Organometallic perovskite absorber layers are regarded as a particularly exciting new material class for solar cells, and Prof Dieter Neher at the University of Potsdam and Dr. Thomas Unold at the Helmholtz Centre in Berlin (HZB) looked in detail at the various defects in solar cells and determine which ones lead to losses and how. This was used to boost the efficiency of a 1cm2 perovskite solar cell to well over 20 percent.
At certain defects in the crystal lattice of the perovskite layer, the 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.
The team used photoluminescence techniques with high precision, spatial and temporal resolution to examoine the materials. 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”, said Unold. 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 chargetransport layers,” he said. 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.
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