Elemental selenium (Se) exhibits a number of low-energy allotropes in the solid state, both crystalline and amorphous, due to its ability to concatenate and form ring-and chain-like structures. While amorphous Se has been studied for photodetection applications, its crystalline form has received research attention as a candidate absorber material in photovoltaic (PV) devices, having been used in the first solar cell reported in 1883. The most stable polymorph at room temperature is the chain-like trigonal phase (t-Se) with space group P3 1 21 (shown in Fig. ), also termed 'hexagonal' (h-Se) or 'crystalline' Se (c-Se) due to the hexagonal lattice system and chain packing. Trigonal Se presents a number of advantageous properties for solar cells: low-temperature solution processing and vacuum deposition (aided by its low melting point of 220 C); good longterm stability in ambient, oxygenated and humid atmospheres; a wide band gap (tunable within 1.2 -2.0 eV via Te alloying ) ideally suited for visible-light photodiodes, indoor PV or top-cells in tandem devices ; a chain-like crystal structure which is likely to yield benign grain boundaries for electronhole recombination, to name a few. Moreover, the high vapour pressure of Se allows efficient recycling through closedspace evaporation, as demonstrated for indoor Se PV devices. Another benefit is the 'simple' chemistry of a single-element system which, in theory, reduces degrees of freedom in processing and characterisation. However, the limited chemical potential space is a double-edged sword, also reducing tunability and the scope of process engineering -particularly for the intrinsic defect chemistry.