In order for photovoltaic to be implemented on a significant scale, the cost of solar electricity needs to be substantially reduced. This cost essentially derives from the raw material employed in the manufacturing of solar cells, the high purity crystalline silicon, and the low density of energy obtained from photovoltaic collectors. Using thinner silicon wafers and higher conversion efficiencies is the clearest path for reaching photovoltaic competitiveness – an idea widely accepted by the photovoltaic community.

The objective of the BiThink project was to develop and demonstrate, at industrial scale, a technology able to exert direct influence on the cost of photovoltaic systems.
BiThink focused on three key aspects:
• the use of bifacial cells and albedo modules as a simple way to increase the amount of energy collected;
• the increase in the number of wafers obtained from the slicing of silicon ingots;
• the use of a simple and efficient manufacturing process, able to combine high mechanical yields with reasonable cell efficiency.

Albedo modules are photovoltaic modules that include bifacial cells in a similar manner to standard flat modules but using a transparent back cover. Because bifacial cells are capable of using the light that falls on any or both of its sides, albedo modules can produce from 30 % to
50 % more electricity than conventional ones, only by placing them close to a wall or a floor painted white. The BiThink project focused on demonstrating the industrial viability of albedo modules based on very thin crystalline silicon wafers.

Results to date

The 1 800 wafers per linear meter of silicon ingot obtained at the beginning of the project rose to the final target of 3 500 wafers. This represents a cost reduction of 50 % in raw material, which currently accounts for more than 70 % of the final cost of solar modules. The Swiss company HCT Shaping Systems is developing the new slicing techniques. The wafer thickness has been decreased down to 90 μm and the wire diameter has been reduced from the standard 160 μm to 120 μm. The final target of 3 500 wafers per metre has been fulfilled
on multi-crystalline wafers of 156 mm x 156 mm.
Solar cell technology must be simple and efficient, being able to be used in standard industrial lines with minor modifications. Technology developed in BiThink is based on an integral screen printing process. TiM-EHU in Spain and Fraunhofer-ISE in Germany have developed the new
manufacturing process for thin bifacial cells. This process produces back surface field (BSF) bifacial cells with phosphorus and boron emitters diffused from screen printed pastes. Contacting the boron emitter with low contact resistance and without short circuiting the p-type
emitter was a challenge solved by the formulation of a new silver paste. Ferro-Holland has developed this new paste, among other pastes used for the n- and p-type emitter formation.

Main steps of the new manufacturing process developed for preparing bifacial cells.

Texturized n or p-type wafers:

Interconnection of thin solar cell requires new technology. Thus Isofoton worked on this task within BiThink. Lastly, to reach high yield values in industrial manufacturing of ultra-thin solar cells requires a better understanding of the mechanical behavior of thin silicon wafers, apart
from the optimisation of existing handling procedures.
CENER from Spain and NPC-Europe from Germany are working on these topics. Finally the University of SouthFlorida (USA) has proven a new technique to detect wafer cracks on an industrial line.
BiThink results show impressive figures in terms of the consumption of silicon, rising to only 3.9 g/W using conservative yield values. In addition, extensive new technology has been developed in the project, in the areas of ingot slicing, wafer un-sticking, screen printing diffusion, mechanical handling, crack detection or interconnection of thin solar cells, technologies which will be used for the next advances on thin silicon solar cells.

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