Electrode cleaning for solar modules

Snow jet technology for electrode cleaning after laser structuring

Automated CO2 snow jet cleaning reliably removes burrs. The compact cleaning module can be adapted to the web width and easily integrated into roll-to-roll production (Image: PV-CO2 research project team)

Roll-to-roll production enables the cost-effective manufacture of printed photovoltaic modules. To avoid short circuits, conductive burrs previously had to be removed manually. In a joint project, an integrable, fully automated CO2 snow jet cleaning solution was developed that eliminates 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.

(Kopie 3)

The compact cleaning module can be adapted to the web width and easily integrated into roll-to-roll production (Image: PV-CO2 research project team)

Automated, integrable cleaning solution

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 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.

The 300R2RCompact R&D platform, into which the cleaning solution can be easily integrated, enables a variety of roll-to-roll coating and printing experiments. It supports the upscaling of laboratory tests to small production series (Image: Sciprios GmbH)

Good deburring and improved performance proven

To clean the laser-structured electrode substrates, a roll-to-roll pilot system was set up 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 thickness of a layer stack of a printed solar cell is < 1 µm - without the PET layer, which serves as a carrier material for the layer stack (ETL electron transport layer, AL active layer, HTL hole transport layer, AgNW silver nanowire electrode, IMI electrode is laser structured) (Image: PV-CO2 research project team)

Effects of deburring on the IV characteristic curve of modules on untreated (d), manually cleaned (e) and cleaned with CO2 snow jet technology (f) substrates. b and c show light and dark characteristic curves of the differently treated modules. In the DLIT images of the modules (d, e, f), the bright areas indicate hot, localized spots, which are particularly visible in the interconnection zones of the uncleaned modules (Image: PV-CO2 research project team)

acp systems AG
ww.acp-systems.com

Sciprios GmbH
www.sciprios.de