Materials optimized for single-junction solar spectral harvesting, such as silicon, perovskites, and large-band-gap colloidal quantum dot solids, fail to absorb the considerable infrared spectral energy that lies below their respective band gap. Here we explore through modeling and experiment the potential for colloidal quantum dots (CQDs) to augment the performance of solar cells by harnessing transmitted light in the infrared. Through detailed balance modeling, we identify the CQD band gap that is best able to augment wafer-based, thin-film, and also solution-processed photovoltaic (PV) materials. The required quantum dots, with an excitonic peak at 1.3 μm, have not previously been studied in depth for solar performance. Using computational studies we find that a new ligand scheme distinct from that employed in better-explored 0.95 μm band gap PbS CQDs is necessary; only via the solution-phase application of a short bromothiol can we prevent dot fusion during ensuing solid-state film treatments and simultaneously offer a high valence band-edge density of states to enhance hole transport. Photoluminescence spectra and transient studies confirm the desired narrowed emission peaks and reduced surface-trap-associated decay. Electronic characterization reveals that only through the use of the bromothiol ligands is strong hole transport retained. The films, when used to make PV devices, achieve the highest AM1.5 power conversion efficiency yet reported in a solution-processed material having a sub-1 eV band gap.

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.