1. Solar Cell Technology (Si)
FIRST AND SECOND GENERATIONS

2. Outlines What is a Solar Cell Generations of Solar Cells History
Basic physics of solar cells
Generations of Solar Cells
First Generation
Second Generation

3. History 1839 Alexandre-Edmond Becquerel 1883 Carles Fritts
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

4. What Is a Solar Cell?
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

5. Illumination and Generation
Ehν < EG : the incident light transparents
Ehν ≥ EG : photons are absorbed and EHP are photogenerated
Ehν > EG : energy generated is lost as heat

6. 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 voltage
drop across the resistive load, which
forward biases the pn junction
Absorption of a photon
Formation of e-h pair (exciton)
Exciton diffusion to Junction
Charge separation
Charge transport to anode (holes) and cathode (electrons)
Supply a direct current for the load

7. Forward Bias vs. Photogeneration
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

8. Cell Structures Homojunction Device Heterojunction 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.

9. 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

10. 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

11. 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

12. 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 substantial
waste
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

13. First Generation: Research Cells
Source: National Renewable Laboratory

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Second Generation: Research Cells
Source: National Renewable Laboratory

22. 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

23. Summary Wafer Si 15 25 2 8 17 a-Si 6.5 13 1.2 4.5 21.7 c-Si 5 10 1.3
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
19.9
CIGS
9.5
19.5
1.8
6.3
22.2
Coal -
5 ~ 8
Nov. 2007