Strong dark current suppression in flexible organic photodetectors by carbon nanotube transparent electrodes

Carbon nanotubes possess outstanding mechanical, optical, and electronic properties and have been proposed as a replacement for indium–tin oxide in curved and flexible organic photodetectors to realize nearly 100% omnidirectional sensing. Because organic photodetectors require dense nanotube arrays to form suitable top layers, it is important to counterbalance their transmittance and packing density to achieve high performance. Carbon nanotubes are introduced as an electrode on a flexible substrate in this study. The carbon nanotube electrode, with an average visible transmittance of 90%, suppresses dark current considerably below the picoampere level, leads to a high detectivity of 2.07 × 1014 Jones, which is 100 times larger than that of the indium–tin oxide-based devices. The deep work function of the carbon nanotube electrode induced a high electron injection barrier, which was investigated by photoelectron yield spectroscopy, considering the energy level bending caused by the electrode–semiconductor contact. The flexible organic photodetectors fabricated with the carbon nanotube electrode operated stably after a cyclic flex test of 500 bendings. This paper demonstrated the effective implementation of curved and flexible image sensors by developing an excellent flexible photodetector.