248 research outputs found
Pathway to oxide photovoltaics via band-structure engineering of SnO
The prospects of scaling current photovoltaic technologies to terawatt levels
remain uncertain. All-oxide photovoltaics could open rapidly scalable
manufacturing routes, if only oxide materials with suitable electronic and
optical properties were developed. A potential candidate material is tin
monoxide (SnO), which has exceptional doping and transport properties among
oxides, but suffers from a low adsorption coefficient due to its strongly
indirect band gap. Here, we address this shortcoming of SnO by band-structure
engineering through isovalent but heterostructural alloying with divalent
cations (Mg, Ca, Sr, Zn). Using first-principles calculations, we show that
suitable band gaps and optical properties close to that of direct-gap
semiconductors are achievable in such SnO based alloys. Due to the defect
tolerant electronic structure of SnO, the dispersive band-structure features
and comparatively small effective masses are preserved in the alloys. Initial
Sn1-xZnxO thin films deposited by sputtering exhibit crystal structure and
optical properties in accord with the theoretical predictions, which confirms
the feasibility of the alloying approach. Thus, the implications of this work
are important not only for terawatt scale photovoltaics, but also for other
large-scale energy technologies where defect-tolerant semiconductors with high
quality electronic properties are required
Perovskite nitrides: A new playground for functional materials
The perovskite crystal is a favorite playground for electroceramists across a wide variety of applications, and recent developments on hybrid metallorganic perovskite photovoltaics has renewed interest in expanding the chemical space of this flexible and multifunctional crystal structure. A survey of experimentally confirmed simple perovskite compounds (ABX3) finds no reports of pure X=N anion chemistries. One challenge of forming nitride perovskite materials is the high valence cations needed to satisfy the high valency of nitrogen; another is limiting oxygen impurities. Computational predictions of energetically favorable nitride perovskites have been reported[1] and DFT+LDA methods[2] suggest that the lowest energy state of LaWN3 is a non-centrosymmetric R3c type distorted perovskite structure with a spontaneous polarization of approximately 60µC/cm2 along the \u3c111\u3e polar axis. A relatively low energy barrier predicted for polarization reversal raises the possibility of ferroelectricity as well. Developing a ferroelectric nitride would greatly simplify integration of a number of functional (e.g., ferroelectric, piezoelectric, and more) properties directly with nitride semiconductors for a variety of integrated sensing and energy conversion applications. Here we report the experimental confirmation of oxygen-free LaWN3 as a perovskite (Fig. 1) using multiple fabrication approaches. Calculations show 5 different symmetries with very similar lattice energies (3 polar and 2 non-polar); refinements of x-ray and electron diffraction in conjunction with property measurements document the complexity of the LaWN3 system in addition to other closely-related perovskite nitrides.
[1] R. Sarmiento-Pérez et. al., Chemistry of Materials, 27, 5957 (2015)
[2] Y. Fang et. al., Physical Review B,95, 014111 (2017)
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