DOI: 10.1002/cphc.200900605 Bandgap Modulation in Efficient n-Thiophene Absorbers for Dye Solar Cell Sensitization Eva M a Barea, [a] RubØn Caballero, [b] Francisco Fabregat-Santiago, [a] Pilar de la Cruz, [b] Fernando Langa, [b] and Juan Bisquert* [a] 1. Introduction Nanocrystalline semiconductor-based dye-sensitized solar cells (DSC) have attracted significant attention as low cost alterna- tives to conventional solid-state photovoltaic devices. [1] The most successful charge-transfer sensitizers employed so far in these cells are polypyridyl-type complexes of ruthenium [2, 3] yielding overall AM 1.5 solar to electric power conversion effi- ciencies up to 11.3%. [4, 5] But the high cost of ruthenium, the necessity of purification treatments and the low molar extinc- tion coefficients, pose difficulties for commercial large DSC modules, hence the increasing interest of research on metal- free organic dyes. [6–8] Organic sensitizers have the advantage of high extinction coefficients and can thus also meet the demand of good light harvesting efficiency with thinner TiO 2 films. New less volatile redox systems such as ionic liquids [9–12] and hole conduc- tors [13–15] require thinner TiO 2 films because of mass transport limitations or insufficient pore filling. A great variety of organic sensitizers based on polyene-triphenylamine, [8, 16–21] coumar- in [22–24] and indoline [25–28] moieties give respectable conversion efficiencies of 5–9 % [29] with the traditional iodide/triiodide redox system. Despite the promising results obtained so far, more research is needed to understand the energetic, kinetic and geometric interplay between dye, TiO 2 and electrolyte, to design an efficient and stable organic dye for large scale appli- cations. Here we present the synthesis and solar cell performance of a series of D-p-A chromophores with prolongation of p conjugation length of n oligothienylvinylene units to increase the spectral re- sponse (Figure 1). The donor unit is an oligomer of thienylvinylene with different lengths and the acceptor is the cyanoacetic acid moiety. The risk of decreased electron injection by p-stacked dye aggrega- tion is prevented with attached alkyl groups [30] to avoid the strong p–p interaction. [31] The dyes will be denoted FLn (n = 2, 3, 4, 5 and 6, n means the number of oligothienylvinylene units) in which FL4 is the compound named RC4-17 in previ- ous work. [32] Experimental Section Synthesis: The general procedure for the synthesis of the FLn dyes from thienylvinylenes (TV) is described in Figure 2 and in the Sup- porting Information. Under Ar, over a stirred solution of the corre- sponding aldehyde and cyanoacetic acid in CHCl 3 , 2 drops of pi- peridine are added and the mixture is refluxed for the indicated time. The solvent is evaporated under reduced pressure and the Five new sensitizers for dye sensitized solar cells have been de- signed consisting of conjugated thienylvinylene units threaded with alkyl chains to improve solubility and cyanoacetic acid as anchoring group. The conjugation length was increased from 2 to 6 thienylvinylene units, which resulted in a red-shift of the optical absorption of the dyes from 550 to 750 nm, improving the spectral overlap with the solar spectrum. The photovoltaic performance of these dyes as sensitizers in mesoporous TiO 2 solar cells shows a clear correlation of increasing photocurrent with the extension of the conjugation up to an optimal length. Further extension of the conjugation increases the absorption but additional effects like self-quenching or recombination processes reduce the photocurrent and photovoltages and consequently the overall efficiency of the DSC. Figure 1. FLn dyes structures, n is the number of thienylvi- nylene units. Figure 2. General synthesis of FLn dyes. [a] Dr. E. M. Barea, Dr. F. Fabregat-Santiago, Prof. J. Bisquert Photovoltaic and Optoelectronic Devices Group Physics Department, Universitat Jaume I, 12071 Castelló (Spain) Fax: (+ 34) 964 729218 bisquert@fca.uji.es [b] R. Caballero, Dr. P.d. l. Cruz, Prof. F. Langa Instituto de Nanociencia, Nanotecnología y Materiales Moleculares (INAMOL) Universidad de Castilla La Mancha, Campus de la Antigua Fµbrica de Armas Avda. Carlos III, s/n 45071. Toledo (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200900605. ChemPhysChem 2009, 10, 1 – 6  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 These are not the final page numbers! ÞÞ 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 36 36 37 37 38 38 39 39 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47 48 48 49 49 50 50 51 51 52 52 53 53 54 54 55 55 56 56 57 57