TiO2 Solar Cells Research Articles

Optical and Photovoltaic Properties of Thieno[3,2-b]thiophene-Based Push-Pull Organic Dyes with Different Anchoring Groups for Dye-Sensitized Solar Cells.

The effect of anchoring groups on the optical and electrochemical properties of triphenylamine-thienothiophenes, and on the photovoltaic performance of DSSCs photosensitized with the prepared dyes, was studied using newly synthesized compounds with cyanoacetic acid or rhodanine-3-acetic acid groups. Precursor aldehydes were synthesized through Suzuki cross-coupling, whereas Knoevenagel condensation of these with 2-cyanoacetic acid or rhodanine-3-acetic acid afforded the final push–pull dyes. A comprehensive photophysical study was performed in solution and in the solid state. The femtosecond time-resolved transient absorption spectra for the synthesized dyes were obtained following photoexcitation in solution and for the dyes adsorbed to TiO2 mesoporous films. Information on conformation, electronic structure, and electron distribution was obtained by density functional theory (DFT) and time-dependent DFT calculations. Triphenylamine–thienothiophene functionalized with a cyanoacetic acid anchoring group displayed the highest conversion efficiency (3.68%) as the dye sensitizer in nanocrystalline TiO2 solar cells. Coadsorption studies were performed for this dye with the ruthenium-based N719 dye, and they showed dye power conversion efficiencies enhanced by 20–64%. The best cell performance obtained with the coadsorbed N719 and cyanoacetic dye showed an efficiency of 6.05%.

Porphyrins as Multifunctional Interconnects in Networks of ZnO Nanoparticles and their Application in Dye‐Sensitized Solar Cells

AbstractZnO nanoparticles of 2.6 to 4.0 nm in size were interconnected by meso‐tetra(4‐carboxyphenyl)porphyrin (TCPP) for the realization of novel ZnO–TCPP networks, reaching lateral dimensions ranging from 50 to 800 nm. The resulting ZnO–TCPP networks were probed by dynamic light‐scattering experiments, transmission electron microscopy, and spectroscopy, and were also tested as photoactive top layers in TiO2 and ZnO dye‐sensitized solar cells (DSSCs). Control over these ZnO–TCPP networks on top of the semiconductor electrodes enables efficiency increases of 10 % and 46 % for TiO2‐ and ZnO‐based photoelectrodes, respectively. In other words, effects, such as reducing the electron recombination and increasing the electron lifetime, document the benefits of using tailored functional hybrid systems in DSSCs.

Fabrication of Inorganic Sensitized Solar Cells by Drop Casting Deposition of PbSe and PbTe on the TiO2 Surface

In this work, PbSe and PbTe sensitized TiO2 solar cells were fabricated. PbSe and PbTe nanostructure was deposited on the TiO2 surface via a drop cast method. The fabricated surfaces were examined by atomic force microscopy (AFM). Also the optical properties of the layers were studied by diffuse reflectance spectroscopy (DRS) spectra. The morphology of the surfaces was obtained by scanning electron microscopy (SEM) images. The structure and phase of the obtained materials were studied by X-ray diffraction pattern (XRD). Furthuremore chemical elements of the fabricated layers were examined by energy dispersive X-ray analysis (EDX) spectra. The solar cells were made by the fabricated layers, Pt as a counter electrode, I3-/I- electrolyte, surlyn as a sealer and silver paste to enhance collection of the produced current.

Enhanced Performance of Nanoporous Titanium Dioxide Solar Cells Using Cadmium Sulfide and Poly(3-hexylthiophene) Co-Sensitizers.

