Improving Adhesion in Organic Photovoltaic Cells with Self-Assembled Monolayers

Self-assembled monolayers (SAMs) are promising interface layers in organic photovoltaic (OPV) cells. Phosphonic acid-based SAMs, such as 2PACz, have shown improved efficiency over the commonly used PEDOT:PSS hole transport layer. This study examines the impact of 2PACz and its variants (MeO-2PACz, Cl-2PACz, Br-2PACz) on not only device performance but also interfacial adhesion. T-peel and shear strength tests show that 2PACz increases interfacial peel and shear strength more than 3-fold compared to PEDOT:PSS, whereas the halogenated SAMs do not enhance adhesion significantly. Molecular dynamic simulations of the photoactive molecules on the SAMs reveal that 2PACz exhibits stronger interaction energy with PM6 than the halogenated SAMs, consistent with adhesion measurements. The improved adhesion when using 2PACz relative to the other SAMs is attributed to greater conformational freedom allowing for more intimate packing with the polymer semiconductor. These findings demonstrate the potential of SAM interlayers to enhance OPV efficiency and mechanical reliability and that adhesion is sensitive to small variations of the SAM structure.