In this study, an ultra‐thin MoO3 layer synthesized by a solution‐based technique is introduced as a promising interfacial layer to improve the performance of kesterite Cu2ZnSnSe4 (CZTSe) solar cell. Solar cells with 10 nm of MoO3 between Mo rear contact and CZTSe had larger minority carrier life time and open‐circuit voltage compared to the reference solar cells. Temperature dependent current density–voltage measurement indicated that the activation energy (EA) of the main recombination is higher (∼ 837 meV) in solar cells with MoO3 layer, as compared to conventional solar cells where EA ∼ 770 meV, indicating reduced interface recombination. A best efficiency of 7.1% was achieved for a SLG/Mo/MoO3/CZTSe/CdS/TCO solar cell compared to the reference solar cell SLG/Mo/CZTSe/CdS/TCO for which 5.9% efficiency was achieved.