Molten and solid metal-iodide-doped trialkylsulphonium iodides and polyiodides as electrolytes in dye-sensitized nanocrystalline solar cells

The conductivity and solar cell performance of metal-iodide-doped trialkylsulphonium iodides and polyiodides have been investigated as electrolytes in dye-sensitized nanocrystalline solar cells (DNSCs). Nine different metal-iodide-containing (R₂R′S)I with additional iodine provided overall solar-to-electric energy conversion efficiencies of over 2%, while used as electrolytes in DNSCs in simulated AM 1.5 solar light at the light intensity of 100 W m⁻². The highest overall conversion efficiency, 3.1%, was achieved by using the electrolyte (Bu₂MeS)I:AgI:I₂ in the proportions (1:0.03:0.05). The effects from 4-tert-butylpyridine treatment of the electrodes were studied. The effects of metal-iodide doping were also investigated with respect to speciation in the electrolytes and potential influence on electrochemical conductivity.     

Ionic liquid electrolytes based on 1-vinyl-3-alkylimidazolium iodides for dye-sensitized solar cells

Five new ionic liquids of 1-vinyl-3-alkylimidazolium iodide were synthesized to develop novel electrolytes for dye-sensitized solar cells. The effects of photovoltaic characteristics of the cell and the ionic liquid features such as viscosity and ionic conductivity were described. The 1-vinyl-3-alkylimidazolium cation volume was calculated by quantum chemistry method. The linear dependence of photon-to-current conversion efficiency on the non-solvated cation volume was revealed. After lithium iodide was added to 1-vinyl-3-alkylimidazolium salts as electrolytes, except the photovoltage, the photocurrent, fill factor and photon-to-current efficiency were improved correspondingly.     

Regenerative effects by temperature variations in dye-sensitized solar cells

The effect of repeated temperature variations on the performance of both fresh and aged dye-sensitized solar cells with liquid and semi-solid electrolytes has been studied. The cell performance was characterized with IV-curves obtained at different cell operating temperatures and electrochemical impedance spectroscopy measurements before and after the temperature treatments. Consecutive temperature rampings of the aged cells did regenerate the cell function, so that the total efficiency drop over the observation period was on average 18%/unit less for the temperature-treated cells than for reference cells aged at constant temperature.     

Pyridinium molten salts as co-adsorbents in dye-sensitized solar cells

The influence of using pyridinium molten salts as co-adsorbents to modify the monolayer of a TiO₂ semiconductor on the performance of a dye-sensitized solar cell is studied. The current-voltage characteristics are measured under AM 1.5 (100 mW cm⁻²). The pyridinium molten salts significantly enhance the open-circuit photovoltage (V_oc), the short circuit photocurrent density (J_sc) as well as the solar energy conversion efficiency (η). 1-Ethyl-3-carboxypyridinium iodide ([ECP][I]) is applied successfully to prepare an insulating molecular layer with N719, and achieve high energy conversion efficiency as high as 4.49% at 100 mW cm⁻² and AM 1.5. The resulting efficiency is 20% higher than that of a non-additive device. This enhancement of conversion efficiency is attributed to the negative shift of the conduction band (CB) edge and the abundant concentration of I⁻ on the surface of the electrode when using [ECP][I] as the co-adsorbent.     

On the modeling of the dye-sensitized solar cell

A simplified electric model of the dye-sensitized electrochemical solar cell (DSC) is presented. It permits the calculation of internal steady-state cell characteristics like particle density distributions or the electric field as a function of the (measured) external current I_ext. The cell is modeled as an one-dimensional pseudo-homogeneous medium of thickness L, where all the electroactive particles involved in the current supporting process move according to different effective transport coefficients (i.e. effective diffusivities D and effective mobilities μ). The electroactive particles are the electrons e⁻ injected into the nanoporous TiO₂ layer after light absorption by the dye, the reduced and the oxidized counterpart of the redox electrolyte El_Red and El_Ox, and the positively charged cation Kat⁺ being brought into the cell together with the electrolyte. By applying the continuity equation, the transport-equation and Poisson's equation to all the electroactive species involved (e⁻, El_Red, El_Ox and Kat⁺) and by assuming a linear Boltzmann relaxation approximation for the back reaction, a system of differential equations is derived, describing particle densities, particle currents and the electric field within the cell. The underlying simplifying assumptions as well as the resulting limits of the model are stated, and some possible extensions are given. This paper aims to outline the general ideas and limitations of the proposed electric modeling, numerical calculations have been successfully implemented, but will be presented in a future paper.     

