New Technique Boosts Efficiency of Eco-Friendly Solar Cells

Researchers at ICFO have introduced a post-deposition in situ passivation technique designed to minimize surface defects in eco-friendly, solution-processed nanocrystal ultrathin solar cells. This innovative approach has achieved the highest recorded power conversion efficiency for this type of solar cell to date.

With climate change intensifying, renewable energy sources have gained momentum, with solar cells at the forefront of this transition. In 2023, for instance, Spain’s installed solar photovoltaic power increased by 28 % compared to the previous year, making up 20.3 % of the country’s energy mix—a trend similarly mirrored across Western nations.

Despite the environmental benefits of solar cells, they often rely on materials that are not fully sustainable.

Expanding solar energy use beyond traditional solar farms to power buildings, infrastructure, IoT systems, and even vehicles is increasingly seen as the future of renewable energy. Achieving this vision calls for solar cell technology that is lightweight, cost-effective, flexible, and eco-friendly. In response, the scientific community is focusing on sustainable alternatives that not only maintain or enhance energy generation efficiency but also lower production costs and simplify the manufacturing processes of existing solar technologies.

Colloidal silver bismuth sulfide (AgBiS₂) nanocrystals have emerged as a promising, environmentally friendly alternative for solar cells. Known for their exceptionally high absorption coefficient, AgBiS₂ nanocrystals enable the creation of ultrathin-film absorbers ideal for solar applications. Although layer-by-layer manufacturing has already produced solar cells with impressive performance, moving from this multi-step deposition method to a streamlined, single-step approach is essential to reduce material waste, lower costs, and enhance scalability in manufacturing.

Developing AgBiS₂ nanocrystal inks could make this single-step manufacturing approach a reality. Since 2020, various studies have explored this direction, yet AgBiS₂ nanocrystals still exhibit significant surface defects, leading to low power conversion efficiency in solar cells.

Current surface passivation techniques, which aim to address these defects, have proven ineffective. These residual defects trap charge carriers generated by sunlight, causing recombination and ultimately reducing the device’s efficiency to levels below those achieved with the layer-by-layer method.

To bring the efficiency of eco-friendly solar cells up to competitive levels, a simpler yet more effective passivation approach for AgBiS₂ nanocrystal ink is required.

In a recent breakthrough, ICFO researchers—Dr. Jae Taek Oh, Dr. Yongjie Wang, Dr. Carmelita Rodà, Dr. Debranjan Mandal, Dr. Gaurav Kumar, and Dr. Guy Luke Whitworth, under the guidance of ICREA Prof. Gerasimos Konstantatos—introduced a promising solution.

The team’s study, published in Energy & Environmental Science, presents a post-deposition in situ passivation (P-DIP) technique that enhances surface passivation and significantly improves the optoelectronic properties of nanocrystal ink films. This innovative method led to ultrathin solar cells with superior power conversion efficiency compared to those produced with multi-step deposition, marking a new efficiency record for sustainable nanocrystal solar cells.

Post-Deposition in situ Passivation For Improved Surface Passivation

Researchers at ICFO were able to successfully passivate surface flaws in their nanocrystal ink sheet.

Imagine a bumpy road that slows down cars. Surface passivation is like repaving the road, making it smoother so cars can move without getting stuck. In our case, the removal of surface defects is very important to facilitate the transportation of charge carriers created from light absorption in nanocrystal films. With our P-DIP method, charge carries could move without ‘bumping into so many obstacles’ within the AgBiS₂ nanocrystals thin film.

Dr. Jae Taek Oh, Study First Author and Researcher, ICFO

Effectively mitigating defects through an improved passivation strategy resulted in higher film quality and, consequently, better-performing solar cells. These cells achieved an efficiency of approximately 10 %, surpassing that of earlier AgBiS₂ nanocrystal-based solar cells produced through both single-step and layer-by-layer deposition methods.

To achieve these results, the research team synthesized AgBiS₂ nanocrystal ink by introducing a multifunctional molecular agent containing chlorine. This agent’s molecular structure helped stabilize the nanocrystals and ensured even dispersion within the solution—both essential for creating smooth coatings.

Following film deposition, the team applied further passivation to the AgBiS₂ nanocrystal surfaces. This specific in situ passivation technique extended the carrier lifetime and optimized carrier transport within the film, both critical for enhancing solar cell efficiency. Together, these factors enabled the unprecedented performance that ICFO researchers demonstrated in this study, setting a new benchmark for sustainable solar cell technology.

Journal Reference:

‌Oh, J. T., et al. (2024) Post-Deposition In-Situ Passivation of AgBiS₂ Nanocrystal Inks for High-Efficiency Ultra-Thin Solar Cells. Energy & Environmental Science. doi.org/10.1039/d4ee03266g.