On the photophysical and electrochemical studies of dye-sensitized solar cells with the new dye CYC-B1

In this study, the photoelectrochemical characteristics of a ruthenium photosensitizer with an alkyl bithiophene group, designated as CYC-B1, are studied. The effect of mesoporous TiO₂ film thickness on the photovoltaic performance of CYC-B1 and N3 dye-sensitized solar cells was investigated. The performance of the dye-sensitized nanocrystalline TiO₂ solar cells (DSSC) fabricated using CYC-B1 dye-anchored TiO₂ photoelectrode showed a convincing enhancement in cell efficiency when the TiO₂ film thickness was increased from 3 μm (eff.=5.41%) to 6 μm (eff.=7.19%). The efficiency of the CYC-B1-sensitized DSSC was maximum at 6 μm of the TiO₂ film thickness, reached its limiting value and remained constant up to 53 μm, although a similar trend was also observed for N3 dye-sensitized DSSC, however, the maximum efficiency achieved was only at 27 μm thickness (eff.=6.75%). As expected, the photocurrent density generated in the DSSC modified by CYC-B1 dye is larger than that from N3 dye. The effect of guanidinium thiocyanate (GuSCN) (additive) addition to the electrolyte on the photovoltaic performance of DSSCs based on CYC-B1 was also investigated. Furthermore, the electrochemical impedance spectroscopy (EIS) technique and photo-transient laser method have been employed to analyze the charge transfer resistances (R_ct) and the lifetime of the injected electrons on the TiO₂ containing different thicknesses.     

Solar module using dye-sensitized solar cells with a polymer electrolyte

Dye-sensitized TiO₂ solar cells assembled with a polymer electrolyte were investigated, aiming at the construction of an 8 V solar module. The individual solar cells were assembled with 4.5 cm² active area and were characterized under outdoor conditions, exhibiting an average efficiency of 0.9% per cell (at 12:00 noon). The solar module was built by connecting 13 cells in series. The integrated average daily power was estimated to be 183 mW. The present paper discusses the performance of the individual cells and the module.     

A study on the electron transport properties of TiO2 electrodes in dye-sensitized solar cells

The influences of annealing temperature and different poly (ethylene glycol) (PEG) contents in nano-crystalline TiO₂ electrodes with and without N3 dye on the electron transfer in a dye-sensitized solar cell (DSSC) were investigated. It is found that the power conversion efficiency increases with the increase in annealing temperature and becomes saturated at 400–500 °C, and further increase lowers the performance which is consistent with the enhancement of the crystalline TiO₂ particles observed in X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images. Electrochemical impedance spectroscopy (EIS) also confirms this behavior. These results have been further verified by studying the electron lifetimes (τ_e) and electron diffusion coefficients (D_e) of a bare TiO₂ and a dye-sensitized TiO₂ film using a pulsed laser spectrometer. It is noted that both the electron lifetime and the electron diffusion coefficient increase with the increase in annealing temperature. However, the evolution of rutile TiO₂ begins beyond 600 °C and this lowers the dye absorbance and the electron diffusion coefficients of TiO₂ electrodes. A similar study was made by varying the content of the PEG in the TiO₂ films. It is found that with the increase in the PEG content, a decrease in the electron lifetimes and a little hike in the electron diffusion coefficients are noted, where the cell performance remains almost the same. In addition, the dye adsorption decreases the electron lifetime and increases the electron diffusion coefficient of the TiO₂ films regardless of the PEG content and the annealing temperature.     

Influences of water in bis-benzimidazole-derivative electrolyte additives to the degradation of the dye-sensitized solar cells

The chemical stability of dye-sensitized solar cells (DSSC) determines both the cell performance and the cell life-time. The presence of water in the solar cell from surrounding leakage through the imperfection packaging sealants causes the decrease in life-time of photogenerated electrons on the working electrodes, which induces the occurrence of the dark current to the electrolytes and thus leakage current significantly deteriorated the life-time of the DSSC. Reliable electrolyte additives diminishing the influences of water to the DSSCs degradation process becomes a critical issue in maintaining an acceptable cell life-time.In this work, the effects of four benzimidazole derivatives a-d as the electrolyte additives in the presence of water were comprehensively examined by time-dependent photovoltaic performance of the cells. As a result, open-circuit voltage (V_oc), short-circuit current (J_sc), efficiency (η), and fill factor (FF) collected from I-V curves were studied. In addition, electrochemical impedance spectroscopy (EIS) technique was implemented to evaluate the effects of the charge-transfer resistance (R_ct) at the interfaces between TiO₂/dye/electrolyte. Results showed that the bis-benzimidazole derivative c gives significant improvement in the long-term stability due to the effective protection of the ligands between dye and working electrodes from the attack by environmental water molecules.     

Novel iminocoumarin dyes as photosensitizers for dye-sensitized solar cell

Novel iminocoumarin dyes (2a-c and 3a-c) having carboxyl and hydroxyl anchoring groups onto the dyes skeletons have been designed and synthesized for the application of dye-sensitized nanocrystalline TiO₂ solar cells (DSSCs). The photophysical and electrochemical studies showed that these iminocoumarin dyes are suitable as light harvesting sensitizers in DSSC application. The dyes having carboxyl and hydroxyl anchoring groups (2a-c) showed better efficiency when compared to the dyes having carboxyl group (3a-c) alone. The cell consisted of dye 2a generated the highest solar-to-electricity conversion efficiency (η) of 0.767% (open circuit voltage (V_oc) = 0.491 V, short circuit photocurrent density (J_sc) = 2.461 mA cm⁻², fill factor (ff) = 0.635) under simulated AM 1.5 irradiation (1000 W m⁻²) with a total semiconductor area of 0.25 cm². The corresponding incident photon-to-current conversion efficiency (IPCE) of the above cell was 21.38%. The overall low efficiency of the dyes is ascribed to the lack of light harvesting ability at longer wavelength region.     

The influence of surface morphology of TiO2 coating on the performance of dye-sensitized solar cells

The optimization of solar energy conversion efficiency of dye-sensitized solar cells (DSSCs) was investigated by the tuning of TiO₂ photoelectrode's surface morphology. Double-layered TiO₂ photoelectrodes with four different structures were designed by the coating of TiO₂ suspension, incorporated with low and high molecular weight poly(ethylene glycol) as a binder. Among these four systems, P2P1, where P1 and P2 correspond to the molecular weight of 20,000 and 200,000, respectively, showed the highest efficiency under the conditions of identical film thickness and constant irradiation. This can be explained by the larger pore size and higher surface area of P2P1 TiO₂ electrode than the other materials as revealed by scanning electron microscopic (SEM) and Brunauer–Emmett–Teller (BET) analyses. Electrochemical Impedance Spectroscopy (EIS) analysis shows that P2P1 formulation displayed a smaller resistance than the others at the TiO₂/electrolyte interface. The best efficiency (η) of 9.04% with the short-circuit photocurrent density (J_sc) and open-circuit voltage (V_oc) of 18.9 mA/cm² and 0.74 V, respectively, was obtained for a solar cell by introducing the light-scattering particles to the TiO₂ nanoparticles matrix coated on FTO electrode having the sheet resistivity of 8 Ω/sq.     

Heteropolyacid-impregnated PVDF as a solid polymer electrolyte for dye-sensitized solar cells

Heteropolyacid (HPA)-impregnated polyvinylidene fluoride (PVDF) with iodine/iodide was prepared as a new polymer electrolyte for bio-mimicking natural photosynthesis. With this new polymer electrolyte, dye-sensitized solar cell was fabricated using N3 dye-adsorbed over TiO₂ nanoparticles (photoanode) and conducting carbon cement coated on conducting glass (photocathode). The fabricated cell generates high open circuit voltage (V_OC 426 mV) and short circuit current (I_SC 3.90 mA) upon illumination with visible light. It is also demonstrated that the polymer electrolytes prevent the back-electron transfer reactions taking place in dye-sensitized hetero-junctions and are highly promising for solar energy conversion to electricity.     

Efficiency enhancement by mixed cation effect in dye-sensitized solar cells with a PVdF based gel polymer electrolyte

This paper reports the effect of using a mixed iodide salt system with two dissimilar cations to enhance the efficiency of dye-sensitized solar cells made with polyvinylidenefluoride (PVdF) based gel electrolyte. Instead of a single iodide salt, a mixture of potassium iodide (KI) with a small K⁺ cation and tetrapropylammonium iodide (Pr₄NI) with a bulky Pr₄N⁺ cation were used to provide the required iodide ion conductivity. Solar cells of configuration FTO/TiO₂/Dye/electrolyte/Pt/FTO were fabricated using a mesoporous TiO₂ electrode sensitized with a Ruthenium dye (N719). With identical electrolyte compositions, the cells with KI and Pr₄NI alone gave efficiencies of 2.37% and 2.90% respectively. The cell with the mixed iodide system, KI:Pr₄NI = 16.6:83.4 (% weight ratio), however, showed an enhanced efficiency of 3.92% with a short circuit current density of 9.16 mA cm⁻², open circuit voltage of 674.4 mV and a fill factor of 63.4%.     

