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 by using a mixed hole transport material, which was a composite of Spiro-OMeTAD and a conjugated small molecule, DRSBDT-2OT. The DRSBDT-2OT with acceptor-donor-acceptor (A-D-A) structure exhibits highly molecule planarity via noncovalent intramolecular S···O interactions, leading to comparatively low reorganization energy and high hole mobility. Thanks to the surface passivation effect 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 using Sprio-OMeTAD as HTM. These findings would pave the way for the rational design of planar hole-transport materials for mixed HTMs toward efficient and stable perovskite solar cells.