If the band gap is too large, many photons possessing insufficient energy shall pass through the material without creating any electron–hole pair. But if the band gap is too small, many will have lot of excess energy which shall be dissipated as heat. Therefore, materials having only specific band energy are used. For efficient harvesting of solar energy band gap should be in range of 1–2 eV. Band gap dependence of conversion efficiency is shown in Fig. 3[2]. Tandem structure can be used for utilizing the most part of the solar spectrum. In this structure, a material having higher band gap energy is deposited over the material having lower band gap. Thus, the first layer is transparent for low energy light photon and the bottom absorber layer with a low band gap can easily absorb the light unabsorbed by the first layer