Influence of iodine concentration on the photoelectrochemical performance of dye-sensitized solar cells containing non-volatile electrolyte

The effect of iodine concentration in the electrolyte with non-volatile solvent of dye-sensitized solar cells (DSCs) on photovoltaic performance was studied. The electron transport and interfacial recombination kinetics were also systematically investigated by electron impedance spectroscopy (EIS). With the iodine concentration increased from 0.025 to 0.1 M, open-circuit voltage (V_oc) and photocurrent density (J_sc) decreased while fill factor (ff) increased significantly. The decline of the V_oc and J_sc was mainly ascribed to increased electron recombination with tri-iodide ions (I₃⁻). The increased fill factor was primarily brought by a decrease in the total resistance. From impedance spectra of the solar cells, it can be concluded that increasing the iodine concentration in electrolytes could decrease charge transfer resistance (R_ct) and the chemical capacitance (C_μ), increase the electron transport resistance (R_t), and hence decrease the electron lifetime (τ) and the effective diffusion coefficient (D_n) of electrons in the TiO₂ semiconductor. With optimum iodine concentration, device showed a photocurrent density of 16.19 mA cm⁻², an open-circuit voltage of 0.765 V, a fill factor of 0.66, and an overall photo-energy conversion efficiency of 8.15% at standard AM 1.5 simulated sunlight (100 mW cm⁻²).     

Enhanced performance of a dye-sensitized solar cell with a modified poly(3,4-ethylenedioxythiophene)/TiO2/FTO counter electrode

ClO₄⁻-poly(3,4-ethylenedioxythiophene)/TiO₂/FTO (ClO₄⁻-PEDOT/TiO₂/FTO) counter electrode (CE) in dye-sensitized solar cells (DSSCs) is fabricated by using an electrochemical deposition method. Comparing with the DSSCs with ClO₄⁻-PEDOT/FTO counter electrode, the photocurrent-voltage (I-V) measurement reveals that the photocurrent conversion efficiency (η), fill factor (FF) and short-circuit current density (J_SC) of DSSCs with a ClO₄⁻-PEDOT/TiO₂/FTO CE increase. The enhanced performances of the DSSCs are attributed to the higher J_SC arising from the increase of active surface area of ClO₄⁻-PEDOT/TiO₂/FTO CE. Electrochemical impedance spectra (EIS) also indicate that the charge-transfer resistance on the ClO₄⁻-PEDOT/electrolyte interface decreases. Cyclic voltammetry results indicate that the ClO₄⁻-PEDOT/TiO₂/FTO electrode shows higher activity towards I₃⁻/I⁻ redox reaction than that of ClO₄⁻-PEDOT/FTO electrode.     

Influence of various cations on redox behavior of I and I3 and comparison between KI complex with 18-crown-6 and 1,2-dimethyl-3-propylimidazolium iodide in dye-sensitized solar cells

In this paper, we studied the influence of different cations on redox behavior of I⁻ and I₃⁻ by cyclic voltammetry and electrochemical impedance spectroscopy. It was demonstrated that increasing cation size tends to give rise to higher reductive activity of I⁻ and lower diffusion coefficient of I₃⁻ in these cations, alkylimidazolium, [Na ⊂ 15-C-5]⁺ and [K ⊂ 18-C-6]⁺. Moreover, we measured the performance of dye-sensitized solar cells (DSCs) with electrolyte containing [K ⊂ 18-C-6]I or 1,2-dimethyl-3-propylimidazolium iodide (DMPII). It was found that dye-sensitized solar cells (DSCs) with [K ⊂ 18-C-6]I give a little higher short-circuit photocurrent density and a little lower fill factor than DSCs with DMPII.     

