FIRST AND SECOND GENERATIONS Solar Cell Technology (Si)

FIRST AND SECOND GENERATIONS

Solar Cell Technology (Si)

Outlines

What is a Solar Cell History Basic physics of solar cells

Generations of Solar Cells First Generation Second Generation

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History

1839 Alexandre-Edmond Becquerel Photovoltaic effect: Light dependant voltage immersing

between two electrodes in an electrolyte

1883 Carles Fritts First solar cell: Coated semiconductor selenium with an

extremely thin layer of gold to form the junctions (1% efficient)

1941 First silicon based solar cell demonstrated

1946 Russell Ohl Patented the modern solar cell

1954 Beginning of modern solar cell research Bell laboratories: Experimenting with semiconductors,

accidentally found that Si doped with certain impurities was very sensitive to light

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A structure that converts solar energy directly to electricity by the photovoltaic effect It supplies voltage and current to a

resistive load (light, battery, motor)

It is like a battery It supplies DC power

It is not like a battery The voltage supplied by the cell changes with the changes of the load

resistance

The solar (photovoltaic) cell fulfills two fundamental functions: Photogeneration of charge carriers (electrons and holes) in a light-

absorbing material Separation of the charge carriers to a conductive contact to transmit

electricity

What Is a Solar Cell?

Illumination and Generation5

Ehν < EG : the incident light transparents

Ehν ≥ EG : photons are absorbed and EHP are

photogenerated Ehν > EG : energy generated is lost

as heat

Photovoltaic Effect

Solar cells are: p-n junctions Minority carrier devices Voltage is not directly applied Itotal = IF - IL = Is{exp(qV/kT)-1} – IL

The photo current produces a voltagedrop across the resistive load, which forward biases the pn junction

1. Absorption of a photon2. Formation of e-h pair (exciton)3. Exciton diffusion to Junction4. Charge separation5. Charge transport to anode (holes) and cathode (electrons)6. Supply a direct current for the load

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Forward Bias vs. Photogeneration

Forward Bias Voltage applied

externally Current is dominated

by diffusion

Photogeneration Voltage is generated

internally from EHP being swept across the junction by and E field

Current is dominated by drift

Cell Structures

Homojunction Device Single material altered so that one side is p-type and the other

sideis n-type p-n junction is located so that the maximum amount of light is

absorbed near it

Heterojunction Device Junction is formed by contacting two different semiconductor Top layer - high bandgap selected for its transparency to light Bottom layer - low bandgap that readily absorbs light.

p-i-n and n-i-p Devices A three-layer sandwich is created Contains a middle intrinsic layer between n-type layer and p-

type layer Light generates free electrons and holes in the intrinsic

region.

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Generations of Solar Cells

First Generation Single crystal silicon wafers (c-Si)

Second Generation Amorphous silicon (a-Si) Polycrystalline silicon (poly-Si) Cadmium telluride (CdTe) Copper indium gallium diselenide (CIGS) alloy

Third Generation Nanocrystal solar cells Photoelectrochemical (PEC) cells

Gräetzel cells Polymer solar cells Dye sensitized solar cell (DSSC)

Fourth Generation Hybrid - inorganic crystals within a polymer matrix

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First Generation: Overview

Dominant technology in the market More than 86% of the commercial production of solar

cells

High-cost, high-efficiency Maximum theoretical efficiency of 33%

Generally, Si based solar cells are more efficient and longer lasting than non-Si based cells. However, they are more at risk to lose some of their efficiency at higher temperatures (hot sunny days), than thin-film solar cells

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First Generation: Crystalline Si-based Cells

Cells are typically made using a crystalline Si wafers Wafers about 0.3mm thick, sawn from ingot with diameter of

10-15cm

Consists of a large-area, high quality and single layer p-n junction diode A single junction for extracting energy from photons

Approaches Ingots can be either monocrystalline or multicrystalline Most common approach is to process discrete cells on

wafers sawed from silicon ingots. More recent approach which saves energy is to process

discrete cells on Si wafers cut from multicrystalline ribbons

Band gap ~1.12 eV

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Crystalline Si-based Cells

Monocrystalline Si (c-Si) Made by Czochralski process, cut from cylindrical ingots

Not completely cover a square solar cell module without a substantialwaste

Expensive Extremely pure refined Si

Poly- or Multi-crystalline Si (poly-Si or mc-Si) Made from cast square ingots; melted Si is poured into a mold. Large

square blocks of molten Si carefully cooled and solidified Less waste of space, more expensive to produce than c-Si, but less

efficient

Ribbon Si A type of mc-Si Formed by drawing flat thin films from molten Si Lower efficiencies than poly-Si Save on production costs due to a great reduction in Si waste

Not require sawing from ingots

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First Generation: Research Cells

Source: National Renewable Laboratory

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First Generation: Evaluation

