Binder-free MWCNT/TiO2 multilayer nanocomposite as an efficient thin interfacial layer for photoanode of dye sensitized solar cell

Multiwalled carbon nanotube/TiO₂ multilayer nanocomposite was successfully deposited on the fluorine-doped tin oxide (FTO) glass via layer-by-layer assembly technique to modify interfacial contact between the FTO surface and nanocrystalline TiO₂ layer as well as carbon nanotube/TiO₂ contacts in photoanode of dye sensitized solar cell. Using this approach, binder-free interfacial thin film was developed with nonagglomerated, well-dispersed MWCNTs on FTO and into TiO₂ matrix and with maximum covering of TiO₂ nanoparticles on MWCNTs. Introduction of MWCNTs/TiO₂ interfacial layer into the TiO₂ photoanode increased short circuit current density (J_sc) from 11.90 to 17.25 mA/cm² and open circuit voltage (V_oc) from 730 mV to 755 mV, whereas there was no notable change in the fill factor (FF). Consequently, power conversion efficiency (η) was enhanced from 5.32% to 7.53%, yielding a 41.5% enhancement. The results suggest that our simple strategy can integrate reduction of back electron reaction at FTO/TiO₂ interface with the effective charge transport ability of carbon nanotubes and possessing high surface area for efficient dye loading.     

Effect of CdS layers on opto-electrical properties of chemically prepared ZnS/CdS/TiO2 photoanodes

In the present work, CdS nanoparticles as a sensitizer were grown on the spin coated nanoporous TiO₂ film by repeated cycles of a Successive Ionic Layer Adsorption and Reaction (SILAR) method. ZnS layer was coated on the CdS/TiO₂ anodes to act as a protection layer on CdS. The crystallite size of CdS nanocrystals is calculated to be 3 nm from XRD spectra. The optical band gap of the film determined from transmittance spectra decreases from 3.46 to 2.15 eV with the increase in the number of CdS SILAR cycles. SEM and TEM analysis depict the enabled penetration of CdS (1 1 1) nanoparticles into the nanoporous TiO₂ (1 0 1) structure. EDX study confirms the presence of all the elements (Ti, Cd, S, Zn and O) found on the photoanode. The attachment of cubic structured CdS on anatase phase of TiO₂ in the photoanode is verified using Raman spectra. Photoluminescence (PL) study shows that the emission peak corresponding to TiO₂ has been slightly blueshifted due to the interaction of CdS nanoparticles in TiO₂ nanoporous structures. The electrical measurement shows that the dark and light illuminated resistivity of the preferred photoanode is 7.91 and 5.65 Ω cm respectively.     

Effect of seed layer on the growth of rutile TiO2 nanorod arrays and their performance in dye-sensitized solar cells

In order to improve the performance of TiO₂ photoanode-based dye sensitized solar cells (DSSCs), rutile TiO₂ nanorod arrays (NRAs) were grown on SnO₂:F (FTO) conductive glass coated with TiO₂ seed layer by a hydrothermal method. The TiO₂ seed layer was obtained by spin-coating titanium tetraisopropoxide (TTIP) isopropanol solution with concentration in the range of 0~0.075 M. Then the effect of the thin TiO₂ seed layer on the crystal structure and surface morphology of TiO₂ NRAs and the photoelectric conversion properties of the corresponding DSSCs were investigated. It is found that TiO₂ NRAs are vertically oriented, about 1.7 μm long and the average diameter is about 35 nm for the samples derived from TTIP in the range of 0.005~0.05 M, which are more uniform and better separated from each other than those without TiO₂ seed layer (average diameter 35~85 nm). The photoelectric conversion efficiency of DSSCs based on TiO₂ NRAs with TiO₂ seed layer is larger than that without TiO₂ seed layer. Typically, the energy efficiency of DSSCs obtained from the seed solution of 0.025 M TTIP is 1.47%, about 1.8 times greater than that without TiO₂ seed layer. The performance improvement is attributed to the thinner, denser and better oriented NRAs grown on seeded-FTO substrate absorbing more dye and suppressing charge recombination at the FTO substrate/electrolyte interface.     

