Temporal and spatial pinhole constraints in small-molecule hole transport layers for stable and efficient perovskite photovoltaics

Small molecules, such as Spiro-OMeTAD, are widely used in photovoltaic devices including perovskite and dye-sensitized solar cells due to their excellent hole transport properties. However, this kind of small molecule is often used with hygroscopic additives and dopants suffering from aggregation and demixing under environmental stress and thus leading to pinholes/voids in the resultant films. By using typical molecular Spiro-OMeTAD as an example, we herein introduced a hydrophobic polymeric poly(4-vinylpyridine) (P4VP) component to substantially reduce the undesired pinholes during device fabrication and operation. It leads to a significant improvement of the long-term stability of devices, which kept 80% of their original PCE for over 6000 h in ambient air and the promotion of light stability. In addition, P4VP can saturate uncoordinated Pb sites in perovskite crystals, resulting in improved device performance (certified PCE of 20.6%) with negligible hysteresis. These results emphasize the effects of pinhole constraints in both the temporal and spatial perspectives, which provides a feasible approach to improve the stability of small-molecule-based hole transport materials for perovskite solar cells and other optoelectronic devices.