Efficient Charge Carrier Injection and Balance Achieved by Low Electrochemical Doping in Solution-Processed Polymer Light-Emitting Diodes

Abstract

Charge carrier injection and transport in polymer light-emitting diodes (PLEDs) is strongly limited by the energy level offset at organic/(in)organic interfaces and the mismatch in electron and hole mobilities. Herein, these limitations are overcome via electrochemical doping of a light-emitting polymer. Less than 1 wt% of doping agent is enough to effectively tune charge injection and balance and hence significantly improve PLED performance. For thick single-layer (1.2 mu m) PLEDs, dramatic reductions in current and luminance turn-on voltages (V-J = 11.6 V from 20.0 V and V-L = 12.7 V from 19.8 V with/without doping) accompanied by reduced efficiency roll-off are observed. For thinner (<100 nm) PLEDs, electrochemical doping removes a thickness dependence on V-J and V-L, enabling homogeneous electroluminescence emission in large-area doped devices. Such efficient charge injection and balance properties achieved in doped PLEDs are attributed to a strong electrochemical interaction between the polymer and the doping agents, which is probed by in situ electric-field-dependent Raman spectroscopy combined with further electrical and energetic analysis. This approach to control charge injection and balance in solution-processed PLEDs by low electrochemical doping provides a simple yet feasible strategy for developing high-quality and efficient lighting applications that are fully compatible with printing technologies.