Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) materials have attracted considerable interest as a promising candidate for future thin film photovoltaic technology. This material family enjoys its compelling features of inexpensive and non/low toxic constituents, thermodynamically stable structure, and theoretical high power conversion efficiency defined by its suitable optoelectronic properties. However, on the other hand, a flexible quaternary kesterite structure induces much more complex defect chemistry in the crystal lattice, which leads to the distinct band- or potential-fluctuation, short minority carrier lifetime, and associated severe bulk as well as interface recombination. This thesis aims to tackle these problems by the addition of extrinsic elements (Ag, Cd, Ge, and Li) to the kesterite matrix (so-called doping/alloying strategy) for improving the electronic properties of CZTSSe solar cells. First, the substitution of Cu by Ag for CZTSSe absorbers has been achieved by annealing Ag contained precursor. The formation mechanism of Ag alloyed CZTSSe has been studied in detail. Second, a small amount of Cd on CZTSSe has been successfully incorporated by depositing a CdS layer on precursor film prior to selenization. The incorporation of Cd dramatically passivated GBs due to Cd segregation. As a result, the electronic properties of CZTSSe have been greatly improved. Third, The incorporation of Ge has effectively improved the VOC and FF of the CZTSSe solar cell. This enhancement is found to be mainly associated with modified defect characteristics instead of improved Na content in the CZTSSe bulk region. Then, we studied the effect of double cations (Cd and Ge) incorporation in electronic properties CZTSSe solar cell. This strategy simultaneously reduced nonradiative recombination at different regions of CZTSSe solar cells. Finally, a feasible solution-based post-deposition treatment process is developed for incorporating lithium into the CZTSSe absorber. The dominant acceptor defects CuZn antisites are replaced by shallower LiZn antisites, thus leading to enhanced effective p-type doping and reduced recombination, especially in the space charge region. Based on the results and analysis, this thesis elucidates new insights into the underlying mechanism of different doping/alloying strategies in kesterite-based materials