Identifying New Solar Absorbers from Abundant Elements

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Apr 4 2022

In an article recently published in the journal ACS Energy Letters, researchers discussed the formation of solar absorbers that are abundant on the Earth.

Study: Emerging Earth-Abundant Solar Absorbers. Image Credit: Gorodenkoff/Shutterstock.com

Background

The holy grail of next-generation photovoltaics (PV) research has long been developing solar absorbers that are efficient, stable, low-cost, and are made of Earth-abundant, non-toxic materials. The introduction and quick rise in performance of lead-halide perovskites which are solution-processable have hampered this endeavor.

The capacity of these halide perovskites to bear point defects is one of its fundamental enabling qualities, allowing for effective PV performance despite large defect densities. This discovery has re-energized efforts in the Earth-abundant PV community to create efficient solar absorbers based on halide perovskites, with a specific focus on defect tolerance and long diffusion length materials.

At the same time, the vast diversity of Earth-abundant solar absorbers offers excellent chances to overcome the lead-halide perovskites' toxicity and stability constraints while remaining outside of the perovskite family of compounds. Chalcogenide perovskites, such as SrZrS₃, BaHfS₃, BaZrS₃, and SrHfS₃, as well as numerous additional Ti, Zr, and Hf-based compounds, are gaining popularity.

About the Study

The recent symposium on "Earth-abundant next-generation materials for solar energy" (Symposium F) at the 2021 Fall European Materials Research Society Meeting highlighted fascinating new possibilities created by the confluence of these two communities (held virtually).

In this study, the authors highlighted some of the important emerging themes presented during Symposium F, i.e., II-IV-N₂ compounds, chalcogenide perovskites, and antimony chalcogenides. The team also presented the computational search for novel defect-tolerant solar absorbers.

Observations

Many studies reported that chalcogenide perovskites were suitable for application as top cells in tandem with photoelectrochemical cells, silicon, and indoor photovoltaics, with typical bandgaps close to 2 eV. On single-crystalline LaAlO₃, SrTiO₃, or SrLaAlO₄ substrates, researchers had grown epitaxial thin films of pure-phase BaZrS₃ using molecular beam epitaxy (MBE), and pulsed laser deposition (PLD). The PLD films were found to have photoluminescence at ambient temperature with PL lifetimes of up to 8 ns. The preparation of these PLD and MBE films, however, necessitated processing temperatures in the 700-900°C range.

One of the studies reported that by using significantly more reactive amide-based precursors, BaTiS₃ colloidal nanorods could be prepared at temperatures as low as 280°C. Because these nanorods may be redispersed and precipitated in chloroform or toluene, they had the potential to create thin films using conventional nanocrystal processing methods. ZnSnN₂ (ZTN) and MgSnN₂ (MTN) were two II-IV-N₂ compounds that had received a lot of interest.

The development of a 0.37% power conversion efficiency heterojunction solar cell made up of n-type ZTN and p-type SnO was reported in one of the studies. The introduction of a thin Al₂O₃ layer with 23.3 nm thickness between SnO and ZTN resulted in a 1.54% increase in efficiency. Carrier mobility in Sb₂Se₃ was highly anisotropic due to its structure of one-dimensional chains, or nanoribbons.

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The current record efficiency was obtained by using close-spaced sublimation in a substrate configuration, in part by controlling Sb₂Se₃ orientation, which was highly dependent on the underlying Mo contact layer thickness. Se vapor flow or the usage of pre-deposited seed layers in a superstrate configuration further influenced crystal orientation control. A unique photoemission approach was utilized based on two-photon energies to investigate the band offset and curvature at the Sb₂Se₃-CdS and Sb₂Se₃-TiO₂ interfaces. It was found that while TiO₂ could operate as a diffusion barrier, it had a less favorable band offset with the absorber than CdS.

The bandgap of thin films of chalco-halides (e.g., SbSeI, BiOI, and Sn₂SbS₂I₃), antimony chalcogenides (e.g., Sb₂Se₃ and Sb₂S₃), binary halides (e.g., InI and BiI₃), ABZ₂ chalcogenides (e.g., NaSbS₂ and AgBiS₂), and perovskite derivatives (e.g., Cs₂AgBiCl₆) formed by reactive co-sputtering were found to match theoretical predictions. According to first-principles defect calculations, except for the nitrogen-vacancy, all native point defects were shallow, and nitrogen-vacancy introduced a shallow state of 0.24 eV below the conduction band minimum. YZn₃N₃ was proposed as a top cell absorber in tandem solar cells, with a bandgap of 1.8 eV.

Conclusions

In conclusion, this study elucidated that the Earth-abundant PV sector is undergoing numerous fascinating breakthroughs, ranging from innovative concepts such as cation disorder effects to unique materials entering new stages of research such as advances in low-temperature processing of chalcogenide perovskites.

The authors believe that in this context, the next few years will be full of opportunities for discovery and technological advancement.

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