Perovskite Solar Cells — CityU's Clean-Energy Frontier in Chemistry and Materials

City University of Hong Kong (CityUHK) integrated information database · Module 04: Research · Materials deep-dive series For an overview and other landmark breakthroughs see materials-and-engineering-research.md; for CityU's other flagship in materials science (high-entropy alloys) see 高熵合金与先进金属.

One laboratory data point tells a commercialisation story: a perovskite solar cell, run continuously at 65°C for over 1,000 hours, still retains more than 90% of its efficiency. This is no isolated laboratory figure. It is the interim report card of a CityU chemistry team that, over three years, has successively broken through the two great barriers of "efficiency" and "stability" — and a sign that this new generation of photovoltaic material is one step closer to leaving the lab and reaching actual rooftops.

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# 1. Why Perovskite Cells Matter

Conventional silicon-based solar cells are a mature technology, but their efficiency gains are increasingly marginal and their manufacture is energy-intensive. Perovskite solar cells, by contrast, promise high photoelectric conversion efficiency at low cost through solution-based processing, and are widely seen as the "next generation" of photovoltaics. But they face two formidable hurdles:

1. Efficiency: pushing the ratio of light converted into electricity ever higher;
2. Stability: perovskite materials degrade under moisture, heat and light exposure — the key to commercialisation is "how long will it last".

The CityU team's research has been built around precisely these two challenges — and it has attacked them layer by layer, in the order "interface → thin film → process".

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# 2. Representative Achievements (all with public sources)

# 2.1 The ferrocene interface strategy: efficiency and stability together (2022)

In April 2022^※, CityU chemists developed a strategy to boost both the efficiency and stability of perovskite solar cells. According to public reports, a CityU professor (Dr Zhu Zonglong) and his team, in collaboration with Professor Long of Imperial College London, used ferrocenes as the interface between the light-absorbing layer and the electron transport layer — an interface-engineering breakthrough that improved both device performance and durability.

# 2.2 A non-volatile additive: inverted cell efficiency raised to about 24.8% (2023)

In May 2023^※, the CityU team developed a multifunctional, non-volatile additive that improves cell efficiency and stability by regulating the growth of the perovskite thin film. Reported results show that an inverted perovskite solar cell built on the modified perovskite achieved a power conversion efficiency of about 24.8% — a notably high figure. The work also drew attention from the international photovoltaic press, with reports noting that Hong Kong researchers had developed an inverted perovskite cell with 25.6% efficiency^※.

# 2.3 One-step solution coating: clearing the path to commercialisation (2023)

However high the efficiency, a cell that requires a complex, hard-to-scale manufacturing process will struggle to reach the market. In April 2023^※, CityU partnered with the US National Renewable Energy Laboratory (NREL) to develop a "one-step solution-coating" method that simplifies the manufacturing process and lowers the barrier to commercialisation. The work shifted the research focus from "laboratory efficiency" to "how perovskite cells can actually be manufactured at scale" — the critical leap from paper to product.

# 2.4 Three strands converge: the October 2023 Science paper

The preceding lines of work came together in a definitive synthesis in October 2023^※: the team of Professor Zhu Zonglong in CityU's Chemistry department designed a self-assembled monolayer (SAM), anchored it on nickel oxide nanoparticles as a hole-selective charge extraction layer, and deployed it in inverted (p-i-n) structured perovskite cells. Test results showed the improved device retained over 90% of its efficiency after running continuously for more than 1,000 hours at about 65°C, with a power conversion efficiency of 25.6%. The paper, Stabilized hole-selective layer for high-performance inverted p-i-n perovskite solar cells, was published in the top journal Science, in collaboration with Professor Zhong'an Li of Huazhong University of Science and Technology. It represents the culmination of the previous two years of interface engineering and thin-film regulation experience — pushing "efficiency" and "thermal stability", two parameters long seen as trading off against each other, simultaneously to a level rarely seen before.

