Monograin layer (MGL) solar cell technology based on CuIn1-xGaxSe2 (CIGSe) monograin powder crystals is a promising approach for the future low-cost production of flexible solar panels. In this study, CuIn1-xGaxSe2 monograin powders (0 <= x <= 1) were prepared from binary compounds in the liquid phase of potassium iodide as flux material in evacuated quartz ampoules at 720 degrees C. The crystal structure and the lattice parameters of the CIGSe monograin powder crystals were determined by using X-ray diffraction analysis. A linear decrease of the lattice parameters with increasing Ga concentration was detected. The photoluminescence (PL) spectra of the CIGSe crystals were dominated by the edge emission band that shifted towards higher energies with increasing Ga content. Moreover, additional deep PL band (below 1.0 eV) appeared for Ga contents above x = 0.21 showing higher relative intensity with increasing Ga content. The effective bandgap energy of the CIGSe monograin powder materials ranged from 1.0 eV to 1.68 eV as the [Ga]/([In] + [Ga]) ratio increased from 0 to 1.0. An efficiency of 12.8% (active area) was obtained with the MGL solar cell based on CuIn1-xGaxSe2 monograin powder with Ga content of x = 0.21.