Finally, turning to the question of the remaining V oc deficit in Se solar cells, we recall our earlier conclusion that intrinsic point defects do not limit carrier lifetimes in t-Se. Extrinsic point defects or their complexes (with V Se or each other) could contribute to recombination. Several (non-chalcogen) impurities exhibit deep defect levels in the band gap as shown in Fig. and tabulated in Section S5. Recently, however, both Chen et al. and Li et al. independently found the amplitude of deep level transient spectroscopy signals for defect levels in Se and Se x Te 1-x alloys to be linearly dependent on the filling pulse widths, which suggests they originate from extended rather than point defects. Our predictions support this conclusion, indicating a significant role of extended defective regions in the defect chemistry of asgrown t-Se. Moreover, several recent experiments have reported large differences between acceptor densities (N A ) from capacitance-voltage (CV) and drive-level capacitance profiling (DLCP) measurements. N A,CV is sensitive to both bulk and interfacial defects, while N A,DLCP is typically sensitive only to bulk defects, and so the large differences (N A,CV N A,DLCP ) ⇠ 10 cm 3 indicate high interfacial defect concentrations. As such, extended defects, interfaces (whether at the edges of t-Se films or internally at non-crystalline inclusions), and/or grain boundaries are proposed to dominate recombination and V oc deficits in t-Se (despite the expectation of benign grain boundaries in vdW-bonded solids). Given these findings, future work could focus on measuring the recombination activity of t-Se devices as a function of impurity content, or predicting these parameters from first-principles, to investigate (and potentially rule out) the impacts of extrinsic species on recombination. Extended defects and interfaces generally present more challenges for characterisation (both theoretically and experimentally), but probing their impact is likely a fruitful route to furthering our understanding of defect chemistry in t-Se and improving device efficiencies. On a positive note, extended defects are often more amenable to materials engineering strategies. Potential strategies include trialling of alternative synthesis (e.g. molecular-beam epitaxy, halogen addition -given the observations of improved crystallinity etc.) or annealing procedures to affect crystallinity and extended defects, and analysing the effects on spectroscopic signals, charge-carrier lifetimes and V oc . Accurate methods for modelling extended defects are active area of research, particularly for electron-hole recombination, and further advances will facilitate their direct prediction. The identification of the key limiting species for open-circuit voltages would then facilitate targeted engineering strategies to avoid their deleterious effects and boost efficiencies for Se photovoltaics.