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Iowa State University researchers have developed a process for producing more efficient polymer solar cells by increasing light absorption through a thin and uniform light-absorbing layer deposited on a textured substrate.

An increase in power conversion efficiency of 20 percent compared to flat solar cells made from polymers, as well as an increase in light captured at the red/near infrared band edge of 100 percent over flat cells has been demonstrated experimentally, and ISU is seeking partners interested in commercializing this technology.

So-called first generation photovoltaic or solar cells are based on the use of crystalline silicon wafers.  While improvements in efficiency have been made with these types of solar cells, their high cost has driven research into materials that would be cheaper to use.  Second generation photovoltaic technologies with the potential to be more economical to manufacture include thin-film, organic (polymer or oligomer), and hybrid organic-inorganic cells.  Organic photovoltaics (OPV) have a number of advantages, including manufacturability (roll-to-roll processes on flexible substrates are possible), low-temperature processing, high optical absorption coefficients, and tunability. Unfortunately, OPVs suffer from low power conversion efficiencies, with 7% being among the highest documented experimentally.  To address this problem, ISU researchers have developed for a process to produce a thin and uniform light-absorbing layer on textured substrates that improves the efficiency of polymer solar cells by increasing light trapping.  While the use of textured substrates is commonly used in conventional, silicon-based solar cells, attempts to use textured substrates in polymer solar cells have not been successful because they require expensive extra processing steps or technically challenging coating technologies that can result in a light-absorbing layer with air gaps or sub-optimal coating thickness in the valleys or on the ridges of the substrate pattern; these solar cells can have poor performance due to a loss of charges and short circuiting at the valleys and ridges.  The technology developed by the ISU team overcomes these drawbacks by optimizing the dimensions of the underlying topographical features, enabling a conformal photovoltaic active layer to be coated on the textured substrate.  As consequence, light trapping is enhanced, resulting in more efficient power conversion compared to flat solar cells. Light captured at the red/near infrared band edge is also increased compared to flat solar cells.

• Efficient (light trapping is more effective compared to flat solar cells without compromising electrical characteristics)

• Economical (does not require extra processing steps or technically challenging coating technologies)

Solar cell manufacturing

1: “On realizing higher efficiency polymer solar cells using a textured substrate platform”, Kanwar S. Nalwa, Joong-Mok Park, Kai-Ming Ho, and Sumit Chaudhary. 2010. Adv. Mat.

2: "Design of Light-trapping Microscale-textured Surface for Efficient Organic Solar Cells", Nalwa, K. S. and S. Chaudhary. 2010. Optics Express 8: 5168-6178.