25. Polymorphous Nano-Si and Radial Junction Solar Cells

Nanostructured silicon (Si) materials are exciting new building blocks for Si-based photovoltaics to achieve a stronger light trapping, absorption, and antireflection with the least material consumption. Constructed upon Si nanowires (NWs), a novel 3D radial junction solar cell architecture decouples the optical absorption thickness from the electric distance that photocarriers need to travel to be collected. This allows a radical reduction of the absorber layer thickness that will benefit a fast photo-carriers separation and extraction. In addition, the light incoupling and absorption distribution among the antenna-like radial junction units can be largely enhanced by the resonant modes in the nanostructured photonic cavities, which necessitates a set of new theoretical models and high-precision simulation capabilities to address and predict the photovoltaic performance of the radial junction units, as a key basis for seeking optimal structural design. Recent progress in radial junction solar cells has accomplished a device performance comparable or even superior to their planar counterparts, with still plenty of room for further improvement. This chapter starts with a presentation of hydrogenated polymorphous silicon, a nanostructured material with enhanced optoelectronic properties with respect to hydrogenated amorphous silicon, and then continues with a review on the major fabrication strategies, growth theories, and key technologies involved in developing a new generation of high performance and low cost Si solar cells, with a particular focus on the radial junction thin film solar cells fabricated upon SiNWs grown via a plasma-assisted low temperature vapor-liquid-solid procedure. Critical issues, such as the geometry, density, and doping control in Si nanowires and the radial junction deposition and optimization, will be addressed in a systematical but concise way.