Charge trapping

Migrating charges can be temporarily or permanently immobilized in trap 

These polymers are mainly known for their conductivity, particularly when doped with suitable electron donors or acceptors. Photoconductivity is marginal, and therefore these polymers are of limited interest for the study of this phenomena. Undoped and doped polyacetylene has been studied in some detail [53-57]. Undoped ds-poly-acetylene is photoconductive in UV light [54] doping with AsFs shifts the photoresponse to the visible range, but the polymer also becomes highly dark-conductive. Under certain conditions, the ratio of photo to dark 

Crystals of poly[2,4-hexadiyne-l,6-diol bis(p-toluenesulphonate)] [59, 60] are highly photoconductive, with high carrier mobilities, near 20m V s even at low electric fields. This corresponds to drift velocities of about 2.2 x 10 m s i.e. close to the velocity of sound in the material. It is interesting to note that the spectral dependence of photoconductivity (the so called action spectrum) in this and other polyacetylenes does not follow the absorption spectrum, but is shifted towards shorter wavelengths. Charge-carrier generation follows the Onsager one-dimensional model, at least for electric fields up to 10 V m [60, 61]. 

A number of photoconductive polyarylene vinylenes of the general structure 

Photoconductive poly-p-phenylene sulphide [65] with hole mobility near 10 m s is included in this category of polymers, even though 

---

In order to study the chaiged photoexcitalions in conjugated materials in detail their contribution to chaige transport can be measured. One possible experiment is to measure thermally stimulated currents (TSC). Next, we will compare the results of the TSC-expcrimenls, which are sensitive to mobile thermally released charges trapped after photoexcilation, to the temperature dependence of the PIA signal (see Fig. 9-17) which is also due to charged states as discussed previously. 

Figure 2. Cyclic voltammograms of a poly(2,2 -bithiophene)-coated electrode in acetonitrile containing 0.1 M Bu4NC 04.34 (Reprinted from G. Zotti, C. Schiavon, and S. Zecchin, Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena, Synth. Met. 72 (3) 275-281, 1995, with kind permission from Elsevier Sciences S.A.)...

Nanocrystals are receiving significant attention for nano-electronics application for the development of future nonvolatile, high density and low power memory devices [1-3]. In nanocrystal complementary metal oxide semiconductor (CMOS) memories, an isolated semiconductor island of nanometer size is coupled to the channel of a MOS field effect transistor (MOSFET) so that the charge trapped in the island modulates the threshold voltage of the transistor (Fig. 1). 

It can be seen from Figure 3.10a and d that the emission spectra of the neat BCzVB and DPVBi doped with BCzVB are essentially the same and can be attributed to an energy transfer process. The emission spectrum of CBP doped with BCzVB is quite different it comes from emission contributed from both CBP and BCzVB molecules owing to both charge trapping as well as a partial energy transfer process. 

R.J. Holmes, B.W. D Andrade, S.R. Forrest, X. Ren, J. Li, and M.E. Thompson, Efficient, deep-blue organic electrophosphorescence by guest charge trapping, Appl. Phys. Lett., 83 3818-3820... 

To increase the ionization probability, a homogeneous weak magnetic field is used to keep the electrons on a spiral path. At the end of the ionization chamber, electrons are collected in a positively charged trap, where the electron current is measured and kept constant by the emission regulator circuitry. 

Hallam T, Lee M, Zhao N, Nandhakumar I, Kemerink M, Heeney M, McCulloch I, Sirringhaus H (2009) Local charge trapping in conjugated polymers resolved by scanning Kelvin probe microscopy. Phys Rev Lett 103 256803... 

---