The bandgap of the ternary chalcogenide Cu2SnS3 (CTS) can be tuned by alloying with Ge. The performance of CTS based solar cell devices with varies band gap grading profiles have been simulated with respect to the solar cells with a uniform band gap absorbing layer. It was revealed that band gap engineering geared to controlling the grading profile in the absorber layer lead to performance enhancement comparing to that of a device without band gap grading. Moreover, bandgap profiles with various back metal working function (phi(m)) were simulated. An over 4-5% efficiency improvement was obtained due to the increasing phi(m) with different band gap grading profile. The optimum PCE of 15.65%, 19.03% and 19.9% have been obtained with uniform, single and double band gap structures respectively. Moreover, the effects of various defects density on solar cell properties were investigated and the results indicate that there is a threshold of 1 x 10(16) cm(-3) for both acceptor/donor and neutral defects. The depth profile of the carrier recombination rate was calculated to understand of the fundamental device physics. The result show that a great improvement of both V-OC and J(SC) was obtained in the back surface grading structure cell comparing to that of the uniform bandgap cell due to the additional quasi-electric field associated to the affinity (conduction band) variation with position benefitting the carrier collection and reducing the back surface recombination and bulk recombination typically characterized by the diffusion length. A slight enhancement of short-circuit current density without significantly sacrificing the open-circuit voltage was obtained in the double band gap grading structure comparing to the single back grading owing to an increasement of front grading within the SCR.