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CdTe semiconductor is an absorbent material used in “tandem” photovoltaic solar cells. This material is commonly deposited by thermal evaporation presenting electrical resistivity values about of 105 W·cm to 109 W·cm. CdTe is applied in thin solar cells as p-type layer which is in contact with metal back electrode in solar cells. In the CdTe/metal junction a Schottky barrier exits; and small number of charge carriers have enough energy to get over the barrier and cross to the metal back contact. To solve part of this problem, nanostructured Te thin films were used as intermediate layers between CdTe and metal contact. Te layers whit different physical properties were deposited on CdS/CdTe structure by thermal evaporation employing different growth parameters. The electrical parameters of CdTe solar cells were influenced by p+ Te regions. p+ Te regions used as intermediate layer with large deposition time increases the FF and VOC values from 30% to 60% and 560 mV to 730 mV respectively. Also, the electrical resistivity is reduced from 106 W·cm to 103 W·cm. In this sense, Te region implemented as nanostructure allows to reduce the series resistance from 99 W to 20 W and increases the shunt resistance from 1445 W to 4424 W; Te region as thin films demonstrated not be adequate.

Recently, advances in the CdTe technology made CdTe a great candidate for commercial thin film solar cells because it is used in infrared windows for optoelectronic devices such as photovoltaic ones. Nowadays, solar cells with nanometric thicknesses are increasingly required due to an enhanced behavior effect. It is common to observe interdiffusion phenomena during several technological processes that are required for the fabrication of photovoltaic devices. It is very important to note that this interdiffusion could strongly affect the final efficiency of devices. In this work, the study of two materials as an interlayer between the CdTe thin film and metallic contact is presented. Te and ZnTe were deposited on the CdTe surface by using a close space sublimation technique. The electrical properties of the devices were evaluated after two and four years to observe the electrical behavior of Te and ZnTe layers implemented as part of the back-contact.

CdTe semiconductor thin film as absorbent material needs to be recrystallized, in order to improve the electrical and crystalline properties of this material. CdCl 2 thermal treatment was used to reduce the defects between CdS and CdTe thin films that helps to obtain the physical properties require. On the other hand, CdCl 2 treatment leads a special blue color in the CdTe surface. The electrical properties of CdTe are influenced for this coloration and confirm a close correlation between the quality of interfaces and its photovoltaic efficiency on CdTe solar cells. A systematic characterization was made to understand the blue color origin. This study revealed TeO X on the CdTe surface.

CdS polycrystalline thin films have been used as a window layer in CdTe, Cu(In,Ga)Se2 and Cu2ZnSnS4 solar cells; the specific optical and electrical properties of the CdS-partner play an important role in the photovoltaic device performance, so that one of the problems to be solved is to reduce the pin-hole density and intergrain cavities on the CdS surface. SnO2:F/soda-lime glass substrates (FTO) were treated with HCl, Ar and O2. CdS films were deposited on FTO substrates by using the chemical bath deposition technique and then CdS thin films were thermally annealed on CdCl2, Ar atmospheres and muffle at atmospheric ambient. Optical and morphological measurements revealed that the HCl treatment improves the CdS growth because the defect levels were reduced while the grain size is increased. The processed CdS-thin films were applied on CdTe solar cells and the photovoltaic efficiency was about 12% when the CdS thin films were thermally annealed in muffle at atmospheric ambient.

article i nfo Available online 8 November 2008 In this work we report the study of the thermal and optical properties of polycrystalline CdS thin films deposited by the gradient recrystallization and growth technique. CdS films were grown on pyrex glass substrates. These studies were carried out using an open photoacoustic cell made out of an electret microphone. From X-ray diffraction, atomic force microscope and photoluminescence measurements we observed polycrystalline CdS films with good morphology and crystalline quality. We obtained a thermal diffusivity coefficient of our samples with values ranging from 3.15 to 3.89×10 −2 cm

Chemical bath deposition (CBD) is an inexpensive and low temperature method (25-90 deg. C) that allows to deposit large area semiconductor thin films. However, the extent of the desired heterogeneous reaction upon the substrate surface is limited first by the competing homogeneous reaction, which is responsible for colloidal particles formation in the bulk solution, and second, by the material deposition on the CBD reactor walls. Therefore, the CBD method exhibits low efficiency in terms of profiting the whole amount of starting materials. The present work describes a procedure to deposit ZnO thin films by CBD in an efficient way, since it offers the possibility to minimize both the undesirable homogeneous reaction in the bulk solution and the material deposition on the CBD reactor walls. In a first stage, zinc peroxide (ZnO{sub 2}) crystallizing with cubic structure is obtained. This compound shows a good average transparency (90%) and an optical bandgap of 4.2 eV. After an annealing process, the ZnO{sub 2} suffers a transformation toward polycrystalline ZnO with hexagonal structure and 3.25 eV of optical bandgap. The surface morphology of the films, analyzed by atomic force microscope (AFM), reveals three-dimensional growth features as well as no colloidal particles upon the surface,more » therefore indicating the predominance of the heterogeneous reaction during the growth.« less

