No abstract

Solid-state (ss) NMR spectroscopy was applied to study the stabilization process of 30 wt % 13 C-labeled atacticpolyacrylonitrile (a-PAN) heat-treated at various temperatures (T s ) under nitrogen and air. Direct polarization magic-angle spinning (DP/MAS) 13 C NMR spectra provided quantitative information about the functional groups of stabilized a-PAN. Two dimensional (2D) refocused 13 C− 13 C INADEQUATE and 1 H− 13 C HETCOR NMR spectra gave through-bond and through-space correlations, respectively, of the complex intermediates and final structures of a-PAN stabilized at different T s values. By comparing 1D and 2D NMR spectra, it was revealed that the stabilization process of a-PAN under nitrogen is initiated via cyclization, while the stabilization under air proceeds via dehydrogenation. Different initial processes lead to the isolated aromatic ring and ladder formation of the aromatic rings under nitrogen and air, respectively. Side reactions and intermediate structures are also discussed in detail. Through this work, the stabilization index (SI) was defined on the basis of the quantified C-1 and C-3 DP/MAS spectra. The former reached 0.87 at T s = 370°C, and further higher T s values did not affect SI; however, the latter continuously increased up to 0.66 at T s = 450°C. All of the experimental results indicated that oxygen plays a vital role on the whole reaction process as well as the final products of stabilized a-PAN.

According to the thinner emitting layer and stronger electric field in perovskite light‐emitting diodes (PeLEDs) than those in perovskite solar cells, the strong electric‐field‐driven ion‐migration is a key issue for the operational stability of PeLEDs. Here, a methylene‐bis‐acrylamide cross‐linking strategy is proposed to both passivate defects and suppress ion‐migration with an emphasis on the suppressing mechanism via in situ investigations. As typical results, in addition to the enhanced external quantum efficiency (EQE, 16.8%), PeLEDs exhibit preferable operational stability with a half lifetime (T50) of 208 h under continuous operation with an initial luminance of 100 cd m−2. Moreover, the EQE of cross‐linked LEDs can maintain above 15% during 25 times scanning as the devices are measured every 4 days. To the authors’ knowledge, this is the highest stability published until now for high‐efficiency PeLEDs with EQE over 15%. The in situ/ex situ mechanism investigation demonstrates that such cross‐linking increases binding energy from 0.54 to 0.92 eV and activation energy from 0.21 to 0.5 eV. Hence, it suppresses ligands breaking away and ion migration, which prevents ions from moving inside and across crystals. The proposed cross‐linking passivation strategy thus provides an effective methodology to fabricate stable perovskites‐based photoelectric devices.