Quasi-Two-Dimensional Blue Perovskite Electroluminescent Devices

Quasi-two-dimensional（quasi-2D）perovskite materials are a type of metal halide perovskite,composed of large organic cations combined with three-dimensional perovskite to form layered perovskite materials.Quasi-2D perovskite materials possess advantages such as high exciton binding energy,narrow emission bandwidth,broad spectral coverage,and simple synthesis,offering vast potential applications in lighting,displays,and photovoltaics.At the early stage of this research,the field of perovskite-based blue light-emitting diodes（LEDs）was in a developing phase.Due to factors such as wide bandgap and poor electroluminescence stability,the performance of blue LEDs was far inferior to red and green devices within the same material system,with the external quantum efficiency（EQE）of perovskite blue LEDs being below 10%.Additionally,the suboptimal performance of blue light devices hindered the development of all-perovskite white light devices,becoming a major obstacle to practical application and commercialization.Quasi-2D perovskite materials can achieve blue emission based on bromide perovskite materials through strong quantum confinement effects,greatly reducing the dependence on chlorine-bromine mixed strategies,thereby avoiding issues such as decreased film quality and luminescence efficiency caused by the introduction of a large number of chlorine atoms.Therefore,quasi-2D perovskite materials have enormous potential to achieve efficient blue emission and develop high-performance blue LEDs.However,there are still many challenges in quasi-2D blue perovskite LEDs,including mismatched energy levels of device transport layers,low charge carrier mobility,unsuitable distribution of emitting layers,poor film conductivity,and defects,which limit further improvement in blue light device performance.The main focus of this paper is to enhance the performance of quasi-2D blue perovskite LEDs.The research is centered around three aspects:modulation of the energy levels and carrier mobility of the transport layer,optimization of the optoelectronic properties of the emissive layer,and passivation of functional layer/interface defects.The following are the specific research contents:（1）Addressing the challenges of mismatched energy levels and low carrier mobility in the hole transport layer of blue LEDs,a strategy is proposed involving the modification of the poly（9-vinylcarbazole）（PVK）hole transport layer（HTL）using metal nanoclusters[Ag₆PL₆].The introduction of Ag₆ nanoclusters（NCs）lowers the highest occupied molecular orbital（HOMO）level of PVK from-5.82 e V to-5.94 e V,achieving better alignment with the emission layer and efficient hole injection.The study reveals the modulation mechanism of metal NCs on the energy level structure of the transport layer,utilizing intermolecularπ-πinteractions to polarize PVK and adjust its energy level structure.Additionally,the metallic properties of Ag₆ NCs enhance hole mobility from 1.26×10^-4 to 4.69×10^-4 cm² V^-1 s^-1,achieving balanced carrier transport and suppressing carrier accumulation and exciton annihilation.These modifications lead to a reduction in device turn-on voltage from 3.18 V to 2.81 V,an increase in maximum brightness from 3276 to 6111 cd m^-2,and an enhancement of external quantum efficiency（EQE）from 10.49%to 14.29%,representing the highest reported values in the field at that time.（2）To address the performance degradation issue in quasi-2D blue perovskite films due to small n-phase,a pre-deposition strategy using guanidinium thiocyanate（GASCN）is proposed to control the distribution of the emissive layer,suppress the formation and aggregation of small n-phase（n=1 and 2）,and reduce its defect density,thereby increasing energy transfer rate.The mechanism of GASCN pre-deposition in regulating the phase distribution of quasi-2D perovskite films is elucidated,demonstrating the diffusion effect of GASCN combined with the binding of phenethylammonium cations（PBA⁺）in the bottom interface to inhibit crystallization of small n-phase.GASCN pre-deposition also avoids interaction with the top perovskite precursor,suppressing redshift in emission.Additionally,GASCN pre-deposition passivates defects,inhibiting non-radiative recombination and improving the photoluminescence quantum yield（PLQY）of quasi-2D perovskite films.Consequently,the EQE of the quasi-2D blue LED is further improved to 16.40%,with a maximum brightness of 8290 cd m^-2.