Solar cell processing takes place using industrial, prototypal or self-build large-scale machines. Being the core of the infrastructure, this equipment is used in a large variety to process the silicon material from the as-sawed wafer to the contact formation of the final solar cell. Thereby applied technology meets the highest requirements used in advanced photovoltaics sector. Processing of any type of solar cell includes a differently large number of process steps, which are used varyingly, but adapted after another depending on the type of silicon material or chosen solar cell design.
Cleaning and surface topography treatment
Any solar cell manufacturing process starts with the conditioning of the raw wafer. Depending on the starting quality, the type of material and the condition of the surface of the silicon wafer it is treated by a certain series of wet-chemical etching steps with the aim to rid the surface of the maximal possible or at least necessary amount of impurities (metal and organic compounds). The idea behind it, is to prevent the parasitic indiffusion of potentially defect-generating impurities into the silicon bulk during the following processing steps, e.g., the purposeful indiffusion of dopants by high-temperature treatment.
Beside the different cleaning steps with different chemical composition (e.g., HF/HCl solution; H2SO4/H2O2 solution; etc.) it is possible to direct-etch the silicon wafer surface to change the topography in a targeted way. This mechanism, known as texturization, is attained using either alkaline or acidic etch solutions resulting in a crystal-orientation dependent surface structuring of inverse pyramid or intertwined worm-like surface shapes, respectively. Changing the surface topography in such a way results in a decrease of reflection and increase in efficiency, yet requires qualitatively high-grade surface passivation due to the increased surface area.
Dopant-diffusion and surface passivation
Directly after cleaning the wafers are commonly doped by indiffusion of dopants in a tube diffusion furnace. Thereby, the used doping source for the high-temperature step can be grown in-situ from the gas phase or be pre-deposited by i.e., CVD. At the end of the diffusion step the pn-junction has been formed necessary for the separation of charge carriers within the silicon bulk. During the indiffusion of dopants gettering of impurities from the silicon bulk occurs, increasing the quality of the material. To maintain this clean state and to saturate the remaining open silicon bonds commonly a dielectric layer is deposited on the surfaces of the wafer. Using multi-purpose layers with additional properties, such as decreasing reflection, increasing passivation and supporting contact formation further increases the cell efficiency. These layers are primarily deposited using CVD reactors in which gases are transitioned into a plasma state to deposit these solids.
The final step in solar cell processing constitutes the contact formation of differently doped areas on the wafer surface. Thereby, common and as industrial standard understood screen-print techniques as well as such being in development are applied. At the end a metal contact between semiconductor and in a module interconnectable material is realized. In case of metal screen-printing pastes, the contact formation to the silicon bulk is achieved in a throughput belt-furnace.
To display all these standard to highly experimental processing steps and sequences a corresponding supply infrastructure is necessary next to the large machine park. Especially in case of this local interconnection of fundamental research and industry-related processing for the development of solar cell concepts a comparable highly variable equipment operation considering possible parameters and flexibility to design new machine prototypes is necessary.
The machine park for processing solar cells consist mainly of large-scale machines, which are operated from manually to fully automated. Depending on the machine the throughput thereby ranges from a single wafer for research purposes to industrial scale of a few hundred wafers to depict real industry circumstances. Some machines include the following:
- Wet chemical process benches
- Tube diffusion furnace
- E-gun and sputtering equipment
- Nano and picosecond laser machine systems
- RTP and annealing ovens
- PECVD, ICP-PECVD, APCVD und ALD tools
- Screen-printing line (screen-printer and dryer)
- Belt firing furnace
- Inkjet system
- Chip saw
- Porosifying equipment
- Assorted large-scale characterization tools (ICP-OES, GD-OES, REM, etc.)
Additionally, the infrastructure includes the upkeep of the functionality of the machines, i.e., the maintenance and repair of the equipment, as well as the media supply (e.g., water, cooling water, deionized water, CDA, nitrogen, specialty gases, power and any other consumables). Special attention is given to working safety, especially in use of specialty gases in e.g., CVD reactors.