Researchers from Central South University, Imperial College London and City University of Hong Kong have reported a new class of tailored ferrocenium oxidants that function as highly efficient dopants for Spiro-OMeTAD in perovskite solar cells (PSCs). By tuning the reduction potential of ferrocenium species, the team achieved near-quantitative conversion of Spiro-OMeTAD to its oxidized form (Spiro-OMeTAD⁺), delivering optimal charge transport and energy-level alignment without the drawbacks of conventional lithium-based schemes.
Conventional PSCs relying on LiTFSI and 4-tert-butylpyridine (TBP) co-doping routinely suffer from hygroscopicity, volatility, and poor long-term reliability. To overcome these limitations, the researchers developed a one-step route to redox-tunable ferrocenium salts with specific anion selectivity. Among these, the dibromo-ferrocenium bis(trifluoromethylsulfonyl)imide (FcBr₂TFSI) compound proved particularly effective, offering strong oxidative power and enabling full Spiro-OMeTAD doping at ultra-low concentrations - just 5 mol%.
Perovskite solar cells fabricated with FcBr₂TFSI reached a certified power conversion efficiency (PCE) of 26.13% for single cells and 22.21% at the module level, outperforming standard LiTFSI/TBP-doped devices. Operational stability was also impressive: devices retained 95% of their initial efficiency after 1,000 hours of continuous operation under room-temperature maximum power point (MPP) tracking, and 87% when held at 65 °C. These results highlight both enhanced intrinsic material stability and improved interface quality at the perovskite/HTM junction.
Spectroscopic studies, particularly time-resolved photoluminescence, revealed that ferrocenium doping leads to faster and more efficient hole extraction at the perovskite/HTM interface, accompanied by extended charge-carrier lifetimes. The combination of low dopant concentration, high efficiency, and excellent durability underscores the balanced electronic and energetic properties of this approach.
Beyond its performance metrics, the work establishes ferrocenium oxidants as a modular, scalable route to doping organic semiconductors from first principles. Given ferrocene’s rich synthetic chemistry and commercial accessibility, this paradigm offers a rational design framework for single-component, Li-free, and O₂-free doping strategies. Such advances not only point to the future of perovskite photovoltaics but could also extend to the broader field of organic electronics where controlled doping remains pivotal.
