Solar energy could soon get a boost with tandem cells

Tandom Cell

While Europe has embraced solar power technology to the point that an eclipse can cause mild panic, the efficiency of the best solar cells is still hovering around a mere 25%. That’s a lot of free energy that isn’t being harvested. Past research has pointed to a plentiful mineral known as perovskite as a possible solution to the poor performance of solar cells, and now a team of scientists from the Massachusetts Institute of Technology (MIT) and Stanford University have put theory into action by constructing tandem silicon-perovskite solar cells.

Tandem solar cells (sometimes known as multi junction cells) are composed of more than one semiconductor material. They have the potential to boost efficiency considerably, but their use has been very limited thanks to manufacturing complications and high cost. The Stanford/MIT team focused on these designs because they felt there is a great deal of room for improvement.

Perovskite is a crystalline organometal mineral that can be produced inexpensively in the lab, and also exists in geological deposits all over the world. Scientists have known that perovskite has light absorbing and semiconductor properties for decades, but only in 2009 was it first used in solar cells. The main advantage of using perovskite in a solar cell is that it can be integrated in layers as thin as one micrometer.

The tandem cells created in this experiment also incorporated advances in manufacturing tech to improve the connection between silicon and perovskite layers. This connecting layer, or “tunnel junction,” is composed of heavily doped p-type and n-type silicon that makes the energy barrier between the two layers almost zero. An additional titanium-dioxide layer allows electrons from the perovskite solar cell to flow freely into the silicon tunnel junction, where they recombine with the electrons from the silicon panel.

A tandem solar cell based on silicon and perovskite can absorb a larger segment of solar energy. Tandem solar panels like these minimize a phenomenon known as thermalization. That’s what happens in a solar cell when the energy of photons is released as heat until it reached the absorbing material’s bandgap. Silicon is great at absorbing photons toward the top of the solar energy spectrum (high bandgap), while perovskite is adept at capturing photons from the lower infrared segment (lower bandgap). These specialized absorbing layers can convert the sun’s light into electricity more efficiently than a single absorber could.

As for the observed efficiency, the cells in this experiment performed at the top of the predicted range. The individual silicon and perovskite designs used were not the most advanced, but the increase in efficiency shows promise. A perovskite solar cell with an efficiency of 12.7 percent was stacked on top of a mid-range silicon cell with an efficiency of just 11.4 percent. The tandem cell was able to reach 17 percent, a 50% increase. The researchers believe that refining the perovskite layer and using more advanced silicon solar cells could result in even better gains down the road.