Preparation of ZnO thin films using MOCVD technique with D2O/H2O gas mixture for use as TCO in silicon-based thin film solar cells

Zinc oxide (ZnO) thin films have been successfully grown by metal organic chemical vapor deposition (MOCVD) technique using deuterium water (D₂O) and water (H₂O) mixtures as oxidants for diethylzinc (DEZ). B₂H₆ was also employed as a dopant gas. It was found that the crystal orientation of ZnO films strongly depends on D₂O/H₂O ratio. As a result, the surface morphology of ZnO changed from textured surface morphology to smooth surface morphology with increase in the ratio of D₂O/H₂O. Moreover, it was also observed that the carrier concentration of ZnO films did not change with the ratio of D₂O/H₂O, while the mobility of these films was strongly dependent on the D₂O/H₂O ratio. Without D₂O addition, the resistivity of films had its lowest value and the minimum sheet resistance was 10 Ω/square. All films showed transmittance higher than 80% in the visible region. Moreover, the haze values of these films could be controlled by the ratio of D₂O/H₂O. These results indicate that the crystal orientation and surface morphology of the low resistivity ZnO films can be modified by using a mixture of D₂O and H₂O without changing the deposition temperature. Thus, the obtained ZnO films are promising for use as a front TCO layer in Si-based thin film solar cells.     

Electrical and optical properties of F-doped textured SnO2 films deposited by APCVD

This paper presents the structural, electrical and optical properties of transparent conducting F-doped textured SnO₂ films prepared by atmosphere pressure chemical vapour deposition (APCVD). Polycrystalline SnO₂:F films having a variable preferred orientation have been obtained with resistivity as low as 5 × 10⁻⁴ Ωcm, with carrier concentrations between 3.5 × 10²⁰ and 7 × 10²⁰ cm⁻³, and Hall mobilities from 15.7 to 20.1 cm²/V/s. The average transmittance (including diffusion transmittance) is as high as 94% in the wavelength range of the visible spectrum and the maximum infrared reflectance reaches 92% for a film 655 nm thick. The figure of merit ƒ_TC = T10/_sh, (7.12 × 10⁻² S) of these films is the highest amongst the results reported on doped SnO₂ films.     

Photoelectrochemical properties of ruthenium dye-sensitized nanocrystalline SnO2:TiO2 solar cells

The photoelectrochemical properties of RuL₂(NCS)₂ dye-sensitized nanocrystalline SnO₂:TiO₂ coupled and composite solar cells are reported. The coupled (bilayer) system shows higher incident photon-to-current conversion efficiency (IPCE) value than the composite (mixture) system. A maximum IPCE value attained 82.4% at 530 nm wavelength in the coupled system with 3.5 μm-thick SnO₂ and 7 μm-thick TiO₂. The higher IPCE value in the coupled system is attributed to the promotion of the charge separation by fast electron transfer process in the SnO₂/TiO₂/RuL₂(NCS)₂ system with different energy levels, different conduction band edge energy positions.     

SnO2:F coated ferritic stainless steels for PEM fuel cell bipolar plates

Ferrite stainless steels (AISI441, AISI444, and AISI446) were successfully coated with 0.6 μm thick SnO₂:F by low-pressure chemical vapor deposition and investigated in simulated PEMFC environments. The results showed that a SnO₂:F coating enhanced the corrosion resistance of the alloys in PEMFC environments, though the substrate steel has a significant influence on the behavior of the coating. ICP results from the testing solutions indicated that fresh AISI441 had the highest dissolution rates in both environments, and coating with SnO₂:F significantly reduced the dissolution. Coating AISI444 also improved the corrosion resistance. Coating AISI446 steel further improved the already excellent corrosion resistance of this alloy. For coated steels, both potentiostatic polarizations and ICP results showed that the PEMFC cathode environment is much more corrosive than the anode one. More dissolved metallic ions were detected in solutions for PEMFC cathode environment than those in PEMFC anode environment. Sn²⁺ was detected for the coated AISI441 and AISI444 steels but not for coated AISI446, indicating that the corrosion resistance of the substrate has a significant influence on the dissolution of the coating. After coating, the ICR values of the coated steels increased compared to those of the fresh steels. The SnO₂:F coating seems add an additional resistance to the native air-formed film on these stainless steels.     

