Sky-Blue-Emissive Perovskite Light-Emitting Diodes: Crystal Growth and Interfacial Control Using Conjugated Polyelectrolytes as a Hole Transporting Layer

A series of poly(fluorene-co-phenylene)-based anionic conjugated polyelectrolytes (CPEs) are prepared with varying sizes of counterions (tetramethylammonium, tetraethylammonium, and tetrabutylammonium (TBA(+))) and studied as a hole-transporting layer (HTL) for sky-blue-emissive perovskite light-emitting diodes (PeLEDs). Ionic CPE HTLs improve the wettability, compatibility, and nucleation of perovskite crystals at interfaces, enabling highly crystalline perovskite crystal growth with enhanced light-emitting properties. By incorporating the CPE HTLs containing bulky TBA(+) counterions (MPS2-TBA) in place of PEDOT:PSS, the decreased phonon-electron coupling and increased exciton binding energy in perovskites are measured by temperature-dependent photoluminescence (PL) measurements. By increasing the size of counterions in CPE interlayers, the PL intensities and lifetimes of perovskite films increase. Through space-charge-limited current measurements, the lowest trap density is measured in the perovskite film on MPS2-TBA, emphasizing a critical role of larger counterions. Using density functional theory, MPS2-TBA is calculated to show the strongest adsorption affinity toward the interstitial defect of lead ions, explaining its pronounced interfacial defect passivation. The counterion size in CPE interlayers is interpreted as a main factor to determine the adsorption affinity onto perovskite, which determines the interacted area as noncovalent adsorption occurs. Finally, the sky-blue-emissive quasi-2D PeLED with MPS2-TBA shows the highest luminance efficiency (a peak EQE of 2.6% at 489 nm) and significantly improved spectral stability.