Di-Spiro-Based Hole-Transporting Materials for Highly Efficient Perovskite Solar Cells

Hole-transporting materials (HTMs) are essential for enabling highly efficient perovskite solar cells (PVSCs) to extract and transport the hole carriers. Among numerous HTMs that are studied so far, the single-spiro-based compounds are the most frequently used HTMs for achieving highly efficient PVSCs. In fact, all the new spiro-based HTMs reported so far that render PVSCs over 20% are based on spiro[fluorene-9,9'-xanthene] or spiro [cyclopenta [2,1-b:3,4b']dithiophene-4,9'-fluorene] cores; therefore, there is a need to diversify the design of their structures for further improving their function and performance. In addition, the fundamental understanding of structure-performance relationships for the spiro-based HTMs is still lagging, for example, how molecular configuration, spiro numbers, and heteroatoms in spiro-rings impact the efficacy of HTMs. To address these needs, two novel H-shaped HTMs, G1 and G2 based on the di-spiro-rings as the cores are designed and synthesized. The combined good film-forming properties, better interactions with perovskite, slightly deeper highest occupied molecular orbital, higher mobility and conductivity, as well as more efficient charge transfer for G2 help devices reach a very impressive power conversion efficiency of 20.2% and good stability. This is the first report of demonstrating the feasibility of using di-spiro-based HTMs for highly efficient PVSCs.