Nitride semiconductors as carrier-selective contacts for silicon heterojunction solar cells

Abstract

This work initially reports the electrical characterization of ZnSnxGe1-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 modelling of the current-voltage characteristics of several device structures, through a self-adaptive Differential Evolution algorithm, 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 differ ent 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. After having identified these loss mechanisms, MgSnN2 (MTN) was envisioned as a good candidate, due to its high electron concentration and bandgap at 50% Mg/(Mg+Sn) (at.%). Thus, we fabricated MTN layers also through a combinatorial sputtering approach, with no substrate heating and at 200 °C, resulting in MgxSn1-xN2 (43% < 𝑥 < 55%) samples, with bandgap around 2 eV, showing dark conductivity and activation energy that decreased towards Mg-rich samples. When applied to SHJ solar cells, JV characteristics similar to that when ZTGN was studied were obtained, and per formance was slightly better. The limiting properties were also of the same kind, with an estimated work function around 4.16 eV, shifting to 4.3 eV for samples grown at 200 °C, and Sn-rich samples showing a too high electron affinity. Mg-rich samples, as Ge-rich ones, resulted in strong s-shapes due to poor doping. Thus, doping these compounds with extrinsic elements appears as the most relevant approach to build efficient devices with a ZTGN or MTN contact layer.