High efficiency flexible dye-sensitized solar cells by multiple electrophoretic depositions

A multiple electrophoretic deposition (EPD) of binder-free TiO₂ photoanode has been developed to successfully fill the crack occurring after air-drying on the first EPD-TiO₂ film surface. With the slow 2nd EPD, high quality TiO₂ thin films are acquired on flexible ITO/PEN substrates at room temperature and the device efficiency of the dye-sensitized solar cell achieved 5.54% with a high fill factor of 0.721. Electrochemical impedance spectroscopy measurements analyze the great enhancement of the photovoltaic performance through multiple EPD. The electron diffusion coefficient improved by about 1 order of magnitude in crack-less multiple-EPD TiO₂ films. With the scattering layer, the device reveals a high conversion efficiency of up to 6.63% under AM 1.5 G one sun irradiation, having a short circuit current density, open circuit voltage, and filling factor of 12.06 mA cm⁻², 0.763 V and 0.72, respectively.     

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.     

Down-converting lanthanide doped TiO2 photoelectrodes for efficiency enhancement of dye-sensitized solar cells

Lanthanide (Ln³⁺) doped TiO₂ down-conversion photoelectrodes (Ln³⁺ = Eu³⁺ and Sm³⁺ ions) are used to enhance the photovoltaic efficiency of dye-sensitized solar cells (DSSC). We report on achieving fill factors of 0.67 and 0.69 and efficiencies of 5.81% and 5.16% for Sm³⁺ and Eu³⁺, respectively. This is compared to the 4.23% efficiency for the undoped-titania photoelectrodes. This enhancement is probably due to the improved UV radiation harvesting via a down-conversion luminescence process by the lanthanide ions. The structure, optical and photoluminescence properties of the down-converting photoelectrode are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDX) and room temperature photoluminescence excitation and emission spectrofluorimetric measurements.     

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.     

Photoelectrochemically polymerized polythiophene layers on ruthenium photosensitizers in dye-sensitized solar cells and their beneficial effects

Beneficial effects of polythiophen layers on ruthenium (Ru) photosensitizer in dye-sensitized solar cells (DSSCs) are observed. The thiophene is photoelectrochemically polymerized by the photoexcitation of Ru photosensitizers on TiO₂. In situ polymerization enables the formation of uniform polythiophene layers on Ru-complexes/TiO₂. The polythiophene layers have hydrophobic nature and bridge the energy level between Ru-complexes and iodine/iodide, which suppress the recombination reaction and improve the concentration of photoinjected electron in TiO₂ layers. The DSSCs with polythiophene protecting layers shows a power conversion efficiency of 6.01%, which is a 30% increase compared to one without polythiophene layers.     

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.     

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.     

Utilization of natural carotenoids as photosensitizers for dye-sensitized solar cells

The dye-sensitized solar cells (DSCs) were assembled by using natural carotenoids, crocetin (8,8′-diapocarotenedioic acid) and crocin (crocetin-di-gentiobioside), as sensitizers and their photoelectrochemical properties were investigated taking a presence or absence of carboxylic group in the dye molecule into consideration. In these carotenoids, crocetin that has carboxylic groups in the molecule can attach effectively to the surface of TiO₂ film so that it performed the best photosensitized effect resulting in the short-circuit photocurrent with 2.84 mA under irradiation of 1.0 cm². On the other hand, crocin that has no carboxylic group in the molecule showed lower photoelectrochemical performance because of its lower affinity to the surface of TiO₂ film. These results indicate that it is possible to apply carotenoid as sensitizers for DSCs at the presence of effective function groups.     

Low-Pt-loading acetylene-black cathode for high-efficient dye-sensitized solar cells

We reported on the synthesis, characterization, and photovoltaic/electrochemical properties of Pt/acetylene-black (AB) cathode as well as their application in dye-sensitized solar cells (DSCs). The Pt/AB electrode was prepared through a thermal decomposition of H₂PtCl₆ on the AB substrate. SEM and TEM observations showed that the Pt nanoparticles were homogeneously dispersed on the AB surface. The Pt-loading content in the Pt/AB electrode was only about 2.0 μg cm⁻², which was much lower than 5–10 μg cm⁻² generally used for the Pt electrode in DSCs. Electrochemical measurements displayed a low charge-transfer resistance of 1.48 Ω cm² for the Pt/AB electrode. Furthermore, when this low-Pt-loading electrode was used as the cathode of DSCs, an overall light-to-electricity energy conversion efficiency of 8.6% was achieved, showing commercially realistic energy conversion efficiency in the application of DSCs.     

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.     

