Effect of carbon nanotubes in conductive adhesives for photovoltaics

Abstract

This study investigated the impact of adding carbon nanotubes (CNTs) to silver-loaded epoxy-based adhesives, focusing on mechanical adhesion, electrical performance, and reliability. CNTs improve electrical conductivity in polymers by forming conductive networks with a low percolation threshold, requiring significantly lower filler loading compared to micron-sized fillers. In this study, a commercial electrically conductive adhesive (ECA) is modified by adding 0.05 to 0.63 wt% of CNTs and several dedicated test structures are manufactured with it. The samples are stressed up to 2000 h under damp-heat and up to 400 thermal cycles. The results indicate that CNTs enhance peel strength, but only above a threshold concentration, with a 0.19 wt% addition increasing adhesion by 82 %, while higher concentrations yield diminishing improvements. The electrical performance is influenced differently: the sheet resistivity of ECA (R_s) improves with increasing CNT content, whereas the contact resistivity (ρ_c) initially increases before decreasing with higher CNT concentration. Under thermal cycling, higher CNT content mitigates R_s degradation, with the 0.63 wt% CNT formulation exhibiting superior long-term stability, though ρ_c degrades in all cases. During damp-heat exposure, R_s improves over time, while ρ_c degrades with increasing stress duration. Interestingly, R_s and ρ_c exhibit opposing trends, suggesting minimal overall impact on module performance (for the particular bill of material used in this study). The study also highlights the challenge of isolating the effects of CNTs from epoxy dilution, emphasizing the need for better control of the ECA formulations. These findings demonstrate the potential of CNT-enhanced ECAs for improved adhesion and electrical stability in photovoltaic applications, provided that trade-offs in contact resistivity are carefully managed to ensure long-term reliability.