Imec/ZSW perovskite/CIGS tandem cell achieves record 24.6% efficiency

At the 35th European Photovoltaic Solar Energy Conference (EU PVSEC 2018) in Brussels, Belgium (24-28 September), nanoelectronics research centre imec of Leuven, Belgium has presented a thin-film tandem solar cell consisting of a top perovskite cell developed by imec within the partnerships of EnergyVille and Solliance, and a bottom copper indium gallium diselenide (CIGS) cell from ZSW (Zentrum für Sonnenenergie- und Wasserstoff-Forschung — or Center for Solar Energy and Hydrogen Research — Baden-Württemberg) in Stuttgart, Germany. The tandem cell resulting from this collaboration (involving an international collaboration between imec, ZSW and Karlsruhe Institute of Technology) achieves a record efficiency of 24.6%.

The perovskite top cell uses light in the visible part of the solar spectrum, while the light in the near-IR spectrum that passes through the perovskite cell is harvested by the underlying CIGS cell. The tandem cell hence significantly outperforms the standalone perovskite and CIGS cells. Moreover, both perovskite and CIGS cells are thin-film solar cells, paving the way to high-efficiency flexible solar cells and building-integrated photovoltaic (BIPV) solutions.

The four-terminal tandem consists of a perovskite solar cell stacked on top of a CIGS cell, based on a fully scalable device concept to enable industrial adoption of the process. The record efficiency of 24.6% was achieved due to several innovations. First, the transmittance of the perovskite cell for near-IR light was improved by adding optical coupling layers to the tandem stack and by optimizing the transparent electrodes. Second, the perovskite itself was optimized in terms of a wide bandgap of 1.72eV for higher tandem efficiency.

The CIGS cell of 0.5cm2 size has been made on the high-efficiency line at ZSW, employing all optimized processes necessary for the preparation of record devices. Further improvements of the technology should ultimately pave the way to thin-film tandem solar cells with efficiencies of more than 30%, it is reckoned.

“We work on two types of tandem cells,” explains Tom Aernouts, group leader for thin-film photovoltaics at imec/EnergyVille. “We combine our state-of-the-art perovskite technology with silicon or CIGS bottom cells. The advantage of CIGS is that it is a thin-film technology, just like perovskite, and that the tandem cells therefore can be fabricated in a wide variety of shapes and sizes. This makes it possible to use this technology in building-integrated PV applications. Future work will focus on upscaling the technology towards larger modules. Moreover, we will also look into developing solutions for two-terminal cells because of their importance to the PV industry,” he adds.

“We owe the record efficiency to two factors,” says professor Michael Powalla, member of the board and head of the Photovoltaics Division at ZSW. “On the one hand to the improved perovskite cell, on the other hand to one of the world’s best efficiencies for CIGS cells,” he adds. “There are several parameters of the CIGS cell which we can optimize with regard to its combination with the perovskite top cell. Thus, we expect even better efficiency values in the future, paving the way to further cost reductions.”

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ZSW regains thin-film solar cell efficiency record with 22.6% CIGS PV cell

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