Solid-state dye-sensitized hierarchically structured ZnO solar cells

A novel solid-state hierarchically structured ZnO dye-sensitized solar cell (DSC) was assembled by using TiO₂ as filler in polyethylene oxide (PEO)/polyethylene glycol (PEG) electrolytes and ZnO nanocrystalline aggregates as photoanode film. Under optimized composite polyelectrolyte containing PEO/oligo-PEG/TiO₂/LiI/I₂ the photovoltaic performance of the solid-state ZnO DSCs was significantly better, with an overall conversion efficiency (η) of 1.8% under irradiation of 100 mW/cm², which was higher than those of the cells with PEO/TiO₂/LiI/I₂ (η = 1.1%) or PEO/oligo-PEG/LiI/I₂ electrolyte (η = 1.5%). Further, the hierarchically structured ZnO-based cell showed a higher η value of 2.0% under 60 mW/cm² radiation. The morphologies, ionic conductivity of three different composite electrolytes and their performance to the DSCs were also studied by FESEM, I–V data, IPCE and EIS.     

Preparation and characterization of core-shell electrodes for application in gel electrolyte-based dye-sensitized solar cells

Core-shell electrodes based on TiO₂ covered with different oxides were prepared and characterized. These electrodes were applied in gel electrolyte-based dye-sensitized solar cells (DSSC). The TiO₂ electrodes were prepared from TiO₂ powder (P25 Degussa) and coated with thin layers of Al₂O₃, MgO, Nb₂O₅, and SrTiO₃ prepared by the sol-gel method. The core-shell electrodes were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscopy measurements. J-V curves in the dark and under standard AM 1.5 conditions and photovoltage decay measurements under open-circuit conditions were carried out in order to evaluate the influence of the oxide layer on the charge recombination dynamics and on the device's performance. The results indicated an improvement in the conversion efficiency as a result of an increase in the open circuit voltage. The photovoltage decay curves under open-circuit conditions showed that the core-shell electrodes provide longer electron lifetime values compared to uncoated TiO₂ electrodes, corroborating with a minimization in the recombination losses at the nanoparticle surface/electrolyte interface. This is the first time that a study has been applied to DSSC based on gel polymer electrolyte. The optimum performance was achieved by solar cells based on TiO₂/MgO core-shell electrodes: fill factor of ∼0.60, short-circuit current density J_sc of 12 mA cm⁻², open-circuit voltage V_oc of 0.78 V and overall energy conversion efficiency of ∼5% (under illumination of 100 mW cm⁻²).     

Dye-sensitised photoelectrochemical solar cells with polyacrylonitrile based solid polymer electrolytes

Novel all solid state dye-sensitised photolectrochemical solar cells of the type, FTO-TiO₂-dye-PAN, EC, PC, Pr₄N⁺I⁻, I₂-Pt-FTO, have been fabricated and characterised using current-voltage characteristics and action spectra. Liquid electrolyte generally used for such solar cells has successfully replaced by a quasi solid electrolyte comprised of polyacrylonitrile (PAN) with ethylene carbonate (EC) and propylene carbonate (PC) as plasticisers and Pr₄N⁺I⁻/I₂ redox couple with tetrapropylammoniumiodide as the complexing salt. For the polymer electrolyte, the optimum conductivity of 2.95×10⁻³ S cm⁻¹ was obtained for the electrolyte composition, PAN:EC:PC=15:35:50 (wt.%). The short circuit current density (J_SC) and the open circuit voltage (V_OC) obtained for an incident light intensity of 600 W m⁻² were 3.73 mA cm⁻² and 0.69 V, respectively. This corresponds to an overall quantum efficiency of 2.99%. From the action spectrum, the maximum incident photon conversion efficiency (IPCE) of 33% was obtained for incident light of wavelength 480 nm.     

Photovoltaic performance enhancement of dye-sensitized solar cells by formation of blocking layers via molecular electrostatic effect

The improvement in photovoltaic performance of dye-sensitized solar cells was found after aging in the dark and was analyzed by linear sweep voltammetry and electrochemical impedance spectroscopy. The promotion was found to arise from the formation of blocking layers on the surface of nanocrystalline TiO₂, resulting most likely from the intermolecular electrostatic action between the 4-tert-butylpyridine and the 1,2-dimethyl-3-propylimidazolium ions. These blocking layers can retard the interfacial reaction of the electron with I₃^? ions without deteriorating the rate of regeneration of the oxidized dye molecules. By virtue of the blocking layers, the retarding recombination of electrons with the I₃^? ions significantly increased the electron lifetime and enlarged the electron diffusion length, resulting in a higher open-circuit voltage and an improvement in charge collection efficiency, and eventually an enhancement of the energy conversion efficiency.     

