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In-and Cd-doped Cu 9 S 5 and reported that the electrical conductivity was reduced and the thermal power was improved when In and Cd were added to Cu 9 S 5 . However, they did not report the thermal conductivity and ZT value. [ 30 ] In our previous work, we reported that the ZT value of pristine Cu 9 S 5 reached 0.3 at 673 K and the ZT for the optimized sintering temperature sample could be further enhanced to 0.5 because of the existence of a second Cu 1.96 S phase and a considerable number of pores. [ 18 ] Dennler et al. [ 31 ] systematically studied the thermal stability and electrical stability of CuS, Cu 9 S 5, and Cu 2 S. The results showed that the CuS is not stable at temperatures beyond 240 °C either in air or N 2 . Experiments also showed that Cu 2 S is not electrically stable because both cracks and copper whiskers were observed in a Cu 2 S sample after long-time current stress tests (24 A cm −2 /24 h). In contrast, CuS and Cu 9 S 5 samples did not show any degradation in the electrical stability test, even when current densities were increased to as high as 48 A cm −2 . However, the Seebeck coeffi cient, electrical resistivity, crystal structure, and stoichiometry remained constant. [ 31 ] This indicates that only Cu 9 S 5 is both thermally and electrically stable.High-performance TE materials should simultaneously own large Seebeck coeffi cients, high electrical conductivity, and low thermal conductivity, but these three parameters are interrelated with each other. It is diffi cult to improve all the parameters at the same time, for instance, an improvement of electrical conductivity will normally lead to the deterioration in the Secbeck coeffi cient and thermal conductivity. [ 4 ] The main challenges for enhancing TE properties of Cu 9 S 5 -based materials are to improve the Seebeck coeffi cient via carrier concentration optimization. In this work, Na x Cu 9 S 5 ( x = 0, 0.025, 0.05, 0.15, 0.25) nanopowders with an average size of 3 nm were prepared by mechanical alloying (MA). The nanopowders were then sintered to bulk materials by spark plasma sintering (SPS) technology. Metallic Na was doped to Cu 9 S 5 to reduce carrier concentration and improve the Seebeck coeffi cient. Besides, nanopores and nanograins were observed unexpectedly in the Na-doped bulk samples, leading to a signifi cantly reduced thermal conductivity. Overall, a peak ZT value of 1.1 was achieved at 773 K in the composition of Na 0.05 Cu 9 S 5 . Figure 1 a,b shows the transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images (inset of Figure 1 b) of Na 0.05 Cu 9 S 5 powders obtained by MA method. Under the low magnifi cation mode, it can be seen that Na 0.05 Cu 9 S 5 powders disperse homogeneously on the microgrid support membrane, show a particle size smaller than 5 nm and an average size of 3 nm. This extremely small particle size obtained directly by MA method is quite astonishing, since the typical particle size of MAed powders is around several hundred nanometer to several micr...

Ruddlesden–Popper halide perovskite (RPP) materials are of significant interest for light‐emitting devices since their emission wavelength can be controlled by tuning the number of layers n, resulting in improved spectral stability compared to mixed halide devices. However, RPP films typically contain phases with different n, and the low n phases tend to be unstable upon exposure to humidity, irradiation, and/or elevated temperature which hinders the achievement of pure blue emission from n = 2 films. In this work, two spacer cations are used to form an RPP film with mixed cation bilayer and high n = 2 phase purity, improved stability, and brighter light emission compared to a single spacer cation RPP. The stabilization of n = 2 phase is attributed to favorable formation energy, reduced strain, and reduced electron–phonon coupling compared to the RPP films with only one type of spacer cation. Using this approach, pure blue light‐emitting diodes (LEDs) with Commission Internationale de l'éclairage (CIE) coordinates of (0.156, 0.088) and excellent spectral stability are achieved.

With the progress in the development of perovskite solar cells, increased efforts have been devoted to enhancing their stability. With more devices being able to survive harsher stability testing conditions, such as damp heat or outdoor testing, there is increased interest in encapsulation techniques suitable for this type of tests, since both device architecture compatible with increased stability and effective encapsulation are necessary for those testing conditions. A variety of encapsulation techniques and materials have been reported to date for devices with different architectures and tested under different conditions. In this Perspective, we will discuss important factors affecting the encapsulation effectiveness and focus on the devices, which have been subjected to outdoor testing or damp heat testing. In addition to encapsulation requirements for these testing conditions, we will also discuss device requirements. Finally, we discuss possible methods for accelerating the testing of encapsulation and device stability and discuss the future outlook and important issues, which need to be addressed for further advancement of the stability of perovskite solar cells.