Mesoscopic dye-sensitized solar cells (DSC) were invented in the 1990s by Brian O’Regan and Michael Grätzel at the École Polytechnique Fédérale de Lausanne (EPFL) (Switzerland). Since then they have been known as “Grätzel cells”, and recently a group of scientists led by Grätzel announced a new breakthrough.
Researchers have increased the power conversion efficiency of dye solar cells (Grätzel cells) greater than 15% in direct sunlight and 30% in ambient light conditions. The main achievement was the development of photosensitizing materials capable of absorbing the entire light spectrum.
Solar cells sensitized with dyes convert light into electricity using photosensitizers. These coloring compounds absorb light and inject electrons into a network of oxide nanocrystals which are then collected as an electric current.
In DSCs, photosensitizers adhere to the surface of mesoporous nanocrystalline titanium dioxide films, which are saturated with redox-active electrolytes or a solid charge transport material. The goal of the entire design is to generate electrical current by moving electrons from the photosensitizer to an electrical outlet, such as an appliance or storage unit.
DSCs are transparent and can be made in different colors for a reasonable price. They are already used in skylights, verandas and glass facades, such as those that adorn the SwissTech Convention Center. In addition, its light and flexible versions are already widely marketed to power portable electronic devices.
Recent advances in the development of sensitizers, redox mediators and device structures improved the performance of DSCs in both sunlight and ambient light conditions. To further improve its efficiency, it is essential to control the assembly of dye molecules on the surface of the films of titanium dioxide nanoparticles which promote the generation of electrical charges.
EPFL researchers have developed a way to improve the packaging of two new photosensitizing dye molecules to improve the photovoltaic performance of the DSC. Together, the two newly designed photosensitizers can quantitatively collect light in the entire visible range.
The new technique consists of pre-adsorbing a monolayer of a hydroxamic acid derivative on the surface of nanocrystalline mesoporous titanium dioxide. This slows the adsorption of the two sensitizers, allowing the formation of a well-ordered and dense layer of sensitizer on the surface of the titanium dioxide.
Using this method, the team was able to create a DSC with a power conversion efficiency of 15.2% under standard global simulated sunlight for the first time, with long-term operational stability tested for 500 hours. Devices with a larger active area of 2.8 square centimeters showed power conversion efficiency of 28.4% to 30.2% over a wide range of ambient light intensities, as well as high stability.
The references: Yameng Ren, Dan Zhang, Jiajia Suo, Yiming Cao, Felix T. Eickemeyer, Nick Vlachopoulos, Shaik M. Zakeeruddin, Anders Hagfeldt, Michael Grätzel. Hydroxamic acid preadsorption increases the efficiency of cosensitized solar cells. Nature 26 October 2022. DOI: 10.1038/s41586-022-05460-z