Organic-inorganic lead halide perovskite semiconductor materials are undoubtedly emerging as low-cost alternatives to traditional materials used in photovoltaics devices. However, real world applications of perovskites are hampered by their intrinsic light instability due to point defects and charge migration as well as their heat and humidity instability, causing irreversible decomposition of the perovskite structure. In this regard, ample research has been carried out studying additives as mediators for prolonged halide perovskite solar cell lifetimes through passivationor dimensionality reductioneffects. On the one hand, thiophene-based molecular additives have shown to effectively passivate trap states by virtue of their Lewis base nature.¹ On the other hand, dimensionality control has been demonstrated with the incorporation of insulating organic cations such as phenylethylammonium or butylammonium. The implementation of organic chromophores that could replace these conventional cations appears to be a promising route to further tailor the optoelectronic properties of perovskites.² Thus we believe that thiophene-based organic cation molecules hold the key to access both of these stabilizing mechanisms by leveraging their Lewis base character, and ability to serve as media for the dimensionality reduction of three-dimensional perovskites, simultaneously bringing about some near-UV light absorption as a plus. In this way, such an approach would bring together two mechanisms for perovskite stability enhancement with a single additive.

In this work, the effect of thiophene-based cationic molecular components (TC) of 3D-2D perovskite materials have been explored. The space-charged limited current-defined hole mobility of the pure novel 2D material has been characterized together with its effect on the overall hole mobility of the 3D-2D composite. These encouraging results lead us to explore the possibility of a 2D perovskite hole-transport-material halide perovskite solar cell structure based on thiophene molecular cations which can simultaneously block water ingress into the sensitive three-dimensional halide perovskite and serve as a passivating agent. We first research several stoichiometries prepared by varying the n number in the preparation solution using the formula (TC)₂(MA)_n-1Pb_nI_3n+1. We then move on to study a gradient structure on perovskites (i.e. a 3D perovskite with a 2D perovskite capping layer) in several lead iodide to 3D cation ratio samples to find an optimum TC composition for maximum passivating effects and enhanced stabilities while keeping dimensionality reduction to a minimum to hamper the large influence of recombination effects from the high exciton binding energies of low-dimensional perovskites. From this we determine the optimized preparation conditions for implementation of a TC-based perovskite component into the conventional 3D perovskite structure. The presence of the novel thiophene-based materials was evidenced by characteristic photoluminescence peaks at 510 nm of the pure 2D perovskite, followed by the higher wavelength photoluminescence peaks of the 2D-3D perovskite derivatives. The out-of-plane orientation of the single 2D perovskite has been characterized with synchrotron GIWAXs data while the effect of thiophene molecular cations on 3D-2D perovskite morphologies and crystallinity have been characterized with scanning electron microscopy and x-ray diffraction, respectively.

This work expands the field of chromophore incorporation into lead halide perovskite structures and solar cells, so far limited to few molecular candidates based on naphthalene, pyrene and perylene³ as well as carbazoles.⁴