Giso Hahn, Chair of the conference, delivers a closing speech for SiliconPV 2022 on Wednesday afternoon. Image: pv magazine/Mark Hutchins
This week saw more than 300 scientists and PV experts converge in Konstanz, Germany for discussion of the latest development in PV cell technology, and for many the first chance for a face-to-face meeting in quite some time. pv magazine reports from the SiliconPV conference, where the PV research community revealed a strong focus on eliminating or optimizing the use of critical materials like silver and indium from cell production, alongside a wealth of improvements in efficiency and longevity that is still possible for silicon PV technology.
From pv magazine
Critical materials are, well, critical
A significant chunk of the research presented this week has focused on the metallization and transparent conductive oxide areas of cell production, which typically involve the use of silver and indium, respectively.
The latest n-type cell technologies, heterojunction in particular, require considerably more silver than earlier generations of cell technology. And as solar manufacturing continues to scale, consumption of these materials becomes ever more problematic. University of New South Wales scientist Brett Hallam outlined these issues clearly in work to be published later this year, under the title “Roadmap towards sustainable HJT solar cell design”, outlining the pros and cons of various approaches including thinner TCO layers and replacing silver screen printing with copper plating.
And there was plenty of innovation in these areas on show throughout the conference: TU Delft researcher Can Han presented work on a heterojunction cell stack without a TCO layer, and Stefan Janke of Helmholz Zentrum Berlin joined the conference virtually to share work on aluminum-zinc oxide as an indium free alternative, finding that some indium might still be needed to ensure stability, but combining ITO with AZO could reduce indium consumption by at least 80%.
On the silver side, several approaches to copper plating were shown during the week: Including one from Sven Kluska of Fraunhofer Institute for Solar Energy Systems, which also found that copper plating in TOPCon potentially allows for other improvements to the cell stack, including a thinner polysilicon layer. And Katharina Gensowski of the same research institute presented a “filament stretching” approach that could greatly optimize the use of silver, reducing the consumption per line from 1.9mg to 0.7mg.
New cells, same hydrogen
Understanding the role of hydrogen atoms in various mechanisms at work within a solar cell has long been a challenge for researchers, and one that has changed with every new development in solar cell technology.
The ability to detect hydrogen and know exactly what it is doing is still high on the list of things PV scientists need – it is known that the presence of hydrogen has both positive and negative effects, but more understanding is still needed to be able to engineer out the negatives and optimize the positives such as improved passivation and performance regeneration after light-induced degradation.
An entire session on the first morning was devoted to this, and the work presented found that hydrogen’s role in heterojunction cells may be quite different for PERC or TOPCOn. And work presented by Abigail Meyer from the U.S. National Renewable Energy Lab/Colorado School of Mines used a technique called electron paramagnetic resonance to confirm hydrogen’s involvement in the light-elevated temperature induced degradation (LETID) mechanism.
Gallium doping: Defects and degradation
As a solution to light-induced degradation (LID), cell manufacturers have begun using gallium rather than boron as a dopant. And since the industry has been relatively quick to adopt this approach at scale, work is still ongoing to fully understand other implications, beyond the removal of the boron-oxygen complex that causes LID.
A presentation from University of Manchester’s Tarek Abdul Fattah spoke of silicon PV “moving into the gallium era”, and noted that the annual ITRPV report – an oft-cited source this week – forecasts 100% of cells to use gallium doping by 2031.
Abigail Meyer’s work also confirmed that LETID can still happen in gallium-doped wafers, and that here as well hydrogen is somehow involved. Elsewhere, work has revealed that the presence of gallium can lead to a material defect, but not in a configuration that causes recombination or performance losses, and that a degradation mechanism related to gallium could potentially be permanently deactivated via heat treatment.
Perovskites: not if, but when (and how)
Though the conference is still called SiliconPV, this year it dedicated an entire morning to tandem cell technologies, placing a perovskite cell on top of the silicon. Stefan de Wolf, professor at the King Abdullah University of Science and Technology, started the day off with an invited talk outlining some of the impressive achievements, and the challenges still in the way for tandem cells.
With the two cells on top of each other and connected in series, much of the discussion in this session focused on the need for accurate current matching between the two cells, and the ability to tell when one is limiting the other, and which one is doing the limiting. A presentation from Fraunhofer ISE’s Alexander J. Bett found that under red light perovskite tends to be the limiting cell, while it is the other way under blue light. And the tunability of perovskites will provide the solution here. Several scientists advocated ‘bromide lean’ perovskite solutions with a bandgap of around 1.64 electron volts as optimal for current matching with a silicon cell.
Wealth of new materials/approaches
Those working on perovskites in particular are often keen to speak of silicon coming close to its limit in terms of performance. However, the work on show this week reveals there is plenty more to be done in terms of understanding and optimizing the minuscule processes going on inside a solar cell: as we saw with new work on light soaking in heterojunction cells – able to bring about a “0.4% absolute efficiency gain in one second”, according to Oxford University’s Matthew Wright.
And there is a wealth of materials seldom seen in silicon cell production that have also shown potential worthy of further investigation, as seen in Tuesday afternoon’s session on passivating contacts and the presentation of both aluminum-doped zinc (from Bart Macco of Eindhoven University of Technology, and silicon carbide (presented by Ezgi Genç of École Polytechnique Fédérale de Lausanne), as possible new assistants in the search for better performance at lower costs.
Author: Mark Hutchins