Lead (Pb) based perovskite solar cells (PSCs) demonstrate a certified power conversion efficiency (PCE of 26.1%). Their toxicity, however, raises environmental concerns and might hinder their commercial deployment. Quest for non-toxic PSCs is still at its infancy: The PCE in Pb-free PSCs is merely around 14%, which is about one third of their radiative limit, which is because of the high voltage loss and a low fill factor in Pb-free PSCs.
In this work, focusing on the properties of tin-perovskites, we first improved their bulk properties. We employed methylammonium chloride (MACl)-assisted crystallization as a route to improve stability and optoelectronic quality of quasi 2D/3D PEA0.08FA0.92SnI3perovskite. For an optimal additive amount (10 mol%), a 37% increase in power conversion efficiency is found. Notably, MACl enhances the films’ stability, evidenced by temporal photoluminescence tracking. We shows that MACl addition causes a shift in the optical bandgap and improves morphology, indicating effects in the bulk crystal structure. X-ray photoelectron spectroscopy confirms the presence of Cl on the surface, but no indication of MA is found. Intriguingly, UV photoelectron spectroscopy shows pronounced changes in the density of states. For the first time, it is shown that MACl promotes the formation of a two-dimensional layer via the surface accumulation of PEA. The MACl additive lowers the absorber’s ionization energy, possibly facilitating hole extraction.
In another work, we demonstrate the importance of energy alignment between charge transport layers and perovskite in further enhancing the photovoltaic performance of tin-based PSCs. We alter the properties of widely used PEDOT:PSS hole transport layer (HTL) via embedding Ti3C2Tx MXene and investigate its impact on the photovoltaic properties of PSCs. Both scanning electron and atomic force microscopies show improved perovskite film formation with reduced pinhole, while Kelvin probe force microscopy concluded a lower variation in contact potential difference when MXene is embedded into PEDOT:PSS HTL. The work function of the HTL was increased according to photoelectron spectroscopy in air measurement, leading to a favourable energy alignment with the HOMO of PEA0.2FA0.8SnI3 perovskite. Coupled with an improved average carrier lifetime, PSCs fabricated using MXene-embedded PEDOT:PSS HTL shows a 13.5% improvement in photovoltaic performance as compared to the pristine PEDOT:PSS HTL counterpart, while retaining ~90% of its initial PCE after 450 hours of storage in nitrogen atmosphere.
