Jinu Park1,Joonyun Kim1,Byungha Shin1
Korea Advanced Institute of Science and Technology1
Jinu Park1,Joonyun Kim1,Byungha Shin1
Korea Advanced Institute of Science and Technology1
All-inorganic metal halide perovskites, CsPbX<sub>3</sub>, are considered as a suitable candidate for the next generation light emitting diodes (LEDs) due to its extremely narrow light emission spectrum. A more challenging, but technologically more significant, task is developing high-performance blue light emitters, in particular, pure-blue in the wavelength range of 460 – 470 nm. In CsPbBr3-based LEDs, the emission wavelength can be easily tuned to blue emission by partially replacing Br with Cl. However, CsPbBr<sub>3-x</sub>Cl<sub>x</sub> has low external quantum efficiency (EQE) and poor emission spectral-stability due to phase separation caused by halide segregation during device operation. Here, we present a synthesis method to produce stable phosphonate-passivated single-halide CsPbBr<sub>3</sub> in the form of nanoplatelets that is capable of true-blue emission without the need for halide-substitution, i.e., free from the operational instability due to the halide segregation. Nanoplatelets with thickness of 3 monolayers were synthesized by emulsion ligand-assisted reprecipitation. These nanoplatelets yielded photoluminescence (PL) peak at 462 nm with an extremely narrow full width half maximum of 16 nm. To improve thermal stability of CsPbBr<sub>3</sub> nanoplatelets, we conducted ligand modification where long-chain alkyl carboxylic acidic ligands was replaced by phosphonate ligands in the synthesis step. The phosphonate-passivated emitters show a higher PL quantum yield (PLQY) of 80% and improved thermal stability--90% of the initial PLQY was maintained after annealing at 70 C for 5 minutes in ambient air, while a control sample without passivation PLQY degraded to 50% of the initial PLQY. With further engineering of the device structure such as the introduction of a conjugated polyelectrolyte layer between a hole transport layer and a CsPbBr<sub>3</sub> nanoplatelet emitter, a maximum EQE of 0.55% and luminance of 0.5 cd/m<sup>2</sup> at the emission wavelength of 462 nm was achieved. A possible mechanism behind the emission enhancement with the phosphonate-passivation and conjugated polyelectrolyte layer will be presented.