Solar Photovoltaic Physics

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Solar Photovoltaic Physics

• Basic Physics and
• Materials Science of Solar Cells

Original Presentation by J. M. Pearce, 2006
Email profpearce_at_gmail.com
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What are Photovoltaics?

• Photovoltaic (PV) systems convert light energy
  directly into electricity.
• Commonly known as solar cells.
• The simplest systems power the small calculators
  we use every day. More complicated systems will
  provide a large portion of the electricity in the
  near future.
• PV represents one of the most promising means of
  maintaining our energy intensive standard of
  living while not contributing to global warming
  and pollution.

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A Brief History Photovoltaic Technology

• 1839 Photovoltaic effect discovered by
  Becquerel.
• 1870s Hertz developed solid selenium PV (2).
• 1905 Photoelectric effect explained by A.
  Einstein.
• 1930s Light meters for photography commonly
  employed cells of copper oxide or selenium.
• 1954 Bell Laboratories developed the first
  crystalline silicon cell (4).
• 1958 PV cells on the space satellite U.S.
  Vanguard (better than expected).

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Things Start To Get Interesting...

• mid 1970s World energy crisis millions spent
  in research and development of cheaper more
  efficient solar cells.
• 1976 First amorphous silicon cell developed by
  Wronski and Carlson.
• 1980s - Steady progress towards higher
  efficiency and many new types introduced
• 1990s - Large scale production of solar cells
  more than 10 efficient with the following
  materials
• Ga-As and other III-Vs
• CuInSe2 and CdTe
• TiO2 Dye-sensitized
• Crystalline, Polycrystalline, and Amorphous
  Silicon
• Today prices continue to drop and new 3rd
  generation solar cells are researched.

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Types of Solar Photovoltaic Materials
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Photovoltaic Materials
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Electronic Structure of Semiconductors

• Silicon
• Group 4 elemental semiconductor
• Silicon crystal forms the diamond lattice
• Resulting in the use of four valence electrons of
  each silicon atom.

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Crystalline Silicon
Amorphous Silicon
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Solar PV MaterialsCrystalline Polycrystalline
Silicon

• Advantages
• High Efficiency (14-22)
• Established technology
• (The leader)
• Stable
• Disadvantages
• Expensive production
• Low absorption coefficient
• Large amount of highly
• purified feedstock

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Amorphous Silicon

• Advantages
• High absorption (dont need a lot of material)
• Established technology
• Ease of integration into buildings
• Excellent ecological balance sheet
• Cheaper than the glass, metal, or plastic you
  deposit it on
• Disadvantages
• Only moderate stabilized efficiency 7-10
• Instability- It degrades when light hits it
• Now degraded steady state

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How do they work?

• The physics view

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Band Theory

• There are 3 types of materials in Band Theory,
  which are differentiated by their electronic
  structure
• insulators,
• conductors, and
• semiconductors.

Ef
Eg
Ef
Ef
Metal Insulator Semiconductor
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Energy Bands in a Semiconductor

• Conduction Band Ec empty
• Valence Band Ev full of electrons

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3 Types of Semiconductors

• Intrinsic
• n-type
• p-type
• Types 2 and 3 are semiconductors that conduct
  electricity - How?
• by alloying semiconductor with an impurity, also
  known as doping
• carriers placed in conduction band or carriers
  removed from valence band.

Note Color Protocol
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Type 1 Intrinsic

• Pure semiconductor (intrinsic) contains the
  right number of electrons to fill valence band,
  therefore, conduction band is empty.
• Because electrons in full valence cannot move,
  the pure semiconductor acts like an insulator.

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Type 2 n-Type

• n-type current is carried by negatively charged
  electrons - How?
• group 5 impurity atoms added to silicon melt from
  which is crystal is grown
• 4/5 of outer electrons used to fill valence band
• 1/5 left is then put into conduction band. These
  impurity atoms are called donors.

Within conduction band the electrons are moving,
therefore, crystal becomes a conductor
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Type 3 p-Type

• p-Type current carried by missing electron holes
  which act as positively charged particles. How?
• group 3 added to silicon melt
• need 4 out of 5 outer electrons but doping
  creates lack of electrons in valence band.
• missing electrons, a.k.a holes, are used to carry
  current.

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What Carries the Current?

• Prevailing charges are called the majority
  carriers
• prevailing charge carrier in n-type electrons
• prevailing charge carrier in p-type holes

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Creating a Junction

• There are four main types of semiconductor
  junctions
• p-n
• p-i-n
• Schottcky barrier
• Heterojunction
• Each has a built in potential

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p-n and p-i-n Junctions
Vbi
Vbi
Ef
Ef
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Schottky Barriers and Heterojunctions
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Semiconductor Junctions

• All the junctions contain strong electric field
• How does the electric field occur?
• When two semiconductors come into contact,
  electrons near interface from n-type, transfer
  over to p-type, leaving a positively charged area
• Holes from p-type by interface transfer over to
  n-type leaving a negatively charged area.
• Because electrons and holes are swapped, a middle
  potential barrier with no mobile charges, is
  formed.
• This potential barrier created does not let any
  more electrons or holes flow through.
• Electric field pulls electrons and holes in
  opposite directions.

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Barrier Changes

• Equilibrium means there is no net current
• Reduced barrier height is called forward bias
  (positive voltage applied to p-side)
• Result- increases current through diode
• Increased barrier height is called reverse bias.
• Result- decreases current to a very small
  amount..

