Longi hits 35.31% tandem cell efficiency with engineered SAM molecules

Longi has pushed a perovskite-silicon tandem solar cell to 35.31% power conversion efficiency using specially engineered self-assembled molecules, with independent certification confirming a stabilized efficiency of 35.13%.

China's National Center of Inspection on Solar Photovoltaic Products Quality (CPVT) verified the result: a reverse-scan efficiency of 35.25% and a fill factor of 84.98%. The champion device posted a short-circuit current density of 20.82 mA/cm², an open-circuit voltage of 2.007 V and a fill factor of 84.53% under standard test conditions.

The advance, published in Nature under the title "Conjugation-regulated self-assembled molecules for tandem solar cells," turns on a deceptively narrow materials problem: how hole-selective contact molecules lay down on rough silicon.

Why the molecule matters

Conventional self-assembled monolayer (SAM) molecules — the industry workhorse is 4PACz — tend to aggregate, leaving uneven coverage on pyramid-textured silicon surfaces. Longi's team designed four derivatives, TC0 through TC3, with controlled molecular twisting and conjugation to suppress excessive stacking while keeping charge transport intact.

TC2 won. Its conjugated backbone paired with a moderately twisted phenyl side group delivered the best balance of molecular packing, substrate adhesion and charge transport. Molecular simulations showed 83.1% surface coverage, against 75.0% for TC1 and 73.3% for TC3. Experimental measurements confirmed TC2 exceeded 80% coverage in practice and anchored strongly.

"This molecular design promotes the formation of a uniform, well-organized self-assembled monolayer, even on rough, pyramid-textured surfaces," Longi researcher Bo He told pv magazine. "By balancing molecular packing and surface coverage, the engineered interface improves energy-level alignment for faster charge extraction, reduces defects through molecular flexibility, and provides a stable foundation for the growth of high-quality perovskite films."

What changes at the device level

The TC2 monolayer modified the indium tin oxide (ITO) work function and aligned energy levels for efficient hole extraction. That strengthened the internal electric field, sped up charge separation and cut recombination at the ITO-perovskite interface.

Downstream effects compounded. TC2 slowed and steadied perovskite crystallisation, producing larger, more uniform grains with higher crystallinity. Photoluminescence measurements showed a quantum yield of 1.46% and a quasi-Fermi level splitting of 1.292 eV — the highest values among the SAMs tested.

The device stack itself is familiar territory: a textured heterojunction silicon bottom cell, ITO recombination layer, TC2-based SAM, perovskite absorber, lithium fluoride interlayer, fullerene (C60) electron transport layer, tin oxide buffer, indium zinc oxide electrode, silver contact and magnesium fluoride coating.

Stability numbers matter as much as headline efficiency for anyone tracking commercial timelines. The tandem devices retained 94% of initial efficiency after 1,500 hours of continuous operation under the ISOS-L-1 protocol, and 82% after 1,000 hours at 65 C under ISOS-L-2.

"Overall, the results demonstrate that optimized molecular interface engineering can simultaneously improve the efficiency, charge transport and operational stability of perovskite-silicon tandem solar cells," He said.

A crowded pipeline of interface chemistry

The result lands in a fast-moving field. Longi and Soochow University recently reported a 34.0%-efficient tandem cell using a dual-anchored interface with zirconium oxide nanoparticles, which retained 84% of initial efficiency after 2,000 hours. In July 2026, Longi raised its own tandem record to 35.5%, certified by the European Solar Test Installation — meaning the 35.31% TC2 result sits just below the company's best mark.

Earlier milestones stack up quickly:

• September 2026: A symmetry-breaking molecular strategy from an international team including Hong Kong Polytechnic and Northwestern Polytechnical University reached 26.32% on an inverted perovskite cell, 25.67% certified, with 93% retention after 1,150 hours.
• December 2025: Northwestern Polytechnical University led a tandem cell to 31.71% certified efficiency using chemical polishing to remove residual lead iodide; Soochow University hit 32.3% (32.12% certified) with a steric-complementary interface design.
• April 2025: Longi achieved 34.85% certified efficiency using bilayer passivation with lithium fluoride and ethylenediammonium diiodide.
• November 2025: Longi detailed a 27.81%-efficient crystalline silicon cell based on a hybrid interdigitated back-contact architecture.

Researchers from Soochow University, Northwestern Polytechnical University and Hong Kong Polytechnic University participated in the TC2 study alongside Longi.

The pattern is clear: interface molecular engineering is now the main lever moving tandem efficiency from the low 30s toward 36% while simultaneously buying operational lifetime. The milestone to watch is whether SAM designs like TC2 translate from champion lab cells to textured, high-throughput production formats — and whether the stability gains hold past the 2,000-hour mark at elevated temperature.