Light-emitting diodes based on molecularly thin films of a conjugated polymer have been fabricated. A few Langmuir–Blodgett layers of poly(3-hexylthiophene) have been used as the active emitting material of the devices. Even the thinnest devices with only 3 monolayers, each having a thickness of ≊3 nm, yield the same luminance as thicker ones. The emission results from excitons in the polymer chains, and has a spectrum similar to that of photoluminescence, with a maximum at about 2.0 eV.