This paper reports the development of morphology-tuned TiO2/ZnO films and their application to the electrode in dye-sensitized solar cells. Amorphous TiO2 precursor films with different morphologies can be synthesized under different processing conditions by our previously developed peroxotitanate-complex deposition process, which employs a supersaturated peroxotitanate aqueous solution. The ZnO nanowire aligns were grown by a two-step method, in which well oriented arrays of ZnO nanowires were prepared on a pre-prepared ZnO seed layer. The cell performance using the prepared TiO2/ZnO films shows the possibility to integrate these films into solar cells. However, the crystallization and morphology should be further optimized to improve the conversion efficiency to be a competitive level.

Germanium nanocrystals have been synthesized using a low-pressure, nonthermal plasma approach. The nanocrystal size can be adjusted between 4-20 nm by varying the plasma parame-ters, and the size distribution is relatively narrow with standard deviations of 10-20% of the av-erage crystal size. Stable colloidal solutions of the germanium crystals have been prepared by grafting organic alkene ligands onto the nanocrystal surfaces. When drop-cast from solution onto TEM grids, the nanocrystals form densely packed films.

A new backside reflector, textured photonic crystal, is introduced into Si thin film solar cells. Scattering matrix method is used to systematically optimize all the parameters of the two components of the backside reflector, grating and distributed Bragg reflector, to achieve the highest power conversion efficiency for a given solar cell thickness. Experimentally, Si-on-insulator solar cells are being fabricated to verify the tremendous efficiency enhancement and optimal design. It is found that while the optimal period and etch depth of the grating, the Bragg wavelength of the distributed Bragg reflector, as well as the antireflection coating thickness all decrease as the cell becomes thinner, the optimum duty cycle of the grating remains almost constant at 0.5. For a 2 μm thick cell, the relative efficiency enhancement can be as high as 52% using the optimized design.

We use SnS and Sb2S3 thin films of about 500 nm in thickness deposited on glass substrates by chemical deposition to develop solar cell structures: glass-SnO2:F/CdS/SnS/CuS/silver paint and SnO2:F/CdS/Sb2(S/Se)3/PbS/silver paint. Here, SnS and Sb2S3, and PbS are absorber materials suitable for large scale production, considering their abundance at 0.2 ppm (Sb) and 2 ppm (Sn) and 8ppm (pb) in the earth's crust according to published data. SnS films deposited through distinct reaction routes have optical band gap of 1.1 eV or 1.7 eV. In SnO2:F/CdS/SnS(1.1eV)/SnS(1.7 eV)/CuS/silver paint, open circuit voltage (Voc) of ≈ 400 mV, and short circuit current (Jsc)of 7 mA/cm2 are obtained with a cell efficiency of 1%. Sb2S3 thin films have optical band gap 1.7 eV, but could be reduced through reaction in Se-vapor, upon which solid solutions of Sb2(S/Se)3 are formed. In SnO2:F/CdS/Sb2(S/Se)3/PbS/silver paint, Voc of ≈ 640 mV, Jsc of 7 mA/cm2 and conversion efficiency of 1.5% are obtained.

In search of an efficient semiconductor material for direct photoelectrochemical (PEC) hydrogen production, chalcopyrite films in the Cu(In,Ga)Se2 system (CIGS) with bandgaps of 1.3-1.65 eV have been evaluated. The films have been fabricated by 2-stage and 3-stage co-evaporation processes. Film samples have been fabricated into CIGS/CdS solar cells for evaluation of solid-state device properties, and into CIGS photocathodes for evaluation of the photoelectrochemical hydrogen-production characteristics. The PEC current-potential scans of the photocathodes in 0.5M sulfuric acid show photocurrents of 18-27 mA/cm2 under simulated AM1.5 global light (100 mA/cm2) at sufficient cathodic potential bias. In terms of fill factor of the photocurrent curves, electrodes with molybdenum back contact are superior to SnO2:F back contact because of better conductivity. The morphology as seen in scanning electron micrographs is unchanged after initial PEC testing in the cathodic regime, suggesting films are stable.

