Multilevel Nonvolatile Organic Photomemory Based on Vanadyl-Phthalocyanine/para-Sexiphenyl Heterojunctions

Organic photomemory based on heterojunction photo-transistor has been fabricated utilizing vanadyl-phthalocyanine (VOPc) on para-sexiphenyl (p-6P) thin films. Under 365 nm ultraviolet light irradiation, the ratio of photocurrent and dark current (I-ph/I-dark) and photoresponsivity of phototransistors are about 1.5 x 10(5) and 87 A/W, respectively. Such devices can transduce the input light signals into electrical signals and the output signals can be stored for recording the light simulation. After applying a light pulse (4.2 mW/cm(2), 100 ms) on the device, the stored current level lasted for similar to 5000 s with only a 20% decrease, indicating a good photomemory behavior. Importantly, the photomemory behavior is effectively modulated by gate voltage. Multilevel photomemory behaviors are observed by modulating light pulse duration and light power intensity. Because of the construction of type-I heterojunction, the superior photomemory characteristics are mainly originated from efficient charge trapping at VOPc/p-6P interface. In situ current sensitive atomic force microscopy (CSAFM) is used for monitoring surface current of the VOPc/p-6P heterojunctions. A change of conductivity in grains is observed upon 365 nm light illumination. After turning off the light, the current of grains did not rapidly decrease, but displayed the behavior of photomemory. This study provides a guide for designing high-performance organic photomemory devices.