  Conjugated polymer films that can conduct ionic and electronic charges are
central to building soft electronic sensors and actuators. Despite the possible
interplay between light absorption and mixed conductivity of these materials in
aqueous biological media, no polymer film has ever been used to realize a
solar-switchable organic bioelectronic circuit relying on a fully reversible,
redox reaction-free mechanism. Here we show that light absorbed by an electron
and cation-transporting polymer film reversibly modulates its electrochemical
potential and conductivity in an aqueous electrolyte, leveraged to design an
n-type photo-electrochemical transistor (n-OPECT). We generate transistor
output characteristics by solely varying the intensity of light that hits the
n-type polymeric gate electrode, emulating the gate voltage-controlled
modulation of the polymeric channel current. The micron-scale n-OPECT shows a
high signal-to-noise ratio and an excellent sensitivity to low light
intensities. We demonstrate three direct applications of the n-OPECT, i.e., a
photoplethysmogram recorder, a light-controller inverter circuit, and a
light-gated artificial synapse, underscoring the suitability of this platform
for a myriad of biomedical applications that involve light intensity changes.
