The unique properties and potential optoelectronic applications of two-dimensional molecular crystals (2DMCs) of organic semiconductors make them fascinating research subjects. With advancements in crystal engineering, it is becoming reality to produce 2DMCs with molecular-level thickness and large areas up to the centimeter scale, enabling us to directly explore the electronic structure of 2DMCs and to correlate them with their electrical properties. Here, we investigated the electronic structure of 2DMCs of C6-DPA using photoemission spectroscopy and electrical properties based on organic field-effect transistors. Our findings indicate that anisotropic band dispersion is present in the ab plane of the 2DMCs of C6-DPA which is in good agreement with the in-plane anisotropic mobility, i.e., the direction of the strongest molecular overlap coincides with the direction of the highest mobility.

Two-dimensional, high-quality perylene single crystals were grown with a space-confined strategy method. The grown films crystallize in the alpha form, as is confirmed by a combination of techniques. Polarization -dependent optical absorption measurements show a strong anisotropy in very good agreement with the literature data, and the anisotropic mobility data in field-effect transistors document the very high crystalline order. Momentum-dependent studies using electron energy-loss spectroscopy reveal a negative dispersion of the first exciton along the crystal b direction with an exciton bandwidth of 72 meV. We argue that this behavior is a result of charge-transfer exciton coupling between the perylene dimers in the unit cell.

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.

Organic electrochemical transistors (OECTs) are a rapidly advancing technology that plays a crucial role in the development of next-generation bioelectronic devices. Recent advances in p-type/n-type organic mixed ionic-electronic conductors (OMIECs) have enabled power-efficient complementary OECT technologies for various applications, such as chemical/biological sensing, large-scale logic gates, and neuromorphic computing. However, ensuring long-term operational stability remains a significant challenge that hinders their widespread adoption. While p-type OMIECs are generally more stable than n-type OMIECs, they still face limitations, especially during prolonged operations. Here, we demonstrate that simple methylation of the pyrrole-benzothiazine-based (PBBT) ladder polymer backbone results in stable and high-performance p-type OECTs. The methylated PBBT (PBBT-Me) exhibits a 25-fold increase in OECT mobility and an impressive 36-fold increase in & mu;C* (mobility x volumetric capacitance) compared to the non-methylated PBBT-H polymer. Combining the newly developed PBBT-Me with the ladder n-type poly(benzimidazobenzophenanthroline) (BBL), we developed complementary inverters with a record-high DC gain of 194 V V-1 and excellent stability. These state-of-the-art complementary inverters were used to demonstrate leaky integrate-and-fire type organic electrochemical neurons (LIF-OECNs) capable of biologically relevant firing frequencies of about 2 Hz and of operating continuously for up to 6.5 h. This achievement represents a significant improvement over previous results and holds great potential for developing stable bioelectronic circuits capable of in-sensor computing.

Persistent photoconductivity (PPC) behavior in organic phototransistors has fascinating potentials in applications of photoelectronic devices. A key issue is how the presence of air affects the PPC behavior. Here, combining with the theoretical and experimental results, the PPC behavior is associated with photogenerated electrons trapped in oxygen atom-induced the reduced Lowest Unoccupied Molecular Orbitals or oxygen molecule-induced new trap state within energy bandgap of organic semiconductor. Inspired by the potential applications arising from the PPC behavior, organic memory phototransistors (OMPTs) are achieved by light programming and electrical erasing. The OMPTs show bistable current states as well as long retention times. Our results suggested that oxygen in air plays a key role in PPC behavior and provides a guidance for controlling the PPC behavior toward integrated multifunctional optoelectronic devices.