3.1. Fundamental Characterization 6672 3.2. Photocurrent Action Spectroscopy 6673 3.3. Optical Characterization 6673 3.4. Characterization of Electron Transfer 6673 4. Electron Transport in Metal Oxide Films 6674 4.1. Mechanism of Photoinduced Carrier Transport 6674 4.2. Characterization of Diffusion Length 6675 4.3. One-Dimensional (1-D) Transport Architectures 6675 4.4. Electrolyte Interactions 6676 5. Recent Trends in Liquid-Junction Solar Cells 6676 5.1. Dye Sensitized Solar Cells 6677 5.2. Quantum Dot Sensitized Solar Cells 6678 5.3. Carbon Nanostructure Based Photochemical Solar Cells 6679

Beyond Photovoltaics: Semiconductor Nanoarchitectures for Liquid-Junction Solar Cells

Among the armoury of photovoltaic materials, thin film heterojunction photovoltaics continue to be a promising candidate for solar energy conversion delivering a vast scope in terms of device design and fabrication. Their production does not require expensive semiconductor substrates and high temperature device processing, which allows reduced cost per unit area while maintaining reasonable efficiency. In this regard, superstrate CdTe/CdS solar cells are extensively investigated because of their suitable bandgap alignments, cost effective methods of production at large scales and stability against proton/electron irradiation. The conversion efficiencies in the range of 6–20% are achieved by structuring the device by varying the absorber/window layer thickness, junction activation/annealing steps, with more suitable front/back contacts, preparation techniques, doping with foreign ions, etc. This review focuses on fundamental and critical aspects like: (a) choice of CdS window layer and CdTe absorber layer; (b) drawbacks associated with the device including environmental problems, optical absorption losses and back contact barriers; (c) structural dynamics at CdS–CdTe interface; (d) influence of junction activation process by CdCl2 or HCF2Cl treatment; (e) interface and grain boundary passivation effects; (f) device degradation due to impurity diffusion and stress; (g) fabrication with suitable front and back contacts; (h) chemical processes occurring at various interfaces; (i) strategies and modifications developed to improve their efficiency. The complexity involved in understanding the multiple aspects of tuning the solar cell efficiency is reviewed in detail by considering the individual contribution from each component of the device. It is expected that this review article will enrich the materials aspects of CdTe/CdS devices for solar energy conversion and stimulate further innovative research interest on this intriguing topic.

Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects

Self-cleaning coatings, with benefits to environment, energy and labor saving, have attracted great attention due to their potential application in many aspects of human's daily life. In this perspective article, the recent efforts to understand and improve photo-induced hydrophilicity and self-cleaning properties of TiO2 are reviewed. Particular attention is paid to the fundamental aspects and mechanism of the photo-induced hydrophilicity effect. The challenges that need to be addressed over the coming years to improve the efficiency and extend the application are also presented.

Photo-induced hydrophilicity and self-cleaning: models and reality

Cellular and molecular interactions between MC3T3-E1 pre-osteoblasts and nanostructured titanium produced by high-pressure torsion

Transparent, pure SiO(2), TiO(2), and mixed silica-titania films were (stochastically) deposited directly onto glass substrates by flame spray pyrolysis of organometallic solutions (hexamethyldisiloxane or tetraethyl orthosilicate and/or titanium tetra isopropoxide in xylene) and stabilized by in situ flame annealing. Silicon dioxide films consisted of a network of interwoven nanofibers or nanowires several hundred nm long and 10-15 nm thick, as determined by microscopy. These nanowire or nanofibrous films were formed by chemical vapor deposition (surface growth) on bare glass substrates during scalable combustion of precursor solutions at ambient conditions, for the first time to our knowledge, as determined by thermophoretic sampling of the flame aerosol and microscopy. In contrast, titanium dioxide films consisted of nanoparticles 3-5 nm in diameter that were formed in the flame and deposited onto the glass substrate, resulting in highly porous, lace-like nanostructures. Mixed SiO(2)-TiO(2) films (40 mol % SiO(2)) had similar morphology to pure TiO(2) films. Under normal solar radiation, all such films having a minimal thickness of about 300 nm completely prevented fogging of the glass substrates. These anti-fogging properties were attributed to inhibition of water droplet formation by such super-hydrophilic coatings as determined by wetting angle measurements. Deactivated (without UV radiation) pure TiO(2) coatings lost their super-hydrophilicity and anti-fogging properties even though their wetting angle was reduced by their nanowicking. In contrast, SiO(2)-TiO(2) coatings exhibited the best anti-fogging performance at all conditions taking advantage of the high surface coverage by TiO(2) nanoparticles and the super-hydrophilic properties of SiO(2) on their surface.

Anti-Fogging Nanofibrous SiO2 and Nanostructured SiO2−TiO2 Films Made by Rapid Flame Deposition and In Situ Annealing

Crystalline TiO 2 layers could be deposited on glass and silicon substrates by reactive pulse magnetron sputtering without additional substrate heating using a long-term stable process with high deposition rate. XRD investigations have revealed that the variation of pulse mode between unipolar and bipolar allows the deposition of layers with anatase and rutile structures, respectively. The SEM micrographs of anatase layers show faceted crystallites on the surface as well as in the fracture. In comparison the rutile layers have a smoother surface and a more glassy like fracture. Cross-section TEM investigations on samples with anatase or rutile phase have shown that the growth of the layers starts with an amorphous sub-layer. With increasing distance from the substrate the nucleation of first crystallites can be recognized. The lateral size of the columnar grains ranges between 100 and 200 nm for anatase and rutile films. In contrast, the defect density of the rutile phase is drastically higher than in films with anatase structure. Spectroscopic ellipsometry measurements have shown that no absorption is present in the visible spectrum range. The refractive index at 550 nm is approximately 2.55 for anatase and 2.68 for rutile phase, respectively. The hardness and Young's modulus measured by nanoindentation is approximately 8 and 170 GPa for anatase and 17 and 260 GPa for rutile layers. Layers with an anatase structure are well suited for photocatalytic applications using the hydrophilic, self-cleaning and antifogging properties. Rutile layers can be used as optical layers with high refractive index and as protective layers with good mechanical properties.

