Abstract This work demonstrates flexible, transparent phototransistors that can detect visible light with nontoxic organic active materials on biodegradable substrates toward environment‐friendly electronics. The molybdenum trioxide (MoO 3 )‐buffered indium zinc oxide as high‐performance hole injector and transparent electrodes is applied for the first time to organic phototransistors on cellulose nanofibrillated fiber substrates to achieve more than 70% of transmittance in the visible range (400–750 nm) while showing high conductivity under multiple bendings. Excellent electrical switching characteristics are obtained from transistors using a pentacene active layer with a saturation mobility value of 1.40 cm 2 V −1 s −1 . The phototransistors, which can detect visible light and perform in two operation modes, exhibit a maximum responsivity of 54.8 A W −1 and a photosensitivity of 24.4 under white light illumination at an intensity of 0.12 mW cm −2 . Moreover, the devices show a stable operation during mechanical bending tests with radii ranging from 100 to 5 mm and cyclic bending tests of up to 2000 cycles at a fixed radius of 5 mm. The results suggest that these flexible phototransistors with properties of transparency and biodegradability have considerable potential for use in low‐cost and eco‐friendly disposable sensor systems.
