Swiss engineers have demonstrated tandem solar cells layered so they can catch more of the solar spectrum, providing a route to cheaper and more efficient solar power. The cells are not yet ready for commercial applications, but could mark a major step forwards for renewable energy.

Traditional solar cells present engineers with a fundamental problem. The colors in sunlight are photons of different energies, but individual cells can only extract the same amount of energy from each photon, leaving designers with a choice. One path involves collecting high-energy photons and missing out on the majority of the Sun’s photons whose energy is too low. The alternative is to harvest a larger portion of the spectrum, but only get a small amount of energy from each photon, so that much of the potential of higher energy photons is wasted.

This problem can be resolved by placing different types of cells on top of each other, with the top layer catching high-energy photons while letting those of lower energy through to be captured by another cell below. Multi-junction cells that stack four layers upon each other have achieved 46 percent efficiency, but at prices not viable for most circumstances. An alternative path is to split sunlight with a prism so that each cell gets the light for which it is most suited.

Professor Ayodhya Tiwari is co-leader of a team at Empa-Swiss Federal Laboratories who have announced in Nature Communications a proof of concept for a way to make the top cell cheaply enough for widespread use, while still letting most of the unused light through.

Tiwari’s version uses the new wonder material perovskite for the top cell, with copper indium gallium diselenide below. Perovskite is a naturally occurring mineral that can also be manufactured for energy-collecting purposes. Although still not as efficient as the best solar cells, progress in perovskite material has happened far more rapidly than any other solar material ever tried.

While most solar cells, perovskites included, require high-temperature manufacturing, greatly adding to the cost, Tiwari created the top layer at 50°C (122°F), opening up the possibility of very cheap mass production.

The test cells used perovskite crystals to collect 14.2 percent of the energy in sunlight, while letting 72 percent through. The cell below captured another 6.3 percent. The total efficiency of 20.5 percent is similar to the best commercial cells and nothing exceptional by laboratory standards. However, Tiwari claimed in a statement that 30 percent efficiency is in sight for the cells made this way. “What we have achieved now is just the beginning,” he said. “We will have to overcome many obstacles before reaching this ambitious goal.”

Higher efficiency, even at the same price per watt, would make solar energy more attractive where space is limited (such as the roofs of electric cars) and reduces the costs of installation and associated infrastructure.

Perovskite cells currently lack the durability of silicon crystals though, particularly when exposed to water, and this remains the biggest obstacle to their widespread use.

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