Quasi-2D metal halide perovskites (MHPs) employed in photovoltaics involve the use of monoammonium or diammonium spacer cations to form the Ruddlesden-Popper or the Dion-Jacobson phases, respectively.¹ Moreover, the chemical nature of the bulky cations plays a significant role in the optoelectronic properties of the quasi-2D MHPs, controlling the orientation of the inorganic octahedral layers, the interlayer distance and thickness of the inorganic layers, phase distribution and the dielectric constant and dipole moment of the organic barriers which all determines the charge transport in the quasi-2D MHPs.² Moreover, additive engineering and processing strategies can also modulate the charge carrier transport.

We have used a p-conjugated bulky cation, 4,4′-diaminostilbene dihydrochloride (Sb), to synthesize a Dion-Jacobson 2D MHPs that is employed to fabricate photovoltaic devices. The relative ratio of the employed precursors is selected to define the n = 5 phase, (Sb)FA_2.8MA_1.2Pb₅I₁₅. However, a distribution of low dimensional phases is found (n = 1, 2 and 3) although other phases with longer n values are also present. The quasi-2D DJ MHP in the film is highly oriented and the deactivation of the photoexcited charge carriers (mostly excitons) is through a radiative recombination pathway assisted by the quantum confinement in the low dimensional phases. This fast deactivation of the excited state prevents a rapid extraction of the charges from the material and therefore, the short circuit current (J_sc) in the solar devices prepared with this perovskite is low, J_sc = 9.03 mA·cm^-2.

On the contrary, the addition of the MASCN additive to the initial precursor solution modifies the distribution of the phases in the film. Thus, the low dimensional phases are scarce now but phases with higher dimensionality are more present in these films. This is clearly demonstrated by the absence of steady-state and transient absorption features of the low dimensional phases. Moreover, the PL decays display a much longer time constant, which is a hint for a less intense quantum confinement in these samples. The reduction of the defect concentration calculated with the space-charge limited current measurements suggests that the excess of the stilbene derivative is localized in between the perovskite grain passivating the superficial defects and, therefore, reducing the non-radiative recombination pathway.