An equivalent circuit for DSCs was studied using electrochemical impedance spectroscopy measurement, and the improvements of conversion efficiency of DSCs for not only single cells but also integrated modules were investigated. Further improvement of cell characteristics of DSCs was also investigated from the view point of modified TiO2 films and series-internal resistance design. The series-internal resistance elements were found to correlate positively with the sheet resistance of the transparent conducting oxide and the thickness of the electrolyte layer and negatively with the roughness factor of the platinum counter-electrode. The short circuit current density (Jsc) of the DSCs was effectively improved by use of a high-haze TiO2 film. In addition, the analysis of TiO2/dye interface by scanning probe microscopy and transient absorption spectroscopy were also useful for the study. As a result, the maximum single cell conversion efficiency of over 11% was obtained. Furthermore, an integrated DSC module composed of many rectangular cells connected in series was fabricated and the efficiency was increased to 8.4% (confirmed by AIST) by realizing high active area and high uniformity.

Ag nanocrystals made by chemical synthesis have been used in solar cell applications as a part of light trapping. The shape, crystal structure, defects and composition of these nanocrystals have been studied in detail. Samples with different ratios of silver solution (AgNO3) and reductant (NaBH4) were made, and a difference in nanocrystal size was observed. HRTEM and diffraction patterns showed that the samples contained mostly Ag nanocrystals, and some of them contained Ag2O nanocrystals as well. Some nanocrystals contained large defects, mostly twinning, which induced facets on the nanocrystal surface.

InGaAs can be used to enhance the response of solar cells past the 1.43 eV cutoff of GaAs. Strained-layer superlattice (SLS) structures with high indium and phosphorus compositions (up to 35% and 68% respectively) have been grown successfully. SLS solar cells with indium and high phosphorus compositions (up to 15% and 85% respectively) have been grown successfully. The spectral response of the solar cells has been extended to as low as 1.27 eV. This enhancement is also shown by an increase in the short circuit current, with a small reduction in the short circuit voltage as compared to standard GaAs p-n junction for AM1.5 and one sun.

Dark current curves show the extent of recombination in the superlattice. The reverse saturation current in the recombination region (0.2-0.8 V) was determined using a non-linear least squares fitting routine. An Arrhenius plot was generated by finding the reverse saturation current over a temperature range of 300-370 K. The low recombination devices show non-ideality constants of 1.7 with activation energies of 1.3-1.4 eV. The high recombination devices have non-ideality constants (˜2.3) and lower activation energies of 1.1 eV.

Nanophase semiconductor composites are widely researched for the development of third-generation photovoltaic (PV) devices. Through quantum-size effects and phase assembly manipulation the optical absorption and carrier transport properties of nanocomposite films can be influenced. We investigate the potential for improved PV-relevant material performance by examining the photo-sensitization of indium-tin-oxide (ITO) with nanophase germanium (Ge). Nanocomposite films are produced by a sequential, RF-magnetron sputter deposition technique. Deposition control and post-deposition annealing are used to demonstrate the manipulation of the extended-assembly of the nanocrystalline Ge phase. Optical absorption characteristics were correlated to variations in the composite film structure as confirmed by transmission electron microscopy. In addition to structure-dependent variation in spectral absorption, spectrally resolved photoconductivity measurements demonstrate enhanced photoconductivity of composite films associated with the incorporation of the Ge phase into the ITO host. These results support the further evaluation of such nanocomposite TCO materials in optoelectronic devices, including PV systems.

Au nanoparticles have been attached to the surface of titania nanotube arrays by means of a deposition-precipitation method. We demonstrate a high deposition density of the gold particles over the nanotube surface and also good control over the size of the gold nanoparticles. Photocurrent measurements using such Au attached titania nanotubes as photoanodes have been measured and compared with blank titania nanotubes.

New materials and structures have been developed for efficient organic solar cells, dye-sensitized solar cells (DCSs) and organic thin-film solar cells (OPVs). Some strategies for achieving high photon-to-electricity conversion efficiency in these solar cells are discussed, focusing on nanostructured materials. In the case of DSCs, unlike TiO2 nanoparticles, TiO2 nanotubes with suitable dimensions are expected to work as efficient light scatterers as well as to give large surface areas for charge separation. A strategy for designing triarylamine-functionalized ruthenium dyes, which display the high efficiency, is also proposed. Furthermore, OPVs based on donor/acceptor (D/A) block copolymers are discussed, focusing on the phase separation of donor and acceptor segments and their domain sizes.

