ISAT 436 Micro-/Nanofabrication and Applications Photovoltaic Cells David J. Lawrence Spring 2004

ISAT 436ISAT 436Micro-/Nanofabrication Micro-/Nanofabrication

and Applicationsand Applications

Photovoltaic CellsPhotovoltaic Cells

David J. Lawrence

Spring 2004

Properties of Light (1)Properties of Light (1)

f = frequency (Hz) o = wavelength in vacuum or air [usually

measured in m, nm, or Angstroms (Å)] c = speed of light in vacuum = 3 108 m/s c = f o

n = refractive index of a material (“medium”) v = c / n = speed of light in material = o / n = wavelength in material

v = f


E = h f = energy of a photonh = Planck’s constant = 6.626 10-34 J-s

= 4.136 10-15 eV-s

E = (h c) / o

h c = 1240 eV-nm = 1.24 eV-m1 eV = 1.602 10-19 J = h / 2 = 1.055 10-34 J-s

Properties of Light (3)Properties of Light (3)A useful equation for the energy of a

photon:

Rearranged, this gives

oo

nmeV1240hcE

E

nmeV1240

E

hco


For the visible portion of the electromagnetic spectrum, the wavelength in vacuum (or in air) ranges from:

Color o (nm) f (Hz) Ephoton (eV)

red 630-760 ~4.5 x 1014 ~1.9orange 590-630 ~4.9 x 1014 ~2.0yellow 560-590 ~5.2 x 1014 ~2.15green 500-560 ~5.7 x 1014 ~2.35blue 450-500 ~6.3 x 1014 ~2.6violet 380-450 ~7.1 x 1014 ~2.9

o 400 nm 700 nm4000 7000 Åto

to

Åo o o


Light with wavelength o< 400 nm is called ultraviolet (UV).

Light with wavelength o> 700 nm is called infrared (IR).

We cannot see light of these wavelengths, however, we can sense it in other ways, e.g., through its heating effects (IR) and its tendency to cause sunburn (UV).

Optical Generation of Free Optical Generation of Free Electrons and HolesElectrons and Holes

Recall that light can generate free electrons and holes in a semiconductor.

See Photovoltaic Fundamentals, p.12 and p.16.

The energy of the photons (hf) must equal or exceed the energy gap of the semiconductor (Eg) .

If hf > Eg , a photon can be absorbed, creating a free electron and a free hole.

Optical Generation of Free Optical Generation of Free Electrons and Holes - - Bond ModelElectrons and Holes - - Bond Model

See Photovoltaic Fundamentals, p.12 and p.16.

Si Si Si Si Si Si Si



free electron

free hole

Photons

Optical Generation of Free Optical Generation of Free Electrons and Holes - - Band ModelElectrons and Holes - - Band Model

If a photon has an energy larger than the energy gap, the photon will be absorbed by the semiconductor, exciting an electron from the valence band into the conduction band, where it is free to move.

A free hole is left behind in the valence band. This absorption process underlies the operation of photoconductive light

detectors, photodiodes, photovoltaic (solar) cells, and solid state camera “chips”.

Ele

ctro

n E

nerg

y

“Conduction Band”(Nearly) Empty

“Valence Band”(Nearly) Filled with Electrons

“Forbidden” EnergyGap

Photoconductive Light DetectorsPhotoconductive Light Detectors

Photons having energy greater than the energy gap of the semiconductor are absorbed, creating free electrons and free holes, and thus the resistivity, , of the semiconductor decreases.

semi-conductor

hf

I Vout

Photoconductive Light DetectorsPhotoconductive Light Detectors

Since Rsemiconductor = /A , the resistance of the semiconductor sample also

decreases.

semi-conductor

hf

I Vout

Recall that:

pn qpqn

11

Photovoltaic CellsPhotovoltaic Cells Photovoltaic cells, also called solar cells, convert sunlight directly

into electricity. A p-n junction is the key element of all efficient photovoltaic

cells. See Photovoltaic Fundamentals, pages 8 and 15.

- -

- -+ +

+ +

n-side p-sideE

junction

Photovoltaic Cells -- Bond ModelPhotovoltaic Cells -- Bond Model Recall that there is an electric field, E, in the depletion region of

a p-n junction. This electric field causes optically generated carriers to move,

enabling a solar cell to generate an electric current.

- -

- -+ +

+ +

n-side p-sideE

depletion regionneutral hereneutral here

Photovoltaic Cells -- Bond ModelPhotovoltaic Cells -- Bond Model If light generates free electrons and holes in the depletion region,

the electric field makes these carriers move. Which way do they go? What direction does the current flow?

