Nanostructured carbon nanotube Schottky junction solar cells

This dissertation explores and exploits the physical processes uncovered during experiments aimed at improving solar cell efficiency in a novel electronically gated solar cell through surface texturing. Besides the increased device efficiency, the findings shed light on the previous limitations in similar devices and may have implications for other semiconductor based devices.;Silicon nanowires have long been known for their excellent antireflection properties, but have suffered substantially from recombination at the surface. Here, we deposit a disperse carbon nanotube network on the tips of a forest of vertical silicon nanowires and exploit electronic gating in a novel Schottky junction solar cell. Previous experiments on carbon nanotube- silicon solar cells made use of an ionic liquid to modulate the nanotube Fermi level via electronic gating. This modulation changed the Schottky barrier height of the device and decreased the carbon nanotube film resistance, leading to power conversion efficiencies of up to 12% for a gate voltage of -0.75V. Further experiments uncovered an additional mechanism in which the ionic liquid induced an inversion layer within the silicon, greatly facilitating hole extraction by repelling electrons from the silicon surface (and consequently reducing recombination). We exploit this induced inversion layer within our silicon nanowire solar cells and show a greatly increased power conversion efficiency exceeding 15%, the highest reported efficiency for silicon nanowire based devices to date.;We also investigate the physical and chemical processes responsible for degradation in these devices. We show that contamination of the ionic liquid with oxygen or water leads to redox reactions for gate voltages previously thought to be well within the electrochemical window. We subsequently demonstrate that by excluding these contaminants, stable performance of the electronically gated nanotube/silicon solar cell is possible. Advanced passivation techniques are used to alleviate such degradation. Specifically, deposition of aluminum oxide via atomic layer deposition was used to create a high quality, conformal, dielectric layer that inhibits electrochemical reactions between the ionic liquid and the silicon, leading to minimal reduction in performance as the gate voltage is applied.