37th EU PVSEC, 07 - 11 September 2020
CCL Lisbon Congress Centre, Lisbon, Portugal

09 July 2019

Preview on Topic 2: Silicon Materials and Cells

The Plenary titled “Silicon PV Highlights” reveals today’s top technologies for high-efficiency Silicon cells. The authors of the first plenary presentation titled “Bulk Defects in Monocrystalline Silicon, Multicrystalline Silicon and Mono-Like Silicon Materials” [2BP.1.1] have been invited in order to provide a comprehensive overview on the bulk defects and their passivation on today’s Si wafer materials. The authors will show that the possible concepts for depositing dielectric layers for passivation, SiN, Aluminium oxide and amorphous Silicon deposited with different PECVD tools, produce similar bulk hydrogenation effects for high performance multi-crystalline Silicon materials when annealed at optimal conditions. The results are important for better understanding of the suitability of multi-crystalline base material for use in e.g. PERC (Passivated Emitter Rear Cell/Contact).

The second plenary, titled “The Versatility of Passivating Carrier-Selective Silicon Thin Films for Diverse High-Efficiency Heterojunction-Based Solar Cells“ [2BP.1.2] gives an overview about the development of SHJ (Silicon Heterojunction) technology. This gains importance as the use of amorphous Silicon for intrinsic passivating films both on the front and back side of solar cells, allows interdigitated back-contact (IBC) cells. The presentation will also demonstrate efficiencies for large-area cells which are consistently above 24%, both for n-type and p-type wafers.

The third plenary quotes in its title “Both Sides Contacted Silicon Solar Cells: Options for Approaching 26% Efficiency” [2BP.1.3] a remarkable record efficiency, achievable by applying a “Tunnel oxide passivating contact” (TOPcon) on the rear surface. So far it represents a result of an extensive3-d-modelling of the structure, analysing all the loss mechanisms, like bulk and surface recombination and surface transport losses. The research group made 2x2 cm2 prototype cells which yielded between 24.5% and 25.8% efficiency, coming close to the modelling target.

The TOPcon approach is also presented in the session named “Development of Industrial n-Type Bifacial TOPCon Solar Cells and Modules” [2BP.1.5] revealing a pre-industrial fabrication process. This concept can increase efficiency by 1% absolute compared to conventional “Passivated Emitter Rear Totally Diffused” (n-type PERT) processes. The “Tunnel-oxide passivating contact” (TOPcon) is created by Ion implantation. Mass produced, large area cells were measured with consistently 22.5% efficiency, those from an optimised process even with 23%. A standard 60 cell module would deliver as much as 330-335 W! We expect that during EU PVSEC many of these efficiency figures will be higher, and we are sure there will be a strive for setting up records this summer!

In the plenary titled “Approaching 23 % and Mass Production of Bifacial p-Cz Q.ANTUM PERC Solar Cells” [2BP.1.4] you will listen to another candidate for record efficiency. The authors employ PERC (Passivated Emitter Rear Cell/Contact) technology, both with mono- as well as bifacial structure. The latter one was tested on a large area and showed an increase to 22.5% in efficiency, with a rear efficiency of almost 15%. A careful analysis of the loss mechanisms and how to overcome them makes the authors confident to increase efficiency further.

In each of the topics at EU PVSEC 2019 there are also several student award finalists. We consider this a very good opportunity to give the floor to the best students in photovoltaic research; the final winners will be selected during the conference. In Topic 2 there are four finalists, and this newsletter points out the student’s oral presentation “Bottom-Up vs Top-Down Approaches for Identifying and Mitigating the Transport Losses in High-Efficiency Silicon Heterojunction Solar Cells” [2CO.10.2]. This presentation is about the complexity of the different interface layers between the bulk material and the solar cell contacts which are very difficult to model. Nonetheless, the student tried it and was successful by choosing two approaches used to develop and optimise n-type amorphous/nanocrystalline multilayers to be integrated into functional Silicon Heterojunction devices. This led to up to 25% efficiency in medium-size devices.

As one of the highlights of the presentations on thin silicon, we are curious to listen to the oral presentation “Bifacial Amorphous Si Quintuple-Junction Solar Cells for IoT Devices with High Open-Circuit Voltage of 3.5V under Low Illuminance” (2AO.6.4). As the “Internet of Things” now seems to be everywhere, it would be great if it could be powered by Photovoltaics! Now, bifacial, and even 5 junctions? The reason the renowned Japanese authors go for many junctions is to increase the voltage to the point at which the PV cell can power Large Scale Integrated Circuits directly without complex power conversion. Such a cell was reported last year, and at EU PVSEC 2019 the presenters will reveal first experimental results and more details on this new thin-film concept consisting of amorphous SiN and Si. An open circuit voltage as high as 3.5 V has already been demonstrated using a device of only 650 nm thickness. It is surprising that the different layers need no separate interconnection steps.

This newsletter also highlights a large European project. In the “Final Report of the EU H2020 Project - NextBase: An European Collaboration for Cost Competitive and High Efficiency Interdigitated Back-Contact Silicon Heterojunction Solar Cell Technology” [2CO.10.1], the consortium of 14 organisations across 8 nations will present the innovation in developing the Silicon Heterojunction (SHJ) cell to efficiency levels as high as 26%, whilst maintaining production costs below <0.35 €/Wp. For this purpose, the consortium developed a new PEVCD (Plasma enhanced Chemical Vapor Deposition) reactor for reaching highly efficient SHJ with a low number of process steps. Using “Smart Wire Connection Technology” (SWCT) large cell efficiencies of 23.2% have been measured in this successful project, which will come to its end just at the time of the EU PVSEC 2019.

If you like to know what the prospects of Europe are to become again a significant manufacturing and technology player, you should listen on Friday to the presentation “The Vision of Large Scale PV Manufacturing in Europe: A Dream or Chance for Execution?” [2EO.1.1]. The authors will demonstrate that this does not only have to be a dream. They will present key aspects from a technology and cost perspective. By comparing technological choices, they opt for manufacturing in a “Smart Green Flex Fab”, meaning that all process steps shall be optimised to allow a truly sustainable production. Within a closed loop material supply chain, key consumables like water, electricity, gas etc. will be significantly reduced compared to standard production. The authors are convinced that this gives Europe a good chance to compete with Asian manufacturing, in particular when the lower transport costs to European markets are taken into account. Based on an annual capacity of 10 GWp, the consortium estimates factory cost-of-ownership only 5% higher than a comparable factory with a similar technology in Asia.