- -

- -+ +

+ +

n-side p-sideE

depletion regionneutral hereneutral here

Photovoltaic Cells -- Band ModelPhotovoltaic Cells -- Band Model Recall that a p-n junction can also be described by an

energy band diagram.

conduction band

n-side

p-side

valence bandE

Eg

depletion region

P-N Junction DiodeP-N Junction Diode Electrons behave like marbles they tend to go downhill. Holes behave like helium-filled balloons they tend to

float uphill.

conduction band

n-side

p-side

valence bandE

Eg

depletion region

P-N Junction DiodeP-N Junction Diode The bent energy bands are a barrier to electron motion. The bent energy bands are a barrier to hole motion.

conduction band

n-side

p-side

valence bandE

Eg

depletion region

Photovoltaic Cells -- Band ModelPhotovoltaic Cells -- Band Model Photons with energy hf > Eg will be absorbed by the

semiconductor. If a photon is absorbed in the depletion region, a free

electron and a free hole are generated there.

conduction band

n-side

p-side

valence bandE

Eg

depletion region

Photovoltaic Cells -- Band ModelPhotovoltaic Cells -- Band Model The optically generated free electron and hole will move in

response to the electric field. Which way do they go? What direction does the current flow?

conduction band

n-side

p-side

valence bandE

Eg

depletion region

Photovoltaic Cells -- Band ModelPhotovoltaic Cells -- Band Model In order for current to flow, we must form a complete circuit. Electrons flow counterclockwise in this circuit. Current flows clockwise in this circuit.

n-side

p-sideE

metalcontact

metalcontac

t

I

V + “load”, e.g., motor

Photovoltaic Cells -- Band ModelPhotovoltaic Cells -- Band ModelLight energy is converted to electrical energy.

n-side

p-sideE

metalcontact

metalcontac

t

I


Photovoltaic CellsPhotovoltaic CellsNotice that the “photocurrent” flows opposite

the diode symbol arrow.

I


Photovoltaic Cells -- Band ModelPhotovoltaic Cells -- Band Model Photons absorbed outside the depletion region can contribute

to the “photocurrent”. The electrons and holes that are generated must diffuse to

the depletion region before they recombine.

conduction band

n-side

p-side

valence bandE

Eg

depletion region

Photovoltaic CellsPhotovoltaic Cells Photovoltaic (solar) cells are designed for energy

conversion, so they usually have a large (> 5 cm2) surface area.

Smaller light detecting p-n junctions, called photodiodes, have numerous other applications, e.g., light measurement scientific instruments light detection in fiber optic communications systems light detection in reading “heads” in optical disc

systems (e.g., CD, CD-ROM, DVD) light sensitive elements in solid state camera “chips”.

Photovoltaic CellsPhotovoltaic Cells Next, let’s consider some practical solar cell

structures. Photovoltaic Fundamentals is a good reference. An essential feature that all efficient solar cells have

is a p-n junction. All solar cells also have metal electrical contacts to

conduct the photogenerated current to the outside world.

Solar cells can be made from single crystal semiconductors polycrystalline (and semicrystalline) semiconductors amorphous semiconductors.

Silicon Photovoltaic CellSilicon Photovoltaic Cell Single crystal

silicon solar cell. Key features to

observe: p-n junction front contact back contact antireflection

coating cross section

not to scale

(Greek “nu”)

f frequency h hf photon

energyLarger diagram on next slide!

Silicon Photovoltaic CellSilicon Photovoltaic Cell

Silicon Photovoltaic CellSilicon Photovoltaic Cell Starting material

Single crystal silicon wafer (2” to 6” diameter) p-type boron-doped1 -cm p ? Wafer is cleaned to remove contaminants. Surface may be “textured” to reduce the reflection of

incident sunlight (see Photovoltaic Fundamentals, page 22). This is done with a chemical etching solution.

We will begin by considering the fabrication of a cell without texturing.

Silicon Photovoltaic CellSilicon Photovoltaic Cell The top ~ 0.3 m of the wafer must be converted

from p-type to n-type. This is usually done by introducing phosphorus from

the wafer surface so that the phosphorus concentration greatly exceeds the background boron concentration from the surface down to a depth of about 0.3 m.

The concentration of added phosphorus is typically 1019 to 1021/cm3.

The process by which phosphorus is introduced is called diffusion.

Diffusion is described in detail in Chapter 4 of Jaeger.

Silicon Photovoltaic CellSilicon Photovoltaic CellEssentials of the diffusion process:

The wafer is heated to 900 to 1200 °C in a furnace with gas (typically N2 or a mixture of N2 and O2) flowing

over the wafer (Jaeger, p. 96). Phosphorus is delivered to the wafer surface by adding a

phosphorus-containing compound (e.g., POCl3) to the gas or by maintaining a solid source containing P2O5 near or in contact with the wafer (Jaeger, p. 98-99).

