Fluorene chemistry as a design platform for polymeric HTLs: Substituent control and core linkages in perovskite photovoltaics

Polymeric hole transport layers (HTLs) are emerging as one of the most promising classes of hole transporting materials for inverted (p–i–n) perovskite solar cells, offering tunable molecular design, reliable film formation, and potential for scalable processing. Within this class, fluorene-based polymers stand out due to their rigid π-conjugated backbone, which imparts thermal stability and optical transparency, and the unique C9 substitution site, which enables precise control over solubility, morphology, interfacial chemistry, and energy alignment. By linking the fluorene core with alkyl, functionalized alkyl, vinylene, biphenyl/spiro, or in situ crosslinkable motifs, researchers have created a diverse family of HTLs that balance mobility, stability, and manufacturability. Recent studies show that well-engineered fluorene polymers can deliver power conversion efficiencies (PCEs) above 20% and retain over 90% of their initial performance after 1000 h of operational stress. Despite advances, challenges remain, as fabrication and stability inconsistencies hinder comparison, and few fluorene-based systems combine efficiency, stability, and scalability. Bridging this gap will require systematic mapping of C9 substitution patterns to device metrics, hybrid designs that merge complementary traits, and ISOS-compliant benchmarking. This review provides a unifying framework to guide the development of next-generation fluorene-based polymeric HTLs for durable, commercially viable perovskite photovoltaics.