Performance analysis of dye solar cell with additional TiO2 layer under different light Intensities

Deployment of dye solar cells (DSCs) for building integration application would require a highly efficient solar cell that work well in diffused light. In order to improve the efficiency of dye solar cell, an additional layer of ultrathin anatase titanium dioxide (TiO₂) has been deposited for strengthening the adhesion of the porous TiO₂-based photo electrode to the conductive transparent substrate, which can lead to an enhancement in electron transportation. Fabricated cells of 1 cm² area were tested under different light intensities (100, 33 and 10 mW cm⁻²) and characterized by scanning electron microscopy (SEM), Raman spectroscopy and electrochemical impedance spectroscopy (EIS). Analysis showed an increment in overall quantum conversion efficiency (η), as high as 35% compared to the standard cell without the additional layer of TiO₂. EIS analysis has proven that the additional ultrathin anatase layer has improved the collection efficiency (Φ_COLL) as the result of the enhancement in both electron transport and lifetime within the porous TiO₂ film which translated into better conversion efficiency of DSCs.     

An investigation on the effect of electrodeposited nanostructured ZnO on the electron transfer process efficiency of TiO2 based DSSC

In DSSCs, ZnO nanostructures can act as carrier trap centers to suppress the recombination reactions of the injected photoelectrons produced by the space charge on the small individual TiO₂ nanoparticles. In the present study, the effect of electrodeposited ZnO nanostructure on TiO₂ nanoparticles was investigated. To achieve the optimum deposition procedure, chronoamperometric (CA) technique was carried out at various precursor concentrations and deposition times. Cyclic voltammetry (CV) technique was utilized to obtain chemical capacitance and density of state values to indicate the ZnO function. DSSCs were fabricated using N719 ruthenium dye and all photovoltaic parameters such as short circuit current density (J_SC), open circuit voltage (V_OC), fill factor (FF) and conversion efficiency (η) were measured as well. Field emission-scanning electron microscopy (FE-SEM) was performed to study the structure and morphology of nanostructured TiO₂/ZnO layers. Furthermore, in order to measure the properties of photoelectrode interfaces, such as charge recombination behavior and electron life time, the electrochemical impedance spectroscopy (EIS) as an informative technique was employed. The results demonstrated the significant enhancement of the electron lifetime (63%) and short circuit current density (22%), due to ZnO charge trapping effect.     

Twisted donor substituted simple thiophene dyes retard the dye aggregation and charge recombination in dye-sensitized solar cells

Two novel thiophene 3,4-disubstituted organic dyes with double acceptors at 2,5-position framework photosensitizers were carefully designed and synthesized for the dye sensitized solar cell (DSSC) application. In this submission, the influence of carbazole (Cz) and triphenylaime (NPh3) units substitution at 3,4-position of thiophene is studied. Both the planar structure of carbazole based dyes and three dimension structure of triphenylamine based dyes are found to effectively inhibit the I⁻/I₃⁻ and Co⁺²/Co⁺³ electrolyte on TiO₂ surface. The results show that varying the donor position in dyes also effectively works for DSSC application, even if it does not broaden the absorption peak in visible region (for both the dyes) due to the blocking of electron delocalization at 2,5-positions by acceptor units. With two twisted donors at 3,4-position in Cz-Th-Dye, NPh3-Th-Dye the dye aggregation and charge recombination retarded. The optical, electrochemical, density functional theory and TRPL studies were used to estimate the photovoltaic properties of the dyes. The dye having two acceptors on both sides (2,5-position) of thiophene moiety showed efficient electron extraction from the electron donor part. Without using any additives, Cz-Th dye and NPh3-Th dye in presence of cobalt (II/III) redox shuttle, resulted in the power conversion efficiency (η, PCE) of ∼3.4% and 3.1% respectively. By replacing one acceptor with any other donor, these thiophene moiety can effectively work as a new π-spacer in DSSC application.     

