Solar cells are fast becoming one of the main ways to produce clean electricity in many countries around the world. Over the past few decades, enormous efforts have been made to make solar energy more popular. However, the technology currently faces several challenges that limit their widespread application.
In the case of dye-sensitized solar cells (DSSCs) – a promising photovoltaic technology – one of the main problems is dye aggregation. By design, DSSCs are electrochemical systems that mimic photosynthesis in plants; they rely on special photosensitive dyes to convert sunlight into electricity. Ideally, the dye should be uniformly deposited on the surface of an oxide electrode behind a transparent layer so that the energy from the absorbed sunlight can be easily transferred to the electrons of the dye. This process generates free electrons that power an external circuit. However, most dyes tend to aggregate on the electrode surface in a way that impedes the desired flow of light and electrical charges. This has an impact on DSSC performance that has proven difficult to overcome.
Fortunately, a team of scientists led by Associate Professor Tomohiko Inomata of the Nagoya Institute of Technology, Japan, may have just found a solution to this problem. In their recent study, published in RSC progress, they showed that certain ionic liquids (molten salts that are in a liquid state at relatively low temperatures) can suppress dye aggregation to an impressive degree. Other members of this research team included Ms. Ayaka Matsunaga and Prof. Tomohiro Ozawa from the Nagoya Institute of Technology and Prof. Hideki Masuda from the Aichi Institute of Technology, Japan.
But how do ionic liquids accomplish this feat? To shed light on the exact mechanism at work, the researchers focused on two ionic liquids with markedly different molecular sizes and two types of dyes. Both ionic liquids have a similar molecular structure, including an anchor that binds well to the electrode (titanium dioxide, TiO2), a main polymer chain connecting this anchor to a phosphorus atom, and three additional short polymer chains protruding from the phosphorus atom and away from the main “vertical” chain.
The researchers dipped TiO2 electrodes in solutions with different ratios of dye to ionic liquid and carefully analyzed how different molecules stick to them. After optimizing the synthesis procedure, they found that DSSCs made using an ionic liquid with a longer molecular structure had remarkably better performance than their counterparts with unmodified oxide electrodes. “The spatially bulky molecular structure of ionic liquids acts as an effective anti-aggregation agent without significantly affecting the amount of dye adsorbed to the electrode,” explains Dr. Inomata.Most importantly, the introduction of the larger ionic liquid improves all the PV parameters of the DSSC.”
Needless to say, improving solar cell technology could give us an edge in the fight against the ongoing energy and climate crisis. Although ionic liquids are typically expensive, the way the team is using it is actually cost-effective. “Simply put, the idea is to apply ionic liquids to only the required part of the device — in this case, the surface of the electrode,” states Dr. Inomata.
The team believes that widespread use of electrodes modified with ionic liquids could pave the way for highly functional yet affordable materials for solar cells and catalytic systems. Because the structure of ionic liquids can be tuned during their synthesis, they offer much-needed flexibility as antiaggregation agents.
Hopefully, these findings will lead to a brighter future for DSSC and ultimately the planet.
Materials provided by Nagoya Institute of Technology. Note: Content may be edited for style and length.