Articolo in rivista, 2016, ENG, 10.1002/pip.2766

Monolithic Si nanocrystal/crystalline Si tandem cells involving Si nanocrystals in SiC

M. Schnabel, M. Canino, K. Schillinger, P. Löper, C. Summonte, P.R. Wilshaw and S. Janz

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany Department of Materials, University of Oxford, Parks Rd, Oxford, OX1 3PH, UK CNR-IMM Sez. di Bologna, via Gobetti 101, 40129 Bologna, Italy École Polytechnique Fédérale de Lausanne, Institute of Microengineering, Photovoltaics and Thin Film Electronics Laboratory, Rue de la Maladière 71b, 2002 Neuchâtel 2, Switzerland

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 °C anneal required to form Si NCs. The cells exhibit mean open-circuit voltages Voc of 900-950 mV, 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 jsc are, at 0.8-0.9 mAcm2, 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 jsc, when accompanied by decreased Voc, can arise from shunts or breakdown in the limiting cell rather than improved current collection from the limiting cell. This indicates that Voc is a better optimisation parameter than jsc for tandem cells where the limiting cell exhibits poor junction characteristics. The high-temperature-stable cell architecture developed in thiswork, coupledwith simulations highlighting potential pitfalls in tandemcell analysis, provides a suitable route for optimisation of Si NC layers for photovoltaics on a tandem cell device level. Copyright © 2016 John Wiley & Sons, Ltd.

Progress in photovoltaics (Online) 24 , pp. 1165–1177

Keywords

tandem cell; silicon nanocrystal; silicon carbide; solid phase crystallisation; external quantum efficiency; equivalent circuit

CNR authors

Canino Maria Concetta, Summonte Caterina

CNR institutes

IMM – Istituto per la microelettronica e microsistemi