Alignment and orientation of Nanowires in a thin film under the influence of an electric field

Sammanfattning: The demand for cheaper and more efficient solar cells is high. One way to decrease cost of material, while maintaining a high degree of efficiency, is to take advantage of nanomaterials. A thin film containing pn-junction nanowires can convert light with an efficiency comparable to traditional planar cells, while using a fraction of the material, and thus reduce costs. At the company Sol Voltaics, a tool for mass producing solar cell nanowires, called Aerotaxy, is used. This method uses aerosol seed particles to grow nanowires, as they flow through a heated pipe. The resulting, randomly distributed, nanowires need to be arranged in arrays such that they can be contacted and used to convert light to electricity. This project focuses on a method to achieve this by using an external electric field. The solar cell nanowires contains a dipole moment due to their pn-junction. When an external electric field is applied over the nanowires, there is an interaction between the field and the dipole. An additional dipole moment is generated in the nanowire under the influence of the electric field, due to the polarization in the material. The junction dipole is directional in the sense that the plus pole and the minus pole is set, while the direction of the polarization dipole will depend on the position of the nanowire relative to the electric field. These two effect also have different dependence on the electric field strength. Therefore, it is here proposed that the degree of orientation of the aligned nanowires depends the strength of the applied field. The aim of this project was to realize the relative magnitude of the junction dipole and the polarization dipole. This was done by a combination of computations and experiments. Modeling of the nanowire dipoles in an electric field show that all nanowires should orient for field strength below 0.26 V/µm, after which the polarization effect becomes dominant, resulting in decreasing orientation among the nanowires. In order to reach alignment, on the other hand, a higher field strength is required. Thin films of pn-junction nanowires in PDMS were produced under the influence of an applied DC electric field. Results indicate a preferred direction of the nanowires in the electric field. However, higher field strengths were required for alignment of the nanowires, than was predicted from computations. The difference in field strength required for orientation of nanowires compared to alignment of nanowires creates a problem. The proposed solution to this problem is a gradually increased electric field strength.