Comparative study on degradation and trap density-of-states of p type and n type organic semiconductors

The OTFTs with both p type and n type channel layers were fabricated using the inverted-staggered (top contact) structure by thermal vapour deposition on Si/SiO2 substrate. Pentacene and N,N’-Dioctyl- 3,4,9,10- perylenedicarboximide (PTCDI-C8) were used as channel layer for the fabrications of p type and n type OTFTs respectively. A comparative study on the degradation and density of states (DOS) of p type and n type organic semiconductors have been carried out. In order to compare the stability and degradation of pentacene and PTCDI-C8 OTFTs, the devices were exposed to air for 2 h before performing electrical measurements in air. The DOS measurements revealed that a level with defect density of 1020 cm-3 was formed only in PTCDI C8 layer on exposure to air. The oxygen adsorption into the PTCDI-C8 active layer can be attributed to the formation of this level at 0.15 eV above the LUMO level. The electrical charge transport is strongly affected by the oxygen traps and hence n type organic materials are less stable than p type organic materials.