This work reports on the changes of solid-state cells dye-sensitized solar cells performance with the variation of concentration of spiro-OMeTAD between 5% and 25% in the fabrication of the cell. The changes in charge recombination and capacitance correlate with the improvement of current-potential characteristics a increasing spiro-OMeTAD content, which is explained by reduction of transport resistance for hole transport, the increase of charge separation in the dye molecules, and importantly, with the increase of the β-factor in the recombination resistance, that causes a reduction of the diode ideality factor.

An ab initio study of several compounds candidates to behave as intermediate band materials is presented. The use of these materials as the active element in solar cells is a promising way to enhance the photovoltaic efficiency. Indeed from this point of view, most interesting compounds are those whose host semiconductor presents a band-gap close to the optimum value of 2 eV. Chalcogenide compounds substituted by light transition metals are solid candidates to this end. While they are being further characterized and experimentally synthesized, another approach is being examined. It consists of using Si as host semiconductor. Ti implantation at concentrations several orders of magnitude above equilibrium solubility has shown a probable intermediate band material behavior, the origin of the intermediate band being related to levels of interstitial Ti. Optoelectronic characterization of this material is completed. A novel possibility consists of combining chalcogen S implantation with boron. In this case preliminary results of electronic structure are shown.

A nitric acid (2M) pre-treatment is shown to increase the efficiency of a standard dye sensitized solar cell mounted on a FTO glass substrate from 4.15% to 5.12%. The pre-treatment involves immersing an FTO glass electrode coated with commercial ethyl cellulose based TiO2 paste for 1-60 minutes prior to sintering at 450°C. The pre-treatment leads to agglomeration of the TiO2 creating a scattering layer which covers the acid treated surface on short term immersion (<30 mins) and penetrates the bulk layer upon long immersion. The scattering layer itself takes up less of the sensitizing (N719) dye but scatters photons in the rest of the film. The optimum immersion time under room temperature conditions was found to be ca 20 minutes since at much longer immersion times the bulk film particle agglomeration reduced efficiency. The choice of anion in the acid is critical with certain species, notably phosphate, resulting in blockage of dye absorption sites in the entire film resulting in reduced cell efficiency.

Solar energy harvesting has been extensively studied in the last three decades to provide a green energy source. Hybrid photovoltaics (HPV) based on titania (TiO2) are researched for their easiness of production and low cost. Nanostructured mesoporous titania films and conductive polymers were used recently to form hybrid solar cells [1]. TiO2, mainly an n-type semiconductor with a band gap of 4.2 eV, is employed in several applications from which paints form the highest world use of titania making it an attractive material to use in HPV industry. On the other side, our targeted conductive polymer is polyaniline (PANI), a hole conductor polymer, which is used in such HPV cells due to its high charge-carriers mobility, absorption coefficient in the visible range and environmental stability. PANI and nanocrystalline TiO2 films fabricated using spin coating or layer by layer assembly techniques behave as a p-n heterojunction diode and can be used as solar cells [2-4].

Precursor solutions are prepared by polymerizing aniline-HCl inside an aqueous solution of titania. To study the effect of the precursor concentration on the PANI-TiO2 composite, polymerization of aniline is held in diverse TiO2 concentrations in water. Industrial grade TiO2 powders with particle size ranging from 200 nm to several μm are used. PANI-TiO2 precursor solutions are dip coated or slot dyed on various substrates such as PMMA, PET and PP, all with metal oxide conductive coatings. Bulk PANI-TiO2 pellets are prepared for comparison. The electrical and photovoltaic properties of the obtained films and pellets are investigated to choose the optimum blend composition for HPV cell. Finally a theoretical study and an analytical model of the HPV cell are presented relating the size of TiO2 and PANI particles and their respective geometrical distribution inside the blend to the transport characteristics of charge carriers and the overall efficiency of the HPV cell.[1] M. McGehee, MRS Bulletin, Vol. 34, No. 2, February 2009.[2] Z. Liu, W. Guo, D. Fu and W. Chen, Synthetic Metals, Vol. 156, pp. 414–416, 2006.[3] Z. Liu, J. Zhou, H. Xue, L. Shen, H. Zang and W. Chen, Synthetic Metals, Vol. 156, pp. 721–723, 2006.[4] X. Zhang, G. Yan, H. Ding and Y. Shan, Materials Chemistry and Physics, Vol. 102, pp. 249–254, 2007.

