A Molecular Mix Pushes Inverted Perovskite Cells Past 25.6% Efficiency

HONG KONG — An international research coalition has developed a scalable chemical method for eliminating structural defects at the internal interfaces of inverted perovskite solar cells. The technique, called symmetry-breaking co-assembly (SBC), combines a widely used symmetric host molecule with a simple asymmetric conjugated additive.

The resulting cells achieved a certified power conversion efficiency exceeding 25.6% and improved durability under continuous electrical stress. The research was published in *Nature Communications* under the title “Symmetry-Breaking Co-Assembly of Conjugated Molecules Boosts Perovskite Photovoltaics.”

Silicon photovoltaic panels account for roughly 95% of the worldwide solar market, while single-junction silicon devices are nearing their theoretical conversion efficiency limit of 29.4%, known as the Shockley-Queisser limit. Metal-halide perovskites, which mirror the crystal structure of calcium titanium oxide, have advanced faster than any other photovoltaic technology since their first solar application in 2009, when conversion efficiencies were only 3.8%.

Researchers have increasingly focused on inverted, or “p-i-n,” architectures as they work toward commercializing perovskites. Conventional “n-i-p” layouts use thick, dopant-dependent hole transport layers such as Spiro-OMeTAD, which degrade rapidly under heat and moisture. In inverted cells, the hole-selective layer sits directly on the front transparent conductive oxide substrate.

That configuration offers improved long-term stability and provides the structural basis for tandem solar cells, which place perovskite layers over existing silicon cells to capture a broader spectrum of sunlight. Inverted devices, however, depend heavily on ultrathin self-assembled monolayers (SAMs) to move electrical charges away from the perovskite absorber.

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Industry-standard carbazole-based SAMs, including MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid), tend to self-aggregate. When deposited on indium tin oxide (ITO) glass substrates, their symmetric molecules clump together instead of forming an even monolayer. Microscopic gaps and pinholes then remain on the conductive surface, exposing regions that trigger non-radiative charge recombination and localized chemical degradation.

The team pursued a low-cost way to address that problem without complex, multi-step organic synthesis. It paired existing compounds in a molecular co-assembly process rather than creating expensive asymmetric molecules.

“Rather than synthesizing complex and costly asymmetric molecules, we achieve the benefits of molecular asymmetry by co-assembling a widely used symmetric SAM (MeO-2PACz) with a simple asymmetric conjugated molecule (DTCA),” corresponding author Tom Wu told *pv magazine*. “This approach suppresses molecular self-aggregation, substantially increases surface coverage, and improves buried-interface quality.”

Two symmetry-breaking carboxylic acid candidates were tested: 2-thiophenecarboxylic acid (TCA) and dibenzo[b,d]thiophene-4-carboxylic acid (DTCA). The compounds were blended into MeO-2PACz formulations. Fabrication used 0.7 milligram-per-milliliter solutions in isopropanol, spin-coated onto plasma-treated ITO substrates at 3,000 revolutions per minute for 30 seconds and thermally annealed at 100 degrees Celsius for 10 minutes.

Molecular dynamics simulations found that DTCA acts as a fast-anchoring guest molecule. Its asymmetric structure and distinct dipole moment allow it to anchor rapidly to the ITO surface, creating physical obstacles that interrupt the self-aggregation of the larger MeO-2PACz host molecules. Testing molar ratios of 3:1, 4:1, and 5:1 identified the 4:1 MeO-DTCA mixture as the optimal formulation.

To measure the structural effect, the researchers combined atomic force microscopy with infrared spectroscopy in a quantitative diagnostic protocol known as AFM-IR. The method directly measured chemical distribution and molecular packing density across the substrate at sub-diffraction nanoscale resolution.

Pure MeO-2PACz produced an incomplete monolayer covering 60.6% of the ITO surface. MeO-TCA increased coverage only slightly, to 62.2%, while the optimized MeO-DTCA co-assembly reached 82.4% coverage. That amounted to more than a 30% increase in active surface coverage compared with single-molecule SAM layers.

“By mixing the industry-standard symmetric SAM MeO-2PACz with a small asymmetric molecule (DTCA), we increased SAM surface coverage by more than 30%, as directly quantified using a newly developed AFM-IR analysis method,” Wu said. “We were amazed to find that strategically breaking molecular symmetry could unlock so many benefits for solar cells, leading to improvements in interface quality, energy conversion efficiency, and operational stability.”

The full inverted solar cell stacks used the optimized MeO-DTCA co-assembled SAM hole-selective layer between an indium tin oxide (ITO) transparent bottom electrode and a metal-halide perovskite absorber layer. The devices also contained a phenyl-C61-butyric acid methyl ester (PCBM) electron-transport layer, a bathocuproine (BCP) buffer layer, and a top silver (Ag) contact electrode.

By removing exposed ITO contact points, the dense co-SAM reduced non-radiative recombination losses and interfacial chemical reactions under bias. Small-area test devices measuring 0.08 square centimeters reached a peak power conversion efficiency of 26.32%, while 1-square-centimeter cells reached 25.34%. Independent testing confirmed certified efficiencies above 25.6%.

Encapsulated MeO-DTCA solar cells also retained 93% of their initial power conversion efficiency after 1,150 hours of continuous operation in stability tests conducted at the maximum power point (MPP).

Because the co-assembly process uses existing chemical inventories instead of expensive bespoke molecules, the development offers a route toward mass manufacturing. Wu said the strategy could apply to many other molecular systems, and that the team plans to develop a broader library of symmetry-breaking molecular combinations.

The researchers are also interested in establishing design rules connecting molecular symmetry, dipole moment, and surface coverage. Wu noted that the team plans to extend these molecular design principles to commercial-scale modules and monolithic perovskite-silicon tandem configurations.

The quantitative AFM-IR technique developed during the project could also be used to study ultrathin molecular monolayers in other semiconductor and electronic technologies. The international research effort included scientists from Hong Kong Polytechnic University, the Shenzhen University of Advanced Technology, the Chinese Academy of Sciences, Northwestern Polytechnical University, and Great Bay University in China; the University of New South Wales in Australia; the University of Surrey in the United Kingdom; Korea University in South Korea; and the Karlsruhe Institute of Technology (KIT) in Germany.