RESUMO
Although colloidal perovskite nanocrystal (PNC) solution has exhibited near-unity photoluminescence quantum yield (PLQY), the luminance would be severely quenched when the PNC solution is assembled into thin films due to the agglomeration and fusion of NCs caused by the exfoliation of surface ligands and non-radiative Förster resonance energy transfer (FRET) from small to large particle sizes, which seriously affected the performances of light-emitting diodes (LEDs). Here, we used Guanidine thiocyanate (GASCN) and Sodium thiocyanate (NaSCN) to achieve effective CsPbI3 PNC surface reconstruction. Due to the strong coordination ability of these small molecules with the anions and cations on the surface of the PNCs, they can provide strong surface protection against PNC fusion during centrifugal purification process and repair the surface defects of PNCs, so that the original uniform size distribution of PNCs can be maintained and FRET between close-packed PNC films is effectively suppressed, which allows the emission characteristics of the films to be preserved. As a result, highly oriented, smooth and nearly defect-free high-quality PNC thin films are obtained, with PLQY as high as 95.1 %, far exceeding that of the original film, and corresponding LEDs exhibit a maximum external quantum efficiency of 24.5 %.
RESUMO
Perovskite nanocrystals have been widely applied in the field of light-emitting diodes (LEDs) due to their excellent optoelectronic properties. However, there is generally a serious degradation of device efficiency when transferring the device from rigid to flexible substrates due to the high roughness, poor wettability, and low endurance temperature of flexible substrates. Herein, a highly flexible perovskite light-emitting diode (PeLED) by utilizing label paper as substrates and poly(methyl methacrylate) (PMMA) as the modified layer was reported. Compared with the reference device based on commonly used polyethylene terephthalate (PET) substrates, the label paper/PMMA-based devices did not show the degraded device performance when transferring from rigid to flexible substrates. This is mainly because of low roughness and good wettability of PMMA-modified label paper, which significantly improve the film-forming ability of the bottom electrode and functional layer. Furthermore, the flexibility of both devices was explored by a three-point bending flexural test, indicating that the label paper-based device has better bending stability than the polyethylene terephthalate-based one due to the lower flexural modulus for label paper. As a result, the label paper-based flexible PeLEDs exhibited the highest external quantum efficiency (EQE) of 14.3% among perovskite nanocrystal-based flexible LEDs and preeminent flexibility with 29% luminance degradation after bending for 1000 cycles at a small radius of 1.5 mm. This extension of the substrate to paper will widen the opportunity of PeLEDs in extremely flexible and inexpensive applications.
RESUMO
Perovskite light-emitting diodes (PeLEDs) can be a promising technology for next-generation display and lighting applications due to their excellent optoelectronic properties. However, a systematical overview of luminescence and degradation mechanism of perovskite materials and PeLEDs is lacking. Therefore, it is crucial to fully understand these mechanisms and further improve device performances. In this work, the fundamental photophysical processes of perovskite materials, electroluminescence mechanism of PeLEDs including carrier kinetics and efficiency roll-off as well as device degradation mechanism are discussed in detail. In addition, the strategies to improve device performances are summarized, including optimization of photoluminescence quantum yield, charge injection and recombination, and light outcoupling efficiency. It is hoped that this work can provide guidance for future development of PeLEDs and ultimately realize industrial applications.