Sensitivity enhancement of NPD: Alq3 based organic humidity sensor via thermal annealing treatment

The present study aims to develop a simple and cost-effective organic-based humidity sensor with a planar Al/organic sensing layer/Al structure. The organic sensing layer, composed of pristine N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) and tris(8-hydroxyquinolinato)aluminum (Alq₃) as well as their evenly blended composite, were deposited via a solution-processed spin coating technique onto aluminum electrodes with a ∼67.5 µm gap. Thermal annealing treatments ranging from 80 to 200 °C were conducted to improve surface properties and enhancing their sensing parameters for the purpose of device optimization. Wettability properties were observed, use being made of contact angle measurement while, the morphology insights were obtained by atomic force microscopy (AFM). The study employed both capacitive and resistive measurement modes, revealing a significant enhancement in humidity sensitivity post-annealing. Notably, the device annealed at 100°C exhibited the highest sensitivity, demonstrating superior repeatability and stability under cyclic humidity variations. The response and recovery times for transitions between 5 % and 100 % relative humidity (RH) were recorded at 11.17 s and 1.76 s, respectively. These findings provide deeper insights into organic-based humidity sensing mechanisms and paving their way for their potential integration of organic thin-film-based sensors for reliable and efficient humidity detection across diverse applications.