Electrolyte-dependent photovoltaic responses in dye-sensitized solar cells

Promoted by the growing concerns about the worldwide energy demand and global warming, dyesensitized solar cells (DSSCs) are currently attracting worldwide scientific and technological interest because of their high energy conversion efficiency and simple fabrication process. Considering long-terms stability and practice applications, growing attentions have been paid to non-volatile, 3-methoxyproprionitrile (MPN)-based electrolyte, ionic liquids (ILs) electrolyte, as well as quasi-solid state electrolyte. In this present review, recent progress in electrolyte for DSSCs made by our group are summarized,including component-optimization of the non-volatile electrolyte, the fluidity-dependent charge transport mechanism in the binary IL electrolytes as well as the structure dominance of the employed ILs. Furthermore,progress on the quasi-solid state electrolyte based on inorganic nanomaterials as gelators in our group has also been outlined.     

Synthesis of novel ruthenium II phenanthroline complex and its application to TiO2 and ZnO nanoparticles on the electrode of dye sensitized solar cells

Novel ruthenium (II) phenanthroline complex, Ru^II(4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine)-(4,7-diphenyl-1,10-phenanthroline-disulfonic acid disodium salt)(thiocyanate), [Ru(L1)(L2)(NCS)], [K328] was designed and synthesized as a photosensitizer for the dye sensitized solar cells (DSSCs) using TiO₂ and ZnO electrodes. The density functional theory (DFT) calculation was used to estimate the photovoltaic properties of the complex in the design stage. In this paper, our aim was to investigate the interaction between semiconductor and anchoring groups like sulfonate group that binds onto TiO₂ and ZnO surface. The influence of the semiconductor type on the performance of TiO₂ and ZnO based photovoltaics has been tested. The solar cell performance of TiO₂ based solar cell shows better efficiencies compared to ZnO based solar cell. Although sulfonate group did offer stronger binding onto the semiconductor surface, this does not help to improve cell performance for ZnO based solar cells.     

Influence of m-fluorine substituted phenylene spacer dyes in dye-sensitized solar cells

A series of new organic dyes based on fluorine substituted phenyl moiety, used as a π-linker in various dyes, were designed and synthesized for dye sensitized solar cell (DSSC) application. These dyes share same anchoring group (Cyano acrylic acid) with different donors such as carbazole, thiopene substituted carbazole, triphenylamine and phenothiazine moieties. For effective electron flow, the dyes were incorporated with novel D-π-A or D-A-A framework. The optical, electrochemical, time-resolved photoluminescence (TRPL) spectra and photovoltaic properties of the dyes were carefully studied. The results reveal that without addition of any co-adsorbent, among all fluoro-phenyl spacer dyes, the Cz-dye showed highest open circuit voltage (V_oc), short circuit current (J_sc) and exhibited enhanced PCE value of 4.2 (±0.2) %, due to the deeper HOMO level of dye, planarity of backbone with better charge transfer occurring form D to A. The optimization calculations for the geometries of all the dyes with fluoro phenyl substituted π-linkers were ascertained by Density functional theory (DFT) using B3LYP/631G (d,p) basis set.     

Catalytic materials manufactured by the polyol process for monolithic dye‐sensitized solar cells

Three types of screen‐printable catalytic pastes were successfully prepared to be used as counterelectrode for monolithic dye solar cells encapsulated with glass frit. The electroless bottom‐up method or so‐called polyol process has been applied to fabricate thermally stable SnO₂:Sb/Pt and carbon black/Pt nanocomposites. The catalytic and electric properties of these materials were compared with a new platinum‐free type of carbon counterelectrode. The layers containing low platinum amounts (less than 5 µg/cm²) exhibit a very low charge transfer resistance of about 0·4 Ω · cm². Also the conductive carbon layer shows an acceptable charge transfer resistance of 1·6 Ω · cm². Additionally the catalytic layer containing porous carbon black reveals excellent sheet resistance below 5 Ω/□; this feature has enabled to work out a low cost counterelectrode which combined suitable catalytic and conductive properties. The layers have been characterized using following methods: electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FE‐SEM), energy filter transmission electron microscopy (EF‐TEM) and inductively coupled plasma mass spectroscopy (ICP‐MS). Copyright © 2008 John Wiley & Sons, Ltd.     

