A large area flexible p-type transparent conducting CuS ultrathin films generated at liquid-liquid interface

Abstract The preparation of highly transparent, electrically conducting materials (TCMs) in the form of thin films possesses many applications, including touch screens, solar cells, smart glass, etc. Designing and making high-performance p-type materials that can match to n-type counter parts are found to be challenging even today. These thin films can be prepared directly on the substrates or more conveniently transferred to the desired substrates after the synthesis. Synthesis of ultrathin films of metal sulfide by simple liquid-liquid interface method has been known previously. However, transferring these films onto a substrate of considerable dimensions has always been a constant challenge as these films tend to break during the lift-off process. Furthermore, the transparency of these films is generally low due to the high roughness of these films. Here, we report CuS ultrathin film synthesis with varying thicknesses (∼15 nm – 40 nm) by a modified liquid-liquid interface method. We have developed two strategies to transfer these ultrathin films produced at a liquid-liquid interface onto any substrate with considerable dimensions (up to 8 cm × 4 cm) without losing electrical continuity. Optical and electrical measurements reveal that CuS thin films show a p-type conductivity with 102–103 S/cm and having transparency between 90–93% at 550 nm, which are among the highest for p-type materials known today. Further, these films on plastic substrates show highly flexible properties, which ITO lacks. We could also extend this technique to transfer large area ZnS thin-films from the liquid-liquid interface to the desired substrates.