Materials Science and Engineering B 159–160 (2009) 44–47 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb Improved homogeneity of microcrystalline absorber layer in thin-film silicon tandem solar cells Vlad Smirnov ∗ , Chandan Das, Thomas Melle, Andreas Lambertz, Markus Hülsbeck, Reinhard Carius, Friedhelm Finger IEF-5 Photovoltaik, Forschungszentrum Jülich GmbH, Leo-Brandt Str., D-52425 Jülich, Germany article info Article history: Received 29 April 2008 Received in revised form 6 October 2008 Accepted 30 October 2008 Keywords: Microcrystalline silicon Solar cells PECVD Nucleation Growth abstract A study of the effects of microcrystalline silicon i-layer modification near p/i interface in tandem con- figuration silicon thin film solar cells is presented. The structural properties of the absorber layers were investigated by Raman spectroscopy at different stages of growth. The results indicate the possibility of improving both the nucleation process and the film homogeneity in the direction of growth, without specific re-optimization of the p-layer, transferred from a single-junction microcrystalline silicon cell. Structural modifications of the i-layer have been correlated with performance of tandem solar cells, lead- ing to improvements in the bottom cell current J sc (up to 11.4 mA/cm 2 ) and initial tandem-cell conversion efficiency (up to 11.3%). © 2008 Elsevier B.V. All rights reserved. 1. Introduction Microcrystalline silicon (c-Si) has received significant atten- tion as intrinsic thin film layer in bottom component of multijunction thin silicon film solar cells due to its enhanced response in longer wavelength part of the solar spectrum and higher stability against light induced degradation. Microcrystalline silicon is a mixed phase material containing columns or clusters of crystalline grains, disordered regions and voids [1], and its elec- tronic and structural properties have been the subject of intensive research. The growth of c-Si films prepared by PECVD is known to be substrate dependent [2–5]. In the early stages of growth its microstructure changes significantly with the deposition time [6] and an incubation layer is frequently observed. In the case of photo- voltaic devices, deposited in a p–i–n sequence, the microcrystalline silicon active layer is deposited on to the p-layer, which therefore must provide nucleation centres for c-Si i-layer growth [7]. In a single junction device, p-layer growth should be optimised directly on TCO. In the case of tandem configuration however, where the p- layer is grown on the n-layer of the top cell, the recipe for an optimal single junction cell can no longer be used, since the p-layer require- ments are different in each case. Thus, the recipe may need to be re-optimized (particularly the p- and i-layer) when transferred into tandem configuration. ∗ Corresponding author. Tel.: +49 2461 614855; fax: +49 2461 613735. E-mail address: v.smirnov@fz-juelich.de (V. Smirnov). It has been shown that the performance of a solar cell, utilizing c-Si as an absorber layer, is strongly dependent on crystalline vol- ume fraction [8]. Additionally, structural homogeneity of the active layer in the direction of growth was suggested as a possible require- ment for improved single junction solar cell performance [9].A method for improving the i-layer homogeneity in microcrystalline silicon cells deposited in n–i–p sequence was suggested by utilizing a hydrogen dilution profiling method [10]. In that case, the i-layer is grown on the n-layer of the cell and the silane concentration ratio is modified over the total growth time. Here, we present a study of the effects of microcrystalline silicon i-layer modification near p/i interface in a tandem solar cell config- uration. The results from Raman spectroscopy are correlated with solar cell performance, and a relatively simple method to maintain a homogeneous i-layer growth with a desirable crystallinity, inde- pendently on p-layer nucleation properties, leading to an improved solar cell performance, is presented. 2. Experimental details Silicon thin films were deposited by plasma enhanced chemical vapour deposition (PECVD) technique using either RF (13.56MHz) or VHF (94.7 MHz) excitation for different intrinsic and extrin- sic layers. In the deposition of microcrystalline silicon intrinsic layer a gas mixture of silane (SiH 4 ) and hydrogen (H 2 ) was used. The intrinsic c-Si layers were deposited using VHF excitation at a power density of 0.13W/cm 2 . The substrate temperature and chamber pressure for the deposition of i-layer was fixed at 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.10.050