Colloquia

Summary

Shading on solar cells reduces their efficiency, as parts of the active area no longer receive direct sunlight. This shading is primarily caused by the front contacts, which are typically made of silver and cover part of the cell, thereby blocking incident light. One way to mitigate this loss is by shaping these contacts with a triangular cross-section. This design redirects some of the incident light onto uncovered, active regions of the cell, effectively reducing shading losses.

A novel technique called string printing enables the fabrication of such triangular contacts. In this method, a wire coated with silver paste is pressed onto the substrate. As the wire is lifted, it leaves behind a triangular contact. A dedicated machine was developed to automate this process.

This thesis investigates how different wire surface materials affect the printability of various silver pastes. Coatings of PDMS, Teflon, and graphene were applied to tungsten wires using various deposition techniques. Tungsten wire was chosen for its high yield strength and low elongation. A tungsten wire was also mechanically roughened to increase surface adhesion. In addition, carbon and aramid fibers were tested as alternative wire surfaces because of their lower surface energy and higher tensile strength.

Uniformly coating the tungsten wires proved technically challenging. While the methods and results of the coating processes are documented, their effect on the printing performance is not further evaluated. As a result, only the uncoated tungsten wire, roughened tungsten wire, and the two fiber types were tested in the string printing machine.

To quantify the printability, a custom image analysis tool was developed. This tool compares top and side-view microscope images taken before and after printing to estimate the volume of deposited silver paste. Several pastes with varying properties such as particle shape and resin type were evaluated alongside the different wire types.

The results show that the uncoated tungsten wire, when combined with a silver paste containing only spherical particles, produced the most consistent and uniform prints. These findings indicate that both wire surface properties and paste composition play a critical role in determining print quality in string printing.