Novel agarose polymer electrolyte for quasi-solid state dye-sensitized solar cell

Quasi-solid state dye-sensitized solar cells (DSSCs) are fabricated with a novel polysaccharide gel electrolyte composed of agarose in 1-methyl-2-pyrrolidinone (NMP) as polymer matrix, lithium iodide (LiI)/iodine (I₂) as redox couple and titania nanoparticles as fillers. The polysaccharide electrolyte with different agarose concentrations (1-5 wt%) and various inorganic filler TiO₂ concentrations (0-10 wt%) are studied systematically by differential scanning calorimetry (DSC) and the AC impedance spectra. The electrochemical and photoelectric performances of DSSCs with these electrolytes are also investigated. It is found that increasing agarose and inorganic filler concentration leads to a decrease in T_g in the range of 1-2 wt% for agarose and 0-2.5 wt% for TiO₂ changed electrolytes, which results in high conductivity in these electrolytes. From the electrochemical analysis, it is observed that the electron lifetime in TiO₂ of DSSCs increases with agarose, while decreases with inorganic filler contents. The prolonged electron lifetime in DSSCs is advantageous to improve open-circuit voltage (V_oc). Based on these results, the cell with the electrolyte of 2 wt% agarose shows the optimized energy conversion efficiency of 4.14%. The optimized efficiency of the DSSC with added titania is 4.74% at 2.5 wt% titania concentration.     

Enhancement in the performance of dye-sensitized solar cells containing ZnO-covered TiO2 electrodes prepared by thermal chemical vapor deposition

A ZnO-covered TiO₂ (denoted as ZnO/TiO₂) film was prepared by incorporating a small quantity of particulate ZnO in a TiO₂ matrix by thermal chemical vapor deposition. When used in a dye-sensitized solar cell, an enhancement was observed in both short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc) by 12% and 17%, respectively, relative to those of a cell containing a bare TiO₂ film. The observed J_sc enhancement is attributed to the increase in the surface area of the ZnO/TiO₂ film, and the V_oc enhancement to the formation of a potential barrier by ZnO at TiO₂/electrolyte interface. The films were characterized by FE-SEM, EDX, and XRD.     

Performance of a new polymer electrolyte incorporated with diphenylamine in nanocrystalline dye-sensitized solar cell

A new solvent-free composite polymer electrolyte consisting of poly(ethylene oxide) (PEO) incorporated into diphenyl amine (DPA) along with KI and I₂ has been developed. The current-voltage characteristics of this nanocrystalline dye-sensitized solar cell measured under simulated sunlight with 1.5 AM at 60 mW/cm² have indicated that this cell generates a photocurrent of 10.2 mA/cm², together with a photovoltage of 810 mV and fill factor of 0.47 yielding an overall energy conversion efficiency of 6.5%. This result suggests that the electron donicity of DPA influences the interaction of nanocrystalline TiO₂ electrode and I⁻/I₃⁻ electrolyte, leading to a high performance of the fabricated solar cell.     

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.     

Influence of alkylaminopyridine additives in electrolytes on dye-sensitized solar cell performance

The influence of alkylaminopyridine additives 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 with an I⁻/I₃⁻ redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 20 different alkylaminopyridines under AM 1.5 (100 mW/cm²). The alkylaminopyridine additives tested had varying effects on the performance of the cell. All the additives decreased the short circuit photocurrent density (J_sc), but increased the open-circuit photovoltage (V_oc) of the solar cell. Molecular orbital calculations imply that the dipole moment of the alkylaminopyridine molecules influences the J_sc of the cell and that the size, solvent accessible surface area, and ionization energy all affect the V_oc of the cell. The highest V_oc of 0.88 V was observed in an electrolyte containing 4-pyrrolidinopyridine, which is comparable to the maximum V_oc of 0.9 V for a cell consisting of TiO₂ electrode and I⁻/I₃⁻ redox system.     

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.     

An analytical study of the porosity effect on dye-sensitized solar cell performance

The porosity in a dye-sensitized solar cell (DSSC) can affect light absorption and electron diffusion that govern the overall electrical current–voltage (I–V) characteristics. In this research, two methods, namely, constant overlap and variable overlap, were developed to determine the connectivity of dye-sensitized TiO₂ particles in high and low porosity levels, respectively. In turn, the light absorption coefficient α and the electron diffusion coefficient D were analytically derived in terms of the porosity P. Subsequently, the electron diffusion differential equation involving α and D was solved for the I–V output as a function of P. A parametric analysis showed that the optimal porosity was equal to 0.41 for maximum I–V output. The analytical results agree well with experimental data reported in the literature. Besides DSSC, the analytical model can be applied to predict the performance of solid-state DSSC as well as dye-sensitized photoelectrochemical cells applied to hydrogen production and water purification.     

