Hole trap formation in polymer light-emitting diodes under current stress


Polymer light-emitting diodes (PLEDs) are attractive for use in large-area displays and lighting panels, but their limited stability under current stress impedes commercialization. In spite of large efforts over the last two decades a fundamental understanding of the degradation mechanisms has not been accomplished. Here we demonstrate that the voltage drift of a PLED driven at constant current is caused by the formation of hole traps, which leads to additional non-radiative recombination between free electrons and trapped holes. The observed trap formation rate is consistent with exciton-free hole interactions as the main mechanism behind PLED degradation, enabling us to unify the degradation behaviour of various poly(p-phenylene) derivatives. The knowledge that hole trap formation is the cause of PLED degradation means that we can suppress the negative effect of hole traps on voltage and efficiency by blending the light-emitting polymer with a large-bandgap semiconductor. Owing to trap-dilution these blended PLEDs show unprecedented stability.

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Fig. 1: PLED degradation under current stress.
Fig. 2: Hole trap formation in a PLED under current stress.
Fig. 3: PLED luminance decay under current stress.
Fig. 4: Hole trap formation as function of ageing time.
Fig. 5: Effect of blending on PLED degradation.


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Parts of the text and results reported in this work have been reproduced from the thesis of Q.N., at the Johannes Gutenberg University Mainz, and accessible at https://publications.ub.uni-mainz.de/theses/frontdoor.php?source_opus=100001527&la=en. We thank C. Bauer, F. Keller and H. Raich for technical support. We acknowledge financial support from BASF SE.

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P.W.M.B. proposed the project, N.I.C. supervised the project. Q.N. and R.R. carried out experiments, G.-J.A.H.W. analysed the transport data, Q.N., N.I.C. and P.W.M.B. wrote the manuscript.

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Correspondence to N. Irina Crăciun.

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Supplementary Sections 1–13, Supplementary Figures 1–10, Supplementary references

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Niu, Q., Rohloff, R., Wetzelaer, G.A.H. et al. Hole trap formation in polymer light-emitting diodes under current stress. Nature Mater 17, 557–562 (2018). https://doi.org/10.1038/s41563-018-0057-x

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