RESEARCH ARTICLE Improved nanocrystal formation, quantum confinement and carrier transport properties of doped Si quantum dot superlattices for third generation photovoltaics Dawei Di*, Heli Xu, Ivan Perez-Wurfl, Martin A. Green and Gavin Conibeer ARC Photovoltaics Centre of Excellence, University of New South Wales, Sydney, NSW 2052, Australia ABSTRACT An all-Si tandem solar cell has the potential to achieve high conversion efficiency at low cost. However, the selection and synthesis of candidate material remain challenging. In this work, we show that the conventional ‘Si quantum dots (Si QDs) in SiO 2 matrix’ approach can lead to the formation of over-sized Si nanocrystals especially when doped with phosphorous, making the size-dependent quantum confinement less effective. Also, our investigation has shown that the high resistivity of this material has become the performance bottleneck of the solar cell. To resolve these matters, we propose a new design based on Si QDs embedded in a SiO 2 /Si 3 N 4 hybrid matrix. By replacing the SiO 2 tunnel barriers by the Si 3 N 4 layers, the new material manages to constrain the growth of doped Si QDs effectively and enhances the apparent band gap, as shown in X-ray diffraction, Raman, photoluminescence and optical spectroscopic measurements. Besides, electrical characterisa- tion on Si QD/c-Si heterointerface test structures indicates the new material possesses improved vertical carrier transport properties. Copyright © 2011 John Wiley & Sons, Ltd. KEYWORDS silicon quantum dots; nanocrystal growth; band gap engineering; doping; carrier transport *Correspondence Dawei Di, ARC Photovoltaics Centre of Excellence, University of New South Wales, Sydney NSW 2052, Australia. E-mail: dawei.di@unsw.edu.au Received 14 October 2010; Revised 28 August 2011; Accepted 27 September 2011 1. INTRODUCTION Self-assembled silicon nanocrystals (Si NCs) (or quantum dots) embedded in a dielectric matrix are believed to be a promising material for applications in optoelectronics [1–3] and photovoltaic solar cells [4–10]. A simple method of fabricating Si quantum dot (Si QD) superlattice was intro- duced by Zacharias et al [11]. One major advantage of Si NCs over bulk Si is the freedom to engineer the material’s effective band gap by varying the size of Si QDs or by modifying the properties of the matrix material. Wide band gap material based on Si QDs that is designed to interact with high energy photons, is particularly important in the realisation of high efficiency low cost all-Si tandem solar cells (Figure 1). As an initial step towards the realisation of nanostruc- tured Si-based tandem solar cells using Si QDs embedded in a dielectric matrix, we demonstrated single junction Si QD solar cells on quartz substrates with open-circuit volt- age (V oc ) exceeding 400 mV [5,7]. These were based on Si NCs in a SiO 2 matrix. However, these cells exhibited low short-circuit currents and high resistivities. This can be improved by optimising device architecture, and the single junction Si QD cell is a promising route to demon- strating V oc greater than the best results achieved to date in Si cells (720 mV [12]). In this work, we will investigate the limitations of the conventional ‘Si QDs in SiO 2 matrix’ approach. We will show that this approach can lead to the formation of over- sized Si NCs especially when doped with phosphorous, making the size-dependent quantum confinement less effective. To resolve this problem and to reduce electrical resistivity, we propose a new design based on Si QDs embedded in a SiO 2 /Si 3 N 4 hybrid matrix. Silicon nitride offers good mechanical stability and stiffness under high tempratures [13–17] and thus can be used as thin confining layers in Si QD materials. Besides, Si 3 N 4 has a lower electronic band gap (E g ~ 5.3 eV) than SiO 2 (E g ~ 9 eV), suggesting that it is more conductive than SiO 2 as quantum tunneling barriers. We will demonstrate that the nanocrystal formation and current transport properties can be improved with the new design. PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. (2011) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.1230 Copyright © 2011 John Wiley & Sons, Ltd.