Solid-state dye-sensitized photocell based on pentacene as a hole collector

A solid-state dye-sensitized photovoltaic cell consisting of vacuum deposited pentacene onto ruthenium dye-coated TiO₂ electrode doped with iodine was fabricated. Cell delivers a short-circuit current of 3.6 mA cm⁻² and an open-circuit voltage of 415 mV at 100 mW cm^–2 (1.5 air mass). The efficiency and the fill factors of the above cell are 0.8% and 0.5%, respectively. Studies of the photocurrent action spectra showed that the dye is mainly responsible for this photocurrent generation. Preliminary results under extended illumination suggested that “long term” stability of the cell is promising.     

TiO2 thin films as protective material for transparent-conducting oxides used in Si thin film solar cells

Nb-doped TiO₂ films have been fabricated by RF magnetron sputtering as protective material for transparent-conducting oxide (TCO) films used in Si thin film solar cells. It is found that TiO₂ has higher resistance against hydrogen radical exposure, utilizing the hot-wire CVD (catalytic CVD) apparatus, compared with SnO₂ and ZnO. Further, the minimum thickness of TiO₂ film as protective material for TCO was experimentally investigated. Electrical conductivity of TiO₂ in the as-deposited film is found to be 10⁻⁶ S/cm due to the Nb doping. Higher conductivity of 10⁻² S/cm is achieved in thermally annealed films. Nitrogen treatments of Nb-doped TiO₂ film have been also performed for improvements of optical and electric properties of the film. The electrical conductivity becomes 4.5×10⁻² S/cm by N₂ annealing of TiO₂ films at 500 °C for 30 min. It is found that the refractive index n of Nb-doped TiO₂ films can be controlled by nitrogen doping (from n=2.2 to 2.5 at λ = 550 nm) using N₂ as a reactive gas. The controllability of n implies a better optical matching at the TCO/p-layer interface in Si thin film solar cells.     

TiO2-assisted degradation of acid orange 7 textile dye under solar light

Photocatalytic degradation of acid orange 7 (AO7) in aqueous systems was successfully achieved by the combination of TiO₂ with potassium persulphate under solar light using a photochemical reactor with recirculation. Degradation of AO7 involves color removal and mineralization. The employment of TiO₂ removed 85% of color from the 0.2 mM AO7 aqueous solution under solar light; while, 66% of color was abated using the persulphate ion as oxidant in the absence of TiO₂ under similar conditions in 2 h. However, over 90% of color removal was achieved by combining TiO₂ and the persulphate ion for the same solution under similar conditions. Color removal was faster at pH 3. Mineralization of AO7 was followed by measuring chemical oxygen demand (COD). Negligible COD abatement of the textile dye was observed in the absence of persulphate ions (S₂O₈²⁻) while over 70% of COD abatement was observed for the initial dye concentrations of 0.2–0.7 mM employing a mix of TiO₂–S₂O₈²⁻ under solar light.     

Structural, electrical and photovoltaic characterization of Si nanocrystals embedded SiC matrix and Si nanocrystals/c-Si heterojunction devices

Thin films of Si nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix (Si-NC:SiC) were prepared by alternating deposition of Si-rich silicon carbide (Si_1−xC_x) and near-stoichiometric SiC mutilayers (Si_1−xC_x/SiC) using magnetron cosputtering followed by a post-deposition anneal. Transmission electron microscopy and Raman spectroscopy revealed that the Si NCs were clearly established, with sizes in the range of 3–5 nm. Optical studies showed an increase in the optical band gap after annealing from 1.4 eV (as-deposited) to 2.0 eV (annealed at 1100 °C). P-type Si-NC:SiC/n-type crystalline silicon (c-Si) heterojunction (HJ) devices were fabricated and their electrical and photovoltaic properties were characterized. The diode showed a good rectification ratio of 1.0×10⁴ at the bias voltage of ±1.0 V at 298 K. The diode ideality factor and junction built-in potential deduced from current–voltage and capacitance–voltage plots are 1.24 and 0.72 V, respectively. Illuminated I–V properties showed that the 1-sun open-circuit voltage, short-circuit current density and fill factor of a typical HJ solar cell were 463 mV, 19 mA/cm² and 53%, respectively. The external quantum efficiency and internal quantum efficiency showed a higher blue response than that of a conventional c-Si homojunction solar cell. Factors limiting the cell's performance are discussed.     

