Deposition pressure dependent structural and optoelectronic properties of ex-situ boron-doped poly-Si/SiOx passivating contacts based on sputtered silicon

https://doi.org/10.1016/j.solmat.2020.110602Get rights and content

Highlights

  • Effects of sputtering pressure on the structural and optoelectronic properties of the as-deposited a-Si films and their corresponding ex-situ boron-doped poly-Si/SiOx passivating contacts were investigated.

  • The Si–H stretching modes observed by FTIR in the as-deposited films are likely a consequence of residual hydrogen incorporated to the oxide layer during its chemical growth.

  • The resultant poly-Si/SiOx contacts show a reasonably good initial passivation quality with Jo of 24 fA/cm2 at 2.5 mTorr, which improves to 9 fA/cm2 after hydrogenation treatments.

Abstract

Among common methods to form polycrystalline silicon (poly-Si) films for passivating-contact solar cells, physical vapor deposition, in particular sputtering, is the safest one as it does not require any toxic gaseous precursors. One of the critical parameters to control the properties of sputtered silicon films is their deposition pressure. In this work, structural and optoelectronic characteristics of ex-situ boron-doped poly-Si/SiOx passivating contacts, formed from sputtered intrinsic amorphous silicon (a-Si) deposited at different pressures on top of SiOx/c-Si substrates and subjected to a high-temperature boron diffusion step, are investigated. The deposition rate and density of the as-deposited a-Si films increase with reducing pressure. Low-temperature photoluminescence spectra captured from the as-deposited samples at different pressures do not show typical emissions from hydrogenated a-Si. Meanwhile, their Fourier-transform infrared absorption spectra all show Si–H stretching modes, indicating that hydrogen had been initially incorporated into the chemical SiOx layers and eventually hydrogenated the a-Si/SiOx interfaces during the sputtering process. After the high-temperature boron-diffusion step, all hydrogen-related peaks disappear. Lower pressure films (1.5 and 2.5 mTorr) show more consistent improved performance after hydrogen treatments, compared to higher pressure films (4 and 5 mTorr). The resultant passivating contacts at 2.5 mTorr achieve a low single-side recombination current density Jo of ~9 fA/cm2, whereas their contact resistivity is still low at 15 mΩ cm2.

Introduction

Passivating-contacts based on a layer of doped polycrystalline silicon (poly-Si) and an ultra-thin interfacial silicon oxide (SiOx) layer on top of a crystalline Si (c-Si) substrate have been shown to be able to overcome the efficiency limitations of the current aluminium back surface field (Al-BSF) and passivated emitter and rear contact (PERC) industrial solar cell technologies. Several groups have demonstrated laboratory efficiencies of >24% [[1], [2], [3], [4]] for solar cells with poly-Si passivating contacts. There are many approaches to form the poly-Si film, and they can broadly be classified in two categories, chemical vapor deposition (CVD) and physical vapor deposition (PVD). In the CVD methods, the poly-Si films are obtained by recrystallizing amorphous Si (a-Si) layers deposited by plasma-enhanced CVD (PECVD) [2], low-pressure CVD (LPCVD) [[5], [6], [7]], atmospheric pressure CVD (APCVD) [8], or hot-wire CVD (HWCVD) [9]. Meanwhile, the PVD method, specifically sputtering, has been used widely in microelectronics, optics and coatings of thin films [10]. In PV, it is often used to deposit thin-film absorbers for CdTe [11] and CIGS [12] solar cells, transparent conductive oxide (TCO) [13] and intrinsic a-Si films for heterojunction Si solar cells [14], and indium zinc oxide (IZO, one type of TCO) for perovskite solar cells [15].

