Hot electrons give up the goods

Harvesting the energy of so-called hot electrons in perovskites is surprisingly easy, suggests a study by the University of Groningen and Nanyang Technological University. The finding may help to increase the efficiency of perovskite solar panels.

Paul van Gerven

The performance of perovskite solar cells has improved spectacularly over the past decade. In 2009, an efficiency of 3.8 percent for a single-junction cell was reported but the current record already stands at 25.2 percent – right behind the best silicon devices. Being a thin-film technology, however, perovskite solar cells will be significantly cheaper to produce and hence are expected to give silicon a run for its money – though perovskites and silicon can reinforce one another as well.

It’s no coincidence that the top efficiencies of perovskites and silicon are very close. The maximum efficiency of a ‘regular’ single-junction solar cell is 31-33 percent, depending on the bandgap. As designs, materials and processing techniques are optimized, any single-junction solar cell will approach that limit. For both silicon and perovskites, the low-hanging fruit simply has been picked.

That’s why researchers have turned their attention to more advanced concepts. Understandably, they’ve set their sights one of the major sources of energy loss in solar cells: the excess energy that photons have with respect to the semiconductor’s bandgap, which is lost as heat. As it turns out, it may be easier to capture that energy portion in perovskites than in silicon, researchers from the University of Groningen (RUG) and Nanyang Technological University Singapore show in Science Advances.

Hot and cold Whenever a photon strikes a semiconductor, an electron is excited only when the photon’s energy is equal to or higher than the bandgap. If its energy is smaller, no photoexcitation occurs and the photon passes right through the device. If its energy is larger, the excess is ‘absorbed’ by the excited electron and then swiftly transferred to crystal lattice vibrations. In other words: the excess is lost as heat.

Key to capturing the energy of so-called hot electrons is slowing down the energy loss to the lattice, so as to at least have a chance of capturing some of it. This has proven extremely challenging, as a complex interplay of disparate processes is involved. The best results have been obtained with nanoparticles, which feature fewer ‘energy sinks’ than bulk materials.

The study led by RUG researcher Maxim Pshenichnikov provides a proof of principle that hot electrons in perovskites can be captured. In fact, merely stacking the perovskite with an organic compound called bathophenanthroline (bphen) and exciting the perovskite’s electrons to energy levels just

Credit: NTU Singapore

above bphen’s bandgap resulted in a remarkably smooth transfer of said electrons to the organic compound. Spectroscopic analysis confirmed that the transfer to bphen is much faster than the undesirable cooling process.

This doesn’t mean we should expect beefed-up perovskite solar cells anytime soon, though. In the experiment, the researchers made sure they only excited perovskite electrons, whereas, in the real world, both the inorganic and organic would absorb light to generate both hot and cold electrons (which have little excess energy). It remains to be seen whether the hot electrons can be extracted without sacrificing the cold ones. “This underscores the complexities of realizing practical hotcarrier perovskite solar cells,” the researchers conclude in Science Advances.