Discrete photoelectrodes with dyes having different absorption wavelengths for efficient cobalt-based tandem dye-sensitised solar cells

Abstract

A pn-tandem dye-sensitised solar cell (pn-DSSC) employing a set of sensitisers with complementary absorption spectra and a less-corrosive cobalt-based electrolyte is presented. We applied three organic sensitisers (denoted C343, DCBZ, and SQ) featuring different absorption wavelengths for the p-DSSC, while keeping the n-DSSC sensitiser (denoted DCA10CN2) constant. Characterisation of the Co^+2/+3-based DSSC devices revealed that SQ dye, with a longer absorption wavelength, showed broader spectra and increased photocurrent activity in the visible and near-infrared region compared to the other two devices with C343 and DCBZ in the pn-DSSCs. As a result, the short-circuit current density increased significantly to 4.00 mA cm^-2, and the devices displayed overall power conversion efficiencies of as high as 1.41%, which is comparable to that of the best pn-DSSCs in the literature. Our results demonstrate that complementary absorption between the two photoelectrodes is important for enhancing the photovoltaic performance of pn-DSSCs.

Figure 1

Schematic representation of tandem pn-DSSC.

Figure 2

Pictorial representation of complementary absorption spectra of the dyes used in this study and chemical structures. Absorption spectrum of n-type DCA10CN2 overlapped with those of C343, DCBZ, and SQ p-type dyes with molecular structure along with molecular structure of cobalt complex.

Figure 3

UV–visible absorption spectra. UV–visible absorption spectra of the dyes DCA10CN2, C343, DCBZ, and SQ (a) in solution and (b) adsorbed on photoelectrode films. (c) Absorption spectra of iodine and cobalt electrolytes.

Figure 4

Photovoltaic characterization of pn-DSSCs illuminated through the n-side under AM 1.5 conditions (100 mW cm⁻²). (a) Current density-voltage (J-V) characteristics and (b) incident photon-to-current conversion efficiency spectra (IPCE).

Figure 5

Energy levels and Cyclic Voltammograms. (a) Energy level diagram of each component in p-type photovoltaic device and (b) cyclic voltammograms of C343, DCBZ, and SQ obtained at a scan rate 100 mV s⁻¹.