In the global race for clean energy, monolithic perovskite/silicon tandem solar cells (PSTSCs) have emerged as the most commercially promising technology to surpass the theoretical single-junction Shockley-Queisser efficiency limit of 29.4%. In May 2026, a groundbreaking research milestone redefined the limits of perovskite solar cell efficiency. A collaborative team of scientists led by Prof. Ye Jichun at the Ningbo Institute of Materials Technology and Engineering (NIMTE), under the Chinese Academy of Sciences, achieved an unprecedented power conversion efficiency (PCE) of 33.33% (with a certified value of 32.89%) on a 1 square centimeter tandem cell.
Published in the prestigious journal Matter on May 21, 2026, the study introduces an innovative “peak-selective passivation” (PSP) strategy. By utilizing 100 nm polystyrene nanospheres as a template, the researchers precisely deposited a 30 nm thin insulating layer of aluminum oxide (Al2O3) onto the peaks of pyramid-textured industrial silicon bottom cells. This strategically blocked localized electrical leakage pathways and minimized the inherent interfacial energy loss at the textured junctions.
However, as tandem architectures scale toward these high-performance regimes, the physical integrity of the active interfaces becomes highly sensitive. To sustain this 33.33% efficiency threshold and prevent rapid degradation, the quality of the electron transport layer (ETL)—historically dominated by fullerene C60—must be held to an uncompromising standard. Transitioning to a metal free fullerene c60 precursor has emerged as a mandatory requirement to protect the delicate passivation layers and prevent the formation of catastrophic charge traps.
1. The Chemistry of Interfacial Energy Loss in Inverted Tandems
The performance of an inverted (p-i-n) perovskite top cell depends heavily on the interface between the polycrystalline perovskite absorber and the overlying C60 ETL. In these devices, light absorption creates bound electron-hole pairs (excitons) that must be separated and extracted at the transport boundaries.
The surface of polycrystalline perovskite, however, is naturally rich in defect states—predominantly undercoordinated lead (Pb2+) centers, halide (iodide) vacancies, and Pb-I antisite defects. These unpassivated dangling bonds introduce localized energy levels within the bandgap, serving as non-radiative recombination pathways.
When electrons and holes recombine non-radiatively at these defect sites, their energy is converted into heat rather than electricity, causing severe interfacial energy loss that limits the open-circuit voltage ($V_{oc}$) of the solar cell.
To suppress this recombination, modern devices employ a sophisticated perovskite passivation strategy. This involves applying molecular bridging agents (such as self-assembled monolayers like Me-4PACz or dicarboxylic acids) or ultra-thin insulating metal oxides (like the Al2O3 peak-passivation designed by NIMTE) to chemically bind and neutralize the surface defects.
While these passivating layers are highly effective, they are structurally delicate and only nanometers thick. If the subsequent C60 ETL contains chemical or metallic impurities, it can degrade or chemically disrupt these underlying passivation layers, negating their beneficial effects and causing the cell’s efficiency to collapse.
2. Transition Metal Impurities: The Saboteur of High-Efficiency Cells
For decades, the bulk production of commercial fullerenes has relied on the Huffman-Krätschmer graphite arc-discharge method. This physical process involves vaporizing solid carbon electrodes under high-voltage electrical currents.
To stabilize the plasma arc and catalyze the yield of the closed carbon cages, manufacturers must dope the graphite anodes with transition metals, primarily nickel (Ni), cobalt (Co), iron (Fe), and copper (Cu).
Consequently, raw arc-discharge fullerene soot contains significant amounts of heavy metals. Even after successive rounds of acid washings and industrial HPLC purification, sub-ppm to high-ppm levels of these transition metals remain associated with the final C60 powder.
In high-efficiency optoelectronic devices, these residual metals act as potent photo-quenchers and “deep-level traps”. While shallow traps near the band edges temporarily capture and thermally release charge carriers, deep traps—positioned near the middle of the semiconductor’s bandgap (0.5 to 0.7 eV)—permanently immobilize electrons.
At the perovskite/C60 interface, these metallic impurities act as Shockley-Read-Hall (SRH) recombination centers. The trapping and subsequent non-radiative recombination of photogenerated carriers can be mathematically modeled by the standard SRH rate equation:
$$U_{SRH} = \frac{n p – n_i^2}{\tau_p (n + n_1) + \tau_n (p + p_1)}$$
Where $n$ and $p$ are the densities of free electrons and holes, $n_i$ is the intrinsic carrier concentration, and $\tau_n$ and $\tau_p$ are the carrier lifetimes. The introduction of metallic deep-level traps dramatically shortens these lifetimes, converting electrical energy into non-radiative heat, which directly increases interfacial energy loss and limits the overall fill factor (FF) and $V_{oc}$.
Furthermore, under continuous operational stress, these metal ions can migrate through the perovskite layer, causing structural lattice distortions, promoting halide segregation, and degrading the newly developed Al2O3 peak passivation structures.
