  The incorporation of even small amounts of strontium (Sr) into lead-based
quadruple cation hybrid perovskite solar cells results in a systematic increase
of the open circuit voltage (Voc) in pin-type perovskite solar cells. We
demonstrate via transient and absolute photoluminescence (PL) experiments how
the incorporation of Sr significantly reduces the non-radiative recombination
losses in the neat perovskite layer and specifically at the perovskite/C60
interface. We show that Sr segregates at the perovskite surface, where it
induces important changes of morphology and energetics. Notably, the
Sr-enriched surface exhibits a wider band gap and a more n-type character,
accompanied with significantly stronger surface band bending. As a result, we
observe a significant increase of the quasi-Fermi level splitting in the neat
perovskite by reduced surface recombination and more importantly, a strong
reduction of losses attributed to non-radiative recombination at the interface
to the C60 electron-transporting layer. The resulting solar cells exhibited a
Voc of 1.18 V, which could be further improved to nearly 1.23 V through
addition of a thin polymer interlayer, bringing the non-radiative voltage loss
to only 110 meV. Our work shows that simply adding a small amount of Sr to the
precursor solutions induces a beneficial surface modification in the
perovskite, without requiring any post treatment, resulting in high efficiency
solar cells with power conversion efficiency (PCE) up to 20.3%. Our results
demonstrate very high Voc values and efficiencies in Sr-containing quadruple
cation perovskite pin solar cells and highlight the imperative importance of
addressing and minimizing the recombination losses at the interface between
perovskite and charge transporting layer.
