Perovskite solar cells, known for their cost-effectiveness and impressive efficiency, have rapidly evolved over the past decade. Their efficiency has soared to over 25%, matching that of traditional silicon-based solar cells. Despite challenges in stability, these cells remain a focal point in photovoltaic research due to their potential for further efficiency improvements.
Thomas Kirchartz, an electrical engineer and head of the organic and hybrid solar cells group at Forschungszentrum Julich's Institute of Energy and Climate Research, emphasizes the importance of understanding charge carrier dynamics. "An important factor here is the question of how long excited charge carriers remain in the material, in other words their lifetime," Kirchartz explains. "Understanding the processes is crucial to further improving the efficiency of perovskite-based solar cells."
In solar cells, photons dislodge electrons, propelling them from the valence band to the conduction band, enabling them to contribute to electrical energy generation. Their effectiveness depends largely on their lifetime, allowing them to traverse the absorber material to the electrical contact. However, defects in the crystal lattice, known as recombination, can cause these electrons to fall back to lower energy levels, ceasing their contribution to current flow. This process is the primary loss mechanism in solar cells.
Kirchartz notes that traditionally, recombination is thought to be mainly triggered by deep defects located between the valence and conduction bands. These defects are accessible to both excited electrons and holes, reducing their utility in electricity generation. This assumption holds for most solar cells.
However, the team's latest research challenges this notion for perovskite solar cells. They discovered that shallow defects, located close to the valence or conduction band, are more critical in determining the efficiency of these cells. "The cause of this unusual behavior has not yet been fully clarified," Kirchartz adds. He speculates that deep defects might not exist in these materials, possibly contributing to their high efficiency.
This groundbreaking observation was made possible through an innovative measurement technique developed by Kirchartz and his team. The new transient photoluminescence measurement method boasts a significantly expanded dynamic range compared to traditional methods. This advancement is akin to HDR (High Dynamic Range) in photography, where multiple images at different exposure levels are combined for a more detailed picture. In this case, signals with varying amplification levels create a richer data set.
Research Report:Shallow defects and variable photoluminescence decay times up to 280 us in triple-cation perovskites
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