In combination with the fact that t-Se films oriented along the caxis (chain direction) are expected to favour benign grain boundaries 103-105 -similar to the case of Sb 2 Se 3 solar cells, , this indicates that t-Se films primarily oriented along the c axis should exhibit greater photovoltaic efficiencies due to increases in both J sc and V oc . For the optical band gap, values in the range of 1.8 -2 eV § Here ? refers to the direction perpendicular to the helical Se chains in t-Se, equivalent to the a, b lattice directions, while k is parallel to the Se chains, equivalent to the c direction. ) with experimental data from Ultraviolet Photoemission Spectroscopy (UPS) measurements. The real-space charge densities of the conduction band minimum (CBM) and valence band maximum (VBM) are shown on the right-hand side. (b) Electronic band alignment to the vacuum level, calculated using surface slab calculations with HSE06+D3+SOC (see Methods) and showing the direct band gap, while those of F-doped SnO 2 (FTO), , Sb 2 Se 3 , and MAPbI 3 are taken from the literature for comparison. The experimentally-determined band positions from UPS measurements are overlaid in red. (c) Optical absorption of single-crystal t-Se, as a function of light propagation direction (i.e. perpendicular to polarization direction), with 0.1 eV Gaussian broadening. Experimental data from spectroscopic ellipsometry with integrating sphere is plotted alongside. The calculated and experimentally-measured polarisation-dependent complex dielectric functions are compared in Fig. , also showing good agreement.