Growth of anatase and rutile TiO2@Sb:SnO2 heterostructures and their application in photoelectrochemical water splitting

We report three-dimensional (3D) nanostructures based on shape- and phase-controlled TiO₂ coated transparent conducting oxide (TCO) nanowire array. Core-shell and branched nanostructures were obtained using an aqueous chemical bath deposition (CBD) method at room temperature. Adjusting the pH of a TiCl₄ solution is a key factor that determines the morphology of the nanostructure. Spherical TiO₂ anatase covered a Sb-doped SnO₂ (ATO) nanowire when pH was maintained at a high level. In contrast, branched nanostructures with TiO₂ rutile nanorods were synthesized by keeping a TiCl₄ solution going down to a low pH. Nanorods were grown epitaxially along the [001] direction on ATO nanowires. Morphological and structural analysis indicates that phases and shapes of the 3D hybrid nanostructure are determined by the pH of the solution and the reaction time. A two-fold higher photoconversion efficiency of rutile TiO₂ rod@ATO was obtained under simulated solar illumination compared to that of the anatase TiO₂ nanoshell@ATO. These 3D hybrid nanostructures can offer (i) a large surface area and efficient charge transport in the TiO₂ nanostructure, and (ii) an effective charge collection path through one-dimensional TCO, which is promising for various areas, including photoelectrochemical water splitting, as well as for application in electronic and photonic nanodevices.     

SnO2:F coated austenite stainless steels for PEM fuel cell bipolar plates

Austenite stainless steels (316L, 317L, and 349™) have been coated with 0.6 μm thick SnO₂:F by low-pressure chemical vapor deposition and investigated in simulated polymer electrolyte membrane fuel cell (PEMFC) environments. The results showed that substrate steel has a significant influence on the behavior of the coating. Coated 316L showed a steadily increasing anodic current in PEMFC environments, indicating that it is not suitable for this alloy/coating combination. Coated 349™ showed a cathodic current in the PEMFC anode environment, demonstrating its stability in the PEMFC cathode environment. Coated 317L exhibited a stable anodic current after a current peak (at ca. 14 min) in the PEMFC anode environment, and showed an extremely stable low current in PEMFC cathode environment, suggesting the possibility of using SnO₂:F coated 317L for PEMFC bipolar plate applications.     

Aging effect of diethanolamine stabilized sol on different properties of TiO2 films: Electrochromic applications

Diethanolamine derived clear precursor sol has been utilized for the deposition of TiO₂ films annealed at 470 °C for 5 min. Effect of the precursor sol's aging on different properties of the films has been examined in the present study. Films obtained from aged sol have exhibited superior electrochemical (diffusion coefficient—2.46×10⁻¹⁰ cm² s⁻¹) and electrochromic characteristics due to enhanced Li ion insertion upon application of electric field. The aged sol derived films have exhibited a higher optical modulation (40% at 550 nm) between the colored and bleached states. The ion storage capacities of the films derived from freshly prepared and aged sols are 4.1 and 8.1 mC cm⁻², respectively, upon applied voltage of ±1.5 V. X-ray diffraction studies have affirmed an increase in the TiO₂ crystallite size upon the use of aged sol for the deposition of films. FTIR investigations have confirmed the conversion of Ti–O–Ti to Ti–O network in the aged sol derived films. SEM studies have evidenced porosity changes in films obtained from the sol aged for different durations. The index of refraction as measured by the ellipsometry method corroborates the SEM results and shows reduced porosity (pore size—38 nm) in films derived from the sol just reaching the state of gelation. Thickness of the aged sol derived film is measured to be the highest i.e. 350 nm. Energy bandgaps of the films for both direct and indirect transitions tend to decrease as a function of sol's aging.     

