near-stoichiometric bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) films were grown by the close space vapor transport (CSVT) method in a relatively simple fashion. The dependence of the film properties on the substrate temperature was explored over a wide range by keeping the source-to-substrate thermal gradient fixed at 300 °C and 350 °C for the Bi2Te3 and Sb2Te3 film growth, respectively. A Seebeck coefficient (α) of 255 µV K−1 and a power factor of 20.5 × 10−4 W K−2 m−1 were obtained for Bi2Te3 at a substrate temperature of 350 °C; meanwhile, the maxima of the above parameters for the antimony telluride films (108 µV K−1 and 3.8 × 10−4 W K−2 m−1, respectively) were reached at a substrate temperature of 450 °C.

Improving the efficiency of CdS/CdTe solar cells by varying the thiourea/CdCl2 ratio in the CdS chemical bath

In this work, the influence of the properties of CdS thin films grown by chemical bath deposition upon the characteristics of CdS/CdTe solar cells, when varying the thiourea concentrations in the CdS bath solution, is studied. The important solar cell parameters such as short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency (η) were measured and it was noted that they improve for thiourea/CdCl2 ratios (in the CdS deposition solution) up to 5, and drop for higher ratios in the range investigated. In addition, the ideal diode factor (n), saturation current (Jo) and series resistence (Rs) were studied under illumination and dark conditions. The results could be related to several factors, among them, the photoconductivity properties of the layers and the change of the CdS1−xTex layer at the CdS–CdTe interface.

Toward a high Cu2ZnSnS4 solar cell efficiency processed by spray pyrolysis method

In this work, a review about the influence of the growth parameters on the chemical and physical properties of Cu2ZnSnS4 (CZTS) deposited by pneumatic spray pyrolysis technique and its impact on the thin film solar cells is presented and analyzed in order to identify the major drawbacks of this technique and the possibility to improve the device efficiency. Our best solar cell using sprayed CZTS shows an open-circuit voltage of 361 mV, a short-circuit current density of 7.5 mA/cm2, a fill factor of 0.37, and an efficiency of 1% under irradiation of AM 1.5 and 100 mW/cm2. Some of the key mechanisms related to the properties of sprayed CZTS layers, as well as those concerning the solar cells mechanisms that limit the cell performance, are also analyzed.

Trap and recombination centers study in sprayed Cu2ZnSnS4 thin films

In this work, a study of trap and recombination center properties in polycrystalline Cu2ZnSnS4 thin films is carried out in order to understand the poor performance in Cu2ZnSnS4 thin film solar cells. Thermally stimulated current has been studied in Cu2ZnSnS4 deposited by pneumatic spray pyrolysis method using various heating rates, in order to gain information about trap centers and/or deep levels present within the band-gap of this material. A set of temperature-dependent current curves revealed three levels with activation energy of 126 ± 10, 476 ± 25, and 1100 ± 100 meV. The possible nature of the three levels found is presented, in which the first one is likely to be related to CuZn antisites, while second and third to Sn vacancies and SnCu antisites, respectively. The values of frequency factor, capture cross section, and trap concentration have been determined for each center.

Influence of the growth conditions in the properties of the CdTe thin films deposited by CSVT

We present in the results of this work, the influence of oxygen and the gradient of temperature in the processing of CdTe thin films, by using the close space vapor transport (CSVT) technique. In general the kinetic process is strongly influenced by these parameters; where a reduction in the growth rate, grain size; an increase in the intergrain barrier height; and structural changes are present.

Passivation properties of CdS thin films grown by chemical bath deposition on GaSb: the influence of the S/Cd ratio in the solution and of the CdS layer thickness on the surface recombination velocity

Evidences of the passivation effect are given when thin films of CdS are deposited on GaSb crystalline substrates, using a bath chemical deposition method. The passivation process is studied through photoacoustic and photoluminescence experiments. The surface recombination velocity calculated from photoacoustic measurements decreases and the radiative recombination rate as measured from photoluminescence spectra increases when the nominal S/Cd ratio in the layer deposition solution increases. The influence of the CdS layer thickness on the surface passivation of GaSb is also studied.

Influence of the growth conditions on the photoluminescence spectrum of CdTe polycrystalline films deposited by the close space vapor transport technique

A study of the influence of some growth parameters, the gradient of temperature between source and substrate and introduction of oxygen in the growth chamber, on the optical properties of CdTe thin films grown by Close Space Vapor Transport (CSVT) is presented. This study was carried out by means of photoluminescence (PL). The combined effect of the temperature gradient and oxygen in the growth chamber on the crystalline quality of CdTe thin films were analyzed. Important changes in the photoluminescence spectra were observed. A decrease of the intensity of the excitonic PL band is observed, as oxygen concentration increases. This accounts for an increase in the defect density due to the presence of oxygen.

High V OC Cu 2 ZnSnSe 4 /CdS:Cu based solar cell: Evidences of a metal-insulator-semiconductor (MIS) type hetero-junction

In this work we report the Cu doping of chemical bath deposited CdS for the preparation of Cu2ZnSnSe4/CdS:Cu hetero-junction. We demonstrate that increasing the Cu concentration in the reaction bath several fundamental properties of CdS are changed (optical, crystalline, morphological), positively impacting in the resulting devices. The improvement of the efficiency from 1.6% to 6.1% is mainly explained by the large increases on VOC, obtaining the higher voltage value reported to now in the literature for this hetero-junction. We propose the formation of a metal-insulator-semiconductor (MIS) type device to explain our experimental results, which opens the possibility to use the MIS structure to solve, at least in part, the voltage deficit problems of kesterites.