A group of researchers led by China's Lingnan University has fabricated an inverted perovskite solar cell through what they called a self-assembled monolayer (SAM) stabilization strategy.
Inverted perovskite cells have a device structure known as “p-i-n”, in which hole-selective contact p is at the bottom of intrinsic perovskite layer i with electron transport layer n at the top. Conventional halide perovskite cells have the same structure but reversed – a “n-i-p” layout. In n-i-p architecture, the solar cell is illuminated through the electron-transport layer (ETL) side; in the p-i-n structure, it is illuminated through the HTL surface.
SAMs are frequently 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 energetics between the SAM and perovskite layer, which results in a more versatile and controlled approach to optimizing energy alignment in the perovskite solar cell, while also having a negligible influence on the phase stability of the wide-bandgap perovskite film.
The research group designed a crosslinkable SAM molecule named JJ24 and combined it with a hole-selective SAM molecule named CbzNaph.
The academics annealed 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 found to improve the conformational stability of SAM molecules, to eliminate degradation in the perovskite absorber, and to down-shift the work function of the substrate, which enhances charge extraction and lowers energy losses.
“The azide-containing SAM can be thermally activated to form a cross-linked and densely assembled co-SAM with a thermally stable conformation and preferred orientation,” they further explained. “This effectively minimizes substrate surface exposure caused by wiggling of loose SAMs under thermal stress, preventing perovskite decomposition.”
Tested under standard illumination conditions, the solar cell incorporating the SAM achieved a maximum power conversion efficiency of 26.92% and a certified efficiency of 26.82%, the group said, without disclosing which third-party entity verified the results.
The device was also found to show “negligible” efficiency loss after 1,000 h at 85 C.
“Furthermore, our experimental results show that this strategy is applicable to various mainstream SAM molecules, demonstrating good universality and excellent scalability in enlarging solar cell areas,” they went on to say. “This should allow the practical deployment and application of large-area perovskite solar modules within the next 3-5 years.”
The cell design was presented in “Toughened self-assembled monolayers for durable perovskite solar cells,” published in nature. The research team comprised scientists from the Chinese Academy of Sciences (CAS) and the City University of Hong Kong.
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