Enhanced light trapping and carrier collection in glancing angle deposited nanostructures

Abstract

Nanostructured materials have become an attractive alternative to their thin film and bulk counterparts in photovoltaic (PV) research. They owe this attention mainly to their superior optical and electrical properties. Light trapping in vertically aligned nanostructures results in high optical absorption and core/shell type of nanostructured devices provide enhanced carrier collection by utilizing a radial junction. Combination of these two features can potentially lead to the development of high efficiency nanostructured solar cells. Here, results from optical absorption properties of indium sulfide (In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ; n-type semiconductor) nanostructures as a model material system in different geometries and their photoconductive properties in an In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -nanorods-core/metal-shell device design are presented and discussed. Glancing angle deposition (GLAD) technique was used to grow In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanostructures in different shapes (i.e., zigzags, springs, screws, tilted rods, and vertical rods). Optical absorption was found to strongly depend on the shapes of semiconducting nanostructures through ultraviolet-visible (UV-Vis) spectroscopy measurements. Numerical solutions of finite difference time domain (FDTD) optical modelling show that diffracted light is distributed uniformly within the 3D nanostructure geometries, indicating an enhanced diffuse light scattering and light trapping. A high pressure sputter deposition method was used to get a conformal silver (Ag) layer around GLAD In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanorods and produce the nanostructured core/shell photoconductive devices. Core/shell geometry was observed to enhance radial interface and shorten charge carrier transit times. This provides efficient carrier collection and results in superior photocurrent and gain. Slow recovery of photocurrent arisen from prolonged carrier lifetimes due to high surface states in nanorods is also eliminated by the metal shell, which provides surface passivation and decreases surface states. Overall, we demonstrate that GLAD nanostructures provide both efficient charge carrier collection and enhanced light trapping, and therefore can lead to the utilization of low quality (i.e. low cost) materials for high efficiency solar cells.