Quasi-solid-state dye-sensitized solar cells with cyanoacrylate as electrolyte matrix

A quasi-solid-state dye-sensitized solar cells (DSSCs) employing a commercial glue (“SuperGlue^®”) as electrolyte matrix was fabricated. The cyano groups of the cyanoacrylate can form a supramolecular complex with tetrapropylammonium cations. This immobilizes the cations and therefore might lead to a favored anionic charge transport necessary for a good performance of the iodide/triiodide electrolytic conductor. Obtaining energy conversion efficiencies of more than 4% under 100 mW/cm² of simulated A.M. 1.5 illumination, the cyanoacrylate quasi-solid-state electrolyte is an ordinary and low-cost compound which has fast drying property and offers significant advantages in the fabrication of solar cells and modules as it is in itself is a very good laminating agent. The influences of different porous layer thicknesses of titanium oxide and various kinds of cations on DSSC performance and long-term stability are presented.     

Doping CuSCN films for enhancement of conductivity: Application in dye-sensitized solid-state solar cells

Construction of dye-sensitized solid-state solar cells requires high band-gap (therefore, transparent) hole collectors which can be deposited on a dye-coated nanocrystalline semiconductor surface without denaturing the dye. Copper (I) thiocyanate (CuSCN) is an important p-type semiconductor satisfying the above requirements. However, the conductivity of this material, which depends on excess SCN, is not sufficiently high and polymerization of SCN prevents incorporation of sufficient amount of excess SCN during the process of synthesis of CuSCN. We have found that the conductivity of solid CuSCN can be increased by exposure to halogen gases which generate SCN or to a solution of (SCN)₂ in CCl₄. The latter method is suitable for doping of CuSCN films in dye-sensitized solid-state solar cells.     

Quasi-solid-state dye-sensitized solar cells employing nanocrystalline TiO2 films made at low temperature

Quasi-solid-state dye-sensitized solar cells with enhanced performance were made by using nanocrystalline TiO₂ films without any template deposited on plastic or glass substrates at low temperature. A simple and benign procedure was developed to synthesize the low-temperature TiO₂ nanostructured films. According to this method, a small quantity of titanium isopropoxide (TTIP) was added in an ethanolic dispersion of TiO₂ powder consisting of nanoparticles at room temperature, which after alkoxide's hydrolysis helps to the connection between TiO₂ particles and to the formation of mechanically stable thick films on plastic or glass substrates. Pure TiO₂ films without any organic residuals consisting of nanoparticles were formed with surface area of 56 m²/g and pore volume of 0.383 cm³/g similar to that obtained for Degussa-P25 powder. The structural properties of the films were characterized by microscopy techniques, X-ray diffractometry, and porosimetry. Overall solar to electric energy conversion efficiencies of 5.3% and 3.2% (under 1sun) were achieved for quasi-solid-state dye-sensitized solar cells employing such TiO₂ films on F:SnO₂ glass and ITO plastic substrates, respectively. Thus, the quasi-solid-state device based on low-temperature TiO₂ attains a conversion efficiency which is very close to that obtained for cells consisting of TiO₂ nanoparticles sintered at high temperature.     

Fabrication and characterization of dye-sensitized solar cells from rutile nanofibers and nanorods

Rutile titania (TiO₂) nanofibers were prepared by electrospinning a polymeric sol containing a titanium precursor and Poly(vinylpyrrolidone) in acetic acid-ethanol mixture and subsequent sintering of the fibers at 800 °C. The resultant continuous, polycrystalline porous fibers contained TiO₂ grains of 15-20 nm sizes. The continuous fibers were broken down into nanorods by mechanical grinding. Morphology of the nanofibers and nanorods was characterized by scanning and transmission electron microscopies. The crystal structure and polycrystallinity of the fibers were further confirmed by X-ray diffraction analysis. Dye-sensitized solar cells (DSCs) fabricated from the nanofibers and rutile nanorods, respectively, showed superior performance with the later.     

