Hybrid halide perovskites are excellent building-blocks for multi-junction architectures, that provide the prospect to overcome fundamental efficiency limits of single-junctions. While perovskite/silicon or all-perovskite tandem cells have shown some remarkable progress, as of yet, perovskite/organic tandem cells show subpar efficiencies of ~20 per cent, limited by the low open circuit voltage of wide-gap perovskite cells, and serious optical/electrical losses introduced by the interconnect between the sub-cells [1].

Organic and perovskite semiconductors share similar processing technologies, which makes them attractive partners in multi-junction architectures. More importantly, the introduction of non-fullerene acceptors has boosted the efficiency of organic solar cells to levels beyond 18 per cent [2].

Here, we demonstrate perovskite/organic tandem cells with an efficiency up to 24 per cent, setting a new milestone for perovskite/organic tandem devices, which now for the first time outperform the most efficient single junction perovskite cells in p-i-n architecture [3].

This achievement draws from progress in all parts of the tandem:

Firstly, our ternary organic sub-cells (PM6:Y6:PC61BM) provide an enhanced efficiency in the near infrared spectral region and complement the perovskite cell. Most strikingly, under the filtered illumination conditions in the tandem, where excitons are solely generated on the Y6 acceptor, they are outstandingly stable even under long-term continuous operation of more than 5000 hours. This result is also in notable contrast to all-perovskite tandems, where stability issues of Sn-based narrow‑gap perovskite cells are a very serious issue.

Secondly, we managed to overcome interfacial losses, that are the predominant factors limiting the performance of wide-gap perovskite cells with high Br content. This allows us to access previously unreached territory of combined high open circuit voltage and fill factor.

Finally, we introduce a novel recombination interconnect for the two sub-cells, that is based on an ultra-thin (~1.5 nanometers) ALD grown indium oxide layer, that shows metallic properties and offers unprecedented low optical and electrical losses.

In an optimistic scenario, we envision that perovskite/organic tandem architectures bear a realistic prospect to reach efficiencies above 31 per cent.