CORDIS - EU research results

Crystalline Silicon PV: Low-cost, highly efficient and reliable modules (CRYSTAL CLEAR)

Final Report Summary - CRYSTAL CLEAR (Crystalline Silicon PV: Low-cost, highly efficient and reliable modules)

Solar energy is clean, can be used almost anywhere, and has a huge potential, but the price of the systems is relatively high. The project aimed to bring down the production costs of the central system component, the solar module, by more than 50 %.

CRYSTALCLEAR was a research and development project dedicated primarily to cost reduction of solar photovoltaic (PV) modules. At the same time the project aimed at increasing the efficiency (electricity yield), improving the environmental quality and improving the applicability of such modules. CRYSTALCLEAR was about crystalline silicon module technology.

The CRYSTALCLEAR project tackled all aspects from the raw materials up to the completed solar module. Key activities concerned:
- Strongly reducing the consumption of expensive materials (especially silicon, but also others) as well as introducing the use cheaper materials.
- Increasing the electricity output of solar modules.
- Developing highly automated, high-throughput, low-cost manufacturing processes.
- Screening materials, processes and products in relation to sustainability and suitability for large-scale use.

The expected results of CRYSTALCLEAR after completion of the project were divided in three main blocks.
1. Availability of innovative manufacturing technologies which allowed solar modules to be produced at a cost of EUR 1/watt-peak (which was a reduction by more than 50% compared to state-of-the-art at the start of the project). This objective was extremely ambitious, but essential to get world-class technology. Manufacturing cost reduction was essential to bring prices of modules and turn-key complete systems down.
2. Improved environmental profile of solar modules by reduction of materials consumption, replacement of undesired materials and designing for recycling. This would strengthen the position of solar energy as a clean and sustainable alternative to conventional electricity generation.
3. Enhanced applicability of modules by tailoring to customer needs and by improving product lifetime and reliability. Since solar modules would be used in very different situations (e.g. on buildings) flexibility of use was crucial. Assured quality was a prerequisite for large-scale, professional use.

The project was divided into the following subprojects:
- Sub Project 1: Feedstock
- Sub Project 2: Wafers
- Sub Project 3: Wafer-equivalent approaches
- Sub Project 4: Cell technology
- Sub Project 5: Modules
- Sub Project 6: Environmental sustainability
- Sub Project 7: Integration.

Subproject 1 was dedicated to the subject of the so-called feedstock, the silicon that solar cells are made from. For solar cells (as well as for microelectronic chips) a high grade (purity) of silicon was required.

Sub Project 2 dealt with the preparation of solar silicon material by ingot growth applying directional solidification. Further it dealt with wafering of these materials by multi wire slurry sawing (MWSS) as well as by diamond wire cutting. The general aim was to reduce the wafer cost by increasing the throughput of ingot fabrication and by reducing the silicon consumption per wafer.

Sub Project 3 dealt with R&D on crystalline silicon thin-film solar cells, which were compatible to state-of-the-art wafer technologies but aimed at a significantly lower cost and an alternative resources supply. The respective approach was called 'wafer equivalent approach'. Initially SP3 started with 3 concepts:
- The free-standing lift-off concept, where very thin layers were detached from a substrate by means of a porous silicon separation layer, and processed after epitaxial thickening to very thin solar cells. Due to major problems and investment needs concerning handling of the thin layers, this subject was stopped already after month 24.
- The epitaxial wafer equivalent (EpiWE), a sole silicon epitaxy on low-cost silicon substrates. This concept turned out to be the one which fitted best into the timeline and aims of CRYSTALCLEAR. It was therefore pursued in CRYSTALCLEAR as Subject 3.2 from month 18 to the very end of the project.
- The Recrystallised Wafer Equivalent concept, which was based on recrystallised silicon layers on mechanically supporting substrates to produce Wafer equivalent solar cells. Investigated in Subject 3.3 it showed its economic suitablility for the CrystalClear cost aims. However the experimentally achieved efficiency was behind the milestones, which made clear that more fundamental development effort was needed. Therefore the activity was stopped after month 48.

The goal of Sub Project 4 was to develop cell designs concepts and manufacturing processes that would enable a reduction in the order of 40% of the cell processing costs per Wp. The major objectives of the subproject within the reporting period were:
1. To demonstrate the concept of photon conversion for Si solar cells and improve the performance of the photon converting structures.
2. To demonstrate stable, high efficiencies on solar cells on low cost materials.
3. To process high efficiency, industrial-type solar cells on thin (150 Pm or less) substrates.
4. To develop innovative structures particularly adapted for large and thin wafers and easy module manufacturing.
5. To develop and test new in-line characterisation tools suitable for novel materials and cells.
6. To test large scale processing of new cell concepts.
7. To produce a sufficient amount of high efficiency large-area cells on very thin substrates for the fabrication of demonstration modules.

The interconnection of very thin cells using conductive adhesives as the interconnection medium was demonstrated reliable in the project was demonstrated in the realisation of a full size module (using 120 Pm thin cells originating from SP4) as the Superslice I demonstrator module. Furthermore, other options for stress free interconnections of such cells based on the use of low temperature solders were demonstrated in another full size module with these cells.

In Sub Project 6, the Life Cycle Assessment (LCA) of the existing crystalline silicon production technology was updated. The CRYSTALCLEAR demonstrator modules showed a decrease of the energy payback time to 1.8-2.4 years (Central Europe). The reduction in the 'CML 2 baseline' environmental impacts on area basis was at least 18% for the multi-crystalline silicon and at least 25% for the mono-crystalline silicon demonstrator modules compared to 2004.

The main objectives of Sub Project 7 in this reporting period were:
1. Finalise CRYSTALCLEAR Technology Roadmap
2. Finalise cost modelling of roadmap scenarios
3. Cost modelling of CRYSTALCLEAR demonstrators
4. Perform overall assessment of project results
5. Dissemination CRYSTALCLEAR results (open workshop, press releases).

After finalising the CRYSTALCLEAR Technology Roadmap the cost modelling for the different roadmap scenarios were updated and also finalised. Furthermore, the technology developments within CRYSTALCLEAR were evaluated economically and ecologically. This evaluation was based on the data resulting from the manufacturing of the CRYSTALCLEAR demonstrators.