Evidence of band bending observed by electroabsorption studies in polymer light emitting device with ionomer/Al or LiF/Al cathode

Abstract

We report electroabsorption studies of indium-tin-oxide ͑ITO͒/poly͓2-methoxy-5-͑2Ј-ethyl-hexyloxy͒-1,4-phenylenevinylene͔ insulating layer/Al light-emitting devices. An ionomer and LiF were used as the insulating layer. The internal electric field was nullified at the same bias voltage of 0.6 V for different cathodes of Al, ionomer/Al, and LiF/Al. The bias voltage is close to the work-function difference between ITO and Al. Average electric field in the emitting layer was reduced by inserting the insulating layers. On the contrary, current density was increased significantly. These observations suggest band bending in the devices with ionomer/Al or LiF/Al cathode.

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

“…3 The same beneficial effect of an inserted LiF layer was noted for several other devices, containing organic compounds such as 4-4Ј-bis͑2,2-diphenylvinyl͒-1,1Ј-biphenyl ͑DPVBi͒, 4 poly ͓ 2-methoxy-5-͑ 2Ј-ethylhexyloxy ͒ -1, 4-phenylenevinylene͔ ͑MEH-PPV͒ 5 and molecularly mixed layers of poly ͓ 2-methoxy-5-͑ 3Ј-7Ј-dimethyloctyloxy ͒1, 4-phenylenevinylene͒ ͑MDMO-PPV͒ and 1-͓3-͑methoxycarbonyl͒ propyl͔-1-phenyl͓6,6͔C 61 ͑PCBM͒. 6 Several mechanisms to explain the beneficial effect of a thin layer of LiF on the electron injection at the cathode have been suggested: The formation of a Li ϩ polymer Ϫ or Li ϩ molecule Ϫ charge transfer complex after the reaction of LiF with Al to AlF 3 and liberated Li, 7-11 tunneling injection, 1,5 formation of a dipolar layer, 4,12 and the protection of the organic layer from reaction with aluminum. 4,8,9,13 The mechanism of electron injection enhancement by the formation of radical anions or charge transfer complexes, which could be responsible for the higher device efficiencies, was first suggested by Kido et al 14 They showed that an organic layer at the cathode interface shows electron injection improvement upon the co-evaporation of a low-workfunction metal and the organic layer, which suggests that doping takes place.…”

Migration-assisted energy transfer at conjugated polymer/metal interfaces

“…3 Present polymer LEDs and photovoltaic devices often employ a cathode that consists of a thin LiF layer with aluminum evaporated on top. 26,27 Time-integrated PL quenching in NRS-PPV/LiF/Al heterostructures has been presented in Fig. 1.…”

Interface formation in K doped poly(dialkoxy-p-phenylene vinylene) light-emitting diodes

“…Contributions in this field arose among others from ultraviolet photoelectron spectroscopy ͑UPS͒, which demonstrated the formation of gap states, ͑bi͒polarons, 2-5 the failing of vacuum level alignment, 6,7 and energy level bending. 8 By photoluminescence ͑PL͒ experiments it was also shown that metal deposition resulted in exciton quenching by either directly introducing gap states 9 or opening a nonradiative decay channel through energy transfer to a metal mirror. 10,11 Recently we have shown that the amount of Ca diffusing into the polymer during cathode deposition influences the electroluminescence.…”