As microcrystalline silicon oxide (μc-SiO) is reported to be more promising material than microcrystalline silicon carbide for the application to solar cells, both of high efficiency amorphous (top cell) and microcrystalline (bottom cell) silicon solar cells fabricated on tin oxide coated glass substrate have been developed by using μc-SiO p-layer and buffer i-layer. High VHF frequency of 60 MHz and carbon dioxide gas are used for their deposition. ZnO deposited by DC sputtering is coated on Asahi's type U tin oxide in order to promote the crystallization of the μc-SiO p-layer. It was found that the top cell with this novel mc-SiO p-layer has higher cell efficiency than the one with normal SiO p-layer. Furthermore, the microcrystalline bottom cell with novel μc-SiO p-layer and μc-SiO buffer layer has higher performance than the one with normal μc-Si p-layer. Up to now, an efficiency of more than 15% has been achieved with a-SiO/a-SiGe/μc-Si tandem cell.

We prepared multilayers of semiconducting half-heusler β-Zn4Sb3 and skutterudites CeFe(4-x)CoxSb12 compound thin films by ion beam assisted deposition (IBAD) system for the application of thermoelectric (TE) materials. Rutherford backscattering spectrometry (RBS) was used to analyze the composition of thin films. The thin films were then bombarded by 5 MeV Si ions for generation of nanodots in the films. We measured the cross-plane thermal conductivity by a house developed 3ω-method system, cross-plane Seebeck coefficient by a (MMR) Seebeck system, and cross plane electrical conductivity of these nanolayered systems by a (MMR) Hall system before and after bombardment. Both β-Zn4Sb3 and CeFe(4-x)CoxSb12 systems have been identified as promising thermoelectric materials for the application of thermal-to-electrical energy conversion. The nanodots produced by MeV ion beam can cause significant change in both electrical and thermal conductivity of thin films, thus improving the efficiency. The MeV ion-beam bombardment resulted in decrease in the thermal conductivity of thin films and increase in the efficiency of thermal-to-electrical energy conversion.

Nanorods/nanoparticles TiO2 with mesoporous structure were synthesized by hydrothermal method at 150 °C for 20 h. The samples characterized by XRD, SEM, TEM, SAED, HRTEM, and BET surface area. The nanorods had diameter about 10-20 nm and the lengths of 100-200 nm, the nanoparticles had diameter about 5-10 nm. The prepared material had average pore diameter about 7-12 nm. The BET surface area and pore volume of the sample are about 203 m2/g and 0.655 cm3/g, respectively. The nanorods/nanoparticles TiO2 with mesoporous structure showed higher photocatalytic activity (I3- concentration) than the nanorods TiO2, nanofibers TiO2, mesoporous TiO2, and commercial TiO2 (ST-01, P-25, JRC-01, and JRC-03). The solar energy conversion efficiency (η) of the cell using nanorods/nanoparticles TiO2 with mesoporous structure was about 7.12 % with Jsc of 13.97 mA/cm2, Voc of 0.73 V and ff of 0.70; while η of the cell using P-25 reached 5.82 % with Jsc of 12.74 mA/cm2, Voc of 0.704 V and ff of 0.649.

Here we will compare four techniques that are commonly used for diagnosis of the lifetime of photovoltaic materials. The strengths and weaknesses of these techniques will be shown and discussed.

We have fabricated transparent ZnO nanorod arrays on the fluorine-doped SnO2 transparent conducting oxide (FTO) glass substrates and used them as the wide band gap semiconductor in dye-sensitized solar cells. Our objectives are to introduce and demonstrate new possibilities in designing the semiconductor morphology. The best performance with this cell structure produced an open circuit voltage (Voc) of 0.64 mV, a short circuit current density (Jsc) of 5.37 mA/cm2, a fill factor (FF) of 0.49, and a conversion efficiency (η) of 1.69 %, primarily limited by the surface area of the nanorod array.