Deposition pressure effects on material structure and performance of micromorph tandem solar cells

Tandem solar cells represent an elegant way of overcoming the efficiency limits of single-junction solar cells and reducing the light-induced degradation of amorphous silicon films. Stacked structures consisting of an amorphous silicon top cell and a microcrystalline silicon bottom cell allow a good utilization of the solar spectrum due to the band gap values of the two materials. These devices, firstly introduced by the IMT research group, were designated as “micromorph” tandem solar cells. To better exploit this concept, it is important to tune parameters like the band gaps and the short-circuit currents.In this work, we have realized micromorph tandem solar cells on Asahi U-type TCO-covered glass substrates. The intrinsic layer of both the amorphous top cell and the microcrystalline bottom cell is grown by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at 100 MHz at low substrate temperature (150 °C). Finally, a ZnO reflector and a metal contact complete the structure. No intermediate optical mirror between the two cells is used at this stage. Undiluted a-Si:H, with reduced band gap when compared to H₂-diluted amorphous silicon, is used as absorber layer in the top cell. As for the bottom cell, the high-pressure–high-power regime (up to 267 Pa–80 W) has been explored aiming at growing high-quality microcrystalline silicon at large deposition rates. The effect of the structural composition of the microcrystalline absorber layer on the current–voltage characteristic and spectral response of tandem devices has been investigated. An efficiency of 11.3% has been obtained with short-circuit current densities around 13 mA/cm², open-circuit voltages 1.34 V and fill factors 66%.     

Dye-sensitized titania aerogels as photovoltaic electrodes for electrochemical solar cells

We describe the fabrication and performance of dye-sensitized photoanodes derived from TiO₂ aerogel. Nanocrystalline titania aerogel is a bicontinuous, nanostructured pore–solid architecture featuring specific surface areas of 85–150 m²/g and a continuous mesoporous network, allowing chemisorption of high concentrations of sensitizing dye and rapid mass-transport of electron-transfer mediators. Considerable design and processing flexibility arises with aerogels because the continuous pore–solid networks are fixed by the supercritical drying process, allowing the creation of multifunctional, nanostructured films of single or multiple layers. Titania aerogels can be processed as powders and deposited as nearly opaque films from 2 μm to >35-μm thick while retaining their bicontinuous nanoscale networks. Two-layer, 30-μm-thick TiO₂ aerogel films yield incident photon-to-electron conversion efficiency (IPCE) values of 85% in the 500–600 nm range and 52% at 700 nm with N719 as a sensitizing dye and after correcting for transmittance of the 3.2-mm-thick FTO-coated glass substrates at these wavelengths.     

Pt–NiO nanophase electrodes for dye-sensitized solar cells

A new type of counter electrode comprising of Pt and NiO biphase was prepared an RF magnetron cosputtering system for a dye-sensitized solar cell (DSSC). Transmission electron microscope images, transmission electron diffraction patterns, and X-ray diffraction patterns of the Pt–NiO electrodes confirmed the formation of a nanosized Pt polycrystalline phase of 7 nm mixed with porous amorphous NiO phase. The short-circuit current density and cell efficiency were increased from 0.22 to 0.30 mA/cm² and from 2.1% to 2.8%, respectively, and almost constant open-circuit voltage and fill factor, 0.53 V and 63%, respectively, were observed.     

