We have carried out microstructural studies of secondary phase formation at the Mo/Culn(Se,S)(2) interface, and of grain structure in hydrazine solution-processed CuIn(Se,S)(2) films. The CuIn(Se,S)(2) layers were deposited on Mo-coated glasses followed by thermal annealing under a nitrogen ambient. In our previous work, we identified [Cu6S4](2-) and [In-2(Se,S)(4)](2-) as the two major molecular species present in hydrazine Culn(Se,S)(2) precursor solutions. The CuIn(Se,S)(2) films prepared by the precursor solution containing only above two molecular complexes exhibited the undesired secondary interfacial phases such as CuSe and CuIn5S8. It is likely that the reactions between the Mo bottom electrode with sulfur in [Cu6S4](2-) can result in the formation of a MoS2 phase, triggering the phase separation of Culn(Se,S)2 into CuSe and CuIn5S8. To make matters worse, the final CuIn(Se,S)(2) films contain a lot of voids that reduce the structural and electrical integrity of the device. The addition of polyselenide molecules [Se](n) into the above CuIn(Se,S)(2) precursor solutions introduced MoSe2 rather than MoS2 at the back interface which in turn allows sulfur to remain in [Cu6S4](2-) complexes. As a result, only the desired phases, Culn(Se,S)(2) and MoSe2, were formed near the back interface. Furthermore, The formation of voids within the film can be also prevented by adding [Se](n) into the Culn(Se,S)(2) precursor solution, leading to the formation of a dense film structure. (C) 2013 Elsevier B.V. All rights reserved.