Science for You: Doubling the efficiency of solar cells

A new material has been shown to have the capability to double the efficiency of solar cells by researchers at Purdue University and the National Renewable Energy Laboratory.

Conventional solar cells are at most one-third efficient, a limit known to scientists as the Shockley-Queisser Limit. The new material, a crystalline structure that contains both inorganic materials (iodine and lead) and an organic material (methyl-ammonium), boosts the efficiency so that it can carry two-thirds of the energy from light without losing as much energy to heat.

In less technical terms, this material could double the amount of electricity produced without a significant cost increase.

Enough solar energy reaches the earth to supply all of the planet’s energy needs multiple times over, but capturing that energy has been difficult – as of 2013, only about 1 percent of the world’s grid electricity was produced from solar panels.

Libai Huang, assistant professor of chemistry at Purdue, says the new material, called a hybrid perovskites, would create solar cells thinner than conventional silicon solar cells, and is also flexible, cheap and easy to make.

“My graduate students learn how to make it in a few days,” she says.

The breakthrough was published in the journal Science.

The most common solar cells use silicon as a semiconductor, which can transmit only one-third of the energy because of the band gap, which is the amount of energy needed to boost an electron from a bound state to a conducting state, in which the electrons are able to move, creating electricity.

Incoming photons can have more energy than the band gap, and for a very short time – so short it’s difficult to imagine – the electrons exist with extra energy. These electrons are called “hot carriers,” and in silicon they exist for only one picosecond (which is 10-12 seconds) and only travel a maximum distance of 10 nanometers. At this point the hot carrier electrons give up their energy as heat. This is one of the main reasons for the inefficiency of solar cells.

Huang and her colleagues have developed a new technique that can track the range of the motion and the speed of the hot carriers by using fast lasers and microscopes.

“The distance hot carriers need to migrate is at least the thickness of a solar cell, or about 200 nanometers, which this new perovskite material can achieve,” Huang says. “Also these carriers can live for about 100 picoseconds, two orders of magnitude longer than silicon.”

Huang’s research at Purdue was funded by National Science Foundation grant DMR- 1507803. The work at the National Renewable Energy Laboratory was funded by the Solar Energy Technologies Office of the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy.

– Steve Tally, Purdue News Service, bit.ly/2p9fq61