In summary, we characterise the intrinsic and extrinsic point defect chemistry of t-Se by both theoretical and experimental means. We first analyse the intrinsic structural and electronic properties of Se thin films; including their allotropic and highlydeformable nature, likely inducing tolerance to non-crystalline inclusions or ring-like formations; imperfect band-edge absorption, making achievable PV efficiency sensitive to film thickness and light-trapping strategies; strong thermal fluctuations in the band gap, possibly contributing to moderate Urbach energies (Fig. ) 146 ; and highly-anisotropic optical and electronic properties, highlighting the importance of controlled grain orientation in t-Se films. We find the crystal dimensionality to play a key role in the behaviour of defects in this system, favouring selfinterstitials and chalcogen substitutions while disfavouring vacancies and heterovalent defects, due to its structural flexibility, high energy cost of chain rupturing and pure-covalent bonding. Selfinterstitials are low-energy and electrically-neutral, while vacancies are electrically-active but higher energy, thus do not significantly contribute to (self-)doping. Analysing the effect of a range of relevant extrinsic impurities in t-Se samples, we show the presence of F, Cl, O and Te using ToF-SIMS measurements (Figs. 8 and S22), while Br and I are also expected to be present. Computing the electronic and energetic properties of these species and other likely impurities, we find that most extrinsic species are inactive for doping, either due to electrical-inactivity (chalcogens) or self-compensation (hydrogen, halogens, pnictogens). F interstitials are the most doping-active species, though only yielding modest hole concentrations of ⇠10 12 cm 3 , and so neither intrinsic nor extrinsic point defects are found to contribute significantly to doping.