Effects of Work Function and Electron Affinity on the Performance of Carrier-Selective Contacts in Silicon Solar Cells Using ZnSnx Ge-1 (-) N-x(2) as a Case Study

This work reports the electrical characterization of ZnSnx Ge1 - xN2 (ZTGN) layers (10% < x < 90%) 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.