Jatin K. Rath

Upconversion in solar cells

The possibility to tune chemical and physical properties in nanosized materials has a strong impact on a variety of technologies, including photovoltaics. One of the prominent research areas of nanomaterials for photovoltaics involves spectral conversion. Modification of the spectrum requires down- and/or upconversion or downshifting of the spectrum, meaning that the energy of photons is modified to either lower (down) or higher (up) energy. Nanostructures such as quantum dots, luminescent dye molecules, and lanthanide-doped glasses are capable of absorbing photons at a certain wavelength and emitting photons at a different (shorter or longer) wavelength. We will discuss upconversion by lanthanide compounds in various host materials and will further demonstrate upconversion to work for thin-film silicon solar cells.

Gas phase synthesis of two ensembles of silicon nanoparticles

Dusty plasmas provide a very favorable environment for the growth of silicon nanocrystals. For application of silicon nanocrystals in a solar cell, the fabrication of monodisperse silicon quantum dots has been challenging. We report a single step method to synthesize silicon (Si) nanoparticles in a custom designed dedicated plasma reactor. The nanoparticles produced in the gas phase belong to two different phases exhibiting different structural and optical properties. Particles made in the bulk of the plasma are aggregates of crystalline particles with a mean size of 100 nm. Particles made in locally enhanced plasma regions produced at holes present in the grounded electrode contain free-standing quantum sized particles with crystallites (with mean size of 2.95 nm) embedded within an amorphous matrix. We provide insight on different plasma processes leading to the formation of aggregates and free-standing particles. We hypothesize that the free standing particles are formed due to the excess energetic electrons present in locally enhanced discharges.

Electrical Characterization of HIT Type Solar Cells

The silicon heterojunction solar cell (SHJ) has made rapid progress in reaching high efficiency and it is already developed as an industrially viable product. However, much of its progress has come through process development while there is scarce knowledge on the microscopic nature of the functioning of this device. Although this device as a whole can be considered as bulk type, the parts of a SHJ solar cell that control the charge transport behavior are limited to very thin regions, either interface or a very thin layer. This poses problems on accurate determination of the physical quantities, such as defect densities and energetic positions, conductivity, carrier recombination and the overall charge transport behavior. This chapter gives the present understanding of electrical characterization of SHJ solar cells and provides a study of defects in the interesting regions of the device.

Development of SnS/In 2 S 3 core-shell nanoparticles for solar cell application

SnS nanoparticles (NPs) were synthesized by a colloidal route at low temperatures and analyzed by several characterization techniques. Whereas transmission electron microscopy (TEM) and atomic force microscopy (AFM) imaging conclusively proved quantum dot sized particles (~4 nm) with a narrow size distribution, Torsional Resonance-Tunneling AFM and Peak Force AFM proved the conductive nature of the particles. The chemical composition, studied by energy dispersive X-ray spectroscopy (EDX) showed the ratio S:Sn of 1:1, confirming that the semiconductor is SnS and not any other compound. To passivate the QD surface and protect it from reaction to ambient, core-shell structures were made. The SnS nanoparticles (NPs) were immersed in a CBD bath for deposition of In₂S₃ layers. The formed core/shell SnS/In₂S₃ nanoparticles were separated by centrifugation and washed with ethanol. The structure of the core-shell SnS/In₂S₃ NPs has been studied by High Resolution TEM which showed the lattice fringes of the SnS core, surrounded by amorphous matrix, tentatively attributed to In₂S₃. The EDX confirms the presence of the elements expected. The absorption spectra of SnS/In₂S₃ nanoparticles with increasing time of CBD In₂S₃ clearly showed increasing band gap, attributed to thicker In₂S₃ shell. Research on SnS QDs embedded in CIS solar cells is in progress.

Optical Transmission in a Silicon Nitride/Polymer Multilayer Permeation Barrier made by Hot-Wire CVD: Model and Experiment

The optical transmission of silicon nitride/polymer multilayer permeation barriers was measured and compared with model calculations. With this model the individual layer thicknesses can be tailored to create specific transmission spectra.

Electrical and Optical Properties of Indium and Aluminium Doped Zinc Oxide Films Prepared by RF Magnetron Sputtering

ZnO: In (IZO, 10wt % In2O3) and ZnO: Al (AZO, 1wt % Al2O3) films were deposited on Corning glass substrates by RF magnetron sputtering. The samples were either prepared on unheated substrates and post annealed in N2 at different temperatures, or prepared at elevated temperatures. Electrical, optical and structural properties were investigated as a function of deposition temperature and annealing temperature. Increasing the substrate heater temperature would lead to a decline in the electrical conductivity of IZO films, while AZO films showed unchanged performance in the substrate heater temperature range of 150 - 300°C. Post annealing appears to be an effective way to improve the electrical properties of both IZO and AZO films without sacrificing transparency. In this work, AZO films have higher conductivity and light transmission than IZO films.