German Researchers Set New World Record with 25.5% Efficiency Perovskite-CIGS Tandem Solar Cell

In a significant breakthrough for photovoltaic technology, researchers from Germany's Helmholtz-Zentrum Berlin (HZB) and Humboldt-Universität have achieved a new world record power conversion efficiency of 25.5% for a perovskite-CIGS tandem solar cell. This milestone, certified by the European Solar Test Installation (ESTI), represents the highest efficiency ever recorded for this specific material combination and marks a major step forward in the development of thin-film solar technologies.

This device is capable of achieving a world record in efficiency due to its design utilizing both an upper cell (perovskite) and a lower cell (CIGS) that together make up what is known as a "monolithic tandem structure". Therefore, each part of the cell absorbs and converts solar energy at different wavelengths within the solar spectrum-i.e., the top cell absorbs high energy (ultraviolet) light; while the bottom cell absorbs low energy (infrared) light. As a result of this structure, the upside of the upper cell and the downside of the lower cell will absorb different parts of the solar spectrum with higher efficiency than either single device would achieve by itself; thus creating an increase in total efficiency when the two devices work together in a tandem arrangement.

To achieve this record, the HZB and Humboldt-Universität researchers systematically optimized multiple aspects of the device architecture. They explored CIGS-based bottom cells with carefully tuned band gaps of 1.05 eV and 1.1 eV, alongside two different thicknesses of aluminium-doped zinc oxide layers. This meticulous bandgap engineering allowed them to better match the current generation between the top and bottom subcells-a critical factor in tandem device performance.

The team also conducted extensive screening of hole transport layer materials, testing various combinations of nickel oxide (NiOx) and self-assembled monolayers (SAMs). These interface layers play a crucial role in extracting photogenerated charge carriers while suppressing recombination losses-a primary efficiency-limiting mechanism in solar cells. By identifying optimal interface configurations, the researchers were able to simultaneously improve charge selectivity and minimize energy losses at the contacts.

Perhaps most notably, the team refined the electron-selective contact formation by precisely controlling the initial thermal evaporation rate of buckminsterfullerene (C60) deposited onto an ultra-thin 1 nm lithium fluoride (LiF) passivation layer. This sophisticated approach enabled what they described as a "more controlled interface formation and improved electronic alignment," demonstrating that nanoscale engineering of contact layers can yield substantial performance gains.

While the record efficiency was achieved on a small-area device of 1.081 cm², the researchers demonstrated that their optimizations could be translated to larger formats. Using a similar stack of materials, they fabricated a mini-module with an area of 2.25 cm² that achieved an efficiency of approximately 19.7%. Although this represents a significant drop from the small-cell efficiency, it provides encouraging evidence that the underlying interface and contact strategies can be scaled beyond the laboratory setting.

Perhaps the most exciting aspect of this announcement is what it suggests about future potential. HZB researcher Guillermo Farias Basulto noted that "the physics underlying our current cell architecture indicates that 25.5% is only an initial milestone". In-house testing of similar designs has already achieved efficiencies of up to 27.5%, implying that the fundamental device concept still has considerable untapped potential.

The research was conducted as part of the "SOLMATES" project, an EU-funded initiative led by HZB to explore the integration of CIGS and perovskite technologies. HZB's previous record under this program stood at 24.6%, making the jump to 25.5% a meaningful, albeit incremental, step forward.

This accomplishment holds important meaning for the solar cell industry. A CIGS thin-film cell will have several advantages over conventional thin-film solar cells, such as being flexible, being able to perform well in partially illuminated conditions, and having low manufacturing costs. By combining the advantages of CIGS cells with the advantages of perovskite cells, CIGS cells have the potential to be used for new applications in flexible electronics and BIPV, where silicon cells cannot be used.

The study results also indicate that perovskite-CIGS tandems are competitive with silicon-based tandem systems. In addition to being capable of being manufactured using only a thin film, they can also be manufactured using less than one micrometer of material. Their ultra-lightweight and flexible design will provide new opportunities for their use as building-integrated solutions or portable electronic applications.

While 25.5% is a noteworthy achievement, the theoretical efficiency limit for perovskite-CIGS tandems exceeds 30%. The gap between the current record and this theoretical ceiling suggests that substantial improvements remain possible through continued optimization of material quality, interface engineering, and optical management. The German team's systematic approach-exploring multiple bandgaps, transport layer combinations, and contact engineering strategies-provides a methodological blueprint for future efforts.

As the global solar industry continues its rapid expansion, innovations in tandem cell technology will play an increasingly important role in driving down the cost of solar electricity. The German researchers' achievement of 25.5% efficiency for perovskite-CIGS tandems represents not just a new world record, but a demonstration of the extraordinary potential that lies at the intersection of these two remarkable material systems. With in-house results already suggesting 27.5% is within reach, and with the theoretical limit still far higher, the journey toward truly transformative solar efficiency is far from over.