V. Canevari

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.

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.

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.

HgCdTe thin films for solar cells application prepared by multisource evaporation

Hg1−xCdxTe thin films in their cadmium-rich composition were prepared by a multisource evaporation system. In the composition range 0.05 < 1 − x < 0.15 the films exhibit a forbidden gap between 1.4 and 1.2 eV, close to that corresponding to the theoretical maximum photovoltaic conversion efficiency. In the same composition range they are p type and exhibit a resistivity which varies between 104 and 10 Ω cm.

High quality ZnS:Mn thin films grown by quasi-rheotaxy for electroluminescent devices

Abstract The object of the present paper is to describe the morphological and crystalline characteristics of ZnS:Mn thin films grown on different low melting point metallic substrates such as Pb, Bi and Bi (1− x ) Sb x alloys by a new method based on the quasi-rheotaxy technique. In this paper we also prove that the quasi-rheotaxy deposition technique yields high quality ZnS:Mn thin films without the so called dead layer and without the conical pattern typical of the most common deposition techniques.

Thin polycrystalline silicon films grown by quasi- rheotaxy on aluminium-covered glass substrates

Abstract This polycrystalline silicon films with a crystalline grain size of 50–100 μm were grown by quasi-rheotaxy on glass substrates covered by a thin previously grown aluminium layer. The aluminium and silicon were both deposited by high vacuum electron beam evaporation. The thin silicon films exhibit only the (111) orientation for thicknesses of less than 1 μm, whereas for thicker films the (220) orientation begins to appear. The latter orientation prevails for films thicker than 10 μm. Crystalline grains as large as 10–20 μm were obtained.

Large grain electroluminescent ZnS:Mn thin films grown by quasi-rheotaxy on insulating materials

Abstract In a previous paper we proved the possibility of growing thin ZnS:Mn films of good morphological quality on metal substrates with the aid of the quasi-rheotaxy technique. In the present paper, besides confirming the previous results, we demonstrate that the growth of ZnS:Mn by means of the quasi-rheotaxy technique is also possible on insulating materials. In particular we show that the insertion of the ZnS:Mn film between two very thin layers of As 2 S 3 , Sb 2 S 3 and B 2 O 3 in a metal–insulator–semiconductor–insulator–metal device, allows the growth of the active layer with superior morphological characteristics if compared with the already rather satisfactory results previously obtained on metal substrates. Finally we discuss the luminance–voltage characteristics of one of our electroluminescent devices, which shows good symmetrical brightness with respect to the positive and negative pulses of the a.c. excitation voltage.