Excited Nanocrystals Yield More Solar Power

LOS ALAMOS, New Mexico, May 10, 2004 (ENS) - University of California scientists working at Los Alamos National Laboratory have demonstrated a process that could lead to a new generation of solar cells producing up to 35 percent more electrical output than current solar cells. Today the most efficient solar cells known convert sunlight to electricity at 32 percent efficiency.

In a paper published April 30 in the journal "Physical Review Letters," scientists Richard Schaller and Victor Klimov describe their observations of high efficiency carrier multiplication in nanoparticles of less than 10 nanometers in diameter made from lead and selenium (PbSe nanocrystals). A nanometer equals one billionth of a meter.

Schaller and Klimov showed how semiconductor nanocrystals respond to photons by producing multiple electrons. This increase in the number of electrons being produced can lead to a greater electrical current output from solar cells.

Exposed to light at the green-blue end of the spectrum, the nanocrystals reacted to absorbing solar photons by producing twice the electrons of conventional bulk semiconductors through a process known as carrier multiplication, the scientists found.

The basic operation of solar cells has remained unchanged over the past 40 years. The absorption of a photon by the solar cell material generates a single exciton - a bound state of a negatively charged electron and a positively charged hole - which undergoes charge separation and produces electrical current.

Traditionally, the single photon produces only one exciton. The rest of the photon's energy is lost as heat. Over the decades, scientists have proposed various methods for improving the efficiency of solar panels, including a method called carrier multiplication.

Carrier multiplication was discovered in the 1950s, but has always been considered an inefficient method for solar energy conversion since it produced, at best, an increase in solar energy conversion efficiency of less than one percent.

But now the Los Alamos scientists have demonstrated that the use of nanoscale semiconductor particles can improve the efficiency of carrier multiplication through an enhancement of the effect called impact ionization.

Impact ionization is a process where an exciton, created in a semiconductor by absorbing a photon, transfers the excess energy that would normally have been lost as heat to another electron. The result of this energy transfer process is that two excitons are formed for one absorbed photon.

Schaller and Klimov have not yet built a working PbSe nanocrystal solar cell, but they are the first to demonstrate the ability to use impact ionization to generate more excitons and a greater electrical current in the solar cell configuration.

Schaller and Klimov are researchers in the Physical Chemistry and Applied Spectroscopy group of the Laboratory's Chemistry Division. For more information on their work, go to: http://quantumdot.lanl.gov/online.