N. Romeo

Back contacts to CSS CdS/CdTe solar cells and stability of performances

Abstract High efficiency CdTe/CdS thin film solar cells with a good stability can be obtained if a low gap, high conductivity p-type semiconductor, such as Sb2Te3, is used as a back contact for CdTe. However, even though Sb2Te3 is used as a back contact, some cells can exhibit a low fill factor. This, at first sight, could suggest that the back contact is not the right one. Here we would like to show that a low fill factor could depend not only on the back contact but also on the front contact and on the amount of Na-diffusion from the soda lime glass substrate.

Structural and chemical interface characterization of CdTe solar cells by transmission electron microscopy

Abstract CdTe/CdS thin film solar cells have been grown by high-vacuum evaporation (HVE) and close-space sublimation. To understand the role of Cl on the microstructure and composition of the CdTe/CdS layers and interfaces, the cells were annealed for different durations after different amounts of CdCl 2 had been deposited. Transmission electron microscopy shows a loss of orientational relationship between CdTe and CdS after annealing under the influence of Cl. The interdiffusion of S and Te across the interface is measured quantitatively and segregation of Cl, Te and O at the CdTe/CdS interface is detected by energy-dispersive X-ray mapping in the electron microscope. The results show a strong correlation with the diffusion of Cl along the CdTe grain boundaries, which is directly proved for the first time. It is suggested that recrystallization of CdTe grains starts from the CdTe surface and proceeds towards the interface. Cells grown by HVE show all the features expected from the predominance of recrystallization.

The distribution of impurities in the interfaces and window layers of thin-film solar cells

We report a systematic multielement study of impurities in CdS window layers by dynamic and quantitative secondary-ion-mass spectrometry (SIMS) with high depth resolution. The study was carried out on CdTe∕CdS solar cell structures, with the glass substrate removed. The analysis proceeded from the transparent conductive oxide free surface to the CdTe absorbing layer with a view to examining the influence of the CdCl2 heat treatment on the distribution and concentration of impurities in the structures. Special attention was paid to the impurities present in the CdS window layer that may be electrically active, and therefore affect the characteristics of the CdTe∕CdS device. It was shown that Cl, Na, and Sb impurities had higher concentrations in CdS following cadmium chloride (CdCl2) heat treatment while Pb, O, Sn, and Cu conserved the same concentration. Furthermore, Zn, Si, and In showed slightly lower concentrations on CdCl2 treatment. Possible explanations of these changes are discussed and the results...

A combined SIMS and ICPMS investigation of the origin and distribution of potentially electrically active impurities in CdTe/CdS solar cell structures

Quantitative, dynamic secondary ion mass spectrometry (SIMS) combined with inductively coupled plasma mass spectrometry (ICPMS) was used to study the origin and distribution of impurities in CdTe/CdS/In2O3:F/glass solar cell structures. Particular emphasis was put on the potentially electrically active impurities that may possibly originate from either the CdTe starting material or the cadmium chloride (CdCl2) post-deposition annealing-induced activation, and that are likely to affect the device performance. Structures were grown using CdTe starting material of 5N and 7N purity, and were analysed by SIMS, both before and after CdCl2 heat treatment. Depth profiles of the elements: Cl, O, Cu, Na, In, Sb, Sn, Si, Zn, Pb and S were made. Around 91% of the impurities detected by ICPMS in the CdCl2 powder used were found to consist of species known to show electrical activity in CdTe. The origin of most of the impurity species present in the cell structures was elucidated, and it was shown that Pb, Sn, Cu and Zn were not coming from the CdTe or CdCl2 starting materials. However, Na and Cl turned out to originate from the CdCl2 processing. The potential interdiffusion of elements, such as S, Te, Si, In and Na from the layers of the structures was also highlighted and is discussed.

LARGE CRYSTALLINE GRAIN CdTe THIN FILMS FOR PHOTOVOLTAIC APPLICATION

A simple new method suitable to grow large crystalline grain CdTe thin films on glass substrates has been developed. CdTe films which exhibit a grain size larger than 20 µm have been obtained. The films are p-type with a resistivity of about 100 Ωcm. Backwall CdTe/CdS thin film solar cells with an efficiency above 9% have been prepared.

In situ oxygen incorporation and related issues in CdTe/CdS photovoltaic devices.

