Self-assembled monolayers as hole transport layers for efficient thermally evaporated blue perovskite light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) based on thermal evaporation technique have demonstrated great potential in newly-developed wide color gamut display applications. However, it is still a challenging task to obtain high efficiency thermally evaporated blue PeLEDs. One of the key issues is the lack of desirable functional materials with both hole transport function and perovskite buried interface modification function, which can alleviate the accumulation of high density of deep-level defects between hole transport layer (HTL) and perovskite emitting layer (EML) and weaken the interfacial nonradiative recombination process. In this study, we developed a new multifunctional wide-bandgap self-assembled monolayers (SAMs) suitable for thermally evaporated blue PeLEDs, named (2-(3-bromo-6-(4-formylphenyl)-9H-carbazol-9-yl)ethyl)phosphonic acid (CZPC). The interactions between the carbonyl groups (C = O) contained in CZPC and uncoordinated Pb2+ are critical for improving crystallinity and suppressing nonradiative losses at the buried perovskite-hole transporter interface. In addition, the phosphonic acid groups bonding with indium tin oxide (ITO) can form a self-assembled monolayer to facilitate carrier injection and transport. Finally, simplified thermally evaporated sky-blue (488 nm) PeLEDs utilize CZPC as HTL and no additional functional layers such as hole injection layer are used show excellent device performance with a maximum external quantum efficiency (EQE) of 5.42 %, accompanied by the CIE color coordinates of (0.07, 0.25) and a full width at half maximum (FWHM) of 24 nm. This work lays a foundation for the further exploration of SAMs on high-performance thermally evaporated blue PeLEDs towards ultra-high resolution display applications.