Efficient Far Red Sensitization of Nanocrystalline TiO₂ Films by an Unsymmetrical Squaraine Dye

Abstract

Dye-sensitized solar cells (DSSCs) have attracted significant attention as low-cost alternatives to conventional solid-state photovoltaic devices. [1][2][3] In these cells, the most successful chargetransfer sensitizers employed are ruthenium polypyridyl complexes, yielding 9-11% solar-to-electric power conversion efficiencies under AM 1.5. 4 The majority of the ruthenium complexes reported to date show absorption in the visible region at around 535 nm. Essential for efficient conversion of solar energy by DSSC is the spectral match of the sensitizer absorption to the solar radiation, and in this regard, the ruthenium complexes are inadequate. Therefore, development of sensitizers with extended absorption and spectral sensitivity into the infrared region is essential. Squaraines are well-known for their intense absorption in the red/near-IR regions, and for that reason, they are an excellent option to explore for solar cell applications. 5 Various groups have tested squaraines as sensitizers on wide band gap oxide semiconductors and obtained rather low power conversion efficiencies. [6][7][8][9][10] The reported low efficiencies of squaraines are due to aggregation and lack of directionality in the excited state. 11 There are several basic requirements guiding the molecular engineering of an efficient sensitizer. The excited-state redox potential should match the energy of the conduction band edge of the oxide. Light excitation should be associated with vectorial electron flow from the light-harvesting moiety of the sensitizer toward the semiconductor surface, providing for efficient electron transfer from the excited dye to the TiO 2 conduction band. Finally, a strong conjugation across the chromophore and anchoring groups is required for a good electronic coupling between the lowest unoccupied orbital (LUMO) of the dye and the TiO 2 conduction band. In order to satisfy these essential requirements, we have designed and developed a novel asymmetrical squaraine sensitizer that has a carboxylic acid group directly attached to the chromophore. In this paper, we report on the synthesis, electronic, and photovoltaic properties of the squaraine sensitizer. Scheme 1 shows the synthetic strategy used to obtain squaraine sensitizer (see Supporting Information for synthetic details). The UV/vis absorption spectrum (see Figure S1 in Supporting Information) of the squaraine sensitizer in ethanol shows an absorption maximum at 636 nm with high molar extinction coefficient ( ) 158 500 dm 3 mol -1 cm -1 ) corresponding to π-π* charge-transfer (CT) transitions. When the squaraine sensitizer is excited within the CT absorption band at room temperature in an air-equilibrated ethanol solution, it exhibits a strong luminescence maximum at 659 nm. The absorption spectrum of the squaraine sensitizer adsorbed on a 4 µm TiO 2 film shows features similar to those seen in the corresponding solution spectrum but exhibits a slight red shift of 15 nm due to the interaction of the anchoring group with the surface (see Figure S2 in Supportin...

Examining architectures of photoanode–photovoltaic tandem cells for solar water splitting

“…The dyes were selected to allow a broad transparency window in the visible spectrum for the first cell and maximizing the absorption range for the second cell. A squaraine dye coded as SQ1 13 and the black dye (N749) 11 were chosen based on their complementary properties over their absorption spectra. The electrolyte A6141 14 was selected for its electrochemical properties allowing a high current density with low concentration and therefore high light transmission.…”

“…All-organic dyes, such as the squaraine dye, have a narrow absorption bandwidth extending into the far red region of the visible spectrum and have demonstrated solar power conversion efficiencies up to 4.5% under AM 1.5. 13 In addition to considerably reducing costs when compared with a ruthenium-based dye, two new distinct tandemcell configurations become available when using this type of dye. First we can propose a trilevel tandem device [ Fig.…”

A TD-DFT study of the effects of structural variations on the photochemistry of polyenedyes

“…Different π-conjugated linker groups such as the -C=C-chain, [9][10][11][12][13]16,24 thiophene units (and its derivatives), 11,[14][15][16][17][18][19][20][21][22]26 etc have also been investigated. In particular two types of acceptor/anchor groups, namely cyanoacrylic acid, [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] and carboxylic acid [27][28][29] are widely used in organic dyes for DSSC.…”

Dye Solar Cells: Basic and Photon Management Strategies

“…It consists in a multiple dyes co-sensitization in order to enlarge photonic response via panchromatic absorption, hence to increase efficiency. There have been already proposed works focalizing on the panchromatic feature of a dye solar cell (Ogura et al, 2009;Yum et al, 2007;Park, 2010). The way to get improvement is by the use of two (up to three) dyes adsorbed on the nanocrystalline titania that are responsible for broad spectral response of the device.…”

“…It has been considered with particular attention to some organic dyes having complementary spectral response in the red with respect to the ruthenium-based dyes (largely used for standard DSC), such as squaraine (SQ1) (Clifford et al, 2004), cyanine (Pandey et al, 2010), phthalocyanine (Ono et al, 2009), hemicyanine (Cid et al, 2007). Indeed in other studies the co-sensibilization approach has shown high device performances toward red and violet as well in the electromagnetic spectrum (Yao et al, 2003;Kuang et al, 2007;Yum et al, 2007Yum et al, , 2008Clifford et al, 2004). The scope of co-sensitization is to enlarge the absorbance spectrum of the cell toward the Near Infra Red (NIR), thus to increase the Incident Photon to Current Efficiency (IPCE) by enhancing the LHE (Light Harvesting Efficiency) and the efficiency of injection inside the TiO 2 (see IPCE section).…”