Photophysical Reappraisal of Additives for Photovoltaic Systems: A Case Study on Three Hole Transporting Materials and Two Dopants

Abstract

A study on the dissimilar interaction of two photovoltaic additives, FK209 or the combination of LiTFSI and TBP dopants, with three structurally different molecular hole-transporting materials (HTMs) previously used in perovskite solar cells (PSCs) is presented. The investigated HTMs are Spiro-OMeTAD and a toroidally delocalized oligotriarylamine-hexaarylbenzene derivative (HAB1), both globular and structurally rigid, as well as a globular but structurally fluxional oligotriarylamine-[60]fullerene hexakis-adduct (FU7). Photophysical characterisation with/without the additives in solution has been performed considering the changes in UV-vis absorption and fluorescence excitation/emission spectra of the HTMs, modulation of their emission quantum yields and lifetimes, analysis of the excited state quenching by the dopants, results of time-resolved fluorescence anisotropy assays accounting for changes in the size of the HTMs behaving as fluorophores, as well as evaluation of the singlet oxygen production by the HTMs. Whereas FK209 efficiently promotes charge transfer by association to the molecular HTMs, as evidenced by the observation of static quenching and variations of their rotational lifetimes due to increased fluorophore sizes; the charge transfer process with the LiTFSI and TBP system is only based on diffusion-controlled dynamic quenching of the excited HTMs, suggesting a weaker interaction, which is modulated by the accessibility of small hydrophobic molecules such as LiTFSI and TBP to the inner domains of the molecular HTMs.Remarkably, Spiro-OMeTAD shows competitive singlet oxygen photosensitisation by energy transfer, with nonnegligible quantum yield of 0.35. This study shows excellent agreement with previously reported results in PSCs, providing a better understanding of the underlying interactions between HTMs and dopants and, in turn, facilitating the optimisation of decisions regarding additives employed in photovoltaic devices.