High efficiency and stable hydrogenated amorphous silicon radial junction solar cells built on VLS-grown silicon nanowires Soumyadeep Misra a , Linwei Yu a,b,n , Martin Foldyna a , Pere Roca i Cabarrocas a a LPICM, Ecole Polytechnique, CNRS, 91128 Palaiseau, France b School of Electronics Science and Engineering, Nanjing University, 210093 Nanjing, China article info Article history: Received 24 April 2013 Received in revised form 12 June 2013 Accepted 26 July 2013 Keywords: Solar cell Radial junction Silicon nanowires VLS method abstract Hydrogenated amorphous silicon (a-Si:H) radial junction solar cells, built over a dense matrix of Si nanowires (SiNWs), benefit from strong light trapping. This allows the use of a very thin absorber layer without sacrificing the solar cell performance, while improving its stability. By optimizing the density of SiNWs grown via a plasma-assisted vapor–liquid–solid process on glass, we have achieved radial junction a-Si:H solar cells with an open circuit voltage of 0.80 V, short circuit current density of 16.1 mA/ cm 2 and a high power conversion efficiency of 8.14%. Furthermore, we present experimental evidence of the excellent stability of such radial junction a-Si:H solar cells with a light-induced degradation of only 6%, compared to the typical degradation of 15% to 20% in planar cells. These results indicate the feasible and promising approach towards a new generation of stable and high performance a-Si:H thin film solar cells. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Tapping into the ultimate clean energy source, by employing cost-efficient and environment-friendly photovoltaic (PV) solar cells, can help to reverse the unsustainable energy consumption pattern based on the burning fossil fuels [1–4]. In terms of technical maturity and fabrication cost, hydrogenated amorphous silicon (a-Si:H) solar cells represent an ideal choice. This is also because silicon based solar cells can reach Tera-Watt scale PV capacity without fundamental limitations in materials supply and environmental issues [5,6]. However, a major challenge for a-Si:H materials lies on its disordered matrix, resulting in limited photo- carrier collection and poor solar cell stability under sunlight exposure [5,7]. An effective solution to address this problem is to build up radial junction PIN thin film solar cells on silicon nanowire (SiNW) array to benefit from a strong light trapping and absorption [8]. Recently, Fontcuberta et al. have shown that nanowire based solar cell has the potential to go beyond Shockley– Queisser limit [9]. Thanks to the strongly enhanced light trapping among the Radial Junction (RJ) forest [1,2], a much thinner absorber (i-layer) can be used to increase the built-in electric field for a better carrier separation and minimized light-induced degradation, which is well known for planar Si thin film devices [10]. Although reasonable RJ thin film solar cells with 48% efficiency have been demonstrated on etched crystalline silicon wafer [11,12], the efficiency of most of the early RJ prototypes has been mostly limited by a series of problems, including catalyst contamination, conformal coating and electrical quality in radial junction formation [13–17], leading to a poor conversion efficiency ( o2%). Moreover, the stability of RJ a-Si:H solar cells, built on randomly oriented SiNWs, has never been studied experimentally mainly due to the lack of a reasonable RJ solar cell platform to evaluate this critical aspect. Over the past few years, we have been developing a one-pump- down process to grow SiNWs via a plasma-assisted Vapor–Liquid– Solid (VLS) process [18–21], as an industrially viable platform to frame up a-Si:H RJ solar cells on top of low cost glass or metal foil substrates. A group of low melting point metals, including tin (Sn) [22–25], indium (In) [23,26,27], gallium (Ga) [28] and bismuth (Bi) [29], have been adopted to mediate the VLS growth of SiNWs in a Plasma Enhanced Chemical Vapor Deposition (PECVD) system. In this way, we have demonstrated Sn assisted SiNW growth at a substrate temperature as low as 240 1C [23], with yet a very important bonus that after growing the SiNWs into a desired length, the remnant Sn can be removed by a simple hydrogen (H 2 ) plasma etching without the need of any ex situ cleaning [30]. All these unique abilities enabled us to fabricate a-Si:H radial PIN junction solar cells showing conversion efficiencies up to 6% [31,32]. Our further investigation has identified the density of the random SiNW array as a key parameter for seeking a best trade-off between strong light-trapping and uniform junction Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.07.036 n Corresponding author at: LPICM, Ecole Polytechnique, CNRS, 91128 Palaiseau, France. Tel.: +33 169334353. E-mail addresses: Linwei.yu@polytechnique.edu, yulinwei@nju.edu.cn (L. Yu), pere.roca@polytechnique.edu (P. Roca i Cabarrocas). Solar Energy Materials & Solar Cells 118 (2013) 90–95