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.     

Effects of tetrabutoxytitanium on photoelectrochemical properties of plastic-based TiO2 film electrodes for flexible dye-sensitized solar cells

The flexible DSSCs based on conducting plastic substrates are fabricated using electrodes made of tetrabutoxytitanium (TBOT) mixed with P25 TiO₂ nanoparticles at low temperature. To investigate the effects of TBOT on the flexible dye-sensitized solar cells, electrochemical impedance spectroscopy (EIS) is performed in the dark and under illumination conditions. Resistances for electron transport through TiO₂, charge-transfer resistance related to the TiO₂/redox electrolytes interface recombination, electron transport time and electron lifetime are quantified under different weight ratios of TBOT/P25. Additionally, the photovoltaic characteristics I-V curves and incident photon to current conversion efficiencies (IPCE) of flexible anodes made of different weight ratios of TBOT/P25 are obtained as well. It is found that the electrode under weight ratio 0.17 has the smallest inherent resistance, longest electron transport time and electron lifetime, lowest recombination rate and best performance with conversion efficiency 3.94%. These results indicate that after the weight ratios of TBOT/P25 is optimized, TBOT could enhance the interconnection between the TiO₂ particles, improve the conductivity of the electrode and decrease the charge recombination. Above results demonstrate that adding TBOT to TiO₂ is an easy and efficient method to improve the performance of the flexible DSSC fabricated at low temperature.     

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.     

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.     

The effect of pre-thermal treatment of TiO2 nano-particles on the performances of dye-sensitized solar cells

We have demonstrated the effect of pre-thermal treatment of TiO₂ nano-particles on the performances of dye-sensitized solar cells (DSCs) by using high specific surface area and anatase only TiO₂ nano-particles (ca. 340 m²/g, Sachtleben Chemie GmgH, represented as HK). TiO₂ particles and thin films were characterized with X-ray diffraction, FT-IR, UV–Vis diffuse reflectance spectroscopy and FE-SEM. The photoelectrochemical properties of the thin films and the performances of DSCs were measured by photocurrent densities, AC impedance spectra and photocurrent–voltage curves. Before coating the raw TiO₂ of HK (HK-raw) on transparent conducting oxide (TCO) glass for DSC fabrication, pre-thermal treatment of HK-raw by calcining at 450 °C (HK-450) was an essential step to achieve the optimum properties in terms of morphological feature, crystallinity, specific surface area and photocurrent density. HK-450 film showed the high adsorption of dye, high photocurrent density and low interface resistance between TiO₂ and TCO glass, R_TiO2/TCO and TiO₂ and redox electrolyte, R_CT, resulting in the superior photovoltaic performance on the DSC fabricated with HK-450 and Eosin Y (or ruthenium 535 bis-TBA) at AM 1.5: open-circuit voltage of 0.62 V (0.77 V), short-circuit current of 3.03 mA/cm² (22.80 mA/cm²), fill factor of 0.57 (0.44) and overall conversion efficiency of 1.06%, (7.52%). Accordingly, the optimization between the morphological feature, specific surface area and photocurrent density of TiO₂ substrate is promising to accomplish the improved overall conversion efficiency of DSC.     

Doubled layered ITO/SnO2 conducting glass for substrate of dye-sensitized solar cells

The effects of indium tin oxide (ITO) and ITO/SnO₂ conducting substrates on photovoltaic properties of dye-sensitized solar cells (DSCs) using nanocrystalline TiO₂ were studied. The decrease in fill factor of the DSCs was correlated to the increase in resistance of conducting substrate. The heat stability of ITO conducting glass was improved by depositing SnO₂ on ITO layer. The efficiency of the cells using double layered ITO/SnO₂ substrate remarkably increased comparing with that of the cells using ITO substrates. It is worth mentioning that increasing in sintering time, which enhanced the electronic contact between substrate and TiO₂, also modified the cell performance of MP-TiO₂ cells. Our experimental finding suggests that 3000 Å ITO substrate, which was covered by 1000 Å SnO₂ layer, exhibited the best properties for the DSCs.     

Stability of the SnO2/MgO dye-sensitized photoelectrochemical solar cell

Dye-sensitized solar cells made of TiO₂ are extensively studied as a cheap alternative to conventional photovoltaic cells. The other familiar stable oxide material of similar band gap suitable for dye sensitization is SnO₂. Although cells based only of SnO₂ are prone to severe recombination losses, the cells made of SnO₂/MgO films where the SnO₂ crystallite is surface covered with an ultra-thin shell of MgO, deliver reasonably high efficiencies. It is found that SnO₂/MgO cells resist dye and electrolyte degradation better than TiO₂ cells. Furthermore, the ultra-thin barrier of MgO on SnO₂ remains intact during prolonged usage or storage of the cell.     

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.     

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.     

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.     

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.     

