Bipolar tunnelling injection into single-layer organic light emitting devices: analytical solution using the regional approximation

We present an analytical model to describe the electrical characteristics o f single-layer organic light emitting devices. The electrons and holes are assumed to be injected by tunnelling at the electrode/polymer interfaces. T he spatial distribution of electric field intensity, charge carrier concent ration and recombination ratio are calculated using the regional approximat ion method. We analyse the electrical characteristics of devices with diffe rent barrier heights for charge carrier injection and different electron an d hole mobilities. We find that the increase of the barrier heights increas es the voltage threshold for light emission. Also, we show that a lower ele ctron mobility compared to the hole mobility concentrates the emission dist ribution in the regions near to the cathode. A simple method of comparing t he efficiency for different devices at each applied voltage is derived. Fro m this we find that devices with a balanced injection of electrons and hole s are more efficient than the unbalanced ones before they reach maximum eff iciency. The increase of electron mobility slightly decreases the device ef ficiency for each voltage. Finally, the current-voltage (J-V) characteristi cs of the regional approximation model are compared with experimental resul ts showing a close agreement in the applied voltage range of the model vali dity.