Solar Photovoltaic
(Photovolaic cell - PV cell)
- A device that converts solar energy directly to electricity
- The term solar cell is designated to capture energy from sunlight, whereas PV cells are referred to unspecified light sources
Applications of Solar Cells
- renewable energy
- can be powered for remote locations
- Its free, limitless, and environmentally friendly
Silicon (role + advantages)
Silicon is the most important material for solar cells production.
Si is the most common material used for solar cell mass production. As the most often used semiconductor material it has some important advantages.
Advantages:
- It is very abundant: SiO2 forms almost 1/3 of the earths crust
- It is not poisonous
- it is environmentally friendly, and its waste does not represent any problems
- it can be easily melted, handled, and it is fairly easy formed into mono-crystalline form
Its electrical properties with endurance of 125 degree C allow the use of Si-based semiconductor devices even in the most harsh environment and applications
Silicon production
Pure silicon (SiO2) is produced from sand
In production the following steps are used:
- Pure silicon is produced from SiO2 by reduction in specially designed furnaces at 1800 degree C. The produced material contain 98%-99% of pure Si. Carbon electrodes are used to reduce the SiO2
SiO2 + C => Si + CO2
additional steps :
Si + 2Cl_2 => SiCl4
or
SiCl2 + HCl => SiHCl3
Reduction in hydrogen atmosphere at 950 degree C
4SiHCl3 + H2 => 2Si + SiCl4 + SiCl2 + 6HCL
At 1200 degree C, conversion into tri-chloro-silae using trhe following reaction:
SiCl4 + H2 => SiHCl3 + HCl
Crystalline solar cells production
- Raw material
- Ingot
- Ingot squaring
-Wafer slicing - wafer
-cells
-module
-system
Disadvantages to Si- based solar cells
Processing SiO2 to produce Si is a very high energy process, and it takes over two years for a conventional solar cell to generate as much energy as was used to make the silicon it contains
1.5 tons of CO2 are emitted for each ton of Si (about 98% pure) produced
Other materials which can be used in place of Si
- Gallium aresenide (GaAs)
GaAs is used for the production of high efficiency solar cells, it is often utilized in concentrated PV systems and space applications. The efficiency is up to 25%, and up to 28% at concentrated solar radiation. Special types have efficiency over 30%
- Cadmium telluride (CdTe)
Thin film material produced by deposition or by sputtering is a promising low cost foundation for PV applications in the future. The procedures disadvantage is poisonous material used in production. Lab solar cells efficiency is up to 16%, whilst commercial types efficiency is up to 8%
- Copper-indium-diselenide (CulnSe2, or ClS)
Thin - film material with efficiency up to 17%. Promising material but production technology is not mature yet.
PV technology classification (6)
- Silicon Crystalline technology
-Mono-crystal;line PV cells
Multi-crystalline PV cells (silicon based)
- Thin film technology
Amorphous silicon PV cells
Poly-crystalline PV cells (non-silicon based)
Silicon crytalline technology
- makes up 86% of PV market
- very stable with module efficiencies 10-16%
- Mono crystalline PV cells
made using saw cut from single cylindrical crystal of Si
Operating efficiency up to 15 %
- Multi Crystalline PV cells
Caste from ingot of melted and recrystallized silicon
Cell efficiency - 12%
Accounts for 90% of the crystalline Si market
Amorphous solar cells
advantages + disadvantages
method
Amorphous solar cells are produced with technological procedures similar to that of integrated circuits. Due to the procedure, these modules are also known as thin -film solar cells (thin film modules)
The most advanced thin film technology
operating at 6% efficiency
Makes up of 13% PV market
Advantages:
mature manufacturing technologies available
Disadvantages:
initial 20-40% loss in efficiency
Method:
Glass substrate is thoroughly cleaned
Lower contact layer is applied
The surface is then structured- divided into bands
In vacuum, under high frequency electric field amorphous silicon layer is applied
The surface is re-banded
Upper metal electrodes are fixated
Thin film technology
Advantages + disadvantages
Advantages : low cost substrate and fabrication process
Disadvantages: Not very stable
How are thin film modules constructed?
