Chinese solar module manufacturer Longi has developed a perovskite-silicon tandem solar cell using specially designed molecules that suppress excessive molecular stacking while maintaining efficient charge transport.
“This molecular design promotes the formation of a uniform, well-organized self-assembled monolayer (SAM), 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.”
The researchers explained that conventional SAM molecules often suffer from excessive aggregation, resulting in uneven coverage on textured silicon surfaces. To address this limitation, they designed molecules with controlled molecular twisting and conjugation. They developed four SAM derivatives – TC0, TC1, TC2 and TC3 – based on the commonly used 4PACz molecule.
TC2 offered the best balance between molecular packing, substrate adhesion and charge transport, as its structure combines a conjugated backbone with a moderately twisted phenyl side group. Molecular simulations showed surface coverage of 83.1%, compared with 75.0% for TC1 and 73.3% for TC3. Experimental measurements confirmed that TC2 achieved more than 80% coverage and exhibited strong anchoring stability.
The improved molecular arrangement also enhanced the electronic properties of the interface between the indium tin oxide (ITO) layer and the perovskite absorber. In particular, TC2 modified the ITO work function and provided favorable energy-level alignment for efficient hole extraction. This strengthened the internal electric field, accelerating charge separation and transport, while improved interface coverage reduced defects and suppressed charge recombination.
The analysis also showed that TC2 promoted slower, more controlled perovskite crystallization, producing larger, more uniform grains with higher crystallinity. The resulting films exhibited improved phase uniformity and reduced nonradiative recombination. Photoluminescence measurements showed a quantum yield of 1.46% and a quasi-Fermi level splitting of 1.292 eV, the highest values among the tested SAMs.
Using this molecular engineering approach, the researchers fabricated a two-terminal monolithic tandem solar cell combining a perovskite top cell with a heterojunction (HJT) bottom cell.
The device features a textured silicon bottom cell coated with an ITO recombination layer, followed by a TC2-based SAM serving as the hole-selective contact. The perovskite absorber is deposited on the SAM and covered with a lithium fluoride (LiF) interlayer, a fullerene (C60) electron transport layer and a tin oxide (SnOx) buffer layer. The stack is completed with a transparent indium zinc oxide (IZO) electrode, a silver (Ag) contact and a magnesium fluoride (MgFx) optical coating.
Under standard test conditions, TC2-based devices achieved an average open-circuit voltage of 2.007 V and a fill factor of 84.30%, outperforming cells based on 4PACz, TC1 and TC3. The champion device reached a power conversion efficiency of 35.31%, with 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%.
Independent testing by China’s National Center of Inspection on Solar Photovoltaic Products Quality (CPVT) certified a reverse-scan efficiency of 35.25%, a fill factor of 84.98% and a stabilized efficiency of 35.13%. External quantum efficiency measurements confirmed effective light absorption and current generation in both subcells.
Stability tests also demonstrated improved resistance to light-induced degradation and phase segregation in TC2-based perovskite films. The tandem devices retained 94% of their 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 the ISOS-L-2 protocol.
“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.
The cell technology is described in “Conjugation-regulated self-assembled molecules for tandem solar cells,” published in Nature. Researchers from China’s Soochow University, Northwestern Polytechnical University and Hong Kong Polytechnic University participated in the study.
The new development builds on a series of advances in molecular interface engineering for perovskite photovoltaics. In September, an international research team involving Hong Kong Polytechnic University and Northwestern Polytechnical University developed a symmetry-breaking molecular strategy that improved SAM coverage by combining symmetric and asymmetric molecules. The resulting inverted perovskite solar cell achieved an efficiency of 26.32%, with a certified value of 25.67%, while retaining 93% of its initial performance after 1,150 hours of operation.
In December 2025, researchers led by Northwestern Polytechnical University developed a perovskite-silicon tandem cell with a certified efficiency of 31.71%. Their approach relied on chemical polishing to selectively remove residual lead iodide from the perovskite absorber, reducing recombination losses and improving film uniformity. Around the same time, a team led by Soochow University achieved 32.3% efficiency using a steric-complementary interface design combining small, flexible molecules for defect passivation with larger molecules providing protection against environmental degradation. The device achieved a certified efficiency of 32.12%.
More recently, Longi and Soochow University developed a 34.0%-efficient tandem cell using a dual-anchored interface incorporating zirconium oxide (ZrO2) nanoparticles between the transparent conductive oxide and the SAM. The approach reduced interfacial recombination and leakage while maintaining efficient charge extraction. The device achieved an open-circuit voltage of 1.997 V and retained 84% of its initial efficiency after 2,000 hours of operation.
Longi has also continued to advance tandem cell efficiency. In April 2025, the manufacturer achieved a certified efficiency of 34.85% for a two-terminal perovskite-silicon device, using a bilayer passivation strategy based on lithium fluoride and ethylenediammonium diiodide. In July 2026, the company raised its tandem cell efficiency record to 35.5%, with the result independently certified by the European Solar Test Installation (ESTI).
Alongside its tandem research, Longi has made progress in conventional crystalline silicon technology. In November 2025, the company disclosed the technical details of its 27.81%-efficient silicon solar cell, based on a hybrid interdigitated back-contact architecture combining passivated tunneling contacts, dielectric passivation layers and optimized carrier transport.
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