Photoelectrochemical decomposition of bio-related compounds at a nanoporous semiconductor film photoanode and their photocurrent-photovoltage characteristics

Photoelectrochemical decomposition of bio-related compounds such as amino acids was investigated with a biophotochemical cell comprising a mesoporous TiO₂ thin film photoanode and an O₂-reducing cathode. It was concluded that a kind of Schottky junction formed at the surface of the TiO₂ (called as liquid junction) induced the photodecomposition followed by generation of photocurrent/photovoltage. Complete photodecomposition was investigated by the CO₂ formation yield. The photocurrent-photovoltage (J-V) characteristics of amino acids and other typical bio-related compounds were investigated, and the short circuit photocurrent (J_sc), open circuit photovoltage (V_oc), and Fill factor (ff) were exhibited. Effect of pH on the photodecomposition of phenylalanine and cysteine were studied; for cysteine alkaline conditions gave a high efficiency, which was interpreted by the high electron-donating ability of the dissociated -S⁻ group. The incident light-to-current conversion efficiency (IPCE) of cysteine was 25% at 350 nm. It was for the first time shown that organic acids gave high internal quantum efficiency (η′) over 8 (=800%) in the photodecomposition; for oxalic acid it was 9.3 (=930%) and for butyric acid 8.2. The alternating current impedance spectroscopy of glycine showed that the cell performance is determined by the chemical reactions at TiO₂ or Pt electrodes.     

Memristive Switching Characteristics in Biomaterial Chitosan-Based Solid Polymer Electrolyte for Artificial Synapse

This study evaluated the memristive switching characteristics of a biomaterial solid polymer electrolyte (SPE) chitosan-based memristor and confirmed its artificial synaptic behavior with analog switching. Despite the potential advantages of organic memristors for high-end electronics, the unstable multilevel states and poor reliability of organic devices must be overcome. The fabricated Ti/SPE-chitosan/Pt-structured memristor has stable bipolar resistive switching (BRS) behavior due to a cation-based electrochemical reaction between a polymeric electrolyte and metal ions and exhibits excellent endurance in 5 × 10² DC cycles. In addition, we achieved multilevel per cell (MLC) BRS I-V characteristics by adjusting the set compliance current (I_cc) for analog switching. The multilevel states demonstrated uniform resistance distributions and nonvolatile retention characteristics over 10⁴ s. These stable MLC properties are explained by the laterally intensified conductive filaments in SPE-chitosan, based on the linear relationship between operating voltage margin (ΔV_switching) and I_cc. In addition, the multilevel resistance dependence on I_cc suggests the capability of continuous analog resistance switching. Chitosan-based SPE artificial synapses ensure the emulation of short- and long-term plasticity of biological synapses, including excitatory postsynaptic current, inhibitory postsynaptic current, paired-pulse facilitation, and paired-pulse depression. Furthermore, the gradual conductance modulations upon repeated stimulation by 10⁴ electric pulses were evaluated in high stability.     

The use of 2,6-bis (N-pyrazolyl) pyridine as an efficient dopant in conjugation with poly(ethylene oxide) for nanocrystalline dye-sensitized solar cells

Poly(ethylene oxide) (PEO)/2,6-bis (N-pyrazolyl) pyridine (BNPP) polymer electrolyte based photoelectrochemical cells have been fabricated with [cis-dithiocyanato-N, N-bis (2,2′ bipyridyl-4, 4′ dicarboxylic acid)ruthenium(II)] dihydrate (N3 dye) dye complex as the sensitizer and nanoporous TiO₂ film as photo anode. The introduction of 2,6-bis (N-pyrazolyl) pyridine into the poly (ethylene oxide) matrix reduces the crystallinity of the polymer and enhances the mobility of I⁻/I₃⁻ redox couple resulting in an improved performance with a higher conversion efficiency of 8.8% in terms of light energy to electric energy when compared to that of the corresponding dye-sensitized nanocrystalline TiO₂ solar cell.     

