Application of a heterojunction organic photodiode to radiation measurement

Organic optical photodiodes (OPDs) have attracted considerable interest in fields using photonic devices because they present the potential for use as flexible photodetectors for large areas. To elucidate their applicability to radiation measurements, OPDs with a classical heterojunction structure were fabricated and used for X-ray measurements. The device, whose structure was ITO (150 nm)/ ¿-NPD (50 nm) / td-PTC (50 nm, 110 nm) / Al (80 nm) with sensing area of 2 mm × 2 mm, showed rectifying properties under forward and reverse voltage. Under irradiation of white light with intensity of 100 mW/cm2, the measured current was 103-104 times larger than the dark current. We conducted X-ray irradiation experiments, revealing that X-rays increased the current, although some problems persist, such as low photon-to-current conversion efficiency. At acceleration voltage of 200 kV and tube current of 2 mA, the current increased to about 40 pA from about 5 pA in a dark condition. The X-ray-induced current increased concomitantly with increasing X-ray tube current, although the curve showed a saturating tendency. Subsequently, X-ray shielding experiments with Cu plates and X-ray intensity profile measurements were conducted; results show that the OPDs functioned as a radiation detector. An inorganic scintillator on the OPD enhanced the X-ray-induced current, thereby demonstrating the possibility of a combined system of OPD and usual scintillating material as a new radiation detector. Although further study is necessary for elucidating X-ray-induced processes in devices and for enhancing their charge collection efficiency, we can conclude that OPD can be useful as a new radiation detector.