The use of n-type dye sensitized solar cells for the conversion of solar photons into electrical energy is now a mature field of research with the conversion efficiency extending beyond 11% to approximately 14%. This has been made possible by the implementation of ruthenium-based, zinc(II), and organic porphyrin-based dyes. Unfortunately, the low earth abundance and high cost of ruthenium-based sensitizers have continued to limit their application despite them taking the lead among inorganic compounds. However, copper-based dyes are sustainable and appear to be promising alternatives to ruthenium.
In order to achieve high performance of dye-sensitized solar cells, the absorbance of the incident rays, injection of electrons into the semiconductor, and the interconnection between the electrolyte and the dye must be optimal. When the absorption range of the dye overlaps with the spectrum of the incident rays, there is a possibility of attaining higher conversion efficiencies. It has been observed that the most efficient copper-based dyes attain >3% photoconversion efficiency, which is relative to the values in the range of 7.12-7.63% for the benchmark ruthenium dyes.
Contrary to the panchromatic absorption of ruthenium dye N719 as well as related dyes, the metal-to-ligand charge transfer band of copper(I) sensitizers is typically in the range of 430 to 570 nm. However, this range can be extended by broadening the π-system of the copper-bound ligands. This would result in higher short-circuit current density values, but would not necessarily lead to enhanced global efficiency.
Frederik Malzner, Markus Willgert, Edwin Constable and Catherine Housecroft at the University of Basel in Switzerland successfully reported the first co-sensitization of a copper(I)-based dye-sensitized solar cell with a complementary organic dye in a bid to realize improved dye-sensitized solar cell photo-conversion efficiency. Their research work is published in Journal of Materials Chemistry A.
The authors described the first example of co-sensitization in dye-sensitized solar cells implementing copper(I)-based sensitizer as well as a commercially available dye. The authors combined the heteroleptic copper(I) dye with a commercially available squaraine derivative. They then matched the external quantum efficiency maxima from the two dyes in complementary parts of the visible spectrum.
Through this combination, the authors realized the highest photo-conversion efficiency of 65.6% so far reported for a copper-based dye-sensitized solar cell. This is the confirmation of the potential implementation of earth-sustainable copper as a sensitizer in dye-sensitized solar cells.
The Swiss research team found that the sequences in which the photo-anodes of the n-type dye-sensitized solar cells were exposed to the heteroleptic copper(I) dye and the squaraine derivative, as well as the time the electrodes were exposed to the dyes, immensely affected the overall performance of the solar cells. The aggregation of the squaraine molecules on the surface of the electrode was necessary for realizing panchromatic light harvesting for the co-sensitized dye-sensitized solar cells. However, excess aggregation would have been detrimental. The outcomes of the external quantum efficiency measurements done with varying wavelength-range filters were consistent with the two dyes operating separately.
Frederik J. Malzner, Markus Willgert, Edwin C. Constable and Catherine E. Housecroft. The way to panchromatic copper(I)-based dyesensitized solar cells: co-sensitization with the organic dye SQ2. Journal of Materials Chemistry A, volume 5 (2017), pages 13717–13729.
Go To Journal of Materials Chemistry A