The efficiency of a solar cell can decisively be increased by a surface structuring process. A novel laser based technology to structure organic solar cells has proved itself as a very fast and efficient process and is thus very promising for industrial applications.

In recent years the photoelectric efficiency of organic photovoltaics has made enormous progress. The technology offers the scaling potential to large and flexible photovoltaic substrates. Further efficiency improvements are expected from optimizing the intrinsic properties of the organic material. Micro- and nanostructured surfaces are very advantageous for organic solar cells since they increase the optical path length for the incoming light within the active material.

The scientists at the Fraunhofer IWS apply a very fast and efficient technology with short processing times at increased resolutions, the so-called direct laser beam interference pattering method (DLIP). The method transfers nanostructures in one step into metals, ceramics or polymers. Making an interference structure requires at least two collimated and coherent laser beams, which superimpose on the substrate surface. Interference occurs throughout the entire superposition volume of the beams. A single laser pulse can process a surface area of up to several square centimeters, which leads to an effective structuring speed of several square centimeters per second.

In close cooperation with the Institute for Applied Photo Physics (IAAP) of the Technische Universität Dresden the IWS scientists investigated the influence of the DLIP structuring procedure on the efficiency improvement of organic cells on PET foils. It proved that the efficiency of solar cells on structured substrates could be decisively improved. The superposition of two laser beams results in a linear structure. This procedure achieves an efficiency increase of appr. 10 % compared to the non-structured PET substrate. Three beams that are not within the same spatial plane form a point structure. The structuring period is defined by the angle between two laser beams and their wavelengths. Crossed structures are made in a two-step process. First a linear structure is imprinted and then the substrate is rotated by 90° prior to repeating the same structuring step. This technology turns out to be especially advantageous for organic cells on PET foils and leads to an efficiency increase of 21 %.

For the industrial implementation of this procedure the IWS engineers developed a compact laser interference system, which enables an inline structuring process.

More information at http://www.iws.fraunhofer.de/