Mid-infrared photoconductivity of polypyrrole doped with cadmium sulfide quantum dots

Abstract We report the facile (one-step) in situ doping of soluble conducting polypyrrole (PPy) with cadmium sulfide quantum dots (CdS QDs) aimed at tuning the mid-infrared (MIR) (2.5–25 μm) photoconductivity of PPy films with a planer structure. The doping was done during chemical polymerization of PPy with different mass ratios (0–1800 ppm) of the as-synthesized CdS QDs. The PPy/CdS QD films were fabricated by electrospray coating between gold interdigitated electrodes. The incident light in the MIR region (7.62 mW/cm2) was chopped, the prepared film was illuminated with it, and the photocurrent generated was recorded. Doping with CdS QDs at 2–15 ppm increased the photoconductivity of PPy as they generated greater photocurrent than the undoped PPy, which is attributed to the conductivity enhancement in the doped polymers. More addition of CdS QDs (>15 ppm) decreased the photoconductivity of PPy. So, the best doping concentration was found to be 15 ppm, which produced an average photocurrent of 177.3 μA, which exhibited a large photosensitivity of 1772 and photoresponsivity of 23.25 mA/W. We showed that the in situ doping not only increases the mobility of generated charge carriers in the polymer but also narrows the optical band gap of PPy by about 0.13 eV, enabling more absorption in the MIR region. Essential to this success was the UV–visible characterization of PPy doped with CdS QDs followed by Tauc calculations, which verified the narrowing of the band gap. The high-performance photoconductivity of PPy/CdS QD films in the MIR region suggested their potential applicability in organic electronic devices.