The large band gap of t-Se (1.8 -2 eV) makes it an ideal absorber for single-junction indoor PV or a tandem PV topcell, with theoretical maximum power-conversion efficiencies of ⇠ 50 % (Fig. ) and ⇠ 40 % (if combined with a suitable low band gap material such as Si) respectively. While research on Se-based tandem PV is still in its infancy, a number of recent studies have demonstrated impressive efficiencies up to 18 % for single-junction indoor PV devices -surpassing those of market-dominant a-Si cells and lead-free perovskites. The first single-junction outdoor Se solar cell achieved a modest power-conversion efficiency h of less than 1% in 1883. Inorganic solar cell research moved on to diamond-like semiconductors, such as Si, CdTe, CuIn x Ga 1-x Se 2 (CIGS), and Cu 2 ZnSnS 4 (CZTS), and the efficiency of selenium solar cells stagnated for nearly a century. In the mid-1980s, work from two separate groups in Japan bumped record efficiencies to just over 5 %, with research interest then stalling once again. In 2017, the IBM team of Todorov et al. fabricated t-Se solar cells with h = 6.51 % and V oc = 0.97 V by overhauling the prevailing device architecture to give improved front and back contacts, using a FTO/ZnMgO/Se/MoO x /Au heterojunction stack and an ultrathin 100 nm Se film. The current certified record efficiency of 7.2 % was then achieved by Lu et al. a few months ago in 2024, using heterostructure similar to that of Nakada et al. , but featuring an FTO-coated glass with antireflective coating to boost the short-circuit current density.