MULTI-JUNCTION DYE-SENSITIZED SOLAR CELL (MJ-DSC): AN ELECTROCHEMICAL DEVICE FOR EFFICIENT CONVERSION OF SUNLIGHT INTO ELECTRICITY

The absorption process of solar radiation by the traditional dyes used in the DSC is not very efficient because the spectral band in which they absorb is relatively narrow (FWHM less than 200 nm and, in general, centered in the blue-green part of the visible spectrum), whereas the spectrum of the solar radiation at sea level (AM1.5) shows a maximum around 550 nm and a FWHM of 600 nm (from 550 nm to 1100 nm). It is therefore necessary to study new dyes, whose absorption band would be preferably shifted to the red. With such molecules absorbing in the red we can implement the strategy of using a combination of several colors for the DSC, either by adopting the co-sensitization of the same electrode with two complementary dyes, or by sensitizing two electrodes with different dye molecules having complementary absorption spectra. We have investigated the combination of the N719 dye with another blue-green dye of the family of squaraines (e.g. the commercial dye SQ2), to prove the new concept of a “multi-junction” DSC. First, we perform a calculation based on the ASTM G173-03 Reference Solar Spectrum and the absorption spectrum of each dye, to evaluate the maximum number of photons that each junction would be able to convert into photoelectrons inside the MJ-DSC device. Thus, we can derive an upper limit for the solar conversion efficiency of this device, to be compared with the efficiency of each DSC, single-junction cell alone. We then stack two DSCs on top of each other in the direct (true or simulated) sunlight, to experimentally measure the real parameters of such cells under potentiostatic control. The discussion of the results takes into account: the optical properties of all the components of the real DSCs (glass+thin film electrodes, adsorbed dyes and redox electrolytes); the electrochemical properties of a MJ-DSC vs. that of a single-junction DSC, in terms of matching of ionic and electronic currents, optimization of the cell Fill-Factor and photopotential. We shall discuss the results obtained for the tested DSCs, in the frame of the guidelines for the bandgap combinations and absorption windows for organic tandem and triple-junction solar cells [1] and taking into account the existing literature on DSC-tandem solar cells [2].