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Advanced Photovoltaics > Seminar 2 > Flashcards

Seminar 2 Flashcards

1
Q

Why is crystalline silicon still dominant in commercial solar cells?

A
  • High efficiency potential due to its bandgap.
  • Unlimited availability.
  • Non-toxic materials.
  • Reliably mass produced.
  • Broad knowledge of the material already exists.
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2
Q

What are the 6 main types of loss mechanisms in crystalline silicon solar cells?

A
  • Reflection. -Resistance factors.
  • Shadow. -Absorption & transmission.
  • Recombination. -Other factors.
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3
Q

What causes reflection loss in crystalline silicon solar cells?

A
  • The metal circuit path on the front of a solar cell reflects the light.
  • The solar cell itself reflects the light.
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4
Q

What causes shadow losses in crystalline silicon solar cells?

A

The metal circuit path obscures the front of the solar cell.

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5
Q

What causes recombination losses in crystalline silicon solar cells?

A
  • On the surface -> dangling bonds.

- Inside the volume: defects.

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6
Q

What causes resistance factor losses in crystalline silicon solar cells?

A
  • Short circuit between the front and back of the solar cell.
  • Transport of the charge carriers through the cables & contacts.
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7
Q

What causes absorption & transmission losses in crystalline silicon solar cells?

A
  • Other layers of the solar cell can sometimes absorb photons.
  • Light can be completely transmitted through the solar cell.
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8
Q

What is the highest efficiency PERL cell produced to date?

A

24.7%

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9
Q

What is done to provide the PERL cell with a low resistance back contact and back surface field?

A

Aluminium is diffused into localised regions etched in the back oxide.

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10
Q

How can transmission and absorption losses be countered?

A
  • Use a backside Al layers to reflect light back and prevent sunlight from passing through.
  • Make the solar cell thicker to increase absorption.
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11
Q

What is the purpose of minimising top contact coverage of a solar cell?

A

Reducing optical losses.

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12
Q

What is the purpose of surface passivation and insulating SiO2 and Si3N4 layers

A

Reducing optical losses in solar cells.

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13
Q

What “other factors” cause losses in crystalline silicon solar cells?

A
  • Dirt on the solar cell.

- Not ideal conditions (STC)

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14
Q

How can optical path length be increased and what benefits will that have?

A

Through a combination of surface texturing and light trapping, reducing defects and therefore optical losses.

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15
Q

How can reflection losses be minimised?

A
  • Using anti-reflection coatings on the top of the cell.

- Surface texturing to reduce reflection.

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16
Q

What is the PERL cell and why was it made?

A

The passivated emitted rear locally diffused cell was made to determine how efficient silicon solar cells could be made.

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17
Q

How are PERL cells designed to reduce reflection.

A
  • The top surface of the cell is textured to produce inverted pyramids.
  • It is painted with an anti-reflection coating.
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18
Q

What is used to passivate the top and bottom of each PERL cell?

A

Silicon dioxide - SiO2

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19
Q

What is used to define the top and rear contacts on a PERL cell?

A

Photolithography

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20
Q

What are the first 5 steps in the process of producing a single crystalline Silicon solar cell?

A 
1 - starting material
2 - Saw damage etch
3 - Texture etch
4 - phosphorous diffusion
5 - edge isolation

SSTPE

21
Q

What are the final four steps in the process of producing a single crystalline Silicon solar cell?

A

6 - ARC deposition
7 - front contact print
8 - AlBSF & back contact print
9 - Co-firing

AFAC

22
Q

What is a photovoltaic array?

A

Two or more modules formed together.

23
Q

How are parallel connections made in photovoltaic modules?

A

By joining each cells “n-type” contacts together and “p-type” contacts together.

24
Q

How are series connections made in photovoltaic cells?

A

By connecting one cells n-type to the p-type of the next cell.

25
Q

How do you calculate the voltage of multiple photovoltaic cells connected in parallel?

A

Take the average of all the voltages.

26
Q

How do you calculate the voltage of multiple photovoltaic cells connected in series?

A

Add the cells voltages together.

27
Q

How do you calculate current of multiple cells connected in series?

A

It is equal to the current of the cell generating the lowest current.

28
Q

How do you calculate the current of multiple cells connected in parallel?

A

Add the cells currents together.

29
Q

How does the indirect bandgap of Silicon negatively affect a solar cell?

A

It results in a low value absorption coefficient and therefore > 200 microns of silicon is required to absorb most of the incident light.

30
Q

What are the disadvantages of using silicon to make solar cells?

A
  • Indirect bandgap.

- Defects cause recombination.

31
Q

What is the difference in efficiency between mono-crystalline and multi-crystalline silicon solar cells?

A

Efficiency is reduced ~ 2-3% in multi-crystalline.

32
Q

How do the production costs of mono and multi crystalline silicon solar cells compare to each other?

A

Multi crystalline production costs are ~ 20% less due to a simpler method of growth resulting in a higher throughput.

33
Q

Why is multi-crystalline silicon cheaper than mono-crystalline silicon?

A
  • It has a simpler method of growth resulting in a high throughput.
  • Screen-printing can be used for multi-crystalline.
34
Q

Which has a greater tolerance to residual impurities, mono or multi-crystalline silicon?

A

Mono-crystalline silicon.

35
Q

What shape are the wafers used in multi-crystalline silicon cells?

A

Square

36
Q

What shape are the wafers in mono-crystalline silicon cells and how are they produced?

A

They are circular and are produced via the czochralski method.

37
Q

Why are the circular wafers in mono-crystalline silicon cells worse than the square wafers in multi-crystalline silicon?

A

The square wafers can be packed closer together.

38
Q

What are the initial and stable efficiencies of amorphous silicon solar cells?

A 
initial = 15.2%
Stable = 13%
39
Q

What new applications are being sought after for amorphous silicon solar cells?

A
  • Building integrated PV.
  • Space power.
  • Consumer electronics.
  • Grid integration.
  • Large scale power generation.
40
Q

What is being done to provide a-Si cells with efficiencies comparable to c-Si cells?

A

Research into light degradation remedies.

41
Q

For a given layer thickness, what absorbs more energy a-Si or c-Si?

A

For a given layer thickness, a-Si absorbs about 2.5 times more energy than c-Si.

42
Q

What are the advantages of a-Si as compared to c-Si?

A
  • Deposited on a wide range of substrates.
  • More energy per given layer thickness.
  • Less expensive.
  • Lighter weight.
  • Less material required.
43
Q

What substrates can a-Si:H be deposited on?

A

A wide range, including flexible, curved and roll-away types.

44
Q

What is the approximate efficiency of a-Si:H solar cells?

A

10%

45
Q

What method of a-Si:H manufacturing allows for such large scale production?

A

Continuous “roll-to-roll” manufacturing.

46
Q

What are the four continuous processes in the “roll-to-roll” manufacturing of a-Si:H?

A
  • Substrate washing.
  • Sputter deposition of back reflector.
  • a-Si semiconductor deposition.
  • ITO top electrode deposition.
47
Q

How is roll-to-roll manufacturing used to produce a roll of a-Si:H?

A

A roll of flexible substrate (stainless steel) is unrolled and fed into the manufacturing process and rolled back up at the end.

48
Q

What is the benefit of producing a-Si:H in one large roll?

A

The roll can be cut into a variety of different sizes for different applications.

Advanced Photovoltaics (7 decks)

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