Application of mesoporous carbon to counter electrode for dye-sensitized solar cells

The mesoporous carbons were prepared by the carbonation of the triblock copolymer F127/phloroglucinol-formaldehyde composite self-assembled in an acid medium and employed as the catalyst for triiodide reduction in dye-sensitized solar cells (DSCs). The characteristics of mesoporous carbon were analyzed by scanning electron microscopy, transmission electron microscopy, N₂ sorption measurement and X-ray diffraction. The mesoporous carbon with low crystallinity exhibited Brunauer-Emmett-Teller surface area of 400 m² g⁻¹, pore diameter of 6.8 nm and pore volume of 0.63 cm³ g⁻¹. The photovoltaic performances of DSCs with mesoporous carbon counter electrode were improved by increasing the carbon loading on counter electrode due to the charge-transfer resistance of mesoporous carbon counter electrode decreasing with the increase of the carbon loading. However, further carbon loading increase has no obvious effect on the photovoltaic performance of DSCs with carbon electrode when carbon loading exceeds 300 μg cm⁻². The overall conversion efficiency of 6.18% was obtained by DSCs composed of mesoporous carbon counter electrode with the carbon loading of 339 μg cm⁻². This value is comparable to that of DSCs with conventional platinum counter electrode.     

A novel UV-mediated low-temperature sintering of TiO2 for dye-sensitized solar cells

Titania pastes were fired at 450 °C in oxygen to give white titania that was used to prepare dye-sensitized solar cells (DSSC). Titania fired at lower temperature and/or under inert atmosphere have brown stripes and cells made from these stripes had no measurable efficiency. When the titania paste was screen printed and then heated and simultaneously irradiated with UV light, white stripes were obtained. Improved efficiency was noted for PV cells made from pastes heated at lower temperature under irradiation vs. cells made from low-temperature heated paste but without irradiation. UV irradiation appears to facilitate clean oxidation of residual organic materials in the titania precursor pastes. The best cells in our study made with our titania paste treated at 450 °C in oxygen had the following characteristics: efficiency=3.45%; V_oc=630 mV; J_sc=8.5 mA/cm²; and a fill factor=0.64.     

Influence of alkyl dihalide gelators on solidification of dye-sensitized solar cells

Quasi-dye-sensitized solar cells were prepared by using ionic liquid-type electrolytes and gelators consisting of polyvinylpyridine and alkyl dihalides. Gelation occurred by the reaction of polyvinylpyridine and alkyl dihalides. When the chain length of the dihalides was varied, the short-circuit current (J_sc) increased with an increase in the chain length. However, the open-circuit voltage (V_oc) and fill factor (ff) slightly decreased. The increase in J_sc was brought about by the decrease in the interfacial resistances between the gel electrolyte and the counter electrode. In addition, the increase in the J_sc was explained by increases in the apparent diffusion coefficient of I⁻/I₃⁻ when the chain length increased. Decreases in V_oc and ff were explained by back-electron transfers from TiO₂ to iodine in the electrolytes. V_oc of the cells solidified by alkyldiiodide was lower than that solidified by alkyldichloride or alkyldibromide. It was explained by negatively shifted redox potential of I⁻/I₃⁻, compared with those for Cl⁻/Cl₂ or Br⁻/Br₂.     

Ionic liquid electrolyte based on S-propyltetrahydrothiophenium iodide for dye-sensitized solar cells

A new ionic liquid S-propyltetrahydrothiophenium iodide (T₃I) was developed as the solvent and iodide ion source in electrolyte for dye-sensitized solar cells. The electrochemical behavior of the /I⁻ redox couple and effect of additives in this ionic liquid system was tested and the results showed that this ionic liquid electrolyte revealed good conducting abilities and potential application for solar devices. The effects of LiI and dark-current inhibitors were investigated. The dye-sensitized solar cell with the electrolyte (0.1 mol L⁻¹ LiI, 0.35 mol L⁻¹ I₂, 0.5 mol L⁻¹ NMBI in pure T₃I) gave short-circuit photocurrent density (J_sc) of 11.22 mA cm², open-circuit voltage (V_oc) of 0.61 V and fill factor (FF) of 0.51, corresponding to the photoelectric conversion efficiency (η) of 3.51% under one Sun (AM1.5).     

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.     

Comparison of charge accumulation and transport in nanostructured dye-sensitized solar cells with electrolyte or CuSCN as hole conductor

The charge transport properties of the dye-sensitized solar cells consisting of Ru(dcbpyH₂)₂(NCS)₂-sensitized nanostructured TiO₂ with either redox electrolyte or CuSCN as hole conductor have been compared. The electron transport time and the electron charge in the TiO₂ varies in a similar way with the incident light intensity for both hole conductors: electron transport becomes faster and electron accumulation increases with increasing light intensity. Electron transport in the CuSCN-based cells is significantly faster than in electrolyte cells under conditions where the accumulated charge is equal. An ultra-thin aluminum oxide layer on the nanocrystalline titanium oxide has a beneficial effect as it reduces the recombination and increases the open-circuit potential.     

