Sensitizers for Solar Cells

Dye sensitized solar cells (DSSCs) appear to be highly promising alternatives to more expensive solar cell technologies.  Considering the current maximal level of overall conversion efficiency (h) under simulated sunlight for DSSCs (11%), improvements in efficiency and durability would certainly facilitate widespread utilization of this technology. 

It is clear that there are a number of factors determining the efficiency of solar cells, but the structural and physical properties of the sensitizer is clearly an important one.  As a result, many laboratories around the world have active research programs in optimizing the dye component of the DSSCs.  Ruthenium dyes, while holding the record for conversion efficiencies, have relatively low extinction coefficients, and they are also considered to be expensive and hard to purify.  Motivated by the possibility of finding a replacement for metal-complex dyes, a number of chromophores including, coumarins, indolines, oligoenes, merocyanines, hemicyanines, oligothiophenes, squaraines, benzothiadiazoles, perylenetetracarboxylic acid derivatives and phthalocyanines have been studied, with varying degrees of success. 

Boradiazaindacenes, commonly known as BODIPY dyes, are recognized as useful fluorescent labels for biomolecules for sometime.  Within the last decade, there is much renewed interest in these dyes, due to the development of new avenues for derivatization and novel applications in a highly diverse field, including chemosensors, logic gates, light harvesters, energy transfer casettes  and photodynamic therapy.

We proposed that, when judiciously designed, BODIPY dyes have certain unique features that could make them highly advantageous compared to most other organic dyes and thus, they are highly promising in this regard.  i) BODIPY dyes have high extinction coefficients (70,000-80,000 M-1cm-1) and can easily be modified with any desired functionalities ii) absorption peak can be moved to longer wavelengths through simple modifications, keeping strong absorption cross-sections iii) BODIPY dyes have inherent in charge redistribution when they undergo S0→S1 transition upon excitation, increasing the charge density on the meso-carbon (C-8), while decreasing it in most other positions in the boradiazaindacene system (supporting information).  This inherent directionality of charge redistribution pinpoints C-8 as the optimal position of charge injection.  iv) the directionality observed in excitation can be further enhanced with strategically placed electron-withdrawing and electron donating groups, and based on the earlier work, cyanoacrylic acid and 4-N,N’-diphenylaminophenyl groups are to be of tremendous utility in this regard. 

Our initial results showed that especially in the red to near IR region, our BODIPY dyes displayed unprecedented efficiency.  Further work is being carried out to improve the overall efficiency.

Relevant Publications: [1]