High-performance carbon counter electrode for dye-sensitized solar cells

Here, we reported that a new carbon electrode prepared with an activated carbon was superior to a Pt sputtered electrode as the counter electrode of dye-sensitized solar cells. The photovoltaic performance was largely influenced by the roughness factor of carbon electrode. The open-circuit voltage increased by about 60 mV using the carbon counter electrode compared to the Pt counter electrode because of positive shift of the formal potential for I₃⁻/I⁻ couple.     

High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells

Polypyrrole (PPy) nanoparticle was synthesized and coated on a conducting FTO glass to construct PPy counter electrode used in dye-sensitized solar cell (DSSC). Scanning electron microscope images show that PPy with porous and particle diameter in 40–60 nm is covered on the FTO glass uniformly and tightly. Cyclic voltammograms of I₂/I⁻ system measurement reveals that the PPy electrode has smaller charge-transfer resistance and higher electrocatalytic activity for the I₂/I⁻ redox reaction than that Pt electrode does. Overall energy conversion efficiency of the DSSC with the PPy counter electrode reaches 7.66%, which is higher (11%) than that of the DSSC with Pt counter electrode. The excellent photoelectric properties, simple preparation procedure and inexpensive cost allow the PPy electrode to be a credible alternative used in DSSCs.     

A phenylcarbazole functionalized ruthenium dye for efficient dye-sensitized solar cells

A new heteroleptic Ru(II) complex of [Ru(Hcpip)(Hdcbpy)(NCS)₂]⁻·[N(C₄H₉)₄]⁺·H₂O {where Hcpip = 2-(4-(9H-carbazol-9-yl)phenyl)-1H-imidazo[4,5-f] [1,10]phenanthroline, Hdcbpy = 4-carboxylic acid-4′-carboxylate-2,2′-bipyridine} has been synthesized and demonstrated to function as an efficient sensitizer for nanocrystalline TiO₂-based dye-sensitized solar cell (DSSC). The DSSC based on this Ru(II) complex showed a short-circuit photocurrent density of 19.2 mA cm⁻², an open-circuit photovoltage of 630 mV, a fill factor of 57.7%, corresponding to an overall light to electricity conversion efficiency of 6.98% under simulated solar light irradiation at 100 mW cm⁻². This efficiency value is 2.81- and 1.08-fold efficiency values of 2.48% and 6.47% observed for carbazole-free parent complex [Ru(Hpip)(Hdcbpy)(NCS)₂]⁻·[N(C₄H₉)₄]⁺·H₂O {where Hpip = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline}- and cis-bis(isothiocyanato)bis(4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) N3-based solar cells respectively, under identical experimental conditions. The molecular structures and electronic properties of the Ru(II) complexes were also investigated by means of density functional theory calculations in an effort to understand the device performance observed.     

Faradaic impedance of dye-sensitized solar cells

Theoretical equations of the Faradaic impedance of the photoelectrode and the counter electrode of dye-sensitized solar cell (DSC) were derived. The Faradaic impedance is the frequency dependent resistance related to the time constants of elementary electrode processes like photoexcitation, electron transfer, charge transfer reaction and diffusion. The typical cell impedance spectrum describes the locus of three semicircles on the Nyquist plane. The locus of three semicircles is generally analyzed by using the equivalent circuit composed of charge transfer resistance (R_ct,1) and capacitance (C_dl,1) of counter electrode, charge transfer resistance (R_ct,2) and capacitance (C_dl,2) of photoelectrode, the finite diffusion impedance due to the diffusion of I₃⁻ on the counter electrode (Z_w), and total resistance of the substrate and solution (R_s). The physical meanings of R_ct,1 and R_ct,2 can be elucidated by the interpretations of Faradaic impedance derived in the present paper. The R_ct,1 is represented as the function of the potential-dependent rate constants of I₃⁻ reduction and I⁻ oxidation. On the other hand, the R_ct,2 is the function of the photoelectrode potential, the surface concentration of I₃⁻ and the potential-independent rate constant of the back electron transfer reaction. The theoretical expressions of the current-voltage (I-V) curve of the DSC can be also derived. In the present paper, the relations between the impedance and I-V curve of the DSC are discussed.     

