Halogenated and Dopant-free Hole Transport Materials for <i>n-i-p</i> Perovskite Solar Cells

Hole-transport materials (HTMs) are essential for the efficient extraction and transport of holes from the perovskite layer to the electrodes, thus playing a crucial role in enhancing the performance of perovskite solar cells (PSCs). The introduction of halogens into the polymer-based HTMs contributes to the excellent charge transport and photovoltaic properties of the devices, which provides a promising avenue for improving PSC performance. In this study, three polymer-based HTMs, PM-2F (PM6), PM-4F, and PM-4Cl, were synthesized by strategically incorporating different numbers and types of halogen atoms. These polymers feature thiophene-substituted benzodithiophene (BDT-T) donor units and benzodithiophene-4,8-dione (BDD) acceptor units, along with longitudinal side-chain conjugation extensions. The effect of varying numbers and types of halogen atoms on the properties of HTMs and device performance was systematically investigated. The results indicated that PM-2F and PM-4F, which contain fluorine atoms in the branched chains, exhibit lower highest occupied molecular orbital energy levels, enhanced hole mobility, and superior electrical conductivity compared to PM-4Cl, which incorporates chlorine atoms in the branched chains. Therefore, due to the highest charge transport ability, energy levels highly compatible with perovskite materials, and effective defect passivation, the fluorine-substituted PM-2F and PM-4F achieved device efficiencies of 24.27 and 21.08%, respectively, with PM-2F outperforming PM-4F due to reduced fluorine-induced steric hindrance and optimized energy alignment. Furthermore, after 3500 h in a N₂ environment, devices based on fluorine-substituted groups maintained 85% of their initial efficiency, while devices based on chlorine-substituted groups showed a decrease to approximately 30% of their initial efficiency after only 700 h. The findings suggest that fluorine-substituted organic HTLs have a significant effect on the photovoltaic performance of PSCs, highlighting their potential in developing high-performance organic HTMs for future applications.