A high-yield vacuum-evaporation-based R2R-compatible fabrication route for organic electronic circuits

Abstract

Advances are described in a vacuum-evaporation-based approach for the roll-to-roll (R2R) production of organic thin film transistors (TFTs) and circuits. Results from 90-transistor arrays formed directly onto a plasma-polymerised diacrylate gate dielectric are compared with those formed on polystyrene-buffered diacrylate. The latter approach resulted in stable, reproducible transistors with yields in excess of 90%. The resulting TFTs had low turn-on voltage, on–off ratios ∼106 and mobility ∼1cm2/Vs in the linear regime, as expected for dinaphtho[2,3-b:2′,3′-f] thieno[3,2-b]thiophene the air stable small molecule used as the active semiconductor. We show that when device design is constrained by the generally poor registration ability of R2R processes, parasitic source–drain currents can lead to a >50% increase in the mobility extracted from the resulting TFTs, the increases being especially marked in low channel width devices. Batches of 27 saturated-load inverters were fabricated with 100% yield and their behaviour successfully reproduced using TFT parameters extracted with Silvaco’s UOTFT Model. 5- and 7-stage ring oscillator (RO) outputs ranged from ∼120Hz to >2kHz with rail voltages, VDD, increasing from −15V to −90V. From simulations an order of magnitude increase in frequency could be expected by reducing parasitic gate capacitances. During 8h of continuous operation at VDD=−60V, the frequency of a 7-stage RO remained almost constant at ∼1.4kHz albeit that the output signal amplitude decreased from ∼22V to ∼10V. Over the next 30days of intermittent operation further degradation in performance occurred although an unused RO showed no deterioration over the same period.