Organic Light-Dependent Resistors with Near Infrared Light-Absorbing Conjugated Polymer Films

Here we demonstrate near-infrared (NIR) light-sensing organic light-dependent resistors (OLDRs), so-called organic photoresistors (OPRTs), which consist of solution-processed NIR light-absorbing films of poly[{2,5-bis(2-octyldodecyl)-3,6-bis(thien-2-yl)-pyrrolo[3,4-c]pyrrole- 1,4-diyl}-co-{2,2'-(2,1,3-benzothiadiazole)-5,5'-diyl}] (PODTPPD-BT). The present NIR-OPTRs were fabricated in a planar geometry by forming two aluminum electrodes in parallel on the PODTPPD-BT films with three different thicknesses (40, 100, and 160 nm). The sensing characteristics of NIR-OLDRs were examined using a NIR laser diode featuring a wavelength (lambda) of lambda = 905 nm that is one of the popular wavelengths employed for light detection and ranging (LiDAR) systems. In the dark condition, the electrical resistance (R-H) of devices in the horizonal direction was marginally changed with the thickness of PODTPPD-BT films. However, the R-H values were remarkably reduced upon the NIR light illumination for all devices and were strongly dependent on the NIR light intensity. Thermal annealing of devices delivered greatly improved current responses leading to a huge R-H reduction (by ca. 80%) and excellent sensing stability, even though only 49% improvement in R-H was achieved by thermal annealing of films.