There have been significant efforts to enhance the performance of CZTS thin-film solar cells. However, the increase in series resistance, formation of the unfavorable MoS2 between the absorber layer and back-contact, and recombination losses are some of the most effective factors for the low values of the efficiency and open-circuit voltage parameters. In this paper, five experimental CZTS solar cells with high efficiency of 8.4-11% have been mainly studied and discussed. Afterwards, two experimental works have been reproduced to validate our results. Additionally, with introducing the thin CZTS(x)Se((1-x)) layer between the Mo back-contact and CZTS absorber layer, the efficiency and open-circuit voltage of solar cells have been notably improved. There is a significant decrease in recombination losses (less than 12 mA/cm(2)) after using CZTS(x)Se((1-x)) intermediate layers between the absorber layer and back-contact. Using the CZTSe, and CZTS(0.2)Se(0.8) thin layers and optimizing the structure as p(+)pn structure (p(+)-CZTS(x)Se((1-x))/p-CZTS/n-CdS), the following efficiencies (and open-circuit voltage) are achieved as: 15.98% (921 mV) and 17.81% (986 mV), respectively. Simulations demonstrate that at 0.8-V bias for p(+)-CZTS(0.2)Se(0.8)/p-CZTS/n-CdS structure, the recombination current total density is reduced to 6.47 mA/cm(2). To obtain the highest performance in the proposed structures, the absorber layer carrier concentration should be 1 x 10(16) cm(-3) and the intermediate layer carrier concentration needs to be within the range of 1 x 10(17) cm(-3) to 1 x 10(18) cm(-3). Furthermore, the optimum thickness of CZTSe and CZTSSe intermediate layers in the proposed structures is 50 nm.