Cu2ZnSn(S,Se)4 thin film solar cell has attracted significant attention in thin film solar cell technologies considering its low-cost, non-toxicity, and earth-abundance. However, the highest efficiency still remains at 12.6%, far below the theoretical efficiency of Shockley-Queisser (SQ) limit of around 30%. The limitation behind such shortcoming in the device performance was reported to stem primarily from a high V oc deficit compared to other thin film solar cell technologies such as CdTe or Cu(In,Ga)Se2 (CIGS), whose origins are attributed to the prevalence of band tailing from cation disordering as well as to the high recombination at the interfaces. In this report, systematic studies on the causes of a high V oc deficit and associated remarkable approaches to achieve high V oc have been reviewed, provided with a guidance on the future direction of CZTSSe research in resolving the high V oc deficit issue.