Researchers from the Northwestern Polytechnical University, in China, have developed a planar thin-film solar cell based on antimony trisulfide (Sb2S3) with a power conversion efficiency of 5.84%. As a way of comparison, most of the antimony trisulfide cells developed to date have efficiencies close to 4%.
This kind of cell typology has, so far, been far from reaching commercial production, due to the low crystallinity and high resistivity of the Sb2S3 film, which affects the device's performance in terms of efficiency. Sb2S3, however, has a good bandgap, ranging from 1.70 to 1.90 eV, and a remarkable light absorption coefficient, and the scientists built their device by applying an additive known as 4-Chloro3-nitrobenzenesulfonyl Chloride (CSCl), which they claim has been able to alleviate these issues.
The additive was spin-coated on a titanium oxide (TiO2) substrate and annealed to crystallization. Spiro-OMeTAD was used for the hole transport layer. “The introduction of CSCl enhances the crystallinity of Sb2S3 film with [a] large grain size,” they explained, in the paper Effective additive for enhancing the performance of Sb2S3 planar thin film solar cells, which was recently published in The Journal of Materiomics. “And the two terminal Cl of CSCl molecul[es] have interaction with Sb atoms, which is conducive to increasing electron density around Sb atoms.”
Furthermore, the scientists explained the use of CSCl did not change the thicknesses of Sb2S3 film and that it raises the Fermi levels of Sb2S3 film, which in turn increases the driving force for the electron transport from the cell's Sb2S3 and TiO2 layers. The Fermi levels define the efficient conversion of the energy of radiation into electrochemical energy.
Tested under a standard illumination of AM 1.5G, two devices with concentrations of 0.5% and 3% CSCl additive showed an average power conversion efficiency of 4.35% and 4.98%, respectively. When the CSCl concentration was set at 1.5%, the cell achieved an efficiency of 5.84%, an open-circuit voltage of 0.62 V, a short-circuit current density of 15.85 mA cm2 and a fill factor of 59.61%. “The Sb2S3 films with 0.5%, 1.5%, [and] 3% CSCl additive show an obvious improvement in the light absorption capacity, which originates from the enhanced crystallinity of Sb2S3 film,” the Chinese group stated.
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