Electron Donating Functional Polymer Dielectrics to Reduce the Threshold Voltage of n‐Type Organic Thin‐Film Transistors
Bahar Ronnasi, Benjamin King, Samantha Brixi, Sufal Swaraj, Jukka Niskanen, Benoît H. Lessard
Abstract
Abstract Low‐cost and high‐performance electronics based on synthetically simple materials are required to fuel the deployment of smart packaging and wearable electronics. Metal phthalocyanines (MPcs) are promising semiconductors for use in n‐type organic thin film transistors (OTFTs) but often require high operating voltages. The first silicon phthalocyanine‐based OTFT with a polymer dielectric is reported as an alternative to traditional metal oxide dielectrics. Incorporating poly(methyl methacrylate) (PMMA) as the dielectric successfully reduces the threshold voltage ( V T ) of bispentafluorophenoxy SiPc (F 10 ‐SiPc) from 14.9V to 7.3V while retaining high mobility. Further reduction in V T is obtained by using copolymers and blends of PMMA and dimethylamino ethyl methacrylate (DMAEMA)‐containing polymers, where a higher molar fraction of DMAEMA leads to a consistent drop in V T to ‐0.7 V. The electron‐donating groups of the tertiary amines in the DMAEMA show clear interfacial doping of the semiconductor, reducing the voltage required to populate the dielectric/semiconductor interface with charge carriers and turn on the device. Blending trace amounts of DMAEMA‐containing copolymers with PMMA proves to be an effective strategy for reducing the V T while keeping the charge mobility high, unlike when using pure copolymers with elevated DMAEMA content. Time of flight secondary ion mass spectroscopy (ToF‐SIMS) and X‐ray photoelectron spectroscopy (XPS) demonstrate that the DMAEMA‐containing copolymer is floating to the surface of the PMMA blend at the dielectric–semiconductor interface, which explains the reduced V T . Synchrotron scanning transmission X‐ray microscopy (STXM) demonstrates that PMMA promotes a more edge‐on orientation of F 10 ‐SiPc films, compared to the more face‐on orientation when deposited on the DMAEMA containing copolymer. This study demonstrates a straightforward process for designing dielectric polymers and their blends for the reduction in V T for n‐type OTFTs.