Hybrid solar cells based on MEH-PPV and thin film semiconductor oxides (TiO2, Nb2O5, ZnO, CeO2 and CeO2–TiO2): Performance improvement during long-time irradiation

Performance improvement of hybrid solar cells (HSC) applying five different thin film semiconductor oxides has been observed during long-time irradiation in ambient atmosphere. This behavior shows a direct relation between HSC and oxygen content from the environment. Photovoltaic devices were prepared as bi-layers of thin film semiconducting oxides (TiO₂, Nb₂O₅, ZnO, CeO₂–TiO₂ and CeO₂) and the polymer MEH-PPV, with a final device configuration of ITO/Oxide_{thin film}/MEH-PPV/Ag. The oxides were prepared as thin transparent films from sol–gel solutions. The photovoltaic cells were studied in ambient atmosphere by recording the initial values of open circuit voltage (V_oc) and current density (I_sc). Solar decay curves presented as the measurement of the short circuit current as a function of time, IV curves and photophysical analyses were also carried out for each type of device. Solar cells with TiO₂ thin films showed the best performance with maximum V_oc as high as −0.74 V and I_sc of 0.4 mA/cm². Solar decay analyses showed that the devices require a stabilization period of several hours in order to reach maximum performance. In the case of TiO₂, Nb₂O₅ and CeO₂–TiO₂, the maximum current density was observed after 15 h; for CeO₂, the maximum performance was observed after 30 h. The only exception was observed with devices applying ZnO in which the current density decreased drastically and degraded the polymer in just a couple of hours.     

ITO/ATO/TiO2 triple-layered transparent conducting substrates for dye-sensitized solar cells

A novel transparent conductive oxide film based on the triple-layered indium tin oxide (ITO)/antimony-doped tin oxide (ATO)/titanium oxide (TiO₂) has been developed for dye-sensitized solar cells by using radio frequency magnetron sputtering technique. Effects of the absence and presence of TiO₂ layer and the ITO layer thickness were investigated. Deposition of ATO layer was found to stabilize the thermal instability of ITO. Little change in sheet resistance and optical transmittance was observed by introduction of insulating thin TiO₂ layer on top of the ATO layer, whereas photovoltaic performance was significantly influenced. The conversion efficiency was improved from 4.57% without TiO₂ layer to 6.29% with TiO₂ layer. The enhanced photovoltaic performance with addition of TiO₂ layer was attributed mainly to the improved adhesion and partially to the reduced electron loss at the ITO/ATO conductive layer. Increase in the ITO layer thickness resulted in a slight decrease in photocurrent due to the reduced optical transmittance. When compared with the conventional fluorine-doped tin oxide (FTO), the ITO/ATO/TiO₂ conductive material exhibited similar photocurrent density but higher photovoltage and fill factor, resulting in better conversion efficiency.     

Theoretical modeling of TiO2/TCO interfacial effect on dye-sensitized solar cell performance

A theoretical model based on an integration of both Schottky barrier model and electron diffusion differential model was developed to determine the TiO₂/TCO interfacial effect on the current–voltage (J–V) characteristics of a dye-sensitized solar cell (DSSC). The thermionic-emission theory was appropriately applied to describe the electron transfer at the TiO₂/TCO interface. A parametric analysis was conducted to study how the photoelectric outputs varied with multiple independent variables, such as Schottky barrier height (φ_b) and temperature. It was found that the variation of the maximum DSSC power output (P_max) was insignificant when φ_b varied at a low value; however, an increase in φ_b exceeding a critical value caused an apparent decrease in the maximum DSSC power output. The theoretical results were quantitatively compared and agreed very well with published theoretical results. The experimental data from literature were found to agree well with the present theoretical results, qualitatively validating the present model. The theoretical model can be applied to facilitate selection of suitable TCO material in DSSC design to avoid the adverse TiO₂/TCO interfacial effect.     

