Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)
Publication date: 5th February 2015
The most common redox couple used in contemporary DSSC is the iodide I-/I3- couple. However, due to its 2-electron nature and position of redox levels it sets limits on DSSC Voc and, consequently, global efficiency. In order to overcome this obstacle Co-based single electron redox couples were introduced. They led to the increase of Voc of about 0.3V resulting in efficiencies over 12% when combined with porphiryn based dyes[1]. Yet, the information about their performance in the fs-ns scale is very scarce. We decided to investigate the cells based on the highly efficient MK-2 dye (over 8% efficiency)[2] as well as its recently proposed derivative, ADEKA-1 (Si-O-Ti bridge introduced, over 12% efficiency)[3] with both, iodide based and cobalt based, redox couples. Moreover, the influence of the TBP (4-tert-butylpyridine) addition to both electrolytes is analyzed too. In general, it leads to increase in Voc for MK-2 with the Isc remaining at the similar level. ADEKA-1 is expected to act likewise. Complete solar cells sensitized with MK-2 dye and filled different electrolytes and additives are measured with several time-resolved techniques to study different charge separation processes occurring on time scales spanning over 13-orders of magnitude. Femtosecond transient absorption in the visible, near-infrared and mid-infrared ranges is used to probe electron injection dynamics. Nanosecond flash photolysis gives the information about dye regeneration dynamics. Finally, electrochemical impedance spectroscopy results reflect the characteristic time constants of electron recombination. Figure presents the representative kinetics measured close to the absorption maximum of the oxidized dye and showing the decay of the initially excited state of the dye. For electrolytes without TBP the spectral changes connected with electron injection are completed after several tens of ps. On the contrary, upon adding TBP, the overall kinetics become almost 10 times slower. With the similar composition of additives, the dynamics are similar for iodide- and cobalt-based electrolytes. Acknowledgements This work was supported by NCN (National Science Centre, Poland) under project 2012/05/B/ST3/03284.
[1] Yella A. et al., Porphyrin-Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent Efficiency. SCIENCE 2011, 334,629-633 [2] Zhong-Sheng Wang, Nagatoshi Koumura et al., Hexylthiophene-Functionalized Carbazole Dyes for Efficient Molecular Photovoltaics: Tuning of Solar-Cell Performance by Structural Modification. Chem. Mater., 2008, 20 (12), pp 3993–4003 [3] Kenji Kakiage et al., An achievement of over 12 percent efficiency in an organic dye-sensitized solar cell. Chem. Commun., 2014, 50, 6379
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