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 of single-layer organic light emitting devices. The electrons and holes are assumed to be injected by tunnelling at the electrode/polymer interfaces. The spatial distribution of electric field intensity, charge carrier concentration and recombination ratio are calculated using the regional approximation method. We analyse the electrical characteristics of devices with different barrier heights for charge carrier injection and different electron and hole mobilities. We find that the increase of the barrier heights increases the voltage threshold for light emission. Also, we show that a lower electron mobility compared to the hole mobility concentrates the emission distribution in the regions near to the cathode. A simple method of comparing the efficiency for different devices at each applied voltage is derived. From this we find that devices with a balanced injection of electrons and holes are more efficient than the unbalanced ones before they reach maximum efficiency. The increase of electron mobility slightly decreases the device efficiency for each voltage. Finally, the current-voltage (J-V) characteristics of the regional approximation model are compared with experimental results showing a close agreement in the applied voltage range of the model validity.