In situ preparation of CuInS2 films on a flexible copper foil and their application in thin film solar cells

Abstract

The in situ preparation of semiconductor films on a flexible metal foil has attracted increasing attention for constructing flexible solar cells. In this work, we have developed an in situgrowth strategy for preparing CuInS₂ (CIS) films by solvothermally treating flexible Cu foil in an ethylene glycol solution containing InCl₃·4H₂O and thioacetamide with a concentration ratio of 1 : 2. The effects of solvothermal temperature, time and concentration on the morphology and phase of the CIS films are investigated. Solvothermal temperature has no obvious effect on the morphology of the final films, but higher temperature is favorable for the growth of CIS films with higher crystallinity. Reactant concentration plays a significant role in controlling the morphology of CIS films; if InCl₃·4H₂O concentration is relatively low (≤0.042 M), single-layered CIS films can be produced, which are composed of high ordered potato chips shaped nanosheets, otherwise, it prefers to form a double-layered film, for which the lower layer is similar CIS ordered nanosheets while the upper layer is composed of flower shaped superstructures. A possible mechanism of the CIS films is also investigated. UV-vis measurements show that all these CIS films possess a direct bandgap energy of 1.48 eV, appropriate for the absorption of the solar spectrum. Finally, single-layered CIS films on Cu foil were employed for fabricating flexible solar cells with a structure of Cu foil/CuInS₂/CdS/i–ZnO/ITO/Ni–Al, and the resulting cells yield a power conversion efficiency of 0.75%. Further improvement of the efficiencies of the solar cells can be expected by optimizing the morphology, structure and composition of the CIS films, as well as the fabrication technique.