The microwave-assisted chemical bath deposition (MACBD) process was successfully developed in this study for the preparation of cadmium sulfide (CdS) films as the buffer layers in Cu(In, Ga)Se-2 solar cells. The MACBD process reduces the reaction time to just 8 min. During the MACBD process, increasing the concentration of NH4OH in the solution effectively reduced the thickness of the films as well as the particle sizes of in CdS films. At high NH4OH concentrations, the heterogeneous nucleation of CdS dominated, and the formation of films was controlled via the ion-by-ion growth mechanism. Increasing the concentration of NH4OH from 1 to 2.5 M significantly increased the conversion efficiency of the fabricated CIGS solar cells from 7.15% to 9.12%. The increase in the efficiency was attributed to an increase in the absorption of incident light and the enhancement of the carrier collection due to a reduction in the thickness of the prepared CdS films. According to diode analysis, the improvement in the diode ideality factor and leakage current was owing to the uniform coverage of CdS films and a reduction in series resistance associated with a decrease in the thickness of CdS films. When the concentration of ammonia was further increased to 3 M, incomplete coverage of CdS films on CIGS layers resulted in the formation of shunt paths and degraded the cell performance. This study demonstrated that CdS films prepared using the MACBD process with the optimum concentration of ammonia effectively improved the photovoltaic performance of Cu(In, Ga)Se-2 solar cells.