Temperature dependence of cesium carbonate-doped electron transporting layers on organic light-emitting diodes

Abstract

The temperature dependence and electronic transport properties of 1, 3, 5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl) phenyl (TPBI) and 8-hydroxyquinoline aluminum (Alq) electron transporting layers (ETL) have been investigated as a function of cesium carbonate (Cs2CO3) doping for organic light emitting devices. The current-voltage and light emission characteristics were measured as a function of the Cs2CO3 doped ETL thickness at both room temperature and cryogenic (10–300K). The current density (J) for the Alq:Cs2CO3 ETL device increased for an ETL thickness between 100 and 300 Å, with no further increase in the ETL beyond 300Å, indicating an electron injection limited contact. Conversely, the J for the TPBI:Cs2CO3 ETL device did not saturate for increasing ETL thicknesses confirming the TPBI:Cs2CO3 devices have a near-ohmic cathode contact. The correlation of current density–voltage (J–V) and luminance-voltage (L–V) for both Alq:Cs2CO3 and TPBI:Cs2CO3 devices were studied over temperatures from 10 to 300K. Both increased with increasing temperature; however, Cs2CO3-doped TPBI devices were more effective than Cs2CO3-doped Alq devices. The observed differences between Alq and TPBI may be attributed to the exposed nitrogen electron pair in the electronic structure.