Built-in field

However, the effective internal electric field in the device deviates from the field calculated using Eq. (9.10) due to the internally built-in field (see above) because of defect charging in the active layer [55] and interface effects [57],... 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

Esclc = external field (V/d) + internal built in field (Eint) Ohm current... 

An explanation for this unexpected behavior can be found by analyzing the distribution of the ions within the active layer of the LEC by secondary ion mass spectroscopy (SIMS) (a detailed analysis of the SIMS results is published elsewhere).154 For an active-layer spin-cast onto ITO, the overall concentration of the Liions and triflateions ions is significantly higher near the ITO electrode than near the surface of the active layer (by a factor 15 and 2.5, respectively). After evaporating A1 as the top electrode on the active layer, the negative triflate ions drift toward the top electrode (which acts as the anode) due to the internal electric built-in field. A detailed theoretical description of this scenario can be found in Ref. 155. 

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... 

The two metals then have different work functions. This leads to a contact potential and produces in the semiconductor an internal, built-in field Fb , when the external voltage V is switched off (V=0). For V 0, the internal field is then a superposition of the built-in field and the applied field (see Sect. 8.4.3.2). 

Fig. 8.21). To make charge transport possible, a counter-voltage Vext must therefore be applied to contacts with different work functions, which is at least sufficient to compensate the built-in field, Vext > - bi- The (internal) field strength in this greatly idealised case would then be... 

Brown, T. M. Kim, J. S. Friend, R. H. Cacialli, R Daik, R. and Feast, W. J. 1999. Built-in field electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene dioxythiophene) hole injection layer. Appl. Phys. Lett. 75 1679-1681. 

In what follows, we will now further review how EA spectroscopy has been employed, in some of the available literature, to gauge the built-in fields, the energy level lineup at organic semiconductor/electrode interfaces, and the physics of device operation [4,5,40,42,43,46,69,70,77,109,112,114,116,117,119-143]. 

One can see from the first term in the upper Eq. (3.15), that Pw z) = P v Considering the contributions of the second and third terms in the above Eq. (3.15) to be small, we have P v Pw(z) in general case. We emphasize that surface polarization Pm leads to appearance of built-in field Em dependent on film thickness. Besides that we neglect the terms with P y (with the coefficient proportional to Pm) in Gv due to their smallness. The built-in field Em and thickness induced phase transition temperature 7/(/) can be written in the form [11] for the case Asi = Xs2 = h, / / /g 1, As /n/g 1 ... 

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