A 4.2% efficient flexible dye-sensitized TiO2 solar cells using stainless steel substrate

An efficient flexible dye-sensitized solar cells (DSSCs) using stainless steel supporting substrate for fabricating nanocrystalline TiO₂ film electrodes were developed, intending to improve the photoelectrochemical properties of plastic substrate-based DSSCs. The most important advantage of a stainless steel-based TiO₂ film electrode over a plastic-based electrode lies in its high-temperature sinterability. Optimal photovoltaic properties were obtained with a cell where the TiO₂ film was coated on both ITO- and SiO_x-sputtered stainless steel (denoted as TiO₂/ITO/SiO_x/StSt). The photocurrent of the flexible cells with a TiO₂/ITO/SiO_x/StSt electrode increased significantly, leading to a much higher overall solar conversion efficiency η of 4.2% at 100 mW/cm², based on short-circuit photocurrent density, open-circuit voltage and fill factor of 11.2 mA/cm², 0.61 and 0.61 V, respectively, than those reported for cells with plastic substrates.     

Optical simulation of transmittance into a nanocrystalline anatase TiO2 film for solar cell applications

Optical simulation has been employed, for the first time, for rigorous evaluation of transmittance into the TiO₂ nanocrystalline film, entering from the fluorine-doped SnO₂ (F-SnO₂) coated glass side, in dye sensitized solar cells. The refractive index of the TiO₂ film with various porosities was determined theoretically, and was in agreement with the data obtained by ellipsometric measurements. The simulation clearly indicates that the transmittance into the TiO₂ film is 85–90% at 450–800 nm, on adjusting the porosity to 0.5–0.75. In contrast, transmittances experimentally determined for the TiO₂ film deposited on F-SnO₂ exhibits 70–83% at 450–800 nm, under-estimating the transmittance by about 10% compared to the simulated results. The simulation method was further substantiated by observing the high IPCE value (85% at 530 nm) for the solar cell using the same TiO₂ film sensitized by ruthenium dye.     

The effect of pre-thermal treatment of TiO2 nano-particles on the performances of dye-sensitized solar cells

We have demonstrated the effect of pre-thermal treatment of TiO₂ nano-particles on the performances of dye-sensitized solar cells (DSCs) by using high specific surface area and anatase only TiO₂ nano-particles (ca. 340 m²/g, Sachtleben Chemie GmgH, represented as HK). TiO₂ particles and thin films were characterized with X-ray diffraction, FT-IR, UV–Vis diffuse reflectance spectroscopy and FE-SEM. The photoelectrochemical properties of the thin films and the performances of DSCs were measured by photocurrent densities, AC impedance spectra and photocurrent–voltage curves. Before coating the raw TiO₂ of HK (HK-raw) on transparent conducting oxide (TCO) glass for DSC fabrication, pre-thermal treatment of HK-raw by calcining at 450 °C (HK-450) was an essential step to achieve the optimum properties in terms of morphological feature, crystallinity, specific surface area and photocurrent density. HK-450 film showed the high adsorption of dye, high photocurrent density and low interface resistance between TiO₂ and TCO glass, R_TiO2/TCO and TiO₂ and redox electrolyte, R_CT, resulting in the superior photovoltaic performance on the DSC fabricated with HK-450 and Eosin Y (or ruthenium 535 bis-TBA) at AM 1.5: open-circuit voltage of 0.62 V (0.77 V), short-circuit current of 3.03 mA/cm² (22.80 mA/cm²), fill factor of 0.57 (0.44) and overall conversion efficiency of 1.06%, (7.52%). Accordingly, the optimization between the morphological feature, specific surface area and photocurrent density of TiO₂ substrate is promising to accomplish the improved overall conversion efficiency of DSC.     

Energy analysis of silicon solar cell modules based on an optical model for arbitrary layers

An optical model for arbitrary layers is developed and a one-dimensional steady-state thermal model is applied to analyze the energy balance of silicon solar cell modules. Experimental measurements show that simulations are in good agreement, with a maximum relative error of 8.43%. The wind speed v_wind, ambient temperature T_amb and irradiance G are three main factors influencing the temperature of a photovoltaic panel. Over the course of a day the electrical output is reduced by the module temperature to only 32.5% of the rated value. Optical studies reveal that before 8:00 hours and after 16:00 hours, significant incident energy is lost by reflection because of the large angle of incidence θ_in, while at other times of day optical losses are nearly the same due to only small changes of transmission for θ_in < 45°. In addition, some optical losses result from the mismatched refractive indexes of encapsulating materials, especially at the ethylene-vinyl-acetate (EVA)/anti-reflection coating (ARC) and the ARC/Si interfaces. The uses of SiO₂ and TiO₂ as ARC materials for un-encapsulated and encapsulated Si solar cells are investigated by simulation. Comparing the results indicates that TiO₂ as ARC reduces the reflective optical loss within λ = 0.4–1.1 μm after encapsulation, while SiO₂ as ARC increases the loss by 5%. Energy allotment analysis shows that from 9:00 to 15:00, the reflective and transmissive optical losses are relatively steady at 26% and 13% of the incident energy, while the convective and radiative heat losses account for a further 30% and 24%, respectively. Thus, only 7% of incident energy is converted to electrical power.     

