Abstract
The trap-assisted charge injection in polyfluorene-poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) model systems with an Al or Al/LiF cathode is investigated. We find that inserting 1.3 nm LiF increases electron and hole injections simultaneously and the increase of holes is greater than electrons. The evolution of internal interfaces within polymer light-emitting diodes is observed by transmission electron microscopy, which reveals that the introduction of LiF improves the interface stability at both the cathode (cathode/polymer) and the anode (indium tin oxide (ITO)/PEDOT:PSS). Above-mentioned experimental results have been compared to the numerical simulations with a revised Davids model and potential physical mechanisms for the trap-assisted charge injection are discussed.
Highlights
Polymer light-emitting diodes (PLED) have attracted widespread attention ever since they were first reported by Burroughes et al in 1990 [1], for their advantages such as solution-process method, low-cost, light weight, and flexibility
We study the PFO-PEDOT:PSS model systems with an Al or Al/LiF cathode
In region II, electron injection occurs and the current through the LiF device begin to increase much sharper than that through the pristine device. This implies that the influence of LiF interlayer on electron injection is the origin for the difference in J-V profiles
Summary
Polymer light-emitting diodes (PLED) have attracted widespread attention ever since they were first reported by Burroughes et al in 1990 [1], for their advantages such as solution-process method, low-cost, light weight, and flexibility. Knowledge of the physical processes governing the operation of PLED is essential for optimizing the device performance. Due to the low carrier concentration present in semiconducting polymers, crucial for efficient operation of PLEDs are the adequate and balanced charge injections of both electron and hole, which are controlled by the injection barriers from respective electrode contacts. Depending on the magnitude of barrier height, the current can be either injection limited or space-charge limited, and a big interface barrier at either the electron or hole contact may hinder the injection process which results in excess carriers of one polarity and decreasing the recombination efficiency significantly. In order to Materials 2019, 12, 2427; doi:10.3390/ma12152427 www.mdpi.com/journal/materials
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