Lead acetate (PbAc₂) offers exceptional advantages as lead (Pb²⁺) source for solution processing of homogeneous and pinholes-free MAPbX₃ perovskite films over large areas at low temperatures for photovoltaic applications. However, PbAc₂-derived MAPbI₃ films usually exhibit small crystallites (<200 nm) and high densities of grain boundaries with their associated trap states, hindering the charge carrier transport which consequently leads to lower photovoltaic performance compared to state-of-the-art lead iodide (PbI₂)-derived films. In this study, we show that the use of chloride additives into PbAc₂-based precursor solutions overcome these issues and significantly improve the carrier mobility in the films, which leads to high fill factors (FF). It is also found that some chloride ions remain in the final films at the grain boundaries, passivating defect-generated trap states. Consequently, planar inverted perovskite solar cells (PSCs), with PEDOT:PSS and PCBM as charge-transporting layers, based on optimized Cl-doped MAPbI₃ layers showed improved FF up to 82%, among the best FF values for PSCs, along with a PCE over 16% and improved ambient stability, compared to the devices employing pristine PbAc₂-based MAPbI₃ films with an average FF of 74.5%. This investigation provides an insight into the factors influencing the FF in PSCs and proposes a strategy to further improve this parameter towards its radiative limit.