For over a decade, a large number of researchers have been investigated organic and inorganic perovskites as potential materials for next-generation solar cells. In order to achieve a higher theoretical efficiency and possibly lower the negative attitude of the community towards the usage of lead, during that period scientists started to look into a Pb-less of even Pb-free perovskite materials, which could possess an ideal bandgap in a range of 1.1 to 1.3 eV.
So far, Sn-Pb (1:1) mixed perovskite is one of a few currently existing alternatives, that can be comparable to the regular Pb-based perovskites. Additionally, due to the possible bandgap of 1.25 eV of Sn–Pb perovskites, the development of perovskite/perovskite tandem solar cells was made possible. However, when fully replacing lead and having only Sn\(^{2+}\) in the perovskite crystal structure, a number of disadvantages occurs, such as more susceptible surface characteristics, inhomogeneous thin film morphologies, and easy Sn\(^{2+}\) oxidation [1]. A number of advancements in surface treatments, precursor design, compositional design, and structural optimization have been done so far, but as the efficiency record of Pb-free PSCs lags behind of Pb-based solar cells (14.81% and 27%, respectively [2]), there is still much more room for further improvement.
Self-assembling monolayer forming compounds have contributed significantly to the development of p-i-n architecture PSCs by increasing the efficiency and stability of the devices. However, when Sn is introduced into the perovskite system, SAMs, which have been found to work optimally in Pb-based perovskites, do not exhibit equally good performance. In succession, carbazole-based SAM material (BrNH\(_{3}\)-4PACz) containing ionic groups demonstrated high performance in Pb-Sn mixed PSC because of well-matched energetic alignment between the Pb-Sn perovskite and ITO/BrNH\(_{3}\)-4PACz, and perovskite passivation at the buried interface [3]. According to these results, ionic self-assembled monolayers show promising potential for Pb-Sn-mixed PSCs, furthermore, ionic SAMs have the potential to be suitable in lead-free perovskites development.
In this work, we have synthesized and investigated self-assembled monolayer-forming benzimidazole ionic compounds that can be used to construct Sn-based perovskite solar cells.