Electromagnetic
radiation emitted from hot objects represents a
sizable source of energy, one that in most applications is not harvested
efficiently. Even for a blackbody at 800 °C, the radiation intensity
peaks near 2.7 μm wavelength, and this requires a semiconductor
absorber having a band gap in the short-wavelength infrared and beyond
to enable thermophotovoltaic (TPV) heat recovery. Here we report the
first solution-processed TPV device to harvest efficiently 800 °C
heat. The active layer consists of colloidal quantum dots (CQDs),
infrared-absorbing nanoparticles synthesized using a scalable solution-based
method, having 0.75 eV band gap. We construct rectifying junction
devices based on controllably p- and n-doped CQD solids that benefit
from a gradient in electron affinity that extends over the devices’
thickness. The gradient-doped architecture relies on engineered charge
carrier drift and overcomes the existing limitations of small band
gap CQD solids. The devices provide 2.7% efficiency in the conversion
of optical power from above-band gap photons from a blackbody source
at 800 ± 20 °C into electrical power. The cells were thermally
stable up to 140 °C, increasing the promise of CQD solids for
TPV applications.