Cadmium sulfide, a poor partner for selenium tandem solar cells


Scientists from the Technical University of Denmark and Germany's Helmholtz Center Berlin (HZB) have investigated how cadmium sulfide (CdS) may be used as an n-type partner in tandem solar cells based on selenium.
“We deposited CdS using a chemical bath, which is extremely simple, cheap and scalable, but we found that CdS is intrinsically a poor choice for use in selenium thin-film solar cells,” researcher Rasmus Nielsen told pv magazine. “The reason for this poor relationship between CdS and selenium (Se) is very similar to that of kesterite (CZTS), which more specifically relates to the electronic band structure of CdS; our study, therefore, discourages researchers from going down the rabbit hole in which the research on CZTS has been stuck for a decade!”
The scientists improved chemical bath conditions to achieve full coverage of the CdS film on the rough surface of the cell's fluorine-doped tin oxide (FTO) substrate.
“The morphology of the Se layer grown on CdS is similar to the case of Se grown on the standard n-type partner TiO2, with crystal grains extending from top to bottom and no apparent pinhole,” the Danish-German group said.
They analyzed the energy band positions of CdS and polycrystalline selenium through photoelectron spectroscopy and optical transmission measurement. Further measurements showed that the solar cell reached an efficiency of only 1.14%, an open-circuit voltage of 472 mV, and a fill factor of 30.8%.
“The difference in electron affinities combined with the proximity of the bandgap of CdS to that of poly-Se suggests that pure CdS is fundamentally a poor heterojunction partner for poly-Se, and while a chemical bath-deposited buffer layer may enable an inversion of the device architecture, our findings indicate that an altogether different partner material is needed to realize the full potential of poly-Se,” the researchers concluded.
They presented the solar cell in “Selenium Thin-Film Solar Cells with Cadmium Sulfide as a Heterojunction Partner,” which was recently published in ACS Publications.
Earlier this year, researchers at the Technical University of Denmark developed a bifacial selenium solar cell that could be used as a top cell in tandem PV devices. The 300 nm bifacial device showed a power conversion efficiency of 5,2% on the front side through an n-type contact and 2.7% on the rear side through a p-type contact. Its short-circuit current was 10.96 mA for the front side and 7.40 mA on the backside, and the respective open-circuit voltage values were 0.91 V and 0.88 V.
According to Nielsen, low material use in selenium solar cells suggests that world production of selenium is sufficient to allow scaling to several hundreds of GWp of high-efficiency tandem solar cells with efficiencies well above the fundamental limit of silicon.
“Only about 300 nm of Se is enough to absorb and efficiently collect all photons above its bandgap so the amount of selenium needed for meaningful implementation is modest,” he told pv magazine. “Three times less material is needed when compared to most other promising thin-film PV materials, including perovskites, CIGS, CZTS, and CdTe.”

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