DOI: 10.1002/ejic.201501068 Full Paper Dye-Sensitized Solar Cells Iridium(III) 2-Phenylbenzimidazole Complexes: Synthesis, Structure, Optical Properties, and Applications in Dye- Sensitized Solar Cells Stanislav I. Bezzubov,* [a] Yuri M. Kiselev, [b] Andrey V. Churakov, [a] Sergey A. Kozyukhin, [a] Alexey A. Sadovnikov, [a] Vitaly A. Grinberg, [c] Viktor V. Emets, [c] and Vladimir D. Doljenko [b] Abstract: A series of bis-cyclometalated iridium(III) complexes, [Ir(LH) 2 (H 2 dcbpy)][PF 6 ] (1), [Ir(LMe) 2 (H 2 dcbpy)][PF 6 ] (2), and [Ir(LPh) 2 (H 2 dcbpy)][PF 6 ](3), where LH = 1-H-2-phenylbenzimid- azole, LMe = 1-methyl-2-phenylbenzimidazole, LPh = 1,2-di- phenylbenzimidazole, and H 2 dcbpy = 2,2′-bipyridine-4,4′-di- carboxylic acid, has been synthesized and fully characterized by elemental analysis, 1 H and 31 P NMR spectroscopy, mass spec- trometry, and single-crystal X-ray analysis. The complexes show strong luminescence in the yellow–orange region in ethanol at room temperature (quantum yield is up to 22 %), and Introduction Since the publication of the first efficient dye-sensitized solar cell (DSSC) in 1991, [1] hundreds of transition-metal complexes, organic dyes, quantum dots, and other substances have been tested as photosensitizers in these devices. [2] The most widely used dyes are ruthenium(II) polypyridine complexes, for example, N3 and “black dye” (Figure 1, a, b) because of their intense metal-to-ligand charge-transfer bands (MLCT) in the visible range with molar absorptivities of approximately 15000 M –1 cm –1 . [3] However, the main drawback of sensitizers like N3 and “black dye” is the presence of two (or three) labile isothiocyanate groups, which cause poor long-term stability of the complexes. [4] Cyclometalated ruthenium(II) complexes are considered to be more stable, and they have been intensively studied in recent years, [5] but their synthesis is often accompa- nied by difficulties and can be carried out only with a limited number of ligands. [a] Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii Prosp. 31, Moscow 119991, Russian Federation E-mail: stas.bezzubov@gmail.com http://www.igic.ras.ru/ [b] Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory 1/3, Moscow 119991, Russian Federation [c] Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow 119071, Russia Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201501068. Eur. J. Inorg. Chem. 2016, 347–354 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 347 absorb light up to 550 nm with molar absorptivities of 1500– 2000 M –1 cm –1 . Complexes 1 and 2 possess very similar optical properties, whereas the introduction of the phenyl ring (com- plex 3) causes a hypsochromic shift (≈ 30 nm) of the lumines- cent maximum as well as resulting in an almost 50 % increase in the extinction coefficient at 490 nm compared with 1 and 2. A dye-sensitized solar cell (DSSC) based on complex 3 exhibits a short-circuit photocurrent of 2.8 mA cm –2 , an open-circuit photovoltage of 0.44 V, and a power conversion efficiency of 0.7 %. Figure 1. Ru II and Ir III photosensitizers: (a) N3, (b) “black dye” (TBA = tetra- butylammonium cation), (c) the best Ir III dye. [7] In contrast, iridium(III) easily forms bis- or tris-cyclometalated complexes that demonstrate high thermal and chemical stabil- ity, long excited-state lifetimes, and emission color tunability by cyclometalated (C N) ligands variations. [6] Unfortunately, only a relatively low device performance (2.2 %) has been achieved by