The effect of 4-tert-butylpyridine and Li + on the thermal degradation of TiO 2 -bound ruthenium dye N719 Phuong Tuyet Nguyen, Poul Erik Hansen, Torben Lund ⇑ Department of Science, Systems and Models, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark Received 2 August 2012; received in revised form 24 October 2012; accepted 21 November 2012 Communicated by: Associate Editor: Sam-Shajin Sun Abstract Thermal stability experiments were performed at 100 °C of the dye-sensitized solar cell ruthenium dye N719. The experiments were performed as simple test-tube experiments carried out with colloidal solutions of N719-loaded TiO 2 particles. The dye degradation was followed by the use of HPLC-coupled electrospray mass spectrometry. The longest half life of N719 of 84 h at 100 °C was obtained in 3- methoxypropionitrile based electrolytes containing 0.5 M 4-TBP, no Li + ions and 0.25 M I  3 . If 4-TBP was removed from the solution the degradation rate of N719 increased up to 3–15 times depending on the I  3 concentration. If Li + ions were added to the electrolyte the degradation increased by a factor of 4–16 times and the thermal degradation product mixture became more complex. It is suggested that an adsorbed layer of 4-TBP on the TiO 2 protects the N719 dye against solvent substitution and oxidation processes. A complexation constant of Li + ions with 4-TBP equal to 9 M 1 was obtained by 13 C NMR. The Li + complexation with 4-TBP is thought to destroy the adsorbed protection layer of 4-TBP on the TiO 2 surface. It is concluded that addition of 4-TBP or other N-additives to the DSC electrolyte is important to enhance the dye life time at elevated temperatures. However, addition of Li + to the electrolyte should be avoided if the wish is to construct DSCs with high thermal stability. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: N719; Dye-degradation; Dye-sensitized solar cells; Electrolytes 1. Introduction In the last decade, dye-sensitized solar cells (DSCs) have extensively been studied (Peter, 2011). From an economical point of view, DSCs are of high interest because the man- ufacturing costs of DSCs devices are significantly lower in contrast to the costs of other solar devices such as silicon cells (Gra ¨tzel, 2006; Hagfeldt et al., 2010). One of the suc- cess criteria required for commercial use of DSCs is a suf- ficient stability in order to pass the IEC 61646 standard test of 1.000 h thermal stress at 85 °C in dark (IEC, 1996-11, Hinsch et al., 2001; Wang et al., 2005; Harikisun and Desilvestro, 2011). The dye sensitizer is one of the key com- ponents of a DSC device. Consequently, the thermal stabil- ity of DSCs is directly linked to the dye stability which is in turn linked to its degradation on the surface of a semicon- ductor anode (TiO 2 ). In order to be able to design new dyes, better electrolyte compositions and semiconductor surfaces, it is crucial to understand the underlying degrada- tion process as a function of the DSC electrolyte composi- tion. So far the most efficient DSCs are prepared with low viscosity liquid electrolytes, which typically are comprised of the redox couple I  =I  3 , a nitrogen-containing additive such as 4-tert-butylpyridine (4-TBP) and Li + ions dissolved in 3-methoxypropionitrile (3-MPN) (Nazeeruddin et al., 2001; Kroon et al., 2007; Gao et al., 2008). DSCs which are prepared with ionic liquids are typically 1–2% less efficient (Gao et al., 2008; Gorlov and Kloo, 2008). 0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.11.012 ⇑ Corresponding author. Tel.: +45 46742472. E-mail address: tlund@ruc.dk (T. Lund). www.elsevier.com/locate/solener Available online at www.sciencedirect.com Solar Energy 88 (2013) 23–30