S’Tile process: silicon sintered substrates for the PV industry

Summary

A large number of processes have been proposed to realize CSiTF solar cells. The presentation has focused on high temperature substrates which are expected to give the higher solar cell efficiencies and a competitive production cost.

Lower efficiencies are usually obtained on foreign substrates. This is attributed to thermo- mechanical mismatches between the substrate and the grown layer which generates stresses and defects inside and at the interface with the grown layer that are detrimental to its electrical properties.

The conclusion is that silicon materials are the best substrates for the epitaxial growth of silicon. The issue then relies on the production cost and crystalline quality of the substrate. In the next section, a new process for the low cost production of silicon substrates is proposed.

- A RexWE-like process can also be proposed. Although the elaboration is comparatively more complex than the EpiWE-like process, it bypasses the need for high crystalline quality (large grain size, no dislocations…) and high purity levels. Silicon powders obtained from metallurgical routes or recycled from the standard wafering process can be used.

Process

Impurities

Grain size Density Advantages / Drawbacks Metallic

ppm

O / C ppm

Conventional (bulk)

< 1 < 10 > 1 mm Full density

The conventional process is highly reliable.

Complete wafer re- crystallization needed

EpiWE

< 1 < 100 > 100 µm at the surface

Pore closure 92 % TD_Si

High potential efficiencies (19 % on mono-crystalline Si)

Surface re-crystallization needed

RexWE

< 10-100 < 1000 - Pore closure 92 % TD_Si

Allow the use of recycled silicon powders

Complex elaboration process

Table I.1: Material requirements depending on the silicon solar cell elaboration process envisaged.

The material requirements for the elaboration of conventional solar cells are highly difficult to achieve by low cost processing like the Powder Metallurgy route. As will be shown in this manuscript and as already shown in previous thesis and literature [MW85, Der06, Bel10], it is impossible to obtain a wafer of high crystalline quality using only the sintering route. Full wafer re-crystallization through melting processes has then been proposed to enlarge the grain size. This usually leads to high level of contamination or defects in the substrate which are inappropriate for the production of bulk silicon solar cells. The solar cell efficiency is then limited to 8.9 % with a production cost that is drastically increased due to the re- crystallization step [Bel10].

Wafer Equivalent concepts have comparatively a higher potential efficiency. Using the RexWE concept, S’Tile achieved 9.2 % efficiency with a non-optimized (no texturing and no light trapping) 4 cm² solar cell [GBL+09]. The open circuit voltage obtained is as large as 580 mV which is a very encouraging result as CSiTF on low cost substrates usually suffer from too small open voltage values. It indicates that the sintered silicon substrate is appropriate to obtain high quality wafer equivalent. Using this approach the company estimates that the production cost of a solar cell can be divided by two [Gra12] (Table I.2).

Conventional solar cell S’Tile solar cell

Silicon raw material 0.30 0.05

Ingot 0.05 -

Wafer 0.29 0.05

Cell 0.29 0.35

Total 0.93 0.45

Table I.2: Estimated production cost in €/Wc of a RexWE S’Tile solar cell compared to a conventional solar cell.

To our knowledge, the EpiWE approach has never been investigated on low cost crystalline silicon substrates. However, this approach would be very competitive as only one processing step, namely the silicon epitaxy, is needed for its manufacture. The issue actually relies on the low cost realization of a large grain seeding layer at the top surface with low-defect and low impurity levels for the epitaxial growth of the thin film material.

I.4.2 Substrate elaboration process – Technological and scientific issues

Sintering is a solid state thermal process in which isolated particles bond together to give a consolidated material. The basic phenomena occurring during this process are neck growth between particles, densification and coarsening.

The main advantage of this process is that it avoids silicon waste originating from wire sawing. For a given purity of raw silicon material, this makes the sintering process cost lower than the conventional block casting and sawing process. Also, there is no theoretical limit for the size of the sample, so that the limitation would be the same as for regular wafers.

The elaboration process investigated by S’Tile is a hot pressing method. The silicon sample composed of micrometric powders is processed below the melting point of silicon under an applied pressure about of 30 to 40 MPa. Under the effect of the pressure, plastic deformation occurs leading to neck growth between particles and to densification up to 100 % of the theoretical density (TDSi).

The method investigated in this thesis is a pressure-less sintering route. Neck growth between particles occurs only under the effect of the temperature and curvature gradients at the scale of the particles. The pressure-less sintering route will not allow full densification of the sample.

However, regarding the application, the elaboration of pore free wafers is not always necessary (Table I.1). Bulk solar cell processes require full densification mainly because the wafer must be completely re-crystallized by melting. More precisely, melting of the substrate creates bubbles that drastically damage the wafer shape.

EpiWE or RexWE processes are not as stringent as regards the porosity of the substrate. If surface melting re-crystallization is needed (EpiWE), it is assumed that the porosity must be closed (density higher than 92 % TDSi), so that infiltration of the substrate by the melt does not occur.

The contamination of the specimen by the specimen holder is thought to be less stringent in the case of pressure-less sintering. But as a major difference with hot-pressing, the sample must be shaped before the sintering process. Eventually, pressure-less sintering is comparatively more competitive as the material can be produced and processed continuously.