Objectives and problems to be solved:
The readiness issue of silicon is a medium to long-term threat for PV industry to move beyond the growth constraints imposed by the current supply of feedstock. The main objective of the research project SWEET is to develop "wafer-equivalents", i.e. cost-effective thin film silicon solar cells grown on low-cost silicon substrates, represented either by recycled silicon wafers or by wafers cast from highly doped "off-spec" silicon feedstock not used by the IC industry. The aim is to achieve 16% cell and 14% module efficiency. With this new source of silicon feedstock for the production of PV power, a medium-term cost target of <2.0 €/Wp, and a long term cost target <1 €/Wp shall be achieved.
Description of work:
For the fabrication of a PV module made from crystalline silicon solar cells, highest cost reduction potential lies in the wafer and cell processing steps. Using reduced-quality silicon feedstock and thin silicon layers are two important steps to realize a cost reduction. Both steps will be addressed in the proposed project work. Main goal is to achieve high efficiencies and lowest cost for a wafer equivalent, which can be introduced directly into a standard solar cell production line. This requires removing a number of technical/economic constraints before the technology can be implemented in a large scale.
The work programme therefore includes:
- casting of crystals from off-spec silicon best suited for a high-quality substrate;
- cost reduction of crystal growth and use of larger and thinner wafers;
- usage of re-cycled wafers from IC industry as "feedstock" source from "waste";
- optimisation of innovative high-throughput epitaxial CVD growth of silicon layers;
- application of surface structuring methods to increase light trapping;
- application of innovative passivation techniques to increase cell efficiency;
- investigation of applicability of wafer-equivalents in standard cell production lines;
- fabrication of modules made from wafer-equivalents based on crystalline silicon thin-film solar cells;
- calculation of cost saving potentials given by this technology.
Expected results and exploitation plans:
Key deliverables of the project are:
1. a 16% efficient solar cell 10x10cm2 on a wafer equivalent made from low-cost silicon;
2. a 14% efficient solar cell module from wafer equivalents made from low-cost silicon;
3. An economic assessment of thin film silicon solar cells/modules from low-cost Si substrates with epic layers.
As a result of the project, a technology will be available to manufacture cost-efficient silicon thin-film solar cells based on double-usage of silicon wafers or otherwise wasted highly pure off-spec silicon.