A solution for every application

In order to meet the brisk global demand for energy while at the same time reducing the carbon emissions produced by burning fossil fuels, more renewable energy sources must be utilized in the future, for example solar power. Photovoltaic technology is expected to play a central role in this connection.

Suspensions as components
The production of virtually all types of solar cells requires some form of suspensions or pastes. They consist of solid particles in the micrometer to nanometer range which are finely dispersed in a liquid. The particles give the components a large specific surface area and improved optical properties in order to enhance the efficiency of the solar cells.

Another significant benefit of using suspensions as components for manufacturing solar cells lies in the processing of the substances. Unlike vacuum coating processes, suspensions can be cost-effectively applied at ambient pressure and temperature to a wide variety of substrates. For this purpose, it is possible for example to use conventional printing processes, which additionally allow continuous processing (roller-to-roller process).

Broad range of dispersed substance systems
A broad range of different materials can be used as suspensions in manufacturing solar cells. For example, electrically conductive silver and aluminum pastes are applied for producing the contact points for discharging the photo-generated free charge carriers on the front and rear of the cells to the load. The dye required in dye-sensitized solar cells (“dye cells”) is adsorbed on a nanoporous semiconductor material. For producing this layer, titanium dioxide pastes are typically applied. But other semiconductors such as zinc and tin oxides are also available as alternatives.

The photoactive layers in CIGS (copper-indium-gallium-diselenide/disulfide) thin-film cells can also be applied by printing. For this purpose, nanoparticles of the required elements are homogeneously dispersed in a liquid in the desired proportions.

In addition to conductive and semiconductive systems, solar cells also use electrically insulating materials. Such structures can be applied for instance in the form of glass solder pastes by printing, followed by solidification on the basis of a sintering process.

Dye cells also use components with chemical functions. Platinum or carbon nanoparticles serve as catalysts and can therefore be applied as pastes.

Buhler solutions
The industrial manufacture of particulate solar cell components with specific properties poses a number of challenges. For example, in order to generate a high specific surface area in a particulate layer, the particle size should be in the nanometer to micrometer range and have a narrow particle size distribution. This demands high-energy-input grinding processes in order to satisfy the stringent product requirements. Another important criterion in the processing of photovoltaic components, in particular semiconductor materials, is the prevention of product contamination by abraded particles. The grinding equipment must therefore be made of wear-resistant and corrosion-resistant materials such as ceramic. In addition, temperaturesensitive materials require efficient cooling and temperature control during grinding.

For meeting these specific requirements, Buhler provides a wide range of wet-grinding solutions for processing fluid and pasty materials. The machine designs range from laboratory and pilot mills to high-capacity production systems. These comprise mills based on both horizontal and vertical configurations. They also include machines designed for continuous large-scale operation as well as smaller user-friendly solutions for customers with frequent product changes. What is more, special cooling concepts allow the processing of temperature-sensitive products.

From MicroMedia® to Trias®
If particles in the nanometer range are demanded, the MicroMedia® Perl Mill® will be found to be the ideal solution for achieving gentle dispersion of shear-sensitive or hard photovoltaic materials. High-resistance metallic engineering materials and extremely small grinding media with sizes ranging from 300 microns down to 20 microns ensure contamination-free material grinding down to the nanometer range. This fineness is especially made possible by the efficient shearing processes taking place between the grinding beads. The rotor and stator of the MicroMedia® Perl Mill® are also available as ceramic versions, which are ideal for metalfree grinding processes.

The grinding of different photovoltaic material systems requires the use of wear-resistant and corrosion-resistant equipment. The grinding chamber of the Centex® Perl Mill® for example can be made of DraisResist ® (hardened steel), DraisElast® (polyurethane), and different types of ceramic. The special design of this mill increases the number of grinding-effective collisions between the material particles and the grinding beads. This reduces the specific energy requirements for achieving the desired particle size distribution. For processing pasty materials such as metal pastes, the Buhler three-roller mills are the preferred technology. The new Trias® three-roller mill allows optimal control over the product quality. The design version equipped with ceramic rolls and allowing oil-free operation is excellently suited for clean-room applications and thus satisfies the most stringent sanitation and quality standards. Moreover, the VIVA® technology allows a variable roll pressure and a uniform product quality across the entire roll length.

Depending on the product formulation, it is possible to vary the viscosity of suspensions across a very wide range in the manufacture of solar cell components. The conical bead mills (K Series) allow the grinding of materials across a wide range of viscosities and offer a high level of flexibility in terms of grinding formulation quality.