Feb 5 2018
EPFL scientists systematically study the path of the sequential deposition reaction employed for building perovskite solar panels. The study is featured in Science Advances and offers the essentially needed fundamental understanding of perovskite formation and its varied stages.
Image credit: M. Grätzel/EPFL
Perovskite solar cells are an alternative to standard silicon solar cells, composed to enter the market along with their lower capital expenditure and manufacturing costs and high power-conversion efficiencies (above 22%). One of the key methods for depositing perovskite films onto panel structures is a process called sequential deposition, which was created in 2013 by Michael Grätzel and coworkers at EPFL. A number of studies have made attempts to control this process with compositional changes, additives and temperature effects. However, none of these have offered a comprehensive understanding of the whole sequential deposition reaction. This prevents sufficient control over film quality, which defines the performance of the solar cell.
A study carried out by Michael Grätzel and Amita Ummadisingu at EPFL presently offers the most systematic and complete study of the sequential deposition reaction to-date. The scientists started with X-ray diffraction analysis and scanning electron microscopy (SEM) to thoroughly study the crystallization of lead iodide (PbI2), which is the first stage of the reaction. This was followed by using, for the very first time, SEM-cathodoluminescence imaging in order to study the nano-scale dynamics of perovskite film formation.
We have combined two powerful tools to obtain compositional information about the surface of the film during perovskite formation. This technique enables us to achieve stunning nano-scale resolution meaning that we can see, for the first time, that mixed crystalline aggregates composed of perovskite and PbI2 are formed during the reaction.
Amita Ummadisingu
The scientists next used cross-sectional photo-luminescence mapping, which exposed the directionality of the conversion reaction. This type of information has so far been unachievable with standard surface imaging since layers lying beneath one another are inaccessible. However, with the help of modern hybrid high-definition photon detectors, the researchers succeeded in simultaneously imaging PbI2 and perovskites in these cross-sections. “We identified trapped, unreacted PbI2 inside the perovskite film using this technique, which is very useful,” says Ummadisingu.
Our findings finally answer several open questions regarding the location and role of residual PbI2 in perovskite solar cells. On a broader note, our innovative demonstration of this technique’s uses opens the door for understanding the properties of perovskites in vertical cross sections of solar cells, not just the perovskite surface as currently shown in the literature.
Michael Grätzel
Funding was provided by the Swiss National Science Foundation (SNSF).