Molecular dipole engineering enabled TiO₂ electron transport layers for stable quantum-dot light-emitting diodes

Colloidal quantum dot LEDs (QLEDs) offer great potential for next-generation displays and lighting but suffer stability issues linked to the chemically reactive zinc oxide (ZnO) electron transport layer (ETL). While chemically inert titanium dioxide (TiO₂) is a stable ETL alternative, the performance of TiO₂-based QLEDs is significantly inferior to their ZnO-based counterparts due to the low electron injection efficiency caused by the poor crystallization and high work function of TiO₂. This work demonstrated a low-temperature process to yield highly crystalline TiO₂ nanoparticles, which are further modified with monoethanolamine (MEA) to lower the work function. The resulting TiO₂-based QLEDs show performance comparable with the ZnO-based ones, and much better stability than the latter. MEA forms bridging coordination with undercoordinated surface Ti atoms, creating interfacial dipoles that drastically lower the work function and boost electron injection. Simultaneously, protonated amino groups neutralize the residual acidic species on the TiO₂ ETL, reducing exciton quenching. This synergistic approach gives rise to red QLEDs with record high current efficiency (22.18 cd/A, EQE 14.52%) and power efficiency (29.46 lm/W), a 5.24-fold improvement over the unmodified devices. Crucially, the devices show exceptional shelf stability (−2.3% to +6.5% efficiency fluctuation) after 5 months, overcoming the stability-efficiency trade-off in QLEDs.