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Results of iodine doping of an Al/MEH-PPV/ITO-based LED.
Un-doped Doped
Turn on voltage (V) 10 foreword 5, reversed 12
External efficiency (%) 4 x 10 -4 8 x 10 -3

Polymer leds on a silicon substrate: an application advantage over inorganic leds

In the initial research polymer LEDs were in direct competition with the inorganic LEDs and tried to achieve the existing LED standard. This is a difficult task as polymer LEDs have a lower long term stability. However, there are some applications in which polymer LEDs have a clear advantage over their more traditional inorganic analogs. One of these is to incorporate LEDs with the silicon integrated circuits for inter-chip communication.

It is difficult to build inorganic LEDs on a silicon substrate, because of the thermal stress developing between the inorganic LED (usually a III-V based device) and the silicon interface. But polymer LEDs offer a solution, since polymers can be easily spin-coated on the silicon. The operating voltage of polymer LED is less than 4 V, and the turn on voltage can be as low as 2 V. Together with a switching time of less than 50 ns, make polymer LED a perfect candidate.

Reliability and degradation of polymer leds

In terms of the efficiency, color selection, and driving voltage, polymer LED have attained adequate level for commercialization. However, the device lifetime is still far from satisfactory. Research into understanding the reliability and degredation mechanisms of polymer LEDs has generally been divided into two area:

  1. Photo-degradation of polymer.
  2. Interface degradation.

Polymer photo degradation

Photoluminescece (PL) studies of the photo-oxidation of PPV have been undertaken, since it is believes that EL is closely related with PL.

It was found that there is a rapid decay in emission when PPV is exposed to oxygen. Using time resolved FTIR spectroscopy an increase in the carbonyl signal and a decrease in C=C signal with time ( [link] ). It was suggested that the carbonyl group has a strong electron affinity level to charge transfer between molecules segment in the polymer, thereby dissociating the excition and quenching the PL.

FTIR as a function of photo-oxidation of PPV. Adapted from M. Yan, L. J. Rothberg, F. Papadimitrakopoulos, M. E. Galvin and T. M. Miller, Phys. Rev. Lett. , 1994, 73 , 744.

Similar research was performed by Cumpston and Jensen using BCHA-PPV and P3OT ( [link] ) and exposing them to dry air in UV irradiation. In BCHA-PPV, there is an increase in carbonyl signal with time, while the P3OT remain intact. A mechanism proposed for the degradation of BCHA-PPV involves the transfer of energy from the excited triplet state of the PPV to oxygen to from singlet oxygen which attack the vinyl double bond in the PPV backbone. And P3OT dose not has vinyl bond so it can resist the oxidation .

Structure of (a) BCHA-PPV and (b) P3OT.

The research described above was all performed on polymer thin films deposited on an inert surface. The presence of cathode and anode may also affect the oxidation mechanism. Scott et al. have taken IR spectra from a MEH-PPV LED in the absence of oxygen. They obtained similar result as in Yan et al., however, a decrease in ITO’s oxygen signal was noticed suggesting that the ITO anode acts like a oxygen reservoir and supplies the oxygen for the degradation process.

Polymer led interface degradation

There are few interface degredation studies in polymer LEDs. One of them by Scott et al. took SEM image of the cathode from a failed polymer LED. The polymer LED used ITO as the anode, MEH-PPV as the polymer layer, and an aluminum calcium alloy as cathode. SEM images showed “craters” formed in the cathode. The craters are formed when the cathode metal is melted and pull away from the polymer layer. It was suggested that a high current density will generate heat and result in local hot spot. The temperature in the hot spot is high enough to melt the cathode. And when it melt, it will pull away from the polymer. This process will decrease the effective cathode area, and reduce the luminescence gradually.


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Questions & Answers

how do you translate this in Algebraic Expressions
linda Reply
why surface tension is zero at critical temperature
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
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Damian Reply
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abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
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I'm interested in nanotube
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Ramkumar Reply
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preparation of nanomaterial
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Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Chemistry of electronic materials. OpenStax CNX. Aug 09, 2011 Download for free at http://cnx.org/content/col10719/1.9
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