Researchers in China have developed an inverted perovskite solar cell that approaches the 27% efficiency threshold. The device contains a specially designed, self-assembled monolayer that passivates perovskite defects and improves efficiency.
A group of researchers led by China’s Lingnan University has fabricated an inverted perovskite solar cell via what they called a self-assembled monolayer (SAM) stabilization strategy.
Inverted perovskite cells have a device structure known as “pin”, where hole-selective contact p is at the bottom of the intrinsic perovskite layer i with electron transport layer n at the top. Conventional halide perovskite cells have the same structure, but in reverse: a ‘nip’ arrangement. With nip architecture, the solar cell is illuminated via the electron transport layer (ETL) side; in the pin structure it is illuminated by the HTL surface.
SAMs are commonly used in hole transport layers (HTLs) in perovskite PV devices to passivate defects and increase efficiency. In particular, the electron density within the SAMs can be modulated to shift the interfacial energies between the SAM and the perovskite layer, resulting in a more versatile and controlled approach to optimizing the energy alignment in the perovskite solar cell, while also having negligible influence on the phase stability of the wide bandgap perovskite film.
The research group designed a cross-linkable SAM molecule called JJ24 and combined it with a hole-selective SAM molecule called CbzNaph.
The academics tempered JJ24 at 160°C and found that it formed stable covalent bonds with the alkyl chains of neighboring CbzNaph molecules. This interaction suppressed the formation of defects and voids in the SAM during the self-assembly process.
The proposed combination was shown to improve the conformational stability of SAM molecules, eliminate degradation in the perovskite absorber, and shift the work function of the substrate downward, improving charge extraction and reducing energy losses.
“The azide-containing SAM can be thermally activated to form a cross-linked and tightly assembled co-SAM with a thermally stable conformation and preferential orientation,” they further explained. “This effectively minimizes substrate surface exposure caused by wobbling of loose SAMs under thermal stress, thus preventing perovskite dissolution.”
Tested under standard lighting conditions, the solar cell with the SAM achieved a maximum energy conversion efficiency of 26.92% and a certified efficiency of 26.82%, the group said, without disclosing which third party verified the results.
The device was also found to exhibit “negligible” efficiency loss after 1000 hours at 85°C.
“Moreover, our experimental results show that this strategy is applicable to several regular SAM molecules, demonstrating good universality and excellent scalability in expanding solar cell areas,” they continued. “This should enable the practical deployment and application of large-area perovskite solar panels within the next three to five years.”
The cell design was presented in “Hardened self-assembled monolayers for sustainable perovskite solar cells”, published in nature. The research team included scientists from the Chinese Academy of Sciences (CAS) and the City University of Hong Kong.
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