Hierarchical Nanostructures for Solar Cells

Solar cells have been getting tremendous attention as the most reliable clean energy source. However, widespread use of solar cells is still limited by two major challenges: conversion efficiency and cost. As a promising solution, hierarchical nanostructures, such as branched nanoforests and nanoporous structures, have changed the recent research trend in developing high efficiency solar cells. Traditional research had focused on the development of new materials such as dyes, electrolytes and catalysts. However, research into the conversion efficiency enhancement that could be obtained from the development of new materials was slow, so a new research trend to enhance solar cell efficiency by smart nanostructuring from the same materials started to get tremendous attention. Nanowire-based solar cells ignited this nanostructuring research and further progress with 2D and 3D hierarchical nanostructures have shown noticeable solar cell efficiency enhancements. The major objectives of hierarchical nanostructuring in solar cells are high carrier mobility (mostly electron mobility in photo-anodes) along the nanowire structures with less recombination, a large surface area to capture more sunlight and adsorb more dye molecules, and light scattering layers to capture the sunlight more efficiently by multiple scattering. A large surface area and high carrier mobility are required for most energy related devices. Therefore, it is evident that hierarchical nanostructures can be applied to the emerging energy conversion and storage fields such as photocatalysis, photoelectrochemical water splitting, Li ion batteries, supercapacitors, fuel cells, thermoelectric devices, piezoelectric devices as well as solar cells. In this chapter, various research trends will be introduced including how smart material structuring will lead to an increase in photo-conversion efficiency in solar cells especially by introducing hierarchical nanostructures.