The performance of solid-state dye-sensitized solar cells based on spiro-MeOTAD (2, 2’7, 7’-tetrakis(N, N-di-p-methoxyphenyl-amine)-9, 9’-spirobifluorene) was considerably improved by decreasing charge recombination across the interface of the heterojunction. This was achieved by blending the hole conductor matrix with a combination of 4-tert-butylpyridine (tBP) and Li[CF3SO2]2N. Open circuit voltages (Uoc) over 900mV and short circuit currents (Isc) up to 5.1 mA were obtained, yielding an overall efficiency of 2.56 % at AM1.5 illumination. Further improvement of the device performance was observed when conducting stripes of silver were deposited onto the devices as charge collector. The beneficial effect however could be assigned to the contamination of the dye-sensitized TiO2 film with silver during the dyeing process.

We show that electron transport mechanisms in TiO2 solid-state dye-sensitized solar cells (SDSCs) with spiro-OMeTAD as hole conductor are similar to those of high-performance DSCs with liquid electrolytes and ionic liquids. Impedance spectroscopy provides the parameters for transport and recombination at different conditions of steady state in the dark. The recombination rate is much higher in the solid solar cell, this being a main limiting step to obtain high-efficiency SDSCs. Thus, the expected gain in photovoltage, due to a lower hole Fermi level, is prevented by recombination losses. Under low potentials the transport is limited by the electron transport in the TiO2, but at high potentials spiro-OMeTAD transport resistance reduces the fill factor and hence the efficiency on high-current devices.

A high molar extinction coefficient charge transfer sensitizer tetrabutylammonium [Ru(4,-carboxylic acid-4'-carboxylate-2,2'-bipyridine)(4,4'di-(2-(3,6-dimethoxyphenyl)ethenyl)-2,2'-bipyridine)(NCS)(2)], is developed which upon anchoring onto nanocrystalline TiO2 films exhibit superior power conversion efficiency compared to the standard sensitizer bistetrabutylammonium cis-dithiocyanatobis(4,4'-dicarboxylic acid-2,2'bipyridine)ruthenium(II) (N719). The new sensitizer anchored TiO2 films harvest visible light very efficiently over a large spectral range and produce a short-circuit photocurrent density of 18.84 mA/cm(2), open-circuit voltage 783 mV and fill factor 0.73, resulting remarkable solar-to-electric energy conversion efficiency (eta) 10.82, under Air Mass (AM) 1.5 sunlight. The Time Dependent Density Functional Theory (TDDFT) excited state calculations of the new sensitizer show that the first three HOMOs have ruthenium t(2g) character with sizable contribution coming from the NCS ligands and the pi-bonding orbitals of the 4,4'-di-(2-(3,6-dimethoxyphenyl)ethenyl)-2,2'-bipyridine. The LUMO is a pi* orbital localized on the 4,4'-dicarboxylic acid-2,2'-bipyridine ligand. (c) 2006 Elsevier B.V. All rights reserved.

A new ruthenium(II) complex, tetrabutylammonium [ruthenium (4-carboxylic acid-4'-carboxylate-2,2'-bipyridine)(4,4'-di(2-(3,6-dimethoxyphenyl)ethenyl)-2,2'-bipyridine)(NCS)(2)] (N945H), was synthesized and characterized by analytical, spectroscopic, and electrochemical techniques. The absorption spectrum of the N945H sensitizer is dominated by metal-to-ligand charge-transfer (MLCT) transitions in the visible region, with the lowest allowed MLCT bands appearing at 25 380 and 18 180 cm(-1). The molar extinction coefficients of these bands are 34 500 and 18 900 M-1 cm(-1), respectively, and are significantly higher when compared to than those of the standard sensitizer cis-dithiocyanatobis(4,4'-dicarboxylic acid-2,2'-bipyridine)ruthenium(II). An INDO/S and density functional theory study of the electronic and optical properties of N945H and of N945 adsorbed on TiO2 was performed. The calculations point out that the top three frontier-filled orbitals have essentially ruthenium 4d (t(2g) in the octahedral group) character with sizable contribution coming from the NCS ligand orbitals. Most critically the calculations reveal that, in the TiO2-bound N945 sensitizer, excitation directs charge into the carboxylbipyridine ligand bound to the TiO2 surface. The photovoltaic data of the N945 sensitizer using an electrolyte containing 0.60 M butylmethylimidazolium iodide, 0.03 M I-2, 0.10 M guanidinium thiocyanate, and 0.50 M tert-butylpyridine in a mixture of acetonitrile and valeronitrile (volume ratio = 85:15) exhibited a short-circuit photocurrent density of 16.50 +/- 0.2 mA cm(-2), an open-circuit voltage of 790 +/- 30 mV, and a fill factor of 0.72 +/- 0.03, corresponding to an overall conversion efficiency of 9.6% under standard AM (air mass) 1.5 sunlight, and demonstrated a stable performance under light and heat soaking at 80 degrees C.