2,6-Conjugated Bodipy sensitizers for high-performance dye-sensitized solar cells

A series of novel 2,6-conjugated Bodipy metal-free organic dyes (UY5–8) with phenothiazine (PTZ) moiety as electron donor for the dye-sensitized solar cells (DSSCs) have been designed and synthesized. The optical, electrochemical properties and photovoltaic performances are extensively investigated. The structure–property relationship shows that the introduction of various auxiliary conjugated spacers and anchoring groups are favorable to changing the efficiency of DSSCs. Among these dyes, UY7 comprised of furan with lower resonance energy as linker and cyanoacetic acid unit as electron acceptor possesses a flatter structure and longer electron recombination lifetime. Hence, a DSSCs using UY7 showing best photovoltaic performance with a short-circuit photocurrent density (J_sc) of 13.64 mA cm⁻², an open-circuit photovoltage (V_oc) of 590 mV and a fill factor (ff) of 0.66, corresponding to an overall conversion efficiency (η) of 5.31% under 100 mW cm⁻² simulated AM 1.5 G solar irradiation. This is the best reported result in the solar cell with a Bodipy dye as photosensitizer.     

Synthesis and photo-electrochemical properties of novel thienopyrazine and quinoxaline derivatives,and their dye-sensitized solar cell performance

Light absorption from visible to NIR region is required to increase the photocurrent and to enhance the photo-energy conversion efficiencies in dye-sensitized solar cells (DSSCs). We have now developed novel thienopyrazine dye TP1 which has absorption up to 700 nm. Quinoxaline dye QX2 with absorption at shorter wavelengths than TP1 has been synthesized for comparisons. The power conversion efficiencies of DSSCs with TP1 and QX2 showed 4.4% and 3.2%, respectively. The absorption edge in IPCE of TP1 reached 800 nm and the open circuit voltage (Voc) of QX2 was high (0.77 V). To improve the device performances, QX2 was used as a co-adsorbent dye with TP1. In the mixed sensitizer based DSSC, a high power conversion efficiency of 6.2% was achieved due to the effective light harvesting and steric effect of QX2.     

Effects of carboxylic acid and phosphonic acid anchoring groups on the efficiency of dye sensitized solar cells: A computational study

In axially coordinated DSSCs the dye is attached to the surface via linker and the linker play a role of a conductor of electron and a bridge to the surface which has a very important effect on the efficiency of the cell. It has been shown that using one particular dye with different linkers could result in different efficiencies. In this paper, using density functional theory, different carboxylic acid (CA) linkers, and different phosphonic acid (PA) linkers are studied and their effects on the electronic structure of the surface and changes in the HOMO and LUMO of the linkers due to adsorption on the surface are investigated. For each family of linkers, based on calculations, the best one to be used in axially coordinated DSSCs is suggested. Comparing these two families, carboxylic acid linkers show superior properties in terms of cell efficiencies. phosphonic acid linkers, however, are shown to strongly bond to surface while resulting in lower conversion efficiencies compared to the carboxylic acid linkers the reason of which is still a question. Based on calculation of the changes in properties of the surface and properties of phosphonic acid linkers some reasons behind their low performances are suggested.     

Polypyrrole-coated cotton fabrics prepared by electrochemical polymerization as textile counter electrode for dye-sensitized solar cells

Textile-based wearable electronics provides the combined advantages of both electronics and textiles, such as flexibility, stretchability and lightweight. Much effort has been dedicated to achieve flexible photovoltaic power for wearable electronics. Here, we have demonstrated polypyrrole (PPy) coated cotton fabrics as textile counter electrode (CE) in dye-sensitized solar cells (DSSCs). PPy is deposited on the Ni-coated cotton fabrics as catalytic material by electrochemical polymerization of pyrrole. The highly conductive PPy-coated fabric electrode with a surface resistance of 5.0 Ω sq⁻¹ shows reasonable catalytic activity for the reduction of triiodide ion. The DSSC fabricated with the PPy-coated fabric CE exhibits a power conversion efficiency as high as ∼3.83% under AM 1.5 illumination.     

