Spin-orbit coupling was not included during defect geometry relaxation (adding unnecessary cost with negligible impact on structures as confirmed here), but included in a final static totalenergy calculation in each case. To account for spurious finite-size supercell effects, the Kumagai-Oba 66 (eFNV) charge correction scheme was used, as automated in doped 59 via the pydefect 67 API. Charge-carrier capture coefficients were calculated using the one-dimensional configuration coordinate approach of Alkauskas et al. , with electron-phonon coupling matrix elements computed using nonrad . CarrierCapture.jl was used to fit the potential energy surfaces (PES) and solve the 1D vibrational Schrödinger equation, allowing the use of anharmonic PES which are found to significantly impact recombination rates here. For more details on these calculations, the reader is directed to Refs. 61,68,69,73. AMSET 74 was used for deformation potential analysis, and aTLC 75 was used to analyse the effect of nonradiative recombination on PV parameters. Potential secondary phases in the presence of extrinsic species, which determine the chemical potential limits which enter the defect formation energy equation, were generated and parsed using the algorithms in doped . Here, all compounds which could border t-Se on the phase diagram, taking the Materials Project 76 energies and assuming an error range of 0.1 eV/atom, were re-computed with the HSE06+D3+SOC hybrid DFT functional. Pre-relaxed crystal structures of competing phases, using HSE06(+D3), were taken from calculations performed in Refs. 3,77-79 to expedite geometry optimisations. The SLME tools from pymatgen were combined with reference indoor light spectra from Ref. 83 to obtain thickness-dependent efficiency plots (Fig. ), and colour maps from cmcrameri were used in Fig. .