This summary of progress on ACAP's work on tandem solar cells is drawn from a presentation delivered by ACAP Program Lead Professor Klaus Weber (ANU) at the ACAP 2024 Conference (06/12/2024). Professor Weber shared a snapshot of the current technology status, challenges, opportunities and highlights.
Tandem solar cells aim to optimise energy output by using a two-layer structure: a top cell that absorbs high-energy photons and a bottom cell for low-energy photons, improving efficiency compared to single-material cells.
Despite current challenges in stability and scalability, the tandem solar cell field shows immense promise for achieving high efficiencies and supporting global energy transition initiatives. We do not yet know what the optimal architectures are.
ANU, UNSW, CSIRO and the University of Sydney are active partners in this program and it’s likely most of the nodes will eventually have activity in this space.
Current status of tandem solar cell development
The potential of tandem solar cell technology to achieve over 30% efficiencies has been demonstrated, including by ACAP nodes. Recent advancements in materials, particularly perovskites, have pushed tandem solar cell efficiencies beyond silicon’s, with records of up to 34.6% under lab conditions.
Both two-terminal and four-terminal tandem cell configurations have demonstrated efficiencies exceeding 30%. Excellent results have also been reported using high band gap chalcogenide cells.
ACAP nodes have achieved excellent stability for several key tests (including thermal cycling, humidity freeze).
Stability is a critical issue. Existing silicon-based stability tests are being adapted for perovskites and tandem cells, but new testing protocols are needed to meet long-term durability standards.
There is no consensus as to which tandem configuration is most advantageous for commercial exploitation.
There is active involvement of at least 49 research institutions and 32 startup companies globally in tandem solar cell research and development.
Challenges
Stability Issues: Current stability tests for silicon are inadequate for perovskites. Tandem cells need new testing protocols to ensure long-term durability. Heat and light, and day/night cycling are important failure modes.
Material complexity: The wide variety of materials and structural configurations dilutes research efforts, compared to silicon’s more standardised development pathway.
Scaling up: Processes for large-scale production, including transparent conductive oxides (TCOs) that avoid rare metals like indium, require further development and investment.
Sustainability: Recycling and end-of-life management for perovskite-based tandem cells are underdeveloped and need more focus.
Opportunities
Solar SunShot: ACAP’s collaborative activities combined with the Solar SunShot initiative provides an opportunity for Australia to lead development of commercial tandem technology.
ACAP’s collaborative network: ACAP nodes are well placed to address the key challenge of perovskite stability, using existing equipment and expertise, plus additional ACAP resourcing.
ACAP’s strong research community provides a platform for addressing challenges through coordinated efforts across fundamental research and industrial scalability. Doing so provides an opportunity to develop and control crucial new IP.
Higher efficiency: Tandem structures leveraging perovskites have demonstrated clear potential to outperform silicon in energy yield.
Broad applications: While utility-scale PV and rooftop systems are the primary targets, niche applications like building-integrated photovoltaics (BIPV) and electric vehicles can drive innovation. Initial product may prioritise shorter lifetimes for early market entry.
Scaling up: Developing scalable manufacturing techniques offers an opportunity to build a competitive local PV industry in Australia.
Leverage stability work from single junction devices: We can do a lot of the work on single junction devices for more efficient use of resources.
Highlights
ACAP’s stability achievements:
Encapsulated perovskite solar cells achieved a device lifetime of over 7,000 hours with up to 80% of initial efficiency of 20.7% in ambient conditions.
Success in adapting and passing silicon-based stability tests, such as the IEC 61215 thermal cycling test with a perovskite-Si tandem cell. Some structures have passed this at double the length of time.
Efficiency: ACAP nodes demonstrated 4 terminal tandem efficiency of over 30%, and a 2 terminal tandem efficiency of over 31%.
ACAP’s community has positioned itself well to tackle global challenges in tandem solar technology and contribute to sustainable energy solutions.
Watch the video
Readers can find the details on progress and activities in each of ACAP's research programs in Chapter 5 of ACAP’s Annual Reports. The 2024 report will be published in April 2025.
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