The LEEMONS project is researching nanostructured silicon that uses low-energy electron multiplication (LEEM) to allow one high-energy photon to generate multiple electrons, reducing energy losses in solar cells. By creating ultra-thin amorphous silicon layers inside crystalline silicon via ion implantation, the scientists aim to boost solar cell efficiency beyond the Shockley–Queisser limit while keeping compatibility with existing manufacturing methods.
Silicon mask for discontinuous implantation
Image: LEEMONS
A European research consortium is investigating a new nanostructured silicon approach that could help crystalline silicon solar cells overcome efficiency constraints. The initiative, known as Low-Energy Electron Multiplication on Nanostructured Solar Cells (LEEMONS), focuses on exploiting a mechanism called low-energy electron multiplication (LEEM), which allows a single high-energy photon to generate multiple low-energy electrons, reducing energy losses that typically occur when excess photon energy is dissipated as heat.
LEEM is a type of carrier multiplication (CM), which is a very promising process that, if applied with success, may dramatically increase the efficiency of PV devices. The process occurs when the absorption of a single photon results in the excitation of multiple electrons. In conventional solar cells, a single photon is able to excite only one electron across the bandgap of the cell, which makes all high-energy free carriers dissipate as heat.
Nanostructuring silicon
Carrier multiplication is claimed to have the potential of bringing photovoltaics closer to exceeding the Shockley–Queisser – the maximum theoretical efficiency that a solar cell using a single p-n junction can possibly reach, calculated by examining the amount of electrical energy that is extracted per incident photon. To date, however, it has been applied to solar only in experimental research. Compared to classical CM, LEEM enables carrier multiplication at lower excess electron energies, reducing thermalization losses. The proposed LEEM approach modifies the response of silicon itself and does not require additional semiconductors, as in tandem solar cells using perovskite or other absorber materials.
Image: LEEMONS, Frederic Milesi CEA
For their experiments, the scientists are using nanostructuring silicon through controlled ion implantation. The process creates ultra-thin amorphous silicon regions embedded within a crystalline silicon matrix. In these nanostructured regions, high-energy carriers are more likely to generate additional electron–hole pairs through impact ionization before losing their energy through thermalization. A key step in the technology involves forming buried amorphous silicon layers through ion implantation followed by controlled annealing.
Ion implantation
“The nanostructured silicon used in the LEEMONS project is produced through a controlled ion implantation process applied to conventional crystalline silicon wafers,” the project coordinator, Brice Rouffie, told pv magazine. “During implantation, energetic ions are introduced into the silicon lattice, locally damaging the crystalline structure and creating thin amorphous regions below the surface. Because the implanted ions all have a well-defined kinetic energy, they penetrate the silicon to a predictable depth before losing their energy. As a result, the lattice damage is concentrated in a very narrow region, allowing the formation of ultra-thin amorphous layers.”
A subsequent thermal annealing step partially recrystallizes the silicon while preserving extremely thin buried amorphous layers embedded within the crystalline matrix. Recrystallization occurs faster along the crystallographic direction than along other directions, which naturally smooths and flattens the amorphous and crystalline interfaces. These layers typically have nanometer-scale thickness and can be positioned at controlled depths.
Using transmission electron microscopy, the research group found that the produced nanometer-scale amorphous layers within crystalline silicon wafers were suitable for first experiments in solar cells. To precisely define the implantation zones, they use hard masks, including ultra-thin silicon masks with micrometer-scale apertures and metallic mesh masks with openings as small as 7 µm. These techniques allow for patterned ion implantation without relying on photoresist processes, avoiding contamination or damage to the wafer surface.
Manufacturing techniques
The scientists explained that integrating the silicon nanostructures into practical devices requires careful adjustments to several manufacturing steps. One key challenge is metallization, as standard solar cell firing processes typically exceed 400 C, which could alter the implanted nanostructures. To address this, they are exploring low-temperature metallization methods, including silver contacts deposited by magnetron sputtering at temperatures below 100 C. Passivation is another focus, with teams at Germany’s ISC Konstanz and the Swiss Center for Electronics and Microtechnology (CSEM) evaluating dielectric layers and diffusion processes to preserve carrier lifetimes while accommodating the implanted structures.
