Traps for Electrons and Holes Limit the Efficiency and Durability of Polymer Light-Emitting Electrochemical Cells

Polymer light-emitting electrochemical cells (PLECs) and light-emitting diodes (PLEDs) receive interest for large-area lighting and signage applications. During operation of a PLEC, a p-i-n junction develops where electrons and holes are injected into the film and are transported along n- and p-doped regions to the intrinsic (i) region, where they recombine under light emission. Conceptually, this resembles the PLED device architecture equipped with Ohmic charge-injection and transport layers. The similarity between the i-region of the PLEC and the emissive layer of the PLED is obvious; however, implication of this has not been examined in detail so far. For example, for PLEDs it is known that electron traps hinder the electron transport, and that hole trap formation dictates the long-term durability. Here, for PLECs the electrical and optical response to electrical driving and breaks are studied, the current response to external light irradiation is probed, and degradation is followed with long-term absorption and capacitance measurements. The electron traps in PLECs are identified and it is found that hole trap formation limits the device lifetime, in the same manner as established for PLEDs. It is concluded that charge traps in semiconducting polymers present important, but so far overlooked intrinsic performance limitations for PLECs.