Detrimental Effect of Unreacted PbI2 on the Long-Term Stability of Perovskite Solar Cells

Excess/unreacted lead iodide (PbI₂) has been commonly used in perovskite films for the state-of-the-art solar cell applications. However, an understanding of intrinsic degradation mechanisms of perovskite solar cells (PSCs) containing unreacted PbI₂ has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI₂ crystals under continuous light illumination. Unreacted PbI₂ undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI₂ is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI₂ crystals in perovskite films is very important to improve device operational stability for diverse opto-electronic applications in the future.     

Synthesis of Ca–Co hydroxides and their use in facile fabrication of textured CayCoO2 thermoelectric ceramics

A brucite-type hydroxide of Ca_zCo_1?z(OH)₂ with a high Ca content was synthesized and utilized as a single-source precursor for the fabrication of bulk ceramics of a semiconducting layered calcium cobaltite Ca_yCoO₂. Plate-like Ca_zCo_1?z(OH)₂ particles with z up to 0.22 were reproducibly obtained by a reverse co-precipitation method using KOH as an alkaline source. Uniaxial pressing of the plate-like Ca_zCo_1?z(OH)₂ particles (z?=?0.22), which was calcined at 373?K beforehand, at a pressure as high as 500?MPa produced a highly dense green pellet where the plate-like particles were stacked along the pressing direction. The green pellet was then converted by a heat treatment at 923?K into an oxide ceramic of preferentially (010)-oriented Ca_yCoO₂ via a topotactic-like pyrolytic reaction maintaining edge-sharing CoO₆ octahedra. Although the ceramic also contained a minor insulating Co₃O₄ phase (16.7?±?0.7?mol% estimated from thermal analysis), the electrical measurements proved its p-type semiconducting behavior resulting from the Ca_yCoO₂ phase with enhanced Seebeck coefficient exceeding 160 μV/K at room temperature.