Hydrogen energy in changing environmental scenario: Indian context

This paper deals with how the Hydrogen Energy may play a crucial role in taking care of the environmental scenario/climate change. The R&D efforts, at the Hydrogen Energy Center, Banaras Hindu University have been described and discussed to elucidate that hydrogen is the best option for taking care of the environmental/climate changes. All three important ingredients for hydrogen economy, i.e., production, storage and application of hydrogen have been dealt with. As regards hydrogen production, solar routes consisting of photoelectrochemical electrolysis of water have been described and discussed. Nanostructured TiO₂ films used as photoanodes have been synthesized through hydrolysis of Ti[OCH(CH₃)₂]₄. Modular designs of TiO₂ photoelectrode-based PEC cells have been fabricated to get high hydrogen production rate (10.35 lh⁻¹ m⁻²). However, hydrogen storage is a key issue in the success and realization of hydrogen technology and economy. Metal hydrides are the promising candidates due to their safety advantage with high volume efficient storage capacity for on-board applications. As regards storage, we have discussed the storage of hydrogen in intermetallics as well as lightweight complex hydride systems. For intermetallic systems, we have dealt with material tailoring of LaNi₅ through Fe substitution. The La(Ni_l − xFe_x)₅ (x = 0.16) has been found to yield a high storage capacity of 2.40 wt%. We have also discussed how CNT admixing helps to improve the hydrogen desorption rate of NaAlH₄. CNT (8 mol%) admixed NaAlH₄ is found to be optimum for faster desorption (3.3 wt% H₂ within 2 h). From an applications point of view, we have focused on the use of hydrogen (stored in intermetallic La–Ni–Fe system) as fuel for Internal Combustion (IC) engine-based vehicular transport, particularly two and three-wheelers. It is shown that hydrogen used as a fuel is the most effective alternative fuel for circumventing climate change.     

A note on fast protonic solid state electrochromic device: NiOx/Ta2O5/WO3−x

In the present investigation, the electrochromic properties of a fast protonic solid state device: NiO_x/Ta₂O₅/WO_3−x prepared at room temperature (300 K) is reported. The non-stoichiometric tungsten oxide thin film is prepared by reactive DC magnetron sputtering technique on ITO coated glass; the oxides of tantalum (300 nm) and nickel (100 nm) are prepared by electron beam evaporation. This proton device has a coloration efficiency of 82.4 cm²/C and coloration and bleaching time of 6 and 5 s, respectively, and a transmittance variation of 60%. The work function of WO_3−x thin films by Kelvin probe in uncolored and colored states are 4.73 and 4.30 eV, respectively.     

Thermal thiocyanate ligand substitution kinetics of the solar cell dye N719 by acetonitrile, 3-methoxypropionitrile, and 4-tert-butylpyridine