Incorporating carbon nanotube in a low-temperature fabrication process for dye-sensitized TiO2 solar cells

Dye-sensitized solar cells (DSSCs) incorporating TiO₂ porous films, prepared at a low temperature (150 °C), along with multi-wall carbon nanotubes (MWCNTs) were studied using two different electrolytes, namely LiI and THI. Electrochemical impedance spectroscopy (EIS) was employed to quantify the charge transport resistance and electron lifetime (τ_e) under different levels (wt%) of MWCNTs and electrolytes. The charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) increased as a function of the MWCNT concentration, which ranged 0.1-0.5 wt%, due to a decrease in the surface area and decreased dye adsorption. The characteristic peak shifted to a lower frequency at 0.1 wt% of MWCNT, indicating a longer electron lifetime. The DSSC with the TiO₂ electrode containing 0.1 wt% of MWCNT resulted in a higher short-circuited current density (J_SC) of 9.08 mA/cm², an open-circuit voltage (V_OC) of 0.781 V, and a cell conversion efficiency of 5.02%. EIS was also conducted under dark conditions. The large value at a middle frequency represented electron transport at the TiO₂/dye/electrolyte interface (R_rec). The R_rec for 0.1 wt% MWCNT/TiO₂ was found to be 114 Ω, and for those with 0.3 and 0.5 wt% were 35 and 30 Ω, respectively. The significantly higher value of R_rec suggested that the charge recombination between injected electrons and electron acceptors in the redox electrolyte, I₃⁻, was remarkably retarded. Finally, electrolytes with LiI and THI were used to compare the cell conversion performance under the same conditions. It was found that more electrons were injected in the TiO₂ electrode and the electron recombination reaction was faster in the DSSC with THI than that with LiI.     

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.     

Enhanced DSSC performance with high surface area thin anatase TiO2 nanoleaves

Thin anatase TiO₂ nanoleaves (NLs) with high surface area (∼93 m²/g) are synthesized by hydrothermal route and dye-sensitized solar cells (DSSC) made using such NLs are compared with those made with hydro-thermally synthesized TiO₂ nanoparticles and the Degussa P25 powder. The NLs-based DSSCs show increment of 16% and 24% in the total conversion efficiency over the cells made with NPs and P25 respectively. Interestingly, the highest increment in the conversion efficiency (∼35%) is achieved for cells made using a 50:50 (wt/wt) mixture of NLs and NPs. In this case the dye loading is found to be >50% higher than the case of NP films. Moreover the DC resistance of diffusion of in electrolyte (as revealed by the electrochemical impedance spectroscopy) is substantially lower than the NP film case, other cell parameters being nominally comparable.     

Thickness effect of RF sputtered TiO2 passivating layer on the performance of dye-sensitized solar cells

We report on the characteristics of a TiO₂ passivating layer grown by radio frequency (RF) magnetron sputtering on F-doped SnO₂ (FTO) electrodes as a function of its thickness. The optical transparency, surface roughness and passivation properties of the TiO₂ layer passivating the FTO electrode depend on the thickness of the TiO₂ passivating layer. In addition, it was found that the power conversion efficiency of the dye-sensitized solar cells (DSSCs) is critically dependent on the thickness of RF sputtered TiO₂ layer inserted between FTO electrode and nanoporous TiO₂ layer. The DSSC fabricated on 50 nm thick TiO₂ passivating FTO electrode showed the maximum power conversion efficiency of 4.42% due to effective prevention of the electron transfer to electrolyte. This indicates that the thickness optimization of the TiO₂ passivating layer is one of the important parameter to obtain high performance DSSCs.     

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.     

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.     

Influences of different TiO2 morphologies and solvents on the photovoltaic performance of dye-sensitized solar cells

The effects of TiO₂ photoelectrode's surface morphology and different solvents on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were studied. By successive coating of TiO₂ suspension, composed of low and high molecular weight poly(ethylene)glycol (PEG) as a binder, double layered TiO₂ photoelectrodes with four different structures were obtained. Among the DSSCs with different TiO₂ electrodes, DSSC with P2P1 electrode (P2 and P1 correspond to PEG molecular weights of 20,000 and 200,000, respectively) showed higher performance under identical film thickness at a constant irradiation of 100 mW cm⁻², which may be correlated with large pore size and high surface area of the corresponding TiO₂ electrode. This was confirmed by electrochemical impedance spectroscopy (EIS) analysis of the DSSC and the transient photovoltage measurement of electrons in the TiO₂ electrode. Among the different solvents investigated here, the DSSC containing acetonitrile showed high conversion efficiency and the order of performance of the DSSCs with different solvents were AN > MPN > PC > GBL > DMA > DMF > DMSO. Better correlation was observed between the donor number of solvents and photoelectrochemical parameters of the DSSCs containing different solvents rather than the measured viscosity and dielectric constant of solvents. The reasons for the low performance of the DSSCs containing DMA, DMSO and DMF, respectively, were due to the negative shift of TiO₂ conduction band and the desorption of dye molecules from the TiO₂ photoelectrode by those solvents.     

ITO/ATO/TiO2 triple-layered transparent conducting substrates for dye-sensitized solar cells

A novel transparent conductive oxide film based on the triple-layered indium tin oxide (ITO)/antimony-doped tin oxide (ATO)/titanium oxide (TiO₂) has been developed for dye-sensitized solar cells by using radio frequency magnetron sputtering technique. Effects of the absence and presence of TiO₂ layer and the ITO layer thickness were investigated. Deposition of ATO layer was found to stabilize the thermal instability of ITO. Little change in sheet resistance and optical transmittance was observed by introduction of insulating thin TiO₂ layer on top of the ATO layer, whereas photovoltaic performance was significantly influenced. The conversion efficiency was improved from 4.57% without TiO₂ layer to 6.29% with TiO₂ layer. The enhanced photovoltaic performance with addition of TiO₂ layer was attributed mainly to the improved adhesion and partially to the reduced electron loss at the ITO/ATO conductive layer. Increase in the ITO layer thickness resulted in a slight decrease in photocurrent due to the reduced optical transmittance. When compared with the conventional fluorine-doped tin oxide (FTO), the ITO/ATO/TiO₂ conductive material exhibited similar photocurrent density but higher photovoltage and fill factor, resulting in better conversion efficiency.     

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.