  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.
