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...

Lead sulfide (PbS) and cadmium sulfide (CdS) quantum dots (QDs) are prepared over mesoporous TiO2 films by a successive ionic layer adsorption and reaction (SILAR) process. These QDs are exploited as a sensitizer in solid‐state solar cells with 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) as a hole conductor. High‐resolution transmission electron microscopy (TEM) images reveal that PbS QDs of around 3 nm in size are distributed homogeneously over the TiO2 surface and are well separated from each other if prepared under common SILAR deposition conditions. The pore size of the TiO2 films and the deposition medium are found to be very critical in determining the overall performance of the solid‐state QD cells. By incorporating promising inorganic QDs (PbS) and an organic hole conductor spiro‐OMeTAD into the solid‐state cells, it is possible to attain an efficiency of over 1% for PbS‐sensitized solid‐state cells after some optimizations. The optimized deposition cycle of the SILAR process for PbS QDs has also been confirmed by transient spectroscopic studies on the hole generation of spiro‐OMeTAD. In addition, it is established that the PbS QD layer plays a role in mediating the interfacial recombination between the spiro‐OMeTAD+ cation and the TiO2 conduction band electron, and that the lifetime of these species can change by around 2 orders of magnitude by varying the number of SILAR cycles used. When a near infrared (NIR)‐absorbing zinc carboxyphthalocyanine dye (TT1) is added on top of the PbS‐sensitized electrode to obtain a panchromatic response, two signals from each component are observed, which results in an improved efficiency. In particular, when a CdS‐sensitized electrode is first prepared, and then co‐sensitized with a squarine dye (SQ1), the resulting color change is clearly an addition of each component and the overall efficiencies are also added in a more synergistic way than those in PbS/TT1‐modified cells because of favorable charge‐transfer energetics.

droxide and nitric acid. The wafers were stored for several days under these conditions and the changes were observed by measuring the water contact angle in air using a Gonionometer from Dataphysics (OCA 15+).

An optimized unsymmetrical squaraine dye 5‐carboxy‐2‐[[3‐[(2,3‐dihydro‐1, 1‐dimethyl‐3‐ethyl‐1H‐benzo[e]indol‐2‐ylidene)methyl]‐2‐hydroxy‐4‐oxo‐2‐cyclobuten‐1‐ylidene]methyl]‐3,3‐dimethyl‐1‐octyl‐3H‐indolium (SQ02) with carboxylic acid as anchoring group is synthesized for dye‐sensitized solar cells (DSCs). Although the π‐framework of SQ02 is insignificantly extended compared to its antecessor squaraine dye SQ01, photophysical measurements show that the new sensitizer has a much higher overall conversion efficiency η of 5.40% which is improved by 20% when compared to SQ01. UV‐vis spectroscopy, cyclic voltammetry and time dependent density functional theory calculations are accomplished to rationalize the higher conversion efficiency of SQ02. A smaller optical band gap including a higher molar absorption coefficient leads to improved light harvesting of the solar cell and a broadened photocurrent spectrum. Furthermore, all excited state orbitals relevant for the π–π* transition in SQ02 are delocalized over the carboxylic acid anchoring group, ensuring a strong electronic coupling to the conduction band of TiO2 and hence a fast electron transfer.