yale university photovoltaic properties of a revolutionary new solar cell drew mazurek advisor:...

yale university

Photovoltaic Properties of a Revolutionary New Solar Cell

Drew Mazurek Advisor: Jerry Woodall

April 30, 2002

solar cells in real life

• Cost-effective way to provide power to remote areas

solar cells in real life

• Cost-effective way to provide power to remote areas

• Environmentally-friendly renewable energy source

solar cells in real life

• Cost-effective way to provide power to remote areas

• Environmentally-friendly renewable energy source

• Power source for outer space applications

how solar cells work

The simple idea: photons in, current and voltage out

how solar cells work

A closer look: photons above the semiconductor’s band gap energy generate hole/electron pairs…

pn-junction

p

n

- holes

- electrons

h > Eg

how solar cells work

which then diffuse across the cell’s concentration gradients

pn-junction

p

n

- holes

- electrons

how solar cells work

Some holes and electrons recombine before they can reach the other side of the junction. In good cells,

however, there is very little recombination.

pn-junction

p

n

- holes

- electrons

how solar cells work

Most holes and electrons make it to the other side, resulting in a net charge increase on each side. This net charge increase is

realized outside the cell as current and voltage, or power.

pn-junction

p

n

- holes

- electrons

solar cells in space

To go into space, solar cells must be

• efficient – want to produce as much power as possible

• lightweight – launching satellites into space costs $5,000 per pound

Additionally, we’d like them to be

• inexpensive to manufacture – $$$

• radiation-hard – Van Allen Belt ideal place for satellites, but high radiation environment

solar cells at yale

Strong electric field (~1,000-10,000 V/cm)

n++ n p

Indium Phosphide drift-based design

surface

solar cells at yale

n++ n p

h > Eg

Indium Phosphide drift-based design

Strong electric field (~1,000-10,000 V/cm)

surface

Strong electric field (~1,000-10,000 V/cm)

solar cells at yale

n++ n p

Indium Phosphide drift-based design

surface

solar cells at yale

• motion of carriers due to electric field

• not as susceptible to material defects

• motion of carriers due to concentration gradient

• material defects shorten carrier lifetime, causing more recombination

Diffusion (theirs)

Drift (ours)

vs.

solar cells at yale

Strong electric field… so what?

• holes are immediately swept into the junction, producing power

• fewer hole/electron pairs are lost due to recombination – no time to recombine!

n++ n p

solar cells at yale

Strong electric field… so what?

• radiation damage decreases carrier lifetimes. Carriers swept by drift (electric) fields, however, aren’t affected as much.

n++ n p

solar cells at yale

Why Indium Phosphide?

• very high ideal efficiency: ~37% at concentration of 1,000 suns

solar cells at yale

Why Indium Phosphide?

• very high ideal efficiency: ~37% at concentration of 1,000 suns

• absorbs most light at small thicknesses – lightweight! 0

20

40

60

80

100

0.01 0.1 1 10 100 1000

Thickness (μm)

Abs

orbe

d P

ower

(%)

Si GaAs InP

solar cells at yale

Why Indium Phosphide?

• very high quantum efficiency across all wavelengths of visible light and UV – highly efficient and it makes good use of almost the entire spectrum 0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

100 200 300 400 500 600 700 800 900 1000

Wavelength (nm)

Inte

rnal

Qua

ntum

Eff

icie

ncy

1 µm InP (10 Å dead region)1 µm GaAs (100 Å dead region)1 µm Si (10 Å dead region)10 µm Si (5 Å dead region)

solar cells at yale

Yale’s InP solar cells are ideal for outer space applications:

• lightweight

• radiation-hard

• highly efficient

• low cost (~$1/cm2 vs. $10/cm2 for current high-efficiency solar cells)

summary

• Solar cells are simply pn-junctions in which hole/electron pairs are created from photons.

• The holes/electrons diffuse into the junction, and are immediately swept to the other side.

• The net charge gain is seen outside the cell as current and voltage, or power.

summary

• At Yale, we have designed and perfected the first ever drift-dominated solar cell.

• By collecting carriers with an electric field, we are able to create solar cells that are robust in the strong radiation of outer space.

• Additionally, our cells are lightweight and inexpensive.

acknowledgements

Many thanks to:

Professor Jerry Woodall

Professor Janet Pan

Yanning Sun