Dye-sensitized solar cells fabricated using ordered arrays of titania nanotubes (tube lengths 5, 10, and 20 microm) grown on titanium have been characterized by a range of experimental methods. The collection efficiency for photoinjected electrons in the cells is close to 100% under short circuit conditions, even for a 20 microm thick nanotube array. Transport, trapping, and back transfer of electrons in the nanotube cells have been studied in detail by a range of complementary experimental techniques. Analysis of the experimental results has shown that the electron diffusion length (which depends on the diffusion coefficient and lifetime of the photoinjected electrons) is of the order of 100 microm in the titania nanotube cells. This is consistent with the observation that the collection efficiency for electrons is close to 100%, even for the thickest (20 microm) nanotube films used in the study. The study revealed a substantial discrepancy between the shapes of the electron trap distributions measured experimentally using charge extraction techniques and those inferred indirectly from transient current and voltage measurements. The discrepancy is resolved by introduction of a numerical factor to account for non-ideal thermodynamic behavior of free electrons in the nanostructured titania.

CdS/CdSe-sensitized nanostructured SnO(2) solar cells exhibiting record short-circuit photocurrent densities have been fabricated. Under simulated AM 1.5, 100 mW cm(-2) illumination, photocurrents of up to 17.40 mA cm(-2) are obtained, some 32% higher than that achieved by otherwise identical semiconductor-sensitized solar cells (SSCs) employing nanostructured TiO(2). An overall power conversion efficiency of 3.68% has been achieved for the SnO(2)-based SSCs, which compares very favorably to efficiencies obtained by the TiO(2)-based SSCs. The characteristics of these SSCs were studied in more detail by optical measurements, spectral incident photon-to-current efficiency (IPCE) measurements, and impedance spectroscopy (IS). The apparent conductivity of sensitized SnO(2) photoanodes is apparently too large to be measured by IS, yet for otherwise identical TiO(2) electrodes, clear electron transport features could be observed in impedance spectra, tacitly implying slower charge transport in TiO(2). Despite this, electron diffusion length measurements suggest that charge collection losses are negligible in both kinds of cell. SnO(2)-based SSCs exhibit higher IPCEs compared with TiO(2)-based SSCs which, considering the similar light harvesting efficiencies and the long electron diffusion lengths implied by IS, is likely to be due to a superior charge separation yield. The resistance to charge recombination is also larger in SnO(2)-based SSCs at any given photovoltage, and open-circuit photovoltages under simulated AM 1.5, 100 mW cm(-2) illumination are only 26-56 mV lower than those obtained for TiO(2)-based SSCs, despite the conduction band minimum of SnO(2) being hundreds of millielectronvolts lower than that of TiO(2).

Varying Li+ concentration in the electrolyte of dye-sensitized solar cells equipped with compact TiO 2 blocking layers is found to alter the mean slopes of semilogarithmic open-circuit photovoltage-intensity and dark current-voltage plots. Almost identical values of ideality factor or transfer coefficient are required to fit data in the dark and under illumination for each Li + concentration. It is found that cell characteristics become progressively more "ideal" as Li + concentration is increased, with a transfer coefficient of ca.1 for 1 M Li + in the electrolyte. We find that trends in photovoltage-intensity data are well fitted using a model which assumes that electron transfer to acceptor species in the electrolyte occurs from both the conduction band of the TiO 2 and an exponential distribution of band gap surface states. Changes in the mean ideality factor and linearity of semilogarithmic photovoltage-intensity plots can be rationalized by considering the variation in overlap between occupied donor states (conduction band and surface states) with electron acceptor states in the electrolyte, as the conduction band edge is shifted positive by increasing Li + concentration. In accordance with previous studies, this positive shift in conduction band edge is also found to cause a dramatic increase in the photocurrent generation efficiency of the cells, especially in the long-wavelength region of the photocurrent action spectrum. It is argued that this improvement in photocurrent is predominantly due to an increase in wavelength-dependent electron injection efficiency, as opposed to an increase in electron collection efficiency.

Dye-sensitized solar cells with power conversion efficiencies of up to 6.5% have been fabricated using a cobalt tris-bipyridyl redox mediator with the cis-diisothiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(2,2′-bipyridyl-4,4′-dinonyl) ruthenium(II) (Z907) sensitizer. This represents a significant improvement in efficiency compared with previous reports using ruthenium sensitizers. In situ near-IR transmittance measurements in conjunction with electrochemical impedance spectroscopy have been used to explain the difference in performance between DSCs using Z907 and another benchmark sensitizer cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) bis(tetrabutylammonium) (N719). It is found that the small-perturbation electron diffusion length (L n ) is significantly longer in Z907 cells compared with that in N719 cells, which can explain most of the difference in performance. It is also shown that the longer L n in Z907 cells is caused by inhibited recombination, as opposed to faster transport, and possible reasons for this are discussed. Our methodological approach is especially useful for accurately determining L n when it is shorter than the TiO2 layer thickness, where standard dynamic techniques start to become unreliable.

Reliable quantification of parameters influencing the efficiency of dye-sensitized solar cells (DSCs) is essential to guide device optimization and improve the fundamental understanding of device operation. The small-perturbation electron diffusion length (L n ) in DSCs has been determined by electrochemical impedance spectroscopy and by analysis of incident photon-to-collected-electron conversion efficiency (IPCE) spectra. When measurement conditions are chosen so that recombination can be treated as first-order in the free electron concentration, L n values obtained by the two quite different techniques are found to be in good agreement. This result provides an important experimental validation of the simple diffusion−recombination model that is commonly used to explain DSC operation. Knowledge of L n facilitates deconvolution of the charge separation efficiency (ηsep) from IPCE. ηsep is found to decrease significantly with increasing V OC for all DSCs studied here. This phenomenon is likely to be caused by a decrease in sensitizer regeneration efficiency as electrons accumulate in the TiO2, and sensitizer regeneration by iodide can no longer effectively compete with electron transfer from the TiO2 to the oxidized sensitizer.