（3）Further improvement is achieved by employing the synergistic effect of（2-（9H-carbazol-9yl）ethyl）phosphonic acid（2PACz）and 2-aminopyridine-4-carboxylate methylester（MAC）.This strategy effectively increases the relative proportion of the emitting phase（n=4）in the quasi-2D blue perovskite film,forming a dominant distribution of the emitting phase and enhancing energy transfer efficiency while improving film conductivity.Through density functional theory analysis and nuclear magnetic resonance characterization,the synergistic regulation mechanism of 2PACz and MAC on the phase distribution of quasi-2D blue perovskite films is clarified.The synergistic effect enhances the interaction between 2PACz and PBA⁺,inducing the formation of the n=4 phase and significantly increasing energy transfer efficiency.Simultaneously,the synergistic regulation of 2PACz and MAC optimizes film orientation,increasing film conductivity from 1.35×10^-2 to 4.41×10^-2 S cm^-1,establishing balanced carrier transport.Moreover,the synergistic regulation of 2PACz and MAC effectively passivates film defects,raising the PLQY of the film to 63.4%.Ultimately,a high-performance quasi-2D blue perovskite LED is achieved with a maximum brightness of 10142 cd m^-2 and a maximum EQE of 17.08%.（4）Building upon the achievements,a multi-emissive layer LED device structure is proposed,combining the perovskite layer with a phosphorescent ultra-thin layer of yellow phosphorescent bis（4-phenylthieno[3,2-c]pyridine-C2,N）iridium（III）（PO-01）to realize a perovskite/organic molecule hybrid white LED.By adjusting the thickness of the spacer layer,the energy transfer process between the quasi-2D perovskite film and PO-01 is controlled,achieving warm white emission with Commission Internationale de l’éclairage（CIE）coordinates of（0.41,0.46）and an EQE of 6.8%.The study reveals that the quenching of exciton density in the phosphorescent ultra-thin layer reduces the overall performance of the white LED.To address this,a strategy involving hole transport material 4,4’,4’’-tris（carbazol-9-yl）triphenylamine（TCTa）and host-guest doping is proposed to optimize the concentration and thickness of the phosphorescent material,achieving a warm white LED with CIE coordinates of（0.351,0.385）,brightness of 28503 cd m^-2,and EQE of 13.54%.This optimized structure is further applied to a flexible poly（ethylene glycol）2,6-naphthalate（PEN）substrate,resulting in a quasi-2D perovskite flexible white LED with an EQE of 7.92%,showcasing the potential applications of quasi-2D perovskite LEDs in lighting and display technologies.The innovations of this research are as follows:（1）A high-performance hole transport layer with scarce matching for wide-bandgap blue perovskite LEDs is proposed.To address this challenge in the field,the use of metal NCs,which possess both molecular and metallic properties,is suggested to modify the hole transport layer and regulate its energy levels,thereby enhancing carrier injection efficiency.The metal properties of the metal nanoclusters enhance hole transport capability,establishing balanced carrier transport,resulting in a highly efficient（EQE=14.29%）and high-brightness(6111 cd m^-2)quasi-2D blue perovskite LED.（2）A strategy of GASCN pre-deposition to improve thin film phase distribution is proposed to address the problem of excessive small n-phase distribution and interface accumulation,which reduces device efficiency in quasi-2D blue perovskite film.GASCN pre-deposition not only inhibits the formation of small n-phase in quasi-2D perovskite films and interface aggregation but also effectively reduces its defect density,significantly improving device efficiency.（3）The effective formation and distribution of emitting phases in quasi-2D blue perovskite films are crucial for enhancing device performance.Based on this,a method of dual-ligand synergistic effect using 2PACz and MAC is proposed to optimize film phase distribution,regulate thin film orientation,and enhance thin film conductivity.Ultimately,this method increases the EQE of blue light devices to 17.08%.（4）By combining the perovskite layer with the yellow organic material PO-01,a white light emitting layer LED is constructed,achieving warm white light emission with an EQE of 13.54%.At the same time,quasi-2D perovskite flexible white LEDs with an EQE of 7.92%are realized on flexible substrates.