Monday, June 28, 2010

Searching for better solar cell dyes

This morning I looked over a nice review article on Metal free organic dyes for dye-sensitized solar cells. Here are a few highlights and comments.

The figure below is a schematic of what one is trying to design: a dye molecule D-pi-A in which upon excitation by a photon the excited state (D-pi-A)* decays non-radiatively into a charge separated state D+pi-D- which leads to charge injection into the TiO2.

Many of the dyes have the structural motif of methine dyes and are prone to cis-trans isomerisation in their excited state. It is implied that this is a bad thing one wants to stop. I actually thing an optimised/directed isomerisation could be good because it can lead to a twisted intramolecular charge transfer (TICT) state.

I am curious to what extent many of these dyes can be described by the type of effective Hamiltonians that Seth Olsen and I (and also independently by Anna Painelli and collaborators) have been exploring for other methine dyes. This could help answer questions about TICT states and design principles.

The following summary point by the authors is noteworthy:
It is therefore important to determine design rules for organic dyes to achieve improved properties and performance so that they may in the future compete or supersede ruthenium(II) sensitizers. Clearly, the development and optimization of materials for organic solar cells in general is not (yet) rational, but rather empirical. This is due to the multiple parameters which have to be taken into account when novel dyes and materials are designed for organic solar cells. The efficiency of the solar cells does not only depend on the molecular structure of the dye, but sometimes even more so on solid-state properties, such as aggregation, morphology, and self-assembly. In the case of DSSCs, the efficiency additionally depends on the type of photoelectrode, anchoring group of the dye, electrolyte, and mediating redox couple.
To me it raises questions about whether
-so much effort should be expended on making new dyes rather than on understanding solid state interactions.
-authors should be more cautious about attributing variations in device efficiency to properties of specific dyes rather than solid state properties.

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