Band gap grading in microcrystalline silicon germanium thin film solar cells Yu Cao a,b,c,d , Jing Zhou e , Yijun Wang a , Jian Ni b,c,d , Jianjun Zhang b,c,d,⇑ a College of Electrical Engineering, Northeast Dianli University, Jilin 132012, China b Institute of Photo-electronic Thin Film Devices and Technique, Nankai University, Tianjin 300071, China c Key Laboratory of Photo-electronics Thin Film Devices and Technique of Tianjin, Tianjin 300071, China d Key Laboratory of Optoelectronics Information Science and Technology, Chinese Ministry of Education, Tianjin 300071, China e College of Chemical Engineering, Northeast Dianli University, Jilin 132012, China article info Article history: Received 14 October 2014 Received in revised form 21 December 2014 Accepted 4 January 2015 Available online 2 February 2015 Keywords: Hydrogenated microcrystalline silicon germanium Band gap grading Solar cell abstract In this work, hydrogenated microcrystalline silicon germanium (lc-SiGe:H) thin film solar cells with a novel band gap grading profile have been designed. By comparing different profile types (normal profile, reverse profile and no profile), the normal profile was formed in sequence by the superposition of a high Ge content layer, a Ge content grading layer and a lc-Si:H layer has been proposed. This structure exhibits higher short-circuit current density (J sc ) than conventional cell design with the similarly Ge content owing to the enhancement of the infrared response. Finally, an initial efficiency of 6.53% was achieved by lc-SiGe:H solar cell with this novel cell structure. The results have demonstrated a great potential of the lc-SiGe:H solar cells as the infrared absorber in multi-junction silicon based thin film solar cells. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction The photovoltaic performance of silicon based thin film solar cell has progressed remarkably in recent years [1–4]. Multi- junction technology is one of the key issues for improving the performance of the silicon based thin film solar cells [5–7]. Nowadays, the triple junction silicon based thin film solar cell shows the state of art of conversion efficiency of 16.1% for p–i–n type [6] and 16.3% for n–i–p type [7]. The theoretical analysis of triple junction silicon thin film solar cell suggested that an opti- mum band gap of the bottom sub-cell absorber should be 0.9 [8] or 1.0 eV [9] depending on the type of the top and middle sub-cells. However, the band gap of the hydrogenated microcrystalline silicon (lc-Si:H) that generally has been used as the bottom sub- cell absorber was 1.1 eV which is not the optimal value. Besides, due to its weak infrared absorption, a very thick lc-Si:H i-layer (>2 lm) that is much thicker than the top and middle amorphous silicon based sub-cells has to be fixed to match the photocurrent of the other sub-cells, and this leads to a long deposition time, which proves to be unfit for the industrial process [10]. Hence to further improve the conversion efficiency and reduce the production costs, lc-SiGe:H [11] which exhibits narrower variable band gap and higher absorption coefficient than the conventional lc-Si:H mate- rial has been developed. A few groups have reported on the lc-SiGe:H material [12–16] and its application on the thin film solar cells [17–19]. The advantage of the lc-SiGe:H solar cell is the capacity of infrared absorption with thinner thickness due to its narrow band gap. However, a trade-off between optical and electrical properties needs to be concerned, for defect density tends to increase with narrowing the band gap of the lc-SiGe:H film [20], and quite a few efforts are underway to improve the device performance as the infrared absorber [21,22]. The band gap profile structure designed to reduce the carrier recombination and optimize the internal electric field has been well applied to the hydrogenated amorphous silicon germanium solar cells [23–26]. In this paper, the experimental study was reported to improve the performance of the lc-SiGe:H solar cells by applying this band gap multiple profiled technique. The struc- tural properties of the lc-SiGe:H i-layer with different profile types were studied. The performance of the lc-SiGe:H solar cells with different profile structures was compared. Finally, an improved lc-SiGe:H solar cell performance was presented. http://dx.doi.org/10.1016/j.jallcom.2015.01.224 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Institute of Photo-electronic Thin Film Devices and Technique, Nankai University, Tianjin 300071, China. Tel./fax: +86 22 23508032. E-mail address: jjzhang@nankai.edu.cn (J. Zhang). Journal of Alloys and Compounds 632 (2015) 456–459 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom