Improved performance of a dye-sensitized solar cell using a TiO2/ZnO/Eosin Y electrode

TiO₂/ZnO/Eosin Y structure films were prepared by a one-step cathodic electrodeposition method and used as a photoanode in a dye-sensitized solar cell (DSSC). Using this TiO₂/ZnO/Eosin Y electrode in DSSC, the degradation of the cell with time was reduced and I_SC, V_OC and fill factor values were increased. The use of a thin ZnO layer, permitted the formation of an energy barrier at the electrode/electrolyte interface, thus reducing recombination rate and improving cell performance. In addition, the adsorbed dye molecules prepared by one-step cathodic electrodeposition with ZnO were very stable compared with that prepared by conventional immersing method, as evidenced by UV/vis absorption spectroscopy measurements.     

Ruthenium dye-sensitized SnO2/TiO2 coupled solar cells

The photoelectrochemical behaviors of RuL₂(NCS)₂ dye-sensitized SnO₂/TiO₂ coupled solar cell was studied and compared with TiO₂ single system. The coupled system shows higher incident photon-to-current conversion efficiency (IPCE) value than the single system. A maximum IPCE value in the coupled system with 3.5 μm-thick SnO₂ and 7 μm-thick TiO₂ attained 82.4% at 530 nm wavelength. The higher IPCE value in the coupled system is attributed to the charge separation by fast electron transfer process from the excited RuL₂(NCS)₂ dye to TiO₂ to SnO₂ in the system with different energy level.     

Single- and double-layered mesoporous TiO2/P25 TiO2 electrode for dye-sensitized solar cell

Nanocrystalline mesoporous titania (MP-TiO₂) was synthesized by surfactant-assisted templating method using tetraisopropyl orthotitanate modified with acetylacetone and laurylamine hydrochloride as template. The short-circuit photocurrent density (J_sc) of the cell made from MP-TiO₂ was much higher than that of the cell made from commercial P25 titania. The incident photon to current conversion efficiency (IPCE) spectra of thin MP-TiO₂ cell were higher than that of thick P25 cell in the region between 400 and 475 nm but lower than that of thick P25 cell in the red region, because the thickness of thin transparent MP-TiO₂ film was not enough to scatter the light leading to low absorbed spectra in red region. IPCE spectra of MP-TiO₂ can be improved by using the cell made from blended MP-TiO₂ with P25. The cell performance was improved with increasing sintering temperature. Double-layered titania cells were also fabricated to further improve the cell performance by increasing light scattering and amount of adsorbed dye. The solar energy conversion efficiency (η) up to 8.1% was obtained by using the double-layered titania cell sintered at 450 °C for 2 h.     

Influence of pyrazole derivatives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized TiO2 solar cell performance

The influence of pyrazole additives in an I⁻/I₃⁻ redox electrolyte solution on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics of the cell were measured using 18 different pyrazole derivatives. All of the pyrazole additives enhanced the open-circuit photovoltage (V_oc) and the solar energy conversion efficiency (η), but reduced the short-circuit photocurrent density (J_sc). Most of the pyrazoles improved fill factor (ff). The physical and chemical properties of the pyrazoles were computationally calculated in order to elucidate the reasons for the additive effects on cell performance. The greater the partial charge of the nitrogen atom at position 2 in the pyrazole group, the larger the V_oc, but the smaller the J_sc values. As the dipole moment of the pyrazole derivatives increased, the V_oc value increased, but the J_sc value decreased. The V_oc of the cell also increased as the ionization energy of the pyrazoles decreased. These results suggest that the electron donicity of the pyrazole additives affected the interaction with the nanocrystalline TiO₂ photoelectrode, the I⁻/I₃⁻ electrolyte, and the acetonitrile solvent, which changed the Ru(II)-dye-sensitized solar cell performance.     

Theoretical modeling of TiO2/TCO interfacial effect on dye-sensitized solar cell performance

A theoretical model based on an integration of both Schottky barrier model and electron diffusion differential model was developed to determine the TiO₂/TCO interfacial effect on the current–voltage (J–V) characteristics of a dye-sensitized solar cell (DSSC). The thermionic-emission theory was appropriately applied to describe the electron transfer at the TiO₂/TCO interface. A parametric analysis was conducted to study how the photoelectric outputs varied with multiple independent variables, such as Schottky barrier height (φ_b) and temperature. It was found that the variation of the maximum DSSC power output (P_max) was insignificant when φ_b varied at a low value; however, an increase in φ_b exceeding a critical value caused an apparent decrease in the maximum DSSC power output. The theoretical results were quantitatively compared and agreed very well with published theoretical results. The experimental data from literature were found to agree well with the present theoretical results, qualitatively validating the present model. The theoretical model can be applied to facilitate selection of suitable TCO material in DSSC design to avoid the adverse TiO₂/TCO interfacial effect.     

Influence of aminothiazole additives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized nanocrystalline TiO2 solar cell performance

The influence of aminothiazole additives in acetonitrile solution of an I⁻/I₃⁻ redox electrolyte on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′- bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics were investigated under AM 1.5 (100 mW/cm²) for nine different aminothiazole compounds. The aminothiazole additives tested had varying influences on the solar cell performance. Most of the additives enhanced the open-circuit photovoltage (V_oc), but reduced the short circuit photocurrent density (J_sc) of the solar cell. Both the physical and chemical properties of the aminothiazoles were computationally calculated in order to determine the reasons that the additive influenced solar cell performance. The larger the calculated partial charge of the nitrogen atom in the thiazole, the higher the V_oc value. The V_oc value increased as the dipole moment of aminothiazoles in acetonitrile increased. Moreover, the V_oc of the solar cell also increased as the size of the aminothiazole molecules decreased. These results suggest that the electron donicity of the aminothiazole additives influenced the interaction with the TiO₂ photoelectrode, which altered the dye-sensitized solar cell performance.     

DFT investigation of the TiO2 band shift by nitrogen-containing heterocycle adsorption and implications on dye-sensitized solar cell performance

A density functional theory (DFT) method (periodic DMol³) with full geometry optimization was used to investigate the adsorption of nitrogen-containing heterocycles such as 4-t-butylpyridine (TBP) and imidazole on a TiO₂ anatase (1 0 1) surface. Negative shifts of the TiO₂ Fermi level by N-containing heterocycle adsorption were observed. Imidazole adsorption shifted the Fermi level of TiO₂ more negatively than TBP. This shift corresponded to the enhancement of the open-circuit photovoltage (V_oc) and the reduction of the short-circuit photocurrent density (J_sc) in a dye-sensitized TiO₂ solar cell. We are the first to theoretically discover a TiO₂ band shift upon N-containing heterocycles adsorption, and have successfully related this shift to the effect as an additive in an electrolyte solution on dye-sensitized solar cell performance.     

The influence of acid treatment of TiO2 porous film electrode on photoelectric performance of dye-sensitized solar cell

A dye-sensitized solar cell (DYSC) was assembled by adsorbing cis-dithiocyanato-bis (2,2^′-bipyridyl-4,4^′-dicarboxylate) ruthenium (II) onto TiO₂ porous film. The influence of acid treatment of TiO₂ electrode with different kinds and concentrations on the photoelectric performance of DYSC was investigated. It was found that DYSC had better photoelectric performance when the TiO₂ electrode was treated by hydrochloric acid than that by sulfuric acid, nitric acid and phosphoric acid. When the concentration of hydrochloric acid to treat TiO₂ electrode increases from 0 to 0.10 M, the fill factor of DYSC increases, the short-circuit current decreases, the open-circuit photovoltage increases and the absorption amount for TiO₂ porous film to dye molecules decreases. The acid treatment of TiO₂ electrode provides useful information on the mechanism of energy conversion of DYSC.     

Electrodeposition of TiO2/SiO2 nanocomposite for dye-sensitized solar cell

For the working electrode of dye-sensitized solar cell (DSC), TiO₂/SiO₂ nanocomposite materials were electrodeposited on transparent fluorine doped tin oxide-coated glass by cathodic electrodeposition at room temperature. The electrode and DSC fabricated with TiO₂/SiO₂ nanocomposite were characterized with photocurrent density, X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) and a photovoltaic performance test. On the electrodeposition, the addition of an appropriate amount of SiO₂ in the bath containing TiO₂ slurry was essential to achieve the superior crystallinity, photocurrent density and photovoltaic performance of the resulting TiO₂/SiO₂ electrode, which was significantly superior to a bare TiO₂ electrode. This enhanced performance of optimized TiO₂/SiO₂ electrode was ascribed to the role of SiO₂ as an energy barrier, increasing the physical separation of injected electrons and oxidized dyes/redox couple, and thereby retarding the recombination reactions in the resulting DSC.     

Unique TiO2 paste for high efficiency dye-sensitized solar cells

A novel titanium oxide paste based on Pechini sol-gel method and nanocrystalline titanium oxide powder have been successfully developed. Titanium oxide layers possess high inner surface area assuring high dye loading and well-connected nanocrystalline grains assuring good electron transport within the layer. The dye-sensitized layers have been used to assemble dye-sensitized solar cells with acetonitrile- and ionic liquid-based electrolyte. Overall conversion efficiencies of dye-sensitized solar cells (DSSCs) determined under standard test conditions (100 mW/cm², 25 °C and AM 1.5 G) are 10.2% for acetonitrile and 7.3% for ionic liquid-based electrolyte.     

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 electrolyte on the photovoltaic performance of a dye-sensitized TiO2 solar cell based on a Ru(II) terpyridyl complex photosensitizer

We have investigated the influence of electrolyte composition on the photovoltaic performance of a dye-sensitized nanocrystalline TiO₂ solar cell (DSSC) based on a Ru(II) terpyridyl complex photosensitizer (the black dye). We have also spectroscopically investigated the interaction between the electrolyte components and the adsorbed dye. The absorption peaks attributed to the metal-to-ligand charge transfer transitions of the black dye in solution and adsorbed on a TiO₂ film, were red-shifted in the presence of Li cations, which led to an expansion of the spectral response of the solar cell toward the near-IR region. The photovoltaic performance of the DSSC based on the black dye depended remarkably on the electrolyte composition. We developed a novel efficient organic liquid electrolyte containing an imidazolium iodide such as 1,2-dimethyl-3-n-propylimidazolium iodide or 1-ethyl-3-methylimidazolium iodide (EMImI) for a DSSC based on the black dye. A high solar energy-to-electricity conversion efficiency of 9.2% (J_sc=19.0 mA cm⁻², V_oc=0.67 V, and FF=0.72) was attained under AM 1.5 irradiation (100 mW cm⁻²) using a novel electrolyte consisting of 1.5 M EMImI, 0.05 M iodine, and acetonitrile as a solvent with an antireflection film.     

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.     

Increase in photovoltaic performances of dye-sensitized solar cells—Modification of interface between TiO2 nano-porous layers and F-doped SnO2 layers

In order to improve the physical and chemical contacts between a porous TiO₂ layer and an F-doped SnO₂ transparent conductive layer (FTO), the surface of the FTO layer is polished. After polishing, the surface roughness decreased. However, light transmittance and sheet resistance did not vary largely. The short circuit current (J_sc) and efficiencies increased after the FTO was polished. It was found that the interfacial charge transfer between a TiO₂ layer and an FTO layer decreased by impedance measurement, which suggests that contacts between an FTO and a TiO₂ layer are improved because of the flatted surfaces or removal of electrical impurities. We propose one of the industrially important phenomena that surface polishing of FTO is one of the ways to increase photovoltaic performances for DSCs.     

A 4.2% efficient flexible dye-sensitized TiO2 solar cells using stainless steel substrate

An efficient flexible dye-sensitized solar cells (DSSCs) using stainless steel supporting substrate for fabricating nanocrystalline TiO₂ film electrodes were developed, intending to improve the photoelectrochemical properties of plastic substrate-based DSSCs. The most important advantage of a stainless steel-based TiO₂ film electrode over a plastic-based electrode lies in its high-temperature sinterability. Optimal photovoltaic properties were obtained with a cell where the TiO₂ film was coated on both ITO- and SiO_x-sputtered stainless steel (denoted as TiO₂/ITO/SiO_x/StSt). The photocurrent of the flexible cells with a TiO₂/ITO/SiO_x/StSt electrode increased significantly, leading to a much higher overall solar conversion efficiency η of 4.2% at 100 mW/cm², based on short-circuit photocurrent density, open-circuit voltage and fill factor of 11.2 mA/cm², 0.61 and 0.61 V, respectively, than those reported for cells with plastic substrates.     

Nanostructured photoelectrode consisting of TiO2 hollow spheres for non-volatile electrolyte-based dye-sensitized solar cells

A photoelectrode consisting of titania hollow spheres for dye-sensitized solar cells (DSSCs) is prepared by a paste method and the effect of the nanostructure on the performance of DSSCs with non-volatile electrolytes is investigated. The structure of the hollow sphere (HS) electrode with a large pore size and a high porosity allows highly viscous non-volatile electrolytes to penetrate the electrode thoroughly. Furthermore, its outstanding light-harvesting efficiency and long electron diffusion length make the efficiency of the DSSCs with the HS electrode comparable with those of a conventional nanocrystalline electrode, in spite of the smaller amount of the adsorbed dye, when oligomer electrolytes are used. The results show that the structure of a photoelectrode highly improves the performance of the device and the HS electrode is an effective structure for the use of non-volatile electrolytes in DSSCs.     

Effects of silver particles on the photovoltaic properties of dye-sensitized TiO2 thin films

To evaluate the possibility of using the plasmon resonance effect to enhance the efficiency of photochemical cells, cis-(SCN)₂Bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) dye-sensitized cells were used to measure the photoresponse of TiO₂ film electrodes before and after deposition of Ag particles. The deposited Ag particles created a film with Ag islands. We found that the photoresponse in the visible region increased as the mass-equivalent Ag-island film thickness, t_Ag, increased to 3.3 nm, but decreased when t_Ag was further increased to 6 nm. On the other hand, compared with bare TiO₂ films, the photoresponse in the UV region decreased for any level of Ag islands. These results suggest that under proper conditions, enhancement of the optical absorption of the dye by the Ag plasmon resonance effect contributes to the photocurrent, and indicates the possibility of improving the energy conversion efficiency of photoelectrochemical cells with Ag-island films.     

Influence of alkylpyridine additives in electrolyte solution on the performance of dye-sensitized solar cell

The influence of alkylpyridines additive to an I⁻/I₃⁻ redox electrolyte in acetonitrile on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell was studied. I–V measurements were performed using more than 30 different alkylpyridines. The alkylpyridine additives showed a significant influence on the performance of the cell. All the additives decreased the short-circuit photocurrent (J_sc), but most of the alkylpyridines increased the open-circuit photovoltage (V_oc) and fill factor (ff) of the solar cell. The results of the molecular orbital calculations suggest that the dipole moment of the alkylpyridine molecules correlate with the J_sc of the cell. These results also suggest that both the size and ionization energy of pyridines correlate with the V_oc of the cell. Under AM 1.5 (100 mW/cm²), the highest solar energy conversion efficiency (η) of 7.6% was achieved by using 2-propylpyridine as an additive, which was more effective than the previously reported additive, 4-t-butylpyridine.     

Temperature dependence and the oscillatory behavior of the opto-electronic properties of a dye-sensitized nanocrystalline TiO2 solar cell

A dye-sensitized TiO₂ solar cell was developed and characterized. The I–V (current–voltage) characteristics were studied at different temperatures from −40°C to 80°C. The opto-electronic properties of the cell depend on factors like ambient temperature and the time constants of the redox processes at the cell interfaces. The temperature dependence of V_oc and I_sc were clearly demonstrated. I_sc increased with increasing temperature above room temperature, where as V_oc increased with decreasing temperature below room temperature. The opto-electronic properties showed oscillatory behavior especially at low temperatures, which may be attributed to the different velocities of the redox processes occurring at the TiO₂/dye, dye/electrolyte and the electrolyte/counter electrode interfaces.     

Low-temperature oxygen plasma treatment of TiO2 film for enhanced performance of dye-sensitized solar cells

The effects of low-temperature O₂ plasma treatment of a TiO₂ film are studied with the objective of improving the performance of dye-sensitized solar cells (DSSCs). X-ray photoelectron spectra (XPS) reveal that the ratio of titanium dioxide to titanium sub-oxides is increased in the O₂ plasma-treated TiO₂ film, compared with that of the untreated TiO₂ film. This increase suggests that the oxygen vacancies in the film are effectively reduced. The near-edge X-ray absorption fine structure (NEXAFS) spectra results agree with the XPS result. It is proposed that there is a correlation between the shifts of the peaks in the NEXAFS spectra and the adsorption of N719 dye on the TiO₂ particles. A DSSC having an O₂ plasma-treated, 4 μm thick TiO₂ film electrode renders a short-circuit photocurrent of 7.59 mA cm⁻², compared with 6.53 mA cm⁻² for a reference cell with an untreated TiO₂ electrode of the same thickness. As a result of these changes, the solar-to-electricity conversion efficiency of the O₂ plasma-treated cell is found to be 4.0% as compared with 3.5% for the untreated cell. This improvement in the performance is rationalized on the basis of increased N719 dye adsorption on to the TiO₂, due to the reduction in the number of oxygen vacancies caused by the oxygen plasma treatment.