A mixed hole transport material employing a highly planar conjugated molecule for efficient and stable perovskite solar cells

We successfully demonstrated a simple and facile way to improve the efficiency and stability of perovskite solar cells using a mixed hole transport material, which was a composite of spiro-OMeTAD and a conjugated small molecule, DRSBDT-2OT. DRSBDT-2OT, with an acceptor-donor-acceptor (A-D-A) structure, exhibits a high molecular planarity via noncovalent intramolecular SMIDLINE HORIZONTAL ELLIPSISO interactions, leading to comparatively low reorganization energy and high hole mobility. Owing to the surface passivation effects of DRSBDT-2OT in the mixed HTM and the increased hole mobility in comparison with the spiro-OMeTAD, the mixed HTM-based device exhibits a significantly enhanced efficiency of 21.31%, which is much higher than that of single-component HTM-based devices. Furthermore, the mixed HTM-based devices are much more stable than the control devices that use Sprio-OMeTAD as the HTM. These findings pave the way for the rational design of planar hole-transport materials for mixed HTMs toward efficient and stable perovskite solar cells.