Thin films of tungsten oxide were investigated for use as a top junction in a hybrid photoelectrode. To increase the solar to hydrogen efficiency, the tungsten oxide requires a bandgap reduction to the range of 2.2 to 2.4 eV. Nitrogen doping of WO3 films was employed to reduce the bandgap via valence band modification. For low levels of doping, the bandgap was observed to increase, an effect attributed to decreased size of the polycrystals in the film. The photoelectrochemical efficiency was found to decrease from 80% for pure WO3 to 56% for films deposited at a nitrogen partial pressure of 1.5 mTorr. For even higher doping levels (to 5 mTorr N2), the bandgap was shown to decrease to a value of ∼1.9 eV, but the structural data indicates that significant disorder had been introduced. This disorder is consistent with recurrent dislocations, an effect that is common for the tungsten oxide material.

Wet chemically prepared arrays of ZnO nanorods in electrolyte solution were investigated with respect to their capacitive behavior. The ZnO nanorod electrode could be well described by a frequency independent capacitive and a conductive element in parallel. The capacitive element is determined as space charge capacitance in the nanorods. However the dependency of the nanorod electrodes capacitance on the applied voltage bias is more complicated than for flat semiconductor electrodes.

Several new photovoltaic semiconductor materials and technologies have been developed due to the increasing need for renewable energy sources. Quantum dot (QD) based solar cell is potentially one of the best contenders. We have developed a thermodynamically stable nanocomposite (stable up to 900°C) titania-germanium (TiO2-Ge) which shows promise as the active layer for QD solar cells. In TiO2-Ge nanocomposites Ge nanodots are distributed in a TiO2 matrix. Due to the 3-D quantum confinement effect, tailoring of the optoelectronic properties is relatively easily done by simply varying the Ge nanodots size. Ge is particularly advantageous since its Bohr radius is relatively large, 25 nm. In this paper results of the variation of the optoelectronic properties of TiO2-Ge nanocomposites as a function of the nanostructure parameters are presented. TiO2-Ge nanocomposite thin films were synthesized using RF magnetron sputtering. The structural studies (by XRD, HRTEM) established the fact that the Ge concentration in the composite target governs the size of the Ge nanodots whereas RF sputtering power controls the density of Ge nanodots in the nanocomposite films. The optical spectroscopic studies showed that the variation of the size of Ge nanodots in the TiO2-Ge films shifts the absorption edge from ∼ 0.66 eV (infrared) to ∼ 2.3 eV (blue-green). Similarly, dark conductivity also varies in a wide range of 10−7 - 10−2 Scm−1 by altering the concentration as well as the structural phase (amorphous or crystalline) of Ge.

We demonstrate that conjugated polymers (CPs)-based flexible solar cells with well-defined interdigitated donor-acceptor interfaces enhance charge separation and transport. The welldefined straight donor-acceptor interfaces are achieved successful application of nanoimprinting technology to rationally designed energy harvesting and hole transporting conjugated polymers. Nanoimprinting enables the precise and direct nano-scale control of the shape of the donoracceptor interface on both rigid and flexible substrates. Comparison between the performances of the solar cells having imprinted different feature sizes revealed that the short circuit current can be systematically increased by the interfacial area of the heterojunction without affecting the open circuit voltage. The results also showed that the vertically oriented heterojunction facilitate charge transport and allow synergistically improved fill factor, open circuit current, and ensuing energy conversion efficiency beyond the gain of the interfacial area of the heterojunction.

We report an enhancement of photovoltaic response by dispersing phosphorescent dye fac tris (2-phenylpyridine) iridium (Ir(ppy)3) in organic solar cells of poly[2-methoxy-5-(2¡¦-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) doped with surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6] C61 (PCBM). It is known that photoexcitation generates both singlet and triplet states through intersystem crossing caused by hyperfine or spin-orbital coupling. Due to long diffusion length the triplet excitons can migrate from their generation sites to the interfaces of donor-acceptor interaction and directly dissociate into charge carriers. We found, based on the studies of magnetic field-dependent photocurrent, that the dispersed Ir(ppy)3 molecules increase the spin-orbital coupling strength and triplet density in the MEH-PPV matrix due to the penetration of MEH-PPV π electrons into the large field of orbital dipoles of the Ir(ppy)3. Especially, the triplet excitons facilitate the direct dissociation into charge carriers at the donor-acceptor interacting interfaces in the composite of MEH-PPV and PCBM, and consequently improve the photovoltaic response in organic solar cells.