1 -cm p-type Si p 1.5 1016/cm3

P P P P P P P P P

Silicon Photovoltaic CellSilicon Photovoltaic Cell Diffusion process (continued):

The depth to which phosphorus diffuses is controlled by adjusting the temperature (900 - 1200 °C) and duration (minutes to hours) of the diffusion process.

Typical diffusion depths are 0.2 to 1.0 m. Since the phosphorus concentration in the diffused layer

(1019 to 1021/cm3) greatly exceeds the background boron concentration, the diffused layer is converted to n-type.


phos.-doped n-type Si,n 1020/cm3 10-3 -cm

Silicon Photovoltaic CellSilicon Photovoltaic CellWe now have the required p-n junction.We need a metal electrical contact to the p-side

>> the back contact.We need a metal electrical contact to the n-side.

>> the front contact.



Silicon Photovoltaic CellSilicon Photovoltaic Cell PV cell is a large area p-n junction. of most semiconductors (e.g., silicon) is substantially

greater than for a metal (metal ~ 10-6 to 10-5 -cm).

A small wire contact to each side is insufficient. Metal must extend over much of both surfaces in order to

collect the photocurrent efficiently. A metal grid front contact on the n-side allows light to

enter the semiconductor, where it is absorbed.



Silicon Photovoltaic CellSilicon Photovoltaic Cell In the process of diffusing phosphorus into a p-type

silicon wafer to form a p-n junction, the surface may have been oxidized or otherwise “contaminated”.

Before metal contacts are deposited, any SiO2 or surface contamination is removed by etching.

The etching process consists of immersion in a liquid solution containing hydrofluoric acid (HF).



Silicon Photovoltaic CellSilicon Photovoltaic Cell A metal back contact can be deposited over the entire p-

type substrate using a process called evaporation. See Jaeger, pp. 129-134. For example, aluminum in a ceramic crucible is heated by

a tungsten filament until it evaporates. The silicon wafer is placed above the crucible and the

aluminum vapor condenses on the p-type side, forming a thin film, 100-1000 nm thick.

In order to ensure the purity of the deposited metal, evaporation is carried out in an evacuated chamber. (If any oxygen were present in the chamber, it would immediately react with the aluminum vapor.)

Silicon Photovoltaic CellSilicon Photovoltaic Cell Evaporation

Aluminum Vapor

Aluminum Film

Silicon Wafer

Silicon Photovoltaic CellSilicon Photovoltaic CellThe metal front contact is usually in the form of a

grid pattern, as shown on the next slide and on pages 21 and 23 of Photovoltaic Fundamentals.

A grid contact on the n-side allows light to enter the semiconductor, through the spaces between narrow metal “fingers”.

The metal fingers must extend over every part of the cell’s surface in order to collect the photocurrent efficiently.

The front contact can be produced by evaporation of silver or aluminum.

Silicon Photovoltaic CellSilicon Photovoltaic Cell Metal grid pattern on top surface of a photovoltaic cell:

Silicon Photovoltaic CellSilicon Photovoltaic Cell In order to produce the grid pattern, the metal is

evaporated through a “shadow mask”. See page 23 of Photovoltaic Fundamentals.

Silicon Photovoltaic CellSilicon Photovoltaic Cell The shadow mask is in contact with the wafer.

Silicon Wafer

Metal Vapor Metal FilmShadow Mask

Silicon Photovoltaic CellSilicon Photovoltaic Cell The metal contacts are usually annealed in an inert atmosphere

at a temperature of 400 to 500°C. This causes the metal and silicon to interdiffuse, reducing the

contact resistance (the electrical resistance of the interface between metal and semiconductor).

Processes other than evaporation are frequently used to apply metal contacts to solar cells.

The most common process is screen printing, which doesn’t require a vacuum and is far less expensive to implement.

Shadow masks and screen printing cannot produce the small features required for integrated microelectronic circuits.

A patterning process called photolithography is used.

>> More about this later.

Silicon Photovoltaic CellSilicon Photovoltaic Cell An antireflection coating (silicon monoxide = SiO, SiO2, or

Si3N4) is applied by evaporation, chemical vapor deposition, or other techniques to be described .

p-type silicon

n-type silicon

antireflection coating

P-N Junction DiodeP-N Junction Diode The electrical characteristics of a p-n junction diode are

given by a “current-voltage” graph -- a graph of electric current through the diode as a function of applied voltage across the diode. I

Vforward bias

+

reverse bias

+

“reverse

breakdown”

Documents

ISAT 436 Micro-/Nanofabrication and Applications Photovoltaic Cells David J. Lawrence Spring 2004