This work reports the effect of co-sensitization of nanoporous titanium dioxide using Cadmium Sulfide (CdS) and poly(3-hexylthiophene) (P3HT) on the performance of hybrid solar cells. CdS nanolayer with different thicknesses was grown on Titanium Dioxide (TiO2) nanoparticles by chemical bath deposition technique with varying deposition times. Both atomic force microscopy (AFM) and UV–Vis–NIR spectroscopy measurements of TiO2 electrode sensitized with and without CdS layer confirm that the existence of CdS layer on TiO2 nanoparticles. AFM images of CdS-coated TiO2 nanoparticles show that the surface roughness of the TiO2 nanoparticle samples decreases with increasing CdS deposition times. Current density–voltage and external quantum efficiency (EQE) measurements were carried out for corresponding solar cells. Both short circuit current density (JSC) and fill factor were optimized at the CdS deposition time of 12 min. On the other hand, a steady and continuous increment in the open circuit voltage (VOC) was observed with increasing CdS deposition time and increased up to 0.81 V when the deposition time was 24 min. This may be attributed to the increased gradual separation of P3HT and TiO2 phases and their isolation at the interfaces. The higher VOC of 0.81 V was due to the higher built-in voltage at the CdS–P3HT interface when compared to that at the TiO2–P3HT interface. Optimized nanoporous TiO2 solar cells with CdS and P3HT co-sensitizers showed external quantum efficiency (EQE) of over 40% and 80% at the wavelengths corresponding to strong absorption of the polymer and CdS, respectively. The cells showed an overall average efficiency of over 2.4% under the illumination of 70 mW/cm2 at AM 1.5 condition.

Studying the role of CdS on the TiO2 surface passivation to improve CdSeTe quantum dots sensitized solar cell

Recently, CdSeTe Quantum Dot Sensitized Solar Cells (QDSSC) have shown remarkable advances in the photo conversion efficiency. In this work, the synthesis of CdSeTe QD is demonstrated and its performance as a sensitizer evaluated in a TiO2 solar cell device. A photoconversion efficiency of 6.4% was obtained with the configuration TiO2/CdS/CdSeTe/ZnS that represent a fivefold increment compared to TiO2/CdSeTe/ZnS. The corresponding photocurrent was 20.3 mA/cm2 as a result of the increase in the absorption range with the introduction of CdSeTe QDs and the electron injection of CdS. Electrochemical impedance spectroscopy (EIS) analysis confirms that such improvement was obtained as a result of the surface passivation of TiO2 with the deposition of CdS acting also as a sensitizer if the appropriate energy level alignment between CdS and CdSeTe QDs is observed.

Carbon‐Dot/Natural‐Dye Sensitizer for TiO2 Solar Cells Prepared by a One‐Step Treatment of Celery Leaf Extract

AbstractAttempts have been made to create artificial photovoltaic (PV) devices that can harness sunlight in a clean and efficient way by mimicking natural photosynthesis processes. Inspired by the role of chlorophyll in photosynthesis, we introduced pheophytin‐ or pyropheophytin‐capped carbon dots (CDs) into the dye‐sensitized solar‐cell system. The CD hybrids were synthesized by means of a single‐step hydrothermal treatment of celery leaf extract. The effects of hydrothermal temperature on the physical properties of CDs and the survival rate of the natural dyes were studied. CDs heated at 120 °C (C‐120) display a relatively homogenous smaller particle size distribution and lower photoluminescence emission intensity relative to their counterparts obtained at higher temperatures. Heated at lower temperatures, the achieved CD hybrids contain a higher amount of surviving dye molecules which would be converted into pheophytins or pyropheophytins. The nanocrystalline TiO2 solar cells based on C‐120 gave the highest power conversion efficiency (PCE) of 0.48 % under AM 1.5G one full sun illumination, which can be compared favorably to the average value of 0.20 % normally reported for other biomass‐derived carbon‐dot‐sensitized devices. Besides functioning as a light sensitizer, C‐120 CDs with a lower amount of trap states can also work as an effective electron‐transfer intermediate to improve the interaction between the dye and TiO2 and suppress the photogenerated carrier recombination, thus improving solar‐cell performance.

