Perovskite solar cells (PSCs) are a promising alternative to silicon-based photovoltaics, offering high efficiency and potentially low-cost manufacturing [1]. However, their widespread use remains hindered by degradation under moisture, oxygen, heat, bias, and illumination [1,2]. Passivation using spacer cations is a well-known strategy to improve stability. Nevertheless, the relation between the chemical features of the spacer, perovskite properties, charge-carrier dynamics, and photovoltaic (PV) performance remains incompletely understood [3].

Here, we investigate two aromatic spacer cations, m-toluidinium (To⁺) and m-phenylenediammonium (PDA²⁺), associated with Ruddlesden–Popper and Dion–Jacobson two-dimensional (2D) perovskite phases, respectively. Both spacers are incorporated into FA-based perovskite films [4] either as surface treatments using isopropanol (To-IPA and PDA-IPA) or by direct addition to the precursor solution for bulk incorporation (To-PSK and PDA-PSK).

The structural and optical features of these materials are assessed through X-ray Diffraction (XRD) and UV-vis spectroscopy, respectively, showing that all conditions preserved the α-phase and band gap of perovskite. Top-surface scanning electron microscopy (SEM) images reveal that, despite similar morphology, all passivated perovskites exhibit enlarged grains. In addition, Williamson-Hall analysis indicates that the incorporation of 2D spacers reduces the micro-strain in comparison to the control perovskite. Among the series, To-PSK exhibits the lowest micro-strain.

Photovoltaic performance is evaluated using J–V curves. To-based devices exhibit slightly reduced power conversion efficiencies (PCEs) compared to the control (21.6% for To-IPA, 21.5% for To-PSK, and 22.17% for the control). They also show superior stability under reverse-bias and light-stress conditions, retaining over 95% of their initial PCE, compared to 76% for the control. In contrast, PDA-based devices show a pronounced reduction in photocurrent density (23.3 mA cm⁻² for PDA-IPA and 19.3 mA cm⁻² for PDA-PSK, versus 24.5 mA cm⁻² for the control), resulting in lower PCEs of 16.6% and 13.7%, respectively. Notably, both PDA-containing devices exhibit enhanced reverse-bias stability relative to the control, particularly PDA-IPA, which retains 96% of its initial PCE.

Steady-state photoluminescence (PL) spectra reveal a subtle blue shift in bulk-modified films, likely related to small changes in the local structural or defect environment. Time-resolved PL (TRPL) measurements show that control films exhibit longer carrier lifetimes (151.6 ns), whereas PDA-modified films display faster decay (30.8 ns for PDA-IPA and 43.0 ns for PDA-PSK), consistent with enhanced non-radiative recombination. In contrast, To-containing films show lifetimes similar to the control (~140 ns), in agreement with their PV performance.

Electrochemical impedance spectroscopy (EIS) measurements performed in the dark at V_OC reveal pronounced low-frequency (LF) arcs for control and surface-treated devices, with LF resistances of 551 kΩ (control), 662 kΩ (To-IPA), and 322 kΩ (PDA-IPA), indicative of suppressed slow processes and reduced ion-assisted recombination. Conversely, bulk spacer incorporation reduces the LF resistance to 56 kΩ for PDA-PSK and 278 kΩ for To-PSK, suggesting enhanced ionic mobility and increased ion-assisted recombination within the perovskite bulk.

These findings demonstrate that spacer cation composition and incorporation route can establish a balance between stability and charge-carrier dynamics in 2D/3D perovskites. To⁺-modified films enhance operational stability while preserving charge transport; however, PDA²⁺, particularly when incorporated in the bulk, enhances the recombination and reduces current density. Thus, it highlights the need for spacer designs that improve stability without compromising charge transport in 2D/3D PSCs.