I. Introduction
INORGANIC semiconductors are used for many photodetector devices, but because of their disadvantages of high cost and low flexibility, demands for new photonic devices are increasing in wide areas of application. Consequently, organic semiconductors have attracted considerable interest in the fields of photonics devices because of their potential use as flexible photodetectors for large areas. Active studies have continually improved organic photodiode (OPD) properties in recent years [1]–[12].