Syntheses and photovoltaic properties of polymeric sensitizers using thiophene-based copolymer derivatives for dye-sensitized solar cells

We synthesized the thiophene-based copolymers (P(3TAF-co-3TAa)-A-n and P(3TAF-co-3TAa)-B-n) using two different kinds of thiophene monomers, (N-(3-thienylmethylene)-2-aminofluorene and 3-thiophene acetic acid), as sensitizers on the DSSCs. P(3TAF-co-3TAa)-A-n (n=1, 2, 3) was synthesized with different molar ratios (3TAF:3TAa=1:5, 1:10, 1:20) of monomers at room temperature, respectively. Also, P(3TAF-co-3TAa)-B-n (n=1, 2, 3) was synthesized with above molar ratios of monomers at 0 °C, respectively. The DSSCs devices were fabricated using the thiophene-based copolymers as sensitizers and their photovoltaic performances were measured by using a solar simulator under AM 1.5. In the DSSCs devices using polymeric sensitizers, V_oc is 0.53-0.60 V, J_sc is 1.9-4.5 mA/cm², FF is 0.51-0.63 and the power conversion efficiency is 0.63-1.53%, respectively.     

An effective use of nanocrystalline CdO thin films in dye-sensitized solar cells

Thin films of cadmium oxide (CdO) were synthesized by layer-by-layer deposition method on indium doped tin oxide (ITO) substrates. Post-deposition annealing at 250 °C for 24 h produced pure phase CdO films by removal of trace amount of cadmium hydroxide, as confirmed from X-ray diffractogram. First time employment of CdO in place of TiO₂ in dye-sensitized solar cells is reported to check feasibility and cell performance. A dye-sensitized nanocrystalline CdO photo-electrode was obtained by adsorbing cis-dithiocyanato (4,4′-dicarboxylic acid-2,2′-bipyridide) ruthenium (II) (N3) dye by keeping at 45 °C for 20 h. The efficiency of dye-sensitized nanocrystalline CdO thin film solar cell was increased from 0.24% to 2.95% due to dye adsorption. This must be highest reported conversion efficiency for other metal oxides than TiO₂based dye-sensitized solar cells.     

Reaction between oxide sealant and liquid electrolyte in dye-sensitized solar cells

The iodide and tri-iodide reduction-oxidation couples in the liquid electrolyte of dye-sensitized solar cells (DSSCs) are very reactive and corrosive with the other components of the DSSCs, especially the sealing material. A leakage of the liquid electrolyte to cells generally occurs in the DSSCs because of a chemical reaction between the sealing material and the electrolyte. This leakage is correlated to the long-term stability and energy conversion efficiency of the dye-sensitized solar module. This study examined the reactions between the electrolyte and the glass using a leaching test with the aim of using this material as the matrix of the sealing materials in DSSCs. The prepared samples were evaluated using an inductively coupled plasma spectroscope, a nanoindentation test, an atomic force microscope and a contact angle analyzer. After the leaching test, the zinc and boron ions from the glass were leached into the electrolyte after immersion for 1 h. The elastic modulus, hardness, and surface roughness of the glass changed after the formation of the leached layer of the glass surface. The results suggested that the compositions of the oxide sealant should be considered in terms of the chemical stability in the electrolyte.     

Enhancing the performance of dye-sensitized solar cells by incorporating nanosilicate platelets in gel electrolyte

Two kinds of gel-type dye-sensitized solar cells (DSSCs), composed of two types of electrolytes, were constructed and the respective cell performance was evaluated in this study. One electrolyte, TEOS-Triton X-100 gel, was based on a hybrid organic/inorganic gel electrolyte made by the sol–gel method and the other was based on poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) copolymer. TEOS-Triton X-100 gel was based on the reticulate structure of silica, formed by hydrolysis, and condensation of tetraethoxysilane (TEOS), while its organic subphase was a mixture of surfactant (Triton X-100) and ionic liquid electrolytes. Both DSSC gel-type electrolytes were composed of iodine, 1-propy-3-methyl-imidazolium iodide, and 3-methoxypropionitrile to create the redox couple of I₃⁻/I⁻. Based on the results obtained from the I–V characteristics, it was found that the optimal iodine concentrations for the TEOS-Triton X-100 gel electrolyte and PVDF-HFP gel electrolyte are 0.05 M and 0.1 M, respectively. Although the increase in the iodine concentration could enhance the short-circuit current density (J_SC), a further increase in the iodine concentration would reduce the J_SC due to increased dark current. Therefore, the concentration of I₂ is a significant factor in determining the performance of DSSCs.In order to enhance cell performance, the addition of nanosilicate platelets (NSPs) in the above-mentioned gel electrolytes was investigated. By incorporating NSP-Triton X-100 into the electrolytes, the J_SC of the cells increased due to the decrease of diffusion resistance, while the open circuit voltage (V_OC) remained almost the same. As the loading of the NSP-Triton X-100 in the TEOS-Triton X-100 gel electrolyte increased to 0.5 wt%, the J_SC and the conversion efficiency increased from 8.5 to 12 mA/cm² and from 3.6% to 4.7%, respectively. However, the J_SC decreased as the loading of NSP-Triton X-100 exceeded 0.5 wt%. At higher NSP-Triton X-100 loading, NSPs acted as a barrier interface between the electrolyte and the dye molecules, hindering electron transfer, hence, reducing the cell's photocurrent density. The same behavior was also observed in the PVDF-HFP gel electrolyte DSSC system.