An electrochemical co-deposition method and solution to plate uniform, defect free and smooth (In,Ga)—Se films with repeatability and controllable molar ratios of (In,Ga) to Se are provided. Such layers are used in fabrication of semiconductor and electronic devices such as thin film solar cells. In one embodiment, the present invention provides an alkaline electrodeposition solution that includes an In salt, a Se acid or oxide, a tartrate salt as complexing agent for the In species, and a solvent to electrodeposit an In—Se film possessing sub-micron thickness on a conductive surface.