Influence of a partially oxidized calcium cathode on the performance of polymeric light emitting diodes

We investigated the influence of the presence of oxygen during the depositi on of the calcium cathode on the structure and on the performance of polyme ric light emitting diodes (pLEDs). The oxygen background pressure during de position of the calcium cathode of polymeric LEDs was varied. Subsequently, the oxygen depth distribution was measured and correlated with the perform ance of the pLEDs. The devices have been fabricated in a recently built ult raclean setup. The polymer layers of the pLEDs have been spincoated in a dr y nitrogen atmosphere and transported directly into an ultrahigh vacuum cha mber where the metal electrodes have been deposited by evaporation. We used indium-tin-oxide as anode, OC1C10 PPV as electroluminescent polymer, calci um as cathode, and aluminum as protecting layer. We achieved reproducibilit y of about 15% in current and brightness for devices fabricated in an oxyge n atmosphere of < 10(-9) mbar. For further investigations the calcium depos ition was carried out in an oxygen atmosphere from 10(-8) to 10(-5) mbar. W e determined the amount of oxygen in the different layers of the current-vo ltage-light characterized pLEDs with elastic recoil detection analysis and correlated it with the characteristics of the devices. The external efficie ncy of the pLEDs decreases continuously with increasing oxygen pressure, th e current shows a pronounced minimum. The brightness mostly decreases with increasing oxygen with an indication of a slight minimum. PLEDs with comple tely oxidized calcium are not operational. The first contact of the pLEDs w ith the dry glove box environment leads to an immediate reduction of curren t and brightness which is caused by the cooling of the devices by several d egrees. Determining reproducible characteristics of pLEDs in the vacuum req uires the measurement of their temperature. (C) 2001 American Institute of Physics.