ISAT 436 Micro-/Nanofabrication and Applications

1
ISAT 436Micro-/Nanofabrication and Applications

• Photovoltaic Cells
• David J. Lawrence
• Spring 2004

2
Properties of Light (1)

• f frequency (Hz)
• lo wavelength in vacuum or air usually
• measured in mm, nm, or Angstroms (Å)
• c speed of light in vacuum 3 108 m/s
• c f lo
• n refractive index of a material (medium)
• v c / n speed of light in material
• l lo / n wavelength in material
• v f l

3
Properties of Light (2)

• E h f energy of a photon
• h Plancks constant 6.626 10-34 J-s
• 4.136 10-15 eV-s
• E (h c) / lo
• h c 1240 eV-nm 1.24 eV-mm
• 1 eV 1.602 10-19 J
• h h / 2p 1.055 10-34 J-s

4
Properties of Light (3)

• A useful equation for the energy of a photon
• Rearranged, this gives

5
Properties of Light (4)
6
Properties of Light (5)

• Light with wavelength lo lt 400 nm is called
  ultraviolet (UV).
• Light with wavelength lo gt 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).

7
Optical Generation of Free Electrons 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 gt Eg , a photon can be absorbed, creating a
  free electron and a free hole.

8
Optical Generation of Free Electrons and Holes -
- Bond Model

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

9
Optical Generation of Free Electrons 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.

10
Photoconductive 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, r, of the semiconductor decreases.

11
Photoconductive Light Detectors

• Recall that

• Since Rsemiconductor rl/A , the resistance
  of the semiconductor sample also decreases.

12
Photovoltaic 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.

junction
13
Photovoltaic 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.

depletion region
neutral here
neutral here
14
Photovoltaic 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?

15
Photovoltaic Cells -- Band Model

• Recall that a p-n junction can also be described
  by an energy band diagram.

16
P-N Junction Diode

• Electrons behave like marbles Þ they tend to go
  downhill.
• Holes behave like helium-filled balloons Þ they
  tend to float uphill.

17
P-N Junction Diode

• The bent energy bands are a barrier to electron
  motion.
• The bent energy bands are a barrier to hole
  motion.

18
Photovoltaic Cells -- Band Model

• Photons with energy hf gt 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.

19
Photovoltaic 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?

20
Photovoltaic 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.

21
Photovoltaic Cells -- Band Model

• Light energy is converted to electrical energy.

22
Photovoltaic Cells

• Notice that the photocurrent flows opposite the
  diode symbol arrow.

23
Photovoltaic 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.

24
Photovoltaic Cells

• Photovoltaic (solar) cells are designed for
  energy conversion, so they usually have a large
  (gt 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.

25
Photovoltaic Cells

• Next, lets 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.

26
Silicon 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
• n (Greek nu)
• f frequency
• hn hf photon energy

Larger diagram on next slide!
27
Silicon Photovoltaic Cell
28
Silicon Photovoltaic Cell

• Starting material
• Single crystal silicon wafer (2 to 6
  diameter)
• p-type Û boron-doped
• r _at_ 1 W-cm
• p _at_ ?
• 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.

29
Silicon Photovoltaic Cell

• The top 0.3 mm 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 mm.
• 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.

30
Silicon Photovoltaic Cell

• Essentials 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).

31
Silicon 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 mm.
• 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.

32
Silicon Photovoltaic Cell

• We now have the required p-n junction.
• We need a metal electrical contact to the p-side
• gtgt the back contact.
• We need a metal electrical contact to the n-side.
• gtgt the front contact.

33
Silicon Photovoltaic Cell

• PV cell is a large area p-n junction.
• r of most semiconductors (e.g., silicon) is
  substantially greater than for a metal (rmetal
  10-6 to 10-5 W-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.

34
Silicon 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).

35
Silicon 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.)

36
Silicon Photovoltaic Cell

• Evaporation

37
Silicon Photovoltaic Cell

• The 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 cells surface in order to collect the
  photocurrent efficiently.
• The front contact can be produced by evaporation
  of silver or aluminum.

38
Silicon Photovoltaic Cell

• Metal grid pattern on top surface of a
  photovoltaic cell

39
Silicon Photovoltaic Cell

• In order to produce the grid pattern, the metal
  is evaporated through a shadow mask.
• See page 23 of Photovoltaic Fundamentals.

40
Silicon Photovoltaic Cell

• The shadow mask is in contact with the wafer.

41
Silicon Photovoltaic Cell

• The metal contacts are usually annealed in an
  inert atmosphere at a temperature of 400 to
  500C.
• 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
  doesnt 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.
• gtgt More about this later.

42
Silicon Photovoltaic Cell

• An antireflection coating (silicon monoxide
  SiO, SiO2, or Si3N4) is applied by evaporation,
  chemical vapor deposition, or other techniques to
  be described .

43
P-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.