A research team from China has developed a novel approach to mitigate the self-aggregation of self-assembled molecules (SAMs) in co-deposited inverted perovskite solar cells.

Co-deposited inverted cells are fabricated by mixing SAM directly into the perovskite precursor, but SAM tend to aggregate, leading to poor interfacial coverage and reduced device performance. To address this issue, the researchers designed an asymmetric SAM, PhBr-4PACz, which suppresses aggregation and promotes SAM accumulation at the bottom interface, improving adhesion and coverage. They also introduced the grain-boundary crosslinking additive 1-allyl-3-vinylimidazolium chloride (AVIMCl) to suppress SAM diffusion and improve device stability.

Their work is presented in the paper “Co-deposited inverted perovskite photovoltaics towards 27% efficiency via vertical redistribution of self-assembled molecules and in-situ crosslinking,” published in nature communications.

Corresponding author Chun-Chao Chen told pv magazine the work introduces a dual strategy that combines the design of an asymmetric self-assembled molecule with in-situ grain boundary crosslinking.

“The bulky phenyl and bromine groups in PhBr-4PACz suppress self-aggregation and promote vertical redistribution of SAMs, enhancing its adhesion at the buried interface while reducing excess at the top surface of the perovskite layer,” Chen explained. “We further used a crosslinkable ionic liquid, AVIMCl, that penetrates grain boundaries and crosslinks at low temperature, reducing residual stress and suppressing upward diffusion of SAMs.”

Chen added the approach enabled a certified power conversion efficiency of 27.03% (quasi-steady-state 26.50%) in co-deposited inverted devices, which he said is currently the highest reported for this architecture to date. “Unencapsulated devices retained >90% efficiency after 2,000 hours at 85 C and 96.6% after 2,000 hours under maximum power point tracking at 65 C,” he added.

The team mixed the PhBr-4PACz directly into a perovskite precursor solution containing formamidinium iodide (FAI), lead iodide (PbI₂), caesium iodide (CsI), methylammonium chloride (MACl), and formamidinium chloride (FACl).

The resulting solution was spin-coated onto fluorine-doped tin oxide (FTO) substrates, followed by antisolvent treatment and annealing at 100 C to form the perovskite absorber layer while simultaneously enabling SAM deposition. A solution of AVIMCl containing was spin-coated onto the perovskite surface and annealed at 100 C for 30 min, allowing the additive to diffuse along grain boundaries and undergo in situ crosslinking. The researchers coated the device with two electron-transport materials, PCBM and BCP, and then deposited a thin 120-nm copper electrode by thermal evaporation.

The resulting device architecture was FTO/perovskite(PhBr-4PACz)/AVIMCl/PCBM/BCP/Cu. The device was compared with co-deposited devices based on the widely used reference SAM Me-4PACz, as well as devices incorporating PhBr-4PACz without AVIMCl. To demonstrate the versatility of the approach, the researchers fabricated devices on indium tin oxide (ITO) substrates, achieving a maximum efficiency of 26.55%. Devices on flexible PET/ITO substrates reached 25.03%, and an 8.905 cm² mini-module with an efficiency of 23.68%.

Chen said two notable findings emerged from the study. “First, co-deposited SAMs were observed to migrate not only to the top interface but also to the bottom interface during crystallization. Second, PhBr-4PACz significantly enhances buried interface coverage and adhesion, as evidenced by XPS, C-AFM, and peel tests,” Chen explained. “Additionally, thermal aging induces significant upward diffusion of SAMs along grain boundaries, and AVIMCl crosslinking almost entirely suppresses this effect, as confirmed by ToF-SIMS 3D profiling.”

Chen added his team plans to scale the co-deposition strategy to large-area modules and adapt it to industrial coating techniques such as slot-die and blade coating. The researchers also aim to optimize the SAM and crosslinker further to increase open-circuit voltage and reduce recombination losses and are conducting studies to better understand the mechanisms of SAM redistribution and crosslinker penetration.

Scientists from China’s Shanghai Jiao Tong University and Shandong Normal University contributed to the study.