基于掺杂PECVD硅膜的钝化接触的N型高效硅太阳膜电池

Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat High efficiency n-type silicon solar cells with passivating contacts based on PECVD silicon films doped by phosphorus diffusion Di Yan 1 , Sieu Pheng Phang 1 , Yimao Wan, Christian Samundsett, Daniel Macdonald ⁎ , Andres Cuevas ⁎ Research School of Engineering, Australian National University, Canberra, ACT 2601, Australia ARTICLE INFO Keywords: Carrier-selective passivating contacts PECVD Amorphous silicon High efficiency silicon solar cells ABSTRACT Carrier-selective contacts based on silicon films deposited onto a thin SiO x layer combine high performance with a degree of compatibility with industrial solar cell metallization steps. This paper demonstrates an approach to form electron-selective passivating contacts that maximises the overlap with common industrial equipment; it is based on depositing an intrinsic amorphous silicon (a-Si) layer by PECVD and then doping and re-crystallizing it by means of a thermal phosphorus diffusion. By optimizing the intrinsic a-Si thickness and the phosphorus diffusion temperature, a low recombination current density J oc ≈3 fA/cm 2 and a low contact resistivity of ρ c ≈3mΩ-cm 2 have been achieved. Additionally, these electrical parameters have been found to be sensitive to the work function of the outer metal electrode. The application of these optimized electron-selective passivating contacts to n-type silicon solar cells has permitted to achieve a conversion efficiency of 24.7%. A loss analysis has been conducted through Quokka 2 simulations, which together with quantum efficiency measurements, indicate that further optimization should focus on the front boron-doped region of the device. 1. Introduction Reducing the already low fabrication costs of silicon PV is chal- lenging, but there are promising opportunities and an increasing need, to improve the performance of average production silicon cells and modules. Among them, passivating contacts based on a layer of doped re-crystallized silicon and an ultra-thin layer of silicon oxide formed on top of the silicon wafers can achieve a very high performance [1–5]. Although the deposited silicon layer may be initially amorphous, it is subsequently re-crystallized at a high temperature, becoming largely polycrystalline. This high temperature step makes the polycrystalline silicon/SiO x approach radically distinctive from that used for silicon heterojunction solar cells, which is based on depositing at relatively low temperatures layers of doped and intrinsic amorphous silicon [6,7].An attraction of the high temperature passivating contact approach is that it can have a relatively large overlap and be generically compatible with current industrial solar cell manufacturing equipment and pro- cesses. The approach presented in this paper is to use μW/RF PECVD (mi- crowave/radio-frequency plasma enhanced chemical vapour deposition) equipment and silane, both of which are commonly used for SiN x antireflection coatings, to deposit an intrinsic a-Si layer and then dope it by conventional phosphorus diffusion from POCl 3 . Such a doping step simultaneously serves to crystallize the deposited silicon, and slightly in-diffuse the dopant into the silicon wafer, both of which are critical to achieve good contact and passivation properties [8,9]. Examples of such diffusion optimization of intrinsic Si layers are given in earlier work by Romer et al. [9] and Yan et al. [8–10]. Recently, Liu et al. have demonstrated that adopting this approach to form the pas- sivating contacts is accompanied by strong impurity gettering effects, which provides additional benefits at no extra cost [11,12]. In this paper, we report further optimization of these electron-selective pas- sivating contacts, we study the impact of metals with different work functions, and we demonstrate the potential of this approach by fab- ricating n-type silicon solar cells with a 24.7% conversion efficiency. 2. Experimental methods The recombination current density J oc and contact resistivity ρ c test samples were prepared separately: high resistivity (50Ω-cm) (100) https://doi.org/10.1016/j.solmat.2019.01.005 Received 16 July 2018; Received in revised form 21 December 2018; Accepted 3 January 2019 ⁎ Corresponding authors. E-mail addresses: di.yan@anu.edu.au (D. Yan), pheng.phang@anu.edu.au (S.P. Phang), yimoa.wan@anu.edu.au (Y. Wan), chris.samundsett@anu.edu.au (C. Samundsett), daniel.macdonald@anu.edu.au (D. Macdonald), Andres.Cuevas@anu.edu.au (A. Cuevas). 1 Equal contribution as the first authors Solar Energy Materials and Solar Cells 193 (2019) 80–84 0927-0248/ © 2019 Elsevier B.V. All rights reserved. T n-type FZ Si wafers with symmetrical surface structure of electron-se- lective passivating contacts were used to obtain J oc ; Cox and Strack structures were fabricated on low resistivity (~0.5Ω-cm) (100) n-type CZ Si wafer to facilitate the extraction of ρ c values [13]. After saw damage etching and standard RCA cleaning, a thin oxide layer was grown on the substrates by immersion in hot nitric acid (68wt%) solution at a temperature of ~90°C for 30min. The final oxide thickness was measured to be approximately 1.4nm by ellipso- metry. Subsequently, intrinsic a-Si layers with thicknesses ranging from 50nm to 200nm were deposited using a laboratory-scale μW/RF PECVD system (Roth we found that increasing the diffusion temperature is effective for thicker as-deposited a-Si layers. By ad- justing the diffusion temperature, a very low recombination current density J oc ≈3 fA/cm 2 together with a low contact resistivity ρ c ≈3mΩ-cm 2 have been achieved for a wide range of a-Si thicknesses. By exploring contact formation with different metals, we found that low work function metals, such as Ag, are good candidates for such elec- tron-selective passivating contacts fabricated with the approach pre- sented in this paper. 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