The combination of optical transparency and bipolar conductivity in a single material, implying the possibility of a transparent p-n homo-junction, is a lucrative prospect, as it would simplify deposition and minimise strain at the interface between the p-and n-type layers within a device (compared to a p-n hetero-junction such as SnO-SnO 2 ). Achieving bipolarity in BaSnO 3 is particularly appealing due to its perovskite structure, which could enable a transparent p-n homo-junction as part of a larger perovskite oxide device with relative ease. In this Letter, the basic crystal and electronic structure properties of BaSnO 3 are reported, and group 1 and group 13 cations are considered as p-type defects using hybrid and 8.57 m e . The indirect and direct gaps are 3.19 eV and 3.59 eV, respectively, and are in excellent agreement with previous computational studies. (a) Crystal structure of BaSnO 3 . Defect chemistry. -To make p-type BaSnO 3 , low energy p-type acceptor defects must be introduced close to the valence band maximum (VBM), while n-type donor defects must be minimised. Previous hybrid DFT calculations have identified the oxygen vacancy, V O , as the lowest energy n-type species -this defect is recalculated in the present work (with excellent agreement in formation energy and transition level position) and is assumed to be the main charge compensation centre. The group 1 metals Li, Na and K (referred to as M I ), as well as the group 13 metals Al, Ga and In (referred to as M XIII ), are considered as candidate dopants, and the types of defect they can form are summarised as follows: M I Ba , M I Sn and M XIII Sn substitutions can accept electrons from the VBM and act as p-type species; M XIII Ba , M I int and M XIII int can donate electrons to the CBM and act as n-type species. The transition level diagrams for M I and M XIII doping under the most p-type growth conditions (metal poor, oxygen rich) are shown in Figure , and a summary of Shannon ionic radii and Goldschmidt tolerance factors can be found in Table -Considering first the M I dopants, it can be seen that the lowest energy p-type species is the K Ba substitution, with a formation energy of just over 2 eV and a relatively shallow transition level around 300 meV from the VBM. However, as highlighted at point A in Figure , the hole generated by the K Ba defect is charge compensated by the electrons introduced by V O , pinning the Fermi level in the band gap. In fact, the oxygen vacancy is low enough in energy to charge compensate all the p-type M I substitutions, severely compromising their ability to generate net p-type charge carriers in BaSnO 3 .