Growth and physical properties of crystalline rubrene - BOA Bicocca ...

2.7 Growth of rubrene thin films 23
Figure 2.11: (a) Dark I-V curves measured from oxidized and non-oxidized rubrene
single crystals (left) and photocurrent spectral responses of the same samples
(right). From [69]. (b) Charge carrier mobility of a rubrene single crystal as a
function of exposure time of its surface to hydrogen and oxygen. From [31].
surface to a pure oxygen atmosphere and to a pure hydrogen atmosphere,
respectively[31]. They first exposed the surface of rubrene single crystals
to pure oxygen, noticing a clear increase in mobility, and then introduced
pure hydrogen in the experimental chamber, measuring a steep decrease in
the mobility. By exposing the crystals once again to oxygen the starting
mobility value could then be recovered, demonstrating both the relevant
role of oxidation and the reversibility of the reaction. A plot exemplifying
this behavior is shown in figure 2.11b.
On the other hand Najafov et al. have shown in [32] that in organic
single crystals exposed to light and oxygen at the same time there is a clear
decrease in photoconductivity with increasing oxygen exposure time. The
authors attribute such an effect to an increase of the trap density in the
crystal due to oxygen diffusion.
Finally, a prominent role of oxygen related defects in the enhanced free
charge carriers production from long lived excitons in the surface region
of rubrene crystals, accounting for their large photoconductivity, has been
suggested several times[62].
2.7 Growth of rubrene thin films
Single crystals are often the best choice for the study of the intrinsic prop-
erties of a material, since they represent ideal solid samples and can be easily