Solar Cells: Picosecond Lasers Save 20,000 Tons of CO2

Heinz Huber is researching procedures for the production of thin film photovoltaic cells with the picosecond laser which save 20 kt/a of CO2 emissions  at the HM Lasercenter. (Source: J. Lesser, HM)

There is a hundred times more solar energy available worldwide than energy derived from wind power or renewable resources. That is the reason why solar power is an essential component of the renewable energy mix promoted by the German sustainable energy policy.

While mass production of presently used silicon cells has made them affordable, their production process is more complex and generally less resource-friendly than that of thin film cells. Furthermore, if these cells are produced using the picosecond laser technique, their efficiency increases even more by another ten to fifteen percent. Although these cells so far have garnered only a rather modest share of the market, they already save 20,000 tons of CO2 annually, which is equivalent to the amount emitted on average by three thousand people in an industry nation per year.

Production of CIGS thin film cells with a picosecond laser

CIGS thin film photovoltaic cells are comprised of layers only a few micrometers thick. The wafer-thin layers are composed of a sandwich made from four layers: the carrier material glass at the bottom, a molybdenum layer only one micrometer thick, a three micrometers copper-indium-gallium-diselenid layer and a one-micrometer zinc oxide window layer on top.

In order to produce energy the large area layer of the CIGS cell needs a structure of fine grooves subdividing the large CIGS area into approximately one hundred single cells. This way the approximately one volt strong voltage of the single cell is increased to 100 V for the entire module, just like in batteries connected in series. It is crucial though that this process does not damage the molybdenum layer.

Confocal microscope images of P3 trenches, where transparent ZnO was indirectly ablated with ps lasers, 10 kHz, λ = 1064 nm. Left: only ZnO removed at a speed of 2.4 m/s. Right: ablation of ZnO and CIS down to the Mo layer at a speed below 200 m/s. Bottom: profile of the cross section of both trenches (click for larger version; source: HM, Huber Group)

And this is where the picosecond laser comes into play. The mechanical scratching of the grooves called scribing that has been used so far has disadvantages: “The scribing needle produces wide grooves and material of lower conductivity remains at the bottom of those grooves. But if you produce those grooves with a picosecond laser, the grooves are finer and the current flows more evenly. The efficiency of the cell is increased without any noteworthy increase in cost”, says Huber.

By using laser pulses with a duration of picoseconds and a high rate of repetition Huber made industrial production of CIGS thin film cells possible: “A nanosecond laser burns and melts all three layers. Only with an ultra short pulse laser like the picosecond laser it is possible to apply a structure to the upper CIGS layer without damaging the molybdenum layer,” states Huber. The connection between the molybdenum and the transparent zinc oxide layer is stronger. This also reduces internal energy losses which in turn increases efficiency.

Huber thinks that streamlining the production process should improve efficiency and reduce production costs even further. The plan is to consolidate the alternating application of the individual CIGS layers and the structuring of the layer surfaces into one joint application of all layers and their collective laser structuring. (Source: HM)

References: J. Winter et al.: Ultrafast pump-probe ellipsometry and microscopy reveal the surface dynamics of femtosecond laser ablation of aluminium and stainless steel, Appl. Surface Sci. 511 (2020) 145514; DOI: 10.1016/j.apsusc.2020.145514F. Kessler et al.: CIGS Thin Film Photovoltaic – Approaches and Challenges, in: “High-Efficient Low-Cost Photovoltaics”, January 2020, Springer. Series in Optical Sciences 140; DOI: 10.1007/978-3-030-22864-4_9

Link: Lasercenter of Munich University of Applied Sciences (Laserzentrum Hochschule München, LHM, Prof. Dr. Heinz P. Huber), Munich, Germany


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