Breakthrough efficiency of 34.58% achieved with new HTL201 molecule
In the solar sector, innovation never stands still. Quite the opposite, technological progress moves quickly, often marked by a series of world records. The latest? A certified conversion efficiency of 34.58% for silicon–perovskite tandem photovoltaic cells.
A group of 54 researchers, mostly from LONGi Green Energy Technology, has set a new benchmark in solar energy. The team improved the performance of perovskite/silicon tandem cells by focusing on a key component: the self-assembled monolayer, or SAM.
Let’s take a step back to understand why this matters.
What are SAMs?
In recent years, both standalone perovskite solar cells and tandem perovskite/silicon cells have achieved efficiency levels suitable for real-world applications. One critical enabler has been the growing use of self-assembled monolayers (SAMs) as selective hole transport contacts.
These are molecules typically made up of three parts:
- an anchor or head group, with a specific chemical affinity for a target substrate
- a spacer group, usually a chain of carbon atoms
- a functional tail group, whose chemistry defines the final surface properties
Carefully designing these molecular components allows SAMs to enhance solar cell performance by tuning energy level alignment for faster charge extraction, or by adjusting the surface energy of the substrate to improve perovskite crystallization. Studies have shown that SAMs can be formed in solution or by thermal evaporation, offering promising scalability.
To work effectively, SAMs need to be densely packed and orderly, with minimal defects. In silicon–perovskite tandem cells, they must also uniformly cover the textured substrate surface beneath.
However, precisely controlling the thickness and orientation of self-assembled monolayers has proven to be a significant challenge.
The HTL201 breakthrough
Researchers at LONGi Green and partner institutions achieved a major breakthrough in controlling SAM properties. They developed an asymmetric SAM molecule called HTL201, which combines an anchor group and spacer flanking a carbazole core.
HTL201 can directly form on the transparent conductive oxide layer, which serves as the recombination interface between the perovskite and silicon layers.
The molecule creates a more complete and uniform protective layer across the textured surface, bonding tightly to the overlying perovskite layer. This strong bond helps prevent the recombination of electrons and holes, reducing energy loss.
Moreover, the energy levels of HTL201 are finely tuned to match those of the perovskite, enabling an open-circuit voltage of up to 2 volts. The result is a certified tandem cell conversion efficiency of 34.58%.
The research, titled Efficient perovskite/silicon tandem with asymmetric self-assembly molecule, was published in Nature (2025).