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Surfaces, Interfaces, and Applications x
Bifunctional hybrid a-SiO(Mo) layer for hole-selective and interface passivation of high efficient MoO/ a-SiO(Mo)/n-Si heterojunction photovoltaic device x
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Ming Gao, Dongyun Chen, Baichao Han, Wenlei Song, Miao Zhou, Xiaomin Song, Fei Xu, Lei Zhao, Yonghua Li, and Zhongquan Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07001 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018
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ACS Applied Materials & Interfaces
Bifunctional hybrid a-SiOx(Mo) layer for hole-selective and interface passivation of high efficient MoOx/a-SiOx(Mo)/n-Si heterojunction photovoltaic device
Ming Gaoa, Dongyun Chena, Baichao Hana, Wenlei Songa, Miao Zhoua, Xiaomin Songa, Fei Xua, Lei Zhaoa, Yonghua Lia, Zhongquan Ma∗, a, b a
SHU-SOEN’s R&D Lab, Department of Physics, College of Sciences, Shanghai University, Shanghai 200444, China
b
Instrumental Analysis & Research Center, Shanghai University, Shanghai 200444, China
Abstract: The promising n-Si based solar cell is constructed for the purpose of realizing holeand electron-selective passivating contact, using a textured front ITO/MoOx structure and a planar rear a-SiOx/poly-Si(n+) structure severally. The simple MoOx/n-Si heterojunction device obtains an efficiency of 16.7%. It is found that the accompanying ternary hybrid SiOx(Mo) interlayer (3.5~4.0 nm) is formed at the MoOx/n-Si boundary zone without pre-oxidation and is of amorphous structure, which is discriminated by a high-resolution transmission electron microscope with energy dispersive X-ray spectroscopy mapping. The creation of lower oxidation states in MoOx film indicates that the gradient distribution of SiOx with Mo element within interlayer occurs, acting as a passivation of silicon substrate, which is revealed by X-ray photoelectron spectroscopy with depth etching. Specifically, the calculations by density functional theory manifest that there are two half-filled levels (localized states) and three unoccupied levels (extended states) relating to Mo component in the ternary hybrid a-SiOx(Mo) ∗
Corresponding author, E-mail address:
[email protected];
[email protected] (Zhongquan Ma) 1
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interlayer, which play the roles of defect-assisted tunneling and direct-tunneling for photo-generated holes, respectively. The transport process of photo-generated holes in the MoOx/n-Si heterojunction device is well-described by the tunnel-recombination model. Meanwhile, the a-SiOx/poly-Si(n+) has been assembled on the rear of device for a direct-tunneling of photo-induced electrons, and blocking photo-induced holes.
Keywords: ternary hybrid interlayer, MoOx/n-Si heterojunction device, passivated contact, hole-selective contact, defect-assisted tunneling
1. Introduction For the most silicon heterojunction solar cells, the efficient separation, extraction and transport of photo-generated carriers are largely achieved by carriers (holes and electrons) -selective contact and passivating contact.1-3 The typical one is a dopant-free asymmetric hetero-contacts (DASH) solar cell, in which the electron transport layer (LiFx) and the hole transport layer (MoOx) on the opposite surface of n-Si wafer are used. Meanwhile, the intrinsic hydrogenated amorphous silicon (a-Si:H(i)) is inserted to reduce the interface recombination rate.2 Another example is the tunnel oxide passivated contact (TOPCon) photovoltaic (PV) device, which is based on poly-Si(n+) layer and ultrathin SiO2 layer on the n-Si bulk, obtaining electron-selective and passivating contact, respectively.3 More precisely, the thermally stable carrier-selective passivating contact can suppress minority-carrier recombination and facilitate majority-carrier transfer, which essentially improves the open-circuit voltage (Voc) and fill factor (FF) of PV devices.1-3 However, the effective selectivity for majority-carrier and the minimizing recombination for minority-carrier require two types of materials (carrier-selective material and passivation material) in the PV devices mentioned above. Therefore, the best optimization scheme for a Si-based heterojunction PV device would be the assembly of a newly bifunctional (selective/passivated contact) material.1, 4 2
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ACS Applied Materials & Interfaces
In recent years, we devoted to researching advanced electronic materials as hole-selective passivation layer of the ITO/n-Si structure solar cell.5-9 Fortunately, the accompanying a-SiOx(In) layer was formed during sputtering ITO film on n-Si substrate, which played multiple roles including induced inversion layer, interface passivation, hole-selective and hole-tunneling.6-7 The ITO/n-Si solar cell obtained a Voc of 540 mV and an efficiency of 12.2%. However, the Voc was limited by built-in-field, which was essentially confined by the work-function difference between ITO (5.06 eV) film and n-Si (4.31 eV) bulk.6 In order to solve the issue, the replacement of ITO with a higher work function material is initially needed. Remarkably, the sub-stoichiometric molybdenum oxide (MoOx, x
