Charge Injection into Thin Conjugated Polymer Films

Abstract

We review several results on the charge injection into thin conjugated polymer films and discuss tunnelling and space-charge limited current models. The analysis of the currently available models reveals an apparent divergence in the literature which we try to eliminate via the consideration of both effects, tunnelling and space-charge formation, simultaneously. We analyse experimental results using a tunnelling model, which includes the contribution to the electronic potential originated by the space-charge effects due to the low mobility of charge carriers in conjugated polymers. As a consequence, the empty levels accessible for charge carriers in the polymer are shifted and the potential barrier for charge injection at the electrode/polymer interface is modified. This modification implies in a different tunnelling current and consequently in different current versus voltage dependences. We use our results to estimate the mobility of negative charge carriers in poly (p-phenylene vinylene) and to determine the trap centre concentration in the polymer layer of devices which present hysteresis in the current versus voltage curves.