Extremely Efficient Thin-Film Tandem Cell

A team at the Helmholtz Centre Berlin HZB has fabri­cated and charac­terised a thin-film tandem solar cell made of perov­skite and CIGSe. They relied on a simple, robust fabri­cation process that is also suitable for scaling up to large surface areas. The tandem solar cell is a fully thin film device with an impressive effi­ciency of 21.6 %. With further improve­ments it might reach efficiencies above 30 %.

An extremely thin layer between CIGSe and Perovskite improves the efficiency of the tandemcell. (Source: HZB)

Tandem solar cells consist of two semi­conductors with different band gaps, enabling them to use a larger fraction of the solar spectrum for generating elec­tricity. This concept is particularly successful when conventional absorber layers such as silicon or copper indium gallium selenide (CIGSe) are combined with the new metal-halide semi­conductor perovskite. This is because perov­skites convert the blue, high-energy portions of sunlight into electrical energy very effi­ciently, while silicon or CIGSe convert the red and near-infrared portions effec­tively.

The researchers have now combined a thin-film solar cell made of CIGSe with a thin layer of perovskite. The CIGSe thin film was grown on a substrate, resulting in a CIGSe surface that is typi­cally rough and irregular. This has made the appli­cation of the perovskite top cell using wet-chemical methods more difficult thus far. The team in coopera­tion with Eindhoven Univer­sity of Tech­nology has now for the first time improved the properties of the tandem solar cell by applying an ultra-thin yet conformal inter­mediate layer to the CIGSe layer, then spin-coating the perovskite layer onto it at the HySPRINT lab of HZB. The tandem solar cell produced in this way converts 21.6 % of the solar spectrum into electrical energy. This power conversion effi­ciency remains stable.

Although tandem cells made of silicon and perov­skite achieve even higher effi­ciencies so far, theo­retically, CIGSe perovskite tandem cells could also achieve these efficiencies. In addition, the new CIGSe perovskite tandem cell is a fully thin film device. This means that the new tandem cell is made from thin layers of CIGSe and perovskite so that the material and energy consumed during its manu­facture are extremely low.

“This tandem cell was fabri­cated on a rough, untreated CIGS bottom cell, which simplifies manufacture and represents an enormous advantage in terms of mass production“, Rutger Schlat­mann, Director of the HZB Institute PVcomB, points out. The tandem solar cell was realised on an area of 0.8 square centimeters, which is consi­derably larger than the square-millimetre areas commonly used in labora­tory research. “Record values are only recog­nised for areas of one square centi­meter or more, yet our cell area is just slightly below this threshold. Therefore, we now focus to certify this tandem solar cell effi­ciency and its enormous performance by an inde­pendent insti­tution“, says Steve Albrecht, head of a Young Inves­tigator Group at HZB.

Using electron micro­scopy and other measure­ments, first author Marko Jost, a postdoc in Steve Albrecht’s team, and his colleagues analysed the conformal layer structure of the tandem cell and were also able to determine the contri­butions of the indi­vidual sub-cells to the perfor­mance of the tandem cell. Their study points out ways to further improve mono­lithic perov­skite/CIGSe tandem cells and achieve effi­ciencies of over 30 %. (Source: HZB)

Reference: M. Jošt et al.: 21.6%-Efficient Monolithic Perovskite/Cu(In,Ga)Se2 Tandem Solar Cells with Thin Conformal Hole Transport Layers for Integration on Rough Bottom Cell Surfaces, ACS Energy Lett., 4, 583 (2019); DOI: 10.1021/acsenergylett.9b00135

Link: Competence Centre Photovoltaics Berlin (PVcomB), Helmholtz-Centre Berlin HZB, Berlin, Germany

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