New Architecture towards Ultrathin CdTe Solar Cells for High Conversion Efficiency

Abstract

Solar Cell Capacitance Simulator in 1 Dimension (SCAPS-1D) is used to investigate the possibility of realizing ultrathin CdTe based solar cells with high and stable conversion efficiency. In the first step, we modified the conventional cell structure by substituting the CdS window layer with a CdS:O film having a wide band gap ranging from 2.42 to 3.17 eV. Thereafter, we simulated the quantum efficiency, as well as the parameters of J - V characteristics, and showed how the thickness of CdS:O layer influences output parameters of Glass/SnO 2 /ZTO/CdS:O/CdT<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:msub><mml:mrow><mml:mi mathvariant="normal">e</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>-</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>/CdTe/Ni reference cell. High conversion efficiency of 17.30% has been found using CdT<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:msub><mml:mrow><mml:mi mathvariant="normal">e</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>-</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"><mml:mi>x</mml:mi><mml:mo>=</mml:mo><mml:mn>0.12</mml:mn></mml:math>) and CdTe layers of thickness 15 nm and 4 μ m, respectively. Secondly, we introduced a BSR layer between the absorber layer and back metal contact, which led to Glass/SnO 2 /ZTO/CdS:O/CdT<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M4"><mml:msub><mml:mrow><mml:mi mathvariant="normal">e</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>-</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>/CdTe/BSR/Ni configuration. We found that a few nanometers (about 5 nm) of CdT<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M5"><mml:msub><mml:mrow><mml:mi mathvariant="normal">e</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>-</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>layer is sufficient to obtain high conversion efficiency. For BSR layer, different materials with large band gap, such as ZnTe, Cu 2 Te, and p + -CdTe, have been used in order to reduce minority carrier recombination at the back contact. When ZnTe is used, high conversion efficiency of 21.65% and better stability are obtained, compared to other BSR.