have been experimentally reported, 4,20-24 though a value around 1.95 eV is most common, which is slightly underestimated by our calculated direct gap of 1.83 eV. † Given the close match in calculated structural and dielectric parameters with experiment, it is suggested that this remaining mismatch is likely due to electronphonon coupling (band gap renormalisation at room temperature) and/or minor inaccuracies in the hybrid DFT functional (further discussion in Section S3.3). Notably, along with the low phonon frequencies shown in Fig. , we find quite a small bulk modulus of 15.0 GPa and inter-chain surface energy of 0.177 J/m 2 for t-Se (in good agreement with the experimental values of 14.9 GPa 88 and 0.175 J/m 2 108 ), indicating the facile compressibility and weak inter-chain binding. Despite a low enthalpic cost, the band gap remains sensitive to volume deformation, varying by ±0.25 eV while the energy varies by only 2 meV/atom within the ±5% volume range, as shown in Fig. . Strong temperature effects are common in vdW solids, and this combination of high deformation potentials and low elastic constants 4,30,88,109 is likely to yield significant thermal fluctuations of the band gap in t-Se. Indeed, this is evidenced by the close match of the calculated and measured electronic DOS (Fig. ) at high levels of thermal broadening (s = 0.5 eV; Fig. ), as well as the broad onset of absorption -with DFT showing a slightly higher energy onset despite the slight under-estimation of the band gap discussed above. Such strong thermal fluctuations are expected to contribute to the moderate Urbach energies measured for t-Se (E U = 44 meV; Fig. ) and reduced carrier mobilities, along with the possibility of locally-strained regions near interfaces with spatially-varying † Though notably Lu et al. found a direct band gap of 1.85 eV following their critical melting-annealing (CMA) strategy to obtain improved crystallinity for their t-Se samples. electronic potentials. If we account for the finite absorption of t-Se at energies just above the direct band gap, the short-circuit current density (J sc ) is still thickness-dependent at typical values of 100 -300 nm as shown in Fig. , despite the steep absorption onset (due to high band edge degeneracies as discussed above). Using the spectroscopically-limited maximum efficiency (SLME) metric, the maximum terrestrial solar PV efficiency is predicted to drop from 22.3 % at the radiative limit, to 21.1 % at absorber thicknesses t = 300 nm and 18.2 % at t = 100 nm ‡ Under indoor light sources (FL10 or WLED), a higher sensitivity to absorber thickness is found, with h max ' 51 %, 48 %, 38 % at thicknesses t = 1 µm, 300 nm and 100 nm. Importantly, this metric assumes perfect carrier collection and neglects non-radiative recombination, and so the optimal absorber thickness for real-world devices will be a trade-off between J sc -which can be boosted using structured surfaces and/or antireflective coatings 111,112 -and carrier collection. Se has only two possible native point defects: vacancies and interstitials. The calculated charge-dependent formation energies are shown in Fig. . Briefly, these diagrams plot the calculated formation energies of defects as a function of the Fermi level (electronic chemical potential). As defects can adopt various charge states depending on the occupation of their associated electronic states, and the formation energies of charged defects depend linearly on the Fermi level position (E f X,q µ qE F ; Eq. (S1)), the formation energy lines are sloped and only the lowest energy charge state at a given Fermi level is shown. The Fermi level positions at which the ground-state defect charge changes are termed thermodynamic charge transition levels, corresponding to the defect levels often represented in energy level diagrams as shown on the right-hand side of Fig. .