The role of gel electrolyte composition in the kinetics and performance of dye-sensitized solar cells

Gel-type polymer electrolytes based on the copolymer poly(ethylene oxide-co-epichlorohydrin) and the plasticizer γ-butyrolactone (GBL) were optimized and applied in dye-sensitized solar cells. The plasticizer added to the electrolyte allowed the dissolution of a higher concentration of salt, reaching conductivity values close to 1 mS cm⁻¹ for the sample prepared with 30 wt% of LiI. Raman spectroscopy confirmed polyiodide formation in the electrolyte when the salt concentration exceeds 7.5 wt%, introducing a significant contribution of electronic conductivity in the electrolyte. The devices were characterized under AM 1.5 conditions and the I-V curves were fitted using a two diode equation. Increasing the concentration of LiI-I₂ accelerates dye cation regeneration as measured by transient absorption spectroscopy; however, it also contributes to an increase in the dark current of the cell by one order of magnitude. The best performance was achieved for the solar cell prepared with the electrolyte containing 20 wt% of LiI, with efficiencies of 3.26% and 3.49% at 100 and 10 mW cm⁻² of irradiation, respectively.     

Effects of 1,3-dialkylimidazolium cations with different lengths of alkyl chains on the Pt electrode/electrolyte interface in dye-sensitized solar cells

The electrochemical behaviors of the tri-iodide (I₃⁻)/iodide (I⁻) redox couple of symmetric cells were investigated by cyclic voltammetry, steady-state polarization, chronocoulometry, and electrochemical impedance spectroscopy. 1,3-Dialkylimidazolium cations affected the characteristics of the Pt electrode/electrolyte interface by adsorbing on the Pt electrode, as a result of electrostatic interactions, and further affected the reduction of I₃⁻ on the Pt electrode. Capacitance of the double layers of the Pt electrode/electrolyte interface was chiefly determined by capacitance of the compact layer according to the Helmoholtz theory.     

Thin film deposition of metal oxides in resistance switching devices: electrode material dependence of resistance switching in manganite films

The electric-pulse-induced resistance switching in layered structures composed of polycrystalline Pr_1−xCa_xMnO₃ (PCMO) sandwiched between Pt bottom electrode and top electrodes of various metals (metal/PCMO/Pt) was studied by direct current current–voltage (I-V) measurements and alternating current impedance spectroscopy. The I-V characteristics showed nonlinear, asymmetric, and hysteretic behavior in PCMO-based devices with top electrode of Al, Ni, and Ag, while no hysteretic behavior was observed in Au/PCMO/Pt devices. The PCMO-based devices with hysteretic I-V curves exhibited an electric-pulse-induced resistance switching between high and low resistance states. Impedance spectroscopy was employed to study the origin of the resistance switching. From comparison of the impedance spectra between the high and low resistance states, the resistance switching in the PCMO-based devices was mainly due to the resistance change in the interface between the film and the electrode. The electronic properties of the devices showed stronger correlation with the oxidation Gibbs free energy than with the work function of the electrode metal, which suggests that the interface impedance is due to an interfacial oxide layer of the electrode metal. The interface component observed by impedance spectroscopy in the Al/PCMO/Pt device might be due to Al oxide layer formed by oxidation of Al top electrode. It is considered that the interfacial oxide layer plays a dominant role in the bipolar resistance switching in manganite film-based devices.     

