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The fabrication of crystalline silicon thin films on foreign substrates is an attractive and alternative approach to the ingot casting aiming to the reduction of the total costs of photovoltaic cells and modules. The purpose of this work is to describe the CRYSTALSI process which aims at forming polycrystalline silicon films thanks to the thermal crystallization of amorphous silicon layer deposited on aluminium based substrates. The latest are used as a catalyzer for silicon crystallization but also as a back metal contact and reflector for photovoltaic solar cells. Two types of aluminium substrates were applied in these studies: a pure aluminium substrate (99.7% purity) and a silicon rich aluminium substrate containing about 12% of silicon. Silicon thicknesses between 1 and 10 μm were deposited and then annealed at temperatures of 490 °C, 520 °C and 550 °C and for duration times from 5 min to 12 h. The crystallized silicon films were then characterized by Raman spectroscopy, by scanning electron microscopy and by electron backscatter diffraction. The analyses show that the resulting annealed film is composed of two distinct layers: a thin polycrystalline silicon film located just above the substrate and a thicker layer made of a mixture of silicon and aluminium. Contrary to the case of the pure aluminium substrate, the silicon rich aluminium substrate allow to obtain thick and continuous polycrystalline silicon layers due to a controlled diffusion of the silicon within the substrate. As a result, the crystallization at 550 °C of 5 μm thick amorphous silicon on silicon rich aluminium substrate led to the formation of a thick polycrystalline silicon layer composed of grains of few micrometers in size. A low activation energy of about 2 eV is extracted suggesting that the silicon rich aluminium substrate is a catalyzer for the crystallization of amorphous silicon. As for the AIC process, it can be noticed that the limiting step of the CRYSTALSI process is the diffusion of the silicon in the aluminium. A chemical etching using a HNO 3 , HF, H 2 O (72.5 ml/1.5 ml/28 ml) solution is found to be appropriate to remove the residual top layer, in order to have access to the polycrystalline silicon layer. This work demonstrates that the CRYSTALSI process can lead to the formation of polysilicon films that can serve as a seed layer for the growth of a thicker absorbing silicon film for photovoltaic applications.

This paper describes the growth mechanism and structure of Si whiskers grown on a Si substrate in a tungsten hot filament chemical vapor deposition reactor with pure hydrogen as source gas using a two step process. In the first step, atomic hydrogen etched the silicon surface, forming silicon hydrides that react with tungsten from the filament. The resulting silicide particles deposited on the silicon surface forming a mesh-like pattern. The particles work as an etching mask against hydrogen radical etching of the silicon surface and inverted-pyramids or V-groove-shaped surface texture were obtained. In the second step the filament current was reduced and whiskers grew onto the substrate due to the interaction of the silicon hydrides with the particles and subsequent precipitation of saturated Si. The whiskers were found to have tungsten silicide particles on their tip, suggesting the whisker growth was through the Vapor–Solid–Solid (VSS) mechanism. A balance between the hydrogen radical etching effect and the supply of silicon hydride from the etching reaction on the silicon surface is crucial for the growth of dense silicon whiskers.

The goal of this work is to investigate the morphology, electrical and optical properties of undoped ZnO (i-ZnO) thin layers deposited on Si substrates with (100) and (111) orientations. Plasma enhanced metalorganic chemical vapor deposition (PEMOCVD) was used for the deposition of i-ZnO layers at different temperatures. Atomic force microscopy (AFM), ellipsometry and four-probe method were used for the analysis. It is found that substrate orientation and growth temperature determine the morphological (grains size, surface roughness) as well as electrical properties of ZnO films. It is shown that the refractive index value depends on the surface morphology. It is concluded that properties of i-ZnO layers deposited on different Si substrates at different conditions exhibit some trends and peculiarities, which have to be taken into account for the processing of heterojunction solar cells by the PEMOCVD method.

The goal of this work is to investigate morphology, electrical and optical properties of indium-tin-oxide (ITO) deposited by pyrosol on glass and Si substrates at different temperatures and to implement such layers for the processing of Si-based solar cells. The influence of the methanol/H2O ratio on general properties of ITO was investigated. Atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission spectra, ellipsometry and resistivity measurements were used for the analysis. It is shown that properties of ITO layers depend dramatically on the substrate used. It is shown that the resistivity of ITO layers deposited on a glass substrate is higher up to 2.5 times, compared to that of ITO layers deposited on a Si substrate at the same conditions, but in both cases decreases if the deposition temperature increases. Moreover, ITO layers deposited on a glass substrate are more flat and their refractive indexes are always lower for all deposition temperatures. An increase of the H2O concentration in a film-forming solution leads to a decrease of the ITO film resistivity and to a slight increase of the roughness. An application of pyrosol deposited ITO films as the top transparent electrodes for the (p+nn+)Si and heterojunction ITO/n-Si solar cells is demonstrated.

