Understanding wrinkle formation in roll-to-roll top encapsulant production process for thin-film solar cells

Abstract

Thin-film flexible solar panels can be utilized on many surfaces where conventional solar panels are hard to be used such as side walls of buildings, ship roofs or curved surfaces. In conventional solar cells, glass is used to protect the panels from external damage. However due to its heavy weight and rigidity, glass is not an option for thin-film flexible solar cells. HyET Solar, a company based in the Netherlands that produces flexible thin-film solar cells uses a transparent foil produced using a roll-to-roll process to encapsulate and protect their solar panels. In order to reduce the amount of materials used and cut the costs, a thinner foil is being developed. Producing the thinner foil in a roll-to-roll process causes different defects in the foil, among which half sinusoidal waves forming mostly in the bottom side of the foil, these waves are called wrinkles. The process of manufacturing the top encapsulant layer involves changing temperatures and mechanical stresses applied on the foil. Therefore, two models were developed to understand the effect of three process parameters (web force, web speed and temperatures of each production zone) on wrinkle formation. First, a thermal model of the process using ANSYS program was developed to understand the temperature profile of the produced foil along the process. Then, a mechanical model that uses the output of the thermal model and the applied mechanical forces to study the effect of the investigated production parameters on wrinkle formation was worked out. The results show that operating at higher temperatures and reducing the thickness of the foil are directly responsible for decreasing the threshold of the critical stress that causes wrinkle formation and thus reducing the range and the magnitude of the force to 1.4 N is needed to perform the process without forming wrinkles in the foil. Varying web speed between 0.3 m/min and 3m/min is found to have a minor effect when varied under high temperatures. Finally, decreasing the temperature of the first and second zones by 10 °C is found to extend the range of the web force that can be applied. Performing the process under lower temperatures or reducing the mechanical forces that act on the foil will reduce wrinkle formation in the developed foil; web speed should not be considered in mitigating wrinkle formation.