Recently, the role of nanostructured materials in addressing the challenges in energy and natural resources has attracted wide attention. In particular, oriented nanostructures demonstrate promising properties for energy harvesting, conversion, and storage. In this Review, we highlight the synthesis and application of oriented nanostructures in a few key areas of energy technologies, namely photovoltaics, batteries, supercapacitors, and thermoelectrics. Although the applications differ from field to field, a common fundamental challenge is to improve the generation and transport of electrons and ions. We highlight the role of high surface area to maximize the surface activity and discuss the importance of optimum dimension and architecture, controlled pore channels, and alignment of the nanocrystalline phase to optimize the transport of electrons and ions. Finally, we discuss the challenges in attaining integrated architectures to achieve the desired performance. Brief background information is provided for the relevant technologies, but the emphasis is focused mainly on the nanoscale effects of mostly inorganic-based materials and devices.

Oriented nanostructures for energy conversion and storage.

The status of a-Si solar cell technology is reviewed. This review includes a discussion of the types of solar cell structure that are being used in commercial products. An overview of the development efforts under way involving new materials, such as alloys and microcrystalline films, and their impact on device performance is given. The status of stability in a-Si solar cells and projections for costs for large-scale manufacturing facilities are reviewed. The development of markets for a-Si photovoltaics is also discussed. >

Amorphous silicon solar cells

Processes used to grow hydrogenated microcrystalline silicon (μc-Si:H) from a H 2 /SiH 4 -glow discharge plasma are explained in comparison to those for hydrogenated amorphous silicon (a-Si:H). Differences and similarities between μc-Si:H and a-Si:H-growth reactions in the plasma and on the film-growing surface are discussed, and the nucleus-formation process followed by epitaxial-like crystal growth process is illustrated as unique processes for the formulation of μc-Si:H. Determination of the effect of dangling-bond defect density on the propagation of the resulting μc-Si:H films is also discussed in parallel with the effect on a-Si:H in order to obtain a clue to improve opto-electronic properties of those materials for device applications especially for thin-film-silicon solar cells. Material issues to produce low cost and high efficiency solar cells are described, and finally recent progress in those issues is demonstrated.

Microcrystalline silicon.. Growth and device application

Secondary ion mass spectroscopy (SIMS) has been used to measure the diffusion of deuterium in hydrogenated amorphous silicon. For a film deposited in a dc glow discharge in SiH4 at a substrate temperature of 315 °C, the diffusion data fits D (T) =1.17×10−2 exp(−1.53 eV/kT) cm2/s. This result implies that degradation of these films due to hydrogen out‐diffusion at 100 °C will not be significant until after more than 104 years.

A SIMS analysis of deuterium diffusion in hydrogenated amorphous silicon

Optical and photoconductive properties of discharge‐produced amorphous silicon (a‐Si) of the type used in efficient thin‐film solar cells have been studied as a function of a wide range of deposition conditions The optical absorption, optical band gap, photoconductivity, hydrogen content, and the characteristics of the Si‐H vibrational mode in a‐Si were determined Both substrate temperature in the range ∼200–400 °C and the type of discharge used are found to be important factors in determining the measured optical and photoconductive properties of a‐Si For films produced at substrate temperatures near 200 °C, dihydride bonding occurs, and the optical band gap is about 17 eV As the substrate temerature increases, monohydride bonding is favored, the optical band gap decreases, the optical absorption increases, and the photoconductive properties improve These properties are, in part, associated with the presence of bonded hydrogen For substrate temperatures between 300 and 400 °C, the photoconductive 

Optical and photoconductive properties of discharge‐produced amorphous silicon

Metal–amorphous‐silicon Schottky barriers, similar to those used in thin‐film amorphous‐silicon solar cells, exhibit nearly ideal diode behavior in the dark. The current‐voltage characteristics have been studied in the range from 270 to 370 °K and are found to be in agreement with the diffusion theory of metal‐semiconductor rectification. Barrier heights, which were measured by two different methods, depend on the metal work function, and barriers as high as ∼1.1 eV are obtained with Pt films.

Schottky‐barrier characteristics of metal–amorphous‐silicon diodes

Solar cells using Schottky barriers on amorphous silicon

The principal electrical properties that contribute to the operation of efficient metal/a-Si solar cells have been evaluated. Various metal/ a-Si Schottky barriers have been characterized in terms of barrier height, saturated current density, generation-recombination, built-in potential, and depletion width. The photovoltaic and photoconductive properties of the cell structures have been investigated. The dependence of photocurrents on wavelength and depletion width is presented and minority carrier diffusion effects are discussed.

D. E. Carlson

Papers

Schottky‐barrier characteristics of metal–amorphous‐silicon diodes

Solar cells using Schottky barriers on amorphous silicon

Electrical properties of a-Si solar cells