CdTe∕CdS∕SnO2∕ITO:F solar cell devices were investigated using quantitative secondary ion mass spectrometry (SIMS) depth profiling. They were grown on sapphire substrates and potentially active impurity species were analyzed. The SIMS data were calibrated for both CdS window layer (grown by sputtering) and CdTe absorber layer (deposited by close-space sublimation). For comparison, some of the samples were grown with and without oxygen incorporation into the CdTe layer during its deposition, and with and without postgrowth cadmium chloride (CdCl2) annealing in air and chemical etching. These devices were back contacted using Mo∕Sb2Te3 sputtered layers. It was shown that for CdTe and CdS layers there was a correlation between the concentrations of oxygen and chlorine. In situ oxygen incorporation in the CdTe layer yielded a substantial improvement in the device parameters and achieved an efficiency of 14% compared to 11.5% for devices fabricated in the same conditions without oxygen incorporation in CdTe. I...

Efficiency improvement in thin-film solar cell devices with oxygen-containing absorber layer

The CdTe∕CdS solar cell devices were grown using a dry process consisting of sputtering for the transparent conducting oxide and CdS window layers, and close-space sublimation for CdTe absorber layer. These devices were back contacted using Mo∕Sb2Te3 sputtered layers following the CdCl2 activation process carried out in air. It was shown that when oxygen is intentionally introduced in the CdTe layer during its growth, this leads to a significant improvement in all the device parameters yielding an efficiency of 14% compared to 11.5% for devices fabricated in the same conditions but without intentional oxygen incorporation in CdTe. The data obtained were not altered following a light soaking. The devices were investigated by quantitative secondary ion mass spectrometry, which allowed insight into the distribution and amount of oxygen and chlorine within the entire device structure. Both impurities showed an increased concentration throughout the CdTe absorber layer.

Role of substrate and transparent conducting oxide in impurity evolvement in polycrystalline thin-film devices

A comparison of as-grown and processed CdTe∕CdS solar cell structures deposited on sapphire substrate has been undertaken with those grown on glass. The device structures were depth-profiled using quantitative secondary ion mass spectrometry. It was shown that while Si concentration profiles are similar to those for structures grown on glass, Na was more than one order of magnitude lower when sapphire was used instead of glass, showing that Na diffused from the glass. It was also found that there was no measurable diffusion of Sn from the SnO2 layer into CdTe, and that the former played an important role in preventing the diffusion of In from In-containing transparent conducting oxide layer. Cl, O, Br, and F species were also investigated and while Cl and O were found to be independent of the nature of the substrate used, Br and F were shown to be affected by the processing.

P-type CdTe thin films doped during growth by neutral high energy nitrogen atoms

Abstract P-type CdTe films with a resistivity of about 102 Ω cm were prepared by evaporating cadmium and tellurium from two separate crucibles onto heated glass substrates. P-type doping was obtained by directing to the substrate neutral high energy nitrogen atoms shot from an atomic gun during the CdTe film growth. The best results were obtained when the substrate temperature was kept at 350°C and the ratio between the fluxes of cadmium and tellurium was larger than 2.0. Back-wall CdS/CdTe thin film solar cells prepared on nickel-covered glass substrates have so far exhibited an efficiency of about 5% without an antireflecting coating.

Large-grained (111)-oriented CdTe thin films grown by “quasi-rheotaxy” on steel substrates☆

Abstract Large-grained (111)-oriented CdTe thin films were grown on low cost substrates such as steel by a new method which we called quasi-rheotaxy. This method comprises the deposition of the semiconducting material onto a thin metal layer, such as lead or bismuth, kept at a temperature a few degrees below its melting point. Since, according to the theory of melting, a material kept a few degrees below its melting point exhibits one or two melted monolayers, the deposition is made onto a liquid surface of a solid material. In this way, there is a high surface diffusion coefficient which is one of the most important conditions for large-grained growth with oriented crystallites. This method is very similar to rheotaxy and indeed it could be considered as a surface rheotaxy. However, it has an important advantage over rheotaxy which is that it avoids the high surface tension of a bulk melted material. Homojunction CdTe thin film solar cells prepared by quasi-rheotaxy have up to now had an efficiency of 2%. The low efficiency is attributed to the high reverse current which is probably due to the loss through the grain boundaries which cross the junction.