| Abstract Scope |
Basic organic electronics such as capacitors, transistors, and memory devices focus on novel organic materials and concepts for future applications. Processability at low temperatures from solution (T<150°C) is the key feature of organic electronics, allowing the use of low-cost flexible polymeric substrates (Polyethylene naphthalate, Polyethylene terephthalate). Solution processes enable large area fabrication of organic electronics using printing techniques and molecular self-assembly, respectively. The presented work will focus on a hybrid dielectrics formed by an aluminum/aluminum oxide layer (Al/AlO<SUB>x</SUB>) in combination with a self-assembled monolayer (SAM; here: alkyl phosphonic acids). The key benefit of this hybrid dielectric is low-voltage operation in organic electronics due to reduced dielectric layer thicknesses (7nm). Low-voltage operation and processability on flexible substrates are crucial for low energy consumption and adaption to mobile devices. Recent results show the impact of nano-scale variations on molecular lengths in the formation of alkyl phosphonic acid monolayers (C6-, C10-, C14-, C18-alkyl phosphonic acids). The alkyl chain length is correlated with device performance and leakage currents in organic electronics, specifically capacitors and transistors. Non-linear behavior of leakage current versus alkyl chain length is observed in experiments and correlated with Molecular Dynamics (MD) Simulations. MD Simulations display an increasing gap formation within the monolayer – without displaying pinholes – resulting in a reduced effective monolayer thickness. The theoretical calculations link the nonlinear correlation of the alkyl chain length and the leakage currents to the experimental results. Subsequently, hybrid gate dielectrics with alkyl phosphonic acids on Al/AlO<SUB>x</SUB> were used to develop flexible memory cells (bottom electrode/SAM/Al/AlO<SUB>x</SUB>/SAM/top electrode). A hybrid dielectric stack is used in a floating gate memory transistor geometry, integrating the functionality of a capacitor and transistor in a single device. Devices which are processed, stored, and characterized in air (without encapsulation) display high retention times on the order of 2 hours at low-programming voltages (±2V). Utilizing these results a novel memory approach based upon hybrid gate dielectrics incorporating a new molecular design was developed. The new molecular design unites the previously discussed insulation behavior of a monolayer (alkyl phosphonic acid as SAMs anchor group and spacer) and the redox activity of a fullerene (SAMīs head group). Low-voltage programming behavior is obtained (±2V) at retention times higher than 2 hours. The programming effect is based upon the fullerene moiety. In addition, studies demonstrate a highly controlled deposition of mixed monolayer at different molecular ratios. The film formation is studied by amplitude modified atomic force microscopy. Electronic characterization shows that the memory behavior of programmable thin film memory transistors strongly correlates with the amount of fullerene-based molecules within the monolayer. The retention time and the programming ratios are controlled by the quantity of fullerene alkyl phosphonic acids within the monolayer. |