Trap-dominated nitrogen dioxide and ammonia responses of air-stable p-channel conjugated polymers from detailed bias stress analysis

Abstract

The improvement of conjugated polymer-based gas sensors involves fine tuning the backbone electronic structure and solid-state microstructure to combine high stability and sensitivity. We had previously developed a series of diketopyrrolopyrrole (DPP)-based polymer semiconductors by introducing a variety of fluorene linkers to study the trends and mechanisms governing gas sensitivities and electronic stability in air and under gate and drain bias stress. The proportional on-current change of organic field-effect transistors (OFETs) using a dithienyl DPP–fluorene polymer reached ∼600% for a sequential exposure from 0.5–20 ppm of NO₂ for 5 minutes and also a high response-to-drift ratio under dynamic bias stress. In the present work we specify the roles of static bias stress and traps in the sensing process for the first time. Apart from electronic structure, defects at the molecular and microstructural levels govern the ability to form and sustain traps and subsequent backbone dopability. A polymer with a twisted backbone was observed to be capable of creating an energetically broad trap distribution while a polymer with a high degree of solid-state order shows a tendency to form an energetically narrow trap distribution and a fast passivation of traps on exposure to air. The stability and energetic distribution of traps on subjecting the polymers to bias stress was related to electronic structure and solid-state packing; and the ability of NO₂ and NH₃ to fill/create traps further was evaluated. At a bias stress condition of V_G = V_D = −80 V, the polymers retain their NO₂ sensitivity both post NO₂-aided recovery and air-aided recovery. In order to verify the ability of NH₃ to create traps, traps were erased from the OFET sensors by charging with the aid of a positive gate voltage leading to an increase in the NH₃ response when compared to air controls. This work demonstrates that the charge-trap filling and generation response mechanism is predominant and can even be leveraged for higher responses to vapors. Backbone dopability appears to be a minor contributor to responses in this category of polymeric semiconductors with engineered defects. Finally, bias stress generally does not preclude this category of OFET vapor sensors from recovering their original sensitivities.