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-(<i>N</i>,<i>N</i>-di-<i>p</i>-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% (<i>J</i> _sc = 23.9 mA cm^-2, <i>V</i> _oc = 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.