Wideband dye-sensitized solar cells employing a phosphine-coordinated ruthenium sensitizer

Low-cost renewable energies are necessary for the realization of a low-carbon society. Organic photovoltaics such as organic thin-film solar cells1,2 and dye-sensitized solar cells (DSSCs)3,4 are promising candidates for realizing low-cost solar cells. However, device efficiencies are still considerably lower than those of traditional inorganic solar cells. To improve organic photovoltaic performance, approaches are needed to extend the absorption of organic compounds to longer wavelengths. Here, we report efficient DSSCs that exploit near-infrared, spin-forbidden singlet-to-triplet direct transitions in a phosphine-coordinated Ru(II) sensitizer, DX1. A DSSC using DX1 generated a photocurrent density of 26.8 mA cm, the highest value for an organic photovoltaic reported to date. A tandem-type DSSC employing both DX1 and the traditional sensitizer N719 is shown to have a power conversion efficiency of >12% under 35.5 mW cm simulated sunlight. To extend the absorption spectrum of the organic compounds used in organic photovoltaics (OPVs) to longer wavelengths, the gap between the highest occupied molecular orbital (HOMO) and the–lowest unoccupied molecular orbital (LUMO) of these compounds must be reduced. However, organic compounds with lower HOMO–LUMO gaps cannot perform efficient photoelectric conversion due to the inevitable loss incurred during photoinduced charge separation. In organic thin-film solar cells (OTFSCs), efficient charge separation by visible-light absorption is achieved using donor/acceptor bulk heterojunction materials2, and charge separation efficiency decreases when low-bandgap materials are used to extend the spectral sensitivity to the near-infrared region5,6. The Ru bipyridine complex7 N719 (cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium(II)) and the Ru terpyridine complex8 black dye (BD) (tri(isothiocyanato) (2,2′;6′,2′′-terpyridyl-4,4′,4′′-tricarboxylic acid)ruthenium(II)) (Fig. 1a) can achieve excited states (triplet metal-to-ligand chargetransfer, MLCT, states) with long lifetimes, and DSSCs that utilize these sensitizers have shown high quantum efficiencies in the visible region. However, DSSCs based on such sensitizers still have lower spectral responses than inorganic solar cells9–15, because a relatively high energy is needed to excite MLCT (singlet MLCT) states, which lose a large amount of energy to MLCT states. In the case of conventional Ru complexes, short-lived MLCT states immediately relax to long-lived MLCT states through intersystem crossing, which is associated with a large energy loss estimated to be 5,000 cm ( 0.6 eV)16. Finding a way to reduce this energy loss is a key means by which to enhance the photoelectric conversion efficiency of DSSCs. With this in mind, we have tried to utilize the forbidden absorption from the singlet ground state to the MLCT excited state, which is rarely observed17. Fortunately, we discovered that an intense singlet-to-triplet (S–T) transition is exhibited by a newly synthesized sensitizer—trans-dichloro(phenyldimethoxyphosphine)(2,2′;6′,2′′-terpyridyl-4,4′,4′′-tricarboxylic acid)ruthenium(II) (referred to as DX1; Fig. 1a). DX1 was isolated as the trans-isomer18 and did not undergo isomerization in N,N-dimethylformamide (DMF) under solar illumination. Figure 1b shows that DX1 in DMF solution (at a temperature of 298 K) has an intense ultraviolet absorption band at a wavelength of 320 nm, which is assigned to the ligand centred p–p* transition of the terpyridyl ligand. In addition, broad absorption bands in the 500–900 nm region are assigned to MLCT transitions, which are characteristic of typical Ru(II) polypyridyl complexes. Notably, the MLCT band of DX1 at 620 nm is weaker in intensity than the MLCT bands of BD, but the DX1 exhibits higher-intensity and broad electronic absorption peaks at longer wavelengths centred at 792 nm. In spite of the absorption differences between DX1 and BD, the air-sensitive phosphorescence spectra observed for both complexes at 298 K have similar wavelength peaks at 950 nm. In a 9:1 (vol/vol) ethanol/2-methyltetrahydrofuran solution, the absorption peak of DX1 at the lower temperature of 77 K is sharper than that at 298 K, and the lowestenergy absorption peak is observed at 1.66 eV (740 nm) (Fig. 1c). Because the absorption spectrum is the mirror image of the phosphorescence spectrum, the low-energy absorption spectrum of DX1 is attributed to S–T excitation. To eliminate the broadening effects of the deprotonation of the carboxyl groups, the absorption and emission spectra of esterified DX1 and BD (DX1E and BDE) were measured in toluene at 77 K (Supplementary Fig. S2). The spectral shapes of DX1E and BDE are similar, but sharper and redshifted compared to those of DX1 and BD. In DX1E, a very small Stokes shift (900 cm) is observed, as is a clear absorption peak in the area overlapping with the phosphorescence spectrum. On the other hand, a very low-intensity absorption peak is observed for BDE in the region where the emission spectrum and the absorption spectrum overlap. The intensity of the peak is roughly one order smaller than that of DX1E (DX1E, 4× 10 M cm; BDE, 0.6× 10 M cm). Measuring the absorption spectra of DX1E and BDE in methylene dihalide (CH2X2: X1⁄4 Cl, Br, I) solutions at 298 K, the intensities of absorption in the emission overlap region increase in the following order: CH2I2. CH2Br2. CH2Cl2 (Supplementary Fig. S3, Table S3). The enhancements are considered to be due to the external heavy-atom effect, which leads to an increase in spin–orbit coupling and enhancement of the S–T transition19. Similarly, by this effect, the phosphorescence lifetimes of both complexes decrease with increasing atomic number of the halogen atom. In the various methylene dihalide solutions, a difference in optical density (DOD) was observed in the near-infrared area, and for DX1E these differences are larger than for BDE. The excitation energy derived from time-dependent density functional theory (TD–DFT) calculations (details of the calculations are summarized