3. The Metal-Free Imperative: How Combustion Synthesis Unlocks 33.33% PCE
To achieve the stringent requirements of a 33.33% perovskite solar cell efficiency threshold, researchers must utilize C60 precursors that are completely free from transition metal deactivation risks. This is achieved through the transition to a gaseous continuous combustion synthesis process.
Developed as a highly controlled chemical alternative, this method injects a carbon-rich hydrocarbon gas (such as a benzene-oxygen mixture) into a low-pressure reactor chamber. Under a steady-state, laminar flat flame, the precursors undergo partial thermal pyrolysis rather than combustion.
At temperatures maintained between 1200 °C and 1500 °C, the carbon atoms self-assemble natively into the closed-cage, highly symmetric icosahedral structures of C60 and C70 without requiring any transition metal catalysts.
This continuous chemical pathway offers distinct advantages for advanced perovskite R&D:
- Intrinsically Zero-Metal: Because the synthesis operates entirely without solid graphite anodes or transition metal promoters, the resulting fullerene soot is natively free from heavy metals (Ni, Co, Fe, Cu < 0.1 ppm), preventing any deep-level trap state formation.
- Preserving the Passivation Layer: The complete absence of corrosive metal ions protects the structural integrity of the delicate perovskite passivation strategy (e.g., Al2O3 and Me-4PACz), maintaining the optimized energy-level alignment and suppressing non-radiative recombination.
- Enhanced Phase Stability: Metal-free fullerenes demonstrate superior structural packaging and morphological stability, resisting the phase segregation and crystallization decay that typically occurs under continuous light and thermal stress.
4. B2B Sourcing and Quality Assurance Standards
To support global laboratories and manufacturers aiming to reproduce the historic NIMTE tandem efficiency benchmarks, Carbonsphere (Xiamen Carbonsphere Trading Co., Ltd.), in partnership with biotechnology pioneer Healthyking, supplies 99.95% ultra-pure, pharmaceutical/research-grade fullerenes.
By leveraging Healthyking’s advanced, patented continuous combustion technology, Carbonsphere offers bulk quantities of C60 that easily satisfy the strict zero-metal requirements of high-efficiency organic and perovskite devices.
Every single batch is accompanied by comprehensive analytical verification:
- HPLC Peak Area Integration: High-Performance Liquid Chromatography (using specialised columns like Develosil RPFULLERENE) is used to verify the exact chemical purity (99.95%+) and ensure the complete separation of C60 and C70 peaks.
- MALDI-TOF Mass Spectrometry: Positive-ion mass spectra resolve a single, sharp peak at $m/z = 720$ with a clear carbon isotopic distribution, certifying the total absence of metallic adducts, solvent residues, or amorphous soot.
FAQ
Why is traditional C60 contaminated with transition metals?
Traditional C60 is manufactured using the arc-discharge method, which vaporizes solid graphite electrodes under high currents. To catalyze carbon vaporization and increase the yield of fullerene structures, manufacturers impregnate these graphite rods with metal catalysts like nickel, cobalt, and iron. These metals remain associated with the final product as difficult-to-remove impurities.
How do metal impurities in C60 lead to interfacial energy loss?
Residual transition metals introduce localized defect states near the middle of the semiconductor’s bandgap (0.5 to 0.7 eV), acting as “deep traps”. These states immobilize mobile electrons and serve as highly active Shockley-Read-Hall recombination centers. Photogenerated electrons and holes annihilate at these sites, converting electrical energy into heat and leading to significant energy loss.
What is the peak-selective passivation (PSP) strategy used by NIMTE?
Developed by Prof. Ye Jichun’s team at NIMTE, the PSP strategy uses 100 nm polystyrene nanospheres as a template to deposit a 30 nm thin insulating layer of Al2O3 onto the peaks of pyramid-textured silicon bottom cells. This precisely blocks localized electrical leakage pathways and enhances the coverage of the top perovskite layer, driving tandem efficiency to 33.33%.
How does continuous combustion produce metal-free fullerenes?
Continuous combustion is a steady-state chemical process that pyrolyzes gaseous or liquid hydrocarbons in a low-pressure flat flame. The carbon atoms thermally decompose and naturally self-assemble into stable fullerene cages in the gas phase. This completely bypasses the need for transition metal catalysts, resulting in an intrinsically metal-free product.
References
- Chinese Academy of Sciences. (May 28, 2026). Chinese Researchers Design Fresh Strategy to Boost Solar Cell Performance. “
- Yang, W., Yang, Z., Lin, Z., Ye, J., et al. (May 21, 2026). Selective passivation of pyramid peaks for 32.9%-efficient perovskite/silicon tandem solar cells. Matter, DOI: 10.1016/j.matt.2026.102824.() “
- Angewandte Chemie. (April 2026). Interfacial Energetics Reconstruction via Bridging Engineering for Efficient Inverted Perovskite Solar Cells and Modules. “
- Physical Chemistry Chemical Physics. (2025). Continuous Synthesis of Fullerenes and Other Carbon Nanomaterials by Low-Pressure Combustion. “
- Carbonsphere High-Purity Nanomaterials. (2026). Technical Specifications for Fullerene C60 and C70 Photovoltaic Grades.