Stacked graphene–TiO2 photoanode via electrospray deposition for highly efficient dye-sensitized solar cells

A series of TiO₂–graphene stacked photoanodes for dye-sensitized solar cells (DSSCs) were fabricated by electrospray (E-spray) deposition. Among devices incorporating single graphene layer with different deposition times, device with 1 min graphene deposition gave the best performance. For multi-graphene-layer involved devices, best result was obtained with 3 layers of graphene. The working principles were analyzed by scanning electron microscopy, transmittance spectra, electrochemical impedance spectroscopy and incident-photo-conversion efficiency data. We found that although graphene layers incorporated in TiO₂ photoanode slightly decreased dye adsorption, they were able to significantly improve the electron transport, and the charge recombination at the interfaces of TiO₂/dye and TiO₂/electrolyte were greatly suppressed, leading to dramatic improvement in power conversion efficiency. When inserting three layers of pure graphene into the TiO₂ photoanode, high efficiency of 8.9% was obtained, constituting an over 23.6% improvement. Further increasing graphene layers to five, although electron lifetimes is the longest, both the largest charge transfer resistance and the least amount of the dye loading lead to the lowest device efficiency. Our work demonstrated, that pure graphene layer can be successfully incorporated into TiO₂ photoanode by E-spray method with easiness of thickness control and the photoanode with graphene/TiO₂ alternatively layered structure is an excellent candidate for DSSCs.     

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).     

Low-temperature low-pressure die attach with hybrid silver particle paste

New types of die attach pastes comprising micron-sized Ag particles hybridized with submicron-sized Ag particles were considered as lead-free die attach materials for SiC power semiconductors. Micron-sized Ag particles in alcohol solvent were prepared by mixing the die attach paste with submicron-sized Ag particles. The alcohol vaporizes completely during sintering and no residue exists in the bonding layer. The Ag layer has a uniform porous structure. The electrical resistivity of the printed tracks decreases below 1 × 10^?5 Ω cm when sintered above 200 °C. When sintered at 200 °C for 30 min, the average resistivity reaches 5 × 10^?6 Ω cm, which is slightly higher than the value obtained by using Ag nanoparticle paste. A SiC die was successfully bonded to a direct bonded copper substrate and the die-shear strength gradually increases with the increase in bonding temperature up to 300 °C. The Ag die attach bond layer was stable against thermal cycles between ?40 °C and 300 °C.     

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.     

Application of sputter-deposited amorphous and anatase TiO2 as electron-collecting layers in inverted organic photovoltaics

We demonstrate the deposition of amorphous and anatase TiO₂ on indium tin oxide (ITO) substrates via the process of sputtering, and the use of these materials as electron-collecting layers (ECLs) in inverted-type organic photovoltaics (OPVs). Anatase TiO₂ was obtained via vacuum-annealing of as-deposited amorphous TiO₂ at 300 °C. No deterioration of optical and electrical properties of ITO was observed after both sputter-deposition of TiO₂ and annealing process. The anatase TiO₂ proved to be an effective ECL when employed in inverted OPVs using bulk heterojunction photoactive layer of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, achieving a power conversion efficiency of 3.3% (J_SC = 9.0 mA cm^?2, V_OC = 0.62 V and FF = 0.60).     

Preparation of TiO2 paste starting from organic colloidal suspension for semi-transparent DSSC photo-anode application

We investigated the effect of different formulation strategies on the preparation of TiO2 screen printable pastes starting from the same organic colloidal suspension for dye-sensitized solar cells (DSSCs) application. As first strategy, we developed TiO2 pastes starting from nano-crystalline powder dried from the synthetized suspension, with and without an additional acid treatment, obtaining non-transparent scattering sintered layers. Instead, we demonstrate that starting from the sol solution is a key factor to obtain a well nano-dispersed TiO2 paste. The novel paste process resulted in a final homogeneous and semitransparent porous layer (T>80% @450–550 nm). In order to evaluate the properties of TiO2 powders and inks we performed thermal analysis, BET and FTIR measurements. The films were screen printed on FTO (fluorinedoped tin oxide) glass and characterized after sintering via profilometry, FE-SEM analysis and UVVis spectra measurements. The photo-anode was further validated in small (2 cm2) and large (90 cm2) active area DSSCs, giving an average photovoltaic efficiency respectively of 5,22% and 2,39%, comparable with commercially available TiO2 paste with similar composition.     

