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EpiSil-IBC Report Summary

Project ID: 657270
Funded under: H2020-EU.1.3.2.

Periodic Reporting for period 1 - EpiSil-IBC (Epitaxial silicon foil solar cells with interdigitated back contacts)

Reporting period: 2015-11-01 to 2017-10-31

Summary of the context and overall objectives of the project

The leading technology in the photovoltaic market today is the single junction crystalline silicon (c-Si) solar cell. Since there is little room for further improvement beyond current efficiency records the industry is focused on reducing manufacturing costs. To reduce material costs the c-Si technology is pushed towards thinner and thinner cells, which are harder to handle without breaking. One way to make thin solar cells without kerf losses is to grow the silicon foil epitaxially on a parent wafer followed by a lift off step. Since such thin foils are mechanically very fragile, new processing techniques are required in which the foils are bonded to a glass substrate, such that they are never handled free-standing.

Such new device concepts directly contribute to the competitiveness of low-carbon energy production and therefore support the EU’s climate and energy goals to reduce greenhouse gas emissions by 20% below 1990 levels by 2020 and provide 20% of EU energy from renewable sources. The advantage of the epitaxial growth is that it can directly grow crystalline material from the precursor gas which has the potential to produce cheaper and higher quality wafers and foils, providing new market opportunities for European companies and making the transition to low-carbon society economically even more stimulating.

The aim of this proposal is to realize a solar cell based on ultra-thin (<50 μm) silicon foils bonded to glass and with all contacts on the back side of the cell. The goal of this innovative solar cell is to surpass the current state-of-the art efficiencies of thin silicon foil solar cells by using low-cost and high yield processing.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The work carried out towards achievement of the project’s objectives was structurally organized into four work packages. The first work package focused on designing and demonstrating a cell prototype based on initial experience and understanding of the thin foil preparation and cell processing. The second work package focused on in-depth characterization of the used materials, the manufacturing process and the prototype device. The third work package focused on device simulation using state of the art simulation tools, theoretical analysis and device optimization. Finally the fourth work package focused on a novel device design, where based on insights gained from previous work packages we present the process flow resulting in best devices obtained during the project duration.

Additionally the simulation models developed for the epifoil solar cell were adapted to explore advanced topics such as epitaxially grown emitters for nPERT solar cells, combinations with tandem solar cells and selective contacts.

In parallel to the research work, the training of the researcher took place. The researcher was given a series trainings provided by senior scientists, which enabled him independent access to the required equipment.

The results of the project include a filled European patent application, publication of four journal papers and several presentations at photovoltaic conferences and workshops.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

With the developed process, the solar cell using thick wafers bonded to glass and all contacts on the rear side achieved an efficiency of 22.4%. Using thinned down wafers, with thickness similar to the epitaxial foils the achieved efficiency was 20%. Using epitaxially grown silicon foils and the same processing steps the highest efficiency achieved was 16.1%. The relatively low efficiency of epitaxial foil is due to its low lifetime, related to the epitaxial growth process. The details of low epifoil lifetimes are not clear at the moment and the research on improving the epifoil material quality is ongoing. Nevertheless the project demonstrated that the developed process can be used to create high efficiency ultra-thin solar cells bonded to glass with rear side contacts.

Realizing the positive and negative polarity contacts on the rear side of the solar cell adds to device and processing complexity. To address this issue we improve a patterning process based on laser ablation. Our innovative idea (patent application filled) uses a distributed Bragg reflector that prevents damaging the surface passivation layers of the solar cell, resulting in better device quality.

As mentioned before the new device concept facilitates the transition to low-carbon society support the EU’s climate and energy goals and provides new market opportunities for European companies.

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