Enhanced Coherence Length and Optical Disorder in PBTTT/SnS ₂ Composites: A Structure–Property Analysis for NO ₂ Sensing

The realization of low-power, room-temperature gas sensors with high sensitivity and operational stability remains a major challenge in organic electronics. Metal-oxide sensors require elevated temperatures, and therefore, high power, polymer-based organic field-effect transistor (OFET) sensors are often constrained by limited stability and insufficient molecular ordering. In this study, a rapid floating film transfer method was used to study the air–liquid interface self-assembly of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2- b ]thiophene] (PBTTT)/tin disulfide (SnS 2 ) composite thin films. Structural characterization (GI-XRD) indicates enhanced molecular ordering in the PBTTT/SnS 2 film, while UV–vis absorption analysis shows a bathochromic shift in the vibronic features and a marginal change in the extracted optical gap, accompanied by a pronounced Urbach tail that reflects an increased short-range energetic disorder. A top-contact, bottom-gate organic field effect transistor on a SiO 2 -coated p +2 Si substrate of PBTTT/SnS 2 was fabricated for the sensing of NO 2 gas at room temperature. The composite exhibits modified charge-transport behavior relative to pristine PBTTT. A small range of NO 2 concentrations from 0 to 11.5 ppm was exposed on the sensing layer to study the sensitivity and selectivity of the fabricated device. The PBTTT/SnS 2 OFET had a better sensing response of ∼215.27% at 11.5 ppm with a limit of detection of 0.18 ppm (180 ppb). A dynamic response measurement was conducted at an 11.5 ppm concentration of NO 2, and response-recovery times were found to be 98.2 and 284.6 s, respectively. All the electrical characterizations and sensing measurements were done at RT and 55% relative humidity.