Quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range of 1-20 nm, where 1 nm equals one-billionth of a meter. At this small size, semiconductors exhibit dramatic effects because of quantum physics, such as:
• rapidly increasing bandgap with decreasing quantum dot size,
• formation of correlated electron-hole pairs (called excitons) at room temperature,
• enhanced coupling of electronic particles (electrons and positive holes) through Coulombic forces,
• and enhancement of the MEG process.
Quantum dots confine the charges and harvest excess energy
Quantum dots, by confining charge carriers within their tiny volumes, can harvest excess energy that otherwise would be lost as heat – and therefore greatly increase the efficiency of converting photons into usable free energy.

The researchers achieved the 114 percent external quantum efficiency with a layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode. https://cleantechnica.com/2011/12/20/solar-cell-with-114-external-quantum-efficiency/