Mobile ionic impurities in organic semiconductors

We study the stability in time of the current–voltage characteristics of organic thin-film devices on glass substrates. We find for poly(3-hexylthiophene) and for pentacene that the resistance of the devices gradually changes under the application of an electrical bias depending on the sodium content of the glass substrates used in the experiment. For devices on a very common type of glass (with a ${\mathrm{Na}}_2\mathrm{O}$ content of about 6%) and on sodalime glass $\text{(14\%\hspace{0.05em}}{\mathrm{Na}}_2\mathrm{O})$ substrates, the prolonged application of a voltage bias results in a substantial decrease (up to two orders of magnitude) of the bulk and contact resistances, whereas for sodium-free glass substrates the gradual changes in current–voltage characteristics are much smaller. A systematic study of the electrical behavior complemented by chemical analysis shows that the instabilities observed are due to ${\mathrm{Na}}^{+}$ ions diffusing from the substrate into the organic film, and moving inside the organic material as a result of the applied electric field. Our results show in detail how ion motion in organic materials results in substantial hysteresis and device instabilities.