Polymer Light-Emitting Electrochemical Cells Enhanced with Bipolar Electrode Arrays: In Situ Doping and Formation of Multiple p-n Junctions
Polymer Light-Emitting Electrochemical Cell , Bipolar Electrode , Bipolar Electrochemistry , p-n Junction , Doping
Polymer light-emitting electrochemical cells (PLECs) are organic solid-state light-emitting devices operating on in situ electrochemical doping and p-n junction formation in a mixed conductor active layer. PLECs have been extensively researched for their attractive device characteristics and potential in low-cost lighting and display technologies. The majority of PLECs, however, contain only a single light-emitting junction that is very narrow and highly inefficient. Adding bipolar electrodes (BPEs) into the active material of planar PLECs dramatically changes the doping process and improves the device performance in terms of response speed and efficiency. My research focuses on the study of how various BPEs and BPE arrays affect the doping and electroluminescence (EL) of PLECs. Using arrays of metal BPE disks, we demonstrate for the first time wireless EL in a PLEC, and we show that the BPE arrays can be used to assess both the metals' ability to inject charges and the redox properties of the PLEC polymer. By constructing a large, closed BPE array, we present bulk homojunction PLECs that exhibit vastly improved EL output from over a thousand highly emissive light-emitting junctions. The clear doping and light-emission patterns greatly improve our understanding of the fundamental operation processes of the PLECs, and demonstrate that bipolar electrochemistry, realized with metallic wireless BPE, is an efficient way to engineer next-generation high performance PLECs. Towards more practical devices, we investigate the stability and degradation mechanisms of sandwich PLECs. The conventional approach to realize long-lasting PLECs is to have a very low electrolyte content in the active layer. Such PLECs are slow to activate and plagued by black spots. We discover that PLECs made with silver triflate salt and ultralow salt concentration, can avoid the trade-off between device lifetime and response speed. Moreover, the devices exhibit a record peak luminance, a long lifetime, and a high efficiency. The high performance is attributed to the partial chemical doping of the light-emitting polymer by the silver cations and the high ionic conductivity of the electrolyte. This finding opens up new avenues for designing high performance PLECs and potentially other solid-state electrochemical devices.