Improving the Mechanical Resilience of Organic Photovoltaics Using Self-Assembled Monolayers

While mechanical flexibility and stretchability are often touted as a benefit of OPV cells, they can still be prone to mechanical failure. OPV cells are subject to a variety of internal and external stresses throughout their lifecycle, from fabrication to operation. Under flexing a common failure pathway is through delamination between the heterogeneous layers that make up the cell. Among the interfaces within the OPV stack, the electron transporting layer and hole transport layer (HTL) have been identified as the weakest links, prone to delamination failure. Various methods have been employed to enhance the interfacial toughness of these layers, such as the addition of additives like D-sorbitol or through UV-ozone treatment of oxide layers. However, these methods often come at the expense of device performance or show limited improvements in mechanical reliability. In this paper, we show that self-assembled monolayers (SAMs) can act as efficient interlayer materials that not only improve optoelectronic functionality but also improve mechanical adhesion.
The phosphonic acid SAM 2PACz has been shown to be an effective interface layer improving device performance compared to the commonly employed PEDOT:PSS HTL. The latter is known to have poor stability and can also suffer from weak interfacial adhesion. Thus, we investigate 2PACz and its variants X-2PACz (X = MeO-2PACz, Cl-2PACz, and Br-2PACz) on optoelectronic performance and mechanical behavior. We apply these SAMs as the hole transport interlayer in OPV cells composed of PM6 and L8-BO. We find that the use of the halogenated 2PACz SAMs increased the power conversion efficiency (PCE) by nearly 1% compared to PEDOT:PSS. In addition, mechanical peel and shear tests reveal that employing 2PACz results in a 3-fold increase in peel and shear strength compared to PEDOT:PSS. To gain insight into the differences in adhesion, we perform molecular dynamic simulations and calculate the interaction energy between the SAM and organic semiconductors. We find that the interaction energy correlates well with adhesion measurements. The largest interaction energy was found for 2PACz which is attributed to the more face-on orientation of the carbazole unit leading to stronger van der Waals attractive forces at the interface. In summary, this work shows that SAMs can be an effective strategy to improve OPV performance, operational stability, and mechanical resilience. Based on this study, we will outline guidelines that drive improved adhesion in OPV cells enabling mechanically robust devices.