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.     

Exploring the role of the spacers and acceptors on the triphenylamine-based dyes for dye-sensitized solar cells

Six triphenylamine-based dyes were explored for their application in dye-sensitized solar cells (DSSCs). Dyes 1–3 and dyes 4–6 possess cyanoacrylic acid (C-acceptor) and rhodanine-3-acetic acid (R-acceptor), respectively. Stilbene (in dyes 2 and 5) and bis(styryl)benzene (in dyes 3 and 6) were used as π-spacers. There is no π-spacer in the dye 1 and 4. To elucidate the role of π-spacers, optical, electrochemical, and photovoltaic properties of the dyes were studied. Among C-acceptor dyes, dye 2 exhibits the highest light-to-electricity conversion efficiency of 4.45%, followed by dye 3 (4.16%). Similarly, among R-acceptor dyes, dye 5 is the best. These results indicate that stilbene is a better π-spacer over bis(styryl)benzene. Although bis(styryl)benzene could extend the light absorption range (in dye-adsorbed TiO₂ film), its tendency to promote intermolecular π-π stacking is possibly the reason for its poor performance in DSSCs. Furthermore, the conjugation break in the R-acceptor moiety attached to the TiO₂ surface limits the electron injection of R-acceptor dyes poorer than C-acceptor dyes. Density functional theory calculations were performed for the dye-(TiO₂)₈ cluster, assuming a bidentate chelation of a carboxylic acid group with Ti⁴⁺ of TiO₂ anatase. The natural bond orbital (NBO) analysis indicated relatively more electron-accepting ability of cyanoacrylic acid over rhodanine-3-acetic acid.     

Enhancing photoelectrical performance of dye-sensitized solar cell by doping with europium-doped yttria rare-earth oxide

The doped rare-earth oxide europium-doped yttria (Y₂O₃:Eu³⁺) is introduced into the TiO₂ film electrode in a dye-sensitized solar cell. As a luminescence medium, Y₂O₃:Eu³⁺ improves the light harvesting via a conversion luminescence process and increases the photocurrent, as a p-type dopant, Y₂O₃:Eu³⁺ elevates the energy level of the oxide film and increases the photovoltage. When the TiO₂ electrode is doped by 3 wt.% Y₂O₃:Eu³⁺, the cell light-to-electric conversion efficiency is improved by a factor of 1.14 compared to that of a cell without Y₂O₃:Eu³⁺ doping.     

Effect of a redox electrolyte in mixed solvents on the photovoltaic performance of a dye-sensitized solar cell

The effect of the iodide/triiodide redox electrolyte in various organic solvents on the photoelectrochemical properties of bis(tetrabutylammonium) cis-bis(thiocyanato)bis(4-carboxy-2,2′-bipyridine-4′-carboxylato)ruthenium(II)-sensitized nanocrystalline TiO₂ solar cells was studied. Solvents with large donor numbers dramatically enhanced the open-circuit voltage (V_oc), but usually reduced the short-circuit photocurrent density (J_sc). For a mixed solvent of tetrahydrofuran (THF) and acetonitrile, V_oc increased and the fill factor decreased with increasing THF concentration, but J_sc remained relatively constant. As the partial charge of the N or O atom of the solvent molecule increased, V_oc increased, but J_sc was unchanged up to a certain value of the partial charge (for THF, −0.46). For cells using 0.3 M 4-tert-butylpyridine and 20 vol% THF in the electrolyte, a short-circuit photocurrent density of 18.23 mA cm⁻², an open-circuit voltage of 0.73 V, a fill factor of 0.73, and an overall conversion efficiency of 9.74% were obtained.     

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.     

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.     

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.     

A quasi-solid-state dye-sensitized solar cell based on the stable polymer-grafted nanoparticle composite electrolyte

A novel gel electrolyte was prepared by dispersing the polymer-grafted ZnO nanoparticle into liquid electrolyte. This gel electrolyte behaves long-term stability as the poly(ethylene glycol methyl ether) molecules are strongly connected to ZnO nanoparticles with covalent bond in polymer-grafted ZnO nanoparticle. A quasi-solid-state dye-sensitized solar cell (DSC) based on this gel electrolyte yields the energy transfer efficiency of 3.1% at AM 1.5 direct irradiation of 75 mW cm⁻² light intensity. Addition of 4-tert-butylpyridine into the electrolyte results in dramatically improved short circuit current density I_sc, and the overall efficiency is also improved to 5.0%, while the open circuit voltage (V_oc) and fill factor (ff) are insensitive to the presence of 4-tert-butylpyridine. DSC fabricated with this novel gel electrolyte displays better thermal stability than those solidified with the conventional nanoparticle ZnO(Ac).     

