Surface and bulk molecular modulators are the key to
improving
the efficiency and stability of hybrid perovskite solar cells. However,
due to their low concentration, heterogeneous environments, and low
sample mass, it remains challenging to characterize their structure
and dynamics at the atomic level, as required to establish structure–activity
relationships. Nuclear magnetic resonance (NMR) spectroscopy has revealed
a wealth of information on the atomic-level structure of hybrid perovskites,
but the inherent insensitivity of NMR severely limits its utility
to characterize thin-film samples. Dynamic nuclear polarization (DNP)
can enhance NMR sensitivity by orders of magnitude, but DNP methods
for perovskite materials have so far been limited. Here, we determined
the factors that limit the efficiency of DNP NMR for perovskite samples
by systematically studying layered hybrid perovskite analogues. We
find that the fast-relaxing dynamic cation is the major impediment
to higher DNP efficiency, while microwave absorption and particle
morphology play a secondary role. We then show that the former can
be mitigated by deuteration, enabling 1H DNP enhancement
factors of up to 100, which can be harnessed to enhance signals from
dopants or additives present in very low concentrations. Specifically,
using this new DNP methodology at a high magnetic field and with small
sample volumes, we have recorded the NMR spectrum of the 20 nm (6
μg) passivating layer on a single perovskite thin film, revealing
a two-dimensional (2D) layered perovskite structure at the surface
that resembles the n = 1 homologue but which has
greater disorder than in bulk layered perovskites.