Advantages Broad spectral absorption range High carrier mobilities

Disadvantages High costs: Expensive manufacturing technologies

Extracting Si from sand and purifying it before growing the crystals Growing and sawing of ingots is a highly energy intensive process

Fairly easy for an electron generated in another molecule to hit a hole left behind in a previous photo excitation

Much of the energy of higher energy photons, at the blue and violet end of the spectrum, is wasted as heat

Not more energy-cost effective than fossil fuel sources With the max efficiency of 33%, it achieves cost parity with fossil fuel

energy generation after a payback period of 5-7 years

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Second Generation: Overview

Thin-film solar cells Based on the use of thin-film deposits of semiconductors

Intense development for the 90s and early 2000s

Developed to reduce the costs of the first generation cells Alternative manufacturing techniques to reduce high temperature

processing evolves production costs Production costs will then be dominated by material requirements Inherent defects due to lower quality processing methods reduces

efficiencies compared to the first generation cells

Low-cost, Low-efficiency cells

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Second Generation: Thin-Film Cells

Use minimal materials and cheap manufacturing processes Compared to crystalline Si based cells they are made from

layers of semiconductor materials only a few micrometers thick

Reduces mass of material required for cell design

Deposition of thin layers of materials on inexpensive substrates Mounted on glass or ceramic substrates

Devices initially designed to be high-efficiency, multiple junction photovoltaic cells

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Second Generation: Types (a-Si)

Amorphous Si cells deposited on stainless-steel ribbon Non-crystalline-Si deposited over large areas by PECVD

Used to produce large-area photovoltaic solar cells Hydrogenated amorphous Si (a-Si:H)

Plasma-deposited amorphous Si contains a significant percentage of H atoms

Essential to the improvement of the electronic properties of the material Cells are built up in the sequence from bottom to top

Metal base contact, n-layer, intrinsic layer, p-layer, transparent contact, glass substrate

Instead of one layer, several thinner layers are used to prevent efficiency drop

Complex production methods, but less energy intensive For a given layer thickness, absorbs much more energy than c-Si

(×2.5) Not stable, less efficient than c-Si Bandgap~1.7eV

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Second Generation: Types (poly-Si)

Polycrystalline (Micro Crystalline) Si Consists solely of crystalline silicon grains(1mm), separated by grain

boundaries Use antireflection layers to capture light waves with wavelengths

several times greater than the thickness of the cell itself Using a material with a textured surface both in front and back of the cell Light change directions and be reflected, and thus travels a greater

distance within the cell thickness Carrier mobilities can be orders of magnitude larger than amorphous

Si Material shows greater stability under electric field and light-induced

stress Low efficiency Fragile: Can be broken if hit by a falling branch or reasonably heavy

object flying through a strong wind Bandgap~1.1eV

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Second Generation: Types (CdTe)

Cadmium telluride (CdTe) cells deposited on glass Represents the second most utilized solar cell material in the world Crystalline compound formed from Cd and Te with a zincblende (cubic)

crystal structure Usually sandwiched with cadmium sulfide (CdS) to form a p-n junction

photovoltaic solar cell Simplified manufaturing compared to the multi-step process of joining two

different types of doped Si CaTe absorbs sunlight at close to the ideal wavelength, capturing

energy at shorter wavelengths than is possible with Si panels Perfectly matched to the distribution of photons in the solar spectrum in

terms of optimal conversion to electricity Cheaper than Si, especially in thin-film technology Low efficiency levels (10.6%) Toxicity of Cd Bandgap~1.58eV

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Second Generation: Types (CIGS)

Copper indium gallium diselenide (CIGS) alloy cells One of the best light absorber known

About 99% of the light is absorbed before reaching 1μm into the material

Deposited on either glass or stainless steel substrates More complex hetero-junction than CdTe

The most common material for the top/window layer is CdS hard to produce in mass quantities at competitive prices

Highest efficiency among the thin film material Reached efficiency levels of 20% in the laboratory

Better resistance to heat than Si-based solar cells Less toxic than CdTe solar cells

Uses a much lower level of Cd in CdS So far the cost cannot compete with the other solar cells Bandgap~1.38eV

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Second Generation: Research Cells21

Source: National Renewable Laboratory

Second Generation: Evaluation

Advantages Lower manufacturing costs Much less material require

Lower cost/watt can be achieved Lighter weight (reduced mass)

Their flexibility allows fitting panels on curved surface, light or flexible materials like textiles

Less support is needed when placing panels on rooftops Even can be rolled up

Disadvantages Typically, the efficiencies are lower than first generation

cells

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Summary

Technology

Com Eff (%)

Champ Eff(%)

Module ($/W)

Installed($/W)

LCOE(cents/kWh)

Wafer Si 15 25 2 8 17

a-Si 6.5 13 1.2 4.5 21.7

c-Si 5 10 1.3 4.8 18.3

CdTe 9 16.5 1.21 4.5 19.9

CIGS 9.5 19.5 1.8 6.3 22.2

Coal - - - - 5 ~ 8

Nov. 2007