A novel diphenylphosphinic acid coadsorbent for dye-sensitized solar cell

Diphenylphosphinic acid (DPPA) was adopted as a novel coadsorbent in dye-sensitized solar cells (DSCs) based on nanocrystalline TiO₂ sensitized with N719 dye [(Bu₄N)₂[Ru(dcbpyH)₂(NCS)₂]], leading to a significant enhancement of the cell's performance. Different ratios of dye-to-coadsorbent caused varying results, including a 12.5% increase in overall conversion efficiency coupled with a 10.6% increase in short-circuit current with a ratio of 2:1. Electrochemical impedance spectroscopy (EIS) results indicated that the augment ascribes to inhibited interfacial charge recombination between the conduction band electrons and triiodide ions in the electrolyte. On the other hand, different ratios caused different shift directions of the TiO₂ conduction band, which was confirmed by charge transport resistance obtained also from EIS analysis. To be specific, when the ratio of N719-to-DPPA was 2:1 and 1:1, the conduction band of TiO₂ film was positively shifted, while it was negatively shifted when the ratio was 4:1.     

Enhancing the performance of dye-sensitized solar cells based on an organic dye by incorporating TiO2 nanotube in a TiO2 nanoparticle film

The photoelectrochemical properties of a high molar extinction coefficient charge transfer organic dye containing thienylfluorene segment called FL, and the effect of incorporating TiO₂ nanotube (TiNT) in TiO₂ nanoparticle film along with the above dye on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were investigated. The influence of soaking time of the TiO₂ electrode in dye solution and the effect of varying its concentration, on the solar cell efficiency was also studied. Cyclic voltammetric (CV) analysis revealed the linear relationship between the anodic peak current and the scan rate, indicating a surface-confined diffusion process.The surface morphology of TiNT was characterized using SEM, TEM and XRD. The open-circuit voltage (V_OC) of the DSSC increased with the increase in the wt% of TiNT and shows optimal value at about 5 wt%, which is correlated with the suppression of the electron recombination as found out from the electron lifetime studies.The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance (R_ct) and electron lifetime under different ratios of the TiNT/nanoparticle. The electron lifetimes of the DSSCs based on FL and N3 dye were very close to one another and the DSSC based on the FL showed respectable photovoltaic performance of ca. 7.8% under the light intensity of 100 mW cm⁻² (AM 1.5G).     

EIS-assisted performance analysis of non-noble metal electrocatalyst (Fe-N/C)-based PEM fuel cells in the temperature range of 23-80 °C

A carbon-supported non-noble metal catalyst, Fe-N/C, was used as the cathode catalyst to construct membrane electrolyte assemblies (MEAs) for a proton exchange membrane (PEM) fuel cell. The performance of such a fuel cell was then tested and diagnosed using electrochemical impedance spectroscopy (EIS) in the temperature range of 23-80 °C. Based on the EIS measurements, individual resistances, such as charger transfer resistance and membrane resistance, were obtained and used to simulate polarization curves (current-voltage (I-V) curves). A close agreement between the simulated I-V curves and the measured curves demonstrates consistency between the polarization and EIS data. The temperature-dependent parameters obtained via EIS, such as apparent exchange current densities and electrolyte membrane conductivities, were also used to acquire activation energies for both the oxygen reduction reaction (ORR) catalyzed by an Fe-N/C catalyst and the proton transport process across the electrolyte membrane. In addition, the maximum power densities for such a fuel cell were also analyzed.     

Anodic TiO2 Nanotube Arrays for Dye-Sensitized Solar Cells Characterized by Electrochemical Impedance Spectroscopy

This paper reports on the microstructure of anodic titanium oxide (TiO₂) and its use in a dye-sensitized solar cell (DSSC) device. When voltages of 60 V were applied to titanium foil for 2 hr under 0.25 wt% NH₄F+ 2 vol% H₂O+C₂H₄(OH)₂, TiO₂ with a nanotube structure was formed. The film, which had a large surface area, was used as an electron transport film in the DSSC. The DSSC device had a short-circuit current density (J_sc) of 12.52 mA cm⁻², a fill factor (FF) of 0.65, an open-voltage (V_oc) of 0.77 V, and a photocurrent efficiency of 6.3% under 100% AM 1.5 light. The internal impedance values under 100%, 64%, 11%, and 0% (dark) AM 1.5 light intensities were measured and simulated using the electrical impedance spectroscopy (EIS) technique. The impedance characteristics of the DSSC device were simulated using inductors, resistors, and capacitors. The Ti/TiO₂, TiO₂/Electrolyte, electrolyte, and electrolyte/(Pt/ITO) interfaces were simulated using an RC parallel circuit, and the bulk materials, such as the Ti, ITO and conducting wire, were simulated using a series of resistors and inductors. The impedance of the bulk materials was simulated using L₀+R₀+R_b, the impedance of the working electrode was simulated using (C₁//R₁)//(R_a+(C₂//R₂), the electrolyte was simulated using C₃//R₃, and the counter electrode was simulated using C₄//R₄.     