Enhanced performances of dye-sensitized solar cells based on Au-TiO2 and Ag-TiO2 plasmonic hybrid nanocomposites

Novel double-layer films were prepared and applied to dye-sensitized solar cells (DSSCs) using commercial TiO2 nanoparticles as a bonding underlayer and noble metal (Au and Ag) nanoparticles (NP) and nanowires (NW) incorporated to hybrid TiO2 composites, consisting of 3 dimensional (3D) hierarchical microspheres, 3D hollow spheres, 2 dimensional (2D) nanosheets and commercial P25 nanoparticles, as multifunctional light scattering overlayer. The influence of Au NP, Ag NP, Au NW, and Ag NW on of microstructures of the film electrodes and the photovoltaic (PV) performances of DSSCs was investigated. The result revealed that the ranges and intensity of sunlight absorption, the photo capture ability for dye molecules of the hybrid nanocomposite film electrodes, and the photoelectric conversion efficiency (PCE) of the cells were all significantly enhanced due to the plasmonic effect of the noble metal nanostructures. All composite DSSCs with noble metal nanostructures have higher PCE than the pure TiO2 solar cell. This is attributed the improved electron transport of the noble metal nanostructures, and the improvement of light absorption because of their local surface plasmon resonance (LSPR) effect. Under optical conditions, a PCE of 5.74% was obtained in the TiO2-AgNW DSSC, representing a 25.3% enhancement compared to a reference solar cell based on pure TiO2 film (4.58%). The main reason of the advancement is the improved electron transport of AgNW, the light absorption enhancement on account of the LSPR effect of AgNW, and increased light scattering due to the incorporation of the large one dimensional AgNWs within the photo-anode.

Synthesis and characterization of push-pull bithiophene and thieno[3,2-b]thiophene derivatives bearing an ethyne linker as sensitizers for dye-sensitized solar cells

Five push-pull heterocyclic dyes 4–7, and 9 were synthesized and characterized in order to study the variations in the optical, electronic and photovoltaic properties induced by structural modifications, i.e. different spacer units and anchoring groups. The final push-pull conjugated dyes 6–7 and 9 are formed by a N,N-dimethylaniline donor moiety conjugated with bithiophene, ethynyl-bithiophene or ethynyl-thieno[3,2-b]thiophene spacers and cyanoacetic acid or rhodanine-3-acetic acid as anchoring group. The synthesis of the precursor aldehydes 4–5 was accomplished through a Sonogashira coupling, on the other hand, compound 8 was prepared by Suzuki coupling. Knoevenagel condensation of aldehydes 4, 5 and 8 with cyanoacetic acid or rhodamine-3-acetic acid afforded the final push-pull dyes 6, 7 and 9. Information on conformation, electronic structure and electron distribution was obtained by DFT and TDDFT calculations. This multidisciplinary study regarding the evaluation of the optical, redox and photovoltaic properties of the dyes reveals that compound 7, bearing an ethynyl-bithiophene spacer conjugated with a cyanoacetic acid anchoring group, has the highest conversion efficiency (3.51%) as dye sensitizer in nanocrystalline TiO2 solar cells. Co-adsorption studies were also performed for dyes 6–7 with N719 as co-adsorbent, and this enhanced dye efficiencies by 33–45%. The best cell performance was obtained by co-adsorbing N719 and dye 7 (75/25 vol%) with 4.66% efficiency.

Synthesis of new dyes containing double tetrazole groups for sensitization of TiO2 nanoparticles in dye-sensitized solar cells

Di(1H-tetrazol-5-yl)methane is employed as a new electron acceptor group in the synthesis of two metal-free organic dyes containing triphenylamine donor group. Dye-sensitized nanocrystalline TiO2 solar cell (DSSC) applying these novel dyes is constructed for consideration of their photovoltaic properties. The electronic properties of the dyes are also considered with the aid of theoretical calculations. The DSSC constructed from 4-(2,2-di(1H-tetrazol-5-yl)vinyl)-N,N-diphenylaniline (T1) shows a short-circuit photocurrent density of 13.38 mA cm−2, an open circuit voltage of 578 mV, and a fill factor of 0.54, with a resulted solar energy-to-electricity conversion efficiency of 4.18% under simulated 1 sun irradiation (100 mW cm−2). This result reveals that the dye with the di(1H-tetrazol-5-yl)methane anchoring group injects more electrons to the conduction band of TiO2 in comparison with its analogs with single tetrazole ring in their anchoring group. It is found that in spite of a red-shift of the absorption spectrum resulted from the lengthening of the molecule, the dye with two di(1H-tetrazol-5-yl)methane groups gives lower performance than the dye with a single electron acceptor.

