Development of atmospheric plasma Enhanced SPatial ATomiC layer deposition (SALD) for application to silicon Heterojunction solar cells – DESPATCH

In addition to the specific requirements that alternative ZnO-based TCOs must fulfil, in the case of HET processing must also be fast and at temperatures limited to = 200 °C, in order to preserve the amorphous state of silicon. Atomic layer deposition (ALD) would seem the method of choice since it offers the unique capability of depositing high-quality thin films with precise thickness control, exceptional uniformity, and excellent step coverage on non-planar surfaces, thus allowing an unrivalled potential for surface nanoengineering. A novel ALD approach that is receiving much attention in the last years is the so called spatial ALD (SALD). In SALD the precursors are supplied continuously, but in different physical locations. This makes SALD much faster than conventional ALD (up to two orders of magnitude) since no purge steps are involved, and the lack of vacuum processing makes it much more attractive for large-scale production. SALD is therefore very suitable for new generation photovoltaic.
Plasma enhancement or assistance for conventional ALD system (PEALD) has been well studied in the past and is now a robust and conventional tool for microelectronics applications. In the case of SALD, an atmospheric plasma is required. In the same way than in PEALD, atmospheric plasmas are thus interesting since they can deliver a high, diverse but selective reactivity to a surface without heat, and can therefore access a parameter space in materials processing, which is not easily accessible with strictly chemical methods. During the DESPATCH project, an atmospheric plasma activated SALD has been designed and assembled and it has been used to deposit TE for application to HET cells. Simulations and in situ characterization have been used for the optimisation of the system and the deposition protocols.

The major results obtained during the ANR project DESPATCH are the following:

- The design and fabrication of a customizable SALD system equipped with atmospheric plasma activation. The LMGP is thus one of the few labs in the world having this innovative deposition technology
- The deposition of SiO2 thin films by plasma enhanced spatial chemical vapour deposition at temperatures close to room temperature and using an affordable and safe precursor
- The utilisation of 3D printing to fabricate low-cost, custom SALD deposition heads, and atmospheric plasma sources.
- The development of new deposition heads and protocols for the area-selective deposition of functional materials. This include the deposition by Spatial Chemical Vapour Deposition in static mode and the utilization of deposition heads with concentric channels
- The elucidation of a more complete conductivity mechanism model for semiconductive polycrystalline films including the possibility of charge tunnelling at the grain boundaries, which had been ignored by the traditional model used for the last 40 years by the scientific community
- The fabrication, study and modelling of composite electrodes based on metallic nanowires and oxide coatings.
- The utilization of SALD for the passivation of the edges of HET cells and for the deposition to p-type oxides as alternative to the amorphous Si layers

The results of the DESPATCH project have motivated the application of 3 patents. The first deals with the custom designed 3D printed heads developed at LMGP, as detailed above. The second involves the protection of the edges of HET cells with the SALD and involves two partners of DESPATCH (INES and LMGP). Finally, a third patent with the CEA covers the protection of new generation TE with SALD coatings.
For the first patent, the CNRS has entered in contact with a company potentially interested in licensing and exploitation.
The DESAPTCH project has yielded multiple, high-impact results that go beyond the initial objectives and fields of applications of the project. While ZnO-based TE with low enough resistivity to be used in HET cells have not yet been achieved by SALD, there is still the possibility of further optimize the project and combine it with other activation and post deposition processes.
Thanks to DESPATCH, the LMGP has consolidated his top position among the laboratories developing SALD by i) being one of the very few labs developing atmospheric plasma SALD, ii) implementing 3D printing for the custom design and fabrication of SALD close proximity heads, and iii) developing new routes to ASD.
The possibility to passivate the edges of HET cells with SALD has been an unexpected result of the project that has yielded a patent application, and a collaboration of the LMGP and INES in the framework of the EU Highlite project. INES and LMGP are also applying for new projects to develop functional layers in HET based devices.
The project is expected to have a huge impact in the TE community as a result of i) the new, more complete model developed and ii) the works done on composite TE based on oxides and metallic NWNs.
From the industrial point of view, there are several companies already interacting with the LMGP and interested in the possibility to develop and apply SALD in their targeted applications.

There have been five articles so far, in common between different partners of DESPATCH. Several additional joint papers are in progress and should be submitted within the next months. The list of published articles is the following:

Article involving LMGP, LTM and GREMI:

- Nguyen, V. H. et al. Chem. Mater. 32, 5153–5161 (2020).

Article involving LMGP and Annealsys:

- Muñoz-Rojas, D. et al. Mater. Today Chem. 12, 96–120 (2019).

Article involving LMGP and GREMI:

- Zoubian, F. et al. J. Phys. Conf. Ser. 1243, 012002 (2019).

Articles involving LMGP and INES:

- Nguyen, V. H. et al. Mater. Horizons 5, 715–726 (2018).
- Nguyen, V. H. et al. Chem. Eng. J. 403, 126234 (2021).

Articles related to DESPATCH involving LMGP on composite electrodes with metallic nanowires and other transparent conductive materials:

- Nguyen, V. H. et al. Nanoscale 11, 12097–12107 (2019).
- Papanastasiou, D. T. et al. Adv. Funct. Mater. 30(1), 1910225 (2020).
- Aghazadehchors, S. et al. Nanoscale 11, 19969–19979 (2019).
- Resende, J. et al. Small 17, 2007344 (2021).
- Hanauer, S. et al. ACS Appl. Mater. Interfaces 13, 21971–21978 (2021).

Articles related to DESPATCH involving LMGP on the modelling and development of SALD heads and system:

- Masse de la Huerta, C. A. et al. Coatings 9, 5 (2018).
- Masse de la Huerta, C. A. M. et al.Adv. Mater. Technol. 5(12), 2000657 (2020).

Articles related to DESPATCH involving LMGP on the deposition of functional SALD coatings for different applications:

- Nguyen, V. H., et al. ACS Appl. Nano Mater. 1, 6922–6931 (2018).
- Alshehri, A. H. et al. Adv. Funct. Mater. 29, 1805533 (2019).

The results obtained in the DESPATCH project have been featured as covers or frontispieces of different journals

DESPATCH has motivated the application of 3 patents. (1 of them between INES and LMGP)

The aim of this project is threefold:
1.- To develop an atmospheric plasma enhanced spatial atomic layer deposition (APE-SALD) setup. SALD is a recent variation of conventional ALD in which the precursors are separated in space rather than in time, thus allowing processing at atmospheric pressure and orders of magnitude faster deposition rates. Atmospheric plasma assistance would further expand the possibilities of this appealing technique thanks to processing at lower Ts (=150 ºC) with a larger precursor choice.
2.- To apply the developed APE-SALD system for the deposition of ZnO based transparent conductive oxide (TCO) electrodes on Si heterojunction (HET) solar cells, as an alternative to currently used sputtered ITO. SALD is an ideal technique for this HET technology since the presence of active amorphous Si layers on this type of cells requires soft deposition conditions, together with high throughput processing, in order to be industrially competitive. Reciprocally, the stringent processing requirements imposed by HET cells constitute an ideal test bench allowing the evaluation of the full potential of APE-SALD.
3.- On a more fundamental level, to characterize in-situ the plasma and reaction intermediates to understand the reaction mechanisms and the plasma species being generated.