High-Performance Organic Single Crystal /Heterojunction Based Devices

Organic semiconducting materials and devices have attract tremendous attention, due to their advantages of flexibility, low-cost and large-scale processing. In organic semiconducting devices, such as organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs) and ambipolar organic field-effect transistors (OFETs), p-n junction is a very essential component to realize different functions. However, the ordering of molecular packing is usually low, which limits the performance of organic semiconductors. Thus organic semiconductors with highly ordered molecular packing are desired. Moreover, solution-processing is a main advantage of organic materials over inorganic ones. Therefore, preparation and investigation of organic single-crystalline semiconductors and their heterojunctions will accelerate the application of single crystals for high performance organic semiconducting devices.In this thesis, we reported a facile yet efficient solution method to prepare organic semiconducting single crystals and single-crystalline p-n heterojunctions. Ambipolar charge transport characteristics and photovoltaic behavior of the heterojunctions were also investigated.In Chapter 1, preparation approaches of organic semiconducting single crystals and single crystal based heterojunctions are summarized. The applications of single crystal based heterojunctions in ambipolar OFETs and OPVs are also reviewed.In Chapter 2, we reported five organic small molecule well-aligned single crystals grown from solutions using the droplet-pinened crystallizaiton (DPC) method. The morphology of the aligned crystals were characterized and field-effect charge transport characteristics were investigated. Charge carrier mobilities obtain from these single crystals are much higher than that from amorphous films.In Chapter 3, we described the growth of single-crystalline p-n junctions in a single step from a mixed solution of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) (p-type) and C6o (n-type), using the DPC method. Morphology and crystalline structures were characterized and the formation mechanism of the junctions was studied under optical microscope. By choosing proper solvents, the shape of C6o crystals can be varied thus grown into different p-n junctions. With this method, several p-n junctions based on different p-n pairs were also prepared.In Chapter 4, single-crystalline p-n junctions based on C8-BTBT and C60 grown from solutions of different solvents and concentrations were investigated. FETs based on these junctions exhibited ambipolar charge transport characteristics with the best performance of 0.16 cm2V-1s-1 for hole mobility and 0.17 cm2V-1s-1 for electron mobility, respectively, from C8-BTBT ribbon/C60 ribbon single-crystalline junctions. C8-BTBT ribbon/C6o needle p-n junctions gave relatively low electron mobility of 10-5 cm2V-1s-1 order due to low coverage of C60 crystals and air instability.In Chapter 5, we demonstrated that extended single-crystalline heterojunctions with a consitent donor-top and acceptor-bottom structure throughout the substrate can be easily obtained from a mixed solution of 3,6-bis(5-(4-n-butylphenyl)thiophene-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP-PR) (donor) and C60 (acceptor). Morphology and crystalline structures were well characterized. Based on the donor-top and acceptor-bottom structures, an inverted structure of ITO/ZnO/DPP-PR-C60/Mo03/EGaIn was constructed.46 photovoltaic cell devices were studied with power conversion efficiency of 0.26+0.10% under 1 sun, which is significantly higher than the reported value of vapor-grown organic single-crystalline heterojunction (0.007%).