Transitioning global energy production away from carbon-emitting sources to renewable technologies is one of the foremost issues faced by the scientific community. Given the vast amount of solar energy incident on the planet, photovoltaic technologies will need to play a crucial role in decarbonization. To this end, metal halide perovskite solar cells (PSCs) have emerged as promising candidates for next-generation commercial photovoltaics primarily thanks to the exceptional power conversion efficiencies (PCE) achieved (>26%).

         Defect-induced non-radiative recombination and energy-level misalignment at the perovskite/electron transport (ETL) interface remain as key bottlenecks to further improving PSC performance. To systematically address these issues, we developed a novel class of highly tuneable ferrocene-based organometallic interlayers.

         Our first report focused on the excellent efficiency (25%) and stability (meeting IEC61215 standards) achieved using the FcTc₂ molecule and was largely attributed to surface passivation through C=O substituents.¹ Aiming to better understand the structural features responsible for the exceptional performance, oligo-ferrocene analogues Fc₂Tc₂ and Fc₃Tc₂ were investigated.² It was found that further improvements in performance (26.1%) could be achieved with additional ferrocene units. However, a direct correlation between molecular properties and their effects on the perovskite surface and photovoltaic performance remained unclear.

         In our latest research, we identified a novel avenue for tuneability in these ferrocene-based molecules which has gone unnoticed in the literature, electrochemical potential (Fc-E_HOMO). The E_HOMO of ferrocene can be tuned through substitution, so we introduced two novel molecules, FcFk₂ and FcFc₂, featuring structurally similar carboxyl-furyl substituents, responsible for surface passivation, whilst differing significantly in E_HOMO. This allowed us to isolate the structural features responsible for the energetic and passivating interaction. It was found that only when the Fc-E_HOMO (as in FcFc₂) lies above the perovskite VB is a proportional decrease in the surface work function observed. A decrease in surface work function promotes interfacial band bending, reducing minority (hole) charge carrier concentration, and thus improving open circuit voltage and fill factor in the resulting PSCs. When probing the literature, this unrecognized trend holds true for all published examples. Furthermore, the unique chemical interactions between the Fc molecules and PbI₂ were probed and the E_HOMO energies of the compounds involved were again found to play a key role. Ultimately, excellent performance (>25%) and stability (T₉₀>1000 h at 65 °C MPPT) were reached by adopting these strategies.

         This work serves to outline design rules for the directed synthesis of next-generation multifunctional ferrocene-based interlayers featuring optimized passivating groups and energetic alignment.

Figure 1: a) PSC structure and key structural features of the developed organometallic interlayers; b) relation of PVK surface work function with Fc-EHOMO; c) Surface potential distribution extracted from KPFM mapping; d) Unrealised correlation between surface work function and EHOMO; e) PSC JV data highlighting the differences in key photovoltaic parameters.

This work: Francesco Vanin, William D. J. Tremlett, Danpeng Gao, Qi Liu, Bo Li, Shuai Li, Jianqiu Gong, Xin Wu, Zhen Li, Ryan K. Brown, Liangchen Qian, Chunlei Zhang, Xianglang Sun, Xintong Li, Xiao Cheng Zeng, Zonglong Zhu, and Nicholas J. Long