As the researchers explain in their paper, solution-processed QD-based LEDs employ a double heterojunction architecture in which the emissive QD layer is sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL). Although NIR QD solutions have been demonstrated with luminescence efficiencies reaching about 40%, their overall photoluminescence quantum efficiency (PLQE) is much lower due to non-radiative recombination and the dissociation of charge carriers at surface defects and material interfaces.
Improving on prior art strategies to prevent these self-quenching effects, they moved away from core−shell quantum dots embedded in a 3D organolead halide perovskite matrix (which suffers from low stability to heat and moisture) to embedding the QDs into more stable two-dimensional perovskites. The 2D perovskite passivation, as they describe the stabilization process, is achieved via an in situ alkylammonium/alkylamine substitution carried out during the quantum dot (QD) ligand exchange process where the longer carbon backbones breaks the symmetry of the 3D structure and introduces two-dimensional (2D) structuring that produces a perovskite structure more resilient to moisture and irradiation.
They then prepared dot-in-2D-perovskite thin films by spin-casting a solution containing QDs and perovskite precursors, and created QD-based LEDs which exhibited external quantum efficiency (EQE) peak values up to 2% and radiances of about 1 W sr −1 m −2. They also found that the devices maintained a high performance (radiance >0.7 W sr −1 m −2) at operation voltages up to 7.5 V, a 1.7 fold improvement over the previous dot-in-3D-perovskite LEDs which would typically break down under a 4.5V bias.
The active layer film could also be made much thinner, with an average roughness reduced from 31nm for the first-generation films to 3.0nm for films based on the novel 2D Perovskite approach, potentially reducing materials consumptions 10-fold.