Giga-Gain at Room Temperature in Functionalized Carbon Nanotube Phototransistors Based on a Nonequilibrium Mechanism

Achieving high gain in a photodetector is critical to detect weak light fields because of the need to amplify the signal. Here, we report the observation of a gain exceeding 10⁹ for a phototransistor composed of an array of aligned semiconducting carbon nanotubes functionalized with a nanoscale layer of poly(3-hexylthiophene-2,5-diyl) (P3HT). In contrast to the expectation based on simple band alignments, the phototransistor operates by transferring holes between the P3HT and the CNT, trapping negative charge near the nanotubes. This mechanism leads to an integrating detector that is shown to detect as little as 490 aW and to resolve as few as 8-13 photons/nanotube at room temperature. A detailed experimental and theoretical investigation of the mechanism shows that the phototransistor is most sensitive when prepared in a nonequilibrium state.