Solution Processed Organic Semiconductor: Devices And Applications

Owing to the advantages of self light-emission, fast response, solid thin film device, easyfabrication, wide view-angle, thin form factor, wide operation temperature, flexibility, etc.,organic light-emitting diodes （OLEDs） are considered as the next generation displaytechnology.There are two variations of the OLED display technology. One is so called “smallmolecule”（SMOLED） technology. The other is “conjugated polymer”（PLED） basedtechnology. Though SMOLED technology has become the most popular technology in theflat panel display, its fabrication requires vacuum deposition with shadow mask, leading to ahigh production cost. PLED are usually fabricated by solution process, such as spin-coating,dip-coating, screen printing, slot-die coating, and inkjet printing. The solution process can becarried out at low or room temperature without any shadow mask, which are more promisingfor large size, high definition, and high resolution displays.High quality film formation is critical in PLED device. The simplest process to achievehigh quality film is spin-coating. Spincoating is fit for fabricating monochrome PLED display.However, since we need to deposive three kind of polymer of different color to specific pixelsrepectively to achieve micro-patterning of full color display, the most promising option isinkjet printing.Inkjet printing is of great potential to patter colors for full color PLED display with theadvantages of material savings, low cost, and being environmentally friendly. Since inkjetprinting is a non-contact printing method, it avoids contamination to the functional layer. Toachieve uniform, pin-hole free, optical-quality solid films by inkjet printing, the most criticalprocess is to achieve high quality jetted solution drops. The quality of the drops depend on themolecular weight of the polymer, and the viscosity and the surface tension of the solution. Weblended the conjugated polymer with solution-procesable small molecule to obtain inkjetprintable inks with high performance. Moreover, the coffee ring was successfully suppressedby using the mixture of high and low boiling point solvents, leading to a more uniform filmand a larger fill factor for PLED displays.In recent years, semiconducting nanostructures have attracted enormous attention fortheir potential applications in the integrated nano-or micro-optoelectronics. Self-assembled,crystalline, organic, one-dimensional （1D） nanostructures become promising building blocksfor high-performance organic field-effect transistors （FETs）, and highly sensitivephotodetectors, due to their tunable electronic and optical properties via molecular design, low-cost device fabrication, and process simplicity. While most efforts have beenconcentrated on FETs, only a few photoconductive organic1D nanostructures have beenreported, most of which focus on the design and synthesis of new molecules. By combining abetter crystalline structure with proper surface engineering, i.e. coating a dielectric layer ofPMMA between the substrate and the organic semiconductor, we achieved the world-recordperformance of organic nano/micro-structure based photodetectors with two-terminal devicestructure. The photoconductivity gain is enhanced to1.3×10⁴, and the responsivity isimproved to4,372A W-1at the field of about2×106V m^-1. The highest on/off ratio reaches104. The extremely high gain is attributed to the high mobility and the long photo carrier’slifetime induced by the dielectric layer.