# 2.5 Simplifying the architecture: making "structure" itself a solvable problem (2024)

Where the first four breakthroughs had largely pursued higher efficiency or stability numbers, the work reported in October 2024^※ posed a different question: can the cell's structure itself be made simpler? According to CityU's official account, a team led by Professor Zhu and Dr Gao Danpeng, working with colleagues from the Department of Materials Science and Engineering, integrated the hole-selective material directly into the perovskite layer and replaced the conventional organic electron transport layer with inorganic tin oxide — yielding "one of the most simplified architectures in the perovskite solar cell field today". The structure held over 25% power conversion efficiency while retaining over 95% of its efficiency after 2,000 hours of continuous operation — a longer test window than the 1,000 hours reported in the 2023 Science paper, and a further improvement in stability data. The team believes that, thanks to lower materials costs and fewer manufacturing steps, this simplified architecture could be ready for mass production within about five years. The work continues the collaboration network with NREL and Imperial College London.

Fewer layers and simpler processing mean higher yield and lower cost — which is precisely the commercialisation pain point that section 2.3's "one-step solution coating" set out to solve, only this time addressed from the "device architecture" end rather than the "coating process" end.

# 2.6 Tandem cells break 31%: "stacking with" silicon rather than "replacing" it (2025)

The efficiency ceiling of single-junction perovskite cells is set by the material's Shockley-Queisser limit (about 33.7%), so the industry's main thrust in recent years has shifted toward perovskite/silicon tandem cells: layering a perovskite thin film on top of a conventional crystalline-silicon cell, exploiting the complementary absorption of different light wavelengths by the two materials, to push past the single-junction theoretical limit. In a study published in January 2025^※ in Advanced Materials, Professor Zhu, in collaboration with Shenzhen Technology University and JinkoSolar, proposed a "halide locking" strategy: introducing a multifunctional ammonium salt additive, thioacetyl acetamide hydrochloride (TAACl), which bonds simultaneously with both cations and anions in the wide-bandgap perovskite layer, suppressing the phase segregation commonly seen in perovskite films and inducing preferred crystal growth along the (001) plane.

According to the paper, a perovskite/TOPCon silicon tandem cell built with this strategy achieved a power conversion efficiency of 31.32%, with an open-circuit voltage of 1.931 V and a fill factor of 81.54%; the perovskite sub-cell tested alone reached 22.95%. After 1,000 hours of continuous tracking tests, the device still retained 95.43% of its efficiency. This is the first time the CityU team has extended its research from "pure perovskite cells" into the tandem track of "stacking with silicon cells" — and it is the highest-efficiency publicly reported result on this research line to date.

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# 3. In the Global Frame: Where This Track Stands, and What Still Blocks It

The one-sentence answer: CityU's laboratory figures are not an isolated phenomenon — they sit inside a track that the whole world is furiously updating, and between 2024 and 2026 this track saw both "efficiency breaking through the ceiling" and "first commercial module shipments", while the environmental question of lead remains unresolved to this day.

# 3.1 Efficiency records: from "single-junction" scrambling to "tandem"

Single-junction perovskite cells face a theoretical efficiency ceiling, the Shockley-Queisser limit — proposed in 1961 by Bell Labs physicists Shockley and Queisser at about 33.7%. To break through it, the industry has broadly turned to perovskite/silicon tandem cells. According to coverage of the NREL efficiency chart^※, as of April 2025, Chinese firm LONGi had achieved an NREL-certified efficiency of 34.85% for a two-terminal perovskite/silicon tandem cell — the first certified tandem device to break the single-junction theoretical limit. The cell uses a lithium fluoride/EDAI dual-layer interface passivation strategy and was supported by Soochow University, Huaneng Clean Energy Research Institute and The Hong Kong Polytechnic University. The efficiency figures from several public reports are set out side by side below for cross-reference:

A note of caution: each efficiency above is a laboratory or small-area certified value for a specific team, specific device area, and specific point in time. The testing conditions differ (single-junction vs. tandem, device area, certifying body), so the figures cannot be ranked or compared directly; when citing them, always return to the original sources to verify the conditions.

# 3.2 From record to retail: Oxford PV's first shipments

However impressive the laboratory efficiency, turning it into rooftop modules is another matter. According to pv magazine (2024-09-05)^※, UK manufacturer Oxford PV shipped its first batch of 72-cell perovskite/silicon tandem commercial modules, rated at 24.5% efficiency, to a utility-scale project in the United States in September 2024 — the first-ever commercial delivery of perovskite tandem modules in the world. The modules were made on a production line in Brandenburg, Germany and, according to the manufacturer, can generate about 20% more electricity than conventional crystalline-silicon modules. This marked the perovskite track's emergence from "nothing but papers and records" — even though shipment volumes remain far below the scale of the silicon industry.

