Silicon Nanostructures-Graphene Nanocomposites: Efficient Materials for Energy Conversion and Storage

Introduction

The increasing energy demand due to increase in population, industrial activity, etc. led to decrease in fossil fuel. According to US energy information administration (EIA), for the year 2011, energy consumed for electricity generation from fossil fuel is about 87% and 13% from renewable energy (Figure 1) [http://www.eia.gov/electricity].

Figure 1.

The percentage of electrical energy generation from different resources (Source: U.S. Energy Information Administration, Annual Energy Outlook 2013 Early Release, December 5, 2012)

Among renewable energy sources, solar energy is the most important because the amount of energy that reaches the earth surface in one hour is about 18TW, and the global consumption of energy per year is about 15TW (Kalyanasundaram, 2012). Solar energy has to be converted to different convenient forms of energy and the conversion efficiency is the measure of practicality of this energy source. Materials and devices to convert solar light energy into electrical energy efficiently are the immediate concerns. Most of the conventional solar cells in the market are based on Si p-n junction diodes and their efficiency had reached 25% (Green, 2009). Advantages of Si are high abundance (second most abundant element in the earth crust), non toxic, and long term stability. But the main disadvantage of Si is that the process of making ultrapure crystalline Si is very costly, but also it is an indirect band gap semiconductor, so for absorbance of sunlight a very thick layer of Si is required. To reduce the cost of the solar cell, researchers started to evaluate heterojunctions of Si. Among the heterojunctions, Schottky diode holds advantage over p-n junction diode. In Schottky diode the thickness of Si layer can be decreased, that eventually decreases the cost of the device. The condition for Schottkydiode is that the semiconductor should have moderate doping (high doping results in electron flows from semiconductor to metal and forms ohmic contact). When metal and semiconductor brought too close to each other, electrons flow from the conduction band of the semiconductor to metal until thermal equilibrium is reached i.e. the Fermi level becomes the same for both metal and semiconductor. This forms a depletion region with the absence of electrons at the junction, and results in barrier for further electron transition at the junction (Lutz, 2011). The Schottky barrier height (SBH) is given by: