Precisely controlled copper indium gallium diselenide (CuIn1-XGaXSe2; x = 0 to 1) nanoparticles (NPs) are synthesized by using a simple low-cost, environment friendly chemical ion reduction method. A versatile polyethylene glycol (PEG) is used as solvent as well as stabilizer, which allowed the synthesis of NPs at higher working temperature (similar to 300 degrees C). CuInXGa1-XSe2 NPs with precisely controlled properties and high degree of crystallinity are reported. The samples revealed tetragonal crystal structure with systematic variation in 'd' values for (1 1 2) reflection demonstrate the synthesis of CIS, CIGS and CGS. The variation in the interplanar distances 'd' for (1 1 2) is further confirmed with HRTEM analysis. The sharp spotted rings observed from SAED pattern demonstrate the polycrystalline grwoth of CuIn1-XGaXSe2 NPs. Non-uniform CuInXGa1-XSe2 NPs of size ranging from 20 to 100 nm are observed from TEM analyses. However, upon annealing the final devices at 450 degrees C for 20 min. the NP layers become the polycrystalline alloy of materials with uniform cluster size between 100 and 200 nm. The optical absorption results revealed the addition of Ga in the crystal structure with systematic blue shift in the absorption wavelength. The superstrate solar cell developed from CIS, CIGS and CGS NPs measured efficiency 7.80, 9.33 and 9.04%, respectively upon illumination with 100 mW/cm(2). The yield of NPs was over 90%, therefore, the synthesis procedure may be suitable for commercialization purpose and for preparation of flexible solar cells.