Poly high carrier concentration

The operating mechanism of PLEDs is quite different from conventional p-n junction LEDs. In a PLED, a pure undoped film of luminescent semiconducting polymer is sandwiched between a high work function metal anode and a low work function metal cathode. The charge carrier concentration in such pure semiconducting films is sufficiently low (>=1014-1015 cm-3) that any residual carriers introduced by impurities etc. are swept out by the built-in field that arises from the difference in work functions of the two electrodes. The depletion depth of pristine poly(phenylene vinylene) (PPV) is approximately 250 qm, which is much larger than the thickness of the polymer layer in an LED (typically < 100 nm). Consequently, the electronic structure of the LED can be approximated by the rigid band... 

To get a moderately high density of carriers per unit volume is relatively easy. For example in poly acetylene, a doping level of 10% per carbon can be achieved, corresponding to a carrier concentration of about 4x10 cm-. This is a typical number for conducting polymers such as polyacetylene, poly(paraphenylenevinylene), poly(paraphenylene), etc. The addition of side chains, as in the poly (3-alky Ithiophenes), reduces the maximum carrier density somewhat, simply because of the smaller fractional volume occupied by the conjugated backbones. For the emeraldine salt of polyaniline, there is one carrier per (B-NH-B-NH)+ repeat unit again n SxlO l. 

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