Hole transport layers (HTLs) are one of the most significant elements influencing the performance of perovskite solar cells (PSCs). PEDOT:PSS (= Poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate)) is regarded to be the most conducting polymer to use as an organic HTL for the inverted structure of PSCs. Due to the highly acidic behavior, PEDOT:PSS-based PSCs have shown limited stability. Additionally, the valence band mismatch of the PEDOT:PSS layer with the adjacent perovskite layer often results in low open-circuit voltage (V_OC) of the overall PSC. To address these issues, we have incorporated a thin layer of ethylenediamine functionalized graphene (EDA-FG) between the indium tin oxide (ITO) anode and the PEDOT:PSS layer yielding a bilayer HTL. The new bilayer successfully matches the energy valence band at the HTLs/perovskite layer interface. The photoluminescence (PL) confirms that the deposited EDA-FG/PEDOT:PSS quenched the perovskite layer more than PEDOT:PSS. Based on the enhanced electrical and optical properties, a champion PCE of 17.66% was obtained for the respective PSCs. Additionally, the PSCs showed stable photovoltaic behavior up to 500 h.

The exceptional photovoltaic properties of Sn-based perovskite solar cells (Sn–PSCs) have caught the interest of researchers as a potential candidate for the development of non-toxic PSCs. Nevertheless, the Sn–PSCs still face some critical drawbacks, such as the easier oxidation of Sn²⁺ to Sn⁴⁺ and the moisture permeation, which reduce their stability and power conversion efficiency (PCE). In this work, to tackle these limitations, we added a Lewis base additive formamidinesulfinic acid (FASO₂H) to the perovskite precursor solution. The FASO₂H Lewis base additive could successfully reduce the oxidation of Sn²⁺/Sn⁴⁺ by 32%, improved the crystallinity and decreased the lattice strain by 66% in the respective films. As a result, the charge carrier recombination minimized and the FASnI₃-FASO₂H (3 mol%) based PSCs produced a PCE of 7.44%. The FASO₂H additive also offers superior moisture protection for Sn–PSCs thanks to its hydrophobic sulfinic acid group. Notably, the Sn–PSCs exhibit a high light soaking stability by maintaining 90% of the initial PCE up to 450 h when tested under maximum power point tracking conditions.