Effect of TiO2 mixtures on the optical, structural and electrochromic properties of Nb2O5 thin films

Metal oxide films are important for various optical devices and especially for solar energy materials. TiO₂-mixed Nb₂O₅ thin films have been produced by sol–gel dip-coating method. Several parameters such as heat treatment, thickness, and mixture percentages are studied for the effect of the optical, structural and electrochromic properties of the materials. Optical parameters of the films were calculated through transmission and reflection measurement by a refractive index, extinction coefficient and thickness analyzer. Structural, electrochromic and surface analyses of the films were done by X-ray diffractometer, potentiostat/galvanostat and atomic force microscope systems.     

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.     

Photoelectrochemistry of semiconductor electrodes made of solid solutions in the system Fe2O3–Nb2O5

Ceramic semiconductor photoelectrodes made from solid solutions in the system Fe₂O₃–Nb₂O₅ were synthesized. The spectral and capacitance–voltage characteristics of the photoelectrodes were determined, and the dynamic polarization with chopped light was investigated. The anodic photocurrent onset potential, the flat band potential and the shallow and deep donor density of these materials were determined. The threshold photon energies corresponding to the inter-band optical transitions near the edge of the fundamental absorption of the semiconductor photoelectrode were calculated. Analysis of the frequency dispersion of the real and imaginary parts of the complex impedance of photoelectrochemical cell was carried out. On the basis of this analysis, equivalent circuits describing the structure of the double electrical layer of the semiconductor–electrolyte interface were proposed and their parameters calculated. The main limiting steps of the electrode processes, which determine the electrode polarization and current, are defined.     

Surface ligand effects in MEH-PPV/TiO2 hybrid solar cells

TiO₂ nanorods (NRs) were synthesized by hydrolysis of titanium tetraisopropoxide (TTIP) using oleic acid (99%) as surfactant at low temperatures (80-100 °C) and are modified with different ligands: oleic acid (OLA), n-octyl-phosphonic acid (OPA) and thiophenol (TP) in order to investigate the effect of surface ligand on the excition dissociation and the charge transport in hybrid MEH-PPV/TiO₂ photovoltaic (PV) cells. The morphology and crystalline form of as-prepared TiO₂ NRs are examined by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD) and Raman spectrometer (RS). The FTIR analysis confirms all the ligands coordinated with the Ti center of TiO₂ NRs. The optical properties of the modified TiO₂ NRs are characterized by UV-vis absorption spectra and photoluminescence (PL) spectra. Thiophenol modified TiO₂ NRs quench the PL of MEH-PPV more effectively than OLA-TiO₂ NRs and OPA-TiO₂ NRs. The power conversion efficiency of hybrid PV cells from thiophenol modified TiO₂ NRs and MEH-PPV is the highest among the investigated TiO₂ NRs.     

Hybrid inverted bulk heterojunction solar cells with nanoimprinted TiO2 nanopores

Photovoltaic devices with highly ordered nanoporous titanium dioxide (titania; TiO₂) were fabricated to improve the photovoltaic performances by increasing TiO₂ interface area. The nanoimprinting lithography technique with polymethyl methacrylate (PMMA) mold was used to form titania nanopores. The solar cell with poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) active layer on nanoporous titania showed higher power conversion efficiency (PCE) of 1.49% than on flat titania of 1.18%. The improved efficiency using nanoporous titania is interpreted with the enhanced-charge separation and collection by increasing the interface area between TiO₂ and active layer.     

Electrochromic behaviour of Nb2O5 thin films with different morphologies obtained by spray pyrolysis

Two different procedures to stabilize the precursor NbCl₅ have been applied to obtain Nb₂O₅ thin films by spray pyrolysis. Depending on the procedure used, determined by the way in which the precursor solution was injected into the air stream of the spray nozzle, niobium oxide thin films with different surface morphologies can be obtained. The structural properties of the Nb₂O₅ thin films depend on the post-annealing temperature because as-deposited films are amorphous, independently of the synthesis procedure used. The electrochromic behaviour has been estimated for all films, where monochromatic colouration efficiency (at 660 nm) of 25.5 cm²/C and a cathodic charge density close to 24 mC/cm² were found to give the best results to date for niobium oxide thin films obtained by spray pyrolysis.     

