FEASIBILITY OF OBSERVING PHOTOEMISSION FROM METAL CONTACTS INTO ANTHRACENE

Two processes can mask, partly or completely, photoemission from metal contacts into anthracene: carrier photoexcitation out of surface traps which are occupied under thermal equilibrium, and photoexcitation out of bulk traps which are continuously being filled by carrier injection from the contact under study. The first process is studied in this paper by photoconductivity measurements in the absence of contact effects. The photocurrents are shown to originate from hole photoexcitation out of surface traps. They are greatly influenced by surface treatments but even under the most optimal treatment the spectral yield is similar both in magnitude and threshold to yield curves obtained by several workers and attributed to photoemission. As to the second process, the arguments presented suggest that with contacts of alkali and alkaline-earth metals (electron injectors) and gold (hole injector) the reported yield curves are probably associated with photo-enhanced space-charge-limited currents. A critical examination of the available data indicates that photoexcitation out of surface or bulk traps in anthracene, rather than contact photoemission, plays the dominant role in some cases while in others this possibility cannot be ruled out. The data and arguments presented here point to the necessity of carefully checking the contribution of surface generation and/or contact injection before any photoemission studies are undertaken.