An electrode thinner than a soap bubble just pushed organic solar cells to a record efficiency no one had reached before
At the center of a new solar cell sits a layer of silver so thin it would take roughly a thousand of them stacked together to match the width of a human hair. Seven nanometers — thinner than most biological membranes, invisible to any conventional lens.
Researchers at ICFO built that near-disappearing electrode into a tandem organic solar cell, and in doing so crossed an efficiency threshold the field had never reached. The challenge they solved is one that has quietly held back solar design for years: how to let enough light pass through while still conducting electricity reliably.
The problem with stacking solar cells
Tandem solar cells work by layering two cells with different bandgaps — each tuned to absorb a different slice of the solar spectrum. One cell captures high-energy photons; the other catches what passes through. Together, they convert more sunlight than either could alone.
Jordi Martorell noted, the design approach could be adapted to any system where distributing light precisely across components is critical to performance.
Two-terminal (2T) designs connect both subcells in series, which simplifies the structure but creates a tight constraint: the current each subcell produces must closely match the other’s. If it doesn’t, the weaker cell limits the whole device. The interconnection layer also has to let light through and handle charge recombination simultaneously — not easy to engineer reliably.
Four-terminal (4T) designs take a different approach. Each subcell has its own electrical connections, so current matching isn’t a requirement. That freedom lets researchers choose materials for each cell independently, optimizing photon absorption across the spectrum without the constraints a series connection would impose.
Building a cell from the right materials
The ICFO team started by testing three different organic blends for the front cell — the transparent one that intercepts high-energy photons first. After evaluating each, they selected PM6:L8-BO as the best performer for that role.
For the back cell, which needs to absorb lower-energy infrared photons the front cell lets through, they chose PTB7-Th:O6T-4F. Its narrow bandgap makes it well-suited for that part of the spectrum. Picking the right materials was only part of the process — to settle on a final device structure, the researchers used matrix formalism combined with inverse problem-solving, a numerical approach that let them model and optimize the configuration before building anything.
A 7-nanometer electrode that changes the equation
The most technically demanding element is the ultrathin silver electrode placed at the back of the front cell. Its job is to conduct electricity from the front cell while transmitting enough light to keep the back cell powered. Conventional transparent silver electrodes for this application typically range from 9 to 15 nm thick. Getting down to 7 nm is meaningful — thinner means more transparent, but also more electrically resistive.
To manage how light is distributed between the two cells, the team paired the silver electrode with three dielectric layers, alternating tungsten trioxide (WO3) and lithium fluoride (LiF). This photonic multilayer structure transmits light strongly in the 750–1000 nm range while reflecting it in the 500–700 nm range.
“The development of a transparent silver intermediate electrode is crucial for the efficient operation of the solar cell,” said Francisco Bernal, first author of the study. “It must present a delicate balance, being transparent enough to allow light to reach the back cell while maintaining high electrical conductivity to ensure the optimal performance of the front cell.” He added that fabricating a 7 nm electrode without observing losses in the front cell represents a significant advance for transparent cell technology.
A record that rewrites the benchmark
Tested under standard 1-sun illumination using a solar simulator, the device reached a power conversion efficiency of 16.94%. The team also measured its quantum efficiency to confirm the result.
That number matters in context. The current official record for organic tandem devices stands at 14.2%, and the last reported efficiency for a four-terminal organic tandem specifically was just 6.5%. The ICFO result clears both benchmarks by a wide margin. The study was published in the journal Solar RRL.
Beyond solar panels: fuel, CO2, and what comes next
The team isn’t stopping at solar panels. Their stated goal is to apply this tandem architecture to photoelectrochemical (PEC) cells — devices that use sunlight to drive chemical reactions like water splitting or CO2 reduction — as part of the broader SOREC2 project, which focuses on solar fuel production.
The 4T methodology could extend further still. As Prof. Jordi Martorell noted, the design approach could be adapted to any system where distributing light precisely across components is critical to performance.
The researchers are currently refining the design for solar fuel applications, with particular focus on CO2 conversion and valorization. If the efficiency gains demonstrated here translate to those systems, the implications could reach well past the solar cell itself — into the broader challenge of storing and using solar energy in chemical form.
Reference: Francisco Bernal‐Texca, Jordi Martorell. Four‐Terminal Tandem Based on a PM6:L8‐BO Transparent Solar Cell and a 7 nm Ag Layer Intermediate Electrode. Solar RRL, 2024; DOI: 10.1002/solr.202300728
Daniel García is an Editor-in-Chief with strong expertise in structural work and engineering principles. He combines this technical foundation with deep knowledge of energy, spatial design, and emerging technologies, bringing a forward-thinking and analytical approach to editorial leadership.
