Paul van Gerven
14 February

Scientists have found a clue for the remarkably high efficiency of perovskite solar cells. Researchers from Forschzungszentrum Jülich (FZJ) discovered that one of the main mechanisms causing energy loss is different in this material than in most semiconductors.

When a photon of sufficient energy hits a solar panel, an electron is dislodged and promoted to the higher-energy conduction band, leaving behind a hole in the valence band. The electron and the hole are pulled into different directions by an electric field inside the material, creating a flow of electricity. Unfortunately, more often than not, electrons and holes recombine before they find their way to an electrical contact. This typically occurs at defects of the crystal lattice.

Julich perovskite
Credit: Julich

“It had previously been assumed that recombination is predominantly triggered by defects that are energetically located in the middle between the valence and conduction bands. This is because these deep defects are similarly accessible to excited electrons and their counterparts, the holes,” says Thomas Kirchartz, who heads up a FZJ research group on organic and hybrid solar cells.

Kirchartz and colleagues have shown that this assumption isn’t true for perovskite solar cells. Instead, defects near the valence or conduction band play an important role. This finding could explain the relatively low rate of recombination occurring in perovskites, increasing the lifetime of charge carriers and therefore their chances to contribute to solar current.

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The cause of the unusual ‘behavior’ of perovskites is still unclear. “It’s reasonable to assume that deep defects simply can’t exist in these materials,” posits Kirchartz.