Improved performance of nanoporous TiO2 film in dye-sensitized solar cells via ZrCl4 and TiCl4 surface co-modifications

In this study, nanoporous TiO₂ films were modified by a dip-coating process using a mixture aqueous solution of ZrCl₄ and TiCl₄ followed by calcination to prepare a photoanode for dye-sensitized solar cells. Compared with the control film modified with 0.04 mol L⁻¹ TiCl₄, the power conversion efficiency of the TiO₂ film modified with a mixed solution of 0.05 mol L⁻¹ ZrCl₄ and 0.04 mol L⁻¹ TiCl₄, was 18.67% higher because of the improved short circuit current (J_sc) and open circuit voltage (V_oc). The improvement in J_sc was due to the suppression of charge recombination, which was demonstrated by a series of measurements, including electrochemical impedance spectroscopy, monochromatic incident photon-to-electron conversion efficiency spectroscopy, and the open-circuit voltage decay technique. The Mott-Schottky measurement results indicated that the negative shift of a flat band led to the increased V_oc.     

Enhanced printed temperature sensors on flexible substrate

In this paper we present the development of enhanced printed temperature sensors on large area flexible substrates. The process flow is a fully screen printed technology that uses exclusively solution-processed materials. These Screen printed temperature sensors are based on resistive pastes integrated in a Wheatstone bridge circuit. Substrate is a commercial Poly Ethylene Naphtalate (PEN) with a thickness of 125 µm. Functional temperature sensors are demonstrated and characterized with good electrical properties, showing a good sensitivity of 0.06 V/°C at V_in=4.8 V. This sensitivity is enhanced by the annealing and the O₂ plasma treatment. Based on this temperature sensor, we have developed a demonstrator for human body temperature detection.     

Low-operating-voltage polymeric transistor with solution-processed low-k polymer/high-k metal-oxide bilayer insulators

We have successfully demonstrated a polymeric semiconductor-based transistor with low-k polymer/high-k metal-oxide (TiO₂) bilayer as gate dielectric. The TiO₂ layers are readily processable from solution and cured at low temperature, instead of traditionally sputtering or high temperature sintering process, thus may suitable for a low-cost organic field effect transistors (FETs) manufacture. The low-k polymer capped on TiO₂ layer could further smooth the TiO₂ dielectric surface and suppress the leakage current from grain boundary of TiO₂ films. The resulting unpatented P3HT-OFETs could operate with supply voltage less than 10 V and the mobility and threshold voltage were 0.0140 cm²/V s and 1.14 V, respectively. The on/off ratio was 1.0 × 10³.     

Blocking layer optimisation of poly(3-hexylthiopene) based Solid State Dye Sensitized Solar Cells

The optimisation study of the fabrication of a compact TiO₂ blocking layer (via Spray Pyrolysis Deposition) for poly(3-hexylthiopene) (P3HT) for Solid State Dye Sensitized Solar Cells (SDSCs) is reported. We used a novel spray TiO₂ precursor solution composition obtained by adding acetylacetone to a conventional formulation (Diisopropoxytitanium bis(acetylacetonate) in ethanol). By Scanning Electron Microscopy a TiO₂ layer with compact morphology and thickness of around 100 nm is shown. Through a Tafel plot analysis an enhancement of the device diode-like behaviour induced by the acetylacetone blocking layer respect to the conventional one is observed. Significantly, the device fabricated with the acetylacetone blocking layer shows an overall increment of the cell performance with respect to the cell with the conventional one (ΔJ_sc/J_sc = +13.8%, ΔFF/FF = +39.7%, ΔPCE/PCE = +55.6%). A conversion efficiency optimum is found for 15 successive spray cycles where the diode-like behaviour of the acetylacetone blocking layer is more effective. Over three batches of cells (fabricated with P3HT and dye D35) an average conversion efficiency value of 3.9% (under a class A sun simulator with 1 sun A.M. 1.5 illumination conditions) was measured. From the best cell we fabricated a conversion efficiency value of 4.5% was extracted. This represents a significant increment with respect to previously reported values for P3HT/dye D35 based SDSCs.     