Fabrication of microcrystalline silicon solar cells on a SnO2 coated substrate using seed layer insertion

We fabricated hydrogenated microcrystalline silicon (μc-Si:H) solar cells on SnO₂ coated glass using a seed layer insertion technique. Since rich hydrogen atoms from the μc-Si:H deposition process degrade the SnO₂ layer, we applied p-type hydrogenated amorphous silicon (p-a-Si:H) as a window layer. To grow the μc-Si:H layer on the p-a-Si:H window layer, we developed a seed layer insertion method. We inserted the seed layer between the p-a-Si:H layer and intrinsic bulk μc-Si:H. This seed layer consists of a thin hydrogen diluted silicon buffer layer and a naturally hydrogen profiled layer. We compared the characteristics of solar cells with and without the seed layer. When the seed layer was not applied, the fabricated cell showed the characteristics of a-Si:H solar cell whose spectral response was in a range of 400-800 nm. Using the seed layer, we achieved a μc-Si:H solar cell with performance of V_oc=0.535 V, J_sc=16.0 mA/cm², FF=0.667, and conversion efficiency=5.7% without any back reflector. The spectral response was in the range of 400-1100 nm. Also, the fabricated device has little substrate dependence, because a-Si:H has weaker substrate selectivity than μc-Si:H.     

Stability of the SnO2/MgO dye-sensitized photoelectrochemical solar cell

Dye-sensitized solar cells made of TiO₂ are extensively studied as a cheap alternative to conventional photovoltaic cells. The other familiar stable oxide material of similar band gap suitable for dye sensitization is SnO₂. Although cells based only of SnO₂ are prone to severe recombination losses, the cells made of SnO₂/MgO films where the SnO₂ crystallite is surface covered with an ultra-thin shell of MgO, deliver reasonably high efficiencies. It is found that SnO₂/MgO cells resist dye and electrolyte degradation better than TiO₂ cells. Furthermore, the ultra-thin barrier of MgO on SnO₂ remains intact during prolonged usage or storage of the cell.     

Ionothermal synthesis of TiO2 nanoparticles for enhanced photocatalytic H2 generation

Photocatalytic hydrogen generation is one of the most promising solutions to convert light energy into green chemical energy. In the present work, methoxy ethyl methyl imidazolium methyl sulphonate ionic liquid is used for the synthesis of i-TiO₂ nanoparticles via ionothermal method at 120 °C. The obtained products were characterized by various spectroscopic techniques like XRD, FTIR, Raman, UV–visible, DRS, TEM and TG-DSC analysis. XRD pattern confirmed the anatase phase with minor rutile phase having average crystallite size of 5 nm. From the FTIR spectrum, the band appeared at ～547 cm^?1 confirmed the Ti–O–Ti stretching and also few bands of ionic liquid. UV–vis spectrum clearly reveals the blue shift due to size effect of TiO₂. The spherical surface structure and particle size (15–30 nm) have been studied in detail using TEM images. Finally, the practical applicability of the as synthesized i-TiO₂ nanoparticles is shown by using it as a photocatalyst towards the generation of H₂ through water splitting reaction and it is found to be 462 μmol h^?1g^?1.     

High temperature annealing of sprayed SnO2: F layers in a silicon solar cell process with screen-printed contacts

In order to improve the solar cell conversion efficiency, a thin film of doped tin oxide (SnO₂: F) has been deposited by the spray-pyrolysis technique on a monocrystalline diffused silicon wafer. Subsequently, the layer must undergo the firing step of screen-printed contacts with temperatures up to 830 °C. After annealing, one notices with the naked eye the appearance of speckles disturbing the uniformity of the as-deposited blue-coloured SnO₂:F. Characterizations such as XPS, FTIR, RBS, XRD, SEM, Hall Effect, four point probe...etc, are all consistent to reveal a net increase of the SnO₂:F layer resistivity which leads to efficiency degradation. Annealing the thin films under CO and 90% N₂–10% H₂ atmospheres was investigated to seek possibilities to preserve the expected improvements. Unlike forming gas, CO reducing ambient was found to be very effective for the high temperature contact firing with no thin film conductivity deterioration.     

