From electronic grade to solar grade silicon: chances and challenges in photovoltaics

Abstract

Dedicated to Professor Horst P. Strunk on the occasion of his 65th birthday 71.55.Cn, 72.20.Jv, 72.40.+w, 72.80.Cw, 84.60.Jt Photovoltaics is a promising but challenging opportunity for the environmentally clean production of electric energy, as the cost of the produced energy is still too high to compete with conventional thermal and nuclear sources, in spite of the scientific and technological progress occurred in this field after the first oil crisis of 1973. Among the problems which should be solved to make photovoltaics fully competitive, a breakthrough concerning the cost reduction of the base material is compulsory. Aim of this paper is to discuss the scientific and technological problems encountered in the development of solar silicon for its use in high efficiency and low cost solar cells, and to give some firm experimental evidences about its potentialities. OriginalPaper Cadmium Telluride; 0,42% CIS; 0,18% Amorphous Si; 8,30% a-Si on Cz Slice; 4,63% Si Film; 0,26% Ribbon Si; 3,50% Single cystal Si; 35,17% Polycrystal Si; 47,54%c) of a kind of a low-grade silicon, called solar grade silicon, which could be adapted to solar cell fabrication, without loosing conversion efficiency.The processes under a) were aimed at the production of crystalline, non-single crystal ingots, wafers and sheet and were developed under the hypothesis that casting processes are intrinsically lower in cost that the Czochralski one. The potential development of die molding processes for ribbon silicon growth looked even more advantageous. The penalty to be paid in this case is associated to the presence of grain boundaries and other structural defects in the material and to unavoidable contaminations due to the casting crucible and die materials.The processes under b) were and are based on the development of low cost variants of the Siemens C route, which, as it is well known, is based on the energy intensive, high temperature reduction of purified chlorosilanes to polycrystalline silicon. Recent studies carried under the auspices of the European Commission within the fifth Framework Program showed that this route hardly could succeed in getting a polycrystalline feedstock at less than 25 €/kg, against the actual price of 90 $/kg of that produced following the original Siemens route.Very complex looked [1] and still looks the solution of problems associated to the production of a low grade silicon using variants of the metallurgical (MG) silicon process, with the aim to find alternative routes for the production of a low cost, low energy intensive polycrystalline feedstock. It was, in fact, figured that any material based on this processes would contain a quite large amount of impurities and structural defects. Therefore, intense basic research studies were spent worldwide, with the objective of investigating the role of impurities and crystal defects on the minority carrier lifetime and minority and majority carriers mobility and to discover/develop remedies in their presence.It is well known that the work carried out in the...

Macrosegregation of Impurities in Directionally Solidified Silicon

“…The purification of Si and the existence of macrosegregation were shown by Khattak et al [33] in ingots grown by the heat exchanger method (HEM), but the concentration profiles of impurities were not given. Yuge et al [34] and Pizzini et al [35] obtained Si ingots grown by an unspecified directional solidification process and showed concentration profiles of several impurities along the Si ingot, revealing macrosegregation. *From Tang et al [8] For these elements, the solvus line was not available; therefore, the reported solubility in this table is that at the solidus line, at its lowest temperature, where the solvus line should begin.…”

Nanotechnology and the environment: A European perspective

“…Its porous structure leads to quantum confinement and an increase in the bandgap, while the increased light absorption results in higher internal quantum efficiency (IQE). The deposition of nanocrystalline Si thin films by low energy plasma enhanced chemical vapour deposition (LEPECVD) reactor is being investigated by the Nanocrystalline Silicon Films for Photovoltaic and Optoelectronic Applications (NANOPHOTO) project [36]. As an alternative to Si photovoltaics, organic solar cells have been developed using TiO 2 nanoparticles coated with an organic dye to convert light into energy by a process analogous to photosynthesis [37,38].…”

Electromagnetic stirring and retention to improve segregation in silicon for photovoltaics

“…This implies to develop low cost production of silicon wafers [2,3], for example ribbon manufacturing processes [4,5] that avoid the loss of silicon when slicing ingots in wafers, or processes using upgraded metallurgical grade (UMG) silicon as a feedstock. Associating ribbon processing with the use of UMG silicon makes it possible to reduce costs twice, because there is no more silicon loss due to slicing, and because one can use less pure silicon feedstock.…”