The kinetics of the thiocyanate substitution of the solar cell sensitizer [Ru(Hdcbpy)₂(NCS)₂]²⁻, 2(n-C₄H₉)₄N⁺ (H₂dcbpy=L=2,2′-bipyridine-4,4′-dicarboxylic acid), known as N719, by acetonitrile, 3-methoxypropionitrile, and 4-tert-butylpyridine (4-TBP) have been determined in both homogenous solutions and colloidal mixtures of N719-dyed TiO₂ nanocrystalline particles. Thiocyanate ligand substitution by the solvents (S) acetonitrile or 3-methoxypropionitrile in homogeneous solutions occurs at elevated temperatures (80–110 °C) by means of a simple slow pseudo-first-order reaction leading to the formation of the product [RuL₂(NCS)(S)]⁺ with a half-life time t_1/2 2000 h of N719 at 80 °C. If tert-butylpyridine (0.5 M) is added, the end product instead becomes [RuL₂(NCS)(4-TBP)]⁺ with a t_1/2 1000 h. When N719 is bound to TiO₂ particles, the reactions with S and 4-TBP give the same products as occur in the homogenous solutions; however, the reactions are approximately 10 times faster. For the reaction of a colloidal mixture of N719-dyed TiO₂ particles in acetonitrile containing 0.5 M 4-TBP, a t_1/2(het) of 120 h was calculated at 85 °C. The N719-based DSSC cells with acetonitrile and 4-TBP as solvent and additive are therefore not expected to be able to pass a 1000-h thermal stress test in the dark at 85 °C due to thermal degradation of the N719 dye. Adding guanidine thiocyanate to the colloidal solutions, however, decreased the rate of [RuL₂(NCS)(4-TBP)]⁺ formation by a factor of 2–10; it thus may be used as an additive to prevent the thermal degradation of thiocyanate-based ruthenium complexes in DSSC solar cells.     

Visible-light-driven TiO2 catalysts doped with low-concentration nitrogen species

Visible-light-driven nitrogen-doped TiO₂ was synthesized using a novel nitrogen-ion donor of hydrazine hydrate. Low-concentration (0.2 at%) nitrogen species and Ti³⁺ were detected in the TiO₂-based photocatalyst by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy. The trace amount of Ti–N would contribute to the minor band-gap narrowing of about 0.02 eV. Those nitrogen-containing species, especially the NO₂²⁻ species, form surface states, which make the catalysts possible to degrade 4-chlorophenol (4-CP) under visible irradiation (λ>400 nm). Moreover, Ti³⁺ species induce oxygen vacancy states between the valence and the conduction bands, which would also contribute to the visible response. The photocatalytic activity of the nitrogen-doped TiO₂ catalyst was thought to be the synergistic effect of nitrogen and Ti³⁺ species. The catalysts showed higher photocatalytic activity for degradation of 4-CP than pure TiO₂ under not only visible but also UV irradiation. The visible response and the higher UV activity of the nitrogen-doped TiO₂ make it possible to utilize solar energy efficiently to execute photocatalysis processes.     

Microcrystallisation in Si:H: the effect of gas pressure in Ar-diluted SiH4 plasma

Using noble gas argon as a diluent of SiH₄ in RF glow discharge, undoped μc-Si:H thin films have been developed at a low power density of 30 mW/cm². It has been found that the gas pressure is a critical factor for the growth of μc-Si:H films. Undoped μc-Si:H films having σ_D10⁻⁶ S/cm and ΔE<0.57 eV have been obtained at and above a critical pressure of 0.8 Torr. When the RF power density is increased to 90 mW/cm², a more crystalline as well as highly conducting (σ_D10⁻⁴ S/cm) μc-Si:H film has been achieved at a deposition rate of 30 Å/min, which is much higher than that attained from H₂-diluted SiH₄ plasma, by conventional approach. The crystallinity of the films has been identified by the sharp Raman peak at 520 cm⁻¹ and a large number of micrograins in the TEM micrographs. The metastable state of Ar, denoted as Ar^*, plays the crucial role in inducing microcrystallisation by transferring its de-excitation energy at the surface of the growing film. A mechanism has been proposed to explain the dependence of the formation of μc-Si:H film on the working gas pressure in the plasma.     

