Imec hits 27.1% on perovskite/silicon tandem cell

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Imec today announced that it has combined a spin coated perovskite layer, developed within its Solliance cooperation, with an interdigitated back contact (IBC) silicon solar cell, and achieved 27.1% conversion efficiency. Similar to other companies and research institutes working on this technology, Imec states that through careful engineering of its perovskite material, efficiencies of than 30% are well withing reach.

The tandem cell that Imec used to achieve the 27.1% efficiency comprised a perovskite layer measuring 0.13cm², stacked on top a 4cm² silicon IBC cell. When the perovskite layer was extended to cover the full 4cm², the researchers recorded cell efficiency of 25.3%, which they point out is still an improvement on the 23% efficiency of the silicon cell alone.

“The main impact is created by the interconnection losses going from small area single cell to series interconnected cells in the 4cm² module,” explains Tom Aernouts, R&D Manager for thin film Photovoltaics at imec. “[There are] small series resistance losses in transparent electrodes but mainly some area loss in between cells for the interconnection. We are working to further reduce these losses.”

The researchers also noted that, if carefully engineered, a semitransparent perovskite layer can serve to minimize thermal losses occurring in the silicon cell, as well converting sunlight itself. The team also points to band-gap tuning as the key development in achieving this level of efficiency. “We have been working on this tandem technology for two years now,” explains imec researcher Manoj Jaysankar. “The biggest difference with previous versions is in the engineering and processing of the Perovskite absorber, tuning its bandgap to optimize the efficiency for tandem configuration with silicon.”

While issues with perovskite material’s stability under field conditions have been a major issue holding it back from commercial development, imec joined other research groups in stating that strong progress has been made in this area.

“The limiting device for long term stability is currently indeed the perovskite cell or module,” explains Aernouts. “The intrinsic stability of the perovskite material used here is sufficient to survive thermal testing at 85°C for more than 1000hrs, a typical IEC standard test condition. Moreover, we haven’t observed any negative impact on stability so far when going from single cell to interconnected cells in module configuration. Further stability testing on the perovskite is ongoing (within our Solliance partnership), and generally good progress is made in improving the stability of perovskite PV devices.”

The team from imec will now focus on pushing its tandem cell to efficiencies beyond 30%, and is inviting companies from across the supply chain to partner with it in doing so. Aernouts also told pv magazine that this path to higher efficiencies would focus on fine-tuning materials and using new configurations: “In this 4T configuration, some further improvement can be obtained by finetuning the transparency of the electrodes even more to reduce the losses and reach a 30% efficiency,” says Aernouts. “we will also look into 2T configurations where indeed the optical boundary conditions for intermediate electrodes are even less strict, allowing even better light incoupling.”

Imec’s latest efficiency record is approaching that of UK/Germany based perovskite specialist Oxford PV, which has hit 27.3% efficiency on a 1cm² tandem cell.