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Amorphous Semiconductors

And

Solar Cells

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Amorphous Semiconductors are used in many applications, including:

•Solar Cells

•Switching devices

•Thin Film Displays

•Electrophotography

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Thin-Film Transistor (TFT) LCD Displays

•Liquid Crystal Displays Developed by RCA Laboratories in 1968

•Work by acting as a “light valve” either blocking light or allowing it to pass

•An electric field is applied to alter the properties of each Liquid Crystal Cell (LCC) to change each pixel’s light absorption properties

•Colors are added through filtering process

•Modern Laptops produce virtually unlimited colors at very high resolution

Thin-Film Transistor (TFT) LCD Displays

Path of light through a TFT LCD

•Light comes from behind – either LED or fluorescent source

•Beam of light is polarized, then goes through TFT matrix, which decides which pixels should be “on” or “off”

•If “on”, molecules in LCC will align in a single direction, allowing light to pass

•Color filters block all wavelengths of light except those within the range of the pixel. Areas between pixels are printed black to

increase contrast.

•Exiting light passes through another polarizer to sharpen image and eliminate glare

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In an TFT display, each LCC is stimulated by a dedicated thin-film transistor matrix, with one transistor at each pixel.

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LCD Addressing Modes

Three types of addressing have emerged since LCD became the display medium in 1971:

•Direct

•Multiplex

•Active Matrix

Active matrix allows charge storage, enabling pixels to refresh enabling real-time video on large screens

In Direct, one signal controls many segments. Useful for numeric displays, e.g., watches and calculators

Wires in Multiplex are shared through a matrix wiring scheme, allowing separate signals to be delivered to each pixel

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Manufacturing and Display Configurations

•Photolithography used to lay insulators, transistors and conductors down on a glass substrate – the lower glass in an LCD

•TFT displays require a transistor and capacitor for each pixel

•For highest fidelity, RGB is replaced by GRGB and RGB Delta Displays

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Three switch technologies: Amorphous Silicon (a-Si), Polycrystalline Silicon )p-Si)

and Single Crystal Silicon (x-Si)Amorphous Silicon is the standard for TFT LCDs because they have:

•Good Color

•Good Grayscale Reproduction

•Fast Response

Advantages: An a-Si TFT production process requires only 4 basic lithography steps, and produces good quality large screens – low cost

Disadvantage: Because a-Si has low mobility, a capacitor must be added to each pixel

TFT Process Mobility (cm^2/V sec)

A-Si 0.3-0.7

conventional p-Si

6

eximer p-Si 329

singe crystal Si (x-Si)

600

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Polycrystalline Silicon

Disadvantage:

•Requires higher process temperatures than a-Si – 600oC softens most types of glass

Advantage:

•Adding only two process steps, NMOS and PMOS transistors can be formed

•Meet requirements for HDTV displays

p-Si junction

a-Si junction

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Breakthrough Technology

The eximer laser annealing process is capable of recrystallizing p-Si film, increasing its mobility 660 times. This is possible because polycrystalline Si absorbs UV light. The absorbed energy raises the temperature of the p-Si film, thus annealing it.

The eximer laser process allows a cheaper and more conventional glass to be used as a substrate, reducing production costs for the mass production of p-Si TFTs.

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Photovoltaics

•Photoelectric effect discovered by Edmund Bequerel in 1830

•Albert Einstein received the Nobel Prize for describing the nature of light and the photoelectric effect in 1905

•Bell Laboratories made the first photovoltaic module in 1954. The space industry in the 1960s and the energy crisis in the 1970s spurred further photovoltaic development

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Photovoltaics – Operating Principles

Photovoltaics, also known as Solar Cells are semiconductors, typically Silicon

A solar cell uses junctions of an n-type semiconductor (freely moving electrons) with a p-type semiconductor (freely moving holes) which creates a type of diode that is in electric equilibrium in the dark

Photons (electromagnetic radiation) from the sun free electrons and holes, causing a DC current to flow from the n- to the p-type material Several cells are placed in series in

modules to achieve higher voltages and power

Two Photovoltaic Cell Types

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•Single crystal or polycrystalline cells – use “doped” crystals for making the cells, much like computer chips

•This is the most common technology

•Crystalline cells are expensive but last many years with little degradation

•Silicon is the most common material, although others are under development, such as Gallium Arsenide and Indium Selenide

Improving Solar Cell Efficiency

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•The energy of a photon is E = h

•Electrons are elevated to the conduction band if the frequency of the light equals or exceeds the band gap energy

This means that light at a lower frequencies do no work

•To get around this, cells with different band gap energies are assembled into multijunction cells

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Multi-junction Solar Cells•The stack at right is a multijunction with descending order of band gap energy, Eg.

