AbstractCharacterizing subcells in two-terminal multi-junction (M-J) solar cells is challenging due to the lack of direct electrical access. This work presents a novel contactless spectral characterization technique for analysing individual subcells. The technique involves probing terahertz (THz) radiation generated by femtosecond laser pulse excitation and varying the exciting wavelength to selectively absorb light in the desired subcell. The registered THz pulse integral is then proportional to the induced photocurrent in that subcell. The THz photocurrent spectroscopy technique is demonstrated on GaAs and AlGaAs single-junction solar cells, as well as on the triple-junction AlGaAs/GaAs/GaAsBi solar cell. The results show that the recently developed GaAsBi-based subcell, with a nominal energy bandgap of 1.0 eV, exhibits improved electron–hole separation efficiency and can enhance energy harvesting by M-J solar cells.