One of the reasons why photovoltaic cells are inefficient is that they cannot absorb photons that have energies below the bandgap of the light-absorbing layer in the device. This problem can be overcome by using a process of up-conversion in which two or more sub-bandgap photons generate a single above-gap exciton. An exciton is an electron-hole pair produced in a photoactive material when it absorbs light. However, traditional approaches to such up-conversion, such as “nonlinear two-photon absorption” (2PA) or “triplet fusion”, suffer from low efficiency at solar light intensities and other drawbacks such as narrow light absorption bandwidths.

Now, a team led by Victor Klimov says that these shortcomings can be alleviated to some extent by using the Auger up-conversion effect in thick-shell lead selenide/cadmium selenide (PbSe/CdSe) quantum dots (tiny, nanometre-sized specks of semiconducting material).

Enhancing the hole-reexcitation Auger pathway

“This process takes advantage of specially-designed quantum dots wherein the core of a narrow bandgap (infrared-active) material (PbSe) is over-coated with a thick shell of a wider-gap semiconductor (CdSe),” explains Klimov. “The process of Auger up-conversion involves the sequential absorption of two infrared photons by a PbSe core, which leads to the formation of two excitons, called a biexciton, in the core. This biexciton decays via Auger recombination, in which the energy of the first exciton is transferred to either the electron or hole of the second exciton.”

The researchers say they engineered the core/shell quantum dots so as to enhance the hole-reexcitation Auger pathway, which leads to the formation of a high-energy shell-based exciton. “This exciton can either be extracted from the quantum dot to then produce a photocurrent or recombine to generate high-energy photons in the visible spectral range,” Klimov tells nanotechweb.org. “This higher-energy photon can be harvested by a standard solar cell.”

Boosting Auger up-conversion efficiencies

The semiconducting quantum dots employed by the Los Alamos researchers have several advantages over previously studied materials, like organic dimers or lanthanide-based structures. These include the fact that they have a continuous absorption spectrum with a spectrally tuneable onset and are photostable. More importantly, they are compatible with traditional solar architectures, which was not always the case before, adds Klimov.

Although the Auger up-conversion efficiencies observed in this proof-of-principle study are fairly low and require considerable improvement to be practical, they could be boosted using a number of approaches, he says. “These include, for example, extending core-exciton lifetimes by improved quantum-dot surface passivation and/or multi-shell strategies, as well as incorporating a barrier at the core-shell interface for stabilizing a shell-based hole.”

The research is detailed in ACS Nano DOI: 10.1021/acsnano.6b04928.