Direct Growth of Highly Mismatched Type II ZnO/ZnSe Core/Shell Nanowire Arrays on Transparent Conducting Oxide Substrates for Solar Cell Applications

Abstract

A number of nanometer-scale photovoltaic (PV) concepts based on semiconductor nanowires have been developed or proposed in recent years, with either inorganic/organic hybrid or all-inorganic approaches. The quasi-one-dimensional (quasi-1D) structure is perhaps the optimized choice for optoelectronic devices such as solar cells and photodetectors, because it allows for maximal advantage to be taken of reduced dimensionality whilst retaining the last and only needed conduction channel. Besides the possibility of exploring quantum effects at the nanoscopic scale, the quasi-1D system could be superior to the bulk material even at the mesoscopic scale, where the lateral size falls below the carrier diffusion length, for instance, by reducing the nonradiative recombination and carrier scattering loss through elimination of the unnecessary lateral transport and the resulting recombination loss. Additionally, a nanowire array constitutes a natural architecture, such as a photonic crystal, for light trapping. The charge separation of the electron and hole is a key step in the generation of solar power in a PV device. In a conventional solar cell, it is typically achieved by a planar p–n homojunction along the path of the current flow or longitudinally. In nanometer-architecture PV devices, however, the charge separation is often facilitated by a type II or staggered energy alignment of a heterojunction, constructed from two materials for which both the valance and conduction bands of one component lie lower in energy than the corresponding bands of the other component. Such heterojunctions have been intensively investigated for solar cell applications, including dyesensitized solar cells (DSSCs), quantum-dot-sensitized solar