Donor-Modified Anthocyanin Dye-Sensitized Solar Cell with TiO<sub>2</sub> Nanoparticles: Density Functional Theory Investigation

Nowadays, the anthocyanin natural dye has been used as a good photosensitizer for TiO2 solar cells due to its availability as well as its performance. However, the dye still has a problem regarding its narrow absorbance at 550 nm. The aim of study will extend the dye’s absorbance up to NIR spectrum, so that it will increase the TiO2 solar cell performance. The cyanidin-3-glucoside-7- diphenilamine-2'-acrylic acid (CGDA) will be introduced as the new modification of D-π-A functional material in anthocyanin dye. The investigation of the material has been carried out by the density functional theory of TD-DFT/UB3LYP/6-31+G(d,p) level. The findings show that first; the dye absorbance has broadened up to 885 nm due to the reduction of gap energy and the increase of electronic transition probabilities from 84.7% to 96.0%. Second, the molecular orbital analysis shows that both diphenylamine and acrylic acid perform as promising donor and acceptor groups for anthocyanin. Third, CGDA has the light harvesting efficiency of 46.7% surrounding NIR spectrum area. Finally, the NIR anthocyanin dye possessing HOMO of -5.536 eV and LUMO of -2.886 eV fulfills the criteria of both the effective charge transfer from LUMO dye into conduction band TiO2 and the good dye regeneration by the electrolyte redox.

The effects of second electron acceptor group on the performance of tetrazole-based nanocrystalline TiO2 sensitizers in DSSCs

Three new organic sensitizers with two electron acceptor groups were synthesized and applied to nanocrystalline TiO2 solar cells. The ethyl 2-(1H-tetrazol-5-yl) acetate, (2H-tetrazol-5-yl) acrylonitrile and 1H-tetrazole-5-acetic acid moieties were introduced to the triphenylamine as electron acceptor groups. The photophysical, electrochemical and photovoltaic properties of the solar cells based on the synthesized sensitizers were studied and compared with their counterparts of single electron acceptor type. Quantum chemical calculations were also carried out to consideration of the electronic and optical properties of these dyes. The dye with the (2H-tetrazol-5-yl) acrylonitrile electron acceptors showed the absorption maxima in the longer wavelength, compared to the dyes with ethyl 2-(1H-tetrazol-5-yl) acetate and 1H-tetrazole-5-acetic acid. The solar cell based on the dye with 1H-tetrazole-5-acetic acid showed the highest conversion efficiency of 3.53% (open circuit voltage=569mV, short circuit photocurrent density=11.50mAcm−2, and fill factor of 54% under AM 1.5G conditions). The results also showed that the dyes with two electron acceptor groups gave the higher performance than the dyes with single electron acceptor.

Hydrothermal growth of highly monodispersed TiO2 nanoparticles: Functional properties and dye-sensitized solar cell performance

Monodispersed anatase TiO2 nanoparticles were synthesized by hydrothermal method using citric acid as a capping agent. The effect of citric acid and the growth time on the formation of TiO2, functional properties and dye-sensitized solar cell performances were investigated. X-ray diffraction pattern (XRD) and Raman spectroscopy results revealed that the TiO2 nanoparticles possess the anatase phase. Transmission electron microscopy (TEM) measurement revealed the formation of spherical nanoparticles with monodispersity in size and morphology. An average size of 14nm was obtained for the growth period of 15h. The maximum efficiency (η) of dye-sensitized solar cell was achieved for TiO2 nanoparticles grown for 15h as 7.66% which was higher than that of commercial P25 TiO2 (5.23%) and uncapped nanoparticles (3.68%).

Impact of nanoparticles preparation method on the synergic effect in anatase/rutile mixtures

The synergic effect observed in dye sensitized TiO2 solar cells cannot be explained simply by the proportion of rutile and anatase nanoparticles in the mesoporous electrode. In fact, subtle differences in the surface chemical composition dramatically influence the adsorption of N719 dye and increase by an order of magnitude the conductivity of sol-gel rutile, enhancing DSSC's efficiency. However, the now similar conductivity of rutile and anatase nanoparticles also changed the charge diffusion mechanism because electrons can flow through both materials avoiding the formation of preferential anatase channels. As a consequence, no maximum was observed in the Dnvs %rutile plot, and η and JSC become a linear function of %rutile been proportional to the flux of photoinjected electrons, or in other words to the surface concentration of N719 ruthenium dye. All those features increased the photoexcited electron lifetime, particularly around 90%rutile in homogeneous mixtures, suggesting a new synergic effect in mixed phase materials for photocatalytic applications.

