Quasi-2D perovskite for efficient and stable green light-emitting diodes

Abstract

Metal halide perovskites (MHPs) have attracted considerable scientific attention over the past few years thanks to their excellent properties, for example tunable emission wavelength (390-1050 nm), high photoluminescence quantum yield (PLQY), remarkable mobility of charge-carrier, exceptional purity of color and solution processability. In addition to the successful demonstrations of perovskite solar cell, halide perovskites exhibit the potential as the exceptional candidate for light-emitting diodes (LEDs) as well. Even though perovskite light-emitting diodes (PeLEDs) have achieved superior external quantum efficiency (EQE), the operational stability is still the limitation impeding their commercialization. In comparison with conventional three-dimensional perovskites, quasi-2D perovskites exhibit strengthened intrinsic stability and achieve higher PLQY at low excitation density through energy funneling process. This thesis is dedicated to investigating different types of quasi-2D perovskites and their applications, aiming for efficient and stable green emission PeLEDs. 2D and Quasi-2D perovskites could be generally categorized into Ruddlesden−Popper (RP) phases and Dion−Jacobson (DJ) phases. They can be achieved by slicing the bulk into layered structure with the spacer cations, and for monovalent spacer cations representing RP structure or divalent spacer cations representing DJ structure. Regarding the first part of this thesis, research efforts were made to explore the crystal structure, morphology, and optical properties for both RP and DJ perovskite materials. Efficient green PeLEDs were successfully demonstrated based on RP and DJ phase perovskites, respectively. Specifically, hexylammonium (HA) was employed as the spacer cation for RP perovskites, and the effect of mixing small cations was discussed in detail. It is notable that mixed cesium (Cs) and formamidinium (FA) small cations enhances the crystallization of HA based perovskites, which significantly affects not only the characteristics of perovskite thin films but also the performance of RP PeLEDs. The effect of mixed spacer cations was investigated on DJ perovskites. DJ PeLEDs were fabricated with the optimized mixed spacer cations 1,6-hexanediammonium (HDA) and 1,10-decanediammonium (DDA), which demonstrated substantial boost in EQE and lifetime. The second part is focusing on the lifetime of PeLEDs. The optimized mixed spacer cations DJ perovskites mentioned above, was utilized as control emitter to further explore the stability issue. Triphenylphosphine oxide (TPPO) and diphenyl-4-tri-phenylsilylphenyl-phosphine oxide (TSPO1) were applied on top control perovskites as passivation agents. The effect of these two types of passivators as well as different antisolvents on PeLEDs performance was elucidated. Interestingly, increased EQE of PeLEDs were achieved for both TPPO and TSPO1, while TSPO1 significantly improve the lifetime and TPPO shorten the device lifetime. Besides the engineering of perovskites, device structure can be another critical factor in operational stability. Here, one of the self-assembled monolayers (SAMs) as hole injection layer was used to replace hole transporting layers in conventional device structure, which enhanced the device lifetime. Devices based on inverted structure were fabricated, acquiring superior stability compared to conventional structure. Some implications regarding inverted devices were summarized.