Aperiodic crystals in biology

Biological systems display a broad palette of hierarchically ordered designs spanning over many orders of magnitude in size. Remarkably enough, periodic order, which profusely shows up in nonliving ordered compounds, plays a quite subsidiary role in most biological structures, which can be appropriately described in terms of the more general aperiodic crystal notion instead. In this Topical Review I shall illustrate this issue by considering several representative examples, including botanical phyllotaxis, the geometry of cell patterns in tissues, the morphology of sea urchins, or the symmetry principles underlying virus architectures. In doing so, we will realize that albeit the currently adopted quasicrystal notion is not general enough to properly account for the rich structural features one usually finds in biological arrangements of matter, several mathematical tools and fundamental notions belonging to the aperiodic crystals science toolkit can provide a useful modeling framework to this end.

Topical Review: Pathways toward cost-effective single-junction III-V solar cells

III-V semiconductors such as InP and GaAs are direct bandgap semiconductors with significantly higher absorption compared to silicon. The high absorption allows for the fabrication of thin/ultra-thin solar cells, which in turn permits for the realization of lightweight, flexible, and highly efficient solar cells that can be used in many applications where rigidity and weight are an issue, such as electric vehicles, the internet of things, space technologies, remote lighting, portable electronics, etc. However, their cost is significantly higher than silicon solar cells, making them restrictive for widespread applications. Nonetheless, they remain pivotal for the continuous development of photovoltaics. Therefore, there has been a continuous worldwide effort to reduce the cost of III-V solar cells substantially. This topical review summarises current research efforts in III-V growth and device fabrication to overcome the cost barriers of III-V solar cells. We start the review with a cost analysis of the current state-of-art III-V solar cells followed by a subsequent discussion on low-cost growth techniques, substrate reuse, and emerging device technologies. We conclude the review emphasizing that to substantially reduce the cost-related challenges of III-V photovoltaics, low-cost growth technologies need to be combined synergistically with new substrate reuse techniques and innovative device designs.