Electrochemical Deposition of Cu-Based Chalcopyrite (CISe/CIGSe) Thin Films for Photovoltaic Applications

Abstract

Among the second generation solar cells, CuIn1-xGaxSe2 (CIGSe) based solar cells fabricated by vacuum based technique have already surpassed the power conversion efficiency of ƞ > 22%. Despite of their outstanding performance, vacuum based techniques are being challenged by their high cost which can be responded by scalable non-vacuum based techniques like electrodeposition. Till date, the maximum power conversion efficiency achieved by CIGSe thin film devices fabricated by electrodeposition technique have exceeded by 15%. These results are motivating to pursue research in this field towards obtaining high quality CISe and CIGSe thin films. In this study, the ternary and quaternary CuIn1-xSe2 (CISe) and CuIn1-xGaxSe2 (CIGSe) semiconducting absorber thin films have been synthesized by electrodeposition technique.
The overall objectives of the present investigations are: (i) to electrodeposit highly adherent Cu-poor CuInSe2 thin film which is close to ideal stoichiometry (1:1:2) with improved crystallinity, less pin holes, (ii) to study the role of sodium dodecyl sulfate (SDS) surfactant in compositional tuning of CuInSe2 thin films and (iii) electrodeposition of CIGSe thin films by stack approach (CuInSe2/Ga-Se) - Synthesis and its characterization. For the electrodeposition of CuInSe2, sodium dodecyl sulfate (SDS) have been used as additive in citrate electrolyte. Detailed electrochemical experiments were performed to understand the deposition mechanism. The effects of deposition potential and SDS concentration have been studied in detail. It is concluded that the initial [Se4+/Cu2+] ion flux ratio is highly important. The addition of SDS improves the film quality and suppresses the undesirable Cu2Se secondary phase which is confirmed by XRD, Raman and compositional analysis. Moreover, without losing the film quality it is possible to tune the electrical conductivity of the as-deposited CISe thin films by just varying the SDS concentration from Cu-rich (p-type) to In-rich (n-type). The variation in the electrical conductivity is substantiated by photoelectrochemical (PEC) and mott-schottky (M-S) analysis. For the electrodeposition of CIGSe thin films, instead of conventional single step deposition, a binary stack approach has been employed by electrodepositing a thin layer of Ga-Se over Cu-rich CISe thin films. The incorporation of gallium in CISe thin films have been confirmed by X-ray diffraction (XRD), Raman and Scanning electron microscopy/Energy-dispersive X-ray spectroscopy (SEM/EDX) studies. The final results were very promising as the proposed route is simple and the electrolyte is highly stable. The PEC studies revealed an enhanced performance due to the enlarged bandgap than CISe thin films.
The synthesis and characterization of ZnO nanostructures by a facile, surfactant free and seedless growth by wet chemical route has been carried. The reported route is simplistic and versatile as by only varying the reaction time it is possible to synthesize ZnO nanostructures of various morphologies such as pure sheets, rods embedded sheets, pure rods and petal shaped rods. The obtained ZnO nanostructures were nitrogen doped inherently due to the presence of ammonia. A plausible reaction mechanism for the formation and transformation of the ZnO nanostructures morphology has been put forward.