Ag–Al alloy thin film on plastic substrate by screen printing for solar cell back contact application

Ag–Al alloy thin films with different thicknesses were screen printed onto polyethylene terephthalate plastic substrates at room temperature. Three different weights of pure Ag paste were mixed with Al paste to create alloy pastes with different viscosities. A uniform composition of Ag and Al was obtained with a 70:30 composition ratio. The variation in the viscosity of the mixed paste enabled production of different thicknesses of printed layers. The deposited films were annealed at 200 °C for 45 min. Structural characterization and elemental content analysis of the films were carried out using X-ray diffraction (XRD), Raman spectroscopy, and energy-dispersive X-ray spectroscopy. The surface morphology of the printed films was studied by scanning electron microscopy and atomic force microscopy. Their electrical properties were investigated by four-point probe measurements. The crystalline sizes and strain along the a and c axes were calculated from the XRD patterns. Both were found to increase with increased film thickness.     

High photosensitivity with enhanced photoelectrical contribution in hybrid nanocomposite flexible UV photodetector

To devise a reliable strategy to develop an ultraviolet (UV) sensitive hybrid photodetector, plasma process is utilized as a single step method for production of large area nanocomposite films based on plasma polymerized aniline–titanium dioxide (PPani–TiO₂). The synthesis of PPani–TiO₂ nanocomposite films are made using reactive magnetron sputtering in combination with plasma polymerization. The deposited PPani–TiO₂ nanocomposite films are characterized and discussed in terms of structural, optical and electrochemical properties. A hybrid flexible nanostructured UV photodetector is constructed from PPani–TiO₂ nanocomposite and its optoelectronic properties are evaluated which exhibits a greatly enhanced photosensitivity resulting in high photoconductive gain (G = 4.56 × 10⁴) and high responsivity (R = 9.36 × 10³ AW⁻¹) under UV illumination of 254 nm. The flexible devices are successfully operated under bending up to 170° (bending radius, R = 8 mm) and showed a good folding strength and stability. The proposed plasma based method provides a green technology where the self-assembly of molecules, that is, the spontaneous association of atomic or molecular building blocks under plasma environment, emerge as a successful strategy to form well-defined structural and morphological units of nanometer dimensions.     

Rapid synthesis of TiO2/MWCNTs nanocatalyst with enhanced photocatalytic activity using modified microwave technique

This work described the rapid synthesis procedure for TiO₂/MWCNT nanocatalyst using a modified microwave method. Transmission electron micrographs suggested TiO₂ nanoparticles were strongly attached to the MWCNT. The surface area of the nanocatalysts determined by the Brunauer–Emmett–Teller method proved that the hybrid nanocatalyst have higher surface area (262.3 m²/g) than pristine TiO₂ (172.3 m²/g). The photocatalytic activity of the nanocatalyst was investigated under visible light (VL) illumination by studying the degradation of methylene blue dye (MB) with concentration of 10 ppm in 100 mL H₂O as a model compound. The results showed that the degradation efficiency obtained by using TiO₂/MWCNTs is about 40% higher than bare TiO₂. This signification enhancement was attributed to higher surface area and the ability of MWCNTs to act as a visible-light photosensitising agent revealing potential application in the treatment of wastewater.     

