Monolayer molecular crystals: controlled-growth technologies, structure-property relationships, and device applications

Monolayer molecular crystals (MMCs) have emerged as a distinctive class of two-dimensional (2D) organic semiconductor (OSC) materials that combine the structural tunability of molecular semiconductors with the interfacial sensitivity of atomically thin systems. In the monolayer limit, molecular packing, substrate coupling, dielectric screening, and charge injection become strongly intertwined, providing a unique platform for exploring structure-dependent charge transport, photoresponse, and multifunctional device behaviour. Recent advances in solution-based self-assembly, meniscus-guided coating, confinement-assisted growth, physical vapor transport (PVT), vacuum thermal deposition (VTD), and van der Waals epitaxy (vdWE) have substantially expanded the toolbox for producing MMCs with controlled thickness, crystalline order, and molecular orientation. Nevertheless, a comprehensive understanding of molecular assembly, defect formation, and interface effects remains incomplete, while achieving reproducible and scalable fabrication and device integration remains challenging. In this review, we present a unified framework connecting controlled growth, structure-property relationships, and device performance, with particular emphasis on how molecular packing, defects, and interfaces collectively regulate charge transport in MMCs. We further critically examine the evaluation and benchmarking of MMC-based devices by considering mobility, contact resistance (R_c), and stability. Finally, we distinguish fundamental scientific challenges from technological barriers and discuss opportunities for scalable fabrication, reliable integration, and flexible electronics. This review provides mechanistic insights and design principles for the rational development of MMC-based electronics.