Method for fabricating the compact layer in dye-sensitized solar cells by titanium sputter deposition and acid-treatments

Dye-sensitized solar cells (DSCs) have been put forward as a potential low-cost alternative to the widely used silicon solar cells, which are subject to cost limitations. However, some problems need to be solved in order to enhance the efficiency of DSCs. In particular, the electron recombination occurred by the contact between the transparent conductive oxide (TCO) and a redox electrolyte is one of the main limiting factors of efficiency. Accordingly, a compact layer plays an important role in realizing highly efficient DSCs because it improves the adhesion of the TiO₂ to the TCO and provides a larger contact area and more effective electron transfer by preventing electron recombination. In this work, the fabrication of a TiO₂ compact layer using Ti sputter deposition and acid-treatment was investigated rather than the conventional method, which uses a TiCl₄ aqueous solution. The acid-treatment of the sputtered Ti film actively oxidized the Ti particles. As a result, such a cell exhibited an additional 1.3% in total efficiency compared to the standard DSC without a compact layer. These improvements are not inferior to those obtained by the conventional fabrication method using a TiCl₄ aqueous solution.     

Improvement of the performance of dye-sensitized solar cells using Sn-doped ZnO nanoparticles

Sn-doped and undoped ZnO nanoparticles were synthesized by hydrothermal method and their performance as the photoanode of dye-sensitized solar cells (DSSCs) was investigated. Energy dispersive X-ray spectroscopy and X-ray diffraction showed that the Sn had been doped into the ZnO lattice. A red shift of photoluminescence spectra which was induced by Sn doping was observed. The photocurrent density-voltage curves of DSSCs indicated that the efficiency was increased by as high as 140% on bare-FTO substrate and 105% on ZnO compact layer/FTO substrate via Sn doping. Also the effect of the ZnO compact layer was discussed by both of Sn-doped or undoped DSSCs.     

Optical properties of nano-structured dye-sensitized solar cells

Radiative transfer computations are carried out to describe the intrinsic and effective optical properties of light diffusing and absorbing materials consisting of anatase titania pigments hosted in an electrolyte medium. The intrinsic visible absorption of some of the pigments has been increased by coating them with an absorbing dye monolayer. A multiple scattering approach is applied to compute average path-length parameters and forward-scattering ratios used in four-flux radiative transfer calculations. It is shown that the effective absorption coefficient of the inhomogeneous medium is maximized when the size of the pigments is around 12 nm in diameter, and the effective scattering coefficient is optimized for diameters of the pigments around 250 nm. The intrinsic solar absorptance of the medium is optimized when the diameter of the pigments is around 60 nm.     

A study of stainless steel-based dye-sensitized solar cells and modules

Stainless steel (StSt) has been applied as substrate material for efficient, flexible, nanoporous TiO₂ dye-sensitized solar cells (DSSCs) with the aim of improving the photochemical properties of current plastic-based flexible DSSCs. DSSCs with a StSt substrate show almost equivalent properties in efficiency and convenience to cells with a F-doped tin oxide (FTO) glass substrate. Specifically, the metal substrate allows application of high-temperature sintering processes and shows high conductance even after sintering. Cells fabricated with the StSt substrates have been investigated as individual cells and as modules. A comparison between conventional DSSCs with a FTO glass substrate and flexible DSSCs with a StSt substrate is presented. In addition, Pt-coated electrodes, which can serve as window electrodes for StSt-based DSSCs, are fabricated via two different methods, i.e., chemical reduction and annealing, and compared.     

Synthesis of nanocrystalline ZnO powder via sol-gel route for dye-sensitized solar cells

This paper reports the growth mechanism of sol-gel-derived nanocrystalline ZnO powder. The influence of pH value of the sol on the crystallite size, morphology and structure of ZnO powder was investigated by using X-ray diffraction, transmission and scanning electron microscopy. Maximum size nanocrystallite (∼14 nm) of ZnO powder was obtained for pH value of 9. An increase in the band gap (blue shift) was observed with decrease in the size of the ZnO nanocrystallites. The variation in band gap was found to be in agreement with theoretical calculations using effective mass model. The growth mechanism of ZnO particles from zinc acetate dihydrate precursor by the sol-gel process has been discussed in terms of solvation, hydrolysis and polymerization. The synthesized ZnO powders were successfully used as the electrode material for dye-sensitized solar cells.     

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cells

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cell (DSSC) has been investigated to optimize the device geometry for reliable energy conversion efficiency assessment. Mesoporous TiO₂ layers with an identical active area (0.40 cm²) and different dimension are prepared on FTO glass substrate by the screen printing method and used as photoanodes for DSSCs. Under 1 sun illumination (AM 1.5G, 100 mW cm⁻²), both the open-circuit voltage and the short-circuit current density are independent of electrode geometry whereas the fill factor and hence energy conversion efficiency show strong dependency. Electrochemical impedance spectroscopy analysis indicates that the distance between active layer and ohmic contact directly contributes to internal series resistance and influence photovoltaic performance.     