Properties and structure of ZnO and ZnO:Al films deposited on c-Si, a-Si:H/Si and glass substrates are studied by various methods. The transmittance of the ZnO:Al was found to be higher when compared to ZnO, and the refractive index lower. X-ray photoelectron spectroscopy (XPS) shows that the screening efficiency in the presence of core holes is enhanced in the Al doped ZnO. The roughness of the ZnO:Al surfaces is strongly substrate dependent. With transmission electron microscopy (TEM) a 2–3 nm thick amorphous interfacial layer was observed independently of substrate and doping. Deposition of ZnO on a-Si:H substrate results in crystallization of the a-Si:H layer independently of Al doping.

In this work the properties of indium tin oxide (ITO) films deposed on glass substrates by magnetron sputtering technique in the temperature range below 200 °C are studied by various methods. The physical properties of ITO thin films have been investigated using optical transmittance, photoluminescence, atomic force microscopy, ellipsometry, Hall-effect and four point probe methods. It is established that properties of ITO layers depend drastically on the temperature and oxygen partial pressure during the deposition process and exhibit some peculiarities of the surface morphology. It is found that the band gap energy of this material varies in the energy range from 4.1 to 4.4 eV and depends on the growth conditions. It is suggested that local deviations from the stoichiometry and defects are the main physical reasons of Burstein–Moss shift of the optical band gap.

The microstructure of state-of-the-art chemical solution deposited indium tin oxide thin films typically consists of small randomly oriented grains, high porosity and poor homogeneity. The present study demonstrates how the thin film microstructure can be improved significantly by tailoring the precursor solutions and deposition conditions to be kinetically and thermodynamically favorable for generation of homogeneous textured thin films. This is explained by the occurrence of a single heterogeneous nucleation mechanism. The as-deposited thin films, crystallized at 800 °C, have a high apparent density, based on a refractive index of ∼ 1.98 determined by single wavelength ellipsometry at 633 nm. The microstructure of the films consists of columnar grains with preferred orientation as determined by X-ray diffraction and transmission electron microscopy. The resistivity, measured by the four point probe method, is ∼ 2 × 10 − 3 Ω cm prior to post-deposition treatments.

Atomic force microscopy (AFM), scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) were used for the analysis of the morphology and electrical properties of heterojunction (HJ) solar cell structures made of Indium Tin Oxides (ITO) layers deposited on p-type polished Si substrates with a buffer layer of n-type hydrogenated amorphous Si (a-Si:H). By means of AFM measurements it is shown that the a-Si:H layer can be deposited on a Si substrate quite homogeneously with a roughness of only a few nanometers. In contrast, the morphology of the ITO layers depends on the deposition temperature and can be varied during the HJ solar cell processing. Cross section analysis of the HJ structures by SCM shows penetration of n-type carriers into the (p)Si bulk substrate and the formation of the resulting n-p electrical junction depends strongly on the ITO deposition temperature and the presence of the (n)a-Si:H layer. Complementary cross section SSRM measurements show only a highly n-type doped region confined to the ITO layer. Finally it is concluded that the properties of the HJ solar cell structures depend dramatically on the properties of the ITO layers and can be modified by variation of the ITO deposition temperature. (c) 2005 Elsevier B.V. All rights reserved.

Secondary ion mass spectrometry (SIMS) analysis of In, O, Si, and H concentrations versus depth distributions in ITO(In-Sn-O)/(n,p)a-Si:H/(p,n)-Si and ITO/(p)-Si heterojunction (HJ) solar cell structures was performed. SIMS measurements show some peculiarities of the element profiles in the interface of ITO/a-Si:H and a-Si:H/Si regions. These peculiarities can be attributed to the non-homogeneities (In-rich nanostructures, small dents or mounds) of the ITO layer. It is shown that the formation of In-rich nanostructures is more pronounced in case of ITO growth on a (p)a-Si:H/(n) Si substrate at 230 °C. SIMS profiles obtained from such non-homogeneous areas can show a penetration of In into the Si bulk, which is not a real distribution. A hydrogen redistribution from the a-Si:H layer into the neighboring ITO and Si layers in case of ITO/(n,p)a-Si:H/(p,n)-Si structures as well as hydrogen gettering to the ITO/(p)-Si interface was detected. It is concluded that the final heterojunction structure of ITO/(n,p)a-Si:H/(p,n)c-Si is rather complex and includes in addition to the individual ITO, a-Si:H and Si layers also intermediate ones with different composition.

Some crucial limiting process steps of ITO/(n)a-Si:H/(p)c-Si solar cell manufacturing were investigated. The influence of amorphous silicon deposition temperature on the efficiency of heterojunction (HJ) solar cells as well as ITO deposition on the properties of ITO/a-Si:H/c-Si interfaces were studied. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and SIMS measurements were employed for the investigations. It was observed that the decrease in a-Si:H deposition temperature from 230 down to 100°C leads to a reduction in solar cell efficiencies to only 1%. It was found that the ITO deposition process itself can significantly modify the ITO/a-Si:H/c-Si interfaces due to the interaction of oxygen, and In and Sn atoms with silicon at the initial stage. It was concluded that this modification has a more significant impact on the quality of the HJ solar cells than on that of the a-Si:H layer.