M. Koehler et al., Bipolar tunnelling injection into single-layer organic light emitting devices: analytical solution using the regional approximation, J PHYS D, 33(17), 2000, pp. 2096-2107
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.