The efficiency of organic light-emitting devices (OLEDs) is often limited by roll-off, where efficiency decreases with increasing bias. In most OLEDs, roll-off primarily occurs due to exciton quenching, which is commonly assumed to be active only above device turn-on. Below turn-on, exciton and charge carrier densities are often presumed to be too small to cause quenching. Using lock-in detection of photoluminescence, we find that this assumption is not generally valid; luminescence can be quenched by >20% at biases below turn-on. We show that this low-bias quenching is due to hole accumulation induced by intrinsic polarization of the electron transport layer (ETL). Further, we demonstrate that selection of nonpolar ETLs or heating during deposition minimizes these losses, leading to efficiency enhancements of >15%. These results reveal design rules to optimize efficiency, clarify how ultrastable glasses improve OLED performance, and demonstrate the importance of quantifying exciton quenching at low bias.
Bibliographical noteFunding Information:
This work was supported by DuPont Electronics and Imaging. J.S.B. and N.M.C. acknowledge support from the NSF Graduate Research Fellowship under grant no. 00074041. R.J.H. acknowledges support from Ronald L. and Janet A. Christenson. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC program.
© 2020 The Authors.
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