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.     

Oxide/polymer interfaces for hybrid and organic solar cells: Anatase vs. Rutile TiO2

In this work, we study the effect of the transparent conducting oxide (TCO) and the polymer applied (MEH-PPV or P3HT) on the photovoltaic properties of TCO/TiO₂/polymer/Ag bi-layer solar cells. The solar cells were analyzed under inert atmosphere conditions resembling an encapsulated or sealed device. We demonstrate that the substrate applied, ITO or FTO, modifies the crystalline structure of the TiO₂: on an ITO substrate, TiO₂ is present in its anatase phase, on an FTO, the rutile phase predominates. Devices fabricated on an FTO, where the rutile phase is present, show better stability under inert atmospheres than devices fabricated on an ITO, anatase phase. With respect to the polymer, devices based on MEH-PPV show higher Voc (as high as 1 V), while the application of P3HT results in lower Voc, but higher J_sc and longer device stability. These observations have been associated to (a), the crystalline structure of TiO₂ and (b) to the form the polymer is bonded to the TiO₂ surface. In-situ IPCE analyses of P3HT-based solar cells show a red shift on the peak corresponding to TiO₂, which is not present on the MEH-PPV-based solar cells. The latter suggest that P3HT can be linked to the TiO₂ though the S-end atom, which results in devices with lower Voc. All these observations are also valid for devices, where the bare TiO₂ is replaced by an Nb-TiO₂. The application of an Nb-TiO₂ with rutile structure in these polymer/oxide solar cells is the reason for their higher stability under inert atmospheres. We conclude that the application of TiO₂ in its rutile phase is beneficial for long-term stability devices. Moreover there is an interplay between low Voc and J_sc in devices applying P3HT, since power conversion efficiency can be partially canceled by their lower Voc in comparison with MEH-PPV. These findings are important for polymer/oxide solar cells, but also for organic solar cells, where a layer of semiconductor oxides are in direct contact with a polymer, like in an inverted or tandem organic solar cells.     

Structural, morphological and electrochromic properties of Nb2O5 films deposited by reactive sputtering

Nb₂O₅ films were prepared successfully by DC reactive sputtering process. The relationships among structural, morphological and electrochromic properties were studied using XRD, AFM, AES and cyclic voltammograms. Results show that the films deposited on heated substrates are composed of columnar TT-Nb₂O₅ microcrystalline with many grain-to-grain boundaries. These structural characteristics provide films strong electrochemical stability, high Li⁺ insertion/extraction reversibility and good electrochromic properties. DC reactive sputtered Nb₂O₅ films are colorless in bleached state and brownish gray in colored state and may be a promising candidate for the application in electrochromic devices.     

Semiconductor-sensitized solar cells based on nanocrystalline In2S3/In2O3 thin film electrodes

A possibility of semiconductor-sensitized thin film solar cells have been proposed. Nanocrystalline In₂S₃-modified In₂O₃ electrodes were prepared with sulfidation of In₂O₃ thin film electrodes under H₂S atmosphere. The band gap (E_g) of In₂S₃ estimated from the onset of the absorption spectrum was approximately 2.0 eV. The photovoltaic properties of a photoelectrochemical solar cell based on In₂S₃/In₂O₃ thin film electrodes and I⁻/I₃⁻ redox electrolytes were investigated. This photoelectrochemical cell could convert visible light of 400–700 nm to electron. A highly efficient incident photon-to-electron conversion efficiency (IPCE) of 33% was obtained at 410 nm. The solar energy conversion efficiency, η, under AM 1.5 (100 mW cm⁻²) was 0.31% with a short-circuit photocurrent density (J_sc) of 3.10 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.26 V, and a fill factor ( ff ) of 0.38.     

