Scientists in Morocco and Spain have found a way to make a promising class of solar cells more efficient by adding lithium. Their method could pave the way for cheaper, transparent solar panels capable of pairing with silicon cells to harvest more sunlight and accelerate the clean energy transition.
As solar energy surges into the mainstream, one question remains: how much more efficient can we make it without driving up costs or relying on scarce materials? Crystalline silicon — the workhorse of the solar industry — is nearing its theoretical limit. Researchers worldwide are therefore looking beyond silicon, towards new materials that can be layered or tuned to capture more of the sun’s spectrum.
A promising candidate among these is kesterite, a family of copper–zinc–based compounds that can be made from earth-abundant, non-toxic elements. In particular, the germanium-based compound Cu₂ZnGeSe₄, or CZGSe, has attracted attention for its stability, adaptability and compatibility with inexpensive thin-film manufacturing.
Now, a new study led from Rabat, in collaboration with European partners, shows that a simple addition — a whisper-thin layer of lithium fluoride (LiF) — can significantly improve the performance of these next-generation solar absorbers.
The research demonstrates how lithium atoms subtly alter the atomic landscape of the kesterite material, smoothing out defects that normally trap electrical charge and hinder efficiency.
Why Lithium Helps
In semiconductor physics, small structural defects — known as vacancies or antisites — can act like potholes for electrons, scattering them and converting valuable energy into heat. Kesterite materials are particularly prone to these flaws, especially where copper and zinc atoms swap places.
Lithium, it turns out, can slip into these imperfections and stabilise the lattice. Its ionic radius is close to copper’s, meaning it can replace or complement it without disrupting the crystal structure. When introduced as a 10-nanometre LiF nanolayer — thinner than a virus particle — the lithium gently diffuses during heat treatment, passivating these defects and encouraging the growth of larger, better-ordered grains.
The result? Improved charge transport and higher open-circuit voltages, both key ingredients for better-performing solar cells.
The Results in Numbers
The team’s best-performing cell, built on a molybdenum (Mo) back electrode, reached an efficiency of 6.4% — modest compared with today’s commercial silicon panels, but significant for a proof-of-concept material that’s still in early development. For context, earlier versions of similar germanium kesterite cells achieved efficiencies below 6%.
Cells built on transparent electrodes — specifically fluorine-doped tin oxide (FTO), a common component in semi-transparent and tandem solar designs — performed less efficiently, around 2.2%. Yet they showed markedly better structural and electronic properties than undoped equivalents. This transparency is crucial for combining such cells with silicon or other semiconductors to create tandem devices, which can capture a broader range of sunlight and surpass silicon’s efficiency ceiling.
In both cases, lithium doping consistently improved crystallinity, reduced recombination losses, and yielded higher voltages. The effect was strongest with a 10 nm LiF layer; doubling that thickness reduced performance slightly — a reminder that in materials science, more isn’t always better.
How It Works
The lithium doesn’t merely sit on the surface; it interacts with the germanium and selenium inside the absorber. During the high-temperature annealing stage, it forms transient Li–Se compounds that briefly melt, acting as a “liquid flux” to heal imperfections and promote uniform crystal growth.
At the atomic scale, lithium replaces copper or zinc at certain sites, forming what scientists call shallow acceptor defects. These introduce positive charge carriers (holes) without creating energy traps. The result is a cleaner, more conductive semiconductor — and thus a more efficient solar absorber.
Analyses using X-ray diffraction (XRD) and Raman spectroscopy confirmed these subtle structural changes: sharper peaks, fewer secondary phases like zinc selenide (ZnSe), and an overall improvement in crystal order.
Why It Matters
Kesterite materials are a key piece in the puzzle of making solar power more sustainable. Unlike perovskites, which contain lead and degrade under moisture, kesterites use only stable, abundant elements such as copper, zinc, germanium, and selenium. Their environmental footprint is low, and they can be manufactured with existing thin-film technology — the same kind used for flexible or building-integrated solar panels.
If researchers can close the performance gap, kesterites could serve as the top layer in tandem solar cells, sitting above silicon and converting higher-energy photons that silicon alone cannot fully exploit. This “two-layer” approach is seen as one of the most promising routes to push solar efficiency well beyond 30%.
Challenges Ahead
Despite the progress, several hurdles remain. Lithium diffusion must be precisely controlled: too little and defects persist, too much and unwanted secondary phases can form. Moreover, transparent back electrodes such as FTO, essential for tandem integration, still create resistive barriers that limit efficiency.
The researchers suggest using ultra-thin transition-metal oxide layers (like molybdenum oxide, MoO₃) between FTO and the absorber to improve electrical contact. They also note that advanced characterisation — including photoluminescence and impedance spectroscopy — will be needed to fully map how lithium alters defect states.
In the long term, the work opens the door to semi-transparent and bifacial kesterite solar cells, suitable for applications from rooftop glazing to portable electronics.
A Step Toward Truly Sustainable Solar Power
What makes this advance especially exciting is its simplicity. Lithium fluoride is inexpensive, non-toxic, and easy to integrate into existing thin-film production lines. By refining such small, elegant tweaks, researchers can steadily transform promising but underperforming materials into commercially viable technologies.
As the study’s senior author, explains, “Lithium acts as a tiny structural sculptor. It smooths imperfections, strengthens the lattice, and lets us extract more electricity from the same sunlight — all while keeping production costs low.”
The discovery highlights how clean-energy innovation often depends not on exotic new materials, but on clever chemistry at the nanoscale. The team’s next step will be to integrate their lithium-doped kesterite films into silicon-based tandem architectures — potentially combining affordability, stability, and transparency in a single, scalable solar solution.
Further Reading:
- Errafyg, A. et al. “Enhancing the performance of Cu₂ZnGeSe₄ solar cells with metallic and transparent back electrodes via lithium doping.” Scientific Reports (2025).
- Kim, S. et al. “Improvement of voltage deficit of Ge-incorporated kesterite solar cell with 12.3% conversion efficiency.” Applied Physics Express (2016).
- Zhou, Y. et al. “11.4% efficiency kesterite solar cells on transparent electrodes.” Advanced Energy Materials (2023).
- He, M. et al. “High-efficiency Cu₂ZnSn(S,Se)₄ solar cells with shallow LiZn acceptor defects.” Advanced Energy Materials (2021).
Source
Enhancing the performance of cu₂zngese₄ solar cells with metallic and transparent back electrodes via lithium doping, Scientific Reports (2025) 15:34625, 2025-10-03
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