Quantum photocells might cheat efficiency limits

Atoms in a solar cell coaxed into a curious simultaneous quantum state may convert sunlight into electrical energy more efficiently than previously believed possible, a new study proposes.

The laws of thermodynamics set the upper limit of solar cell efficiency at around 80 percent, says the work’s author, Marlan Scully of Texas A&M University in College Station and Princeton University. But this estimate doesn’t take certain quantum effects into account. Scully’s new model shows that the ultimate energy efficiency can be pushed even higher, depending on the particulars of the system.

“I think it’s always important to know what the ultimate efficiency is,” says physicist Ting Shan Luk of Sandia National Laboratories in Albuquerque, who was not involved in the study. “Without knowing the limit, you don’t know what to shoot for.”

Photovoltaic cells capture energetic photons from the sun and convert them into electrical energy. In the kind Scully analyzed, photons hit atoms in a semiconductor and knock electrons free, which results in a roaming electron and an electron-hungry area called a “hole.” Ideally, the loose electrons are funneled into a path, creating an electrical current. But sometimes, electrons can fall back into a hole and emit a photon, an energy-squandering process called radiative recombination. 

This energy loss can be circumvented, Scully says. “You can do better under some conditions.”  

In the new work, published May 21 in Physical Review Letters, Scully proposes that photon squandering can be curbed through a counterintuitive process called quantum coherence, in which atoms are in two energetic states simultaneously and are able to interfere with each other. Applying microwave radiation to the photovoltaic cell induces this fuzzy coherence, which diminishes the chances of free electrons finding holes before their work can be harnessed.

As presented, the new scheme doesn’t really cheat the laws of thermodynamics, because getting the atoms into quantum coherence takes energy itself. “The thing he didn’t take into account is, what is the energy cost in establishing this coherence?” Luk says. The coherence-creating microwave field consumes energy, which cuts into the overall efficiency of the process.

Scully says that he has some ideas about how to create coherence without needing energy. “We’re finding, to our amusement, that there are possible scenarios” that wouldn’t require an extra energy-consuming field, he says.

Adding quantum control to current solar cells isn’t his intention, Scully points out. Most solar cells on the market today operate at around 20 percent efficiency. “The present solar cells are operating a long way from the top,” he says. The new research “can take you down the road to some practical implications,” though that’s a long way away, he cautions. “Right now, we’d like to understand the basic science.”