Hierarchical nanostructures for solar cells

Abstract

Hierarchical nanostructures, such as branched nanoforest 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, catalysts and so on. However, conversion efficiency enhancement by new material development has slowed down and new research trends to enhance solar cell efficiency by smart nanostructuring from the same material have started to garner tremendous attention. Nanowire-based solar cells have ignited this nanostructuring research and further progress with 2D and 3D hierarchical nanostructures has drawn noticeable solar cell efficiency enhancement. The major objectives of hierarchical nanostructuring in solar cells are: (1) high carrier mobility (mostly electron mobility in photoanodes) along the nanowire structures with less recombination, (2) a large surface area to capture more sunlight and adsorb more dye molecules, and (3) a light scattering layer to capture the sunlight more efficiently by multiple scattering. A large surface area and high carrier mobility are the requirements for most energy-related devices. Therefore, it is evident that hierarchical nanostructures can be applied to 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. Furthermore, using 3D branched structures to harvest various types of ambient power, such as thermal, wind, vibration and electromagnetic energy, would also be very promising, which provides a potential endless source of energy.5 Even though there are a lot of published papers on 3D hierarchical nanostructured solar cell devices, further developments in this research field require improvements in synthetic methods and novel fabrication processes to provide better control of the structural complexity, composition uniformity, surface chemistry and interface electronics and last but not least, the yield, of hierarchical nanostructures.5 These factors are directly related to the sustainability, high efficiency and production costs at an affordable level for the public for practical applications. This is why developing simple, economic and environmentally friendly hierarchical nanostructure mass production methods are of great interest. One of the most promising economical approaches usually consists of a solution process without using any expensive and complex vacuum-based vapour phase methods. There are also drawbacks to using 3D branched nanostructures in energy applications, in that they bring challenges in quantifying the charge transport and recombination loss in terms of solar devices.5 Understanding the light absorption property, interface electronic band structure, and photocarrier dissociation and recombination are key issues to the overall performance.5 To establish a reliable structure-performance correlation, a wide range of characterizations should be carried out in a consistent way. Overall, these are multidisciplinary topics, for which physics and chemistry experimentalists and theoreticians need to sit together and brainstorm innovative ideas to bring great enhancement to solar cells and energy devices with hierarchical nanostructures. © 2015 The Royal Society of Chemistry.