A single, high-energy photon hitting a photovoltaic material can produce multiple low-energy excitons (electron-hole pairs). This effect occurs in quantum-confined systems, like quantum dots. In bulk semiconductors, the same process is called impact ionization and is used to increase the efficiency of so-called avalanche photodiodes. However, impact ionization in bulk semiconductors is not efficient enough to increase the efficiency of solar-energy conversion in photovoltaic devices.

"Hot carriers" in solar cells could overcome this problem, says Art Nozik of Colorado. These carriers are electrons and holes produced by photons with energies above the semiconductor bandgap.

Energy losses
In bulk semiconductors, hot carriers quickly cool in a matter of just picoseconds, releasing phonons (vibrations of the crystal lattice, or heat). Indeed, such wasted heat can account for up to 50% of the energy losses in present-day solar cells. The problem comes about because radiation from the Sun consists of photons that span an energy range from around 0.3 to 3 eV, so some of these photons have energies above many semiconductors' bandgaps. If the energy of hot carriers could be captured before it is converted into wasted heat, solar-to-electric power-conversion efficiencies could be greatly increased and Nozik's team says that quantum dots could come into their own here.

"Our results demonstrate that the multiple exciton generation (MEG) process is enhanced in quantum dots over that of bulk semiconductors," team member Matt Beard told nanotechweb.org. "If we learn more about this process, then we could design new material systems to enhance solar-energy conversion."

Quantum dots can be incorporated into solar-cell devices in many different ways and Nozik's team suggests using a thin film of semiconducting quantum dots as the active layer area in a solar cell. For example the Colorado researchers have already made a Schottky junction solar cell based on colloidal nanocrystals films.

The team obtained its current results by measuring MEG in quantum dots using an ultrafast spectroscopic technique, first put forward by Victor Klimov and Richard Schaller at Los Alamos National Lab. Klimov's team has been working on understanding and measuring carrier dynamics in quantum dots for many years now. "The carrier dynamics of a multi-exciton state has a unique temporal signature that is very different to that of a single exciton," explained Beard. "Klimov and colleagues found that exciting quantum dots with high-energy photons at low fluences always produces multi-exciton states."

Nozik's team has now utilized this technique to probe the multi-excitons produced in quantum dots as a function of incoming photon energy. The researchers have analysed energy losses in the dots and established a way to describe how efficient the MEG process is. Finally, they have compared their results with previously published data for impact ionization in thin bulk films of PbSe.

Although the work shows that MEG efficiency can be increased using quantum confinement, this efficiency needs to be further improved, says Beard. "We need to do more work in understanding the different competing channels for hot carriers and learn how to improve the MEG channel. We have many ideas on how to do this and are exploring these now."

As well as these basic photo-physical studies, Nozik and colleagues are also busy developing working quantum-dot solar cells. One of the main advantages of using quantum dots in solar-energy conversion is that new types of materials that would not ordinarily work in solar cells could be used in their quantum-dot form. For example, the researchers have proved that solar cells made from bulk PbSe are not very useful (their bandgap is too low) but that solar cells incorporating PbSe QDs can be very efficient.

"We have now joined forces with Klimov's group at Los Alamos and set up an Energy Research Frontier Center – the "Center for Advanced Solar Photophysics" – to collaborate and advance our understanding of this important phenomenon to increase solar conversion efficiencies," added Beard.

The work was reported in Nano Letters.