Colloidal quantum dots (QDs) are semiconductor particles just a few nanometres in size. They can be synthesized in solution, which means that films of the particles can be deposited quickly and easily onto a wide range of flexible or rigid substrates – just like paint or ink.

CQDs could be used as the light-absorbing component in cheap, highly efficient inorganic solar cells. In a solar cell, photons hitting the photovoltaic material can produce excited electrons and holes (charge carriers) that have energies at least equal to or greater than the bandgaps of the material. Another advantage of using CQDs as the photovoltaic material is that they absorb light over a wide spectrum of wavelengths thanks to the fact that the bandgap can be tuned over a large energy range by simply changing the size of the nanoparticles.

n-type CQDs easily oxidize

Devices made from these quantum dots are composed of rectifying junctions that require high-quality CQD solids that are both n-type (rich in electrons) and p-type (poor in electrons). Having both n- and p-type layers in a device boosts how efficiently it absorbs light. However, the problem is that n-type semiconductors made from CQDs easily oxidize within moments of being exposed to air. This is because they bind to oxygen atoms, give up their electrons and turn into p-type semiconductors.

Now, a team led by Ted Sargent has come up with a solution to this problem by developing a new CQD n-type material that does not bind to oxygen when exposed to air. “Using density functional theory, we identified inorganic passivants that strongly bind to the surface of the CQDs and prevent oxidation,” Sargent told nanotechweb.org. “We then used the results from these simulations to develop a materials processing strategy that wards off strong protic attack by polar solvents to synthesize n-type lead sulphide CQD solids that are stable in air.”

Highest current density for any CQD solar cell

The researchers used this material to fabricate a quantum junction device that boasts the highest current density for any CQD solar cell made to date with a solar-power conversion efficiency as high as 8%.

Most solar cells today are made from heavy crystalline materials,” explained team member Zhijun Ning, “but our work shows that light and versatile materials like CQDs could potentially become cost-competitive compared with these traditional technologies. This new form of solid, stable and light-sensitive nanoparticle could be ideal in low-cost photovoltaics that might be fabricated on flexible substrates, for example, using roll-to-roll manufacturing, and mixed into oils and paints or printed onto surfaces, such as roofing.

We have also proved that they make excellent sensors for detecting gases, like NO2. They might also find use in infrared lasers and light-emitting diodes, and a host of other optoelectronics devices.”

The researchers detail their work in Nature Materials doi:10.1038/nmat4007.