Solar energy is currently one of the most important sources
of clean, free, inexhaustible and renewable energy with minimal
environmental impact. The power from the sun intercepted by
the earth is approximately 1.8 × 1011 MW [1]. About 30% of the
solar power actually reaches the earth and at every 20 minutes,
the sun produces enough power to supply the earth with its
needs for an entire year [2]. The solar energy can be defined as
the energy which comes from the sun and can be converted into
electricity and heat. It has produced energy for
billions of years, so the utilization of solar energy and the
technologies of its materials has received much attention
especially in the last ten years [3-4]. For example, some studies
have indicated that about 1000 times from the global energy
requirements can be achieved by using solar energy; however,
only 0.02% of this energy is currently utilized [5].