Spiro[fluorene-9,9′-xanthene]-based hole shuttle materials for effective defect passivation in perovskite solar cells

Abstract

The molecular engineering of the interface modulator between the perovskite and hole transporting material (HTM) is crucial to achieving satisfactory performance and stability of perovskite solar cells (PSCs). Here, cruciform-shaped dual functional organic materials, denoted as SPX-TPA and SPX-BT, are employed for surface passivation and hole transporting interfacial layers (HTILs) in MAPbI₃-based PSCs. The rigid three-dimensional conjugated spiro(fluorene-9,9′-xanthene) (SPX) core is identical to spiro-fluorene (existing in spiro-OMeTAD) except differentiated by one oxygen atom. This core unit can effectively adjust the HOMO level and inhibits intramolecular π–π stacking to extract holes from the adjacent perovskite layer. The small energy gap between SPX-TPA (50 meV) and the perovskite can effectively minimize voltage losses and promotes the hole shuttling process. The hydroxyl (–OH) group in the SPX unit forms hydrogen bonds with undercoordinated iodide (I⁻) and methyl ammonium (MA⁺) ions, suppressing the related defects. Lewis bases (O, N and S) in TPA and BT units can promote the passivation of undercoordinated Pb²⁺ and MA⁺ (Lewis acid) via Lewis acid–base interactions (Pb–S/O/N). As a result, the PSCs with SPX-TPA and SPX-BT exhibit significantly improved PCEs of 20.03% and 18.51%, respectively, while the PCE of the control device is 17.77%. The enhanced PV performance of SPX-TPA treated PSCs is ascribed to the well-aligned energy levels, superior hole mobility, and favorable film morphology.