Electronic excitation and injection of Ru-N3 dye anchored to TiO2 surface

A step-by-step theoretical protocol based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) has been performed to study a Ruthenium polypyridyl complex named cis-dithiocyanatobis-(4,4′-dicarboxy-2,2′-bipyridine) ruthenium(II) (N3) sensitized TiO2 solar cell including dye excitations and electronic injection. Three binding structures of N3 anchored to a TiO2 nanoparticle (TiO2)38 have been adopted. The electronic structures and optical properties of N3 dye in gas phase, in solutions (with the explicit and implicit solvent models), and interfaced with TiO2 have been calculated. The hybrid DFT exchange-correlation (XC) functional B3LYP and PBE0, the newly-modified PBE0 functional PBE0-1/3, and the long-range corrected DFT (LRC-DFT) XC functional Cam-B3LYP have been applied. It is found that both the DFT XC functionals and molecular environments play a crucial role on molecular properties, which affect both the spectral lineshapes and peak positions. Coupled with PCM model, TD-PBE0-1/3 produces the best result compared with the experimental spectrum of N3 in CH2Cl2, TD-B3LYP slightly underestimates the excitation energies, and TD-Cam-B3LYP overestimates the excitation energies of 0.4–0.5eV. As the solvent polarity increases, the electronic absorption spectra of N3 dye blue shift. The absorption manners of N3 anchored to (TiO2)38 affect the electronic excitations, leading to different electronic injection pathways. Two of three anchoring manners favor the interfacial electronic transfer. In the end, three possible electronic injection pathways from the different parts of the excited dye to TiO2 nanoparticle have been suggested, indicating the different timescales of electron injections.

Efficiency enhancement of TiO2 (active material) solar cell by inserting copper particles grown with pulse voltage electroplating method

Here, we report the manufacture of a solar cell using TiO2 nanoparticles as photon absorbers and copper bridges inserted between the TiO2 particles. The copper bridges were synthesized by the pulse voltage electroplating method, and the effect of the pulse duty cycle was explored. The amount of copper deposited between TiO2 particles can be controlled by varying the duty cycles and the deposition time. We found that the cell fabricated by the deposition of copper at duty cycles of 60% and a deposition time of 30s exhibited the highest efficiency (2.21%). Efficiency was improved to 3.5% following the post-treatment of the cell with NaOH. We also proposed a simple mathematical model to explain the dependence of the efficiency on the amount of copper. Efficiencies of more than 3% for solar cells made by a simple method and using inexpensive materials make these solar cells promising competition for the current commercial solar cells.

Design and synthesis of BODIPY sensitizers with long alkyl chains tethered to N-carbazole and their application for dye sensitized solar cells

In this study, three boron dipyrromethenes (BODIPY) dyes with extended conjugation and electron donating carbazole groups with different alkyl chain lengths tethered to N-carbazole were synthesized and characterized for dye-sensitized solar cells. The goal was to study the effect of different alkyl chain lengths on dye aggregation at TiO2 surface. The proposed molecular strategy resulted in BODIPY dyes which showed interesting electronic absorption and fluorescence properties. It was observed that intramolecular energy transfer decreases with the increase in alkyl chain length possibly due to induced changes in molecular geometry caused by long alkyl chains. Additionally, interface analysis by impedance spectroscopy in comparison to N719 sensitized TiO2 solar cell showed significant charge transport related losses (Nyquist plot) most likely due to impedance resulted from aggregated BODIPY dye on TiO2 surface. Femtosecond transient absorption studies showed the loss of excited electrons by recombination with oxidized ground state of the sensitizers.

Enhanced Conversion Efficiencies in Dye-Sensitized Solar Cells Achieved through Self-Assembled Platinum(II) Metallacages.