Structure and properties of crystalline titanium oxide layers deposited by reactive pulse magnetron sputtering

Al x Sc 1 − x N films were deposited by reactive pulse magnetron co-sputtering from aluminum and scandium targets without additional substrate heating at deposition rates between 100 and 150 nm/min. With increasing incorporation of scandium into the hexagonal wurtzite structure, the piezoelectric properties are drastically improved. The piezoelectric charge coefficient d 33 is increased from 8.4 pC/N for AlN up to 23.6 pC/N for Al x Sc 1 − x N with 33% scandium. Between 35 and 43% scandium content a relative broad maximum with high piezoelectric coefficients between 26.9 and 27.3 pC/N was detected. A further increase of scandium concentration above 50% results in the formation of the cubic and centrosymmetric rock salt structure and therefore the complete loss of piezoelectric properties. By FE-SEM, XRD and TEM investigations it was shown that up to up to 43% scandium concentration the wurtzite structure becomes more and more disordered and the c/a ratio is decreased from 1.6 to 1.27. Nevertheless, no aluminum or scandium segregation could be detected by high resolution TEM investigations. The Young's modulus of the wurtzite phase is reduced with increasing scandium concentration from 340 GPa to 185 GPa. The drastic improvement of the piezoelectric properties can be explained by weakening of the chemical bonding and by high distortion of the wurtzite structure.

Effect of scandium content on structure and piezoelectric properties of AlScN films deposited by reactive pulse magnetron sputtering

Titanium oxide layers were deposited by reactive electron beam evaporation with and without additional plasma activation by spotless arc discharge. The influence of substrate temperature and plasma activation on structure and properties of TiO 2 layers are presented. The layers were deposited with very high deposition rates between 40 and 70 nm/s. XRD investigations have revealed that structure of all layers deposited at substrate temperature ( T S ) below 150 °C is amorphous. Without plasma activation and at substrate temperature above 200 °C layers with anatase phase were deposited. On the other hand with plasma activation the degree of substrate ion bombardment controls the formation of anatase or rutile phase. Furthermore a drastic influence of plasma activation on the properties of the layers could be found. Without plasma activation the refractive index at 550 nm only amounts to between 2.02 and 2.29 depending on substrate temperature. With plasma activation a drastic raise of refractive indexes to values between 2.30 and 2.58 can be reached. In addition the hardness of the layer is obviously influenced by plasma activation. At T S below 150 °C hardness is raised from 2.0 to 6.7 GPa. The formation of the rutile phase in layers deposited with plasma activation is linked with a further increase of hardness up to 12 GPa. Measurements of the contact angle show a promising super hydrophilicity of anatase layers. Therefore these layers should be well-suited as coatings with self-cleaning or antifogging properties. At T S below 150 °C the plasma activation enables the deposition of dense, high-refractive layers at very high deposition rates for applications as optical layers.

Structure and properties of titanium oxide layers deposited by reactive plasma activated electron beam evaporation

We report 12% efficient CdS/CdTe thin film solar cells prepared by low temperature close space sublimation (CSS). Both semiconductor films, CdS and CdTe, were deposited by high vacuum CSS in superstrate configuration on glass substrates with fluorine doped tin oxide (FTO) front contact. The CdTe deposition was carried out at a substrate temperature (Tsub) of ≤340 ∘C, which is much lower than that used in conventional processes (>500 ∘C). The CdTe films were treated with the usual CdCl2 activation process. Different optimal annealing times and temperatures were found for low-temperature cells (Tsub≤ 340 ∘C) compared to high-temperature cells (Tsub = 520 ∘C). The influence of the activation step on the morphology of high-temperature and low-temperature CdTe is determined by XRD, AFM, SEM top views, and SEM cross-sections. Grain growth, strong recrystallization, and a reduction of planar defects during the activation step are observed, especially for low-temperature CdTe. Further, the influence of CdS deposi...

12% efficient CdTe/CdS thin film solar cells deposited by low-temperature close space sublimation

An innovative reactive pulse magnetron sputtering (PMS) system allowing to change the pulse mode (unipolar, bipolar or pulse packet) has been used to deposit TiO 2 films at dynamic deposition rates of 8...50 nm m/min. Photo-induced hydrophilicity and photocatalytic degradation by UV-A illumination were investigated by measuring the decrease of the water contact angle and the decomposition of methylene blue organic dye, respectively. It was observed that using the PMS technology the TiO 2 layers can be deposited in situ crystalline at temperatures much lower than the necessary annealing temperature for amorphous deposited layers. Moreover, these in situ crystalline deposited layers have superior photocatalytic properties to that of amorphous deposited post-annealed TiO 2 layers. Thin TiO 2 films (<50 nm) deposited at substrate temperatures as low as 130 °C exhibit sufficient photocatalytic activity for many applications.

Thomas Modes

Papers

Structure and properties of crystalline titanium oxide layers deposited by reactive pulse magnetron sputtering

Effect of scandium content on structure and piezoelectric properties of AlScN films deposited by reactive pulse magnetron sputtering

Structure and properties of titanium oxide layers deposited by reactive plasma activated electron beam evaporation

12% efficient CdTe/CdS thin film solar cells deposited by low-temperature close space sublimation

Photocatalytic titanium dioxide thin films prepared by reactive pulse magnetron sputtering at low temperature