Organic light-emitting electrochemical cells based on polyfluorene - Investigation of the failure modes

The discovery of organic light-emitting electrochemical cells (OLECs) has o pened new directions for organic light-emitting devices based on luminescen t conjugated polymers. However, such systems face two major problems: slow turn-on speed and short lifetime. If the p-n time formation can be efficien tly reduced by increasing the ionic conductivity of the active layer by sim ply adding salt-laden poly(ethylene oxide), especially when the salt is ful ly dissociated, short lifetimes remains a key issue. A study of the decay o f the electroluminescence intensity vs. current ratio as a function of time is representative of the quantum efficiency and can be compared with the e volution of the cyclic voltammograms, representative of the system electroa ctivity. We report that the main degradation process involved in the decrea se of the light emission intensity during device operation is based on a cu rrent-driven degradation of the semiconducting polymer, mainly a loss of ca pacity on the "n'' side. Possibly this is associated with an irreversible r eduction of the anions from the added salt. (C) 2001 The Electrochemical So ciety.