Effect of pyridine in electrolyte on the current-voltage characteristics in dye-sensitized solar cells

This study aimed to investigate the effect of pyridine in electrolyte on the photocurrent density-photovoltage (J-V) characteristics in dye-sensitized solar cells. When the concentration of pyridine (c_p) in an electrolyte (0.1 M LiI and 0.05 M I₂ in acetonitrile) is increased from 0 to 0.4 M, the open-circuit voltage (V_oc) increased by 0.14 V, whereas the short-circuit photocurrent density (J_sc) decreased by 3.0 mA cm⁻². The optimum c_p for the maximum cell performance was around 0.1-0.2 M. The model calculation predicted that by increasing c_p from 0 to 0.4 M, the electron diffusion coefficient (D) decreases by more than one order of magnitude and the lifetime of conduction band-free electrons (τ) increases by more than one order of magnitude, implying that the electron diffusion length () increases by 31%, whereas the electron injection flux from dye molecules to TiO₂ nanoporous films (Φ) decreases by 28%.     

Fe3O4–carbon black nanocomposite as a highly efficient counter electrode material for dye-sensitized solar cell

Iron oxide–carbon black (Fe₃O₄–CB) nanocomposite has been proposed as a cost–effective alternative counter electrode (CE) to the conventional Pt in a dye sensitized solar cell (DSSC). The Fe₃O₄–CB nanocomposite at three different weight ratio (1:1, 1:2 and 2:1) was prepared by a simple solution mixing process and the material was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning and transmission electron microscopy techniques. The performance of the three Fe₃O₄–CB nanocomposite CEs was assessed with respect to Fe₃O₄, CB and Pt in a conventional DSSC consisting of N719 dye sensitized TiO₂ (P25) photoanode in contact with an electrolyte containing the I⁻/I₃⁻ redox couple. The electrocatalytic activities of the various CEs towards triiodide (I₃⁻) reduction were analyzed by cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization analysis. The photocurrent voltage (J–V) characteristics of the DSSCs assembled with various counter electrodes were assessed at a light intensity of 80 mW cm⁻². The DSSC assembled with Fe₃O₄–CB nanocomposite (1:2) CE showed the best photovoltaic performance in terms of a high power conversion efficiency of 6.1% which is superior to that of sputtered Pt (4.1%). The simple preparation, excellent electrocatalytic activity and low-cost nature allow the Fe₃O₄–CB nanocomposite to be a promising alternative CE to Pt for use in DSSCs.     

Influence of 4-N,N-dimethylaminopyridine on the photovoltaic performance of dye-sensitized solar cells with poly(ethyleneoxide)/oligo(ethylene glycol) blend electrolytes

4-N,N-Dimethylaminopyridine (DMAP) was introduced into poly(ethyleneoxide)/oligo(ethylene glycol) (PEO/PEG) electrolytes for dye-sensitized solar cells (DSCs). The improved photovoltaic performance of DMAP-doped DSCs was attributed to the integrated effects of the upward displacement of the TiO₂ band edge and the decrease in the electron recombination rate. Remarkably, the presence of DMAP suppresses electron recombination via two combined pathways involving the dissociation of triiodide to iodide by a complexation reaction and a modification of the surface state distribution in the band gap of TiO₂. With the addition of DMAP, the open-circuit voltage enhances dramatically. The short-circuit photocurrent density has a small increase at low DMAP concentration and drops afterwards. The power conversion efficiency is 4.07%, which corresponds to a 63% increase over that of the DSC without DMAP.