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Two-dimensional (2D) Ruddlesden-Popper hybrid organic-inorganic perovskites are the most promising candidates for highly efficient optoelectronic devices due to their remarkable exciton characteristics, adjustable band structure and long-term environmental stability. However, excitons emission behavior of 2D Ruddlesden-Popper perovskites is still unclear. Here we theoretically-experimentally decoupled the excitons behavior in 2D Ruddlesden-Popper PEA 2 PbI 4 nanosheets. Theoretically, the Elliott theory was used to separate exciton and band edge absorptions of PEA 2 PbI 4 nanosheets and determine an exciton binding energy of about 274 meV. Experimentally, spectral-dependent photoluminescence lifetime and excitation powder density-dependent photoluminescence spectra revealed that a distinct photoluminescence tail emission should be derived from localized excitons with lower emission energy than free excitons. Further, the luminescent component of the free excitons and localized excitons with their corresponding densities of states was theoretically decoupled by line-shape analysis of PEA 2 PbI 4 nanosheets photoluminescence spectra, which is consistent with the experimental results demonstrating the existence of localized excitons. Hence, our work provides understanding on the exciton behavior and luminescence mechanism of 2D Ruddlesden-Popper perovskites, which would benefit the prediction of high-performance excitonic devices based on 2D Ruddlesden-Popper perovskites. • Excitons behavior in 2D Ruddlesden-Popper PEA 2 PbI 4 nanosheets was decoupled. • A band tail state emission derived from localized excitons. • The luminescent component of the free excitons and localized excitons was separated. [ABSTRACT FROM AUTHOR]

To fundamentally understand the underlying interfacial physics of hole transporting layer is a key to rotationally engineer high-performance light-emitting diodes (LEDs). Here, we unravel the hole interlayer-dependent interfacial energetics of LEDs via combining imaged contact potential/polarization difference, and multi-time scale capacitance/voltage response. A universally existing but undesired stagnation of current density with voltages, under the direct contact of light-emitting Poly-TPD with electrodes, was proved to be rooted from the alternant dominance of capacitance/resistance behaviors with alternating strong/weak polarizations of Poly-TPD. The introduction of hole interlayer (PEDOT:PSS) with more disordered polarization direction and greater intensity can build plentiful carrier hopping channels and ensure the rapidly dynamic charge transfer from electrodes to Poly-TPD, conducing to the increased capacitive reactance contribution at low voltages and inductive effects at high voltages. Also, we clarify its frequency-response difference for emerging high-speed modulation application. This discovery might enable us to explore the next-generation high-efficiency display and optical data-communication. [Display omitted] • The blue-emitting buffer layer (poly-TPD) was found to possess regular namatic stripes with alternating strong/weak polarizations. This ordered alternating polarization distribution is the main cause of the alternant dominance of capacitance/resistance behaviors when poly-TPD directly contacts ITO, naturally resulting in a stagnation of current density for LEDs, only at the end of this worthless stagnation can efficient electroluminescence be produced. • The semi-metallic hole-interlayer (PEDOT:PSS) was found to feature disordered polarization direction and greater intensity, reducing electrode polarization and building plentiful carrier hopping channels, ensuring the rapid and dynamic charge transfer between ITO and poly-TPD. This conduces to the prominent capacitive reactance behaviors at low voltages and increases inductive proportion, enabling current density-voltage of LEDs a standard diode exponential change. • The frequency-response difference of hole interlayer-dependent devices was clarified, providing a fresh insight toward emerging high-speed LED-based optical data-communication. [ABSTRACT FROM AUTHOR]

• The electron injection of ETL/perovskite of PeLEDs at molecular level is studied. • The molecular planarity bridging effects are discovered. • Superior molecular planarity of ETL enhances electron transport ability of ETL. • Superior molecular planarity of ETL improves carrier injection of ETL/perovskite. • Superior molecular planarity of ETL leads to superior performances of PeLEDs. Effectively balancing carrier injection of perovskite LEDs (PeLEDs) for next-generation display and illumination urgently needs for molecularly understanding carrier injection behavior of electron transport layer (ETL)/perovskite toward further developing high performance PeLEDs. Here, we molecularly reveal the electron injection behaviors of PeLEDs by theoretically-experimentally decoupling the electron transport of ETLs and electron injection of TmPyPB/perovskite and TPBi/perovskite. Resulted, the better molecular planarity bridging effect (MPBE) of TmPyPB than TPBi can effectively increase electronic-coupling for the improved electron transport ability and significantly decrease injection barrier for the enhanced electron injection capability, intrinsically leading to a more balanced carrier injection and a less electron accumulation of PeLEDs. Compared to TPBi-based PeLEDs, the TmPyPB-based PeLEDs present the obviously-increased average EQE, current efficiency, power efficiency and luminance by 44 %, 63 %, 43 % and 100 %, respectively. Meanwhile, this PeLED shows a higher voltage tolerance characteristic of 7.5 V than that of TPBi-based PeLEDs (6.25 V). Therefore, this work may provide a fundamental insight into carrier injection behavior of PeLEDs for effectively-regulating its carrier injection balance and further for its greatly-facilitated commercialization. [ABSTRACT FROM AUTHOR]