Snapshot of progress in ACAP’s Tandem solar cells research, in 2025
- alisonpotter2
- Dec 23, 2025
- 4 min read
At the 2025 ACAP Conference, Professor Klaus Weber (ANU), Technical Program Lead for Program Package 3: Tandem Solar Cells, described a field that is moving “remarkably fast”, especially for perovskite–silicon tandems.
Tandem solar cells are about stacking different light‑absorbing materials on top of each other to squeeze more electrical energy out of the solar spectrum. Most of the global research focus has been on monolithic tandem cells, where perovskite is grown or deposited on top of a silicon substrate. The two cells are in series and have the same current (as opposed to more complex, mechanically-stacked cells that must be independently connected).
Tandem solar cell efficiencies keep rising
Globally, the headline number keeps climbing with LONGi and Jinko leading. In the last year, LONGi reported a new world record efficiency of 34.85% , while Jinko announced their 34.76% result, both for 1 cm² perovskite–silicon tandem cells.
On large areas, Oxford PV reported a 25% full size module and LONGi reported a 270 cm2 mini module with 33%.
ACAP partner the University of Sydney has pushed the concept further, demonstrating triple‑junction tandems – two perovskite layers with different bandgaps stacked on top of silicon – with 23.3% efficiency on a 16 cm² cell and 27.06% on a 1 cm² cell. The smaller 1 cm² cells show what’s physically possible; the larger 16 cm² devices are an important step towards proving the same ideas work at more practical sizes.
New emphasis on stability, and scalability
The story is no longer just about chasing record numbers; within ACAP and internationally there is increasing focus on stability. More perovskite cells from ACAP are now passing both traditional IEC accelerated tests and the broader family of ISOS stability tests. The latter include more realistic (tougher) combinations of light, heat and light-dark cycling (mimicking cloud cover, and day-night cycling) – stresses that degrade perovskite cells.
ACAP researchers have had promising early-stage outdoor stability results, with tandem devices maintaining performance over multi-month field exposure. Proving stability over many years under harsh outdoor conditions is an essential step toward bankability – making sure that lending institutions are willing to lend money for projects at low interest rates due to a low perceived project risk.
Stability by design
Rather than treating degradation as an afterthought, ACAP researchers at ANU are exploring how tandem architectures themselves can be engineered to reduce degradation. In two-terminal tandem cells, the amount of current generated in each sub-cell can be tuned to moderate damaging stresses during real-world light and temperature cycling – turning a traditional weakness into a potential strength. By shifting some critical stresses to the robust silicon cell and away from the perovskite cell, the lifetime and performance of the tandem solar cell can be increased.
Scalable fabrication focus for tandem solar cells
Many of the reported stability results for perovskites are for single junction perovskite cells rather than for tandems, and mostly use at least some fabrication techniques – such as spin coating for deposition of the perovskite film – that are challenging to scale. ACAP researchers are prioritising deposition methods that align with industrial manufacturing, moving beyond spin-coating towards scalable, repeatable processes. This includes:
A two-step perovskite deposition method for wide-bandgap perovskites, already delivering certified efficiencies above 21%, and an in-house tandem efficiency approaching 30%.
At CSIRO, researchers built an automated vapour‑phase perovskite deposition system that produces large 30 × 30 mm, ultra‑uniform, pinhole‑free films – an important step towards manufacturable tandem technology.
ACAP tackles a key bottleneck for tandem solar cells
Making a scalable two‑terminal perovskite–silicon tandem (the monolithic structure where the two cells are series‑connected) requires specialised facilities and a steady supply of specially designed silicon bottom cells.
“You can’t just take a commercial silicon cell and coat it,” Weber explained – the cells need to be small in size, with tailored surface textures and contact designs. That means even well‑equipped labs produce far fewer tandems than single‑junction devices.
With ACAP’s collaborative model comes the opportunity – ANU will fabricate small area silicon bottom cells specifically for tandem fabrication and make them available to tandem research teams across ACAP. This kind of system level coordination is rare internationally, and critical for progress.
ACAP is developing advanced characterisation tools for tandem solar cells
Characterising tandems is more complex than for single junction cells: two sub‑cells are electrically tied together, and conventional measurements are not as easy to interpret to help in understanding tandem behaviour and diagnosing degradation pathways. A further complication is the lack of standardised structures and processes between labs, making it harder to directly compare results and learn from each other.
ACAP has world-leading expertise in advanced cell characterisation techniques and the development of machine learning algorithms to analyse data. Our teams are developing advanced photoluminescence and electrical techniques, supported by ML, to determine the behaviour of the perovskite and silicon sub-cells and understand the root causes of degradation.
Looking ahead
As tandem solar technology progresses, success will depend less on headline efficiency records and more on durability, manufacturability and system-level understanding.
ACAP’s strength lies precisely in this transition. Our collaborative program, with multidisciplinary teams, world-leading expertise and shared infrastructure, position us well to tackle the complex challenges that no single lab can solve alone.