A new series of panchromatic ruthenium(II) sensitizers derived from carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) have been developed. Black dye containing different degrees of protonation {(C2H5)(3)NH}[Ru(H(3)tcterpy)(NCS)(3)] 1, {(C4H9)(4)N}(2)[Ru(H(2)tcterpy)(NCS)(3)] 2, {(C4H9)(4)N}(3)[RU(Htcterpy)(NCS)(3)] 3, and {(C4H9)(4)N}(4)[Ru(tcterpy)(NCS)(3)] 4 (tcterpy = 4,4',4 " -tricarboxy-2,2':6',2 " -terpyridine) have been synthesized and fully characterized by UV-vis, emission, IR, Raman, NMR, cyclic voltammetry, and X-ray diffraction studies. The crystal structure of complex 2 confirms the presence of a Ru(II)N6 central core derived from the terpyridine ligand and three N-bonded thiocyanates. Intermolecular H-bonding between carboxylates on neighboring terpyridines gives rise to 2-D H-bonded arrays. The absorption and emission maxima of the black dye show a bathochromic shift with decreasing pH and exhibit pH-dependent excited-state lifetimes. The red-shift of the emission maxima is due to better pi -acceptor properties of the acid form that lowers the energy of the CT excited state. The low-energy metal-to-ligand charge-transfer absorption band showed marked solvatochromism due to the presence of thiocyanate ligands. The Ru(II)/(III) oxidation potential of the black dye and the ligand-based reduction potential shifted cathodically with decreasing number of protons and showed more reversible character. The adsorption of complex 3 from methoxyacetonitrile solution onto transparent TiO2 films was interpreted by a Langmuir isotherm yielding an adsorption equilibrium constant, K-ads, of (1.0 +/- 0.3) x 10(5) M-1. The amount of dye adsorbed at monolayer saturation was (n(alpha) = 6.9 +/- 0.3) x 10(-8) mol/mg of TiO2, which is around 30% less than that of the cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)-ruthenium(II) complex. The black dye, when anchored to nanocrystalline;TiO2. films achieves very efficient sensitization over the whole visible range extending into the near-IR region up to 920 nm, yielding over 80% incident photon to-current efficiencies (IPCE). solar cells containing the black dye were subjected to analysis by a photovoltaic calibration laboratory (NREL, U.S.A.) to determine their solar-to-electric conversion efficiency under standard AM 1.5 sunlight. A short circuit photocurrent density obtained was 20.5 mA/cm(2), and the open circuit voltage was 0.72 V corresponding to an overall conversion efficiency of 10.4%.

The performance of solid-state dye-sensitized solar cells based on spiro-MeOTAD was considerably improved by controlling charge recombination across the interface of the heterojunction. This was achieved by blending the hole conductor matrix with a combination of 4-tert-butylpyridine (tBP) and Li[CF3SO2]2N. Open circuit voltages Uoc over 900 mV and short circuit currents Isc up to 5.1 mA were obtained, yielding an overall efficiency of 2.56% at AM1.5 illumination. These values have been fully confirmed at the National Renewable Energy Laboratories for a device with an active area of 1.07 cm2, signifying a dramatic improvement compared to previously reported values for a similar device.