Molecular modification of spiro[fluorene-9,9′-xanthene]-based dopant-free hole transporting materials for perovskite solar cells

Abstract

The molecular engineering of organic hole-transporting materials (HTMs) plays an important role in enhancing the performance and stability of perovskite solar cells (PSCs) as well as reducing their fabrication cost. Here, two low-cost spiro-OMeTAD analogues, namely SP-Naph and SP-SMe, featuring a spiro[fluorene-9,9-xanthene] (SFX) central core and asymmetric subunits are designed and synthesized. Specifically, the SFX core in the SP-Naph molecule is substituted with dimethoxyphenylnaphthylamine subunits to enhance conductivity and charge transport properties by expansion of the π-conjugated structure. On the other hand, in the molecular structure of SP-SMe, the methoxy groups (–OMe) from diphenylamine units were partially replaced with the methylsulfanyl groups (–SMe) to increase interaction with the perovskite surface through the “Lewis soft” S atoms. By combining various experimental and simulation methods, thestructure–property relationship of the newly synthesized HTMs was thoroughly investigated. The suitable HOMO energy level with the perovskite layer together with superior photoelectric properties and enhanced thermostability and humidity resistivity are obtained for the SP-SMe HTM. As a result, the planar n–i–p PSC with the dopant-free SP-SMe HTM yields a maximum power conversion efficiency (PCE) of 21.95%, which outperforms that with SP-Naph (20.51%) and doped spiro-OMeTAD (19.23%). Importantly, the device with SP-SMe also reveals enhanced operational stability under continuous 1 sun illumination and thermal stability at 65 °C. These findings provide valuable insight for the rational design of dopant-free organic HTMs based on the SFX core, which would promote the development of highly efficient and stable devices.