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.     

Enhanced performance of a quasi-solid-state dye-sensitized solar cell with aluminum nitride in its gel polymer electrolyte

The effects of incorporation of aluminum nitride (AlN) in the gel polymer electrolyte (GPE) of a quasi-solid-state dye-sensitized solar cell (DSSC) were studied in terms of performance of the cell. The electrolyte, consisting of lithium iodide (LiI), iodine (I₂), and 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile (MPN), was solidified with poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP). The 0.05, 0.1, 0.3, and 0.5 wt% of AlN were added to the electrolyte for this study. XRD analysis showed a reduction of crystallinity in the polymer PVDF-HFP for all the additions of AlN. The DSSC fabricated with a GPE containing 0.1 wt% AlN showed a short-circuit current density (J_SC) and power-conversion efficiency (η) of 12.92±0.54 mA/cm² and 5.27±0.23%, respectively, at 100 mW/cm² illumination, in contrast to the corresponding values of 11.52±0.21 mA/cm² and 4.75±0.08% for a cell without AlN. The increases both in J_SC and in η of the promoted DSSC are attributed to the higher apparent diffusion coefficient of I⁻ in its electrolyte (3.52×10⁻⁶ cm²/s), compared to that in the electrolyte without AlN of a DSSC (2.97×10⁻⁶ cm²/s). At-rest stability of the quasi-solid-state DSSC with 0.1 wt% of AlN was found to decrease hardly by 5% and 7% at room temperature and at 40 °C, respectively, after 1000 h duration. The DSSC with a liquid electrolyte showed a decrease of about 40% at room temperature, while it virtually lost its performance in about 150 h at 40 °C. Explanations are further substantiated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and by porosity measurements.     

Influence of polymer concentration and dyes on photovoltaic performance of dye-sensitized solar cell with P(VdF-HFP)-based gel polymer electrolyte

A series of gel polymer electrolytes (GPEs) is synthesized using Poly(vinylidenefluoride-hexafluoropropylene) P(VdF-HFP) as the host matrix and propylene carbonate (PC)–diethyl carbonate (DEC) as plasticizers to fabricate dye-sensitized solar cells. Equal amounts of PC and DEC are used to comprehend high dielectric constant and low viscosity of the electrolyte. The as-prepared GPEs are characterized by XRD, FTIR and SEM. Their thermal properties and ionic conductivities are investigated by TGA/DSC analyses and AC impedance measurements, respectively. The optimized gel polymer electrolyte gives a maximum ionic conductivity of 5.25 × 10⁻³ S cm⁻¹ at room temperature. The formation of porous structure in the electrolyte film supports the entrapment of large volumes of liquid electrolyte inside its cavities. The role of N3 and N719 dyes are also investigated for better photovoltaic performance of DSSC. The overall light-to-electrical-energy conversion efficiencies of 3.95% and 4.41% are obtained for N3 and N719 dyes, respectively, under 100 mW cm⁻² irradiation, which are comparable to those obtained from the corresponding liquid electrolyte cell.     

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 role of the buffer layer in the light of a new equivalent circuit for amorphous silicon solar cells

Although the beneficial effect of the buffer layer between the p- and i-layer of amorphous silicon solar cells has been known for many years, the role of this layer is controversial. This paper examines the effect of the buffer layer using a new equivalent circuit for these devices (Merten et al. IEEE Trans. Electron Dev. 45 (1988) 423–429 [1]). The parameters of this model can be easily assessed by variable illumination measurements (VIM) of the devices' I(V)-curve. With the model, collection of carriers in the bulk of the cell is easy and clearly separated from the diode behaviour of the device. The VIM-method allows for a complete analysis of the thin film cells, covering both technological and physical topics. It is shown that the dominant effect increasing the efficiency of the cells with buffer layer is the reduction of the hole injection from the p-layer which leads to a reduced diode term. The buffer layer only slightly reduces the recombination in the i-layer. This reduction mainly occurs in a region close to the p/i-interface and cannot be observed with red light (homogeneous carrier generation).