The problem of dwindling energy can be attributed to the rapidly increasing worldwide energy demand, leading to an urgent need for alternative energy-harvesting technologies to sustain the economic growth by maintaining our appetite for energy. Among them, solar-energy-harvesting is most promising, and the huge demand for clean, cost-effective, and cost-efficient energy can be met by solar energy. The large-scale solar energy utilization has not become practical because of the high cost and inadequate efficiencies of the current solar-energy-conversions. Nanotechnology offers tools to develop cost-effective and cost-efficient technologies for solar-energy conversions. Nanostructures, such as nanowires, nanopillars, nanodomes, nanorods, quatumdots, nanoparticles, etc., facilitate photon absorption, electron transport, and electron collection properties of the solar-energy-conversion devices. This review specifically summarizes the contribution of the nanotechnology to photovoltaics, dye-sensitive solar cells, quantum-dot-sensitized solar cells, and solar hydrogen production devices.
TopIntroduction
Energy supply is arguably one of the most important challenges that the whole world is facing today and a big concern in future. We heavily rely on the fossil fuels for this energy demand, which, being non-renewable energy sources will lead to quick depletion of these resources. As the population is increasing at a faster rate, so is the energy consumption as shown in Figure 1a. The ever increasing energy demand, of which ~ 65% is currently fulfilled by coal and petroleum (Figure 1b) has disturbed the life on all fronts (economically and technically, ecology and health, present and future). These resources are harmful to the atmosphere as their consumption not only provides energy but also releases green house gases into the atmosphere leading to global warming and other harmful effects on living beings (Service, 2005).
Figure 1. It clearly demonstrates that petroleum, coal and natural gas are the most utilized fuels over renewables
Contrarily, renewable energy sources such as solar, wind, geothermal, tidal are actively investigated as alternatives for both energy demand fulfillment and clean energy production. Among all the alternatives, solar energy is the most easily available and abundant source of energy. Besides, being the clean source of energy and available at almost every part of the earth, the total energy that the whole population of the world will consume in sixteen years at present rates of utilization is not more than the energy that earth's surface receives each day. Considering these factors, we can pin our hopes onto this source of energy for our better present and sustainable future.
Even though, solar-energy harvesting is attractive and promising to tackle the global energy problem and environmental pollution, harvesting solar energy in a low cost and high efficient way is not easy. The energy density of solar energy is far lower than the fossil fuels and also that sun light intensity is different at different places and at different times adds to the difficulty of harvesting solar energy.
Furthermore, storing this sort of energy is difficult as it cannot be stored in chemical bonds as in case of fossil fuels, which accounts to why solar energy utilization is as low as ~0.0015% of the total energy used today (Crabtree, 2007). The major hindrance to the large scale use of solar conversion devices such as photovoltaics, dye sensitive solar cells, quantum-dot- sensitized solar cells and solar hydrogen production etc, is their high cost and low efficiencies. Sunlight to electricity conversion has the highest theoretical efficiency and flexibility and hence there is a great potential to explore free and clean solar-energy in the form of electricity by techniques such as photovoltaics(PV's) (Fan, 2011), photo-electrochemical cells (PEC's), electricity-storing solar hydrogen and quantum dot sensitized solar cells (QDSC). Single crystal based commercial solar cells, so called first generation solar cells have the efficiencies of ~18% (Crabtree, 2007; Lewis, 2005). These suffer from high cost of manufacturing and installation. The second generation polycrystalline semiconductor thin film solar cells (CuInGaSe2) even though reduce the price significantly but the challenge to make their efficiency more practical for commercial application is still not overcome. The third generation solar cells (dye sensitive solar cells) (DSSC) (Bach, 1998), based on cheap dye and titanium dioxide (TiO2) semiconductor offer a typical of ~11% efficiency (Gratzel, 2001; Crabtree, 2007) and cells based on even cheaper dye are 2-5% efficient; bulk hetero-junction cells (Yu, 1995; Rowell, 2006; Huynh, 2002) and organic solar cells (Lagemaat, 2006) promise inexpensive and large scale solar conversion while as solar hydrogen production offers an efficiency of ~11% but out of the total commercial hydrogen production, only ~5% comes from the renewable sources of energy, mostly through water electrolysis (Ni, 2007) while as 70-80% of the total hydrogen production comes from steam reforming of natural gas (Service RF, 2004).