Loss Processes in Perovskite Solar Cells

Organo­metallic perovskite absorber layers are regarded as a parti­cularly exciting new material class for solar cells. In just ten years, their efficiency has increased from three per cent to over twenty per cent. Now a team headed by Dieter Neher at the Uni­versity of Potsdam and Thomas Unold at Helm­holtz-Zentrum Berlin HZB has succeeded in identi­fying the decisive loss processes in perovskite solar cells that limit the effi­ciency.

The analysed perovskite cell: With additional layers between the perovskite semiconductor and the hole- and electron-transport-layers it was able to further increase efficiency of the perovskite cell. (Source: Univ. Potsdam)

At certain defects in the crystal lattice of the perovs­kite layer, charge carriers that have just been released by sunlight can recom­bine again and thus be lost. But whether these defects were preferen­tially located within the perovskite layer, or instead at the interface between the perovs­kite layer and the transport layer was unclear until now. To determine this, the scientists employed photo­luminescence techniques with high precision, spatial and temporal reso­lution.

Using laser light, they excited the square-centi­metre-sized perovs­kite layer and detected where and when the material emitted light in response to the exci­tation. “This measure­ment method at our lab is so precise, we can determine the exact number of photons that have been emitted”, explains Unold. And not only that, the energy of the emitted photons was precisely recorded and analyzed as well using a hyper­spectral CCD camera.

“In this way, we were able to calcu­late the losses at every point of the cell and thereby determine that the most harmful defects are located at the inter­faces between the perovskite absorber layer and the charge-transport layers“, reports Unold. This is important infor­mation for further impro­ving perovs­kite solar cells, for instance by means of inter­mediate layers that have a positive effect or through modified fabri­cation methods. With the help of these findings, the group has succeeded in reducing inter­facial recom­bination and thus increasing the effi­ciency of 1 cm2 -sized perovskite solar cells to well over 20 %. (Source: HZB)

Reference: M. Stolterfoht et al.: Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells, Nat. Ener., online 30 July 2018; DOI: 10.1038/s41560-018-0219-8

Links: Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin HZB, Berlin, Germany • Soft Matter Physics Group, University of Potsdam, Potsdam, Germany

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