Self-Assembled Monolayer Enables Hole Transport Layer-Free Organic Solar Cells with 18% Efficiency and Improved Operational Stability

We report on bulk-heterojunction (BHJ) organic photovoltaics (OPVs) based on the self-assembled monolayer (SAM) 2PACz as a hole-selective interlayer functionalized directly onto the indium tin oxide (ITO) anode. The 2PACz is found to change the work function of ITO while simultaneously affecting the morphology of the BHJ deposited atop. Cells with PM6:N3 BHJ and ITO-2PACz anode exhibit a power conversion efficiency (PCE) of 16.6%, which is greater than that measured for bare ITO (6.45%) and ITO/PEDOT:PSS (15.94%) based devices. The enhanced performance is attributed to lower contact-resistance, reduced bimolecular recombination losses, and improved charge transport within the BHJ. Importantly, the ITO-2PACz-based OPVs show dramatically improved operational stability when compared with PEDOT:PSS-based cells. When the ITO-2PACz anode is combined with the ternary PM6:BTP-eC9:PC71BM BHJ, the resulting cells exhibit a maximum PCE of 18.03%, highlighting the potential of engineered SAMs for use in hole-selective contacts in high-performance OPVs.