Composite Hole-Transporting Materials Based on 9,10-Dimethoxyphenanthrene Cores and Spiro-OMeTAD for Efficient and Stable Perovskite Solar Cells

The hole transport material (HTM) in perovskite solar
cells (PSCs)
is a critical component due to its profound influence on the hole
extraction, surface passivation, shielding the perovskite from moisture,
and oxygen directly impacting on the overall performance and stability
of the devices. The widely used HTM, spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirofluorene),
for n-i-p PSCs suffers from low conductivity and poor hole mobility
in its pristine form. In this work, we designed two structurally simple
and cost-effective isomeric small molecules (2,7-OPOT and 3,6-OPOT),
featuring a 9,10-dimethoxyphenanthrene core in a D-π-D structure,
and mixed them with spiro-OMeTAD to form composite HTMs, S-2,7-OPOT,
and S-3,6-OPOT. The champion device with S-3,6-OPOT-based composite
HTM attained a power conversion efficiency (PCE) of 18.8% (Jsc = 23.9 mA cm–2, Voc = 1.05 V, and FF = 74.92%), outperforming
devices based on S-2,7-OPOT (18.6%) and pristine spiro-OMeTAD (17.7%).
The S-3,6-OPOT-based PSC also displayed superior durability, retaining
over 81% of its initial PCE after 60 days of ambient storage condition
without encapsulation. These findings confirm that systematic mixing
of organic small molecules with spiro-OMeTAD is a promising approach
to improve the photovoltaic performance and durability of PSCs, even
with reduced dopant loading in spiro-OMeTAD.