A comparative study on photoelectrochemical activity of ZnO/TiO2 and TiO2/ZnO nanolayer systems under visible irradiation

TiO₂/ZnO and ZnO/TiO₂ nanolayer thin films were synthesized using sol-gel method. Optical analysis revealed high transmittance of the films in the visible range with almost the same bandgap energy for the both systems. XPS technique shows stoichiometric formation of TiO₂ and ZnO on the surface of TiO₂/ZnO and ZnO/TiO₂ layers, respectively. According to AFM observations and its data analysis, the TiO₂/ZnO films exhibited a higher surface roughness and more effective interfaces with electrolyte during redox reactions. Based on photoelectrochemical measurements, TiO₂/ZnO nanolayer photoanode possesses a lower charge transfer resistance and higher transient time for charge carriers (e⁻ and h⁺) and hence a higher photocurrent density under visible light irradiation as compared with the ZnO/TiO₂ nanolayer system.     

Electrical conduction properties of ZnPc/TiO2 thin films

In this paper we have reported the electrical properties of Al/ZnPc–TiO₂/Al thin film device. The composite layers were prepared by the vacuum evaporation of ZnPc and sol–gel-coated TiO₂. The current–voltage characteristics are found to be Ohmic and non-Ohmic at low field and high field, respectively. The charge transport conduction appears to be Poole-Frankel type of conduction. The total number of interface states, which is responsible for the dispersion of capacitance, is found to be increasing with increase in bias voltage.     

A comparison of fluorine tin oxide and indium tin oxide as the transparent electrode for P3OT/TiO2 solar cells

Open circuit voltage (V_oc) and other photovoltaic parameters from fluorine tin oxide (FTO) P3OT/TiO₂ composite solar cells have been investigated in comparison with those from the indium tin oxide (ITO) devices with the same device structure and fabrication process. From the experimental results, the performance of FTO-based devices is better than that of ITO devices in terms of V_oc, short circuit current density (J_sc), and power conversion efficiency. The origin of V_oc and the higher V_oc of FTO can be explained and estimated by metal–insulator–metal model with a non-ohmic cathode contact.     

Manufacturing method for transparent electric windows using dye-sensitized TiO2 solar cells

The transparent electric windows based on dye-sensitized nanocrystalline TiO₂ solar cells have been prepared. The solar cell consists of dye-sensitized TiO₂ electrode with a TiO₂ layer of an about 8 μm thickness and of a 80×80 mm² active area, Pt counter electrode and redox electrolyte. The solar cell shows a transmittance of approximately 60% in the visible range and an open-circuit voltage (V_oc) of 0.64 V and a short-circuit photocurrent (J_sc) of 250 mA. A moderately transparent electric window composed of nine unit solar cells in series generates V_oc of 5.7 V and J_sc of 220 mA at one sun light intensity.     

Visible light-operated saccharide–O2 biofuel cell based on the photosensitization of chlorophyll derivative on TiO2 film

The visible light-operated saccharide–O₂ biofuel cell consisting of zinc chlorin-e₆ (ZnChl-e₆) adsorbed on nanocrystalline TiO₂ layer coated onto optical transparent conductive glass electrode (OTE) as an anode, platinum-coated OTE as a cathode, and the fuel solution containing sucrose as a saccharide, invertase, glucose dehydrogenase (GDH) and NAD⁺ is studied as a new type biofuel cell. The short-circuit photocurrent (I_SC) and the open-circuit photovoltage (V_OC) of this cell are 9.0 μA cm⁻² and 415 mV, respectively. The peaks in the photocurrent action spectrum of this cell are observed at 400 and 800 nm and the incident photon-to-current efficiency (IPCE) values at 400 and 800 nm are estimated to be ca. 17.3% and 10.6%. Thus, a new type of visible light-operated saccharide–O₂ biofuel cell with the visible and near IR photosensitization of ZnChl-e₆ molecules on nanocrystalline TiO₂ film electrode is accomplished.