Stepwise co-sensitization as a useful tool for enhancement of power conversion efficiency of dye-sensitized solar cells: The case of an unsymmetrical porphyrin dyad and a metal-free organic dye

A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H₂[Porph] (ZnP-H₂P), where Zn[Porph] and H₂[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycine moiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO₂ electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H₂P/DC) and D (DC/ZnP-H₂P).The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H₂P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J_sc = 11.72 mA/cm²), open circuit voltage (V_oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J–V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J_sc = 13.45 mA/cm², V_oc = 0.76 V, and FF = 0.75), when a formic acid treated TiO₂ ZnP-H₂P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J_sc value (=13.45 mA/cm²), which resulted from an increased dye loading, and an enhanced V_oc value (=0.76 V), attributed to an upward shift and increased of electron density in the TiO₂ CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO₂ photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (τ_d), and longer electron lifetime (τ_e).     

Band gap engineering and modifying surface of TiO2 nanostructures by Fe2O3 for enhanced-performance of dye sensitized solar cell

Well-crystallized Fe₂O₃-modified TiO₂ nanoparticles are prepared by a hydrothermal method and were successfully used as the photoanode of dye-sensitized solar cell (DSSC). Structural, optical and thermal characterizations were carried out by SEM, XRD, AFM, EDAX, DTG, TG and UV–vis spectroscopy. We show that the solar conversion efficiency, incident photocurrent efficiency (IPCE) and fill factor (FF) of Fe₂O₃-modified TiO₂ are significantly increased, about 40%, compared those of to bare TiO₂. DSSC shows power conversion efficiency of 7.27% based on Fe₂O₃-modified TiO₂ while TiO₂ anatase shows 5.10% solar conversion efficiency. The high improvement in cell performance is attributed to the enhanced light harvesting and high specific surface area for adsorbing more dye molecules in Fe₂O₃-modified TiO₂ nanostructures.     

Novel ruthenium sensitizer with multiple butadiene equivalent thienyls as conjugation on ancillary ligand for dye-sensitized solar cells

A heteroleptic polypyridyl ruthenium complex ‘cis-Ru(4,4′-bis(3,5-bis(5-hexylthiophen-2-yl)phenyl)-2,2′-bipyridine)(4,4′-dicarboxyl-2,2′-bipyridine) (NCS)₂, MC102′, with a high molar extinction coefficient was synthesized and characterized with IR, ¹H NMR, Mass, UV–Vis spectroscopy. The test cell DSSC devices constructed with 0.23 cm² active area photo-electrode in combination with an electrolyte composed of 0.6 M dimethylpropyl-imidazolium iodide (DMPII), 0.05 M I₂, and 0.1 M LiI in acetonitrile yielded solar to electric energy conversion efficiency (η) of 4.42% under Air Mass (AM) 1.5 sunlight, while the reference N719 sensitized solar cell fabricated and evaluated under similar conditions exhibited η-value of 5.84%.     

Dissolution and electrochemical impedance spectroscopy studies of thin copper oxide films on copper in semi-aqueous fluoride solutions

The selective dissolution of thin copper oxide films grown on copper in semi-aqueous formulations containing dimethyl sulfoxide (DMSO), ammonium fluoride (NH₄F) and water was studied. Optimization of the formulations was carried out by systematic evaluation of the effect of solvent content and pH on the removal rates of copper oxide films and selectivity towards copper and carbon doped oxide (CDO) low k dielectric film. Copper oxide removal rate of ∼180 Å/min with a selectivity of ∼130:1 towards copper and ∼20:1 selectivity towards CDO was obtained in a formulation containing 29% DMSO, 1% NH₄F and 70% H₂O at pH 4. Electrochemical impedance spectroscopy studies were performed on this system and the data were analyzed to characterize the copper oxide/electrolyte interface with the ultimate objective of developing an end point detection technique for copper oxide removal.     

Effect of lithium intercalation on the photovoltaic performances of photovoltachromic cells

In the last few years, a new class of smart multifunctional photoelectrochemical devices has been attracting the interest of several academic institutions and industrial companies: photovoltachromic cells, combining the features of photoelectrochromic cells with those of dye‐sensitized solar cells. Here, we report the results of a detailed electrochemical analysis aiming at investigating the electrochemical behavior of these complex photoelectrochemical devices. In particular, we have been focused on the effect of Li⁺ ions displacement during the coloration of the electrochromic tungsten oxide on the performances of the photovoltaic unit. As we had previously observed striking differences between the performances of the barely photovoltaic mode (with the tungsten oxide in the bleached state) and the photovoltachromic mode (with the tungsten oxide in the colored state), we thus attempted to provide a reasonable physical interpretation to the observed phenomena. Copyright © 2013 John Wiley & Sons, Ltd.     

