  Atomically thin materials such as graphene are uniquely responsive to charge
transfer from adjacent materials, making them ideal charge transport layers in
phototransistor devices. Effective implementation of organic semiconductors as
a photoactive layer would open up a multitude of applications in biomimetic
circuitry and ultra-broadband imaging but polycrystalline and amorphous thin
films have shown inferior performance compared to inorganic semiconductors.
Here, we utilize the long-range order in rubrene single crystals to engineer
organic semiconductor-graphene phototransistors surpassing previously reported
photo-gating efficiencies by one order of magnitude. Phototransistors based
upon these interfaces are spectrally selective to visible wavelengths and,
through photoconductive gain mechanisms, achieve responsivity as large as 10^7
A/W and a detectivity of 1.5 10^9 Jones at room temperature. These findings
point towards implementing low-cost, flexible materials for amplified imaging
at ultra-low light levels.
