Bismuth-Catalyzed and Doped Silicon Nanowires for One-Pump- Down Fabrication of Radial Junction Solar Cells Linwei Yu, †, * Franck Fortuna, ‡ Benedict O’Donnell, †,§ Taewoo Jeon, † Martin Foldyna, † Gennaro Picardi, † and Pere Roca i Cabarrocas † † LPICM, Ecole Polytechnique, CNRS, 91128 Palaiseau, France ‡ CSNSM, Universite ́ Paris-Sud, Bâ timent 108, 91405 Orsay Campus, France § R&D Division, Total S.A., Gas & Power, Courbevoie, France ABSTRACT: Silicon nanowires (SiNWs) are becoming a popular choice to develop a new generation of radial junction solar cells. We here explore a bismuth- (Bi-) catalyzed growth and doping of SiNWs, via vapor-liquid-solid (VLS) mode, to fabricate amorphous Si radial n-i-p junction solar cells in a one-pump-down and low-temperature process in a single chamber plasma deposition system. We provide the first evidence that catalyst doping in the SiNW cores, caused by incorporating Bi catalyst atoms as n-type dopant, can be utilized to fabricate radial junction solar cells, with a record open circuit voltage of V oc = 0.76 V and an enhanced light trapping effect that boosts the short circuit current to J sc = 11.23 mA/ cm 2 . More importantly, this bi-catalyzed SiNW growth and doping strategy exempts the use of extremely toxic phosphine gas, leading to significant procedure simplification and cost reduction for building radial junction thin film solar cells. KEYWORDS: Radial junction solar cell, silicon nanowire, vapor-liquid-solid growth, bismuth catalyzed growth, bismuth doping, PECVD thin film deposition B uilding radial junction thin film solar cells on top of silicon nanowires (SiNWs) allows to decouple light absorption path from photocarrier collection distance, making it possible to reduce the absorber thickness to improve carrier collection, while still achieving enhanced light trapping among SiNWs. 1-5 Amorphous and microcrystalline Si thin film technologies are industrially proven, environmentally friendly and capable of delivering Terawatt scale solar energy without fundamental limitation in material supply. 6,7 Marrying the advanced radial junction design to this mature Si thin film technology has thus the potential to build up a high performance and cost-effective thin film photovoltaics (PV). Particularly, by adopting a thinner absorber layer in a radial junction configuration, a stronger built-in field can help to minimize the Staebler-Wronski degradation in hydrogenated amorphous Si (a-Si:H) cells. To this end, a first and primary concern is to establish a SiNWs growth strategy compatible with the industrial-mainstream plasma deposition process, ideally achieving it on top of cost- effective substrates and at low temperature. The vapor-liquid- solid (VLS) mechanism represents a high-throughput and cost- effective way to produce well-defined SiNWs on top of various low-cost substrates. 8-11 However, since the VLS growth has to be catalyzed by metal nanoparticles, the remnant of metal catalysts on/in SiNWs imposes usually a threat to building effective photovoltaics, where clean surface/interfaces and minimum recombination centers are of utmost importance. Widely used gold (Au) is known to introduce midgap defects in c-Si, 12 acting as recombination centers for photocarriers. As a consequence, prototype SiNW solar cells built on top of untreated Au-catalyzed SiNWs show usually a low open circuit voltage (V oc ) < 0.3 V and poor fill factors (FF) < 30%. 3,13-15 High temperature (>900 °C) oxidation and chemical wet etching are therefore required to alleviate/suppress this impact. 2 Fortunately, VLS growth allows for a vast choice of alternative catalysts. 16 We have been working on a group of low-melting-point metals such as indium (In), tin (Sn), and gallium to achieve a low-temperature SiNW growth (down to 240 °C) in a plasma enhanced chemical vapor deposition (PECVD) environment. 17-23 An additional advantage lies in the fact that these catalysts can be eventually removed from the SiNWs by a simple H 2 plasma etching, while the incorporation of catalyst atoms into c-Si introduces only shallow or neutral levels in the Si bandgap. These features make them attractive to fabricate SiNW-based radial junction solar cells in an all-in situ or one-pump-down CVD process. In this work, we explore the use of bismuth (Bi) as catalyst to mediate the VLS growth of SiNWs, and provide the first evidence that Bi-catalyst incorporation in the c-SiNW cores can be utilized to achieve simultaneously effective n-type doping for building radial n-i-p junction solar cells. Bi has been known to introduce a shallow donor level in c-Si, lying 69 meV below the Received: May 7, 2012 Revised: July 13, 2012 Published: July 23, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 4153 dx.doi.org/10.1021/nl3017187 | Nano Lett. 2012, 12, 4153-4158