2, Arizona State University, Tempe, Arizona, United States
The presence of high conductivity ohmic contacts is of paramount importance to the performance of optoelectronic devices such as solar cells. The formation of these highly conductive contacts often requires high temperature treatments for extended periods of time to sinter the conductive particles and to evaporate the organic residues from conductive pastes which inhibit conductivity. This makes this process tedious and time-consuming. Many emerging optoelectronic technologies require low temperatures to accommodate use of thermally sensitive substrates such as flexible and lightweight printed electronics on polymer, cloth or paper in order to potentially overcome the challenges posed by high temperature techniques. Reactive inks enable printing of highly conductive features at low temperatures through Drop-on-demand printing without the need for a post-annealing step.
In this work, the authors evaluate the adhesion performance of these reactive metal inks on Indium Tin Oxide (representative of the top layer of Silicon-Hetero-Junction solar cells). A 180 °, ASTM standard peel testing method was used to evaluate the adhesion performance of these metal reactive ink contacts on ITO. The silver metal contacts were printed using two different reactive ink dilutions: 1:1 and 1:10 by volume with ethanol, and two different droplet sizes (35 µm and 28 µm) were used to further control the amount of silver present in each droplet. The amount of silver within a contact line was varied by printing multiple number of layers. Failure analysis was done on the metal fingers for varying amount of metal in each contact line to study and quantify the different adhesion failure modes of these metal reactive inks on ITO. We learned that the metal contacts printed with higher concentration inks showed lower adhesion to ITO and diluting the ink 1:10 by volume with ethanol showed significant improvements in adhesion strength with less than 5 % of the film failing adhesively during peel tests. This approach introduces new techniques to deposit front contacts on solar cells and understand their adhesion properties.