# 3.3 The unresolved hurdle: lead

Perovskite light-absorbing layers generally contain lead, and this is the environmental controversy that commercialisation cannot avoid. According to industry reviews, the lead content of a perovskite module's absorbing layer is roughly 0.5 to 1 gram per square metre, whereas a conventional-size (about 2.8 m²) crystalline-silicon module contains about 4 grams of lead in its solder. Chinese manufacturer GCL has stated that its perovskite modules, after prolonged water-immersion testing, show lower lead leaching than crystalline-silicon modules. But this claim has not yet reached industry consensus, and lead-free or low-lead perovskite formulations, along with more reliable encapsulation processes, remain widely acknowledged long-term challenges for both academia and industry. This is one reason why, on this research line, CityU's 2024 "simplified architecture" work was officially positioned as "potentially ready for mass production within about five years" rather than "about to enter production".

CityU's patents and licensing pathway on this research line operate through the same mechanism as its "patent → licensing → startup" conversion loop (see 专利产出与技术转移). As of the time of writing, however, no public reports have been found of CityU setting up a separate spin-off company for its perovskite patents.

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# 4. Characteristics of This Research Line

Spliced together, the achievements above reveal several features of CityU's perovskite research:

1. A full chain of "efficiency + stability + manufacturability + architecture". From interface engineering (2.1), thin-film regulation (2.2) and manufacturing processes (2.3), to the synthesising Science paper (2.4), then the simplified architecture (2.5) and tandem cells (2.6) — CityU's research spans the entire chain from perovskite cell to application, rather than chasing only high laboratory efficiency numbers.
2. An intersection of chemistry and materials. This line spans the Chemistry and Materials Science departments. Perovskite research is inherently a cross-disciplinary field of "chemical synthesis + materials engineering + device physics", which fits neatly with CityU's cluster strengths in materials science.
3. Dense international collaboration. Partnerships with Imperial College London, the US NREL, Huazhong University of Science and Technology, Shenzhen Technology University and JinkoSolar echo CityU's internationally co-authored advantage as "the world's most international university" (see 09-international/most-international-university.md).
4. Aligned with the clean-energy agenda. Perovskite photovoltaics serve the "energy transition / net-zero carbon" agenda directly, resonating with CityU's School of Energy and Environment (see 01-academics/school-of-energy-and-environment.md) and its net-zero-carbon campus (see 05-campus/sustainability-and-net-zero.md) — a thread running from basic research, through the School, to campus practice.
5. From "going it alone" to "stacking up". The 2025 tandem work (2.6) marks a shift in CityU's research focus, from pursuing perovskite's own efficiency and stability to the path of "stacking with" the silicon industry — which is also the mainstream direction of the global track after 2024 (see 3.1).

A note of caution: perovskite solar cell efficiency records are being refreshed frequently worldwide. "About 24.8% / 25.6% / 31.32%" are all reported values for specific studies at specific points in time, and laboratory efficiency differs from mass-production efficiency. When citing specific efficiency figures, always return to the original source to verify testing conditions and timing.

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# 5. Summary

• CityU has made sustained breakthroughs on the twin barriers of efficiency and stability in perovskite solar cells: the ferrocene interface strategy (2022)^※, the non-volatile additive raising inverted cell efficiency to about 24.8% (2023)^※, and the one-step solution-coating method with US NREL (2023)^※; the October 2023 Science paper^※ pushed efficiency and thermal stability up simultaneously, followed by the simplified cell architecture of 2024^※ and the 2025 tandem cell efficiency of 31.32%^※.
• Across research covering the full chain of "efficiency + stability + manufacturability + architecture", the thread shows the intersection of chemistry and materials, dense international collaboration, and alignment with the clean-energy agenda.
• In the global frame, CityU's progress, LONGi's 34.85%, and Oxford PV's first commercial shipments form different facets of the same track; the lead controversy is a public problem the entire industry has yet to solve.
• Efficiency figures are reported values from specific studies at specific points in time; when citing, verify the testing conditions and device type (single-junction/tandem).

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