By Surbhi JainApr 4 2022Reviewed by Susha Cheriyedath, M.Sc.
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
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.
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.
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.
Choi, J. W., Shin, B., Gorai, P., et al. Emerging Earth-Abundant Solar Absorbers. ACS Energy Letters 7 1553-1557 (2022). https://pubs.acs.org/doi/10.1021/acsenergylett.2c00516
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