Pt–NiO nanophase electrodes for dye-sensitized solar cells

A new type of counter electrode comprising of Pt and NiO biphase was prepared an RF magnetron cosputtering system for a dye-sensitized solar cell (DSSC). Transmission electron microscope images, transmission electron diffraction patterns, and X-ray diffraction patterns of the Pt–NiO electrodes confirmed the formation of a nanosized Pt polycrystalline phase of 7 nm mixed with porous amorphous NiO phase. The short-circuit current density and cell efficiency were increased from 0.22 to 0.30 mA/cm² and from 2.1% to 2.8%, respectively, and almost constant open-circuit voltage and fill factor, 0.53 V and 63%, respectively, were observed.     

Pore-size effect on photovoltaic performance of dye-sensitized solar cells composed of mesoporous anatase-titania

The effect of the pore size of mesoporous anatase-TiO₂ on the photovoltaic performance of dye-sensitized solar cells (DSSCs) is investigated. The mesoporous TiO₂ particles are synthesized by two different methods using a soft template of tri-block copolymer and a hard template of mesoporous ZnO/Zn(OH)₂-composite. These methods produce the same high surface area (S_BET ∼ 210 m² g⁻¹) but different pore sizes of 6.8 and 3.0 nm, respectively. With the mesoporous TiO₂ having larger pores, the photo-conversion efficiency (η) is increased significantly to 6.71%, compared with 5.62% that is typically achieved using P25 TiO₂ nanopowders. By comparison, only half the performance (3.05%) has been observed with mesoporous TiO₂ that has small pores. Mesoporous TiO₂ with suitable pore sizes (∼6.8 nm) makes the most of its high surface area and thereby allows a high uptake of dye to enhance the current density. In contrast, the low efficiency of mesoporous TiO₂ with small pores is attributed to the low uptake of dye due to the smaller pore size (∼3.0 nm), which blocks the diffusion and adsorption of dye molecules through the pores.     

Anthraquinone dyes as photosensitizers for dye-sensitized solar cells

Three anthraquinone dyes with carboxylic acid as anchoring group are designed and synthesized as sensitizers for dye-sensitized solar cells (DSSCs). Preliminary photophysical and photoelectrochemical measurements show that these anthraquinone dyes have very low performance on DSSC applications, although they have broad and intense absorption spectra in the visible region (up to 800 nm). Transient absorption kinetics, fluorescence lifetime measurements and density functional theory (DFT) calculations are conducted to investigate the cause of such low DSSC performance for these dyes. The results show that the strong electron-withdrawing character of the two carbonyl groups on anthraquinone framework may lie behind the low performance by suppressing the efficient electron injection from the dye to the conduction band of TiO₂.     

Calibration of solar simulator for evaluation of dye-sensitized solar cells

The photo-to-electricity energy conversion efficiencies of ruthenium-dye-sensitized solar cells (DSC) are measured under a solar simulator. The error in conversion efficiencies was compared under a variety of spectral conditions. Measurements of the conversion efficiencies of DSC between a solar simulator and outdoor sunlight result in about 10% error. This error was seen when the spectral intensity of a xenon-lamp solar simulator (imitating an air mass (AM) 1.5 spectrum) was adjusted by the short-circuit photocurrent I_Si of a crystalline silicon (c-Si) standard cell. In order to adjust the energetic intensity of AM 1.5 for DSC that has a spectrum response only in the visible region light, the c-Si reference cell is modified with a glass UV filter (KG-5, Schott) and the solar simulator was adjusted by I_{IR-cut Si}. The energetic spectrum of the solar simulator has a good accuracy over the wavelength range 300–750 nm, giving the conversion efficiency of DSC an accuracy of about 2%. The dependency of the ratio of I_Si to I_{IR-cut Si} on natural sun power is discussed in view of scattering of the visible light under changing natural sun light.     

Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy

The main features of the characteristic impedance spectra of dye-sensitized solar cells are described in a wide range of potential conditions: from open to short circuit. An equivalent circuit model has been proposed to describe the parameters of electron transport, recombination, accumulation and other interfacial effects separately. These parameters were determined in the presence of three different electrolytes, both in the dark and under illumination. Shift in the conduction band edge due to the electrolyte composition was monitored in terms of the changes in transport resistance and charge accumulation in TiO₂. The interpretation of the current–potential curve characteristics, fill factor, open-circuit photopotential and efficiency in the different conditions, was correlated with this shift and the features of the recombination resistance.