In this paper we focus upon the electron injection dynamics in complete dye sensitized nanocrystalline titanium dioxide solar cells (DSSCs) employing the ruthenium bipyridyl sensitizer dye N719. Electron injection dynamics and quantum yields are studied by time resolved single photon counting and the results are correlated with device performance. In typical DSSC devices, electron injection kinetics were found to proceed from the N719 triplet state with an half time of 200 ± 60 ps and quantum yield of 84 ± 5 %. We find that these injection dynamics are independent of presence of iodide / triiodide redox couple and of the pH of the peptisation step used in the synthesis of the TiO 2 nanoparticles. They are furthermore found to be only weakly dependent upon the application of electrical bias to the device. In contrast, we find these dynamics to be strongly dependent upon the concentration of t-butyl pyridine (tBP) and lithium cations in the electrolyte. This dependence is correlated with shifts of the TiO 2 conduction band energetics as a function of tBP and Li + concentration, from which we conclude that a 100 meV shift in band edge results in approximately a two fold retardation of injection dynamics.We find that electron injection quantum yield determined from these transient emission data as a function of tBP and Li + concentration shows a linear correlation with device short circuit density J sc . We thus conclude that the relative energetics of the dye excited state versus the titanium dioxide acceptor states is a key determinant of the dynamics of electron injection in DSSC, and that variations in these energetics, and therefore in the kinetics and efficiency of electron injection, impact directly upon device photovoltaic efficiency. Finally we discuss these results in terms of singlet versus triplet electron injection pathways and the concept of minimisation of kinetic redundancy.

Injection efficiency, η inj , and diffusion length, L, in dye-sensitized solar cells have been derived from the spectral response (incident photon to current efficiency, IPCE) of the cells under front side or backside illumination. Values of L from IPCE are found to be ∼2 times shorter than the values of L derived from the normal small perturbation transient method. IPCE-derived values of L (2 to more than 40 µm) and η inj (63-90%) are found to correlate with the photocurrent (and indirectly with the photovoltage) of the different cells indicating the extent to which each factor limits the cell efficiency. IPCE spectra varied with light intensity, so that diffusion lengths derived from both methods show similar trends, e.g., L from IPCE is found to increase 3 times when the light intensity is increased 10 times up to approximately 0.1 sun where L tends to plateau or peak. The values for η inj derived from the spectral response are shown to be in quantitative agreement with those determined from picosecond transient emission spectroscopy. To illustrate the utility of this method, L and η inj were measured on cells with and without the TiCl 4 chemical bath treatment. The results show that the increase in photocurrent after the TiCl 4 treatment is due to around a 2-fold increase in L despite a 3-fold reduction in the electron diffusion coefficient. The increased L can be explained by a factor of 10 decrease in electron recombination rate.

The optimization of interfacial charge transfer is crucial to the design of dye-sensitized solar cells. In this paper we address the dynamics of the charge separation and recombination in liquid-electrolyte and solid-state cells employing a series of amphiphilic ruthenium dyes with varying hydrocarbon chain lengths, acting as an insulating barrier for electron-hole recombination. Dynamics of electron injection, monitored by time-resolved emission spectroscopy, and of charge recombination and regeneration, monitored by transient optical absorption spectroscopy, are correlated with device performance. We find that increasing dye alkyl chain length results in slower charge recombination dynamics to both the dye cation and the redox electrolyte or solid-state hole conductor (spiro-OMeTAD). These slower recombination dynamics are however paralleled by reduced rates for both electron injection into the TiO2 electrode and dye regeneration by the I-/I3- redox couple or spiro-OMeTAD. Kinetic competition between electron recombination with dye cations and dye ground state regeneration by the iodide electrolyte is found to be a key factor for liquid electrolyte cells, with optimum device performance being obtained when the dye regeneration is just fast enough to compete with electron-hole recombination. These results are discussed in terms of the minimization of kinetic redundancy in solid-state and liquid-electrolyte dye-sensitized photovoltaic devices.

No abstract

We report electron injection dynamics for a series of porphyrin sensitized nanocrystalline TiO2 films, comparing zinc and free base porphyrins with a conjugated or nonconjugated linker group to the carboxylate binding group. Injection dynamics are measured used time correlated single photon counting, using dye sensitized ZrO2 control films. These injection dynamics are correlated with molecular orbital calculations, electrochemical data and device photocurrent efficiencies. The injection dynamics, and overall injection efficiency is found to be increased by linker conjugation and by the use of a zinc central metal. The faster injection dynamics for the Zinc porphyrins is shown to be quantitative agreement with the higher singlet excited state energy of these dyes compared to free base porphyrins. For the most efficient dye studied, addition of a typical redox electrolyte to the dye sensitized film is observed to retard the injection dynamics. Moreover studies of sensitized ZrO2 control films indicated that the redox electrolyte resulted in a reduction of excited state lifetime, indicative of an additional decay pathway competing with electron injection. Overall, a close correlation is found between electron injection dynamics and photocurrent efficiency for this series of porphyrin sensitized solar cells, indicating that for such sensitizer dyes, electron injection is a key factor limiting device performance.

There is great interest in synthesizing organic dyes to replace organometallic dyes as sensitizers in nanocrystalline TiO2 solar cells. We present a detailed comparison of interfacial electron transfer kinetics in dye-sensitized solar cells containing the coumarin based organic dye (NKX2677) against those observed for the ruthenium based organometallic dye, N719, including analysis of excited state lifetimes, injection kinetics, regeneration kinetics, and recombination to both oxidized dyes and electrolyte. We find three primary differences in behavior between these two dyes which limit the performance of NKX2677-sensitized solar cells: a shorter lifetime of the electron injecting state for NKX2677 versus N719 (primarily due to it being a singlet rather than triplet sensitizer); a faster rate constant for recombination to the electrolyte (RCE) for NKX2677-sensitized solar cells; and the greater tendency for NKX2677 to aggregate, reducing the electron injection efficiency. The shorter excited state lifetime results in relatively fast injection kinetics being required for efficient electron injection, with transient photoluminescence measurements indicating <60 ps injection halftime for NKX2677 compared with ∼350 ps injection halftimes for N719. This faster injection is achieved with NKX2677 by virtue of its relatively more negative excited state reduction potential, but is associated with a greater free energy loss driving electron injection. The faster recombination to the electrolyte is assigned to “catalysis” of this two-electron reaction by the sensitizer dye, most probably resulting from a local increase in the concentration of oxidized redox couple at the dye-sensitized interface, and provides a further limitation on the open circuit voltage achieved with NKX2677-sensitized solar cells. We conclude by discussing the extent to which these results are likely to reflect relatively generic differences between organic and rurthenium based organometallic sensitizer dyes and thus the implications for the development of efficient organic dye-sensitized solar cells.

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 TiO(2) 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 TiO(2)-bound N945 sensitizer, excitation directs charge into the carboxylbipyridine ligand bound to the TiO(2) 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.

No abstract