Solution Processed OFET Substrate System Overview
The exploded diagram below illustrates the layers typically used with the evaporation free OFET substrates. The substrates contain pre-patterned ITO source-drain contacts onto which the semiconductor is deposited before a gate insulator is spun on top and finally the gate material finishes the device. All of these layers can be deposited from solution, meaning no evaporation shadow masks are required. However, we stock active area masks, gate masks and gate insulator masks, for when vacuum deposition is required.
The pre-patterned OFET substrates consist of interdigitated ITO fingers to act as the source-drain contacts and have overall channel dimensions of 30 mm x 50 µm. These relatively large channels mean that contact effects are minimised so that matching the energy levels of the organic semiconductor and contacts are less important. As such the intrinsic Poole Frenkel limited mobility of a material can be more easily assessed independent of the HOMO level. The 30 mm channel length also has the additional benefit of producing larger currents making testing and measurement quicker and requiring less sensitive (less expensive) equipment.
The substrate itself is made of high-quality soda-lime float glass with a 20 nm coating of SiO2 (synthetic quartz) which allows for HMDS and OTS treatments to improve mobility. The transparent nature of the ITO/glass also means that opto-electric experiments can be done with ease (photoconductivity, photo induced doping in the presence of oxygen, sensing).
Transistors can be fabricated simply by depositing an organic semiconductor, gate insulator and gate on the substrates (see diagram below for device schematic). By using a synthetic metal (such as PEDOT:PSS) deposited from solution for the gate it is possible to make fully functioning OFETs by solution processing alone and eliminate vacuum evaporation processes. The use of the Ossila OFET test board also means that devices can be measured at the flick of a switch without the need for probe stations.
While all solution based processing allows transistors to be fabricated in a matter of minutes, the substrates are also designed to work with a wide variety of different material systems and deposition techniques. As such, vacuum deposited semiconductors, gate insulators and gates can also be used with the appropriate shadow mask.