${\rm{Cu}}({\rm{In}},{\rm{Ga}}){\rm{S}}{{\rm{e}}_2}$ (CIGS) is a promising light harvesting material for large-area broadband photodetection, but it has been rarely studied up to now. Here an In2S3/CIGS heterojunction photodiode on steel is shown to be highly broadband photo-sensitive, with a photoresponsivity over 0.8 A/W, an external quantum efficiency over 100%, and a detectivity over 8×1010 Jones from 505 to 910nm under a reverse bias of 1 V. Moreover, the CIGS photodiode exhibits an outstanding weak light detection ability (i.e., at light power density of ${{20}}\;\unicode{x00B5} {\rm{W/c}}{{\rm{m}}^2}$), reaching a record responsivity of 2.06 A/W, an impressive EQE of 293%, and a good detectivity of ${2.3} \times {{1}}{{{0}}^{11}}$ Jones at 870 nm under 1 V reverse bias. Importantly, the CIGS photodiode, working as a self-powered photodetector, under 0 V, shows a record detectivity of ${\sim}{3.4} \times {{1}}{{{0}}^{12}}$ Jones with a high responsivity of ${\sim}{0.44}\;{\rm{A/W}}$ and a high EQE of ${\sim}{{63}}\%$, at 870 nm.