•Junction materials can be mixed (e.g., GaAs and Si) provided they are dimensionally compatible to tailor bandgap energy

•Multijunction solar cells have reached efficiencies of up to 35%

Cell materials of interest include:

•Amorphous Silicon

•Copper Indium Diselenide

•Gallium Aresnide and

•Gallium Indium Phosphide

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Amorphous Silicon

•Amorphous materials have no long-range crystalline order

•In 1974, researchers found that photovoltaic devices could be made using amorphous silicon by properly controlling deposition and composition

•Amorphous silicon absorbs solar radiation 40 times more efficiently than single-crystal silicon – a film 1-micron thick can absorb 90% of the usable solar energy

•Amorphous silicon can be processed at relatively low temperatures on low-cost substrates making it very economical

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Amorphous Silicon

The lack of crystalline regularity in amorphous silicon results in “dangling bonds”. Here, electrons recombine with holes. When amorphous silicon is doped with small amounts of hydrogen (“hydrogenation”), the hydrogen atoms combine chemically with the dangling bonds, permitting electrons to move through the amorphous silicon

Cells are designed to have ultra-thin (0.008-micron) p-type top layer, a thicker (0.5 to 1-micron) intrinsic (middle) layer, a very thin (0.02-micron) n-type bottom layer. The top layer is so thin and transparent that most light passes right through. The p- and n- layers create an electric field across the entire intrinsic region

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Solar Cell Processing Steps

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Solar Cell Efficiency

Power is the product of voltage and current: Vmax X Imax = Pmax

A solar cells energy conversion efficiency, ( or “eta”) is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. It is calculated using the ratio of Pmax divided by the input light irradiance under “standard” test conditions (E, in W/m2) and the surface area of the solar cell (Ac in m2)

cAE

P

max

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Economics of Solar Power

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Crystalline PV Cell Economics

•Total cost of conventional crystalline PV cells is about $500/m2

($50/sq.ft) of collector area

•The output of 1-m2 is 125 Watts, so, at a cost of $500/m2, this corresponds to $4/Watt of electricity, not counting necessary auxiliary components

•The lowest reported cost are $3/Watt for photovoltaic cells in 2002 (IEA). Crystalline silicon cells accounted for 80% of the total worldwide in 2002.

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•Efficiencies vary from 6% for amorphous Si cells to up to 35% for exotic GaAs or InSe cells

Efficiencies

•Efficiency is 14-16% in commercially available mc-Si cells

Photovoltaics - Economics

•Power distribution systems include inverters to connect to the grid – system efficiencies are between 5-19%

•A GaAs or InSe cell delivers 4 times the electrical power – at over 100 times the cost!

Costs

•In 2005, photovoltaic electricity cost $0.30 - $0.60/kWh in the US. Compare this to the ~$0.10/kWh from other sources

•The payback period for solar cell implementation can be from 1 to 20 years. A typical value is 5 years

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Alternative energy sources

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Solar Steam Plant – Four Corners, CA

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Optical Memory and Data Storage

•Use amorphous Chalcogen (group VI elements, e.g. Se, S or Te)

•Photo-induced phase transitions between crystalline and amorphous phases

•Photo-induced phase transitions between crystalline and amorphous phases or reversible photostructural changes in the amorphous phase

Light induces cross-linking of neighboring chains in Se. When a photon is absorbed, an electron from one of the non-bonding (lone-pair) orbitals that form the top of the valence band is transferred into the conduction band, leaving the other electron unpaired. This unpaired electron can, through interaction with lone-pair electrons of a neighboring chain, form an additional bond cross-linking the two chains. Inter-chain bonds strain glass, rupturing bonds, resulting in defect pairs to form with a dangling bond. The dangling bonds recombine to form a structure different from the original.

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An originally amorphous layer can be locally crystallized when exposed to a laser pulse. The difference in optical properties between glass and crystal is the basis for optical recording

Crystallization of a GST film by a long lower-intensity pulse and its amorphization by a short

higher-intensity pulse

Structure of a crystallized GST. The larger white circles represent Te sites, smaller

black/white circles represent Sb/Ge sites, and dashed circles represent Sb/Ge vacancies

GST: Ge-Sb-Te chalcogenide glasses

Because there is little volume change

in GST between glass and

crystalline phases, little atomic

movement is required, and it can change shape very

quickly

The GST layer is sandwiched between two protection layers. Each layer is ~20-100

nm thick

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