One of the most important parameters to control the properties of a-Si films deposited by sputtering is the deposition pressure. In previous work [16], we have shown that with a fixed deposition radio frequency (RF) power density of a Si target and varying power densities of a co-doped boron target, the total dose of boron in the films can be controlled. Although we were successful in demonstrating a good efficiency of 23.7% on the finished devices [16], the effects of sputtering conditions and doping process happened concurrently, yielding difficulties in understanding their individual impacts on the contact performance. In this study, we investigate a hole-selective contact formed by sputtering intrinsic a-Si onto a SiOx/c-Si substrate and performing ex-situ boron diffusion in a high-temperature furnace. In this way, we can separate the effects of the a-Si deposition pressure from the doping process, and investigate the impacts of the as-deposited intrinsic Si film properties on their final passivating contact performance. The effects of the deposition pressure on structural, optoelectronic, and passivation properties of the poly-Si/SiOx contacts are studied by a combination of various characterization techniques. Once fully understood, these findings can be employed to fabricate high-quality passivating contacts via the sputtering methods.

Section snippets

Results and discussions

First, we investigate the effects of deposition pressure on the deposition rate of initial a-Si films. The thickness is measured by an ellipsometer and the deposition rate is calculated by dividing the thickness over a fixed deposition time of 30 min, as shown in Fig. 1A. From the figure, increasing the deposition pressure decreases the deposition rate. This can be explained by the fact that, although at higher pressures the increasing Ar ion density and current in the plasma lead to a higher

Conclusions

In conclusion, we have reported the effects of sputtering pressure on the structural and optoelectronic properties of the as-deposited a-Si films and their corresponding ex-situ boron-doped poly-Si/SiOx passivating contacts. Lower pressures yield thicker and denser films with the same sputtering time, due to higher deposition rates. The PL spectra from the as-deposited films do not show the emissions that are typical of hydrogenated a-Si, confirming the absence of hydrogen in the sputtering

CRediT authorship contribution statement

Thien N. Truong: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft, Validation. Di Yan: Resources, Investigation, Writing - review & editing. Wenhao Chen: Investigation. Wenjie Wang: Investigation. Harvey Guthrey: Investigation, Writing - review & editing. Mowafak Al-Jassim: Writing - review & editing. Andres Cuevas: Writing - review & editing. Daniel Macdonald: Supervision, Funding acquisition, Writing - review & editing. Hieu T. Nguyen: Conceptualization,

Declaration of competing interest

No known conflicts of interest.

Acknowledgments

This work has been supported by the Australian Renewable Energy Agency (ARENA) through Research Grant RND017. The authors acknowledge the facility and technical support from the Australian National Fabrication Facility (ANFF), ACT Node and the Department of Electronic Materials Engineering, The Australian National University. H.T.N. acknowledges the fellowship support and collaboration grant from the Australian Centre for Advanced Photovoltaics (ACAP).

References (30)

  • R.H. Cox et al.

    Ohmic contacts for GaAs devices

    Solid State Electron.

    (1967)
  • A. Richter et al.

    Tunnel oxide passivating electron contacts as full-area rear emitter of high-efficiency p-type silicon solar cells

    Prog. Photovoltaics Res. Appl.

    (2018)
  • C. Hollemann et al.

    26.1%-efficient POLO-IBC cells: quantification of electrical and optical loss mechanisms

    Prog. Photovoltaics Res. Appl.

    (2019)
  • A. Merkle et al.

    Atmospheric pressure chemical vapor deposition of in-situ doped amorphous silicon layers for passivating contacts

  • S. Li et al.

    Poly-Si/SiO x/c-Si passivating contact with 738 mV implied open circuit voltage fabricated by hot-wire chemical vapor deposition

    Appl. Phys. Lett.

    (2019)
  • Cited by (17)

    • Blistering-free polycrystalline silicon carbide films for double-sided passivating contact solar cells

      2022, Solar Energy Materials and Solar Cells
      Citation Excerpt :

      Developing a cost-effective method for the fabrication of ultrathin SiOx and doped poly-Si layers is of great significance to promote the commercialization of TOPCon technology. In terms of the doped poly-Si films, the commonly-used fabrication methods [5] include plasma enhanced chemical vapor deposition (PECVD) [1,6–10], low pressure chemical vapor deposition (LPCVD) [11–17], and sputtering [18–20]. Although LPCVD method is relatively more matured, the PECVD technology has been gaining more attentions in recent years due to the high deposition rate, the feasibility of in-situ doping, and the capability of single-side deposition, which might reduce the overall manufacturing cost and accelerates the practical use of TOPCon in mass production.

    View all citing articles on Scopus
    View full text