Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
The efficiency of dye-sensitized solar cells (DSSC) is limited by electron transfer processes proceeding at the oxide semiconductor/dye/electrolyte interfaces. Ultrafast electron transfer to TiO2 conduction band from metal-to-ligand charge transfer (MLCT) photoexcited state of Ru-bipyridyl dyes can occur from a singlet state (1MLCT) or from a triplet state (3MLCT) as a result of heavy metal atom induced efficient intersystem crossing (~ 10-12 s) [1]. Thus, singlet 1(e‑h) and triplet 3(e‑h) electron–hole polaron pairs can be created where the electron occupies the conduction level of TiO2 and the hole is localized on an oxidized dye molecule. A low external magnetic field of tens mT strength can interact with the polaron pairs and this way change the generated photocurrent as observed in aluminum complex (Alq3) films [2] and in polyhexylthiophene (P3HT) : fullerene (PCBM) bulk heterojunction solar cells [3]. The origin of low magnetic field effects in organic solids is currently under heavy debate.
Recently, the authors of ref. [4] reported a significant low magnetic field effect on short circuit photocurrent (~13%) and energy conversion efficiency (~10%) in DSSCs. However, the mechanism of observed magnetic effects was not specified. To explain underlying processes we decided to study in this work TiO2/N719-based DSSCs with the same components and structure under the 1 sun illumination as in ref. [4]. We have investigated also DSSCs based on dinuclear Ru polypyridine dye (B1) with a longer lifetime of a triplet state and more delocalized electron orbitals [5]. In contrast to ref. [4], we did not observe any influence of low magnetic field on photocurrent in the systems studied within a relative error of 1% (Fig. 1).
In our opinion, insensitivity of photocurrent to low magnetic field is due to too short spin coherence time of e‑h pairs in comparison to electron spin flip time (2 ns in magnetic field of 10 mT). In this case strong spin-orbit coupling in Ru-complexes and spin relaxation of electron in TiO2 lattice due to hyperfine interactions and spin-orbit coupling with Ti3+ ions should be considered. Nevertheless, we note here that high magnetic field (of hundreds mT) effects on photocurrent were observed in P3HT : TiO2 hybrid solar cells [6].
Acknowledgements
This work was supported by the Polish Ministry of Science and Higher Education under “Diamond Grant” 0228/DIA/2013/42 and Polish National Science Centre under grant DEC-2011/03/B/ST7/01888.
Fig. 1. Current density – voltage characteristics and electrochemical impedance spectroscopy spectra for a TiO2/N719-based DSSC with and without external magnetic field. Experimental errors are within symbol sizes
[1] Koops, S. E.; Barnes, P. R. F.; O’Regan, B. C.; Durrant, J. R. Kinetic Competition in a Coumarin Dye- Sensitized Solar Cell: Injection and Recombination Limitations upon Device Performance. Journal of Physical Chemistry C 2010, 114, 8054-8061. [2] Kalinowski, J.; Szmytkowski, J.; Stampor, W. Magnetic hyperfine modulation of charge photogeneration in solid films of Alq3. Chemical Physics Letters 2003, 78, 380-387. [3] Shakya, P.; Desai, P.; Kreouzis, T.; Gillin, W. P.; Tuladhar, S. M.; Ballantyne, A. M.; Nelson, J. The effect of applied magnetic field on photocurrent generation in poly-3-hexylthiophene:[6,6]phenyl C61-butyric acid methyl ester photovoltaic devices. Journal of Physics: Condensed Matter 2008, 20, 452203-452206. [4] Cai, F.; Zhang S.; Zhou S.; Yuan Z. Magnetic-field enhanced photovoltaic performance of dye-sensitized TiO2 nanoparticle-based solar cells. Chemical Physics Letters 2013, 591, 166-169. [5] Zalas, M.; Gierczyk, B.; Klein, M.; Siuzdak, K.; Pedzinski, T.; Luczak, T. Synthesis of novel dinuclear ruthenium polypyridine dye for dye-sensitized solar cells application. Polyhedron 2014, 67, 381-387. [6] Zang, H.; Ivanov, I. N.; Hu, B. Magnetic Studies of Photovoltaic Processes in Organic Solar Cells. IEEE Journal of Selected Topics in Quantum Electronics 2010, 16, 1801-1806.
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