On the Feasibility of Observing Photoemission from Metal Contacts into Anthracene

Abstract

Two processes can mask, partly or complctely, photoemission from metal contacts into anthracene: carrier photoexcitation out of surface traps which are occupied under thermal quilibrium. 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 t o 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 t o yield curves obtained by several workers and attributed t o photoemission. As to the second process, the arguments presented suggest that with contacts of alkali and alkalinecarth mct& (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 t o the necessity of carefully checking the contribution of surface generation and/or contact injecuon before any photoemission studies are undertaken.