Molecularly Engineered Self-Assembled Monolayers as Effective Hole-Selective Layers for Organic Solar Cells

Self-assembled monolayers (SAMs) are an emerging class of hole-selective layers (HSLs) to replace the conventional poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) in organic solar cells (OSCs). Despite the wide use of SAMs, it is difficult to directly establish a feedback loop between material design and OSC performance as the SAM quality will also affect the OSC performance and was frequently neglected. In this work, we designed a series of carbazole-derived SAMs by engineering the halogen substituents and the alkyl linker lengths. A SAM stacking model was established to evaluate the SAM qualities in terms of surface morphology, molecular bonding, and packing quality. Consequently, the iodinated carbazole SAM with C2 spacer (Cz-I-2) showed the highest molecular regularity, the top OSC performance in the PM6:Y6 system, and good universal applicability with a power conversion efficiency (PCE) of 18.1% in the D18:Y6 system. It can be concluded that SAMs should benefit from OSC performance by achieving suitable energy level alignment, high packing regularity, and enhanced interactions with adjacent layers. Our work provides insight into designing SAMs for effective HSLs in efficient OSCs.