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Electric Currents in p-n Junction Under External
Bias
Diode I-V Characteristics
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Current in a Solar Cell

• Output current I Il-Io exp(qV/kT)-1
• Illight generated current
• q electric charge
• V voltage
• k Boltzmans constant 1.3807 10-23 J/K
• When in open circuit (I0) all light generated
  current passes through diode
• When in short circuit (V0) all current passes
  through external load
• 2 Important points
• 1) During open circuit the voltage of open
  circuit,
• Voc (kT/q) ln( Il/Io 1)
• 2) No power is generated under short and open
  circuit - but Pmax VmImFFVocIsc

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I-V Curve for Solar Cells

• Fourth quadrant (i.e., power quadrant) of
  the illuminated I-V characteristic defining fill
  factor (FF) and identifying Jsc and Voc

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Light Absorption by a Semiconductor

• Photovoltaic energy relies on light.
• Light ? stream of photons ? carries energy
• Example On a clear day 4.4x1017 photons hit 1 m2
  of Earths surface every second.
• Eph(?)hc/? hf
• h planks constant 6.625 x 10-34 J-s
• ? wavelength
• c speed of light 3 x 108 m/s
• f frequency
• However, only photons with energy in excess of
  bandgap can be converted into electricity by
  solar cells.

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The Solar Spectrum
The entire spectrum is not available to single
junction solar cell
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Generation of Electron Hole Pairs with Light

• Photon enters, is absorbed, and lets electron
  from VB get sent up to CB
• Therefore a hole is left behind in VB, creating
  absorption process electron-hole pairs.
• Because of this, only part of solar spectrum can
  be converted.
• The photon flux converted by a solar cell is
  about 2/3 of total flux.

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Generation Current

• Generation Current light induced electrons
  across bandgap as electron current
• Electron current IpqNA
• N of photons in highlighted area of spectrum
• A surface area of semiconductor thats exposed
  to light
• Because there is current from light, voltage can
  also occur.
• Electric power can occur by separating the
  electrons and holes to the terminals of device.
• Electrostatic energy of charges occurs after
  separation only if its energy is less than the
  energy of the electron-hole pair in semiconductor
• Therefore VmaxEg/q
• Vmax bandgap of semiconductor is in EVs,
  therefore this equation shows that wide bandgap
  semiconductors produce higher voltage.

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Direct vs Indirect Bandgap

• Everything just talked about, where all energy in
  excess of bandgap of photons are absorbed, are
  called direct-bandgap semiconductors.
• More complicated absorption process is the
  indirect-gap series
• quantum of lattice vibrations, of crystalline
  silicon, are used in the conversion of a photon
  into electron-hole pair to conserve momentum
  there hindering the process and decreasing the
  absorption of light by semiconductor.

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The Solar Cell

• Electric current generated in semiconductor is
  extracted by contacts to the front and rear of
  cell.
• Widely spaced thin strips (fingers) are created
  so that light is allowed through.
• these fingers supply current to the larger bus
  bar.

• Antireflection coating (ARC) is used to cover
  the cell to minimize light reflection from top
  surface.
• ARC is made with thin layer of dielectric
  material.

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Different Types of Photovoltaic Solar Cells

• Diffusion
• Drift
• Excitonic

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Diffusion

• n-type and p-type are aligned by the Fermi-level
• When a photon comes in n-type, it takes the place
  of a hole, the hole acts like an air bubble and
  floats up to the p-type
• When the photon comes to the p-type, it takes
  place of an electron, the electron acts like a
  steel ball and rolls down to the n-type

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Diagram of p-n Junction and Resultant Band
Structure
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Drift

• There is an intrinsic gap where the photon is
  absorbed in and causes the electron hole pair to
  form.
• The electron rises up to the top and drifts
  downwards (to n-type)
• The hole drifts upwards (to p-type)

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Excitonic Solar Cell

• Dye molecule
• electron hole pair splits because it hits the dye
• the electron shifts over to the electric
  conductor and the hole shifts to the hole
  conductor

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Power Losses in Solar Cells
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Recombination

• Opposite of carrier generation, where
  electron-hole pair is annihilated
• Most common at
• impurities
• defects of crystal structure
• surface of semiconductor
• Reducing both voltage and current

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Series Resistance

• Losses of resistance caused by transmission of
  electric current produced by the solar cell.
• I-V characteristic of device
• I Il-I0 exp(qVIRs / mkT) 1
• m nonideality factor

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Other Losses

• Current losses- called collection efficiency,
  ratio b/w number of carriers generated by light
  by number that reaches the junction.
• Temperature dependence of voltage
• V decreases as T increases
• Other losses
• light reflection from top surface
• shading of cell by top contacts
• incomplete absorption of light

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Minimize Recombination Lossesby Adapting the
Device
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Tandem Cells

Silver Grid

• Tandem cell- several cells,
• Top cell has large bandgap
• Middle cell mid eV bandgap
• Bottom cell small bandgap.

Indium Tin Oxide p-a-SiH Blue Cell
i-a-SiH n-a-SiH p Green Cell i-a-SiGeH
(15) n p Red Cell i-a-SiGeH
(50) n Textured Zinc Oxide Silver Stainless
Steel Substrate
Schematic diagram of state-of-the-art a-SiH
based substrate n-i-p triple junction cell
structure.
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Solar Photovoltaics is the Future
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Acknowledgements

• This is the first in a series of presentations
  created for the solar energy community to assist
  in the dissemination of information about solar
  photovoltaics.
• This work was supported from a grant from the
  Pennsylvania State System of Higher Education.
• The author would like to acknowledge assistance
  in creation of this presentation from Heather
  Zielonka, Scott Horengic and Jennifer Rockage.