BIFUNCTIONAL TRIS (8-HYDROXYQUINOLINATO) ALUMINIUM ORGANIC SEMICONDUCTOR-BASED THIN FILM FOR ULTRAVIOLET AND HUMIDITY SENSING

Abstract

In the past decades, organic semiconductor materials have proven to be remarkably prospective in the applications of electronic devices. They have been utilized in optoelectronic devices, including solar cells, light-emitting diodes, and light sensors, as well as in various sensing studies. Organic-based devices are attractive due to their low production cost and easy processing technique compared to their inorganic counterparts. In this work, the organic semiconductor materials of 4,4’-bis[N-(1-naphthyl)- Nphenylamino],biphenyl (NPD) and tris (8-hydroxyquinolinato) aluminium (Alq3) of a small molecule group have been selected as the main materials in the fabricated devices. In general, the active layers for electronic devices based on organic small molecules are mostly prepared using the thermal evaporation method, which is expensive and complicated. However, this study explores the possibility of utilizing a solution processing method in preparing an active sensing layer based on inexpensive small molecules of the Donor/Acceptor (NPD/Alq3). In the first phase, a visible-blind organic UV photodetector with a device structure of ITO/PEDOT:PSS/NPD:Alq3/LiF/Al was fabricated, and its electrical performance was investigated. The active layer is in the form of a bulk heterojunction of the binary blend of organic NPD:Alq3 is prepared using a simple solution-processed method. The influence of different NPD:Alq3 blend ratios has been investigated in the optical study of the thin films and the electrical performance of the photodiode. The optical characteristic of the film is almost transparent at the visible wavelengths above 400 nm while exhibiting high absorption within the UV light region. Moreover, it possesses emission quenching of photoluminescence intensity when more Alq3 is added (i.e., 2 and 3) with respect to the NPD host. In the electrical characterization, the optimized device (1:2 blend ratio) exhibits high sensitivity without an externally applied voltage, which reveals that the UV photodetector could operate in self-powered mode. The device showed high sensitivity with a photocurrent to dark current ratio up to 1.30105. Furthermore, the device's electrical performance variation under the illumination of different UV light intensities from 0.1 to 40 mW/cm2 was tested. The maximum values achieved for responsivity and detectivity of the photodiode under zero bias voltage are 5.39 mA/W and 5.251011 Jones, respectively. Finally, the optimized device's rise time and decay time are estimated to be 0.34 and 0.28 s, respectively. In the second phase, the humidity sensor devices were fabricated in the planar geometry of Al/organic sensing layer/Al. The organic layer of pristine NPD and Alq3 and their evenly blended composite are deposited using a solution-processed spin coating technique onto ~67.5 μm gap between aluminium electrodes. Thermal annealing treatment is conducted to improve surface properties and enhance sensing parameters for the purpose of device optimization. The structure-property-performance relationship was investigated and established for organic-based thin films and the consequences on the electrical characteristics of the fabricated humidity sensor. Both capacitive and resistive measurement modes were performed with respect to the humidity level. The optimized sensitivity in both measurements is obtained for the annealed device at 100°C. The sensor shows repeatability behavior and consistency towards cyclic change in the relative humidity levels. The response and recovery time of the change between two distinct relative humidity levels of 5% and 100%RH are measured to be 11.17 s and 1.76 s, respectively.