Pathway to oxide photovoltaics via band-structure engineering of SnO

Abstract

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 absorption 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 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 Sn 1−x Zn x O 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.

Growth and Characterization of Cu2MnSnS4 Thin Films Synthesized by Spray Pyrolysis under Air Atmosphere

“…Nie et al have also observed an increase of the band gap energy in CMTS films due to the crystallization of sulfide impurity which exhibits a band gap energy of about 2 eV as deposited [16]. Furthermore, the decrease of the optical band gap with the deposition temperature is coherent with the crystallization of secondary phases having an increasingly smaller band gap energy with the elevation of the deposition temperature (e.g., CuS~3.5-3.8 eV [45], SnO~2.5-2.9 eV [46], CuO~1.2-2.2eV [47,48]). In addition, other factors could also contribute to the observed bandgap decrease: the Cu enrichment of the tetragonal phase associated with the sulfur loss, as observed by EDS, could change the degree of hybridization [25,49], and the improvement of crystalline quality of the main CMTS phase would lead to a lower number of structural defects and grain boundaries in agreement with XRD results.…”

Towards bipolar tin monoxide: Revealing unexplored dopants

“…Ample experimental studies of nano-and polycrystalline tin monoxide report transparency of 70% or greater in the visible range [14]. Optical properties can also be enhanced further via applied stress/strain or through external doping [46][47][48]. As such, the preservation of transparency may also be a relevant criteria when selecting dopants for SnO targeted towards specific applications.…”

Ambipolar SnOx thin-film transistors achieved at high sputtering power

“…12 The ambipolar behavior of SnO has been thought to be due to its low electron effective mass (~0.4 m0), low hole effective mass (~0.6 m0), and small fundamental bandgap (~0.7 eV). 11,16,17 However, the density of subgap states in some p-type SnO TFTs extracted by temperature-dependent field-effect results can be higher than 10 19 eV -1 cm -3 , 15,18 which supresses the ambipolar behavior. In 2011, Nomura et al fabricated the first SnO-based ambipolar TFT.…”