The use of a high resistivity transparent oxides (HRT) onto low resistivity transparent oxides (TCO) has been found to improve the performance of CdS/CdTe solar cells. In this work is presented and discussed a study about the use of a commercial conductive glass with SnO2:F as TCO and SnO2 as resistive transparent oxide as front contact in CdS/CdTe solar cells. The SnO2 was deposited by pneumatic spray pyrolysis and its electrical and optical properties were controlled and optimized by varying the substrate temperature and deposition time with the application of a post thermal treatment in O2 atmosphere. The transfer length method (TLM) was implemented to characterize electrically the SnO2:F/SnO2 as well the SnO2/CdS and SnO2:F/SnO2/CdS interfaces through the resistivity and specific contact resistivity measurements and their dependence on O2 treatment were analyzed. The use of the TCO/buffer layer on the CdS/CdTe solar cell in combination with the thermal treatment proposed in this work for TCO and HRT reduced the front contact/CdS contact resistance in a 10 factor and the solar cell efficiency was improved from 6.4% to 12.2%.

Cu(In, Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) semiconductors are direct band gap materials; when these types of material are used in solar cells, they provide efficiencies of 22.1% and 12.6%, respectively. Most traditional fabrication methods involve expensive vacuum processes including co-evaporation and sputtering techniques, where films and doping are conducted separately. On the other hand, the chemical bath deposition (CBD) technique allows an in situ process. Cu-doped CdS thin films working as a buffer layer on solar cells provide good performing devices and they may be deposited by low cost techniques such as chemical methods. In this work, Cu-doped CdS thin films were deposited using the CBD technique on SnO2:F (FTO) substrates. The elemental analysis and mapping reconstruction were conducted by EDXS. Morphological, optical and electrical properties were studied, and they revealed that Cu doping modified the CdS structure, band-gap value and the electrical properties. Cu-doped CdS films show high resistivity compared to the non-doped CdS. The appropriate parameters of Cu-doped CdS films were determined to obtain an adequate window or buffer layer on CIGS and CZTS photovoltaic solar cells.

In this work, a typical superstrate structure of a CdS/CdTe solar cell has been developed, incorporating a Te p+ region in the CdTe surface as a back contact. The Te region was grown as a nanostructured thin film material on the CdTe surface by employing the close space vapor transport (CSVT) technique. The incorporation of a nanostructured Te p+ region to the typical CdS/CdTe structure increased the efficiency from 4.5%–12%. A strong dependence on the electrical characteristics of the CdTe/metal junction on the morphological properties of the Te p+ region was observed, and an appreciable reduction in the Cu diffusion into the solar cell was measured by secondary ion mass spectrometry (SIMS). Thus, we demonstrate that a nanostructured Te p+ region is the best option to improve the performance of the back contact.

We present in this work the processing and characterization of two semiconductor thin films (ZnS and Zn x Cd 1− x O) grown by chemical bath deposition (CBD), and used as heterojunction partners on CSVT-CdTe films. The photovoltaic properties of the Zn x Cd 1− x O/CdTe heterojunction are reported here for the first time. The analysis of the basic properties of the films was carried out by standard optical and electrical characterization techniques. The heterojunction systems were studied by means of I–V characteristics, spectral response and quasi-static C–V measurements. Short-circuit currents of 8.9 and 22 mA/cm 2 , open-circuit voltages of 0.489 V and 0.324 V, fill factors of 0.29 and 0.42 and conversion efficiencies of 1.26 and 3.12% were obtained for SnO 2 /Zn 0.9 Cd 0.1 /CdTe and SnO 2 /ZnS/CdTe, respectively, under normalized 100 mW/cm 2 illumination (AM1).

AgInS2 and AgInS2:Sn semiconductors are currently interesting because of their potential applications like solar cells. Earlier results show that electrical and optical properties of AgInS2 appear to be dominated by donor-like intrinsic defects because it usually displays n-type conductivity. Chalcopyrite AgInS2 and AgInS2:Sn thin films were prepared by spray pyrolysis. It was noted that the Sn-doping produces no appreciable effect on the structural neither the optical properties. The remarkable difference between undoped and some Sn-doped samples lies in the conductivity type. Photoluminescence (PL) studies reveal that Sn reduces sulphur vacancies; electron paramagnetic resonance (EPR) studies in Sn-doped samples, show two superimposed signals close to the g ~ 2 region. One of them was assigned to punctual defects in the lattice due to substitution of Sn instead of In or S sites. This promotes a conductivity increase in this material; the second one shows a hyperfine structure which implies a nuclear spin l = 9/2 assigned to In in the lattice. Both signals were used to determine relative Sn quantities which substituting In or S determining the conductivity type.