Porphyrins are very promising sensitizers for dye solar cells (DSC) but narrow absorption bands at 400-450 and 500-650 IntroductionDye-sensitized solar cells (DSCs) have attracted significant attention as low-cost alternatives to conventional solid-state photovoltaic devices. In these cells, a nanoporous material is filled with a thin layer of a sensitizer which can be photoexcited to ultimately provide a photocurrent.1 Ruthenium-based dyes have been used as the most successful sensitizers to date yielding up to 11.3% solar-to-electric power conversion efficiencies under 1 sun ilumination. 2, 3 However, these dyes lack some of the desired properties required for their large scale production: low raw material costs and a good spectral matching with the solar radiation. Thus, there is real need for the development of inexpensive dyes with improved light absorption in the red and near infrared region. Extended -aromatic molecules have the potential to fulfill these two requirements. [4][5][6][7][8][9] Porphyrins are molecules that contain a heterocylic macrocycle with a -aromatic core. These do not rely on precious metals such as ruthenium and benefit from very high molar extinction coefficients. Metaloporphyrins show an intense Soret band at 400-450 nm and moderate Q bands at 500-650 nm. 10Compared to the ruthenium complexes, these narrow bands limit the light-harvesting properties for porphyrins-based DSC. However, it has been demonstrated that elongation of the conjugation and loss of symmetry causes broadening and a red shift of the absorption bands in porphyrins. 9,11,12 Additionally, they present good photostability and high light-harvesting capabilities that favour applications in thin low cost DSC. In this work we describe the impact of using an oligothienylenevinylene to bridge a Zn-porphyrin system and the anchoring group of the sensitizer. We show that this strategy increases the extended system producing an exceptional absorption enlargement between 450 and 650 nm. This translates into a 3-fold photocurrent increase to reach a cell eff Results and DiscussionA Zn-porphyrin sensitizer with extended absorption was synthesised by the structural combination of the Zn-porphyrin (1) and a previously reported dye containing two oligothienylenevinylene units (2). 13 The structures of these sensitizers (including terminal groups for anchoring to TiO2 surface) are shown in Figure 1. Thus, the Zn-porphyrinoligothienylenevinylene (3) was designed having the main features of 1 and 2 linked by a styryl group. The porphyrins 1 and 3 dyes were prepared by Knoevenagel condensation of the corresponding aldehydes 13 and cyanoacetic acid in 72% and 89% yield, respectively. The oligothienylenevinylene dye (2) was prepared according to a previously described procedure. 13Further details of synthesis methods are given in SI as well as full characterization data.[ Figure 1. Structure of Zn-porphyrin dye (1), oligothienylenevinylene dye (2) and Zn-porphyrin-oligothienylenevinylene dye (3).The electrochemical properties (see ...

Two new organic dyes have been synthesized and used as efficient light-harvesting materials in molecular photovoltaic devices. These dyes are based on conjugated thienylvinylene units, with FL-4 consisting of a four-unit thienylvinylene oligomer and its homologue FL-7 which additionally incorporates the electron-donating triphenylamine unit (TPA) into its structure. Upon light excitation both dyes show efficient electron injection into the TiO2 conduction band and slow back electron transfer to the oxidized dye. In fact, for FL-7, the back electron transfer dynamics are slower owing to efficient hole transfer to the TPA moiety situated further from the semiconductor surface. However, the electron recombination kinetics with the oxidized electrolyte for both FL-4 and FL-7 in dye-sensitized solar cells are faster than for devices made using the ruthenium dye N719. We believe that this is a serious limiting factor for devices based on oligothiophenes which, despite showing higher molecular extinction coefficients in the vis-NIR region of the solar spectrum, still cannot challenge the light-to-energy conversion efficiency of N719 or other ruthenium polypyridyl complexes.

Two new organic dyes incorporating triphenylamine as a donor and oligothienylenevinylene as a bridge have been synthesized. The new dyes cover the entire visible region and have a power conversion of up to 6.25%.