Automated cleaning of printed photovoltaic modules

For printed photovoltaic modules in roll-to-roll production, the burrs created during laser structuring of the front electrode can now be automatically removed using a CO2 snow jet cleaning solution.

Roll-to-roll production enables the particularly cost-effective manufacture of printed photovoltaic modules. However, in order to avoid short circuits, conductive burrs resulting from the laser structuring of the front electrode previously had to be removed manually in a way that was not industry-compatible. In a joint project, a fully automated CO2 snow jet cleaning solution that can be integrated into the production line was developed to eliminate this weak point.

With a layer thickness of between 0.5 and one micrometer and a high degree of efficiency even with low solar radiation, flexible, printed photovoltaic cells open up a wide range of applications in solar energy supply. In the production of printed photovoltaics, the roll-to-roll process offers great advantages in terms of production speed, volume and costs. The total of five layers of the modules based on organic and perovskite semiconductors can be processed individually, whereby the bottom layer, a transparent IMI electrode (structure: indium tin oxide, silver, indium tin oxide) is laser structured. Burrs are created along the structuring edges, which are conductive and protrude a few micrometers from the surface. Burrs that are not removed cause damage and short circuits due to the low module thickness. The current state of the art is the mechanical removal of burrs at very low web speeds. There is a risk that the structured layers will be damaged by the mechanical impact.

In order to eliminate this weak point in the roll-to-roll production of printed photovoltaic modules, the Institute Materials of Energy Technology and Electronics (I-MEET) and the Solar Factory of the Future at Friedrich-Alexander-Universität Erlangen-Nürnberg, Sciprios GmbH and Acp Systems AG initiated the "_PV-CO2" research project funded by the German Federal Ministry of Economics and Climate Protection (BMWK). The aim was to develop a fully automated, industrially applicable_{CO2 snow jet cleaning system} based on Acp's QuattroClean snow jet technology.

This is a dry cleaning process for full-surface and localized applications. The cleaning medium is liquid carbon dioxide recycled from chemical production processes and energy generation from biomass. It is fed through a wear-free two-substance ring nozzle and expands into fine snow crystals as it exits. These are bundled by a separate, ring-shaped jet of compressed air and accelerated to supersonic speed. The cleaning effect is based on a combination of thermal, mechanical, solvent and sublimation effects when the easily focused jet of compressed air hits the surface to be cleaned. The crystalline carbon dioxide sublimates completely during the process, leaving the treated surfaces dry.

A roll-to-roll pilot system was set up for cleaning the laser-structured electrode substrates and equipped with several QuattroClean snow jet nozzles arranged above the electrode track. The first step was to optimize the jet parameters so that the burr height is significantly reduced without damaging the electrode. In addition to the capillary diameter, which defines the flow rate of the liquid carbon dioxide, and the pressure of the compressed air jacket, this included the distance between the nozzle and the substrate, the inclination of the nozzles in relation to the substrate and the speed of the path. After each cleaning process, the maximum burr height was determined by confocal microscopy measurement. This procedure was repeated for a large number of parameter combinations until an optimum cleaning result was achieved.

To evaluate the effect of deburring on photovoltaic performance, eight centimeter wide organic photovoltaic modules were produced on the CO2 snow jet treated substrates. They were compared with modules produced on untreated and manually cleaned substrates of the same size. As expected, the modules on the uncleaned substrate exhibited a high leakage current, which reduced the photovoltaic efficiency (PCE value) to 2.3 %. For the manually cleaned modules, the PCE value was 4.8 %, while it was as high as 5.3 % for the modules treated with CO2 snow jet cleaning. This difference in performance can be explained by the fact that manual cleaning causes scratches on the electrode. This can significantly reduce the active area, as not only the scratched area does not generate current, but also areas that are cut off from charge extraction by the scratch. Dark lock-in thermography (DLIT) was used to confirm that the difference in performance of the differently treated substrates is due to deburring.

The fully automated cleaning solution has now been integrated into the standard production process for printed photovoltaics at the Institute for Materials in Energy Technology and Electronics. Here, an array with seven nozzles is used to reliably remove laser-induced burrs on a 25 cm wide web. At Sciprios, too, the CO2 cleaning process is now one of the equipment options for roll-to-roll systems for the production of printed photovoltaic modules.

The cleaning solution, which can be easily integrated into roll-to-roll production, makes the manufacture of any type of printed electronics that involves laser structuring more economical, productive and sustainable. Another area of application is electrode production in battery manufacturing. (OM-10/24)