Interfacial and bulk properties of hole transporting materials in perovskite solar cells: spiro-MeTAD spiro-OMeTAD

Two spiro-MeTAD compounds ( 1 and 2 ) were synthesized, characterized by experimental and quantum mechanical methods, and used as hole transporting materials (HTMs) in perovskite solar cells (PSCs). The new compounds differ from spiro-OMeTAD only by the presence of methyl substituents as compared to methoxy groups. This modification results in the absorption band blue shifting by ∼20 nm as compared to spiro-OMeTAD, increased glass transition temperature for 2, and reduced ionization potentials by 0.02-0.12 eV. Hole mobilities five times larger were obtained for spiro-MeTAD/spiro-MeTAD, which is maintained in the presence of additives. Despite this improvement, J - V measurements in PSCs resulted in a power conversion efficiency (PCE) of 17.2% and 17.05% for 1 and 2 HTMs, respectively, as compared to 19.24% for spiro-OMeTAD. Photoluminescence measurements of perovskite:HTM layers indicate much stronger quenching in the case of spiro-OMeTAD/spiro-MeTAD. These results point to the dominant importance of the perovskite:HTM interfacial properties as compared to the HTM hole-transport properties in the bulk. Given that improved hole-mobility and energy-level alignment are the main targets of the current research efforts in this domain, our results alert to the necessity to prioritize the improvement of perovskite-HTM interaction properties. Improving perovskite-HTM interface properties in solar cells is more important than just improving HTM hole mobility.