Exciton Dynamics and Optically Pumped Lasing in 1-Naphthylmethylamine-Based Quasi-2D Perovskite Films

Films of the quasi-2D perovskite based on 1-naphthylmethylamine (NMA) are promising as the gain medium for optically pumped lasing and future electrically pumped lasing because of its low lasing threshold and small electroluminescence efficiency rolloff. However, reasons for the low threshold and small efficiency rolloff are still unclear. Therefore, exciton dynamics are investigated in NMA-based quasi-2D perovskite films. It is found that quenching of bright excitons by other excitons or charge carriers is unlikely in NMA-based quasi-2D perovskite films, which is one reason for the low lasing threshold and small efficiency rolloff. Moreover, thermally stimulated current measurements reveal that the defect levels inside the band gap of the NMA-based quasi-2D perovskite are shallow, with a depth of ≈0.3 eV, causing a decrease in nonradiative exciton recombination through the defects. Therefore, population inversion can be easily achieved, leading to the low lasing threshold as well. For fabrication of NMA-based quasi-2D perovskite laser devices with even lower lasing thresholds, a circular-shaped optical resonator, and small-molecule-based defect passivation are used. Optically pumped lasing can be obtained from these devices, with a threshold of ≈1 µJ cm⁻², which is one of the lowest values ever reported in any perovskite lasers.     

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.     

Manufacturing nanofibrous bundles and the roller drafting effects on the bundle properties

This research reports a new product form made of nanofibers and the method used to produce it. Nanofibers are electrospun into a web form on a moving collector and the web is continuously transformed into a bundle by passage through a condenser. Since the web is made of loops of nanofibers and peeled off from the collector surface, the condensed bundle deforms very easily and is too weak for further processing. Thus, the bundle is drafted through a roller drafting system to improve the fiber array in the bundle length direction in various levels of draft temperature, since the temperature may affect the bundle drafting process and thus the bundle properties. Using a newly designed system, we tried to produce nanofibrous bundle of polyvinylidiene difluoride. The nanofiber arrangement and the tensile properties of the bundle were measured before and after the roller drafting process under various conditions. Results showed that the technology suggested in this paper had the feasibility to manufacture a new product form composed of nanofibers, which implied the possible improvement of the productivity and product quality. Thickness reduction of the bundle occurring during the roller drafting straightened the nanofiber loops in the bundle and rearranged the nanofibers along the bundle axis. The drafting process was affected by the process temperature. Uni-axial tensile tests demonstrated that the bundle strength reached 4.8 cN/tex, and the strain at breakage was 0.087, when the draft ratio was 4.0 and the temperature 110 °C, whereas the raw nanofiber bundle in process had a strength of 0.014 cN/tex and the strain at breakage of 0.65. The thickness of the constituent nanofibers was not significantly affected by drafting.