Seminar 2 Flashcards
Why is crystalline silicon still dominant in commercial solar cells?
- High efficiency potential due to its bandgap.
- Unlimited availability.
- Non-toxic materials.
- Reliably mass produced.
- Broad knowledge of the material already exists.
What are the 6 main types of loss mechanisms in crystalline silicon solar cells?
- Reflection. -Resistance factors.
- Shadow. -Absorption & transmission.
- Recombination. -Other factors.
What causes reflection loss in crystalline silicon solar cells?
- The metal circuit path on the front of a solar cell reflects the light.
- The solar cell itself reflects the light.
What causes shadow losses in crystalline silicon solar cells?
The metal circuit path obscures the front of the solar cell.
What causes recombination losses in crystalline silicon solar cells?
- On the surface -> dangling bonds.
- Inside the volume: defects.
What causes resistance factor losses in crystalline silicon solar cells?
- Short circuit between the front and back of the solar cell.
- Transport of the charge carriers through the cables & contacts.
What causes absorption & transmission losses in crystalline silicon solar cells?
- Other layers of the solar cell can sometimes absorb photons.
- Light can be completely transmitted through the solar cell.
What is the highest efficiency PERL cell produced to date?
24.7%
What is done to provide the PERL cell with a low resistance back contact and back surface field?
Aluminium is diffused into localised regions etched in the back oxide.
How can transmission and absorption losses be countered?
- Use a backside Al layers to reflect light back and prevent sunlight from passing through.
- Make the solar cell thicker to increase absorption.
What is the purpose of minimising top contact coverage of a solar cell?
Reducing optical losses.
What is the purpose of surface passivation and insulating SiO2 and Si3N4 layers
Reducing optical losses in solar cells.
What “other factors” cause losses in crystalline silicon solar cells?
- Dirt on the solar cell.
- Not ideal conditions (STC)
How can optical path length be increased and what benefits will that have?
Through a combination of surface texturing and light trapping, reducing defects and therefore optical losses.
How can reflection losses be minimised?
- Using anti-reflection coatings on the top of the cell.
- Surface texturing to reduce reflection.
What is the PERL cell and why was it made?
The passivated emitted rear locally diffused cell was made to determine how efficient silicon solar cells could be made.
How are PERL cells designed to reduce reflection.
- The top surface of the cell is textured to produce inverted pyramids.
- It is painted with an anti-reflection coating.
What is used to passivate the top and bottom of each PERL cell?
Silicon dioxide - SiO2
What is used to define the top and rear contacts on a PERL cell?
Photolithography
What are the first 5 steps in the process of producing a single crystalline Silicon solar cell?
1 - starting material 2 - Saw damage etch 3 - Texture etch 4 - phosphorous diffusion 5 - edge isolation
SSTPE
What are the final four steps in the process of producing a single crystalline Silicon solar cell?
6 - ARC deposition
7 - front contact print
8 - AlBSF & back contact print
9 - Co-firing
AFAC
What is a photovoltaic array?
Two or more modules formed together.
How are parallel connections made in photovoltaic modules?
By joining each cells “n-type” contacts together and “p-type” contacts together.
How are series connections made in photovoltaic cells?
By connecting one cells n-type to the p-type of the next cell.
How do you calculate the voltage of multiple photovoltaic cells connected in parallel?
Take the average of all the voltages.
How do you calculate the voltage of multiple photovoltaic cells connected in series?
Add the cells voltages together.
How do you calculate current of multiple cells connected in series?
It is equal to the current of the cell generating the lowest current.
How do you calculate the current of multiple cells connected in parallel?
Add the cells currents together.
How does the indirect bandgap of Silicon negatively affect a solar cell?
It results in a low value absorption coefficient and therefore > 200 microns of silicon is required to absorb most of the incident light.
What are the disadvantages of using silicon to make solar cells?
- Indirect bandgap.
- Defects cause recombination.
What is the difference in efficiency between mono-crystalline and multi-crystalline silicon solar cells?
Efficiency is reduced ~ 2-3% in multi-crystalline.
How do the production costs of mono and multi crystalline silicon solar cells compare to each other?
Multi crystalline production costs are ~ 20% less due to a simpler method of growth resulting in a higher throughput.
Why is multi-crystalline silicon cheaper than mono-crystalline silicon?
- It has a simpler method of growth resulting in a high throughput.
- Screen-printing can be used for multi-crystalline.
Which has a greater tolerance to residual impurities, mono or multi-crystalline silicon?
Mono-crystalline silicon.
What shape are the wafers used in multi-crystalline silicon cells?
Square
What shape are the wafers in mono-crystalline silicon cells and how are they produced?
They are circular and are produced via the czochralski method.
Why are the circular wafers in mono-crystalline silicon cells worse than the square wafers in multi-crystalline silicon?
The square wafers can be packed closer together.
What are the initial and stable efficiencies of amorphous silicon solar cells?
initial = 15.2% Stable = 13%
What new applications are being sought after for amorphous silicon solar cells?
- Building integrated PV.
- Space power.
- Consumer electronics.
- Grid integration.
- Large scale power generation.
What is being done to provide a-Si cells with efficiencies comparable to c-Si cells?
Research into light degradation remedies.
For a given layer thickness, what absorbs more energy a-Si or c-Si?
For a given layer thickness, a-Si absorbs about 2.5 times more energy than c-Si.
What are the advantages of a-Si as compared to c-Si?
- Deposited on a wide range of substrates.
- More energy per given layer thickness.
- Less expensive.
- Lighter weight.
- Less material required.
What substrates can a-Si:H be deposited on?
A wide range, including flexible, curved and roll-away types.
What is the approximate efficiency of a-Si:H solar cells?
10%
What method of a-Si:H manufacturing allows for such large scale production?
Continuous “roll-to-roll” manufacturing.
What are the four continuous processes in the “roll-to-roll” manufacturing of a-Si:H?
- Substrate washing.
- Sputter deposition of back reflector.
- a-Si semiconductor deposition.
- ITO top electrode deposition.
How is roll-to-roll manufacturing used to produce a roll of a-Si:H?
A roll of flexible substrate (stainless steel) is unrolled and fed into the manufacturing process and rolled back up at the end.
What is the benefit of producing a-Si:H in one large roll?
The roll can be cut into a variety of different sizes for different applications.