Thin film modules are constructed by depositing extremely thin layers of photosensitive materials onto a low-cost backing such as glass, stainless steel or plastic
This technology results in lower production costs compared to other crystalline technology, a price advantage which is counterbalanced by lower efficiency rates ( from 4 to 11%)
Thin-film crystalline solar cell consists of layers about 10 micro m thick compared with 200-300 micro m layers of crystalline silicon cells
What are the four types of thin film modules that are commercially available:
- Amorphous Silicon (a-Si)
- Cadmium telluride (CdTe)
- Copper Indium/gallium diselenide/ disulphide (CIS, CIGS)
- Multi-junction cells (a-Si/ m-Si)
ACCM
Poly Crystalline PV cells (non-silicon based technology)
Advantages + disadvantages
- Copper indium gallium selenide (CIGS) with band gap up to 1eV, absorption coefficent 10^5 cm^-1
- high efficiency levels
Advantages:
- 18% lab efficiency
- > 11% module efficiency
Disadvantages:
- Immature manufacturing process
- Slow vacuum process
- CdTe exhibits direct band gap of 1.4 eV and high absorption coefficient
Advantages:
- 16% lab efficiency
- 6-9 % module efficiency
Disadvantages:
- immature manufacturing process
Concentrated PV:
Cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. The main idea is to use very little of the expensive semi conducting PV material while collecting as much sunlight as possible
Efficiencies are between 20-30%
Flexible cells
based on similar production process to thin film cells. When the active material is deposited in a thin plastic, the cell can be flexible. This opens the range of applications, especially for building integration (roof-tiles) and end-consumer applications
Thickness: 10 micro meters
Cell efficiency: as high as 20.3 %
What is the photovoltaic effect?
A phenomenon that certain materials produce electric current when they are exposed to light
E = hv = hc/gamma
A photon can be thought of as a packet of light and the amount of energy in a photon is proportional to the wavelength of light
Solar radiation and Si PV
(advantages)
- converts sunlight directly to electricity
- sunlight is the most abundant renewable resource (175 PW)
- Electricity is a very versatile form of energy
- No moving parts, long lifetime (>20 years)
Si PV (facts)
- only 77% of solar spectrum is absorbed by silicon
- of this around 30% is used as electrical energy
- net effect is 23% maximum efficiency
- silicon is transparent at wavelengths longer than 1.1 microns (1100 nm)
- 23% of sunlight passes right through with no effect
- Excess photon energy is wasted as heat
Silicon Atom facts
- Four valence electrons
- All electrons used in bonds… Not very conductive
p-type silicon
is created by doping with compounds containing one less valence electrons than Si does, such as B, Ga, In, Ti
Only three electrons are available for bonding with 4 adjacent Si atoms, therefore an incomplete bond (“hole”) exists which can attract an electron from a nearby atom. Filling one hole creates another hole in a different Si atom. This movement of holes is available for conduction
n-type silicon
is created by doping the Si with compounds with that contain one more valence electrons than Si does, such as with P, As, ….
Since only 4 electrons are required to bond with 4 adjacent Si atoms, the 5th valence electron is available for conduction
p-n junction
- each side started from an electrically neutral state
- negative charges move left and positive charges move right, there-by a charge imbalance is generated
- This creates an electric field, which in turn erects an obstacle to further flow of electrons
- All the electrons and holes in the depletion layer have combined and annihilated eachother
- This creates a voltage across the junction
Gratzel cells
also known as dye-sensitized solar cells (DSSCs), are a type of photovoltaic device that convert sunlight into electricity
Working Principle: Graetzel cells mimic the natural process of photosynthesis. They consist of a porous layer of titanium dioxide (TiO2) nanoparticles coated with a light-absorbing dye molecule. When exposed to sunlight, the dye absorbs photons and transfers the excitation energy to the TiO2 layer, generating electron-hole pairs (excitons).
Electron Transport: The excited electrons in the TiO2 layer are rapidly injected into a conductive network of nanoparticles, while the positively charged holes remain in the dye molecule. The TiO2 layer acts as an electron transport medium, allowing the electrons to move towards the electrode.
Electrolyte: Unlike traditional silicon solar cells, Grätzel cells use an electrolyte solution as a medium for the transportation of charge carriers. The electrolyte is typically an organic solvent containing a redox couple, such as iodide/triiodide (I-/I3-) or cobalt complex.
Electron Flow: The excited electrons in the titanium dioxide layer move through the porous network and are absorbed by the electrolyte solution. At the same time, the dye molecules are re-oxidized by accepting electrons from the electrolyte. This cyclic electron flow creates an electrical current.
Counter Electrode: A counter electrode, usually made of a conductive material like platinum, is placed in contact with the electrolyte to complete the electrical circuit. At the counter electrode, the electrons from the electrolyte combine with oxidizing species, typically triiodide ions (I3-), to regenerate the iodide ions (I-) needed in the dye regeneration process.
Transparent Conductive Electrodes: Transparent conductive electrodes, typically made of materials like indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), are used to allow sunlight to reach the dye-sensitized layer while facilitating the extraction of generated charge carriers.
Power Generation: When exposed to sunlight, the dye-sensitized layer absorbs photons and converts them into excited electrons, which are then transported through the electrolyte and collected at the counter electrode. This electron flow creates a voltage difference and generates electrical power.