Photoelectrochemical properties of photoelectrodes consisting of pressed layers of electrospun TiO2 nanofibers were investigated by wavelength-resolved photocurrent measurements in LiClO4 (0.1 M) aqueous electrolyte with or without addition of KI as an additional hole scavenger. The photocurrents on nanofiber electrodes were three-times lower as compared to electrodes based on Hombikat nanocrystalline particles. The calcination of electrodes was necessary to observe enhanced efficiencies in the presence of iodide. The most striking difference between nanofiber and particulate electrodes was found in the effect of calcination on the efficiency of water photooxidation.

Nanostructured metal oxides with high surface areas have been shown to be efficient photoelectrodes for light-to-energy conversion in dye-sensitized solar cells (DSCs). In this work we demonstrate the use of nanofibrous mats of transparent conducting oxides (TCOs) as nanostructured electrodes, especially for DSCs. The nanofibers have been obtained by electrospinning suitable inorganic precursors and polymers, followed by calcination to remove the polymer. Afterwards, TiO2 layers were generated on our 3D-electrodes by electrodeposition. An improved performance as DSC was found compared to flat electrodes of similar thickness, validating our approach.

Effects of very high frequency- plasma enhanced chemical vapor deposition (VHF-PECVD) using diluted ultrapure silane at higher dilution ratio (R>30) on microstructures and optical characteristics of hydrogenated nanocrystalline silicon (nc-Si:H) film were studied. Nanocrystalline silicon films were prepared by at RF power ranging from 50 to 300 W. It was found that the transition from amorphous phase to nanocrystalline phase occurred between 100 W and 150 W. The nucleation mechanism toward nc-Si:H near the transition point of amorphous phase was discussed based on transmission electron microscopy with atomic scale. Further, it is suggested from UV-visible spectroscopy that nc-Si:H films with the best optical properties would be obtained near the transition point from the amorphous phase to the crystalline phase.

A near infrared heating method is presented which directly heats metal substrates to very high temperatures within seconds. The technique is used to heat 1mm thick titanium and stainless steel metal coupons onto which 1 cm2 commercial TiO2 pastes have been deposited within 12-25 seconds giving assembled dye sensitized solar cell efficiencies which are equivalent to cells prepared using a convection oven for 1800 seconds. The near infrared method is applicable to different paste thicknesses and paste types as well as different metal substrates. Near infrared sintering for the shortest times 12.5 seconds yielded cells with the highest efficiency compared to convection oven prepared samples. This ultrafast heating seems to drive off binder very effectively and lead to rapid sintering. Ultrafast sintering allows peak metal temperatures of 500-800 °C to be achieved without the massive losses in cell efficiency observed with the conventional heat treatment at temperatures over 600 ◦C.

Quantum dot (QD) solar cells have been actively investigated recently since they have been theoretically shown to have the potential to realize high conversion efficiencies. However, very little research has analyzed the effect the dots have on the transport or recombination effects in the device. In this paper, we report the I-V and spectral response characteristics of InAs/InGaAs “dots-in-a-well” (DWELL) solar cells and compared them with GaAs control cells. The QD cells show higher short circuit density (Jsc) and better long-wavelength efficiency compared to the control cell. By comparing the dark current behavior of the QD cells to the GaAs control cells, we have conservatively estimated the concentration level at which the QD solar cells would surpass GaAs control devices.

The quantum dot solar cells are grown by molecular beam epitaxy using the DWELL technique and a standard pin structure. The control cell structure is similar to the QD one except that there are no InAs dots or surrounding InGaAs quantum wells. The light I-V characteristics were measured under AM1.5G at 100 mW/cm2 illumination. The control cell has a Voc of 0.89V and a Jsc of 9.1 mA/cm2. The InAs QD solar cell has a Voc of 0.68 V and a Jsc of 12.2 mA/cm2. The QD cell has about a 33% larger short circuit current density compared to the GaAs control cell, which is mainly due to the higher photon absorption rate related to the DWELL structure. The spectrum response data show that the GaAs control cell and the QD cell have similar external quantum efficiency (EQE) in the visible to near-IR range (400-870nm). Beyond the GaAs absorption edge (870nm), the QD solar cell shows extended response with much higher measured EQE up to ˜1200 nm. This is strong evidence of the contribution from the InAs QDs and InGaAs QWs, the latter being the primary contributor to the increased Jsc.

