Sol-gel synthesized indium tin oxide as a transparent conducting oxide with solution-processed black phosphorus for its integration into solar-cells

Abstract

In this work, indium tin oxide (ITO) thin films were synthesized using solgel processing with a mixture of InCl3, methanol, and SnCl2, where the solutions were spin coated onto glass substrates. The maximum transmittance of the ITO thin film in the visible region was found to be ∼75% for films annealed at 650 °C, where plasma treatment of the substrate was found to aid in the large-area continuity and homogeneity over the glass substrates compared to films annealed at lower temperatures. Two-dimensional (2D), semiconducting black phosphorus (BP) dispersions were then prepared by liquid exfoliation, where the black phosphorus bulk crystals were finely ground inside a glove box and dissolved in N-cyclohexyl-2-pyrrolidone. Following further treatment, the BP solution dispersions were drop cast onto the transparent ITO thin films to form heterostructures toward transparent electronics and future solar cell applications. Direct electrical probing of the black phosphorus revealed that it was electrically conducting and the currents measured were large on the order of a few microampere at ∼20 V. Raman and photoluminescence measurements on the black phosphorus revealed that the flakes ranged in thickness from few-layers up to bulk. Few-layer black phosphorus can be distinguished from the bulk through the red-shift of the Ag1, Bg2, and Ag2 peaks for bulk black phosphorus flakes compared to the few-layers’ black phosphorus flakes. Electrical measurements made in the heterostructure interfaces showed a higher magnitude of currents at the black phosphorus interface compared to the bare ITO film. The combined architecture of black phosphorus on ITO thin films shows promise in its use for transparent electronics, which can also serve as a stepping stone for future solar cell platforms.