Imaging the doping and electroluminescence in extremely large planar polymer light-emitting electrochemical cells

An extremely large planar polymer light-emitting electrochemical cell with an interelectrode spacing of 11 mm has been demonstrated. The large planar device structure allows for the imaging of doping propagation, photoluminescence, and electroluminescence (EL) with high spatial and temporal resolutions. Several unconventional EL modes have been observed based on the direct imaging of these devices with an interelectrode spacing ranging from 0.6 to 11 mm. These include transient EL in a dynamic-junction device, EL from planar polymer/polymer heterojunction devices, and anomalous reverse-bias EL in a frozen-junction device. Transient EL occurs when the polarity of the applied bias is reversed after the device is fully turned on. The reversal of the applied bias causes the initiation and uneven propagation of fresh p doping within previously n-doped regions, and vise versa. This leads to the formation of transient, discrete, light-emitting p-n junctions near the electrodes before a continuous light-emission zone is formed by the complete reversal of the doping profile. In addition, planar cells consisting of a side-by-side polymer/polymer heterojunction have been demonstrated and imaged. The heterojunction in all working devices is found to be electronically conductive, but exhibits different ion-transport properties. Three types of polymer/polymer interface have been identified based on the imaging of doping and EL profiles. Finally, an anomalous reverse-bias EL mode has been observed in a planar frozen-junction device. The device was turned on at elevated temperature and then cooled to 200 K. Stress under a constant reverse bias leads to the activation of anomalous EL that originates from the same region as forward-bias EL. Furthermore, both forward- and re- verse-bias EL have been found to increase with time under reverse-bias stress.