<< Chapter < Page Chapter >> Page >
Alternation of bond lengths along a conjugated polymer chain (a) results in a material with properties of a large band gap semiconductor (b), where CB is the conductive band gap, and VB is the valence band, and E g is the band gap.
Example of common conjugated polymers.
Polymer Chemical name Structure π-π* energy gap (eV) Emission peak (nm)
PA trans -polyacetylene 1.5 600
PDA polydiacetylene 1.7
PPP poly( para -phenylene) 3.0 (red) 465
PPV Poly( para -phenylenevinylene) 2.5 (green) 565
RO-PPV poly(2,5-dialkoxy-p-phenlyenevinylen) 2.2 (blue) ~580
PT polythiophene 2.0 (red)
P3AT Poly(3-alkythiophene) 2.0 (red) 690
PTV Poly(2,5-thiophenevinylene) 1.8
PPy Polypyrrole 3.1
PAni Polyaniline 3.2

Approaches to improve the efficiency

Efficiency for any LED is defined:

n ext = n esc * n int

where n ext is the external quantum efficiency, n int is the internal efficiency (represents the fraction of injected carrier, usually electron, that is converted to photon), and n esc is the escape efficiency (represent fraction of photons that can reach to the outside).

The most common way to improve the internal efficiency is to balance the number of electrons and holes which arrives at the polymer layer. Originally, there are more holes than electron that arrive of the polymer layer because conjugated polymers have a higher electron affinity, and as a consequence will favor the transport of hole than electron. There are two ways to maintains the balance:

  1. Match the work function of electrode with the electron affinity and ionization potential of the polymer.
  2. Tune the polymer’s electron affinity and ionization potential to match the work function of the electrode.

The escape efficiency is also important because a polymer LED is made up of layers of materials that have different refractive index, and some of the photon generated from the excition may be reflected at the boundary and trapped inside the device.

Improvement in internal quantum efficiency using low working function cathode

Conjugated polymer is electron rich, the mobility for hole is higher than electron, and more holes will arrive in the polymer layer than electrons. One way to increase the population of the electron is to use a lower working function metal as cathode. Braun and Heeger have replaced the aluminum cathode with calcium results in improved internal efficiency by a factor of ten, to 0.1%. This approach is direct and fast but low working function electrode like calcium will be oxidized easily and shorten the devices’ life.

Improvement in internal quantum efficiency using multiple polymer layers

A layer of poly[2,5-di(hexyloxy)cyanoterephthalylidene] (CN-PPV, [link] ) is coated on top of PPV to improve the transport and recombination of electron and holes ( [link] ).

Structure of CN-PPV.
Schematic representation of a CN-PPV and PPV multi-layer LED.

The nitrile group in the CN-PPV has two effect on the polymer.

  1. It increases the electron affinity so electrons can travel more efficient from the aluminum to the polymer layer. And metal of relative high working function like aluminum and gold can be now be used as cathode instead of calcium.
  2. It increases the binding energy of the occupied π and unoccupied π* state but maintain a similar π-π* gap. So when the PPV and CN-PPV is placed together, holes and electron will be confined at the heterojunction.

Questions & Answers

what is mutation
Janga Reply
what is a cell
Sifune Reply
how is urine form
Sifune
what is antagonism?
mahase Reply
classification of plants, gymnosperm features.
Linsy Reply
what is the features of gymnosperm
Linsy
how many types of solid did we have
Samuel Reply
what is an ionic bond
Samuel
What is Atoms
Daprince Reply
what is fallopian tube
Merolyn
what is bladder
Merolyn
what's bulbourethral gland
Eduek Reply
urine is formed in the nephron of the renal medulla in the kidney. It starts from filtration, then selective reabsorption and finally secretion
onuoha Reply
State the evolution relation and relevance between endoplasmic reticulum and cytoskeleton as it relates to cell.
Jeremiah
what is heart
Konadu Reply
how is urine formed in human
Konadu
how is urine formed in human
Rahma
what is the diference between a cavity and a canal
Pelagie Reply
what is the causative agent of malaria
Diamond
malaria is caused by an insect called mosquito.
Naomi
Malaria is cause by female anopheles mosquito
Isaac
Malaria is caused by plasmodium Female anopheles mosquitoe is d carrier
Olalekan
a canal is more needed in a root but a cavity is a bad effect
Commander
what are pathogens
Don Reply
In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s.[1][2
Zainab
A virus
Commander
Definition of respiration
Muhsin Reply
respiration is the process in which we breath in oxygen and breath out carbon dioxide
Achor
how are lungs work
Commander
where does digestion begins
Achiri Reply
in the mouth
EZEKIEL
what are the functions of follicle stimulating harmones?
Rashima Reply
stimulates the follicle to release the mature ovum into the oviduct
Davonte
what are the functions of Endocrine and pituitary gland
Chinaza
endocrine secrete hormone and regulate body process
Achor
while pituitary gland is an example of endocrine system and it's found in the Brain
Achor
what's biology?
Egbodo Reply
Biology is the study of living organisms, divided into many specialized field that cover their morphology, physiology,anatomy, behaviour,origin and distribution.
Lisah
biology is the study of life.
Alfreda
Biology is the study of how living organisms live and survive in a specific environment
Sifune
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Chemistry of electronic materials. OpenStax CNX. Aug 09, 2011 Download for free at http://cnx.org/content/col10719/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chemistry of electronic materials' conversation and receive update notifications?

Ask