Electronic transport in liquid and solid sulphur

Abstract

The transport of generated charge carriers in sulphur has been investigated by drift mobility techniques during the melting of single crystals (at similar 114 °c) and also in the liquid prepared from molten granules of ultra-pure (6N) or laboratory reagent sulphur. The discussion of the results leads to the conclusion that in the ultra-pure liquid the transport is predominantly electronic and that both electrons and holes propagate by an intermolecular hopping mechanism. For electrons the transport mechanism is the same as in the solid, but during melting the intermolecular overlap energy drops by about a factor of 5, leading to an electron mobility μe = 10-4 cm2 s-1 v-1 at 120 °c. The hole conduction, which takes place in a small-polaron band in the solid, changes to a hopping transport upon melting. The magnitude and temperature dependence of μh is then very similar to that of μe. These results are discussed in the light of Holstein's small-polaron theory. Experiments on less pure liquid specimens show additional features which appear to arise from the ionization of impurity molecules during the transit of the generated carriers. Above 160 °c a change in the temperature dependence of μe and μh occurs, connected with the break-up and polymerization of the S8 ring molecules. The efficiency of photogeneration in the liquid is lower than in the solid, but its spectral dependence is substantially the same. The steady dark current in the ultra-pure liquid is ohmic, although on application of the field the initial current is injected under space-charge-limited conditions. The different steady dark current behaviour of the impure liquid has been attributed to the formation of heterocharge layers near the electrodes.