Since the first report on the dye-sensitized solar cell by O'Regan and Grätzel in 1991 [1
], a great number of photovoltaic devices based on nanostructures have been proposed or developed, such as nanostructured dye-sensitized cells [2
], extremely thin absorber (ETA) cells [4
], quantum dot cells [5
], nanowire array cells [6
], organic/inorganic nanostructured cells [7
], and III-VI quantum ring solar cells [8
]. Nanostructured solar cells have several advantages over conventional bulk and thin film solar cells: large surface area, high efficiency for light harvesting, less expensive materials, and low process cost.
The two most frequently used window materials in nanostructured solar cells are highly porous nanocrystalline TiO2
and highly textured ZnO nanorod arrays. Porous nanocrystalline TiO2
particles can provide a large surface area for the absorber material. However, their slow trap-limited diffusion process and short effective diffusion length of electrons are big obstacles in making more efficient cells. ZnO nanowires have higher carrier concentration and electron mobility which favor the electron transport to the collection electrode. As the nanowires are not in direct contact with each other, the electrons transport only along the nanowire axis without any lateral transport, which will reduce the non-radiative recombination and carrier scattering loss dramatically. Solar cells sensitized by organic dye absorbers have shown impressive results, although their long-term stability and bandgap controllability need to be improved further. On the other hand, inorganic narrow bandgap semiconductors, such as Ag2
], CdS [11
], and CdSe [13
], are also promising candidates as sensitizers for nanostructured solar cells.
It has been postulated that ZnO/CdSe can form a type II heterojunction which will accelerate the separation of photoexcited electron–hole pairs and improve the efficiency of solar cells. In a previous study, Leschkies et al. fabricated CdSe quantum dot sensitized ZnO nanowire solar cells [14
]. They recorded a power conversion efficiency of 0.4% and a short-circuit current density of 2.1
, which are still low compared with those of dye-sensitized solar cells. Lévy-Clément et al. prepared a nanostructured ZnO/CdSe/CuSCN ETA solar cell [15
], and a high energy conversion efficiency greater than 2% was demonstrated under a 340-W/m2
illumination using a halogen lamp. However, they did not report the energy conversion efficiency under the air mass (AM)1.5 full sun intensity. Luan et al. reported a CdS/CdSe co-sensitized solar cell using a facile solution growth which resulted in a power conversion efficiency of approximately 1% with a fill factor of 0.55 [17
]. Until now, there have been only a few reports published concerning ZnO/CdSe nanostructure-based solar cells. The mechanisms of such structures have not been systemically studied, and more fundamental researches should be conducted to provide further understanding of the electronic transporting process in these nanostructures. Herein, we reported the fabrication and characterization of open structure ZnO/CdSe core/shell nanoneedle array-based solar cells. High short-circuit current densities and power conversion efficiencies were obtained, which provided significant insight as to how to improve the photovoltaic performance of this type of solar cell.