Monolithic Si nanocrystal/crystalline Si tandem cells involving Si nanocrystals in SiC
Monolithic tandem cells involving a top cell with Si nanocrystals embedded in SiC (Si NC/SiC) and a c-Si bottom cell have been prepared. Scanning electron microscopy shows that the intended cell architecture is achieved and that it survives the 1100 degrees C anneal required to form Si NCs. The cells exhibit mean open-circuit voltages V-oc of 900-950mV, demonstrating tandem cell functionality, with 580mV arising from the c-Si bottom cell and 320mV arising from the Si NC/SiC top cell. The cells are successfully connected using a SiC/Si tunnelling recombination junction that results in very little voltage loss. The short-circuit current densities j(sc) are, at 0.8-0.9 mAcm(-2), rather low and found to be limited by current collection in the top cell. However, equivalent circuit simulations demonstrate that in current-mismatched tandem cells such as the ones studied here, higher j(sc), when accompanied by decreased V-oc, can arise from shunts or breakdown in the limiting cell rather than improved current collection from the limiting cell. This indicates that V-oc is a better optimisation parameter than j(sc) for tandem cells where the limiting cell exhibits poor junction characteristics. The high-temperature-stable cell architecture developed in this work, coupled with simulations highlighting potential pitfalls in tandem cell analysis, provides a suitable route for optimisation of Si NC layers for photovoltaics on a tandem cell device level. Copyright (c) 2016 John Wiley & Sons, Ltd.