2-Ethyl-1-hexanol based screen-printed titania thin films for dye-sensitized solar cells

Nanocrystalline titania thin films were prepared by screen printing in order to efficiently control and optimize the main step of the dye-sensitized solar cells (DSSCs) fabrication process. Different compositions of nanocrystalline titanium dioxide screen-printing pastes are described, based on 2-ethyl-1-hexanol solvent and commercial Degussa P25 TiO₂. The rheological properties of the prepared pastes are presented as the crucial parameter of the deposition procedure. The produced titania thin films are extensively characterized by means of spectroscopy (Raman, XRD) and microscopy (SEM, AFM). The performance (induced photon-to-current efficiency—IPCE% and overall energy conversion efficiency—η%) of the corresponding DSSCs is also reported.     

Characteristics of triphenylamine-based dyes with multiple acceptors in application of dye-sensitized solar cells

We report the synthesis and photophysical/electrochemical properties of triphenylamine (TPA)-based multiple electron acceptor dyes (TPAR1, TPAR2, and TPAR3) as well as their applications in dye-sensitized solar cells (DSSCs). In these dyes, the TPA group and the rhodanine-3-acetic acid play the role of the basic electron donor unit and the electron acceptor, respectively. It was found that introduction of two rhodanine-3-acetic acid groups into the TPA unit (TPAR2) exhibited better photovoltaic performance due to the increase with a red shift and broadening of the absorption spectrum. The monolayer of these TPA-based dyes was adsorbed on the surface of nanocrystalline TiO₂ mesoporous electrode with the thickness of ∼6 μm, polyethylene oxide (PEO) used as the matrix of gel electrolyte, and 4-nm thick Pt used as a counter-electrode. Photovoltaic device can be realized in a single quasi-solid-state DSSC. TPAR2-based gel DSSC had an open circuit voltage and short circuit current density of about 541 and 10.7 mA cm⁻², respectively, at 1-sun.     

Spectral response and IV-characterization of dye-sensitized nanocrystalline TiO2 solar cells

Dye-sensitized nanocrystalline TiO₂ solar cells (nc-DSCs) are based on a fundamentally different working principle than solar cells based on semiconductors. This could have implications for the characterization of nc-DSCs. In this study a comparison is made between two methods for determination of the spectral response of nc-DSCs. The standard method for determination of the spectral response according to the ASTM E1021-84 norm appears to be valid for the nc-DSC. The response of the solar cell to pulsed irradiation plays an important role in this determination, since pulsed illumination of the solar cell is involved. The response time of the nc-DSC is related to electron trapping in the TiO₂ and depends on illumination conditions and also on chemical composition of the cell. For this reason, prior to measurements of spectral response of nc-DSCs, the response time of the cell should be measured under the same illumination conditions that are applied during spectral response measurements.     

Mix-solvent-thermal method for the synthesis of anatase nanocrystalline titanium dioxide used in dye-sensitized solar cell

Nanocrystalline TiO₂ with almost pure anatase form has been synthesized through the Mix-solvent-thermal method (MST) by using TiCl₄ as the starting material. The mean size of the synthesized TiO₂ is 10 nm with narrow distribution. High-performance dye-sensitized solar cell with nanocrystalline TiO₂ electrode formed from MST was achieved. Its I_sc and V_oc values reached 21.62 mA/cm² and 727.9 mV, respectively, and the photovoltaic conversion efficiency reached 9.13%, i.e. 7.5% higher than those of solar cells with TiO₂ made from the traditional precursor of titanium alkoxides. To our knowledge they are the highest values obtained from the solar cells with nanocrystalline TiO₂ electrode formed from the hydrolysis of TiCl₄.     

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.     

Enhancement of the photoelectric performance of dye-sensitized solar cells by using a CaCO3-coated TiO2 nanoparticle film as an electrode

Core-shell-type nanoparticles with TiO₂ cores and CaCO₃ shells were applied as the electrode of dye-sensitized solar cells. The performance of the cell was significantly improved (as high as 26.7%) compared to the case when un-coated TiO₂ particle film was used as electrode. The improved energy conversion efficiency has been ascribed to (i) enhanced dye adsorption due to the high isoelectric point of the overlayer, and (ii) the prevention of the back electron transfer by the insulating nature of the overlayer.     

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.     

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.