As the optical response above 600 nm is mainly due to PbS, the observed enhanced EQE response should come from PbS. If the optical change was considered, where the increase in optical absorption was noted only up to 600 nm due to increase in CdS amount, an increase in EQE should observe only in the 400–600 nm region but not in the 600–1200 nm region. Hence, the observed increase in Jsc with the increase in the thickness of CdS layer while keeping the PbS layer thickness the same, cannot be attributed to increase in light absorption by thicker CdS layer as evident by the light absorption spectra and EQE results.