AbstractCharacterisation of thin films of conjugated oligomers deposited by thermal evaporation and pulsed laser ablation has been carried out through measurements of the optical absorption, dark conductivity and photoconductivity of samples with coplanar and sandwich geometries. The dark conductivity was measured as a function of field and temperature. The photoconductivity was measured with steady state optical excitation, and transient excitation as a function of temperature, field, excitation intensity and excitation energy. It was found that in most experiments all oligomers yielded similar results, suggesting that the specific chemical structure of the oligomers is relatively unimportant in determining the electrical properties of the thin films.
Experimental results have been interpreted using computer simulation techniques and current transport models for organic semiconductors. It is found that the multiple trapping and disorder limited hopping models are not applicable to these materials; and a trap-controlled hopping mechanism is suggested as appropriate.
An apparent density of states for the oligomers has been obtained using the transient photocurrent (TPC) decays and a multiple trapping analysis. Qualitatively, the TPC decays are of a form that suggest a flat density of states at high energies with an exponential at deeper energies. The DOS was also determined quantitatively using a Fourier transform (FT) technique. This resulted in a density of states similar to that predicted qualitatively. Transport in such a density of states is suggested to occur by carriers directly “hopping down” in the flat region of the DOS, followed by trap-controlled hopping in the exponential region.
The validity of the Fourier transform technique in systems not exhibiting multiple trapping is discussed and evaluated using computer simulation. It is concluded that the FT technique must be valid for trap-controlled hopping. Thus the form of the obtained DOS for the oligomers is correct, although the magnitude and energy scale can not be considered to be accurate, in either the “hopping down” regime or the trap-controlled hopping regime. An attempt to escape frequency of approximately 108 s-1 is predicted by fitting DOS plots at different temperatures.
Measurements of the effect of oxygen absorption and the slow relaxation of the steady state photocurrent have also been performed, as these effects are found to be characteristic of the oligomers. Oxygen absorption appears to lead to carrier generation and was found to be more important in sandwich samples. This is attributed to oxygen absorption at the electrode- oligomer interface leading to carrier injection. The slow relaxation of the steady state photocurrent could not be explained using previously developed models. It is tentatively suggested that the effect may be due to very deep trapping of holes.
|Date of Award||Feb 1998|