Achieving high efficiency and stability of inverse perovskite solar cells through a multi-legged anchoring co-assembly strategy

Professor Yongzhe Wun's team at East China University of Science and Technology significantly improved the interfacial contact and mechanical stability of inverted perovskite solar cells by constructing triphenylamine-based co-assembled hole transport monolayers. This work addressed key issues such as poor interfacial wettability and insufficient mechanical stability of traditional carbazole-based self-assembled monolayers (e.g., Me-4PACz) in inverted perovskite solar cells. They designed and synthesized a series of multi-legged triphenylamine derivatives (TPA-nCPA) containing cyanoethylphosphonic acid anchoring groups and innovatively proposed a co-assembly strategy with Me-4PACz. Rigid devices fabricated based on this strategy achieved an excellent photoelectric conversion efficiency of 26.27%, while flexible devices achieved an efficiency of 22.38%. Furthermore, after 5000 cycles at a 5mm bending radius, they maintained 90% of their initial efficiency, demonstrating outstanding mechanical stability. The article was published as an open access research article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background information:

Inverted perovskite solar cells (PSCs) have attracted much attention due to their high efficiency potential and ease of fabrication on flexible substrates. The hole transport layer is crucial for device performance, and self-assembled monolayers (SAMs), as an emerging class of hole transport materials, show great promise due to their advantages such as precise interfacial energy level control, efficient charge extraction, and low recombination loss. However, currently widely used SAMs such as Me-4PACz suffer from surface hydrophobicity and poor wettability with perovskite precursor solutions, leading to uneven film coverage and poor interfacial contact, ultimately limiting device performance, reproducibility, and mechanical stability.

Highlights of this article:

The research team developed a series of triphenylamine-based multi-legged co-assembly molecules (TPA-1CPA, TPA-2CPA, and TPA-3CPA) by introducing tunable numbers of cyanoethylphosphonic acid groups as anchoring units. This molecular engineering strategy increases the number of anchoring sites in the self-assembled monolayer while achieving an optimal balance between substrate anchoring, surface wettability, and interfacial adhesion, ultimately making the three-legged TPA-3CPA an ideal co-assembly component for high-performance devices.

Contact angle tests showed that the TPA-3CPA co-assembled monolayer significantly reduced the contact angle of the perovskite precursor solution from 23.3° in Me-4PACz to 8.8°, achieving a superwetting surface. This excellent wettability greatly promoted the deposition of uniform, dense, and pore-free high-quality perovskite films, thereby effectively reducing the generation of interface defects.

Quantitative characterization through peel tests revealed that the TPA-3CPA co-assembly strategy increased the interfacial fracture energy from 3.91 kJ m⁻³ ^to 12.18 kJ m⁻³, more than doubling the adhesion. This enhanced interfacial bonding stems from the increased anchoring sites provided by the multi-legged molecules and the synergistic coordination with the perovskite, effectively suppressing crack initiation and propagation under bending and thermal stress, thereby significantly improving the mechanical durability of flexible devices.

The TPA-3CPA-based co-assembled monolayer can achieve complete coverage on the ITO substrate, effectively reducing interface defects and optical losses. In rigid devices, this structure achieved a champion efficiency of 26.27%, an open-circuit voltage of 1.19 V, and a fill factor of 86.56%; the corresponding flexible device also achieved a conversion efficiency of 22.38%, which is one of the highest efficiencies reported to date for inverted perovskite solar cells based on self-assembled monolayers.

The TPA-3CPA co-assembled device exhibits excellent thermal stability through its tripod anchoring and stress-buffering structure, maintaining 90% of its initial efficiency after 120 hours of continuous heating at 85 °C , with no interface delamination observed. Furthermore, the flexible device retains 90% of its initial performance after 5000 cycles at a 5 mm bending radius, demonstrating outstanding mechanical durability.

Summary and Outlook:

In summary, this study developed a multi-legged triphenylamine co-assembly strategy through ingenious molecular design, successfully overcoming the bottlenecks in wettability and mechanical stability of SAM-based hole transport layers. This strategy has strong universality and provides a new approach and effective solution for fabricating high-performance, high-stability perovskite solar cells, especially those with high mechanical strength suitable for flexible electronic devices.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.