Optical, Photophysical, and Optoelectronic Characterization of Green Light-Emitting Polymers for Applications in Flexible OLED-Based Optical Sensing

This study explores the integration of printable photoactive polymers poly(9,9-dioctylfluorene-alt-bithiophene)) (F8T2), poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), and spiro copolymer (SPG-01T), as well as the blended system of poly(9,9-dioctylfluorenyl-2,7-diyl) (F8) with F8BT (F8:F8BT), into polymer-based OLEDs (PLEDs) to be applied as actuators in wearable optical sensors. In-depth analysis of the photophysical, optical, and optoelectronic properties is conducted on thin films produced by using spin-coating (SC) and slot-die-coating (SD) techniques. This research aims to develop either rigid or flexible optical sensing devices. By fine-tuning the film thickness, emission profiles near the ideal green color coordinates (0.30, 0.60) were achieved, which are essential for precise optical sensing. Flexible PLEDs were fabricated to validate and examine their thickness-induced differences in emission bandwidths, color coordinates, and operational characteristics. The flexible devices bearing SPG-01T (77.3 nm) exhibited nearly ideal green color coordinates (0.29, 0.55) with a broad electroluminescence spectrum width of 95.4 nm, rendering them suitable for integration into OLED-based sensing platforms with broadband emission requirements. In contrast, flexible PLEDs that utilize thinner SD films of F8:F8BT (31 nm) and F8T2 (21.4 nm) produced narrower EL spectra, with widths of 74.1 nm and 63.8 nm, respectively. The color coordinates for these materials are (0.33, 0.60) for F8:F8BT and (0.32, 0.62) for F8T2. These characteristics make them particularly suitable for optical sensing applications that require spectral specificity, such as analyte detection. As a proof of concept, the detection of varying concentrations of Rhodamine 6G (R6G) was tested. Flexible devices using F8T2 and F8:F8BT exhibited a higher linearity in fluorescence response when detecting increasing amounts of R6G, achieving R2 values of 0.957 and 0.942, respectively. In contrast, devices using SPG-01T showed a much lower linearity, with an R2 value of only 0.587. Our findings show that the thickness of the emissive layer and the electroluminescence emission profile are critical parameters for the functionality and sensitivity of PLED-based flexible optical detectors.