Using a controlled bromine etching on kesterite absorbers, two major results are obtained. We establish the first defect depth profiling and secondary phases depth profiling of a state of the art Cu2ZnSnSe4 (CZTSe) film by using surface sensitive characterization methods (XPS and Raman spectroscopy) on successively etched samples, obtaining a direct insight on the factors hampering the performance of this class of absorber. In a second step, we demonstrate the possibility of significant improvement to the p-n interface in Cu2ZnGeSe4 (CZGSe)/CdS solar cells when a short bromine etching of the absorber is performed, with the Fill Factor improving by more than 7 points. This method offers a simple improvement pathway for state of the art kesterite devices, with a potentially broader application to thin film solar cells where the p-n interface is limiting.