Turning our focus to the chemical purity of our t-Se samples, we perform time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements on photoabsorbers fabricated on RCA-cleaned Si substrates (Fig. ) in parallel with the absorber of the champion device to ensure a representative sample. The JV-curve of the experimental device, the reconstructed JV-curve from Suns-Voc assuming the short-circuit current density from experiment (green) or the detailed-balance (DB) limit (blue), and the DB-limit JV curve -using the photovoltaic band gap of t-Se, E PV g 27 , obtained from the inflection point of the external quantum efficiency (EQE) spectrum. The photovoltaic losses which account for the difference between curves have been explicitly highlighted, including parasitic series resistance (R S ), light management (absorption losses), and carrier management (recombination losses, indirect band edges, etc.). (c) The Urbach band tailing derived from EQE measurements (Fig. ). (d) Cross-sectional scanning electron microscopy (SEM) of the thin-film selenium photo-absorber synthesised on a silicon substrate for impurity analysis. (e) Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) results, integrated while sputtering through the bulk of the selenium thin-film. Among the unintentional impurities, we detect F and Cl, whereas O and Te were only observed at the surface and interface with the substrate (see Section S7)