Two-component self-assembly supramolecular coordination complexes with particular photo-physical property, wherein unique donors are combined with a single metal acceptor, can be utilized for many applications including in photo-devices. In this communication, we described the synthesis and characterization of two-component self-assembly supramolecular coordination complexes (SCCs) bearing triazine and porphyrin faces with promising light-harvesting properties. These complexes were obtained from the self-assembly of a 90° Pt(II) acceptor with 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPyT) or 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine (TPyP). The greatly improved conversion efficiencies of the dye-sensitized TiO2 solar cells were 6.79 and 6.08 respectively, while these SCCs were introduced into the TiO2 nanoparticle film photoanodes. In addition, the open circuit voltage (Voc) of dye-sensitized solar cells was also increased to 0.769 and 0.768 V, which could be ascribed to the inhibited interfacial charge recombination due to the addition of SCCs.

Improved performance of CdSe/CdS/PbS co-sensitized solar cell with double-layered TiO2 films as photoanode

Improving the photovoltaic performance of CdSe/CdS/PbS co-sensitized double-layered TiO2 solar cells is reported. Double-layered TiO2 films with TiO2 microspheres as the light blocking layers were prepared. PbS, CdS and CdSe quantum dots (QDs) were assembled onto TiO2 photoanodes by simple successive ionic layer absorption and reaction (SILAR) to fabricate CdSe/CdS/PbS co-sensitized solar cells. An improved power conversion efficiency (PCE) of 5.11% was achieved for CdSe/CdS/PbS co-sensitized solar cells at one sun illumination (AM 1.5G, 100mWcm−2), which had an improvement of 22.6% over that of the CdSe/CdS co-sensitized solar cells (4.17%). This enhancement is mainly attributed to their better ability of the absorption of solar light with the existence of PbS QDs, the reduction of charge recombination of the excited electron and longer lifetime of electrons, which have been proved with the photovoltaic studies and electrochemical impedance spectroscopy (EIS).

Tailoring band structure of ternary CdSxSe1−x quantum dots for highly efficient sensitized solar cells

Instead of employing numerous binary quantum dots (QDs), engineering ternary alloyed ones has been emerging as a promising approach to tailor their optical and electronic properties. In this paper, composition-tunable CdSxSe1−x QDs have been explored and demonstrated to be efficient sensitizers for quantum dot-sensitized solar cells (QDSCs). The optical bandgap and the corresponding band edges could be varied by controlling the component ratios of the ternary QDs while keeping the particle size constant, and a moderate bandgap was attained to deliver broad light harvesting and ideal band alignment. The successful balance of these two requirements resulted in a power conversion efficiency (PCE) of 3.58% for CdSxSe1−x QD-sensitized TiO2 solar cells. When the engineered CdSxSe1−x interlayer was further incorporated between CdS core and CdSe shell layers, a superior PCE of 5.06% has been reached for the graded CdS/CdSxSe1−x/CdSe structure, which appreciably outperforms the conventional CdS/CdSe one (η=4.41%). The incorporation of CdSxSe1−x interlayer offers two benefits, (1) the construction of a favorable stepwise band structure, facilitating desired charge injection, and (2) the passivation of the interfacial defects and interphase strain, contributing to suppressed charge recombination. This work highlights the broad prospect of ternary alloyed QDs for developing highly efficient QDSCs.

Copper Phenanthroline as a Fast and High-Performance Redox Mediator for Dye-Sensitized Solar Cells

The most commonly used redox mediators in dye-sensitized solar cells (DSCs), iodide/triiodide and cobalt trisbipyridine ([Co(bpy)3]2+/3+), were successfully replaced by bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp)2]1+/2+). The use of the copper complex based electrolyte led to an exceptionally high photovoltaic performance of 8.3% for LEG4-sensitized TiO2 solar cells, with a remarkably high open-circuit potential of above 1.0 V at 1000 W m–2 under AM1.5G conditions. The copper complex based redox electrolyte has higher diffusion coefficients and is considerably faster in dye regeneration than comparable cobalt trisbipyridine based electrolytes. A driving force for dye regeneration of only 0.2 eV is sufficient to obtain unit yield, pointing to new possibilities for improvement in DSC efficiencies. The interaction of the excited dye with components of the electrolyte was monitored using steady-state emission measurements and time-correlated single-photon counting (TC-SPC). Our results indicat...