  Among various types of perovskite-based tandem solar cells (TSCs),
all-perovskite TSCs are of particular attractiveness for building- and
vehicle-integrated photovoltaics, or space energy areas as they can be
fabricated on flexible and lightweight substrates with a very high
power-to-weight ratio. However, the efficiency of flexible all-perovskite
tandems is lagging far behind their rigid counterparts primarily due to the
challenges in developing efficient wide-bandgap (WBG) perovskite solar cells on
the flexible substrates as well as the low open-circuit voltage (VOC) in the
WBG perovskite subcell. Here, we report that the use of self-assembled
monolayers as hole-selective contact effectively suppresses the interfacial
recombination and allows the subsequent uniform growth of a 1.77 eV WBG
perovskite with superior optoelectronic quality. In addition, we employ a
post-deposition treatment with 2-thiopheneethylammonium chloride to further
suppress the bulk and interfacial recombination, boosting the VOC of the WBG
top cell to 1.29 V. Based on this, we present the first proof-of-concept
four-terminal all-perovskite flexible TSC with a PCE of 22.6%. When integrating
into two-terminal flexible tandems, we achieved 23.8% flexible all-perovskite
TSCs with a superior VOC of 2.1 V, which is on par with the VOC reported on the
28% all-perovskite tandems grown on the rigid substrate.

Applied Physics