We calculated the “local” ideality factor from measured dark IV data, and then substituted it into a single diode equation to get the “local” reverse saturation current. Whereas the GaAs control shows the typical monotonically decreasing ideality from 0.3 to 0.8V, a linearly increasing ideality is observed in the QD cell. Based on the measured dark currents, and neglecting series resistance, we extrapolated the IV curves to higher voltages and found that they intercept at ˜2×104 mA/cm2. Dividing the intercept point Jdark by the Jsc of the QD cell conservatively estimates the light concentration (˜1400×) above which the QD cell would have a higher Voc than the GaAs cell assuming additivity applies. This result is mainly attributed to the unique carrier transport properties that are introduced into the solar cell devices that utilize QDs.

Characteristics of strained layer superlattices (SLS) consisting of alternating layers InxGa1-xAs and GaAs1-yPy are examined for use in high efficiency solar cells. The effects of SLS quantum barrier widths on tunneling probability and short circuit current are discussed through analysis of J-V and spectral response measurements. Results indicate a threshold barrier thickness for which tunneling effects are deleterious. Effect of the number of SLS periods incorporated into a p-i-n structure and maximum number of periods are presented through spectral response and CV analysis. It is demonstrated that SLS show increasing responsivity with increasing number of periods due to higher absorption. CV analysis is performed to determine zero bias depletion widths for verifying appropriate number of SLS periods and fully depleted SLS region.

Adsorption behavior of eosinY on ZnO and its complex formation with Zn2+ were studied in detail, changing the Zn2+ concentration, temperature, and pH of mixed solutions. While eosinY undergoes typical Langmuir type adsorption when the solution contains only eosinY, co-presence of Zn2+ lead to a formation of multilayer of eosinY. Red precipitates of mixed aggregates were obtained when ZnCl2 was added to a concentrated eosinY solution. Aggregation was further promoted when the temperature was elevated. While increase of pH resulted in a decrease of monolayer adsorption of eosinY in the absence of Zn2+, it promoted the formation of mixed aggregates in its presence. These aggregation behaviors of eosinY reasonably explained the changes of nanostructures of the electrodeposited ZnO/eosinY hybrid thin films.

The artificial nano-clay powder was newly examined as a gelator of electrolyte of quasi-solid-state dye-sensitized solar cell (DSSC). The size of clay has two main distributions with 1.4 nm and 20 nm in diameter which are confirmed by STEM observation. The gelation point was determined by using Rheometer. The gel state maintained with more than 5wt% nano-clay in the acetonitrile based solvent. The quasi-solid-state DSSC with nano-clay electrolyte (10 wt%) was successfully showed a high photoelectric conversion efficiency of 10.3%, which is equivalent to that using a liquid electrolyte.

Transformation of titanium dioxide (TiO2) nanotubes (NTs) to truncated bipyramidal shape nanoparticles (NPs) with a large fraction of photo-catalytically active {001} facet surface was observed after thermal annealing TiO2 ordered nanotube arrays in fluorine ambient. Size of the formed nanoparticles depended on fluorine concentration and can be controlled from 20 nm to 350 nm. The crystal and optical properties of nanoparticle layers are superior to those of nanotube arrays, which are also annealed but without geometrical transformation. Using nanoparticle layers formed by this method we have fabricated dye-sensitized solar cells (DSSCs) with different size NPs in the range 35-350 nm. The dependence of solar cell performance on NP size is discussed.

Much attention has been directed towards the enhancement of electron transport in dye-sensitized solar cells (DSSC) using one-dimensional nanoarchitectured semiconductors. However, the improvement resulting from these ordered 1-D nanostructured electrodes is often offset or diminished by the deterioration in other device parameters intrinsically associated with the use of these 1-D nanostrucutres, such as the two-sided effects of the length of the nanowires impacting the series resistance and roughness factor. In this work, we mitigate this problem by allocating part of the roughness factor to the collecting anode instead of imparting all the roughness factors onto the semiconductor layer. A microscopically rough Zn microtip array is used as an anode on which ZnO nanotips are grown to serve as the semiconductor component in a DSSC. For the same surface roughness factor, our Zn microtip/ZnO nanotip DSSC exhibits an enhanced fill factor compared to a corresponding planar anode supported ZnO nanowire DSSC. In addition, the open circuit voltage of the Zn-microtip|ZnO-nanotip DSSC is also enhanced due to a favorable band shift at the Zn-ZnO interface, which raises the quasi Fermi level of the semiconductor and consequently enlarges the energy gap between the quasi Fermi level of ZnO and the redox species. The overall improvement demonstrates a new fundamental approach to enhance the efficiency of dye-sensitized solar cells.