Effects of Work Function and Electron Affinity on the Performance of Carrier-Selective Contacts in Silicon Solar Cells Using ZnSn$_\text{} x $ Ge$_\text{} 1-x $N$_\text{2}$ as a Case Study

Abstract

This work reports the electrical characterization of ZnSn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><inline-formula><tex-math notation="LaTeX">$_\text{} x $</tex-math></inline-formula></sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><inline-formula><tex-math notation="LaTeX">$_\text{} 1-x $</tex-math></inline-formula></sub> N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_\text{2}$</tex-math></inline-formula> (ZTGN) layers <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(10\% &lt; x &lt; 90\%)$</tex-math></inline-formula> deposited on glass by combinatorial sputtering and further assesses the performance of silicon heterojunction (SHJ) solar cells featuring them as electron-selective contacts. Bandgap, dark conductivity, and the activation energy of the latter were found to significantly change between Sn- and Ge-rich samples. When applying ZTGN layers as electron-selective contacts for SHJ solar cells, poor solar cell performance was observed, with surprisingly similar results despite changes in material properties. From analysis and modeling of the current–voltage characteristics using several device structures, we show that the work function of the electron-selective contact lies around 4.35 eV for all investigated Sn and Ge contents, which is too high to form an excellent electron-selective contact. By comparing different solar cell architectures, we could further identify that the Ge-rich layer imposes an additional barrier to electron extraction, independently of its poor selectivity, due to its low conductivity. Doping of Ge-rich ZTGN, thus, appears as the most relevant approach to build efficient devices with a ZTGN contact layer.