Electronic band structure serves as the foundation for understanding the physics of semiconductors. The electronic band structure of inorganic semiconductors has been well understood based on ideal single crystal samples, thus actually laid the foundation for the prospering development of inorganic semiconductor devices.

As for organic semiconductors, which are currently being pervasive in our daily life, however, the ‘physics’ of organic semiconductors is still far from complete understanding compared to their inorganic counter-parts which could hinder the rapid development of organic electronics. Only few organic single crystals (e.g., rubrene, pentacene) have been well investigated to probe the electronic structure and their relationship with their electrical properties, sufficient experimental evidence is still lacking for intrinsic properties of organic single crystal, which mainly hindered by the limited crystal size and low conductivity.

Two-dimensional molecular crystals (2DMCs) of organic semiconductors are intriguing materials because their unique advantages, such as long-range molecular packing, low defect density and lack of grain boundaries, make 2DMCs an ideal platform for exploring the structure-property relationship, revealing the intrinsic properties and probable carrier transport mechanism, fabricating high-performance optoelectronic devices. Especially, unique optoelectronic properties exhibited in 2DMCs are not found in their bulk counterparts. With breakthrough in crystal engineering for producing large-area (e.g., millimeter or centi-meter even wafer-sized) 2DMCs and material engineering for designing novel organic semiconductors (e.g., C10-DNTT, C6-DPA), all provide a great opportunity to explore the physical origin behind the novel optoelectronic properties of kinds of organic semiconductors and their correlation with optoelectronic device performance, which further guiding material design and facilitating flourishing of organic electronics.

The aim of this thesis is to investigate the electronic structure of 2DMCs and correlate them with their optoelectronic properties. The 2DMCs were mainly produced by space-confined strategy and layer-defining strategy, the produced 2DMCs could be transferred to any substrates for further characterization. One of selected organic materials is 2,6-Bis(4-hexylphenyl)anthracene (C6-DPA), which belongs to anthracene derivates family, typically known for their high luminescence efficiency and carrier mobility. All characterized 2DMCs of C6-DPA show a high quality. Firstly, we fabricated integrated organic effect field transistors e.g., organic phototransistors, organic memory phototransistors based on 2DMCs of C6-DPA to elucidate the high performance and potential applications. To clarify the physical origin of opto-electronic properties, some advanced surface science experimental techniques were used to determine the electronic structure of 2DMCs of C6-DPA. Resonant photoemission spectroscopy reveals a room temperature band dispersion of C6-DPA, which is well explained by calculated band structure. Angle-resolved photoemission spectroscopy results confirm the room temperature dispersion and exhibit anisotropic band dispersion in plane. The anisotropic charge carrier mobility is 2 in plane, where the highest mobility obtained along the molecular direction with obvious band dispersion, suggesting the electronic property and electrical property quite match well. We then investigated the in-fluence of degree of crystallinity on electronic structure by ultraviolet photoemission spectroscopy, all results indicate that high crystallinity help to overcome Coulomb interaction and facilities charge to be delocalized on whole 2D crystal, while in verse in less degree of thin film. We then selected perylene as the second material to explore the exciton band structure by electron energy-loss spectroscopy. The observed negative band dispersion is rationalized by effective inter-dimer coupling with an additional charge transfer contribution. This result could provide guidance for understanding the in-plane charge transport properties in 2D crystal of perylene.