Session P35: Focus Session: Materials for Photovoltaics and Photocatalysis II

Photovoltaic properties of novel titanium oxide nanotubes

Ultrafast synthesis of high aspect ratio titania nanotubes by anodization in chloride ions containing solutions has been reported and furthermore optimized by our group. We are presenting in this paper the results of relative measurements on photovoltaic properties of the chloride nanotubes samples sensitized with ruthenium dye N3, comparing them with samples obtained by other anodization methods, and with anatase nanopowders. Photoresponse parameters like short circuit current, open circuit voltage, maximum power and overall conversion efficiency have been measured under simulated solar radiation. Preliminary results on absolute measurements on dye sensitized solar cells employing these samples will also be presented.   

Enhanced optical absorption by nanocavities inside titania

Titania, a wide band gap semiconductor, can generate powerful oxidants and reductants by absorbing photon energies. Titania has been extensively used in photoelectrochemical systems, such as dye-sensitized titania, a wide band gap semiconductor, can generate powerful oxidants and reductants by absorbing photon energies. To improve the photoreactivity of titania, several approaches, including doping and metal loading have been proposed. Nanocavities are isolated entities inside a solid and hence are very different from nanoporous, whose pores (often amorphous and irregular) connect together and open to the surface. Dense polyhedral nanocavities inside single-crystalline anatase titania nanorods were successfully synthesized by simply heating titanate nanorods. The size of the nanocavities is typically about 10 nm. The surfaces of the nanocavity polyhedron are determined to be the crystallographic low-index planes of the titania crystal. We found that these dense nanocavities significantly enhance the optical absorption coefficient of titania in the near-ultraviolet region, thereby providing a new approach to increasing the photoreactivity of the titania nanorods in the applications related to absorbing photons. This work is supported by the U. S. DOE under contract DE-AC02-98CH10886 and Laboratory Directed Research and Development Fund of Brookhaven National Laboratory (to W.H.).   

Charge Separation in layered Titanate Nanostructures: Effect of Ion Exchange induced Morphology Transformation

Morphology changes induced by surface chemistry can provide important insights into photoexcitation processes on solids which are critical to photovoltaic and photocatalytic applications.$^{ }$We investigated charge separation processes on Na$_{2}$Ti$_{3}$O$_{7}$ nanowires and scrolled up H$_{2}$Ti$_{3}$O$_{7}$ nanotubes, two types of morphologies which by means of acid/base treatment can reversibly be transformed into each other. Some of the competitive processes photoexcited states undergo can be tracked by means of electron paramagnetic resonance and photoluminescence spectroscopy. A complementarity between efficient charge separation and radiative recombination of photoexcited states [1] was observed and clearly demonstrates the critical influence of morphology and interlayer composition on the photoelectronic properties of layered oxide nanostructures [2]. [1] Riss et al. Nano Lett. \textbf{2007, }7, 433-438. [2] Riss et al. Angew. Chem. Int Ed. \textbf{2007, a}nie.200703817, in press   

Bandgap Narrowing of Titanium Dioxides via Non-Compensated n-p Co-doping for Photocatalysis

Titanium dioxide (TiO$_{2})$ is a promising photocatalyst for solar hydrogen production from water, yet its photocatalytic efficiency is limited by its intrinsic wide-bandgap nature. In this talk, we present a conceptually new and intuitive approach, termed non-compensated n-p co-doping, to narrow the bandgap of TiO$_{2}$. The validity of this approach has been demonstrated using first-principles calculations within density functional theory, showing that extra impurity bands are created in the gap region because of the non-compensated nature of the n-p co-doping, resulting in a narrowed bandgap around 2 eV. Moreover, the electrostatic attraction between the n and p dopants enhances their thermodynamic and kinetic solubility in the host semiconductors. Preliminary experimental results confirming the non-compensated n-p co-doping concept will also be presented, together with its applicability to other wide bandgap semiconductors.   

The role of bond switches in light-induced defects in amorphous silicon

Amorphous silicon(a-Si) thin-film solar cells are promising materials for solar cells, but they suffer from the Staebler-Wronski effect, in which the efficiency degrades over the course of a few hours of light exposure. While there has been progress in mitigating this effect through sample preparation, there is still no clear microscopic explanation for the degradation. Using first principles density functional theory and highly accurate quantum Monte Carlo techniques, we investigate the simplest reaction in a-Si: a bond switch between two neighboring Si atoms. We find that these reactions can create defect states and can be light activated.   

First-principle study of the interfacial rehybridization in organic-inorganic composite photovoltaic devices

Composites of organic conjugated polymers and inorganic nanostructures offer cheap but at present inefficient photovoltaic materials. The efficiency of the photovoltaic device is critically dependent on charge transfer and orbital rehybridization at the donor-acceptor interface. In this work we investigate the P3HT/PCBM interface from density functional theory (DFT) based first-principles calculations. We find a strong rehybridization of the conduction band edge states suggesting an efficient route for exciton dissociation at the interface. Using many-body perturbation theory, we compute the quasiparticle corrections on top of the DFT results. These corrections are critical for accurate predictions and to reach agreement with experiment.   

Pulsed optically detected magnetic resonance of intrinsic a-Si:H at low excitation power

For more than 3 decades, there has been much effort devoted to the investigation of recombination processes in hydrogenated amorphous silicon (a-Si:H). Recently, low-temperature pulsed optically-detected magnetic-resonance (pODMR) studies have shown the presence of a variety of qualitatively different recombination mechanisms that influence the photoluminescence of this material [K. Lips, \textit{et. al.}, JOAM, \textbf{7}, 13 (2005).]. Here, we present similar experiments with comparatively low light excitation densities (60(10)mW/cm$^{2}$, 514nm, cw Ar$^{+}$ Laser). Qualitatively, our measurements confirm the presence of similar spin dependent recombination channels to those seen at high light excitation densities. However, due to the reduced densities of excess charge carriers, the dynamics of these processes are significantly slower. We attribute this behavior to the decreased transition probabilities at increased charge carrier separations.