Light-modulated vertical heterojunction phototransistors with distinct logical photocurrents.

The intriguing carrier dynamics in graphene heterojunctions have stimulated great interest in modulating the optoelectronic features to realize high-performance photodetectors. However, for most phototransistors, the photoresponse characteristics are modulated with an electrical gate or a static field. In this paper, we demonstrate a graphene/C60/pentacene vertical phototransistor to tune both the photoresponse time and photocurrent based on light modulation. By exploiting the power-dependent multiple states of the photocurrent, remarkable logical photocurrent switching under infrared light modulation occurs in a thick C60 layer (11 nm) device, which implies competition of the photogenerated carriers between graphene/C60 and C60/pentacene. Meanwhile, we observe a complete positive-negative alternating process under continuous 405 nm irradiation. Furthermore, infrared light modulation of a thin C60 (5 nm) device results in a photoresponsivity improvement from 3425 A/W up to 7673 A/W, and we clearly probe the primary reason for the distinct modulation results between the 5 and 11 nm C60 devices. In addition, the tuneable bandwidth of the infrared response from 10 to 3 × 103 Hz under visible light modulation is explored. Such distinct types of optical modulation phenomena and logical photocurrent inversion characteristics pave the way for future tuneable logical photocurrent switching devices and high-performance phototransistors with vertical graphene heterojunction structures.