The equipment was built by an international research team. The battery has a buffer layer based on zinc sulfide oxide and a low copper absorbent. The open circuit voltage of the battery is 920 mV. Although the material is different from the widely commercialized CIG selenide PV film, the researchers claim that a similar process can be used for its large-scale manufacturing.
Although it has good optical properties and band gap tunability, the performance of Cu(In,Ga)S2 solar cells is usually limited due to volume and interface recombination loss.
Pure chalcopyrite sulfide Cu(In,Ga)S2 is a promising semiconductor material with a band gap of 1.5 to 2.4 eV, which has been used to construct single junction and tandem solar cells. However, until now, due to the difficulty in reducing the voltage loss occurring at the body and its interface, the application of this material in the development of thin-film photovoltaic devices has been limited.
A team of researchers from the University of Luxembourg and Uppsala University in Sweden tried to meet this challenge by reducing the copper content in the material. "Compared to its close relative, pure selenide CIGS, there are many unanswered questions about pure sulfide CIGS," Sudhanshu Shukla, who led the research, told Photovoltaic Magazine. "Because of the lack of performance, the former has received less attention. We believe that our work will stimulate interest in the field and draw attention to the basic photophysics of materials and efficiency improvements."
The researchers used a type of Cu(In,Ga)S2 with a band gap of 1.6 eV as the battery absorber layer. According to them, this material with lower copper content has significantly reduced defects in quasi-Fermi level splitting (QFLS), which is a standard theoretical tool for describing the operation of semiconductor devices; and represents the maximum open circuit voltage that solar cells can achieve .
They fabricated a battery with a buffer layer made of cadmium sulfide (CdS) and a second device with a buffer layer based on zinc oxide. The latter exhibits a high open circuit voltage of 920 mV and the highest efficiency of 15.2%. The efficiency of the former is only 12.8%.
"The higher open circuit voltage is due to the higher QFLS, that is, the photoelectric quality of the absorber is better, and the loss due to the open circuit voltage is smaller," the scholar explained, adding that the improvement in interface quality also played a role in this process , Because it creates a more suitable conduction band alignment with the Zn(O,S) buffer layer. "In addition, the larger band gap of the Zn(O,S) buffer layer can absorb higher-energy photons, resulting in a higher short-circuit current density, which is consistent with the photocurrent gain."
The scientists added that the existing processes used to make CIG selenide films can easily be applied to similar sulfide materials. “The process developed for the production of commercial CIGSe thin-film solar cells can be transformed into the production of CIGS without the need for additional infrastructure,” Shukla explained. "Companies like Avancis and Flisom are already manufacturing large-area CIGS modules, and scalability is certainly possible."
The solar cell is described in the paper "Broad-gap Cu(In,Ga)S2 Solar Cells with Efficiency More Than 15%: Suppressing Volume and Interface Recombination through Composition Engineering", which was published in Joule.
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