Decoupled luminance decay and voltage drift in polymer light-emitting electrochemical cells: Forward bias vs. reverse bias operation

Abstract

Sandwich polymer light-emitting electrochemical cells (LECs) with low electrolyte loading have been investigated. The LECs can be turned on to emit light at low voltages by applying either a forward- or reverse-bias current. The reverse operation, however, displays significantly higher luminance, frozen-junction behavior, and voltage instability. Moreover, the LECs display de-coupled luminance decay and voltage drift. Under forward-current operation, the luminance decreases for most of the test duration, while the operating voltage remains nearly unchanged. Under reverse-current operation, the luminance remains constant for over 200 h while the operating voltage more than doubles. The highly asymmetric device characteristics between forward and reverse operation can be explained by the different doping profiles created when the initial charge injection barriers are different. Under forward-current operation, the LEC film is predominantly p-doped. This leads to heavy luminescence quenching but a stable operating voltage. Under reverse operation, a predominately n-doped film is responsible for the higher luminance, frozen-junction behavior, and voltage drift. The anomalous device characteristics offer additional insights into the complex operating mechanism of LECs and the behaviours of a doped luminescent polymer.