Highly Stable SnO₂-Based Quantum-Dot Light-Emitting Diodes with the Conventional Device Structure

Abstract

ZnO-based electron-transporting layers (ETLs) have been universally used in quantum-dot light-emitting diodes (QLEDs) for high performance. The active surface chemistry of ZnO nanoparticles (NPs), however, leads to QLEDs with positive aging and unacceptably poor shelf stability. SnO 2 is a promising candidate for ETLs with less reactivity, but NP agglomeration in nonionic solvents makes the conventional device structure abandoned, resulting in QLEDs with extremely low operational lifetimes. The large barrier for electron injection also limits the electroluminescence efficiency. Here, we report one solution to all the above-mentioned problems. Owing to the strong HO−SnO 2 coordination and the steric effect provided by the hydrocarbon groups, tetramethylammonium hydroxide can stabilize SnO 2 NPs in alcohol, while its intrinsic dipole induces a favorable electronic-level shift for charge injection. The SnO 2 -based devices, with the conventional structure, exhibit not only the most efficient electroluminescence among ZnO-free QLEDs but also an operational lifetime (T 95 ) over 3200 h at 1000 cd m −2 , which is comparable with that of state-of-the-art ZnO-based devices. More importantly, the superior shelf stability means that the TMAH−SnO 2 NPs are promising to enable QLEDs with real stability.

Solution-processed quantum-dot light-emitting diodes combining ultrahigh operational stability, shelf stability, and luminance

“…The buffer layer used here is assembled of SnO 2 NPs, an n-type ETL material with similar electronic levels but less surface activity. Notably, the function of SnO 2 proposed here significantly differs from previous reports 17,20 . Through the post-treatment in tetramethylammonium hydroxide (TMAH) solution, NPs can decently disperse in ethanol (see TEM images in Supplementary Fig.…”

“…First, adding the SnO 2 buffer layer does not significantly reduce electron injection. The device exhibits a similar threshold turn-on (V th ) compared with the s-ZnO-based counterpart and a significantly higher maximum luminance (approaching 200,000 cd m −2 ) than any previously reported shelf-stable QLED 16,17 . Second, the SnO 2 buffer layer suppresses the leakage current, mainly from hole leakage through ETL 27 .…”

“…Through the post-treatment in tetramethylammonium hydroxide (TMAH) solution, NPs can decently disperse in ethanol (see TEM images in Supplementary Fig. 3) 17 , which is compatible with the fabrication process of the conventional structure.…”

“…More recently, our group reported that an ETL based on SnO 2 NPs, known for their less active surface chemistry than ZnO, enabled QLEDs with shelf stability. This work also demonstrated ZnO-free QLED with a competitive operational lifetime 17 . However, the mobility of SnO 2 NPs still limits the maximum luminance.…”

“…Therefore, the buffer layer assembled of SnO 2 NPs successfully stabilizes the QDs' PL decay kinetics. The mechanism for reducing the exciton quenching rate is ascribed to the reduction of the photoinduced electrontransfer rate and the lower number of charge acceptors on SnO 2 surfaces 17 .…”