Light harvesting methods in photovoltaic devices with superficial treatments

Photovoltaics is fast emerging as an attractive renewable energy technology due to concerns of global warming, pollution and scarcity of fossil fuels supplies. However to compete in the global energy market, solar cells need to be cheaper and more energy efficient. Silicon is the favourite semiconductor used in solar photovoltaic cells because of its abandoned in nature, well established technology and non toxicity. But due to its indirect band gap, silicon is poor absorber of light and theoretical limiting efficiency of single junction wafer based silicon solar cells is ~31% which is called Shockley Queisser limit. But up to now the maximum achievable efficiency in laboratory for single crystal single junction silicon solar cells is ~ 24.7%. So far the cost of the wafer based silicon photovoltaics is high. Also thin film cells play an important role in low cost photvoltaics, but efficiency of the cost reduced cells is lower compared to wafer based cells. So light trapping into photovoltaic cells is a great issue inorder to increase the carrier generation inside the active layer of both bulk as well as thin film cells with out disturbing their fabrication technology. There many light harvesting methods; among them Surface Plasmon method using metal nanoparticles and spectrum downshifting method using nanocrystals are discussed here. Metal nanoparticles support surface plamson when light is incident on them, which cause the scatter light into the underlying substrate. This process is realized on standard silicon solar cells. The feasible light scattering related enhancement was examined using spectral response and I-V measurements. Relative increases of the total delivered power under simulated solar irradiation were observed for cells both with and without antireflection coating using both silver and gold nanoparticles. The relative enhancement of External Quantum Efficiency derived from the spectral response measurements was observed for both the silicon cells. The better results obtained from both spectral response and I-V measurements were ascribed in the case of cells without antireflection coating. The results from I-V measurements under Air Mass 1.5 irradiation on the cells (without antireflection coating) correspond to a clear increase of the short circuit current due to both silver (relative increase of 7.5%) and gold (relative increase of 6.1%) nanoparticles. Also there is a relative enhancement (1.5%) of short circuit current was ascribed in the cells (with antireflection coating). Further realization of this method on copper indium gallium selenide based thin film solar cells attributed the enhancement of external quantum efficiency in the red wavelength region where these cells have already a poor spectral response. Spectral downshifting method by the nanocrystals was investigated on the silicon based solar cells. Down shifting of photons on the silicon solar cells is realized by the absorption and emission property of the manganese doped zinc sulfide nanocrystals. The variation of band gap and photoluminescence intensity of different nanocrystals due to different doping concentration was observed. Relative enhancement of External Quantum Efficiency has been attributed in UV region (where silicon solar cells have poor spectral response) due to lower concentration of nanocrystals. A strong concentration quenching effect which causes decrease of external quantum efficiency in both UV and visible region has been observed.