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.     

Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen

Nanostructured semiconductor thin films of Zn-Fe₂O₃ modified with underlying layer of Fe-TiO₂ have been synthesized and studied as photoelectrode in photoelectrochemical (PEC) cell for generation of hydrogen through water splitting. The Zn-Fe₂O₃ thin film photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO₂ film. A maximum photocurrent density of 748 μA/cm² at 0.95 V/SCE and solar to hydrogen conversion efficiency of 0.47% was observed for 0.89 μm thick modified photoelectrode in 1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the films were characterized for various physical and semiconducting properties using XRD, SEM, EDX and UV–Visible spectrophotometer. Zn-Fe₂O₃ thin films modified with Fe-TiO₂ exhibited improved visible light absorption. A noticeable change in surface morphology of the modified Zn-Fe₂O₃ film was observed as compared to the pristine Zn-Fe₂O₃ film. Flatband potential values calculated from Mott–Schottky curves also supported the PEC response.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

Efficient hybrid carboxylated polythiophene/nanocrystalline TiO2 heterojunction solar cells

Efficient hybrid solar cells fabricated from TiO₂, novel carboxylated polythiophene poly (3-thiophenemalonic acid) P3TMA as sensitizer as well as hole conductor and poly (3-hexylthiophene) (P3HT) as hole transporter was described. UV-Vis absorption and morphology of the active layer were investigated. Device J/V characterizations with different P3HT layer thickness were measured and discussed. Efficiency improvements were observed in thinner P3HT layer thickness and with poly[3,4-(ethylenedioxy)-thiophene]:poly(styrene sulfonate) (PEDOT:PSS) as charge collection layer, and such device showed a short-circuit current density of 1.32 mA/cm², an open-circuit voltage of 0.44 V, a fill factor of 0.43, and a energy conversion efficiency of 0.25% at A.M. 1.5 solar illumination (100 mW/cm²).     

Thermal cycle stability of BaO–B2O3–SiO2 sealing glass

Thermal cycle stability is very important for glass seals in planar solid oxide fuel cell (pSOFC) applications. In the present study, thermal cycle stability of a thermally stable sealing glass is investigated using a sealing fixture from 150 °C to 700 °C. SS410 alloy with the TEC (thermal expansion coefficient) of 12.2 × 10⁻⁶ K⁻¹ (room temperature to 700 °C) is used to evaluate the effect of TEC mismatch on the thermal cycle stability. The leak rates increase with thermal cycles and appear to be two different stages. Microstructure examinations are performed to investigate the degradation mechanism of the thermal cycle stability. It is found that the sealing glass interacts chemically with the SS410 alloy and the formation of BaCrO₄ new phase results in the rapid increase of the leak rates.     

Tri-layer antireflection coatings (SiO2/SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique

Antireflection coatings (ARCs) have become one of the key issues for mass production of Si solar cells. They are generally performed by vacuum processes such as thermal evaporation, reactive sputtering, and plasma-enhanced chemical vapor deposition. In this work, a sol–gel method has been demonstrated to prepare the ARCs for the non-textured monocrystalline Si solar cells. The spin-coated TiO₂ single-layer, SiO₂/TiO₂ double-layer and SiO₂/SiO₂–TiO₂/TiO₂ triple-layer ARCs were deposited on the Si solar cells and they showed good uniformity in thickness. The measured average optical reflectance (400–1000 nm) was about 9.3, 6.2 and 3.2% for the single-layer, double-layer and triple-layer ARCs, respectively. Good correlation between theoretical and experimental data was obtained. Under a triple-layer ARC condition, a 39% improvement in the efficiency of the monocrystalline Si solar cell was achieved. These indicate that the sol–gel ARC process has high potential for low-cost solar cell fabrication.     

