Solar power is one of the leading technologies to produce clean, renewable energy, but photovoltaic solar cells remain uneconomical in many areas. The key to improving photovoltaic economics is more efficiently utilizing the energy that the cells receive, as recently demonstrated by solar cells that utilize a combination of light and heat.

Photovoltaic cells work by absorbing photons and using that energy to shift electrons to the conduction band, which creates an electron-hole pair called an exciton. The key to generating electricity in a photovoltaic cell is separating the electron-hole pair and transporting the electron out of the cell to provide an electric current (called a photocurrent). The energy required to promote an electron to the conduction band is called the band gap energy, and it is generally specific to the material in use.

If a photon with more energy than the band gap energy is absorbed, the excess energy is converted into heat, making it unavailable for conversion to electrical energy. For silicon, the most common photovoltaic material, the band gap is 1.1 eV, meaning that energy is wasted over the entire visible spectrum (2-3.1 eV). Rather than waste all this energy, researchers in this week's Science have developed a solar cell that converts each high energy photon into multiple electrons through a process called multiple exciton generation (MEG).

MEG meets nanoparticles

MEG has been well known and studied in bulk semiconductor materials for decades, but the process is tremendously inefficient and only operates at light frequencies higher than those produced by the Sun—in other words, the effect is useless for photovoltaic applications. However, recent studies have shown that multiple excitons can be generated by a single photon, provided they're stabilized in nanoscale semiconductor particles, which confine the electron-hole pairs to very small volumes (a strong Coulombic interaction between excitons prevents decay into phonon modes).

While MEG can be highly efficient in nanoscale semiconductor particles, it has been difficult to efficiently extract the electrons from the nanoparticles—so difficult that the process has never been measured as an actual photocurrent. To solve this problem, the researchers chemically bonded PbS nanoparticles to specially prepared TiO 2 surfaces that were exceptionally clean and had large, atomically flat ledges.

The keys to the process appear to be surface preparation of the TiO 2 substrate and choosing appropriate semiconductor chemistries so that there is a strong overlap between the conduction bands of the two materials. Electrons will flow provided that there's a slight energy decrease from the semiconductor conduction band to the TiO2 conduction band.

Photoelectric current as function of photon energy was measured on single crystal TiO2 coated with a monolayer of PbS nanoparticles. The experiments conclusively showed that multiple electrons were generated and extracted from the nanoparticles for each high energy photon that was absorbed. The result is a theoretical efficiency increase from 31 percent in single junction photovoltaic cells to 47 percent in a MEG cell.

Because the photocurrent measurements were taken on monolayers of nanoparticles, most of the incident light was not absorbed. In order to make a truly efficient cell that absorbs most of the incident light, they'll have to demonstrate that MEG works through multilayer ensambles of nanoparticles. Efficient MEG and conduction has been demonstrated in more complex nanoparticle structures, but current has not been successfully extracted.

An exceptional paper, in a good way

The technology demonstrated in this paper is particularly interesting for several reasons. First, it is a true “nanomaterial” application where the size of the semiconductor particles enable truly unique properties by confining the excitons to quantum length scales. During my daily abstract scan, it is all too common to find "nano-" papers that simply involve small particles rather than truly novel properties enabled by the scale of the materials.

The work also concentrated on extracting electrons from the nanoparticles rather than just trying to break efficiency records for electron generation. There are constant reports of efficiency numbers for photovoltaic materials that are generated without separating excitons to produce a useable photocurrent. For many types of photovoltaic cells, charge separation is a far more challenging and important scientific problem than charge generation.

Finally, the experimental setup for this study is largely consistent with dye sensitized solar cells, which are easy to manufacture compared to silicon technologies. In principle, this would suggest that the technology used in the study could be rapidly transferred to more industrial scales. I’m skeptical that easy technology transfer is possible, though, due to the emphasis on the surface treatment and surface morphology of the TiO 2 conductors. It may not be possible to develop similar surface morphologies on the titania particles and porous structures common to commercial dye sensitized cells.

Despite the potential problems listed above, this study is interesting because it clearly demonstrates an incremental but fundamental step forward for one the most exciting next-generation technologies in photovoltaic devices.

Science, 2010. DOI: 10.1126/science.1191462 (About DOIs).