Capturing transiently charged states at the C60/TiO2(110) interface by time-resolved soft X-ray photoelectron spectroscopy

Time-resolved soft X-ray photoelectron spectroscopy is utilized to determine an energy level alignment and the photoexcited carrier dynamics at a C₆₀/TiO₂(110) interface. The interface electronic structure is characterized by a type II junction, which favors an injection of photoexcited electrons from C₆₀ to TiO₂. Ultraviolet (UV) laser pulse irradiation induces transient shifts of both C 1s and Ti 2p core levels towards the higher binding energies. These energy shifts are caused by a laser-induced charge transfer between the C₆₀ layer and the TiO₂(110) surface. Upon UV absorption, valence electrons of C₆₀ are promoted to unoccupied levels, followed by a resonant transfer to TiO₂, leaving C₆₀ in a cationized state. On the TiO₂(110) side, the electrons are injected into the conduction band to raise the carrier density so that downward bending of the TiO₂ band is induced. The UV-excited states of C₆₀ and TiO₂ have sufficiently longer lifetime than the lifetime of the electron–hole pairs in solid C₆₀. The C₆₀/TiO₂(110) interface is, thus, proved to be efficient for separating the electron–hole pairs generated within the C₆₀ layer.     

Highly efficient dye‐sensitized solar cells using phenothiazine derivative organic dyes

Two novel organic dyes have been synthesized using electron rich phenothiazine as electron donors and oligothiophene vinylene as conjugation spacers. The two dyes (2E)‐2‐cyano‐3‐(5‐(5‐((E)‐2‐(10‐(2‐ethylhexyl)‐10H‐phenothiazin‐7‐yl)vinyl)thiophen‐2‐yl)thiophen‐2‐yl)acrylic acid (PTZ‐1) and (2E)‐3‐(5‐(5‐(4,5‐bis((E)‐2‐(10‐(2‐ethylhexyl)‐10H‐phenothiazin‐3‐yl)vinyl)thiophen‐2‐yl)thiophen‐2‐yl)thiophen‐2‐yl)‐2‐cyanoacrylic acid (PTZ‐2) were fully characterized and employed in dye‐sensitized solar cells (DSCs) to explore the effect of disubstituted donors on photovoltaic (PV) performance. The solar cells sensitized by the PTZ1 dye have a high IPCE plateau of 80% and achieve a short‐circuit photocurrent density of 12.98 mA/cm², an open‐circuit voltage of 0.713 V, and a fill factor (ff) of 66.6%, corresponding to a conversion efficiency of 6.17% under AM 1.5 100 mW/cm² illumination. The different performance of the solar cells based on the two dyes can be understood from the studies of the electron kinetics by electrochemical impedance spectroscopy (EIS). These investigations reveal that disubstituted donors in the organic sensitizers of three or more conjugation units deteriorate the PV performance due to enhanced recombination. Copyright © 2010 John Wiley & Sons, Ltd.     

Coumarin based sensitizers with ortho-halides substituted phenylene spacer for dye sensitized solar cells

The performance of DSSCs (dye sensitized solar cells) with a new series of dyes having different halide groups (i.e. F, Cl and Br) on o-position substituted phenyl spacers with same coumarin donor moieties have been reported. Optical, electrochemical, molecular orbital and photovoltaic properties were studied by varying the halide groups using these dyes. The replacement of halide atoms in same coumarin based dye had a significant effect on the short circuit current density (J_sc), open circuit voltage (V_oc), and photo conversion efficiency (PCE). The J_sc and PCE of dye CD-1 (fluorine substituted) are 10.3 mA/cm² and 5.2% respectively, which is higher than CD-2 (chlorine substituted) and CD-3 (bromine substituted) dyes (having PCE 4.1% and 3.5% respectively) devices. The optimized geometry calculation of o-halide phenyl π-spacer dyes were ascertained by density functional theory (DFT) using the B3LYP/6-31G(d,p) basis set. Moreover, we have checked the effect of various substituents in the same dye structure by DFT analysis.