Krishna P. Acharya

Pulsed laser deposition of graphite counter electrodes for dye-sensitized solar cells

We report on pulsed laser deposition of graphite onto flexible plastic and conductive glass substrates for use as a counter electrode in dye-sensitized solar cells. The efficiency of as-prepared graphite electrodes was tested using CdS-sensitized solar cell architecture resulting in external quantum efficiency comparable to that of conventional platinum counter electrodes. This work highlights the possibility of using pulsed laser deposited graphite as a low-cost alternative to platinum, which could be fabricated both on flexible and rigid substrates.

CdS thin films formed on flexible plastic substrates by pulsed-laser deposition

The merger of a transparent plastic foil substrate with a semiconductor CdS film for a photonic application was realized using pulsed-laser deposition. Although plastic is not considered to be a favoured substrate material for semiconductor thin-film formation, the deposited CdS film possesses good adhesion, with a polycrystalline texture, flat surface (roughness/thickness = 0.003), and room-temperature photosensitivity with a blue-shifted peak at 2.54 eV. This work demonstrates the capability of pulsed-laser deposition to form novel heterostructures with appealing and useful technological properties such as plasticity and low weight.

Heteroepitaxial Growth of Colloidal Nanocrystals onto Substrate Films via Hot-Injection Routes

Hot-injection synthesis of colloidal nanocrystals (NCs) in a substrate-bound form is demonstrated. We show that polycrystalline films submerged into hot organic solvents can nucleate the heteroepitaxial growth of semiconductor NCs, for which the ensuing lattice quality and size distribution are on the par with those of isolated colloidal nanoparticles. This strategy is demonstrated by growing lead chalcogenide NCs directly onto solvent-submerged TiO(2) substrates. The resulting PbX/TiO(2) (X = S, Se, Te) nanocomposites exhibit heteroepitaxial interfaces between lead chalcogenide and oxide domains and show an efficient separation of photoinduced charges, deployable for light-harvesting applications. The extendibility of the present method to other material systems was demonstrated through the synthesis of CdS/TiO(2) and Cu(2)S/TiO(2) heterostructures, fabricated from PbS/TiO(2) composites via cation exchange. The photovoltaic performance of nanocrystal/substrate composites comprising PbS NCs was evaluated by incorporating PbS/TiO(2) films into prototype solar cells.

Physical characterization of n-GaAs on p-Si formed by low-temperature pulsed-laser deposition

Stoichiometry, texture, surface features, I-V characteristic, and optical responsivity were studied of thin-film n-GaAs deposited onto p-type Si at room temperature. Low-temperature pulsed-laser deposition using a neodymium doped yttrium aluminum garnet laser (532 nm, 6 ns, 10 Hz) was employed to form the heteropairing. The film is stoichiometric, mainly of amorphous nature including some crystalline sections, and, despite droplets, exhibits an optically smooth surface. Rectification and photodiode properties for alternating as well as direct current experiments were established in an almost scholastic manner, which cannot be attributed to the previously investigated p-GaAs/n-Si. Hence, the ablation of electron rich GaAs is apparently the way of choice to form operative GaAs/Si junction devices. The work further reveals that intrinsic sample features might be different for alternating and direct photocurrent measurements.

Photonic digitizing and pattern alteration with flexible CdS and GaAs film surfaces

The application potential of thin-film CdS and GaAs on polymer-based substrates for photonic operations was investigated employing constant wavelength and pulsed-laser sources. We utilized the flexible film surfaces as media for laser crossing in order to realize photonic digitizing. The extremely straightforward concept has the potential to respond within nanoseconds and faster, while the presently used concept in optical networks operates in the millisecond regime. Furthermore, the concept of laser crossing points to a different way of operation philosophy by employing the photonic alteration of reflection patterns.