In this work, the influence of partial substitution (up to 10%) of tin (Sn) by germanium (Ge) in Cu2ZnSn(S,Se)(4) (CZTSSe) solid solution monograins was studied. The external quantum efficiency measurements of the CZTGSSe monograin layer solar cells showed an increase in the band gap energy of the Ge-substituted CZTGSSe. According to the current-voltage measurements, the higher band gap energy is accompanied with a larger open circuit voltage deficit showing the lowest value for the Ge-free sample of around 785 mV. The temperature dependent current-voltage measurements revealed strong recombination losses at the CdS/CZTGSSe interface contributing to the voltage deficit. The radiative recombination processes in CZGTSSe monograins were studied using low-temperature photoluminescence (PL) spectroscopy. A detailed analysis of the PL spectra indicates that at low temperatures the recombination of free electrons with holes localized at acceptors related to the group IV element deeper than the mean energy depth of potential fluctuations is dominating.