Electron injection studies in TiO 2 nanocrystalline films sensitized with fluorene dyes and photovoltaic characterization. The effect of co-adsorption of a bile acid derivative M. Dori a , K. Seintis a , E. Stathatos b , G. Tsigaridas c,d , T.-Y. Lin e , J.T. Lin e , M. Fakis a,⇑ , V. Giannetas a , P. Persephonis a a Department of Physics, University of Patras, 26504 Patras, Greece b Department of Electrical Engineering, Technological and Educational Institute of Patras, 26334 Patras, Greece c National Technical University of Athens, School of Applied Mathematical and Physical Sciences, Department of Physics, Zografou Campus, GR-15780 Athens, Greece d Department of Informatics and Computer Technology, Technological and Educational Institute of Lamia, GR-35100 Lamia, Greece e Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan article info Article history: Received 27 December 2012 In final form 24 January 2013 Available online 4 February 2013 abstract In this Letter, the electron injection in TiO 2 films sensitized with six fluorene sensitizers is studied by femtosecond time resolved fluorescence spectroscopy using nanocrystalline Al 2 O 3 films as reference. The sensitizers are dipolar organic molecules with the fluorene group utilized as a conjugated bridge. The electron injection efficiency is correlated to the structure, conjugation length and excited state poten- tial of the sensitizers. One of the sensitizers has been studied using different amounts of cheno-deoxy cholic acid as co-adsorbent. In order to correlate the efficiency of electron injection with the device per- formance, quasi solid-state solar cells have been fabricated and characterized. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Dye Sensitized Solar Cells (DSSCs) reaching efficiencies of 12% are currently considered as a low cost alternative to silicon based photovoltaics [1–3]. In DSSCs, electron injection from the excited state of the sensitizer to the conduction band of the semiconductor is the most significant charge transfer process and an electron injection efficiency close to unity is necessary in order to achieve high solar cell efficiency. Electron injection depends on the elec- tronic coupling between the organic sensitizer and the semicon- ductor nanoparticles, the energy difference between the conduction band edge and the redox potential of the adsorbent’s excited state as well as the density of accepting states at the semi- conductor’s conduction band [4]. In order to optimize the electron injection, systematic experimental and theoretical studies should be carried out by using different sensitizers and semiconductors. Towards this purpose, time resolved spectroscopy in the fs and ps timescale can provide a valuable tool. Especially, time resolved fluorescence spectroscopy compared to transient absorption is ideal for the monitoring of only the relaxation process of the dye’s excited state avoiding complex dynamics due to the dye cations or injected electrons [5,6]. Although the majority of DSSCs rely upon the use of sensitizers based on Ru-complexes, there is currently a tendency to replace those sensitizers with all-organic ones [7–10]. In this field, fluorene dipolar molecules are very promising reaching efficiencies as high as 7% [11–16]. The fluorene group provides good planarity, good electronic coupling and stability. In the past, the fluorene group was used as a p-conjugated bridge linking the electron donating and electron accepting groups in a dipolar molecule [11,12], or it was put in the electron donating part [13–16]. An important issue that influences the performance of DSSCs is the formation of aggregates upon the adsorption of dye molecules on the surface of semiconductor nanoparticles. The amount of aggregates formed can be controlled by adding co-adsorbents like cholic, hexadecyl malonic or 1-decyl phosphonic acid [17–20]. These co-adsorbents penetrate among the dye molecules and con- sequently cause a decrease of aggregation. However, on the same time the total amount of the adsorbed dye molecules is reduced. Towards the optimization of a DSSC with co-adsorbents, detailed studies of the ultrafast injection dynamics from the dye to the semiconductor as a function of the co-adsorbent concentration are required [21,22]. In this Letter, the injection dynamics in a series of TiO 2 films sensitized with fluorene dyes have been investigated with femto- second time resolved fluorescence spectroscopy using Al 2 O 3 films as reference. The sensitizers contain di-arylamine electron donat- ing group, cyanoacrylic acid as electron accepting group and a fluorene as well as a heteroaromatic ring as p-conjugated bridge. 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.01.049 ⇑ Corresponding author. Fax: +30 2610 997470. E-mail address: fakis@upatras.gr (M. Fakis). Chemical Physics Letters 563 (2013) 63–69 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett