Analysis of electron traps formed in organic films with a sputtered cathode

To understand why performance degradation is reduced for sputtered cathodes on organic devices when the electron transport layer (ETL) is doped with Li, we analyze electron-only devices using the thermally stimulated current (TSC) technique and modeling of temperature-dependent current-voltage characteristics with a trapped-charge-limited current (TCLC) model. The combined results suggest that the trap density measured by TSC might also include a portion of the density of the hopping sites in the lowest unoccupied molecular orbital levels, which contributes to charge transport. Compared to undoped devices, doped devices maintain a high density of hopping sites even when the Al is sputtered. We propose that the reduced effect of sputtering on electron injection and transport properties is because radical anions of Alq₃ might still be formed by the strong reducer Li even if the organic material is partially damaged. An additional TSC peak and increased driving voltage for doped tris(8-hydroxyquinoline)aluminum (Alq₃) as an ETL with a sputtered cathode suggests the formation of new traps possibly because of damage even though the transport is better compared to the undoped device. Such traps are not found in doped bathophenanthroline (Bphen) as an ETL, which shows no change in driving voltage.