Electrical Reliability Enhancement in Organic Thin-film Transistors and Circuits Using a Hydroxyl-containing Polymer Blend as Gate Dielectric

Abstract

A polymer gate dielectric is one of the key elements of solution-processed organic thin-film transistors (TFTs) for low-cost and large-area electronic applications. In the past few decades, a hydroxyl-containing polymer blend of poly(4-vinylphenol) (PVP) and poly(melamine-co-formaldehyde) (PMF) has been widely used as gate dielectric due to its low leakage current, high dielectric constant, and good chemical compatibility in organic TFTs.1) However, the polymer blend has been compatible only with suitably engineered expensive plastic substrates due to its high annealing temperature (typically higher than 175 ºC) to remove hydroxyl groups that cause the substantial hysteresis phenomenon in the current-voltage characteristics.2,3) In this work, we investigated the influence of various PVP:PMF blend conditions such as weight ratio (0.2:1 to 5:1) and annealing temperature (100 to 200 ºC) on the device performances in 6,13-bis(triisopropylsilylethynyl) organic TFTs (Figs. 1 and 2). We found that the organic TFT using the PVP:PMF blend with the weight ratio of 0.5:1 and annealing temperature of 100 ºC showed negligible hysteresis, high on/off drain current ratio, and low gate-leakage current. Fourier transform infrared spectroscopy measurements provided the evidence that the optimized condition of PVP:PMF blend is strongly related to the minimization of non-hydrogen-bonded hydroxyl groups. Finally, we successfully demonstrated transistors and diode-connected inverters on a 3-μm-thick flexible parylene film. This work shows that the reduction of non-hydrogen-bonded hydroxyl groups in polymer dielectrics can effectively lower the process temperature and enhance the device electrical reliability on inexpensive plastic substrates.