“Preliminary work is still ongoing and the project has not yet reached the stage of testing fully integrated M6 solar cell prototypes,” said Rouffie. “At ISC Konstanz, the current effort focuses on optimizing the metallization process to ensure that the nanostructured regions created by ion implantation can survive the high-temperature firing step used in industrial solar cell manufacturing. Initial tests have shown that the amorphized silicon layers can remain stable during firing and when appropriate dummy wafer configurations and laser-enhanced contact optimization (LECO) are used, which is an encouraging result for process compatibility.”
In parallel, work at CSEM is currently focused on understanding the impact of the LEEM nanostructures on carrier lifetime. “A first experimental campaign in January 2026 did not provide the expected improvements, so a second optimization campaign is now underway to refine implantation and annealing conditions and better understand the mechanisms affecting lifetime,” he went on to say. “Once these lifetime optimizations are completed and the process integration is stabilized, the consortium plans to move toward the fabrication and testing of the first complete M6 cell prototypes incorporating the LEEM nanostructures starting mid 2026.”
Efficiency roadmap
Rouffie explained that, in the detailed balance calculation by Shockley and Queisser, it is assumed that each absorbed photon can generate at most one electron–hole pair. This assumption is one of the main factors limiting the maximum efficiency of conventional photovoltaic devices.
“Carrier multiplication processes challenge this limitation by allowing a single high-energy photon to generate multiple charge carriers,” he emphasized. “In theory, this could push the efficiency limits well beyond the 31.0% and 40.8% calculated for devices without carrier multiplication under one-sun illumination and maximum concentration, respectively. Detailed balance calculations predict that efficiencies as high as 44.7% under one sun and 85.9% under concentrated light could be achieved, assuming the sun behaves as a blackbody at a temperature of 5,762 K.”
“These results confirm earlier calculations by Rofl Brendel, CEO at Germany’s Institute for Solar Energy Research Hamelin (ISFH), who obtained comparable values under slightly different assumptions, namely 43.6% for an optimal bandgap of 0.768 eV and 85.4% for 0.048 eV,” Rouffie said. “Comparable limits have also been obtained for standard illumination conditions. Overall, for single-junction photovoltaic cells, the theoretical efficiency increases from about 33.7% without carrier multiplication to around 44.4% when multiplication effects are included.”
“LEEM is expected to outperform other known carrier multiplication processes, such as multiple exciton generation, because the low-energy activation threshold could allow a larger portion of the solar spectrum to contribute to the effect,” he also stressed. “In addition, reducing thermalization losses would lower the operating temperature of the solar cells and therefore further improve overall device performance. LEEM therefore has the potential to significantly increase the efficiency of silicon solar cells, possibly approaching a doubling of the theoretical efficiency limits without requiring fundamentally different manufacturing processes.”
Looking forward, the researchers aim to demonstrate proof-of-concept devices using established solar cell architectures including Passivated Emitter and Rear Contact (PERC) and heterojunction (HJT) solar cells.
“At this stage, the project does not target a specific commercial efficiency value,” Fourrie concluded. “The main objective is to experimentally verify whether the LEEM mechanism can be induced in nanostructured silicon and whether it can lead to measurable improvements in carrier generation. If such effects can be demonstrated, efficiencies approaching 30-35% for single-junction silicon devices becomes conceivable, considering the higher theoretical limits predicted when carrier multiplication mechanisms are taken into account.”
The LEEMONS project is funded under the EU’s Horizon Europe program and runs from November 2024 to October 2027. The consortium includes six partners: Segton Advanced Technology, CEA-Leti, ISC Konstanz, CSEM, Roltec, and University of Franche-Comté
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