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Novel bifacial single-substrate solar cell utilising reflected solar radiation

Final Report Summary - REFLECTS (Novel bifacial single-substrate solar cell utilising reflected solar radiation)

The project aimed at manufacturing mono-crystalline silicium (c-Si) solar cells with an increased efficiency by also making use of the solar radiation reflected from earth surface. Studies indicate that this energy is in the order of 30-50 % of direct radiation, and by adopting high-reflective surfaces (mirrors) this could be exploited. The technical route foreseen to utilise the reflected solar radiation was making a double-sided (bifacial) solar cell.

The project proposed aimed to prepare the back end of an c-Si solar cell by applying an established Lithuanian technology for producing single-sided solar cells, based on self-formation. In this technology, the + and - conductors were both at the front end, and two-sided processes simply could copy the process to the back-end. This process was far simpler than the manufacturing process of the current two-sided solar cells.

In addition, the project supported following societal and policy objectives:
- Mitigate the environmental effects from the emissions of greenhouse gasses and local pollution caused by the burning of fossil fuels, aiming at a 15% reduction by the year 2010 from the 1990 level.
- Improve the diversity, self-sufficiency and security of EU energy supplies, especially with respect to the dependence on imported energy.
- Reduce the pressures on governments (taxpayers) and utility companies to provide enough electricity to meet future demands.
- Reduce the cost of energy and dependence on utilities for consumers.
- Attract investors that would back an industry that’s largely driven by SMEs that design, manufacture, and distribute renewable energy systems, thus increasing jobs in data acquisition systems, engineering design, manufacturing, and data analysis as well as distribution, marketing, sales and service.

Furthermore, the quality of life for people in under-developed countries would be enhanced by providing access to equipment such as radio / TV, electrical appliances, etc. Other possible benefits outside Europe were reducing the destruction of rain forests and saving threatened species.

Two groups of rectangular grooves were created in the wafer: for emitter and for base. The emitter was formed by phosphorous diffusion. The boron diffused grooves provided a BSF. The distance between grooves was less than diffusion length of minority charge carriers. In this mode, the photo-generated carriers must be effectively separated by space charge region. The structure was “thick” for photon flow direction and 'thin' along emitter-base direction. The metal contacts for emitter and base were formed on the slots walls by the obliquely metal evaporation. Because metal stripes were placed perpendicular to photon direction the shadowing of structure by contacts was minimal. The rear side of structure was free of contacts and effectively passivated by SiNx. So, the ORTO structure acted as a bifacial solar cell.

The work that was performed during the project was divided into nine work packages.

Work package 1: Project definition. The main objectives of this work package were the following:
- Definition of material specifications: requirements for bifacial c-Si solar cells.
- Definition of cell manufacturing requirements: self-formation theory to the manufacturing of bifacial c-Si solar cells.
- Defining reflector performance requirements.

The project was defined in more exact terms. Starting from end user requirements, the project was defined with respect to material specifications, manufacturing route and reflector design.

Work package 2: Reflector Design and Prototype Manufacturing. The objective of this work package was to design and manufacture a prototype reflector to capture as much albedo sunlight as possible.

The reflector design was optimised, including: Optical design, selection and testing of materials, mounting of cell and module, cooling aspects, exact dimensions and setting up Manufacturing Plan.

Work package 3: Bifacial Solar Cell ORTO Structure Development. The objectives of this work package were the following:
- Definition / selection of the best method for mass production of high-efficiency solar cell.
- Definition of the best production process.

Work package 4: Software Development for Selfformation Iterations. The objectives of this work package were the following:
- Development / setting up of main documents related to software development process.
- Architecture design.
- Preparation of documents for entire system integration and qualification testing, based on functional requirements (input for this workpackage).
- Development of computer program for simulation of the solar cell self-formation manufacturing methods by iterative process.
- Development of computer program for synthesis of optimal initial planar system and technological processes.

Work package 5: Experimental Bifacial Solar Cell Manufacturing. The objectives of the fifth work package were the following:
- Elaboration of experimental technology for manufacturing of ORTO solar cells.
- Development and manufacturing of c-Si solar cells with 20 % cell efficiency.
- Manufacturing of functional PV system based on new solar cells.

Work package 6: Bench Tests, Laboratory Testing and Evaluations. The main objective of this work package was testing the REFLECTS prototypes with respect to their performance specification, stability, reproduceability, etc.

Work package 7: System Integration and Field Evaluations. The main objectives of this work package were the following:
- Demonstrate and test system under real life conditions.
- Verifiy that researched concepts indeed work as expected/predicted in practice.

Work package 8: Dissemination and Exploitation. The eighth work package had the following objectives:
- To ensure that the achievements were made known to the targeted potential clients / market segments.
- To prepare plans for future exploitation to ensure that the results were implemented in real-world applications.

Work package 9: Consortium Management. The objective of this work package was to ensure a smooth project management and communication.

In order to analyse the benefits for the SMEs from the anticipated innovation more in detail an Innovation Impact Assessment was carried out.

Dissemination activities included:
- Participation in leading conferences related with PV research.
- Disclosure of information through a project brochure (in pdf). The brochure would be compiled to serve as a tool for the dissemination of the project objectives, its status and project results at external events.
- Publications in relevant magazines, being careful not to reveal sensitive proprietary information.
- Set-up and maintenance of a project website.
- Dissemination of information to the networks and established distribution channels of the individual partners.

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