A new simple environmentally benign water-soluble zwitterion, sulfamic acid (SA), was used as a multifunctional additive to tune the properties of PEDOT:PSS-based hole transporting layer (HTL). A layer of PEDOT:PSS was inserted in-between the ITO electrode and SA-modified PEDOT:PSS to form a pseudo bi-layered PS/SA@PS HTL to protect the ITO electrode from damage by more acidic SA@PS. Inverted tin-based perovskite solar cells based on the pseudo bi-layered PS/SA@PS HTLs achieved the highest efficiency of 10.5% with very small current hysteresis. The cell lost only 5% of the initial efficiency by storing in a glovebox without packing for more than 2000 h. The functions of SA include increasing the conductivity and mobility of the HTL to extract and transport the hole facilely by changing the conformation of PEDOT chains via zwitterion-induced charge screening, elevating the work function of PEDOT:PSS to match the VB of TPsk via increasing the PSS– chains on the HTL surface, creating a more hydrophilic surface for depositing better quality tin perovskite film (TPsk with the chemical formula of FA0.98EDA0.01SnI3), and avoiding the oxidation Sn2+ to Sn4+ via interacting with Sn2+ ions (at the HTL/absorber interface) using the sulfite group on SA, which were revealed with various physicochemical data.