P3HT/TiO2 bulk-heterojunction solar cell sensitized by a perylene derivative

In this work, a new type of dye-sensitized bulk-heterojunction hybrid solar cells has been developed. The heterojunction films were prepared to contain poly(3-hexylthiophene) (P3HT), N,N′-diphenyl glyoxaline-3,4,9,10-perylene tetracarboxylic acid diacidamide (PDI) and TiO₂. In the architecture, TiO₂ and P3HT were designed to act as the electron acceptor and donor. PDI was used as sensitizer to enhance the photon absorption. Results showed that by incorporation of PDI in the P3HT/TiO₂ composite, the light absorption, exciton separation and photocurrent under white light were dramatically enhanced. Solar decay analyses showed that devices contained TiO₂ required 12 h to obtain maximum current density and the addition of PDI did not affect the solar decay behavior and stability of device composed of P3HT/TiO₂. The devices of P3HT, P3HT/TiO₂, P3HT/TiO₂/PDI could work for 5, 42, 45 h under continuous white light illumination (100 mW/m²) under the ambient condition.     

CVD of CuGaSe2 for thin film solar cells with various transport agents

Chemical vapor deposition (CVD) in an open tube system was employed to deposit single-phase CuGaSe₂ thin films on plain and Mo-coated glass substrates. The use of HCl and ternary CuGaSe₂ source material resulted in non-stoichiometric volatilization of the source material. The use of binary source materials – Cu₂Se and Ga₂Se₃ – in combination with I₂ and HCl as the respective transport agents yielded single-phase CuGaSe₂ thin films while the source materials were volatilized stoichiometrically. Mo/CuGaSe₂/CdS/ZnO devices were fabricated from these samples exhibiting an open-circuit voltages up to V_oc=853 mV.     

A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices

In this communication, we report on a technique to fabricate solid-state polythiophene-based dye sensitized solar cells (DSSCs) that can be directly compared to analogous liquid junction devices. The device configuration is based on non-porous TiO₂ thin films and one of the three undoped polythiophene hole conductors: poly[3-(11 diethylphosphorylundecyl) thiophene], P3PUT, poly(4-undecyl-2,2′-bithiophene), P4UBT, or poly(3-undecyl-2,2′-bithiophene), P3UBT. These polymers were spin coated and cast from organic solutions onto the TiO₂ films. The dense TiO₂ thin films (ca. 30 nm) were deposited on conductive glass via facile spray pyrolysis and sol–gel techniques. After that, cis-(SCN)₂ Bis(2,2′ bipyridyl-4,4′-dicarboxylate) ruthenium(II) (a.k.a. Ru N3 dye) was adsorbed on the TiO₂ surface, and the polythiophenes were utilized as hole conductors in a simplified solar cell geometry. The results were compared to the control DSSC device made with dense TiO₂ and a liquid electrolyte, or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (a.k.a. Spiro-MeOTAD). The polythiophenes exhibited bandgaps in the range 1.9–2.0 eV, and HOMO energy levels of approximately 5 eV (vs. vacuum). The P3PUT DSSC device exhibited an AM1.5 V_OC=0.8 V, a J_SC=0.1 mA/cm², as well as an IPCE=0.5–1%. The AM1.5 short-circuit photocurrents and quantum efficiencies for DSSCs made with the polythiophenes, the Spiro-MeOTAD and the standard liquid electrolyte (I⁻/I₃⁻) were found to be identical within the limits of experimental uncertainty and reproducibility. Our results indicate that a solid-state replacement to the liquid junction is not necessarily limited by the fundamental aspect of hole transfer, one of the three fundamental aspects that must be met for an efficient DSSC. Rather than suggest that P3UBT or P4UBT could be used to create efficient “organic solar cells” with the exclusion of the Ru dye, we suggest that transparent thiophene compounds could be attractive candidates for high-surface area solid-state DSSCs, and that the technique presented can be applied to other hole conductors. It can allow a verification of one of the things necessary for the DSSC, so that parallel studies using high-surface area materials can proceed with confidence.