Charge transport characterization in a squaraine-based photodetector by means of admittance spectroscopy

Thanks to their strong absorption in the near infrared, squaraine dyes have raised interest in the field of organic photovoltaics and photodetection. We develop photodetectors based on the bulk heterojunction of an indolic squaraine and of a fullerene derivative, obtaining an external quantum efficiency as high as 12% at 590 nm. To gain insight into carrier transport and recombination phenomena, devices are characterized in the dark by means of admittance spectroscopy. At low applied voltages carrier transport is dominated by holes, which show dispersive transport and an electric field dependent mobility of about 10-5-10-4 cm2 V-1 s-1. At higher voltages electron injection occurs as well and a bipolar, recombination-limited regime sets in. The modeling of the low-frequency negative capacitance contribution suggests a trap-mediated carrier recombination mechanism.