Activation of thermally evaporated CdSe films employing different halides: an evolution to ion size impact on grain growth

Cadmium-based thin films and solar cell devices frequently possess crystallographic defects like dislocations in the form of grain boundaries which deteriorate the quality of films and hinder the performance of the devices. The accomplishment of passivation of such grain boundaries could be carried out by halide activation treatment where segregation of halide ions at grain boundaries results in their passivation. Therefore, in the present comprehensive study, the impact of ion size of halides on grain boundary passivation, grain growth, recrystallization and the physical properties of CdSe thin films is obtained using different halide compounds (MX2 where M = Cd, Mg and X = Cl, I, F). The CdSe thin films having thickness of 600 nm are grown employing thermal evaporative technique and subsequently subjected to halide activation using CdCl2, CdI2, MgF2, MgCl2 and MgI2 compounds at 300 °C. Halide activation persuaded grain growth mechanism, and creation of different complexes are elucidated. The XRD findings reveal the polycrystalline nature of grown CdSe films having mixture of cubic and hexagonal phases where phase conversion is observed from (111) preferred orientation of cubic to (103) preferred orientation of hexagonal CdSe for CdI2- and MgCl2-treated films. The electrical resistivity is decreased with halide activation treatment where minimum resistivity is attained by CdI2-treated films. The surface morphology and topography of films are varied with halide activation, and observed peaks of Cd and Se along with Mg and X elements in EDS patterns corroborate the triumphant deposition of films and halide treatment. Optical energy band gap varied from 1.59 eV to 1.70 eV with halide treatment where MgCl2-activated films exhibit maximum absorbance and least transmittance. All the films exhibited two PL peaks positioned at ~ 625 nm and at ~ 675 nm whose intensity is varied with size and nature of integrated halide ions. Graphical illustration of CdSe thin films, halide treatment procedure and characterizations.