The sputtered-Zn(O,S) and -CdS films were prepared as a buffer layer in CuInSe2 (CIS) in the bottom cell of a Cu(In,Ga)Se2 (CIGS)-tandem-structured solar cell. We characterized the thermal stability and long wavelength response of the CIS solar cell. The thermal stability of CIS solar cell is tested in increasing annealing temperature up to 500 °C in a vacuum for 60 min. The optical response of CIS solar cell was measured by using long-wavelength pass filter to verify availability as a bottom cell. The power conversion efficiency (PCE) of the sputtered-Zn(O,S)/CIS cell showed a slight drop from 9.73% as-fabricated to 9.61% after 60 min of annealing at 300 °C, whereas the 7.97% as-fabricated PCE of the sputtered-CdS/CIS cell fully deteriorated above 300 °C at 60 min of annealing. The PCE of as-fabricated-Zn(O,S)/CIS was 5.46% by using the selective 715 nm long-pass filter. This value remains almost unchanged after post-annealing, whereas the 4.36% PCE of as-fabricated-CdS/CIS solar cell by using the 715 nm long-pass filter continuously dropped after 60 min of 300 °C annealing. The high-temperature-stability of the Zn(O,S) buffer was attributed to the smaller diffusivity of Zn atoms than that of Cd atoms.