This paper discusses the characterization and optimization of organic solar cells based on a bulk heterojunction consisting of an alternating copolymer, containing a fluorene and a benzathiadiazole unit with two neighboring thiophene rings, and a fullerene derivative (PCBM). The resulting power conversion efficiency amounts 3.9±0.2 % (AM1.5, 100 mW/cm2) and these polymer solar cells are therefore considered auspicious for further research.

Tantalate and titanate photocatalysts were prepared by solid-state reaction at 1273 K using various ratios of SrCO3, Ta2O5, and TiO2 as starting materials. The prepared solid photocatalysts were characterized using XRD and SEM analysis. These prepared tantalate and titanate photocatalysts showed high photocatalytic H2 evolution activity by water splitting without co-catalyst loading. The highest H2 evolution rate of prepared photocatalysts was found to be 138 μmolh−1 with the starting materials ratio of 2/0.5/1.5 (Sr/Ta/Ti; mol). Furthermore, this photocatalyst showed photocatalytic activity for H2 evolution from distilled water.

A small p-n diode array was fabricated on a polycrystalline Si substrate and the electrical characteristics were measured for each small diode to evaluate the distribution of energy conversion efficiency in the substrate. The crystal qualities in conjunction with the electrical characteristics were also evaluated. We found large variations in measuring the current-voltage (I-V) characteristics of the p-n diode. We also observed variations in quality even in diodes without any grain boundaries at the p-n junction. Therefore, we evaluated crystalline quality using various techniques to compare the diode characteristics. We found clear evidence in photo-luminescence (PL) mapping, where grains, including degraded diodes, were darker in the mapping, implying lower PL intensities than the others. The PL spectra obtained from the “dark grains” included D-lines indicating the existence of dislocations. We could conclude that the electrical characteristics of p-n diodes were not only affected by grain boundaries but also by crystalline defects evaluation such as dislocations. We observed a Secco-etched surface for crystalline defects evaluation using an optical microscope. The origins of etch pits were also determined by transmission electron microscope (TEM) and three different types of defects were confirmed.

A Titanium dioxide ultrathin film composed of nanoplatelet structures was prepared and characterized with AFM, SEM, and GISAXS measurement, whose photophysical properties are also studied.

Covalently linked electron donor (D) and electron acceptor (A) with conjugated organic building blocks are novel materials for potential solar cell applications, because these molecular p-n junctions can minimize the exciton diffusion and transform the charge separation from interdomain to intramolecular processes. Hence, the bottleneck of the exciton diffusion in many bulk heterojuction materials can be eliminated. Meanwhile, these planar conjugated assemblies, such as supermolecules, multiblock oligomers and polymers, have strong tendency to π-π stacking to form continuous channels for charge carriers to hop/diffuse to respective electrodes. A quartet D-A assembly has been synthesized with bis-oligothiophene (BOTH) and bis-perylenediimide (BPDI) derivatives attached to a benzo template. The electronic structures and dynamics of photoinduced charge separation and recombination of this quartet molecule and reference compounds in solutions and films were studied at isolated the molecular level in solutions as well as at the molecular assembly level in films with stacked structures. Two different dynamics of charge separation and recombination associated with two types of donor/acceptor pair conformations in solution were observed. This molecular system exhibits a more efficient charge separation than charge recombination processes in both polar and non-polar organic solvents, as well as films. More efficient charge separation and slower charge recombination due to the covalent linkage indicating that the material is a potential candidate for photovoltaic studies in solid-state.