Thin Film Solar Cells with Light Trapping Transparent Conducting Oxide Layer
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Thin film solar cells, if film thickness is thinner than the optical absorption length, typically give lower cell performance. For the thinner structure, electric current loss due to light penetration can offset the electric current gain obtained from higher built-in electric field. Light trapping schemes can increase the effective optical absorption length and thus enhance the electric current for thinner solar cells. Here a new light trapping scheme based on light trapping transparent conducting oxide layer (LT-TCO) is proposed to enhance the performance of thin film solar cells. Three different configurations of integrating the LT-TCO layer in solar cells are proposed and evaluated. This research aims to develop the LT-TCO layer with surface texture and good conductivity by pulsed laser deposition (PLD) technique at low temperature. The LT-TCO layer is fabricated by PLD deposition of Al-doped ZnO to achieve multilayer films by tuning of oxygen pressure. The light trapping effect is examined by optical transmittance measurement and the surface texture is characterized by transmission electron microscopy (TEM) technique. The conductivity of LT-TCO layer is measured by resistivity measurement. Thin film CdTe/CdS solar cells are fabricated by PLD technique to develop baseline solar cells for integration of LT-TCO layer. The as-deposited thin film solar cells show relatively low performance and are further processed with various post-deposition treatments to seek efficiency enhancement. The effects of different processes on cell performance are examined by electrical, optical, and microstructure studies. Air annealing of CdS layer and CdCl2 treatment of CdTe layer combined are found to yield the best cell performance. The fabrication issues that limit the cell performance are discussed and future optimizations in fabrication processes are suggested.