Jingbi You explains how precisely adjusting the stoichiometry of planar architectures can affect the performance of perovskite devices. For more comprehensive information, you can also check out the Communication ‘Planar-Structure Perovskite Solar Cells with Efficiency beyond 21%‘ published in Advanced Materials.
Why does the current efficiency of planar structures lag behind that of other perovskite devices?
There are two types of device structure for perovskite solar cells: one is planar and the other is mesoporous. I think there are three reasons leading to the current efficiency of planar structures lagging behind that of the mesoporous structure. One is that TiO2 mesoporous structures have been extensively studied for more than 20 years, deriving from research into dye-sensitized solar cells (DSSCs), and the experience in DSSC research has been successfully transferred to perovskite solar cell research. Although interest in the planar structure came through studies of organic solar cells, the study of planar structures began later than that of mesoporous structures. This could be one reason.
Secondly, perovskite is easily crystallized. In the planar structure, this can lead to pinhole defects, while in the mesoporous structure, the mesopores can act as a scaffold to support the perovskite layer and reduce recombination.
Thirdly, I think the serious hysteresis effect has affected the progress of planar structure perovskite solar cells. The reverse and forward scans show different current–voltage curves, especially in n–i–p planar structure solar cells. This serious hysteresis effect makes it very hard for people to accurately determine the efficiency, which is a headache, and which makes people doubt planar structure solar cells.
What affect does the stoichiometry, particularly the PbI2 content, have?
PbI2 has a very special role in perovskite solar cells. A small amount of PbI2 in grain boundary could passivate the defects, and enhance the device performance. But we found that too much PbI2 in grain boundary could lead to serious hysteresis and stability. Optimization of PbI2 in perovskite is very critical.
You recently achieved a record PCE for planar-structure devices. What needs to be done next?
We will continue to improve the PCE of the planar structure, and also we will put more effort into improving device stability. The issue of pinhole defects has been almost resolved by control over the perovskite growth. The hysteresis issue has also been almost resolved, by electron transport interface engineering. The planar structure has shown significant progress in the last two years. I believe it could catch up to that of the mesoporous structure soon.
If you work on perovskites, you may also like to check out these previous Perovskite Perspectives:
Perovskite Perspectives: Yi Hou and Christoph Brabec Talk Hysteresis
Perovskite Perspectives: Chul-Ho Lee and Min Jae Ko Discuss Moisture Stability