A SnO2/ZnO Nanoparticle Bilayer Electron Transport Layer for Regulated Electron Injection in Quantum-Dot Light-Emitting Diodes

ZnO is widely used as an electron transport layer in quantum-dot light-emitting diodes (QLEDs) because of its high electron mobility and suitable energy levels. However, rapid electron transport may cause excessive electron injection, leading to charge accumulation and parasitic recombination. SnO2 provides weaker electron transport, but using it alone limits device performance. Here, a SnO2/ZnO bilayer electron transport layer was introduced to regulate electron injection. Unlike previously reported structures in which SnO2 directly contacts the QDs, the present configuration places SnO2 on ITO and ZnO adjacent to the QD layer. The low-concentration ZnO overlayer reduced the RMS roughness of the SnO2 film from 1.79 to 1.07 nm and facilitated electron injection, while the underlying SnO2 layer moderated the electron supply. The bilayer also suppressed leakage current and showed the lowest capacitance peak, consistent with improved charge balance. The bilayer QLED achieved a maximum current efficiency of 13.40 cd/A and a maximum luminance of 33,210 cd/m2. Its current efficiency was 100% and 32.7% higher than those of the SnO2 and ZnO devices, respectively. These results demonstrate that the bilayer improves charge balance through facilitated electron injection and controlled electron supply.