Perovskite photovoltaic PV cells have demonstrated power conversion efficiencies PCE that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1 amp; 8208;sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non amp; 8208;radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open amp; 8208;circuit voltage and the internal quasi amp; 8208;Fermi level splitting QFLS , the transport resistance amp; 8208;free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity amp; 8208;dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non amp; 8208;radiative fill factor and open amp; 8208;circuit voltage loss. It is found that potassium amp; 8208;passivated triple cation perovskite films stand out by their exceptionally high implied PCEs gt; 28 , which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limi