Optical and electrical characteristics of 17 keV X-rays exposed TiO2 films and Ag/TiO2/p-Si MOS device

This work presents the effect of varied doses of X-rays radiation on the Ag/TiO₂/p-Si MOS device. The device functionality was observed to depend strongly on the formation of an interfacial layer composed of SiO_x and TiO_y, which was confirmed by the spectroscopic ellipsometry. The XRD patterns showed that the as prepared TiO₂ films had an anatase phase and its exposure to varied doses of 17 keV X-rays resulted in the formation of minute rutile phase. In the X-rays exposed films, reduced Ti³⁺ state was not observed; however a fraction of Ti–O bonds disassociated and little oxygen vacancies were created. It was observed that the device performance was mainly influenced by the nature and composition of the interfacial layer formed at the TiO₂/Si interface. The spectroscopic ellipsometry was used to determine the refractive indices of the interfacial layer, which was 2.80 at λ=633 nm lying in between that of Si (3.87) and TiO₂ (2.11). The dc and frequency dependent electrical measurements showed that the interface defects (traps) were for both types of charge carriers. The presence of SiO_x was responsible for the creation of positive charge traps. The interface trap density and relaxation time (τ) were determined and analyzed by dc and frequency dependent (100 Hz–1 MHz) ac-electrical measurements. The appearance of peak in G/ω vs log (f) confirmed the presence of interface traps. The interface traps initially increased up to exposure of 10 kGy and then decreased at high dose due to compensation by the positive charge traps in SiO_x part of the interface layer. It was observed that large number of interface defects was active at low frequencies and reduced to a limiting value at high frequency. The values of relaxation time, τ ranged from 4.3±0.02×10⁻⁴ s at 0 V and 7.6±0.2×10⁻⁵ s at −1.0 V.     

Enhance the performance of quantum dot-sensitized solar cell by manganese-doped ZnS films as a passivation layer

To make quantum dot-sensitized solar cells (QDSSCs) more attractive, it is necessary to achieve higher power conversion efficiency. A novel Mn-doped ZnS has been successfully fabricated on CdS/CdSe quantum dots (QDs) by simple successive ion layer adsorption and reaction (SILAR) technique. The Mn-doped ZnS is used as a passivation layer in the QDSSCs. The performance of the QDSSCs was examined in detail using sulfide/polysulfide electrolyte with a Pt or copper sulfide (CuS) counter electrode. Here we demonstrated, the fabricated Mn-doped ZnS QDs shows an improved V_oc (0.65 V) compared to bare ZnS QDs (0.60 V). The QDSSC based on a photoanode with Mn-doped ZnS (10 wt% of Zn) shows higher J_sc (15.32 mA cm⁻²) and power conversion efficiency (4.18%) compared to the bare ZnS photoanode (2.90%) under AM 1.5 G one sun illumination. We explore the reasons for this enhancement and demonstrated that it is caused by improved passivation of the ZnS surface by Mn ions, leading to a lower recombination of photo-injected electrons with the electrolyte. The effect of Cu ions in ZnS has been investigated by UV–Vis spectra and current density–voltage analysis.     

Preparation of dye sensitized titanium oxide nanoparticles for solar cell applications

A new method for synthesis of titanium dioxide (TiO₂)-dye nanoparticles is reported. TiO₂ nanocrystals were obtained at 150 and 200 °C by using chemically bonded TiO₂-sensitizer dye as a precursor. Titanium tetraisopropoxide was first modified with a dye molecule and then precipitated by dropping into acidic water. A strongly colored precipitate was obtained. Hydrothermal growth of a colloidal solution was carried out in a Teflon-lined stainless steel autoclave. Dye sensitized solar cell efficiencies obtained were comparable and fill factor values were close to the analogous cells prepared by the use of conventional TiO₂ paste techniques. This method allows the use of different substrates together with nanocrystalline TiO₂ for many technological applications.     

Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer

TiO₂ sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO₂ nanoparticles (TiO₂ NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H₂O in the process of peptization both the crystallinity and particle size of TiO₂ NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO₂ NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO₂ NPs/Al showed the short-circuit current (J_sc) of 12.83 mA/cm² and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO₂ buffer layer. The stability measurement showed that PSC devices with a TiO₂ NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H₂O diffusion into the active layers. The observations indicate that TiO₂ NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs.