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