Effect of a redox electrolyte in mixed solvents on the photovoltaic performance of a dye-sensitized solar cell

The effect of the iodide/triiodide redox electrolyte in various organic solvents on the photoelectrochemical properties of bis(tetrabutylammonium) cis-bis(thiocyanato)bis(4-carboxy-2,2′-bipyridine-4′-carboxylato)ruthenium(II)-sensitized nanocrystalline TiO₂ solar cells was studied. Solvents with large donor numbers dramatically enhanced the open-circuit voltage (V_oc), but usually reduced the short-circuit photocurrent density (J_sc). For a mixed solvent of tetrahydrofuran (THF) and acetonitrile, V_oc increased and the fill factor decreased with increasing THF concentration, but J_sc remained relatively constant. As the partial charge of the N or O atom of the solvent molecule increased, V_oc increased, but J_sc was unchanged up to a certain value of the partial charge (for THF, −0.46). For cells using 0.3 M 4-tert-butylpyridine and 20 vol% THF in the electrolyte, a short-circuit photocurrent density of 18.23 mA cm⁻², an open-circuit voltage of 0.73 V, a fill factor of 0.73, and an overall conversion efficiency of 9.74% were obtained.     

The growth kinetics and optical confinement studies of porous Si for application in terrestrial Si solar cells as antireflection coating

The mesoporous porous-silicon (PS) layers were grown on 1 0 0, 1 1 0, and 1 1 1 oriented wafers at constant current density of 20 mA cm⁻². The pore sizes and surface morphologies were measured by atomic force and scanning electron microscopes. The thickness x of the PS formed and the refractive index were measured by an ellipsometer as a function of time duration t (in min) of anodization. The x vs. t data were fitted into a power law x=at^c where c is a dimensionless constant and growth kinetics was established. The growth is practically independent of orientation. This is due the reason that the growth rate is controlled largely by the availability of holes which exchange their charge with oxidizing species and desirably large concentrations of holes were available at current density of 20 mA cm⁻². For a similar reason the growth of PS layer on the front surface of the n⁺ region of n⁺–p solar cells could also be done at current density of 20 mA cm⁻² nearly at the same rate. A large concentration of holes could be injected from p region into the n⁺ region because the positive contact was made on the p side and thus the junction was forward biased. The PS ARC of thickness 70 nm showed increase 26% in the short circuit current density J_sc and 24% in efficiency of the cells. However, the improvement in the values of the open circuit voltage V_oc were lower than the expected value indicating that the PS layers had enhanced recombination of minority carriers at the front surface or in the front emitter region immediately below the PS layer.     

Miniaturization limits for single-chamber micro-solid oxide fuel cells with coplanar electrodes

Single-chamber solid oxide fuel cells with coplanar microelectrodes were operated in methane–air mixtures (R_mix = 2) at 700 °C. The performance of cells with one pair of NiO–YSZ (yttria stabilized zirconia) anode and (La_0.8Sr_0.2)_0.98MnO₃–YSZ cathode, arranged parallel on a YSZ electrolyte substrate, was found to be significantly dependent on the electrode width. For an interelectrode gap of 250 μm, cells with average electrode widths exceeding 850 μm could establish a stable open circuit voltage (OCV) of 0.8 V, while those with widths less than 550 μm could not establish any OCV. In the intermediate range, the cells exhibited significant fluctuations in voltage and power under our testing conditions. This behavior suggests that a lower limit to electrode dimensions exists for cells with single electrode pairs, below which neither a stable difference in oxygen partial pressure, nor an OCV, can be established. Conversely, increasing the electrode width imposes a penalty in the form of an increase in the cell resistance. However, both size limits can be circumvented by employing multiple pairs of microscale electrodes in an interdigitated configuration.     

Structure, morphology and electrochemical behaviour of manganese oxides prepared by controlled decomposition of permanganate

Hydrothermal decomposition of permanganate, conducted in a range of pH-controlled solutions (from strongly acidic to strongly basic), is used to prepare manganese dioxides that are well-suited for use as supercapacitor electrode materials. While permanganate is thermodynamically unstable, the kinetics of its decomposition in an aqueous environment are very slow, until the temperature is raised to 200 °C. Although the resultant materials are relatively crystalline and have low total pore volume, their prominent meso-porosity leads to good electrochemical performance. Best behaviour is obtained for material from permanganate decomposition in 0.01 M H₂SO₄ solution, for which composite electrodes (150 μm thick) yield 150 F g⁻¹ at 5 mV s⁻¹ in a 9 M KOH electrolyte.